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Superplasticity of AZ31 magnesium alloy prepared by friction stir processing
Abstract
Microstructure and tensile behaviors of AZ31 magnesium alloy prepared by friction stir processing (FSP) were investigated. The results show that microstructure of the AZ31 hot-rolled plate with an average grain size of 92.0 μm is refined to 11.4 μm after FSP. The FSP AZ31 alloy exhibits excellent plasticity at elevated temperature, with an elongation to failure of 1050% at 723 K and a strain rate of 5×10−4 s−1. The elongation of the FSP material is 268% at 723 K and 1×10−2 s−1, indicating that high strain rate superplasticity could be achieved. On the other hand, the hot-rolled base material, which has a coarse grain structure, possesses no superplasticity under the experimental conditions.
... This method is shown in Fig. 9b [140]. The SPD techniques such as accumulative roll bonding (ARB) [141], ECAP [142][143][144][145], FSP [122,[146][147][148], high pressure torsion (HPT) [149], and multidirectional forging (MDF) [150,151] have been extensively used to study the superplasticity of the AZ31 alloy. For instance, ARB of a homogenized AZ31 alloy with an average grain size of 22 µm has been reported [141]. ...
... The application of FSP for the processing of superplastic AZ31 alloy has been shown recently [122,148]. For instance, the hot rolled AZ31 alloy with an average grain size of 92 µm has been subjected to FSP with a rotation rate of 1500 rpm and a traversing speed of 60 mm/min, resulting in the average grain size of the 11.4 µm in the stir zone (SZ) compared to 92 µm for the base metal (BM), as shown in Fig. 12 [148]. ...
... The application of FSP for the processing of superplastic AZ31 alloy has been shown recently [122,148]. For instance, the hot rolled AZ31 alloy with an average grain size of 92 µm has been subjected to FSP with a rotation rate of 1500 rpm and a traversing speed of 60 mm/min, resulting in the average grain size of the 11.4 µm in the stir zone (SZ) compared to 92 µm for the base metal (BM), as shown in Fig. 12 [148]. Accordingly, the tensile deformation of BM and SZ at elevated temperatures led to superplastic elongations in SZ but low elongations in BM, as shown in Fig. 12. ...
The superplastic behavior of medical magnesium alloys is reviewed in this overview article. Firstly, the basics of superplasticity and superplastic forming via grain boundary sliding (GBS) as the main deformation mechanism are discussed. Subsequently, the biomedical Mg alloys and their properties are tabulated. Afterwards, the superplasticity of biocompatible Mg-Al, Mg-Zn, Mg-Li, and Mg-RE (rare earth) alloys is critically discussed, where the influence of grain size, hot deformation temperature, and strain rate on the tensile ductility (elongation to failure) is assessed. Moreover, the thermomechanical processing routes (e.g. by dynamic recrystallization (DRX)) and severe plastic deformation (SPD) methods for grain refinement and superplasticity in each alloying system are introduced. The importance of thermal stability (thermostability) of the microstructure against the grain coarsening (grain growth) is emphasized, where the addition of alloying elements for the formation of thermally stable pinning particles and segregation of solutes at grain boundaries are found to be major controlling factors. It is revealed that superplasticity at very high temperatures can be achieved in the presence of stable rare-earth intermetallics. On the other hand, the high-strain-rate superplasticity and low-temperature superplasticity in Mg alloys with great potential for industrial applications are summarized. In this regard, it is shown that the ultrafine-grained (UFG) duplex Mg-Li alloys might show remarkable superplasticity at low temperatures. Finally, the future prospects and distinct research suggestions are summarized. Accordingly, this paper presents the opportunities that superplastic Mg alloys can offer for the biomedical industries.
... The reduced grain size is a necessary condition for achieving the superplastic behaviour. The superplastic behaviour of the base and processed material, at different temperatures and strain rate, has been studied in the literature [15,16], while free bulge forming of processed material has not been investigated so far. Finite element modeling (FEM) is a well-established tool both for simulation and optimization of industrial process and for academic and scientific research [17]. ...
... where is the effective strain rate, is the effective flow stress, m is the strain rate sensitivity index, d is the average grain diameter, f a is the area fraction of voids, p is the grain growth exponent, and k is a material parameter. The parameters used for modelling the behaviour of the AZ31 were found in literature, including the material processing maps of the alloy in the as rolled condition [22,23] and after the FSP [16]. The model and the evolution equations were then implemented into the commercial finite element (FE) code MSC.Marc™ and different FE simulations were conducted to correlate the numerical data to experimental results. ...
... To simulate the different behaviour of the processed material, a strip of 24 mm width was modified in the model with a different material behaviour; the material parameters were collected from previous work, especially [16]. The correspondence of the numerical results with the experimental results is not so good as in the previous model; figure 14 a,b shows, in particular, the measured and computed thickness of the processed strip for the two directions taken into account as reported in figure 3. The difference between the mean of the measured values and the numerical results could be linked to the particular shape of the processed material in the cross section of the sheet, reported in figure 5: the refined material showed a cone-like shape, while, in the numerical model, the material properties were uniform along the thickness. ...
Magnesium alloys are attractive for lightweight structural applications in the transportation industry because of their low density and high specific strength and stiffness [1]. With an ultrafine-grained microstructure, they exhibit superplasticity at relatively low temperatures and high strain rates [2]. Friction stir processing (FSP) was used to obtain a microstructure with ultrafine grains in the magnesium alloy AZ31. Microstructures obtained using different rotational speeds are studied. Free bulge forming of the FS processed AZ31 sheets are carried out to evaluate the superplastic behaviour [3]. The model and the evolution equations are, then, implemented into a commercial FE code and different simulations are conducted to correlate the experimental and numerical results for the model validation [4]. The purpose of this study is to investigate the effect of the microstructure on the superplastic behaviour using free bulge forming and FE simulations.
... The choice of temperature and straining rate plays a pivotal role in obtaining superplastic elongations. Fig. 14 a) highlights the effect of diverse temperature ranges at two different strain rates for FSP of AZ31 alloy [103]. It can be interpreted that as the strain rate was increased from 5 Â 10 À4 / s to 1 Â 10 À2 /s, the elongation to failure decreased substantially, especially at 723 K. ...
... a) Effect of temperature and strain rate on elongation of AZ31 Mg alloy b) Effect of temperature and strain rate on flow stress of AZ31 Mg alloy[103].Fig. 15 e a)Effect of temperature b) Effect of temperature on elongation of MgeZneYeZr alloy[38]. ...
Friction stir processing (FSP), an adaption of the solid-state joining process friction stir welding (FSW), is now a widely recognized severe plastic deformation (SPD) technique. It induces microstructural refinement in the metallic materials which enhances their formability and other mechanical properties. Dynamic recrystallization occurs during the stirring phase which leads to reduction in the grain size and texture modification. Breaking up of the intermetallics and precipitates with their homogeneous distribution in the matrix is also accompanied. This further improves the material’s ability to attain maximum ductility during plastic deformation at higher temperatures, resulting in very large uniform elongations (>200%) termed as ‘superplasticity’. Optimization of FSP parameters activates superplastic behaviour in different magnesium alloys at low temperatures and high strain rates. It has become the focal point of the recent researches owing to its huge potential in the light-weight structural applications. In addition to the essential aspects of superplasticity, this article highlights the major explorations in the area of superplasticity of magnesium alloys using FSP method and it’s recently developed variants.
... There are several research papers on the FSP of AZ31 magnesium alloy which have dealt with the evolution of the microstructure [16][17][18][19][20][21][22]. Chang et al. [18] used FSP to produce nano grain structure in AZ31 Mg alloy. ...
... Chang et al. [18] used FSP to produce nano grain structure in AZ31 Mg alloy. Zhang et al. [21] correlated the superplasticity of AZ31 magnesium alloy to the rotational tool speed and pin profile during FSP. Moreover, FSP was employed to improve the corrosion performance of the AZ series of magnesium alloys. ...
Friction stir processing (FSP) in different pass number, accordingly one and four, was performed to AZ31 magnesium alloy. Optical and scanning electron microscopy (SEM) were used to investigate the effect of FSP and its pass number on the microstructure of FSPed samples. The hardness of the samples was measured using microhardness measurement. Furthermore, wear behaviors of the samples, including wear rate and friction coefficient, were investigated using a reciprocal wear machine. To deduce the wear mechanism, SEM observations of the worn surface were carried out. Optical microscopy of FSPed samples showed grain refinement in the stir zone. Increasing FSP pass number had a considerable effect on grain refinement. The average grain size of the as-received AZ31 base metal reduced from 11µm to about 4µm after four passes. Microhardness evaluations showed a substantial improvement by increasing FSP pass number, about 70% improvement. Wear tests results revealed enhanced tribological in FSPed samples. SEM observations of the worn surfaces indicated that the abrasion was the dominant wear mechanism governed in the samples.
... Magnesium alloys have a low specific weight, which makes them promising materials for aerospace and automobile industries [1]. However, these alloys possess low properties of corrosion resistance, wear resistance, hardness and deformability [2]. ...
... However, these alloys possess low properties of corrosion resistance, wear resistance, hardness and deformability [2]. The mechanical properties of magnesium alloys can be raised by refinement of grains [1]. Another method for raising their properties is fabrication of a composite on the surface of alloys by friction stir processing (FSP). ...
A nanocomposite obtained by introduction of carbon nanotubes into the surface layer of magnesium alloy AZ31B by the method of friction stir processing is studied. Dependences of the hardness and wear resistance on the speed of motion of the friction tool, the speed of rotation of the friction head, the number of passes of the tool and the relative content of carbon nanotubes in the layer are determined. The method of design of experiment is used to find the optimum modes for production of a nanocomposite with high hardness and wear resistance.
... Several numerical simulations were carried out using a commercial finite element (FE) code MSC.Marc with the aim to obtain the modeling optimization for the SPF of friction stir processed AZ31 Mg alloy. The parameters used to model the behavior of the AZ31 were found in literature, especially by the use of material processing maps of the alloy in the as-rolled condition [15,16] and after the FSP [17]. ...
... A simulation planning was organized for both cases (see Fig. 4), by fixing for the two factors (k and m of Eq. 1) five levels of observation centered around their mean values assumed by previous works available in literature [3,[15][16][17]. The models and the evolution equations were then implemented into the FE code MSC.Marc and the FE simulations were carried out to find that couple of values, respectively for k and m, resulting in the minimum computing error with respect to the experimental results. ...
Superplastic forming is a near net shape process used to produce various items with complex geometry. However in many cases, only some portions of the workpiece undergo superplastic deformation. In these cases, instead of choosing expensive starting sheet material with superplastic properties, a low-cost conventional material can be chosen and a grain refinement process can be performed in the selected regions to enhance superplastic properties locally [1]. This process is known as "selective superplastic forming" [R.S. Mishra, M.W. Mahoney, US Patent 6,712,916, 2002]. In some previous works the use of Friction Stir Processing (FSP) was used to obtain locally a microstructure with ultrafine grains in the AZ31 magnesium alloys [2, 3]. In this study a modeling approach was adopted thanks to a commercial FE code and different simulations were conducted in order to correlate the experimental and numerical results for the model optimization [4, 5]. Free bulge forming tests of friction stir processed AZ31 sheets, in conjunction with numerical simulations, were used to evaluate the proposed optimization approach, with the aim to reduce the time and costs in the design of components with complex geometry.
... Magnesium alloy sheets cannot be formed successfully at room temperature due to a tightly packed hexagonal crystal structure. However, it has been observed that magnesium alloys, once heated, can reach high elongations even if the main deformation mechanism is not grain boundary sliding [6][7][8][9][10]. ...
This work proposes a simple procedure to characterize 1.0 mm thick sheets of superplastic magnesium alloy AZ31. The simplest mathematical function that models the behavior of a superplastic material is a power law between stress and strain rate with two parameters connected to the material: K and m. First, the parameter m (variable with the strain) was defined directly by carrying out free-forming experimental tests at constant pressure and using a simple expression taken from the analytical modeling of the free-forming process. In the second step, an inverse analysis was carried out through a finite element model (FEM) and based on a numerical–experimental comparison between the results of the dimensionless height–time (H–t) curve, which made it possible to identify the variation of the parameter K in the same strain range. Once the m and K parameters were evaluated, it was possible to simulate the free-forming tests at constant pressure in the pressure range used to characterize the material. The proposed procedure to estimate m and K parameters made it possible to best match the numerical with the experimental results in terms of the dimensionless height–time curve. The difference between the forming time estimated by FEM and that measured experimentally along the H–t curve was found to be less than 9%.
... Limited studies have been devoted towards investigating the tensile deformation of friction stirred AZ31 alloy at elevated temperatures. Mohan et al. [23] explored the aspect of multi-pass FSP to examine similar behaviour in AZ31 alloy while another research aimed at analysing the effect of different strain rates on the high temperature deformation (HTD) during tensile stretching [24]. Less deliberation has been concentrated towards the scrutiny of the impact of various deformation temperatures on the plastic elongations of AZ31 alloy. ...
The present study has expounded the effect of high temperatures on the tensile deformation of AZ31 magnesium alloy processed through a single-pass of friction stir processing (FSP). The major operation parameters, namely rotation speed and traverse speed of the FSP tool, were varied which led to extensive dynamic recrystallization (DRX) in the stir zone (SZ) engendering maximum grain refinement of about 63% as compared to the base metal. The lowest average grain size ∼ 5.66 μm was attained after a single FSP pass. Optical microscopy (OM) was followed by the uniaxial tensile tests at three different temperatures of 350, 400 and 450 ºC at a constant strain rate of 1.3 × 10⁻³/s. As the deformation temperature was raised, the flow stress reduced and led to appreciable increments in the processed material’s tensile elongations. The maximal elongation to fracture of 160% was observed in the friction stir processed (FSPed) sample possessing the finest grains.
... FSP has already been used for thermomechanical processing of Mg and its alloys in order to achieve enhanced mechanical properties or improved corrosion resistance (58)(59)(60)(61)(62)(63)(64)(65)(66)(67). A critical analysis of the relevant literature shows that previous research was focused either on deformation, microstructure and mechanical properties or on corrosion resistance. ...
Magnesium alloys have many unique properties, mostly benefitting from the low density of magnesium. However, they are not competitive, when compared with other lightweight materials, such as aluminum alloys, particularly in ductility and corrosion resistance. There is a strong need to improve the mechanical properties and corrosion resistance of magnesium alloys. In the present research, friction stir processing (FSP) as a severe plastic deformation process was applied to the WE43 magnesium alloy. The effect of FSP up to 6 passes on the grain structure, second-phase particle distribution, mechanical properties and corrosion resistance of the alloy was investigated. It was found that a continuous network of second-phase particles was present at the grain boundaries, which was considered to be one of the main causes for the poor ductility of the alloy in the as-annealed state. By applying two passes of FSP, the grain structure was significantly refined, changing from an average grain size of 12.4 to 2.5 μm. By further FSP, the grain structure continued to refine to an average grain size of 1.4 μm after 4 passes and remained unchanged after 6 passes. However, the fragmentation and redistribution of second-phase particles continued to occur during the 4th and 6th passes of FSP. Because of these microstructural changes, the uniform strain to maximum stress and the strength of specimens gradually improved with increasing number of FSP passes. The corrosion resistance of the alloy was found to be improved by applying 6 passes of FSP, compared to that of the alloy in the initial as-annealed state, which was attributed to the fragmentation and redistribution of second-phase particles. By applying FSP, the uniformity of the protective passive layer was improved and, in the meantime, the intensity of micro-galvanic coupling leading to pitting corrosion was decreased.
... The softening of low melting-point Mg17Al12 phase at elevated temperatures facilitates the interphase boundary sliding, and thus assists in developing their superplasticity [8]. Several reported studies have achieved the high-temperature superplasticity in Mg-Al-Zn alloys by friction stir processing (FSP) [9], high ratio differential speed rolling (HRDSR) [10], and controlled rolling [11,12]. However, the room-temperature strength of commercially available Mg-Al-Zn wrought Mg alloys are still difficult to meet the industrial demand [13,14]. ...
The high-temperature superplastic deformation behavior of rolled Mg-8Al-2Sn (AT82) and Mg-8Al-1Sn-1Zn (ATZ811) alloys were investigated in this study. During tensile deformation at 573 K, no obvious grain growth occurred in both alloys, because of the high-volume fraction of second phases located at grain boundaries. Meanwhile, texture weakening was observed, suggesting that grain boundary sliding (GBS) is the dominant superplastic deformation mechanism, which agreed well with the strain rate sensitivity (m) and the activation energy (Q) calculations. The microstructural evolution during tensile deformation manifested that there were more and larger cavities in AT82 than ATZ811 during high-temperature tensile deformation. Therefore, superior superplasticity was found in the ATZ811 alloy that presented a tensile elongation of ~510% under a strain rate of 10−3 s−1 at 573 K, in contrast to the relatively inferior elongation of ~380% for the AT82 alloy. Meanwhile, good tensile properties at ambient temperature were also obtained in ATZ811 alloy, showing the ultimate tensile strength (UTS) of ~355 MPa, yield strength (YS) of ~250 MPa and elongation of ~18%. Excellent mechanical performance at both ambient and elevated temperatures can be realized by using economical elements and conventional rolling process, which is desirable for the industrial application of Mg alloy sheets.
... 24 It has been observed in the literature that, the suitability of a broad variety of filling media including titanium carbide (TiC), alumina (Al 2 O 3 ), silicon carbide (SiC), boron carbide (B 4 C), tungsten carbide (WC), fly ash, and different types of Mg composites could be fabricated. 25,26 Table 1 shows the practical implications of the FSPbased Mg composites. ...
In present research work, an attempt has been made on the development of titanium carbide–reinforced magnesium-based surface composites through friction stir processing technique. Particularly, attempt has been made to observe the influence of input processing conditions, namely tools-pin geometry, travel speed, and rotational speed for the mechanical importance (surface hardness and elastic modulus) of the developed composites. Further, the, incurred modifications in the metallurgical characteristics and corrosion behaviour of the developed composites have also been analysed through microscopic and scanning electron microscopy, and immersion fluid test, respectively. It has been found that the quality characteristics of the composites have been greatly influenced by the selected range of input variables. As noticed, the grain size of the magnesium alloy has been significantly reduced from 22.42 to 6.6 µm. Furthermore, the maximum level of the micro-hardness (180 HV 0.3 ) of the processed composite with square-shaped tool-pin geometry. Moreover, the degradation rate of the processed composite is found to be 45% lesser than the unprocessed magnesium alloy.
... The initial microstructure of the base metal (AZ31) consisted of coarse grains with an average grain size of 92 μm. The microstructures of friction stir processed specimens are greatly refined due to dynamic recrystallization [19,20]. According to figure 2, the grain boundaries in the base metal are more obvious than the friction stir processed alloy, since precipitates in the grain boundaries during the FSP were dissolved due to heat generation [21]. ...
In this work, friction stir processing of AZ31 alloy under pure nitrogen gas and air were studied. For this purpose, the influences of different rotating speeds (700, 900 and 1120 rpm) and environment condition on the metallurgical structure, wear and corrosion resistance were investigated. The results, obtained under optimized experimental conditions, indicated that the surface nitriding results in refining of surface grains to an average of 3 microns and its hardness enhancement by 37%. Also the corrosion potential, carried out by potentiodynamic polarization tests in simulated body fluid (SBF), showed an improvement by 160 mVs. Meanwhile, the dry sliding wear tests showed a 30% wear rate improvement occurred.
... The parameters used to model the AZ31B Mg alloy were chosen according to the literature, especially by the use of stress-strain maps in the unprocessed condition [15], [16] and after the FSP [17]. ...
Superplastic forming (SPF) is considered to be a near net shape manufacturing technique, mainly adopted to realize aircraft and automotive parts, which requires relatively high tooling and assembly costs. Furthermore the tuning of the process is a non trivial operation since very limited reliable models have been developed to predict the complex geometries obtained through SPF. In such context several researches, based on finite element method (FEM,) have been conducted on the numerical optimization of conventional SPF processes. Friction Stir Processing (FSP) can be used combined with conventional SPF to enhance the superplastic material behavior by means of grain refinement treatment locally performed. From this point of view very few models have been developed to simulate the different superplastic behavior distinguishing the materials after the application of FSP. In this work free bulge forming tests of AZ31B Mg alloy was experimentally performed by means of blow forming laboratory-scale equipment as well as FEM analysis were conducted to simulate the SPF in two different cases: unprocessed and friction stir processed (FSProcessed) condition. The most relevant parameters of the constitutive numerical model were optimized by numerical-experimental comparison. More specifically material strength factor (K) and strain rate sensitivity index (m) were considered during the parametric optimization. Strain and thickness distributions were compared to the experimental measurements in order to individuate the optimized constitutive equations governing the superplastic behavior in both case studies.
... A surface composite matrix using FSP was affected by the type of reinforced particles and methods of inserting these particles into the alloys during processing. FSP is considered one of the major techniques used in fabricating surface composites, and the results showed that as the number of FSP passes increases it causes a uniform dispersion of reinforced particles [27][28][29][30][31][32]. Three common methods for inserting reinforcement particles in the fabrication composite matrix are by grooves, drilled holes, and by using cover plate, as specified in one study [26]. ...
In the present work, an aluminum metal matrix reinforced with (Al₂O₃) nanoparticles was fabricated as a surface composite sheet using friction stir processing (FSP). The effects of processing parameters on mechanical properties, hardness, and microstructure grain were investigated. The results revealed that multi-pass FSP causes a homogeneous distribution and good dispersion of Al₂O₃ in the metal matrix, and consequently an increase in the hardness of the matrix composites. A finer grain is observed in the microstructure examination in specimens subjected to second and third passes of FSP. The improvement in the grain refinement is 80% compared to base metal. The processing parameters, particularly rotational tool speed and pass number in FSP, have a major effect on strength properties and surface hardness. The ultimate tensile strength (UTS) and the average hardness are improved by 25% and 46%, respectively, due to presence of reinforcement Al₂O₃ nanoparticles.
... The first pass noticed that, it is insufficient to improve the mechanical properties because some voids or defects are remained in the matrix. These results are in accordance to the previous results [ 27,28,29,30 ]. ...
In the current study, the surface composite sheet of AA2024/al203 has been fabricated using friction stir processing technique. The processing parameters during fabrication process; such as rotation speed, travel speed and number of passes have been investigated. The number of passes has a significant effect on the mechanical properties through tensile test. The results revealed that tensile strength improved with increasing passes number.
... The same approach was followed by another author who used two-pass FSP to achieve an average grain size of 85 nm [4]. A recent publication by [5] presented AZ31 magnesium alloy prepared by friction stir processing which exhibited 268% elongation at 723 K and 10-2 s-1 indicating that high strain rate superplasticity could be achieved. ...
Microstructural modification via dynamic recrystallization (DRX) is the main mechanism involved in grain refinement associated with friction stir processing. Introduced in this work is a methodology for controlling grain size through the manipulation of process parameters in friction stir processes, FSP. The methodology was demonstrated on FSP using a simple shoulder/pin tool on work material comprised twin-roll cast (TRC) magnesium-aluminum-copper, AZ31B. A robust 3D FE model was used to simulate a test matrix the resulting state variables of which were used to relate the desired output (here, average grain size) to the controlled input process parameters. The Zener-Hollomon parameter (temperature compensated strain rate or Z-parameter) was used to calculate the predicted grain size from the state variables estimated from FEM simulations. Then, a power equation that relates average grain diameter, d, to the input process parameters, spindle speed (N) and tool feed (f). The proposed methodology was validated by comparing experimental test cases whose measured average grain size diameter was found to compare favorably with that predicted by the power equation. Evidence of grain size refinement and homogeneity was observed in the friction stir processed samples of Magnesium alloy AZ31B in agreement with the proposed equation over a wide range of these two parameters. Copyright © (2014) by the Society of Manufacturing Engineers All rights reserved.
... The same approach was followed by another author who used two-pass FSP to achieve an average grain size of 85 nm [4]. A recent publication by [5] presented AZ31 magnesium alloy prepared by friction stir processing which exhibited 268% elongation at 723K and 10 -2 s -1 indicating that high strain rate superplasticity could be achieved. ...
Introduced in this work is a relation that captures the behavior of grain size with the varying process parameters in friction stir processing of AZ31B. The relation was based on the results of a 3D FE model that was used to run simulations of the process at different tool rotational and traverse speeds. The model was validated by comparing its state variable outputs to experimental results found in the literature. The coefficients of the proposed relation were determined for magnesium alloy AZ31B. This proposed relation will aid in controlling the output grain size in computerized friction stir processes.
... The same approach was followed by another author who used two-pass FSP to achieve an average grain size of 85 nm [4]. A recent publication by [5] presented AZ31 magnesium alloy prepared by friction stir processing which exhibited 268% elongation at 723K and 10-2 s-1 indicating that high strain rate superplasticity could be achieved. ...
Introduced in this work are correlations that capture the behavior of thrust force and torque vs. input process parameters in friction stir processing (FSP) of twin-roll cast (TRC) AZ3 IB. The correlations are based on the findings of an experimentally validated robust 3D FE model that was used to simulate the FSP process at different values of tool rotational and traverse speeds. The findings are fitted into simple power equations relating thrust force and torque to input parameters of spindle speed and feed. An experimental test matrix was used to validate the proposed correlations. The correlation equations were found to be able to predict the experimentally measured forces during friction stir processing with good statistical significance with average estimate errors of 6.2% and 5.4% for the thrust force and torque, respectively. The thrust force and torque exhibited opposite trends with increasing tool rotational speed. The thrust force increased while the torque decreased as the tool rotational speed increased. Copyright © (2014) by the Society of Manufacturing Engineers All rights reserved.
... It has been also demonstrated that the pressure profile can be improved to speed up the gas forming process on AZ series Mg alloys [5]. Zhang et al. studied the superplastic behaviour of an AZ31 Mg alloy prepared by friction stir processing, and they found elongations to failure above 200 % at 673 and 723 K also with a strain rate of 10 −2 s −1 [6]. They also found that grain growth and cavities coalescence play important roles in the deformation behaviour of the AZ31 Mg alloy at 673 and 723 K, while dynamic recrystallisation dominates the structure evolution at 573 K. Chung et al. investigated the superplastic behaviour of a fine-grained AZ61 Mg alloy sheet during equi-biaxial tensile deformation: Thin circular diaphragms (0.7 mm in thickness) were successfully deformed into hemispherical domes at 673 K applying gas pressures in the range 0.46-1.20 MPa. ...
In this work, the gas forming of AZ31 magnesium alloy 0.75-mm-thick sheets at elevated strain rates (fast gas forming) is investigated through an experimental-numerical approach. First, free inflation tests were carried out to find the conditions, in terms of temperature and forming pressure, able to give the best compromise between the alloy formability and the forming time. The analysis was successively moved to a closed die forming application with a stepped geometry case study in order to analyse the real forming process. Both an axisymmetric model of the free inflation test and a 3D model of the closed die forming process were built to correlate the results from free inflation tests (in terms of optimal strain rate values) to the closed die forming test: Numerical simulations were run to find the pressure value to be applied in gas forming tests. Experimental gas forming trials were finally conducted in order to support the approach and to analyse post-forming characteristics of the formed parts. Results showed that very small fillet radii can be reached on a commercial Mg alloy sheet setting very short forming times (few seconds). The choice of the forming temperature and of the corresponding optimal strain rate strongly affects the grain growth and the cavitation phenomena. Even if the alloy is prone to a strong static and dynamic grain growth at elevated temperatures, a small mean grain size value can be reached in the formed component due to the short forming times.
... So to achieve the best results, one of the promising SPD techniques is equal channel angular pressing (ECAP) which is also known as equal channel angular extrusion (Olejnik and Rosochowski, 2005;Segal, 1995Segal, , 2002. It was developed among all the SPD techniques such as accumulative back extrusion (Fatemi-Varzaneh and Zarei-Hanzaki, 2009;Fatemi-Varzaneh et al., 2010), cyclic extrusion compression Wang et al., 2010), accumulative roll bonding (Del Valle et al., 2005;Mei-yan et al., 2008), friction stir processing (Darras et al., 2007;Xing-hao and Bao-lin, 2008;Zhang et al., 2011), high pressure torsion (Sakai et al., 2005), twist extrusion (Beygelzimer et al., 2009a(Beygelzimer et al., , 2009bLatypov et al., 2012), reciprocating extrusion (Lee et al., 2007;Yang et al., 2012;Yeh et al., 1998), repetitive corrugation and strengthening (Pandey et al., 2012;Rajinikanth et al., 2008), severe torsion straining (Nakamura et al., 2004), cylinder covered compression (Zhao et al., 2004(Zhao et al., , 2007, submerged friction stir processing (Hofmann and Vecchio, 2005), constrained groove pressing (Khakbaz and Kazeminezhad, 2012;Shin et al., 2002) and repetitive upsetting . The aim of ECAP technique is to obtain effective microstructures to enhance the mechanical properties of the material (Wang et al., 2013). ...
Magnesium and its alloys with severe plastic deformation (SPD) techniques are more attractive as structural parts in many industrial applications because of their advantages. In this paper, the importance of wrought magnesium alloys with their applications to accomplish the essential development of components is reviewed. In addition, the different approaches of equal channel angular pressing (ECAP) process for refining the grain size to achieve the ultrafine grained material on the bulk metals are discussed. Recent developments in the ECAP process are outlined clearly with their importance to overcome many complexities. Various factors like processing temperature of a specimen, die geometry, ram speed, back pressure and processing routes influencing during ECAP process of wrought magnesium alloys at different conditions such as channel angle and corner or outer arc angle are discussed. Finally, the properties of ECAP processed wrought alloys are outlined for improving the microstructure in structural parts.
Keywords: SPD, ECAP, wrought magnesium alloys, grain refinement and mechanical properties.
... Friction stir processing has been widely applied to lightweight alloys such as magnesium. Mg is being increasingly used in many industries including aerospace and automotive with processing applications including surface composite fabrication [2], grain size refinement and homogeneity [3], and superplasticity enhancement [4]. ...
... Friction stir processing has been widely applied to lightweight alloys such as magnesium. Mg is being increasingly used in many industries including aerospace and automotive with processing applications including surface composite fabrication [2], grain size refinement and homogeneity [3], and superplasticity enhancement [4]. ...
... Application of FSP to magnesium alloys are challenging due to the base metal's low melting point. When carefully applied, FSP has been applied to magnesium including surface composite fabrication [2], grain refinement, and homogeneity enhancement [3], and enhancing superplasticity [4]. If solely based on traditional experiments, such studies would require extensive and costly investigations to arrive at the optimum processing parameters. ...
... Friction stir processing has been widely applied to lightweight alloys such as magnesium. Mg is being increasingly used in many industries including aerospace and automotive with processing applications including surface composite fabrication [2], grain size refinement and homogeneity [3], and superplasticity enhancement [4]. ...
... Copyright © 2013 by ASME homogeneity [3], and super-plasticity enhancement [4]. The challenge of these applications originates from the low melting point of magnesium and its microstructure"s susceptibility to high temperatures. ...
Controlling the temperature in friction stir processing (FSP) of Magnesium alloy AZ31b is crucial given its low melting point and surface deformability. A numerical FEM study is presented in this paper where a thermo-mechanical-based model is used for optimizing the process parameters, including active in-process cooling, in FSP. This model is simulated using a solid mechanics FEM solver capable of analyzing the three dimensional flow and of estimating the state variables associated with materials processing. Such processing (input) parameters of the FSP as spindle rotational speed, travel speed, and cooling rate are optimized to minimize the heat affected zone, while maintaining reasonable travel speeds and producing uniformity of the desired grain size distribution of the microstructure in the stirred zone. The simulation results predict that such optimized parameters will result in submicron grain sized structure in the stirred zone and at the corresponding stirred surface. These simulation predictions were verified using published experimental data.
An ultrafine-grained AZ31 Mg alloy processed through multipass equal channel angular pressing (ECAP) achieved 5 and 0.9 µm mean grain sizes after two and four passes, respectively. The deformation behaviour of the alloy was studied through tensile testing at high temperatures. A 410% superplastic elongation was achieved at 250°C during the tensile deformation of the four-pass-processed alloy. The Haehner criterion allowed the prediction of the stable uniform deformation of the alloy. The strain rate sensitivity of 0.38 and equiaxed microstructure suggested grain boundary sliding as the rate-controlling mechanism. The slip system’s contribution to superplasticity was discussed using related Schmid factors. Slip activity prevalence at high strains induced a texture-strengthening effect and allowed low-angle grain boundary formation at high strains.
The stir casting method is used to make metal matrix composites and fabricated into plates. The composite material suffers from porosity, uneven distribution of reinforcement particle, and friction stir processing (FSP) is attempted to improve the surface properties of LM25AA-5% SiC p aluminum based metal matrix composite. Tool rotational speed is one of the vital FSP parameters which will influence the processing efficiency predominantly. In this investigation, five different tool rotational speeds of 800, 900, 1000, 1100 and 1200 rpm were used and their effect on tensile strength, microstructure, hardness and ductility of the friction stir processed material were evaluated. From this investigation it is found that the tool rotation speed of 1000 rpm yielded superior tensile strength compared to its counterparts. The formation of finer grains in the stir zone and uniform distribution of reinforcement particles are the main reasons for the superior tensile properties of this material.
In the present research, a composite with a magnesium alloy (WE43) as the matrix and Akermanite as the bioactive and reinforcing agent was fabricated by friction stir processing (FSP), resulting in a microstructure with uniformly distributed fine grains, second-phase particles and micro-sized Akermanite particles. The effect of an addition of Akermanite to the alloy on the mechanical properties and corrosion resistance of the resulting composite was investigated. The compressive strength and ductility of the composite were found to be significantly higher than those of the monolithic WE43 alloy. The value of yield strength of the WE43 sample increased from 75 MPa up to 119 and 225 MPa for WE43-6P and WE43-A-6P samples, respectively. Also, the value of the ultimate compressive strength of the WE43 sample increased from 210 MPa up to 240 and 362 MPa for WE43-6P and WE43-A-6P samples, respectively. The value of elongation for WE43, WE43-6P, and WE43-A-6P samples were 4.5%, 16%, and 22%, respectively. The EIS test showed that the corrosion mechanism of WE43 sample is a combination of localized pitting and uniform corrosion, which shifted towards more uniform corrosion with higher corrosion resistance by applying FSP and adding Akermanite powder. The potentiodynamic polarization and in vitro immersion tests confirmed this finding, as evidenced by the increase in polarization resistance from 0.192 for the monolithic WE43 alloy up to 0.339 and 0.609 kΩ/cm² for WE43-6P and WE43-A-6P samples, respectively. The mass loss rate of the WE43 sample decreased from 20.82 to 10.13 mm per year for the WE43-A-6P sample after 312 h immersion in SBF solution. All tests approved that by applying FSP and adding Akermanite to WE43, the corrosion resistance in the SBF solution could be significantly enhanced.
Magnesium alloys and their composites are fast replacing aluminum alloys and other materials in the aerospace and automotive industries. Significant progress has been made in the fabrication of these composites to make them materials of choice for these industries. The choice of the fabrication process is crucial to realize the composites with properties that can compete with the materials currently in vogue. Conventional methods of fabrication of magnesium alloys and their composites are seriously limited as they lead to defects such as porosity and particle clustering. Friction stir processing (FSP) is turning out to be a promising fabrication technique to surmount these challenges. The process being a solid state technique is highly amenable to production of surface modified composites with very attractive mechanical and tribological properties. The main factor making FSP attractive is the relative ease of modification of the surface layers and the incorporation of reinforcement particles. The underlying plastic deformation in FSP ensures that the reinforcement particles are incorporated and distributed uniformly throughout the matrix. This paper attempts to review the current status of FSP as a technique of enabling the surface modification and fabrication of surface composites of magnesium alloys. The objective is to summarize the progress made towards the realization of surface-modified magnesium alloys, primarily in two systems, namely, Mg-AZ system and Mg/rare earth system. The operating conditions (and process parameters) and their subsequent effect on mechanical and tribological properties of the fabricated composites are summarized through the consideration of fabrication of three representative systems, viz., Mg-metal oxide (Mg-MO), Mg-metal carbide (Mg-MC), and Mg-carbon nano tube (Mg-CNT) systems.
The tensile elongation behavior and deformation mechanisms of superplastic Mg alloys and Mg composites were examined by extensively reviewing the literature published from the time of the first report on the superplasticity of Mg alloys to the present day. Studies on the superplasticity of Mg alloys have been conducted mainly on Mg-Al-Zn (AZ) series alloys, Mg-Zn-Zr (ZK), Mg-Li and Mg-RE (rare earth) alloys, and in recent years, Mg-RE alloys have attracted the greatest attention. The effect of grain size and the type and amount of secondary phase particles on the superplasticity of Mg alloys was systematically examined and reviewed. The alloys processed by severe plastic deformation (SPD) and powder-metallurgy methods have smaller grain sizes and exhibit superior superplasticity compared to conventionally processed (by extrusion and rolling) Mg alloys. For the AZ alloys, as the volume fraction of the Mg17Al12 phase increases, smaller grains are obtained, and the low-temperature superplasticity (LTS) and high-strain-rate superplasticity (HSRS) characteristics become enhanced. The ZK60 alloy with finely dispersed MgZn2 particles exhibits excellent LTS, while the Mg-RE alloys with a high fraction of thermally stable particles exhibit excellent HSRS. Mg-Li alloys can exhibit LTS even at room temperature due to the presence of a high-volume fraction of the body centered cubic (BCC) phase where atomic diffusivity is high. Grain boundary diffusion- and lattice diffusion-controlled grain boundary sliding are found to operate as the dominant deformation mechanisms below ~ 473 K and above ~ 673 K, respectively, at small grain sizes. Deformation mechanism maps were constructed based on the analysis of the deformation behavior of superplastic Mg alloys, and from the maps, the critical conditions for achieving LTS, HSRS and simultaneous achievement of LTS and HRSR were calculated and proposed, and their importance was discussed.
An improved method of friction stir processing (FSP) was introduced for the processing of AZ91 magnesium alloy specimens. This novel process was called “friction stir vibration processing (FSVP)”. FSP and FSVP were utilized to develop surface composites on the studied alloy while SiC nanoparticles were applied as second-phase particles. The effect of reinforcing SiC particles with different sizes (30 and 300 nm) on different characteristics of the composite surface was studied. The results indicated that the microstructure was refined and mechanical properties such as hardness, ductility, and strength were enhanced as FSVP was applied. Furthermore, it was concluded that the effect of reinforcing particles with a size of 30 nm on the microstructure and mechanical properties of the surface composite was more obvious than that of particles with a size of 300 nm. It was also found that mechanical properties and microstructure of FSV-processed specimens were improved as vibration frequency increased. The hardness value in the stir zone was about 157 MPa for the FSV-processed specimen at a vibration frequency of 50 Hz, while this value was around 116 MPa for the FSV-processed specimen at a vibration frequency of 25 Hz.
Magnesium (Mg) alloys have been extensively used in various fields, such as aerospace, automobile, electronics, and biomedical industries, due to their high specific strength and stiffness, excellent vibration absorption, electromagnetic shielding effect, good machinability, and recyclability. Friction stir processing (FSP) is a severe plastic deformation technique, based on the principle of friction stir welding. In addition to introducing the basic principle and advantages of FSP, this paper reviews the studies of FSP in the modification of the cast structure, superplastic deformation behavior, preparation of fine-grained Mg alloys and Mg-based surface composites, and additive manufacturing. FSP not only refines, homogenizes, and densifies the microstructure, but also eliminates the cast microstructure defects, breaks up the brittle and network-like phases, and prepares fine-grained, ultrafine-, and nano-grained Mg alloys. Indeed, FSP significantly improves the comprehensive mechanical properties of the alloys and achieves low-temperature and/or high strain rate superplasticity. Furthermore, FSP can produce particle- and fiber-reinforced Mg-based surface composites. As a promising additive manufacturing technique of light metals, FSP enables the additive manufacturing of Mg alloys. Finally, we prospect the future research direction and application with friction stir processed Mg alloys.
This study presents application of an up-scaled high-pressure torsion (HPT) process to AZ31 and AZ61 Mg alloys for ultrafine grain refinement. Disks with 30 mm diameter were processed at room temperature under 6 to 7 GPa using the up-scaled HPT facility with a maximum capacity of 5 MN (500 ton). Microstructural evolution was evaluated by hardness measurement and microscopy observations including tensile testing. The grain size was well refined to ∼150 nm and ∼100 nm at the saturated state for the AZ31 and AZ61 alloys, respectively. Superplastic elongations of ∼520% and ∼550% were then attained in the corresponding alloys when tested in tension at elevated temperatures because of the grain boundary sliding controlled by grain boundary diffusion. Upsizing of the disk sample makes for a chance to extract the tensile specimens at different radial distance within the same disk and therefore the effect of the equivalent strain on the superplastic elongations was effectively evaluated.
A356 alloy was used as the base metal to produce boron carbide (B4C)/A356 composites using friction stir processing (FSP). The microstructural and mechanical properties of B4C/A356 composites were optimized using artificial neural network (ANN) and non-dominated sorting genetic algorithm-II (NSGA-II). Firstly, microstructural properties of the composites fabricated in different processing conditions were investigated. Results show that FSP parameters such as rotational speed, traverse speed and tool pin profile significantly affect the size of the primary silicon (Si) particles of the base metal, as well as the dispersion quality and volume fraction of reinforcing B4C particles in the composite layer. Higher rotational to traverse speeds ratio accompanied by threaded pin profile leads to better particles distribution, finer Si particles and smaller B4C agglomerations. Secondly, hardness and tensile tests were performed to study mechanical properties of the composites. FSP changes the fracture mechanism from brittle form in the as-received metal to very ductile form in the FSPed specimens. Then, a relation between the FSP parameters and microstructural and mechanical properties of the composites was established using ANN. A modified NSGA-II by incorporating diversity preserving mechanism called the ɛ elimination algorithm was employed to obtain the Pareto-optimal set of FSP parameters.
In this paper, the optimization of the surface composite of Mg AZ31B-carbon nanotub(CNT) via friction stir processing was investigated. Then, the most effective process parameters such as transverse speed, rotational speed, CNT weight percent and welding passes were studied by Response Surface Methodology (RSM) design of experiment. The specimens were also characterized by micro-hardness, tensile, shear punch and pin on disk dry sliding wear tests. The optimization results of hardness and weight reduction responses showed that the best conditions would be achievable with a transverse speed of 24 mm/min, rotational speed of 660 rpm, 4wt.% CNT and 3 welding passes. Moreover, fracture analysis of the surfaces proved a uniform distribution of CNTs in the matrix resulted in higher tensile and shear strength.
H62 copper alloy surface was modified by friction stir surface processing (FSSP) with different processing parameters including rotation rate, penetration depth and processing speed of the stirring tool under the room temperature (25 °C). Then friction and wear experiments of the surface were carried on at different temperatures. The experimental results show that (i) the specimens hardness initially increases with increasing the wear test temperature, then decreases with further increasing the temperature for fixed rotation rate and penetration depth, (ii) the wear resistances of the specimens decrease as the rotation rate of the stirring tool increases for fixed penetration depth and wear test temperature, (iii) the wear resistances of the specimens decrease as the penetration depth increases for constant rotation rate and wear test temperature. It is found that the optimal processing parameters are rotational speed 1200 rpm, temperature 100 ℃ and penetration depth 0.2 mm, respectively, which can greatly improve the wear performance of the H62 surface. Therefore, FSSP is a promising technique for modifying H62 copper alloy, which can be widely applied in ship manufacturing. © 2017, Kauno Technologijos Universitetas. All rights reserved.
In the present study, AZ91-D magnesium alloy based metal matrix surface composites (MMSCs) were fabricated by multi-pass friction stir processing (FSP) technique. The nano-WC–Co–Cr and multi-walled carbon nanotubes (MWCNTs) as reinforcements were added by multi-pass FSP using grooving method. The effect of different volume fractions (%) of nano-reinforcements on the micro-structure, micro-hardness and tribological properties of MMSCs were studied. The AZ91-D parent material showed poor micro-hardness and tribological performance which could be attributed to coarse grain microstructure and secondary β-phase distributed on the grain boundaries. On the other hand, friction stir processed material and fabricated MMSCs exhibited improved micro-hardness and tribological properties. This improved performance could be attributed to the homogeneous dispersion of reinforcement particles, dynamically recrystallized ultra-fine structure, and the dissolution of secondary β-phase. Among the different nano-MMSCs reinforced with pure MWCNTs, pure WC–Co–Cr and its hybrid. The WC–Co–Cr/MWCNTs (1:1 ratio) hybrid composite exhibited a maximum average micro-hardness of 118.05 HV, minimum wear rate of 0.45 × 10−3 mm3/m. But the minimum coefficient of friction (COF) of 0.32 was obtained in case of pure MWCNTs reinforced MMSC. Wear mechanisms like micro-cutting, micro-ploughing, delamination etc. were also evident from the SEM images.
The present research aimed to fabricate Mg–ZrSiO4–Al2O3 hybrid micro/nano composite on the surface of pure cast magnesium plate by friction stir processing (FSP) route. Then, effect of passes sequence as a new FSP parameter on the maximum temperature experienced by heat affected zone (HAZ), distribution of reinforcements and mechanical properties of the stir zone was investigated. Results showed that temperature of the HAZ was influenced by the number of passes and their sequence. Microstructure observations showed the there exist a uniform distribution of micro-ZrSiO4 and nano-Al2O3 particles after four-passes FSP in the stir zone which improved the hardness of the substrate. However, variations in passes sequence had not considerable effect on hardness distribution. Wear resistance of the composite produced by two passes was higher than the composites produced by four passes with different sequences. The main wear mechanisms were light ploughing due to the three-body abrasion and cracking in the composites produced by two passes while sever ploughing as a result of the two-body abrasion for the four-passes FSPed ones.
Purpose
This paper aims to investigate the modification of surface of a copper alloy by friction stir surface processing (FSSP).
Design/methodology/approach
The metallographic condition of the surface modification was observed using microscopy. Electrochemical corrosion tests were carried out on the modified surface and the corroded surface was observed by scanning electron microscopy (SEM).
Findings
The test results showed that FSSP resulted in refinement of the surface grains of the copper alloy. The degree of refinement was increased with rotation speed and increased in the descending distance of the stirring tool. The corrosion resistance of the modified surface was superior to the base metal except for the surface generated by a rotation speed of 800 rpm and a descending distance 0.1 mm. For the surface modification of the rotation speed of 800 rpm, its corrosion resistance was lower than for the other two rotation speeds. When the rotation speed is specified, the corrosion resistance is improved with increased descending distance. When the descending distance is specified, the corrosion resistance is improved with the rotation speed.
Originality/value
In this study, it was confirmed that the corrosion resistance of the surface modification was best at the rotation speed 1200 rpm and descending distance 0.2 mm.
Magnesium alloy AZ31 sheet was processed by isothermal repeated bending technology (IRBT) at different temperatures. Microstructure and texture as well as properties of magnesium alloy sheet were analyzed. The results indicate that the twinning crystal is the dominant deformation mechanism at 443 K and 483 K, and initial recrystallization grain appears near the twinning boundaries. With deformation temperature of 523 K, initial recrystallization is observed at grain boundaries. For initial magnesium alloy sheet, the elongation is 12.4% and the texture intensity reaches 9.8 at room temperature. When magnesium alloy AZ31 sheet was bended 3 passes at 483 K, the elongation reaches 17.1% and the texture intensity is 5.75 at room temperature.
Submerged friction stir processing (SFSP), which means the entire process is carried out under water, is an improved technique to normal FSP for refining the microstructure of metallic materials. In this study, microstructure and tensile behaviors of AZ91 magnesium alloy prepared by SFSP are investigated. Thermal history during SFSP is also measured, and its effect on the microstructure evolution during SFSP is discussed. SFSP results in significant grain refinement due to the enhanced cooling rate, and a fine-grained AZ91 alloy with an average size of 1.2 micrometers is produced. The SFSP specimens exhibit excellent high strain rate superplasticity, with an elongation of 990% at a strain rate of 0.02 per second and 623K. Grain boundary sliding is the main mechanism during superplastic deformation. The excellent superplasticity of the SFSP alloy is attributed to its fine grain structures, which contain a large fraction of high-angle grain boundary.
In order to improve the poor formability and forming limit of the commercial non-fine grain AZ31 magnesium, pulse current was employed during the gas blow forming, and pulse current auxiliary gas blow forming of the commercial non-fine grain AZ31 Mg alloy was studied. The results of the free bulging indicate that the forming limit of the commercial non-fine grain AZ31 magnesium can be improved by the pulse current. The microstructures of the formed parts indicate that the forming mechanism of the commercial non-fine grain AZ31 magnesium under pulse current auxiliary is the combined action of many kinds of deformation mechanisms, such as the grain boundary sliding and diffusion creep, dislocation sliding inside the grain and the twin deformation. The mechanism of the pulse current effect on the deformation was studied. The result shows that the dislocation activity can be enhanced by the pulse current, therefore, the formability of the commercial AZ31 magnesium during gas blow forming can be increased. ©, 2015, Central South University of Technology. All right reserved.
TiN is an exceptionally hard and a wear-resistant ceramic. In the present work, the effect of mixing TiN particulates into the Al7075-T651 alloy was studied by employing a novel material fabrication technique called friction stir processing (FSP). The FSP was carried out using three different tool geometries (namely square, triangular, and threaded taper) with an objective to fabricate the Al/TiN composite with an appropriate set of mechanical and wear properties. A number of microstructural, mechanical, and wear tests were carried out in order to characterize the composite. In comparison to the parent metal, each of the composite specimens showed improved wear and friction performance. However, the improvement in the hardness was realized only with the threaded tool and that in the tensile strength was observed with the square tool, thus revealing a tool-property relationship in FSP. Contrarily, the ductility of all of the composite specimens was lower relative to that of the parent metal. A suitable trade-off among various characteristics was realized when FSP was performed using the square tool. The microscopic and energy dispersive spectroscopy (EDS) analyses showed that the dominant wear mechanism in the composite was adhesion. This study is the first report on the FSPed Al/TiN composite.
Mg-3Al-1Zn (AZ31) alloy was caliber rolled isothermally in the temperature range of 523 K to 723 K (250 °C to 450 °C) to develop fine grains of 3 to 13 µm. Tensile tests by constant initial strain rate as well as differential strain rate test techniques were conducted over the temperature range of 493 K to 723 K (220 °C to 450 °C) and strain rate range of 10−5 to 10−1 s−1. Maximum tensile elongation of 182 pct was obtained at test temperature of 723 K (450 °C) and strain rate of 10−3 s−1 in the sample obtained from caliber rolling at 723 K (450 °C), in spite of its large grain size of 13 µm. The strain rate sensitivity index ‘m’ was found to vary from 0.08 to 0.33 and activation energy for deformation ‘Q’ varied from 30 to 185 kJ mol−1 depending on test condition and caliber-rolling condition. These variations in m and Q values are explained by the difference in prior grain size, texture, and twins developed as a function of caliber-rolling temperature, and further by the concomitant microstructural change occurring during tensile test itself. The presence of twins and orientation of grains influences the parameters of the constitutive relation to varying extent.
A fine-grained AZ91 magnesium alloy prepared by submerged friction stir processing is subjected to high temperature tensile test at 623 K and 2×10−2 s−1 to intermediate strains of 270%, 510%, 750% and failure strain of 990%, and microstructure evolution of the experimental material during tensile test is investigated. The initial grain size is about 1.2 μm. Microstructures within the gauge region are much finer than those of grip region, and the grain aspect ratios remain approximately 1.0 in the whole superplastic deformation. With the tensile strains increasing, the average size of β-Mg17Al12 particles increases, and the density of the β-Mg17Al12 particles decreases. Due to the pinning effect of β-Mg17Al12 particles and the occurrence of DRX, the fine microstructures are maintained in the whole superplastic deformation process. Grain boundary sliding is the main deformation mechanism, and cavities are formed in the triple junctions of grains and around the second phase particles during deformation. The excellent high strain rate superplasticity of the AZ91 magnesium alloy is mainly attributed to its initial fine microstructure and good thermal stability.
Normal and submerged friction stir processing (SFSP) were conducted to AZ91 magnesium alloy plates with 6mm in thickness, and influence of processing speed (ν) on microstructures and mechanical properties of the experimental materials was investigated. The results revealed that fine and equiaxed grains were observed in the stirred zone (SZ). As the processing speed increased from 60mm/min to 150mm/min, the average grain size in the SZ of normal FSP material decreased. However, the grain size of the SFSP specimens first increased with the processing rate increasing from 60mm/min to 120mm/min, and then decreased when the processing rate increased to 150mm/min. Microstructure of the SFSP specimen was much finer compared with the normal FSP one, and the grain size of α-Mg was about 1.2μm when the processing speed was 60mm/min during SFSP. Because of much finer microstructure of SFSP, the microhardness, tensile strength and elongation were all improved. SEM fracture observation showed that fine dimples and tearing edges could be observed on SFSP specimen which showing good ductility. In addition, high temperature tensile tests showed that SFSP AZ91 alloys exhibited excellent superplasticity at high strain rate, with an elongation of 1202% at 623 K with a strain rate of 3×10-3s-1. The present study demonstrated that SFSP possesses great potential in preparing fine-grained materials.
Ultrafine-grained (UFG) microstructures with an average grain size of 100–300 nm are achieved in solution-hardened AZ31 Mg–Al–Zn alloy prepared by friction stir processing equipped with a rapid heat sink. The mean hardness of the UFG region reaches ∼120Hv, which is more than twice as high as that of the AZ31 matrix. The grain refinement kinetics are analyzed and the results are self-consistent.
Friction stir processing (FSP), developed based on the basic principles of friction stir welding (FSW), a solid-state joining
process originally developed for aluminum alloys, is an emerging metalworking technique that can provide localized modification
and control of microstructures in near-surface layers of processed metallic components. The FSP causes intense plastic deformation,
material mixing, and thermal exposure, resulting in significant microstructural refinement, densification, and homogeneity
of the processed zone. The FSP technique has been successfully used for producing the fine-grained structure and surface composite,
modifying the microstructure of materials, and synthesizing the composite and intermetallic compound in situ. In this review article, the current state of the understanding and development of FSP is addressed.
With a view to the current researches of ultrafine grained and nano-crystalline metallic materials processed by severe plastic deformation(SPD), several typical methods, characteristics and principle of SPD were introduced, such as equal channel angular pressing, high pressure torsion, accumulative roll-bonding and multiple forging. The microstructural revolution and grain refinement mechanism of metallic materials fabricated by SPD were discussed. The change tendency of the strength and ductility of the metallic material, and the influence rules of superplastic deformation was also analyzed. The application of SPD for metallic materials was prospected.
Microstructural evolution and mechanical property of ME20M magnesium alloy processed by multidirectional forging (MF) were analyzed through OM, microhardness, SEM and mechanical tensile test at room temperature with the strain rate of 5 × 10 -5/s. The experimental results show that the microstructural evolution is divided into three stages according to different evolution mechanisms: The mechanical splitting mechanism with the grain size fined from 45μm to 12μm; the deformation-induced dynamic recrystallization mechanism with the average grain size of 2.1μm and thermal activated grain growth mechanism with maximum grain size reached to 65μm, when the true strains are ε ≤ 0.60, 0.60 < ε ≤ 0.80 and ε ≥ 1.50, respectively. After MF, mechanical property of ME20M magnesium alloy is greatly improved. The maximum value of elongation, tensile strength and microhardness are 26.25%, 225.52 MPa and HV 55.1, which are 245%, 6.7% and 15.5% more than those of as-received Mg alloy, respectively. The density and size of ductile dimples on tensile fractured surface of the MF magnesium alloy increase with increasing strain before the sixth pass, which demonstrates that the ductility is improved, and then the ductile dimples enlarged result in the decreasing of plasticity.
Magnesium is the lightest of all metals used as the basis for constructional alloys. It is this property which entices automobile manufacturers to replace denser materials, not only steels, cast irons and copper base alloys but even aluminium alloys by magnesium based alloys. The requirement to reduce the weight of car components as a result in part of the introduction of legislation limiting emission has triggered renewed interest in magnesium. The growth rate over the next 10 years has been forecast to be 7% per annum. A wider use of magnesium base alloys necessitates several parallel programs. These can be classified as alloy development, process development/improvement and design considerations. These will be discussed briefly and followed by some examples of the increasing uses of magnesium and future trends.
The mechanical and microstructural properties of AM60B magnesium alloy resulting from the friction stir processing (FSP) were analysed in the present study. The sheets were produced by high-pressure die casting (HPDC) into the form of trial sheets 2.5mm thickness and then friction-stir processed. The tensile mechanical properties were evaluated at room temperature in the longitudinal direction respect to the processing one. Tensile tests were also performed at higher temperatures and different strain rates in the nugget zone parallel to the processing direction, in order to analyse the superplastic properties of the recrystallized material and to observe the differences with the parent material as a function of the strong grain refinement due to the friction stir process. The dynamic recrystallized structure of the material was observed by employing optical and electron microscopy. The high temperature behaviour of the material was studied in the parallel direction, by means of tensile tests in the temperature and strain rate ranges of 150–300°C and 10−2 to 10−4s−1, respectively. The deformation behaviour in the high temperature regime (275–300°C) is related to the grain boundary sliding (GBS) acting in the material parallel to the tensile direction, differing strongly from the lower temperature one in which the deformation is strongly linked to grain triple junctions fracture.
This work aims to investigate whether accumulative roll bonding (ARB) is an effective grain refinement technique for the Mg–Al alloy AZ61. Thus, a number of ARB routes at 300°C and 400°C, using thickness reductions per pass of 25%, 50%, 66%, and 80%, were performed. It was found that both the ultimate grain size achieved, as well as the degree of bonding, depend on the rolling temperature and on the thickness reduction per pass. Higher temperatures and higher reductions promote a larger degree of bonding. Increasing strain also favors the formation of a more homogeneous microstructure. The smallest grain sizes were obtained at the lowest rolling temperature.
Friction stir processing (FSP) was applied to extruded Al-Mg-Sc alloy to produce fine-grained microstructure with 2.6 mu m grains. A maximum elongation of 2150% was achieved at 450 degrees C and a high strain rate of 1 x 10(-1) s(-1). Although the grains obtained by FSP were much larger than those by other techniques, such as equal-channel angular pressing, approximately the same superplasticity was achieved at an even higher strain rate in the FSP alloy. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
The room temperature and hot tensile properties of AZ91 magnesium alloy produced by high pressure die cast after friction stir processing (FSP) were studied in the present paper. Such process is a modification of classical friction stir welding one in which the sheets are not joined but the stirring action of the tool, on the bulk material, is used to refine the microstructure in order to increase the mechanical properties of the metal such as yield strength, fatigue life and possible superplastic properties at relative low temperature and high strain rates [R.S. Mishra, Z.Y. Ma, Mater. Sci. Eng. R 50 (2005) 1–78]. The material was FSP after solution treatment and the superplastic behaviour was analysed by means of hot tensile tests, in longitudinal direction, in the temperature and strain rate ranges of 225–300°C and 10−2 to 10−4s−1, respectively.
The microstructure evolution of a typical hexagonal close-packed (HCP) material (AZ31 magnesium alloy) during friction stir welding was studied in a wide range of welding temperatures. In all cases, the grain structure development was found to be significantly influenced by the formation of a very strong {0 0 0 1} 'u w t v' B-fiber texture. Due to limitations imposed by this texture as well as by symmetry of the HCP crystal structure, an extensive lowering of grain-boundary misorientation was found to occur during deformation.
Friction stir processing (FSP) was used to create a microstructure with ultrafine grains (0.68μm grain size) in an as-cast Al–8.9Zn–2.6Mg–0.09Sc (wt.%) alloy. The ultrafine grained alloy exhibited superplasticity at relatively low temperatures and higher strain rates. Optimum ductility of 1165% at a strain rate of 3×10−2s−1 and 310°C was obtained. Enhanced superplasticity was also achieved at a temperature as low as 220°C. Experimentally observed parametric dependencies and microstructural examinations indicated that the operating deformation mechanism might be the Rachinger grain boundary sliding accommodated by intragranular slip. The FSP microstructure became highly unstable at 390°C onwards, thus, affecting ductility adversely. In situ transmission electron microscopy heating was used to understand the instability phenomenon, which has been attributed to the drop in particle pinning forces due to the dissolution of metastable precipitates and microstructural heterogeneity.
An as-extruded AZ31 alloy was friction stirred to explore the effects of crystallographic texture and twinning factor on the tensile deformation behavior. Compared to the steadily drop of work hardening rate (Δσ/Δɛ) in the as-extruded sample, the work hardening rate in the friction stirred specimen tend to increase after small strains. Based on experimental results the tensile flow curve feature, work hardening rate and an increase in tensile ductility can be correlated with the variations of textural feature. However, in this investigation the emphasis will be on the improvement in tensile ductility of AZ31 alloy by a subsequent friction stir process (FSP).
An annealed AZ31–Mg alloy was given a FSP (friction stir process) at various rotation speeds (region from 1500 to 2800rpm) to explore the room temperature tensile properties first, and the FSP1500 specimen which possessed the lowest average grain size was selected to investigate the tensile deformation mechanism in the range 100–500°C afterwards. The results indicate that increasing the rotation speed not only increased the friction stir heat, but also caused the average grain size of the stirred zones to increase. When the rotation speed of FSP was >2200rpm, the intensity of (0002) reduced, however, (1011) and (1012) increased. Regardless of the variation of rotation speed, the equiaxed grain structure of the stirred zones possessed a micro-texture. The recrystallization of FSP increased the elongation of the specimens at room temperature, but the refinement grain had no contribution to tensile strength to cause an unusual Hall-Petch effect. After high temperature tensile testing, the tensile strength of the AZ31–O specimen was higher than the FSP1500 specimen (
Ultrafine-grained (0.7 μm) Al–Mg–Sc alloy with an approximately random misorientation distribution and predominantly high-angle boundaries of 97% was produced by friction stir processing. A ductility of 235% was obtained at 200 °C. Increasing temperature from 200 to 300 °C resulted in an increase in superplasticity, optimum strain rate and strain rate sensitivity. Low temperature and high strain rate superplasticity with a ductility of 620% was achieved at 300 °C and 3 × 10−2 s−1. Abnormal grain growth occurred at 350 °C, resulting in the disappearance of superplasticity.
Creep and superplasticity were investigated by testing in tension a fine-grained extruded Mg–Zn–Zr Mg alloy in the temperature range 100–300 °C. Analysis of the experimental data suggests that grain boundary sliding becomes the dominating mechanism at 200 °C, leading to a true superplastic behaviour. By contrast, for lower temperatures, the straining process is controlled by dislocation climb. A comprehensive model, taking into account the simultaneous operation of the different mechanisms, was used to describe the dependence of strain rate on the applied stress.
Single-pass friction stir processing (FSP) was used to increase the mechanical properties of a cast Mg-Zn-Zr-rare earth (RE)
alloy, Elektron 21. A fine grain size was achieved through intense plastic deformation and the control of heat input during
processing. The effects of processing and heat treatment on the mechanical and microstructural properties were evaluated.
An aging treatment of 16hours at 200°C resulted in a 0.2pct proof stress of 275MPa in the FSP material, a 61pct improvement
over the cast+T6 condition.
The magnesium AZ31 alloy exhibits excellent superplastic properties after processing by equal-channel angular pressing (ECAP) and testing in tension at a temperature of 623 K. Experiments show that there is an increase in the elongations to failure with increasing numbers of passes in ECAP. X-ray microtomography was used to obtain detailed information on the morphologies of cavities developed during superplastic flow and the results were analyzed in terms of the different possible cavity growth mechanisms. The results show that superplastic elongations are attained in this alloy because the finer grain structure introduced by ECAP is associated with lower flow stresses in tensile testing at elevated temperatures and this leads to a reduction in the rate of growth of internal cavities.
Superplasticity of coarse-grained magnesium alloy has been investigated. Elongation of 320% has been obtained at 773 K and a strain rate of 1×10−3 s−1. The grains were refined from initial 300 to 25 μm when the stress reached its maximum. The grain size maintained about 25–30 μm dynamically in the proceeding deformation. Mechanical behavior and microstructures have also been studied.
Fine-grained alloys of Mg-3Al-1Zn-0.2Mn in wt.% (AZ31B) were obtained by an equal-channel angular extrusion technique and subsequent annealing at elevated temperatures. Tensile tests were performed at room temperature at a strain rate of 1x10-3 s-1. The alloys exhibited an apparent steady-state deformation region and a large tensile elongation of 47%. The deformed microstructure at an elongation of 2% indicated substantial cross-slip to non-basal planes induced by plastic compatibility stress associated with grain boundaries. The non-basal segment of dislocations was found to consist of 40% of the total dislocation density at a yield anisotropy factor of only 1.1 instead of an expected value of 100 obtained from single-crystal experiments. The deformed microstructure at an elongation of 16% indicated recovered regions within twins as well as untwinned matrices. These results indicate that dynamic recovery can occur in Mg alloys at room temperature.
Microstructures of a heat-resistant magnesium alloy prepared by friction stir welding were investigated. In the friction stir zone, the microstructure of the thixomoulded parent material is replaced by fine magnesium grains and small particles of intermetallic compounds (Al2Ca). Due to microstructural refinement, the microhardness of the friction stir zone is higher than that of the base material.
Low temperature superplastic (SP) behavior (mechanical and deformation mechanisms) of two commercial Mg-based alloys (AZ31 and ZK60) was characterized. The two alloys were tested in the as extruded condition with initial grain size of 15 μm (AZ31) and fine (2 μm) and coarse (25 μm) grains mixed randomly for the ZK60. Strain rate was activated in the range 10−5–1 s−1 at 450 K (0.49Tm) in order to determine the deformation capacity curves (elongation to failure vs. strain rate), and to evaluate the strain rate sensitivity coefficient, m, from the stress versus strain rate curves. Optical, scanning and transmission electron microscopy observations (SEM and TEM) were performed to elaborate on the dynamic recrystallization (DRX) grain growth, fracture modes and deformation mechanisms at the SP mode. In addition, X-ray diffraction was utilized to track for microstructural classification. Although low temperature was applied, the ZK60 exhibited superplastic-like behavior and the maximum peak of elongation (220%) was detected at 1×10−5 s−1 with m equal to 0.2. In AZ31 SP behavior was suppressed due to grain growth, while for ZK60, DRX was detected. However, for the latter alloy, it was observed that the coarse/fine grain interface was the trigger for microcracking initiation. Actually, this phenomenon reduces the SP capacity of the ZK60 alloy. Surface observations and TEM findings indicate that grain boundary sliding with homogeneous character is the controlling SP deformation mode. Typical dislocation features have supported this deformation mode, mainly by grain boundary dislocation pile-ups. More sophisticated extrusion processes as equal angular channel extrusion (EACE) is likely to be considered in the future as a mean to improve grain homogeneity and produce ultra fine grain microstructure.
The temperature evolution during friction stir welding (FSW) and the resulting residual stresses of AZ31 Mg alloy were studied to get a better understanding of the mechanisms involved in this process. The relationship between the processing parameters, the heat and plastic deformation produced and the resulting microstructure and mechanical properties was investigated. Increasing the shoulder diameter or the tool rotation speed or decreasing the welding speed produced an increase in the heat generated during the process and then promoted grain growth. The temperature distribution on the advancing side and on the retreating side differed, and stress levels were higher on the retreating side. The grain size heterogeneity produced by FSW was not the prevailing cause of failure.
An introduction to material science
- Xu Zhu-Yao
- Li
- Shanghai
XU Zhu-yao, LI Peng-xing. An introduction to material science [M]. Shanghai: Shanghai Science and Technology Press, 1986. (in Chinese)
An introduction to material science
- L I Xu Zhu-Yao
- Peng-Xing
XU Zhu-yao, LI Peng-xing. An introduction to material science [M].