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Effect of chemical compositions on tensile behaviors of high pressure die-casting alloys Al-10Si-yCu-xMn-zFe

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... However, the as-cast strengths of the currently available die-cast aluminium alloys are usually low, with a yield strength of ~130-170 MPa [8,9]. Explorations have been done to develop die-cast aluminium alloys with higher strength through the conventional micro alloying method, but the improvements are struggling limited and the improved yield strengths are still in the low level of 180-190 MPa [10][11][12][13]. ...
... Fig. 8b presents the tensile properties of the die-cast AlSiMgCu/TiB 2 composites after T6 heat treatment. The yield strength, UTS and elongation of the unreinforced alloy were 320 � 2 MPa, 423 � 2 MPa and 12.1 � 1.0%, [8][9][10][11][12][13], and it also demonstrates at least ~60% increase in UTS over the reported particle reinforcement die-cast aluminium composites [22,37,38]. The super high yield strength of more than 350 MPa and UTS of above 450 MPa in association with the ductility of 5.5% delivered by the 3.0 wt% TiB 2 reinforced composite are milestone mechanical properties for the high pressure die casting industry. ...
... Fig. 11b Fig. 11b. The corresponding FFT pattern in Fig. 11d verified that the TiB 2 nanoparticle had coherent interface with the α-Al matrix, and the crystal orientation relation (OR) between the TiB 2 nanoparticle and the α-Al matrix was Al(11-1)// TiB 2 (0001) and Al[011]//TiB 2 [11][12][13][14][15][16][17][18][19][20]. ...
Article
The structural lightweight in automotive and other industries motivates the usage of high strength die-cast aluminium alloys. However, the improvements of die-cast aluminium alloys are struggling limited by conventional micro alloying, and the improved yield strengths are still in the low level of 180-190 MPa. Here, high performance die-cast AlSiMgCu/TiB 2 composite was fabricated through the reinforcement of TiB 2 nanoparticles and the latest developed cutting-edge super vacuum assisted high pressure die casting (HPDC) process. The composite with 3.0 wt% TiB 2 nanoparticles could deliver the milestone high hardness of 1.56 GPa, yield strength of 356 MPa, tensile strength of 457 MPa and an industrially applicable ductility of 5.5%. The composite was strengthened by TiB 2 nanoparticles and nanoscale Q 0 and θ 0 precipitates that all had highly coherent interfaces with the α-Al matrix, indicative of strong interfacial bonding. The implantation of TiB 2 nanoparticles also induced special grain refinement strengthening of the composite. The fabricated composite demonstrates ~90% increase in strength over the currently available alloys, and it is promising for industrial application.
... The composite with 3.5 wt% TiB 2 nanoparticles delivered the high hardness of 150.2 kg/mm 2 , yield strength of 351 MPa, tensile strength of 410 MPa, and the industrially applicable good ductility of 5.2%, after T6 heat treatment. The strengthening of the T6 heat treated composite was a result of both TiB 2 nanoparticles and nanoscale β′′ precipitates that had coherent interfaces with α-Al matrix, i.e., Al(11-1)//TiB 2 (0001), Al[011]//TiB 2 [11][12][13][14][15][16][17][18][19][20], Al[320]//β″(a-axis), Al[1-30]//β″(c-axis) and Al(020)//β″(b-axis). The T6 heat treated composite reinforced by 3.5 wt% TiB 2 showed ductile fracture. ...
... However, the as-cast strengths of the currently available die-cast automotive aluminium alloys are usually low, with a yield strength of 130-170 MPa [9,10]. Explorations have been done to develop diecast automotive aluminium alloys with higher strength by conventional micro alloying, but the improvements are struggling limited and the improved yield strengths are still in the low level of 180-190 MPa [11][12][13][14]. It is hard to meet the requirement of higher strength die-cast automotive aluminium alloys basing on the currently available alloys. ...
... The ductility of the as-cast 3.5 wt% TiB 2 reinforced composite was 5.5% under non-vacuum assisted HPDC, while the ductility of the composite improved to 6.6% under super vacuum assisted HPDC. The as-cast yield strength of the 3.5 wt% TiB 2 reinforced composite was 214 MPa, and it was at least 25 MPa higher than the as-cast yield strength of the currently available diecast aluminium alloys [9][10][11][12][13][14]. The T6 heat-treated composite reinforced 3.5 wt% TiB 2 demonstrated 17% increase in yield strength over the previously reported [27] T6 heat-treated die-cast AlSiMgMn alloy without particle reinforcement, and it also demonstrated 23% increase in yield strength over the T6 heat-treated A356 Al alloy reinforced by SiC particles [25], under vacuum assisted HPDC, as shown in Fig. 10d. ...
Article
The global carbon emission reduction strongly requires high strength lightweight die-cast aluminium alloys in industry. Here die-cast AlSiMgMn–TiB2 composites with advanced mechanical performance were fabricated by the implantation of TiB2 nanoparticles. Super vacuum assisted high pressure die casting was applied to enable the T6 heat treatment of the composites, and the super vacuum of 20 mbar was achieved in the limited evacuation time of 1.6 s. The composites demonstrated good die castability within the addition of 3.5 wt% TiB2, while the composites could not fill into the chill vent with the addition of N3.5 wt% TiB2. The composite with 3.5 wt% TiB2 nanoparticles delivered the high hardness of 150.2 kg/mm2 , yield strength of 351 MPa, tensile strength of 410 MPa, and the industrially applicable good ductility of 5.2%, after T6 heat treatment. The strengthening of the T6 heat treated composite was a result of both TiB2 nanoparticles and nanoscale β′′ precipitates that had coherent interfaces with α–Al matrix, i.e., Al(11-1)//TiB2(0001), Al[011]//TiB2[11-20], Al[320]//β″(a-axis), Al[1-30]//β″(c-axis) and Al(020)//β″(b-axis). The T6 heat treated composite reinforced by 3.5 wt% TiB2 showed ductile fracture.
... The composite with 3.5 wt% TiB 2 nanoparticles delivered the high hardness of 150.2 kg/mm 2 , yield strength of 351 MPa, tensile strength of 410 MPa, and the industrially applicable good ductility of 5.2%, after T6 heat treatment. The strengthening of the T6 heat treated composite was a result of both TiB 2 nanoparticles and nanoscale β′′ precipitates that had coherent interfaces with α-Al matrix, i.e., Al(11-1)//TiB 2 (0001), Al[011]//TiB 2 [11][12][13][14][15][16][17][18][19][20], Al[320]//β″(a-axis), Al[1-30]//β″(c-axis) and Al(020)//β″(b-axis). The T6 heat treated composite reinforced by 3.5 wt% TiB 2 showed ductile fracture. ...
... However, the as-cast strengths of the currently available die-cast automotive aluminium alloys are usually low, with a yield strength of 130-170 MPa [9,10]. Explorations have been done to develop diecast automotive aluminium alloys with higher strength by conventional micro alloying, but the improvements are struggling limited and the improved yield strengths are still in the low level of 180-190 MPa [11][12][13][14]. It is hard to meet the requirement of higher strength die-cast automotive aluminium alloys basing on the currently available alloys. ...
... The ductility of the as-cast 3.5 wt% TiB 2 reinforced composite was 5.5% under non-vacuum assisted HPDC, while the ductility of the composite improved to 6.6% under super vacuum assisted HPDC. The as-cast yield strength of the 3.5 wt% TiB 2 reinforced composite was 214 MPa, and it was at least 25 MPa higher than the as-cast yield strength of the currently available diecast aluminium alloys [9][10][11][12][13][14]. The T6 heat-treated composite reinforced 3.5 wt% TiB 2 demonstrated 17% increase in yield strength over the previously reported [27] T6 heat-treated die-cast AlSiMgMn alloy without particle reinforcement, and it also demonstrated 23% increase in yield strength over the T6 heat-treated A356 Al alloy reinforced by SiC particles [25], under vacuum assisted HPDC, as shown in Fig. 10d. ...
Article
The global carbon emission reduction strongly requires high strength lightweight die-cast aluminium alloys in industry. Here die-cast AlSiMgMn–TiB2 composites with advanced mechanical performance were fabricated by the implantation of TiB2 nanoparticles. Super vacuum assisted high pressure die casting was applied to enable the T6 heat treatment of the composites, and the super vacuum of 20 mbar was achieved in the limited evacuation time of 1.6 s. The composites demonstrated good die castability within the addition of 3.5 wt% TiB2,while the composites could not fill into the chill vent with the addition of N3.5 wt% TiB2. The composite with 3.5 wt% TiB2 nanoparticles delivered the high hardness of 150.2 kg/mm2, yield strength of 351MPa, tensile strength of 410MPa, and the industrially applicable good ductility of 5.2%, after T6 heat treatment. The strengthening of the T6 heat treated composite was a result of both TiB2 nanoparticles and nanoscale β′′ precipitates that had coherent interfaceswithα–Al matrix, i.e., Al(11-1)//TiB2(0001), Al[011]//TiB2[11-20], Al[320]//β″(a-axis), Al[1-30]//β″(c-axis) and Al(020)//β″(b-axis). The T6 heat treated composite reinforced by 3.5 wt% TiB2 showed ductile fracture.
... However, the as-cast strengths of the currently available die-cast aluminium alloys are usually low, with a yield strength of ~130-170 MPa [8,9]. Explorations have been done to develop die-cast aluminium alloys with higher strength through the conventional micro alloying method, but the improvements are struggling limited and the improved yield strengths are still in the low level of 180-190 MPa [10][11][12][13]. ...
... Fig. 8b presents the tensile properties of the die-cast AlSiMgCu/TiB 2 composites after T6 heat treatment. The yield strength, UTS and elongation of the unreinforced alloy were 320 � 2 MPa, 423 � 2 MPa and 12.1 � 1.0%, [8][9][10][11][12][13], and it also demonstrates at least ~60% increase in UTS over the reported particle reinforcement die-cast aluminium composites [22,37,38]. The super high yield strength of more than 350 MPa and UTS of above 450 MPa in association with the ductility of 5.5% delivered by the 3.0 wt% TiB 2 reinforced composite are milestone mechanical properties for the high pressure die casting industry. ...
... Fig. 11b Fig. 11b. The corresponding FFT pattern in Fig. 11d verified that the TiB 2 nanoparticle had coherent interface with the α-Al matrix, and the crystal orientation relation (OR) between the TiB 2 nanoparticle and the α-Al matrix was Al(11-1)// TiB 2 (0001) and Al[011]//TiB 2 [11][12][13][14][15][16][17][18][19][20]. ...
... In order to promote the mechanical and tribological properties of recycled Al alloys, more and more researches are focused on changing the morphologies of needle-like iron phases by adding the neutralized elements (i.e., Ti, Sr, B, Ce, Cr, etc.) [7,[11][12][13]. Among these neutralized elements, the manganese has been widely added into the secondary Al alloys due to the refinement and modification effect on needle-like phases [14,15]. Although the morphologies of needle-like phases could be remarkably changed by using manganese, the promotion of their mechanical properties were not prominent enough [11,13]. ...
... The EDS testing results given in Figure 7(a) indicate that the coarse dendrite-like phase is mainly composed of Fe and Mn elements. The XRD analysis suggests that the dendritelike phase might be the Al 85 (Mn 0.72 Fe 0.28 ) 14 Si phase. In addition, the Al 3.21 Si 0.47 (eutectic Al-Si) and the Al 86 Fe 14 existed. ...
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Individual and combined addition of Ti and Ce on the recycled Al-Si-Cu-Fe-Mn alloy was conducted. The microstructures and tensile properties of these fabricated alloys were investigated. In the case of Ti or Ce which was individually added, the added amount was ranging from 0.03 wt.% to 0.09 wt.%. The combined addition of Ti and Ce was set at the ratios of 1 : 1, 1 : 3, and 3 : 1 with a total amount of 0.12 wt.%. Microstructures and phases of these alloys were investigated by using an optical microscope, X-ray diffraction testing, and SEM coupled with EDS. The morphologies of these alloys were quantified by analyzing the SDAS value, length of secondary phases, and phases' distribution uniformity. Tensile testing was carried out for understanding the strengthen effect of the modification process. Results show that the addition of Ce was favorable to the strength and % elongation because the coarse needle-like phase and the polyhedral phase were effectively refined. Their SDAS values and distribution factor were remarkably declined with the increase of the Ce level. The Ti addition could also refine the secondary phases and SDAS values. But its effect was not as prominent as the addition of Ce. Combined addition of Ti and Ce elements at the ratio of 1 : 3 resulted in the samples reaching maximum comprehensive tensile properties. In this case, the short needle-like phase was uniformly distributed in the microstructure. Few polyhedral phases could be found in the Al-Si-Cu-Fe-Mn matrix. The strengthening of these fabricated materials was due to the grain refinement for α-Al and modification for coarse secondary phases. In addition, distribution uniformity of secondary phases was also changed by their modification effects.
... According to Zhang et al. [50] hydrogen micropores are formed from hydrogen vacancy groups. Other studies [11,14] however, show that hydrogen micropores are heterogeneously Mackenzie [20] charge hydrogen blisters formed during heat treatment by transition to Al2O3 hydration AlOOH (boehmite) shows in Figure 6a, although this is not formed in solution temperature, although heating does not prevent this compound from being formed [48] shows that the formation of blisters that transform surface oxide into boehmite occurs at temperatures lower than that of solution. ...
... The necessity to enhance the utility properties of materials and increase the longevity of construction components has led research and development in various industrial branches, such as materials processing [1] or construction [2,3], towards the design of new materials and improving the performance of contemporary ones. Generally, enhancement of the mechanical and utility properties can be achieved via variations in the chemical composition of the particular material, as documented, for example, by Zhang et al. [4], and structural modifications (grain refinement), which was demonstrated also for pure and commercially pure metals [5,6]. Verlinden et al. claimed that grain refinement introduces lattice defects, such as dislocations and grain boundaries, which increase the materials' intrinsic energy and act as strengthening agents [7]. ...
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This contribution characterizes the performance of a DESI 11 high-speed disintegrator working on the principle of a pin mill with two opposite counter-rotating rotors. As the ground material, batches of Portland cement featuring 6–7 Mohs scale hardness and containing relatively hard and abrasive compounds with the specific surface areas ranging from 200 to 500 m2/kg, with the step of 50 m2/kg, were used. The character of the ground particles was assessed via scanning electron microscopy and measurement of the absolute/relative increase in their specific surface areas. Detailed characterization of the rotors was performed via recording the thermal imprints, evaluating their wear by 3D optical microscopy, and measuring rotor weight loss after the grinding of constant amounts of cement. The results showed that coarse particles are ground by impacting the front faces of the pins, while finer particles are primarily milled via mutual collisions. Therefore, the coarse particles cause higher abrasion and wear on the rotor pins; after the milling of 20 kg of the 200 m2/kg cement sample, the wear of the rotor reached up to 5% of its original mass and the pins were severely damaged.
... 1.1.1 Non-heat-treated die-cast aluminum alloy Shanghai Jiao Tong University has developed two aluminum alloys for die castings, which are denoted as JDA1 (Al-Si-Mn-Mg-RE) [4] and JDA2 (Al-Mg-Si-Mn) [5]. These alloys do not need high-temperature solution treatment and artificial aging. ...
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The microstructure, mechanical properties and thermal stability of high pressure die casting Al‐12Si‐Sc alloy with 0.1 wt% Sc addition were investigated. Compared with the commercial A380 alloy, the alloy shows a significant refinement of α‐Al and eutectic Si microstructure. Meanwhile, β‐Fe with irregular morphology shows a smaller size and lower volume fraction in Al‐12Si‐Sc alloy. Upon as‐cast state, the elongation of the alloy is improved to 6.7% with an ultimate tensile strength of 332 MPa, which is a better combination of ductility and strength in the literature. After thermal exposure at 200 °C for 200 h, it retains tensile strength of 277 MPa and holds the microhardness up to 90 HV even exposure for 1000 h. These excellent properties may widen the application of high pressure die casting Al‐Si alloy for cylinder block and cylinder head. This article is protected by copyright. All rights reserved.
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Mechanical behavior and microstructure evolution of a newly developed high strength die casting Al–13Si-0.25Cu alloy (in wt%) were investigated under as-cast, peak-aged and thermal-exposed conditions. In the as-cast condition, the alloy has a yield strength (YS) of 213 MPa and an elongation (El) of 4.6% under as-cast condition. The alloy YS is further improved to 265 MPa after T5 ageing treatment. The enhancement of YS after ageing is attributed to the formation of nanoscale β″, Q′ and θ′ phase precipitates from the α-Al matrix. After thermal exposure at 350 °C for 100 hours, however, these precipitates mostly dissolve into the α-Al matrix and the ultrafine eutectic Si networks undergo breaking and coarsening, reducing YS to 117 MPa while enhancing the El to 10.3%.
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High cycle fatigue properties of high-pressure die-cast magnesium alloys AZ91 hp, AM60 hp, AE42 hp, AS21 hp and of similarly produced cast aluminium alloy AlSi9Cu3 have been investigated. Ultrasonic fatigue tests up to 109 cycles show mean fatigue limits of approx. 38–50 MPa (magnesium alloys) and 75 MPa (AlSi9Cu3) in the tested casting condition. Fatigue cracks initiated at porosity in 98.5% of the samples. Considering porosity as initial cracks, specimens fail, if critical stress intensity amplitude, Kcr is exceeded. Kcr of the magnesium alloys range from 0.85±0.05 to 1.05±0.05 MPa√m, and 1.85±0.10 MPa√m was found for AlSi9Cu3. Below Kcr, fatigue cracks may initiate at porosity, however, do not propagate until failure. Using Kcr, the statistical distribution of defects is linked to the fracture probability at different stress amplitudes.
Book
This monograph summar Steel and Alloy during many decades in part together with Alcoa Inc. The research covered areas of the structure, properties, thermal resistance, corrosion and fatigue of aluminum alloys in industrial manufacturing. · Emphasis on interconnection among phase equilibria, thermodynamics and microstructure of alloys; · Systematic overview of all phase diagrams with Al that are important for the development of casting aluminium alloys · Diagrams ("processing windows") of important technological properties such as castability, molten metal fluidity, tendency to hot pre-solidification cracking, porosity · Mathematical models for alloy mechanical properties facilitating the down-selection of best prospect candidates for new alloy development · New principles of design of eutectic casting aluminium alloys · Examples of successful novel casting alloy development, including alloys for high-strength applications, alloys with transition metals, and novel alloys utilizing aluminium scrap.
Chapter
This chapter aims to trace a short history of the logical steps which led to the development of new aluminium materials for automotive applications. The main focus is on high-pressure die-cast (HPDC) aluminium alloys and the driving factors leading to the current use of premium casting alloys. The chemical composition, mechanical properties and specific features of Silafont®-36, Magsimal®-59 and Castasil®-37 are described, with guidelines provided for their processing and casting. Data from practical experiences are reported, answering the most common questions on primary low-iron ductile alloys. Finally, examples of the applications of these materials are briefly described, underlining the innovative aspect of each component.
Article
This book has the stated aim of covering the physical and mechanical metallurgy, interfacial physics and mechanics, and processing of metal matrix composites (MMCs). The term metal matrix composites covers particle, short-fiber, and continuous-fiber reinforced metals. The book covers most of the important topics relevant to the mechanical characteristics of MMCs such as load transfer from the matrix to the fiber, plastic deformation of the metal matrix, effect of thermal mismatch between the fiber and the matrix, and microstructural changes in the matrix caused by the presence of reinforcements. Fracture, transport properties, environmental properties, and tribological characteristics of MMCs are dealt with. Analytical treatments of various topics are provided in a very able manner. The last two chapters include practical information on special techniques such as MMC specimen preparation for transmission electron microscopy, etc. and commercial applications of MMCs. What is surprising is that among the applications of MMCs, no mention is made of filamentary superconductors, conventional or high [Tc] ceramic oxide superconductors. This book is a welcome addition to the literature on metal matrix composites. It is suitable for scientists, engineers, and practicing engineers who deal with metal matrix composites. There is a wealth of line diagrams, micrographs, andmore » references to the literature, and there are subject and author indexes.« less
Article
A method was developed for simultaneous determination of the migration of 28 kinds of primary aromatic amines(PAAs) from food contact materials by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The analytes were separated on a C-3 column, eluted by gradient with methanol and water, identified by electrospray ionization mass spectrometry in positive mode using multiple reaction monitoring (MRM), quantified by external standard method. The limits of detection (LOD, S/N = 3) and quantification (LOQ, S/N = 10) of 28 PAAs were 0.02-0.3 mu g/kg and 0.1-1.0 mu g/kg, respectively. The correlation coefficients of linear calibration curve were over 0.9915 in the ranges varied from 1.0 mu g/L to 1000 mu g/L. The recoveries of PAAs were 77.8%-105.3% in the spiked range of 1.0-100 mu g/kg, and the relative standard deviation (RSD) was 4.5%-11.8%. The experiment results indicated that the method was simple, rapid, sensitive, accurate, and could meet the correlative requirements for determination.
Article
In order to assess the high-temperature performance of aluminum–silicon alloy reinforced with titanium diboride particles as potential piston material, the tensile behaviors and fracture mechanisms of in situ 4 wt% TiB2/Al–Si composite were investigated in the temperature range 25–350 °C. The tensile results revealed that the composite exhibited higher modulus than the matrix alloy at all testing temperatures, but both the matrix alloy and the composite presented similar strength levels above 200 °C. The ductility of the composite was found to be lower than that of the unreinforced matrix alloy at 25 and 200 °C, but no obvious distinction was observed at 350 °C. The effects of temperature and the presence of TiB2 particles on tensile properties of the composite had been evaluated. Fractographic morphology studies were done using scanning electron microscope, which indicated that the fracture of the composite altered from brittle to ductile mode with temperature increasing. At 25 and 200 °C, fracture was dominated by cracked silicon particles and separated TiB2 particles, while decohesion at particle–matrix interface was prevalent at 350 °C. Analysis of the fracture surfaces also showed that regions of clustered TiB2 particles were found to be the locations prone to damage in the composite at both room and high temperatures.
Article
A refined microstructure of Al-Mg-Sc-Zr alloy with an average grain size of ~3.7 μm and a portion of high angle boundaries of 69.2% was produced by free forging. Excellent superplastic ductility of ≥500% was achieved at a wide temperature range of 450~500 °C and relatively high strain rate range of 1×10−3~5×10−2 s−1 in the Al-Mg-Sc-Zr alloy. A maximum elongation of 1593% was obtained at 475 °C and 1×10−3 s−1. Moreover, the electron back scattered diffraction (EBSD) and the transmission electron microscopy (TEM) analyses showed that the excellent superplasticity can be attributed to the high fraction of high angle grain boundaries and the presence of Al3(Sc,Zr) dispersoids in the Al-Mg-Sc-Zr alloy microstructure. The analyses on the superplastic data revealed the presence of threshold stress, the coefficient of strain rate sensitivity of 0.5, and an activation energy of 83.9 kJ/mol-1. It indicated that the dominant deformation mechanism was grain boundary sliding. Based on this notion, a constitutive equation for Al-Mg-Sc-Zr alloy has been developed.
Article
Fracture of Al2O3 particles in an Al2O3 particle/ aluminum alloy metal matrix composite (MMC) under plastic straining was observed by in-situ tensile testing in a scanning electron microscope. The fracture of larger sized particles was found to be preferred to that of smaller sized ones for a given plastic strain. The Young’s modulus (Ec) of plastically strained MMC was reduced with increase in plastic strain (εp). The analytical model based on Eshelby’s equivalent inclusion method was constructed to explain the relationship between Ec and εp, resulting in a good agreement with the experimental results.
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In this paper, the influence of La addition on the aging behavior, microstructure, mechanical properties, and thermal-resistant properties of Al–Mg–Si–Zr aluminum alloy has been studied with the help of optical microscope (OM), Brinell hardness measurements and scanning electron microscopy (SEM) equipped with energy dispersive spectrum (EDS). The results show that La addition decreases the peak-aged hardness during aging process. Electrical conductivity and thermal-resistant properties improve when the La addition exceed 0.2%. Increasing La content to 0.3% leads to a poor elongation at fracture. Most of La element exists in the form of compounds containing Si which result in improvement in thermal-resistant properties. The tensile strength of the material with 0.2% La addition reduces while the thermal-resistant properties and electrical conductivity improve. This amount of La addition added to the experimental alloy could be a compromise between the mechanical properties and others like electrical conductivity and thermal-resistant properties.
Article
A high strength Al–Mg2Si–Mg–Zn based alloy has been developed for the application in high pressure die casting to provide improved mechanical properties. The effect of various alloying elements on the microstructure and mechanical properties including yield strength, ultimate tensile strength and elongation of the alloy was investigated under the as-cast and heat-treated conditions. The typical composition of the high strength alloy has been optimised to be Al–8.0 wt%Mg2Si–6.0 wt%Mg–3.5 wt%Zn–0.6 wt%Mn (Al–11.0 wt%Mg–2.9 wt%Si–3.5 wt%Zn–0.6 wt%Mn) with unavoidable trace impurities. The mechanical properties of the alloy were enhanced by a quick solution treatment followed by ageing treatment. The improved tensile properties were at a level of yield strength over 300 MPa, the ultimate tensile strength over 420 MPa and the elongation over 3% assessed using international standard tensile samples made by high pressure die casting. The microstructure of the die-cast alloy consisted of the primary α-Al phase, Al–Mg2Si eutectics, AlMgZn intermetallics and α-AlFeMnSi intermetallics under the as-cast condition. The AlMgZn intermetallic compound was dissolved into the Al-matrix during solution treatment and subsequently precipitated during ageing treatment for providing the effective improvement of the mechanical properties.
Article
The super ductile diecast aluminium alloys have been developed particularly for application in automotive body structure. On the basis of the reviewing aluminium alloys currently available, the requirement of diecast aluminium alloys is summarized and the Al-Mg-Si system is focused in the development. The effect of various alloying elements on the microstructure and the mechanical properties, such as yield strength, ultimate tensile strength and elongation is assessed. The optimized composition of the super ductile Al–Mg–Si alloy has been found to be at 5.0–5.5 wt% Mg, 1.5–2.0 wt% Si, 0.5–0.7 wt% Mn, 0.15–0.2 wt% Ti with Fe o0.25 wt% for the best combination of strength and ductility, which shows 150 MPa of yield strength, 300 MPa of ultimate tensile strength, and 15% of elongation under as-cast condition. The paint baking hardenability of the optimized alloy is found to be insignificant. Less than a 10% increase in the yield strength was achieved, with a slight decrease in the elongation after aging at 180 1C for 30 min, which is a simulated process of paint baking. Cu is found to slightly increase the yield strength under the as-cast condition and after the heat treatment, but with a significant reduction in the ductility. Therefore, Cu should be limited in the super ductile aluminium alloy. The microstructure of diecast aluminium alloys at the optimized composition consists of the primary a-Al phase, the a-AlFeMnSi intermetallics and the Al–Mg2Si eutectics. There are two types of primary a-Al phase: dendritic or fragmented dendritic a-Al phase solidified in the shot sleeve and globular a-Al particles solidified in the die cavity. The a-AlFeMnSi intermetallics is in the form of compact morphology and with a size of less than 3 mm. The eutectic cells are at size of 10 mm with a typical lamellar morphology of a-Al phase and Mg2Si phase.
Article
Thermal analysis and interrupted quench experiments have been carried out to study the formation of β-FeSiAl5 and (BeFe)-BeSiFe2Al8 phases in Al7Si0.3Mg alloy with and without Be addition. In the base alloy with 0.6% Fe (without Be addition), a needle- and plate-shaped β-phase is present in the interdendritic regions and is formed by a ternary eutectic reaction. In the Be-added alloy with 0.6% Fe, a BeFe phase of Chinese script and polygon shapes grows along with the primary α-Al dendrites, leading to superior mechanical properties. It is proposed that this BeFe phase is formed by a peritectic reaction. Be addition has also resulted in some grain refinement.
Article
The effects of rare earth erbium (Er) additions (0, 0.3, 0.6 and 0.9 wt.%) on the microstructure development and tensile properties of die-cast ADC12 aluminum alloy have been investigated in the present work. The microstructures and fracture surfaces of die-cast samples were examined by optical microscopy and scanning electron microscopy (SEM). It was found that the secondary dendrite arm spacing (SDAS) will decrease with increasing Er content, as the Er content increases to 0.6%, the lowest SDAS was obtained. In addition, the Er modified the eutectic silicon from a coarse plate-like and acicular structure to a fine branched and fibrous one. The microhardness of die-casted alloys were measured, the microhardness corresponding to the die-casted samples with 0, 0.3, 0.6 and 0.9 wt.% Er additions are 100.6, 107.1, 113.6 and 108.5 Hv, respectively. The tensile properties were improved by the addition of Er, and a good ultimate tensile strength (269 MPa) but poor elongation (2%) were obtained when the Er addition was 0.6 wt.%. Furthermore, fractographic examinations revealed that refined pore and spheroidized α-Al dendrite were responsible for the high ultimate tensile strength.
Article
Deformation behavior of Al–6.2%Cu–0.6%Mg alloy and Al–6.2%Cu–0.6%Mg alloy containing 0.06wt.% of Sn was studied by hot compression tests conducted at various temperatures and strain rates. During the deformation process, the flow stress of the Al–Cu–Mg alloy increased due to trace addition of Sn. The peak flow stress for both the alloys increased with increase in strain rate and decrease in deformation temperature. The peak flow stress during deformation can be correlated with temperature and strain rate by a hyperbolic-sine equation. The activation energy for hot deformation of the Al–Cu–Mg alloy was determined to be 183.38kJ/mol, which increased to 223.30kJ/mol when micro-alloyed with 0.06wt.% of Sn. The Zener–Hollomon (Z) parameter for the two alloys was determined for the various deformation conditions. The tendency of dynamic recrystallization increases with low strain rates and high deformation temperatures which correspond to low Z values. The hot deformation behavior of both the alloys was modeled by suitable constitutive equation. The peak flow stresses at various deforming conditions have been predicted and correlated with the experimental values. It was possible to predict 80% of the values for peak flow stress within an error less than ±11.5% for Alloy-A, where as for Alloy-B, 95% of the peak flow stress values could be predicted within an error of ±7%.
Article
The high-temperature tensile properties and microstructural evolution of Al–6Mg–0.4Sc–0.13Zr alloy prepared by conventional cold rolling at various temperatures were studied. At 573K, the Al3(ScxZr1−x) particles stabilized the extremely fine subgrains (SG) and submicrograins (SMG). These SG and SMG boundaries acted as effective dislocation sinks, turning into true high-angled boundaries susceptible to consequent sliding. Therefore, the alloy achieved an acceptable ductility with 300% elongation. At 623–723K, because geometrical sliding was not feasible with rugged grain boundaries and dislocation movement was strongly hindered by the Al3(ScxZr1−x) particles, the alloy exhibited poor ductility. At 773 and 803K, the Al3(ScxZr1−x) particles began to act as grain stabilizers, and retained fine-grain structure at these high temperatures. As a result, a maximum elongation of 1100% was achieved after deformation at 803K with a strain rate of 5×10−3s−1.
Article
In this investigation, an effective approach based on multivariable linear regression (MVLR) and genetic algorithm (GA) methods has been developed to determine the optimum conditions leading to minimum porosity in AlSi9Cu3 aluminium alloy die castings. Experiments were conducted by varying holding furnace temperature, die temperature, plunger velocities in the first and second stage, and multiplied pressure in the third stage using L27 orthogonal array of Taguchi method. The experimental results from the orthogonal array were used as the training data for the MVLR model to map the relationship between process parameters and porosity formation of the die cast parts. With the fitness function based on this model, genetic algorithms were used for the process conditions optimization. By comparing the predicted values with the experimental data, it was demonstrated that the proposed model is a useful and efficient method to find the optimal process conditions in pressure die casting associated with the minimum porosity percent.
Article
The zinc die casting process is undergoing major changes as researchers uncover new knowledge about the material and the process. International Lead Zinc Research Organisation, Inc. (ILZRO) has been sponsoring such research for years in the U.S., U.K. and Australia. The basic objective of these projects is to improve the zinc die casting process itself so that ultimately all major parameters in the process will be controlled automatically to enable production of zinc die castings at the lowest possible cost and with zero defects. This paper reviews some of the specific developments emerging from all three research areas, including thin-walled die casting, alternative gating systems, die coatings and die materials, pressure and thermal studies, metallurgical properties of castings, and runner design, with emphasis on tapered runners.
Article
The influence of casting defects on static and fatigue strength is investigated for a high pressure die cast aluminium alloy. Defects exist in gas and shrinkage pores as well as cold fills, dross and alumina skins. For the three batches of specimens, differing for the sprue–runner design, the influence was straightforward, while no significant variation in the fatigue strength was observed when looking at batches of “acceptable” and “non-acceptable” components, as judged within the foundry quality control. In this case, defects count for their size and location, while quality control often takes no account for component working conditions. The Haigh diagram shows a good matching between the specimen reference material and the component fatigue data.
Article
The elastic-plastic deformation behavior of particulate reinforced metal matrix composites is strongly influenced by not only properties of the metal matrix and reinforcement but also microstructural parameters such as particle size, volume fraction, reinforcement shape and topology that are dependent on the particular processing route used to fabricate the materials. Nan and Clarke have recently developed a hybrid, analytical model by incorporating essential features of both continuum plasticity and dislocation plasticity, which is a combination of the key features of dislocation plasticity with a continuum mechanics approach based on effective medium approximations (EMA). In the present contribution, particular attention will be paid to two problems on the hybrid model. Firstly, the authors introduce a volume-weighted averaging procedure in Nan and Clarke`s methodology and then re-estimate the effect of particle size distribution on deformation in the case of both no damage and including particle fracture. For illustrative and quantitative purposes, the calculations are compared with the experimental results on particular SiC-Al composite by Lloyd. Secondly, the difference between the deformation, secant and the incremental, tangent modulus approaches chosen for EMA modeling is addressed.
Article
The dissolution and melting of Al2Cu phase in solution heat-treated samples of unmodified Al-Si 319.2 alloy solidified at ≈10 °C were studied using optical microscopy, image analysis, electron probe microanalysis (EPMA), and differential scanning calorimetry (DSC). The solution heat treat-ment was carried out in the temperature range 480 °C to 545 °C for solution times of up to 24 hours. Of the two forms of Al2Cu found to exist,i.e., blocky and eutectic-like, the latter type is more pronounced in the unmodified alloy (at ≈10 °C) and was observed either as separate eutectic pockets or precipitated on preexisting Si particles, β-iron phase needles, or the blocky Al2Cu phase. Dissolution of the (Al + Al2Cu) eutectic takes place at temperatures close to 480 °C through frag-mentation of the phase and its dissolution into the surrounding Al matrix. The dissolution is seen to accelerate with increasing solution temperature (505 °C to 515 °C). The ultimate tensile strength (UTS) and fracture elongation (EL) show a linear increase when plotted against the amount of dissolved copper in the matrix, whereas the yield strength (YS) is not affected by the dissolution of the Al2Cu phase. Melting of the copper phase is observed at 540 °C solution temperature; the molten copper-phase particles transform to a shiny, structureless phase upon quenching. Coarsening of the copper eutectic can occur prior to melting and give rise to massive eutectic regions of (Al + Al2Cu). Unlike the eutectic, fragments of the blocky Al2Cu phase are still observed in the matrix, even after 24 hours at 540 °C.
Article
The plastic deformation behavior of aluminum casting alloys A356 and A357 has been investigated at various solidification rates with or without Sr modification using monotonic tensile and multi-loop tensile and compression testing. The results indicate that at low plastic strains, the eutectic particle aspect ratio and matrix strength dominate the work hardening, while at large plastic strains, the hardening rate depends on secondary dendrite arm spacing (SDAS). For the alloys studied, the average internal stresses increase very rapidly at small plastic strains and gradually saturate at large plastic strains. Elongated eutectic particles, small SDAS, or high matrix strength result in a high saturation value. The difference in the internal stresses, due to different microstructural features, determines the rate of eutectic particle cracking and, in turn, the tensile instability of the alloys. The higher the internal stresses, the higher the damage rate of particle cracking and then the lower the Young’s modulus. The fracture strain of alloys A356/357 corresponds to the critical amount of damage by particle cracking locally or globally, irrespective of the fineness of the microstructure. In the coarse structure (large SDAS), this critical amount of damage is easily reached, due to the clusters of large and elongated particles, leading to alloy fracture before global necking. However, in the alloy with the small SDAS, the critical amount of damage is postponed until global necking takes place due to the small and round particles. Current models for dispersion hardening can be used to calculate the stresses induced in the particles. The calculations agree well with the results inferred from the experimental results.
Article
Microstructural evolution under intensive forced convection is of importance to the development of new semisolid processing technologies. Although efforts have been made to understand the nucleation and growth behavior of the primary phase under forced convection, important aspects of microstructural evolution still remain as open questions. Experimental work was undertaken to investigate the effects of a high shear rate and high intensity of turbulence on the solidification behavior of a Sn-15 wt pct Pb alloy using a twin-screw extruder. It was found that increasing the shear rate and intensity of turbulence can give rise to substantial grain refinement, which can be attributed to the increased effective nucleation rate caused by the extremely uniform temperature and composition fields in the bulk liquid at early stages of solidification. It was also found that forced convection increases the growth velocity and stabilizes the solidification interface, resulting in a transition of the growth morphology from rosette to spheroid with increasing shear rate and intensity of turbulence. Discussions are made on the effects of flow conditions on the nucleation behavior, constitutional undercooling, stability of the solidification interface, and growth morphology.
Article
The automotive business is an ideal choice to examine the dramatic impact of improved materials and manufacturing processes on an industry Automakers today are able to combine high-tech materials originally applied in aerospace and other industries with the high-volume manufacture of a mass-marketed consumer product. This paper will detail how many of the changes to vehicles that have resulted from these influences over the past 50 years have been enabled by significant advances in materials and processes.
Article
Constant-amplitude high-cycle fatigue tests (σmax=133 MPa, σmax/σy=0.55, and R=0.1) were conducted on cylindrical samples machined from a cast A356-T6 aluminum plate: The fracture surface of the sample with the smallest fatigue-crack nucleating defect was examined using a scanning electron microscope (SEM). For low crack-tip driving forces (fatigue-crack growth rates of da/dN<1 × 10−7 m/cycle), we discovered that a small semicircular surface fatigue crack propagated primarily through the Al-1 pct Si dendrite cells. The silicon particles in the eutectic remained intact and served as barriers at low fatigue-crack propagation rates. When the semicircular fatigue crack inevitably crossed the three-dimensional Al-Si eutectic network, it propagated primarily along the interface between the silicon particles and the Al-1 pct Si matrix. Furthermore, nearly all of the silicon particles were progressively debonded by the fatigue cracks propagating at low rates, with the exception of elongated particles with a major axis perpendicular to the crack plane, which were fractured. As the fatigue crack grew with a high crack-tip driving force (fatigue-crack growth rates of da/dN>1 × 10−6 m/cycle), silicon particles ahead of the crack tip were fractured, and the crack subsequently propagated through the weakest distribution of prefractured particles in the Al-Si eutectic. Only small rounded silicon particles were observed to debond while the fatigue crack grew at high rates. Using fracture-surface markings and fracture mechanics, a macroscopic measure of the maximum critical driving force between particle debonding vs fracture during fatigue-crack growth was calculated to be approximately K maxtr ≈6.0 MPa √m for the present cast A356 alloy.
Article
Phase equilibria in the Al corner of the Al-Fe-Mn-Si system at 550 °C have been explored. Twenty-six quaternary alloys were prepared and analyzed by microprobe analysis. Elemental powders were mixed under air and molten at 1000 °C under argon flux. These specimens received a heat treatment at 550 °C during 4 to 12 weeks and finally were water quenched. A four-phase region, where Al, Si,α-Al(Fe,Mn)Si, andβ-Al(Fe,Mn)Si are in equilibrium with each other, was found. The presence of a single-phaseα-Al(Fe,Mn)Si region, starting in the Al-Mn-Si subsystem and extending toward the Al-Fe-Si subsystem, could be confirmed. Some problems concerning the fit of the quaternary system with its ternary Al-Fe-Mn subsystem remain to be solved.
Article
The tensile fracture behavior of a cast and extruded 2014 aluminum alloy metal matrix composite (MMC) reinforced with 10, 15, and 20 vol.% aluminum oxide particles was investigated as a function of temperature between 100 and 300°C and hold time, and compared with the unreinforced alloy. In addition, the effect of aging condition was investigated in a 15 vol.% composite tested at 200°C. At lower temperature the composites have higher yield strength and UTS than the unreinforced material, and both decrease with increasing temperature. At higher temperatures all the materials have similar strength levels. The elongation is lower in the composites, decreasing with increasing level of reinforcement and increasing with increasing temperature, except at the highest temperature where all the composites are about the same. The microstructural damage in the composites also varies with temperature: particle fracture dominates at lower temperatures and interparticle voiding is the main damage feature at elevated temperatures. The time at temperature, and hence the degree of overaging, has little effect on the observed trends in the composite, in contrast with the unreinforced material where the density of voids decreases with increasing hold times. The transition temperature where the major damage changes from particle cracking to interparticle voiding increases with volume fraction and particle size, and decreases with overaging. The cracked particle density and void density both increase with strain, and the highest rate of increase occurs in the overaged material. In general, the tendency for particle cracking is reduced and for interparticle voiding is increased by any factor which permits accomodation of strain by the matrix, such as lower volume fraction of particles, small particle size, nonclustered particle distribution, and matrix softening from underaging or overaging.
Article
The effects of particle volume fraction and matrix temper on the flow and fracture characteristics of a series of particle-reinforced metal matrix composites under tensile and compressive loadings have been examined. Under compressive loading, a steady-state regime is attained in which the composite flow stress is proportional to the matrix flow stress at the same level of strain. The strength enhancement associated with the particles increases with increasing particle content and the matrix hardening exponent. The trends are consistent with predictions of finite element calculations of unit cell models, treating the particles as either spheres or cylinders with unit aspect ratio. Under tensile loading, the particles crack at a rate dependent on the intrinsic strength characteristics of the particles as well as the flow characteristics of the matrix. Particle cracking causes local softening, which reduces the work hardening rate as compared with compression deformation. This lowers both the strength and the ductility. Experimental measurements have been combined with finite element calculations to develop a damage law, incorporating the effects of the matrix strength on the particle stress. The damage law has been used to simulate the tensile flow response of the composites, using appropriate cell models under either isostrain or isostress conditions. Though the trends obtained from the simulations are in qualitative agreement with the experimental results, they tend to underestimate the flow stress. In all cases, tensile fracture is preceded by the formation of a neck. The condition at the onset of necking is consistent with the Considère criterion. Differences in necking strains between the composites and the monolithic matrix alloy have been rationalized on the basis of the rate of damage accumulation.
Article
A systematic study of the effect of microstructural parameters on the fracture behaviour of silicon carbide particle reinforced aluminium matrix composites has been carried out. Acoustic emissions have been monitored during tensile testing, giving the size and number of emmissions as a function of strain. This has been shown to be simply related to the rate of void nucleation at the reinforcing phase. Both particle fracture and particle/matrix decohesion mechanisms can be detected. Void nucleation was observed from the onset of plastic deformation and a linear relationship between damage initiation rate and strain was found. The rate of emission increased with reiforcing particle size and volume fraction but was independent of matrix alloy composition and heat treatment. These results show that the failure strain of particulate metal matrix composites is not controlled solely by the onset of void nucleation at the reinforcing phase. Local failure processes in the matrix are shown to promote void coalescence and dominate the ductility. However, suppression of void nucleation at the particles increases the ductility. It is suggested that a critical number of fractured particles is required before failure.
Article
Lightweight magnesium alloys are being increasingly used in automotive and other transportation industries to achieve energy efficiency and environmental protection. Design of magnesium components requires low cycle fatigue (LCF) behavior since these applications are often subjected to cyclic loading and/or thermal stresses. The objective of this investigation was to study the cyclic deformation behavior and LCF life of a large solid extruded section of AZ31 magnesium alloy. It was observed that the alloy was cyclically stable at lower strain amplitudes and exhibited cyclic hardening characteristics at higher strain amplitudes, with a cyclic hardening exponent of about 2.6 times higher than the monotonic strain hardening exponent. A relationship between the plastic strain amplitude and the number of cycles (N), was observed. With increasing total strain amplitude both plastic strain amplitude and mean stress increased and the fatigue lifetime decreased. Bauschinger effect was pronounced at higher strain amplitudes, resulting in asymmetric hysteresis loops due to twinning in compression during unloading and subsequent detwinning in tension during loading. Modulus during cyclic deformation was constant at the low strain amplitude, but it decreased with increasing strain amplitudes and increased with increasing number of cycles at the high strain amplitudes due to the presence of pseudoelastic behavior. Fatigue parameters following the Coffin-Manson and Basquin’s equations were evaluated. Fatigue crack initiation was observed to occur from the specimen surface and crack propagation was characterized by striation-like features coupled with secondary cracks.
Article
The tensile deformation and fracture behaviour of the aluminium alloy 6061 reinforced with SiC has been investigated. In the T4 temper plastic deformation occurs throughout the gauge length and the extent of SiC particle cracking increases with increasing strain. In the T6 temper strain becomes localised and particle cracking is more concentrated close to the fracture. The elastic modulus decreases with increasing particle damage and this allows a damage parameter to be identified. The fraction of SiC particles which fracture is less than 5%, and over most of the strain range the damage controlling the tensile ductility can be recovered, indicating that other factors, in addition to particle cracking are important in influencing tensile ductility. It is suggested that macroscopic fracture is initiated by the SiC particle clusters that are present in these composites as a result of the processing. The matrix within the clusters is subjected to high levels of triaxial stress due to elastic misfit and the constraints exerted on the matrix by the surrounding particles. Final fracture is then produced by crack propagation through the matrix between the clusters.
Article
The pickup of impurity Fe is an inherent engineering problem for the recycling of aluminium alloys, due to the detrimental effects of Fe containing intermetallic compounds on the mechanical properties. This work aims to understand the effects of intensive forced melt convection and alloying on the mechanical properties of Fe containing Al–Si based cast alloys. Varied Fe levels were introduced into widely used commercial Al–Si based cast alloys with different Mn contents. The role of intensive forced melt convection was investigated through comparative study of the microstructure properties relationship of alloys that are processed via rheo-diecasting (RDC) and conventional high-pressure diecasting (HPDC), with the former involving intensively shearing the melt in a slurry maker before diecasting. CALPHAD modelling of the thermodynamic properties of the multi-component alloys was carried out to clarify the role of alloying elements on the formation of different primary Fe containing intermetallic compounds. The experimental results show that intensive forced melt convection during solidification promotes the formation of α-AlFeMnSi compound with a compact morphology. It was also found that Mn as an alloying element assists the conversion of needle-shaped β-AlFeSi compound into more compact α-AlFeMnSi compound.
Article
Sand-cast plates of a commercial AZ91C alloy have been used for the study. Varying the solidification rate by placing large cast-iron chills in the mould produced a range of secondary dendrite arm spacing (SDAS) within the cast plates. The plates were solution heat-treated, quenched and aged at 165 °C for up to 350 h. The SDAS (μm) varied with the solidification time, tf (s), as SDAS=5.3 tf0.43. The tensile ductility in the as-quenched (T4) condition did not depend on the solidification rate whilst in the T6 condition it tended to decrease for slowly solidified material (SDAS>50 μm). The yield strength and hardness increased and the ductility decreased with ageing. The fracture mode changed from predominantly transgranular in the T4 condition to predominantly intergranular in the T6 condition. The properties of the sand-castings are compared with those of high-pressure diecastings and the possible strengthening mechanisms are discussed. A number of areas that require more research are pointed out.
Article
Iron is the most common and detrimental impurity in aluminum casting alloys and has long been associated with an increase in casting defects. While the negative effects of iron are clear, the mechanism involved is not fully understood. It is generally believed to be associated with the formation of Fe-rich intermetallic phases. Many factors, including alloy composition, melt superheating, Sr modification, cooling, rate, and oxide bifilms, could play a role. In the present investigation, the interactions between iron and each individual element commonly present in aluminum casting alloys, were investigated using a combination of thermal analysis and interrupted quenching tests. The Fe-rich intermetallic phases were characterized using optical microscope, scanning electron microscope, and electron probe microanalysis (EPMA), and the results were compared with the predictions by Thermocalc. It was found that increasing the iron content changes the precipitation sequence of the beta phase, leading to the precipitation of coarse binary beta platelets at a higher temperature. In contrast, manganese, silicon, and strontium appear to suppress the coarse binary beta platelets, and Mn further promotes the formation of a more compact and less harmful a phase. They are therefore expected to reduce the negative effects of the phase. While reported in the literature, no effect of P on the amount of beta platelets was observed. Finally, attempts are made to correlate the Fe-rich intermetallic phases to the formation of casting defects. The role of the beta phase as a nucleation site for eutectic Si and the role of the oxide bifilms and AIP as a heterogeneous substrate of Fe intermetallics are also discussed.
Article
Banded defects are often found in high-pressure die castings. These bands can contain segregation, porosity, and/or tears, and changing casting conditions and alloy are known to change the position and make-up of the bands. Due to the complex, dynamic nature of the high-pressure die-casting (HPDC) process, it is very difficult to study the effect of individual parameters on band formation. In the work presented here, bands of segregation similar to those found in cold-chamber HPDC aluminum alloys were found in laboratory gravity die castings. Samples were cast with a range of fraction solids from 0 to 0.3 and the effect of die temperature and external solid fraction on segregation bands was investigated. The results are considered with reference to the theological properties of the filling semisolid metal and a formation mechanism for bands is proposed by considering flow past a solidifying immobile wall layer.
Article
The strain dependence of particle cracking in aluminum alloys A356/357 in the T6 temper has been studied in a range of microstructures produced by varying solidification rate and Mg content, and by chemical (Sr) modification of the eutectic silicon. The damage accumulates linearly with the applied strain for all microstructures, but the rate depends on the secondary dendrite arm spacing and modification state. Large and elongated eutectic silicon particles in the unmodified alloys and large pi-phase (Al9FeMg3Si5) particles in alloy A357 show the greatest tendency to cracking. In alloy A356, cracking of eutectic silicon particles dominates the accumulation of damage while cracking of Fe-rich particles is relatively unimportant. However, in alloy A357, especially with Sr modification, cracking of the large pi-phase intermetallics accounts for the majority of damage at low and intermediate strains but becomes comparable with silicon particle cracking at large strains. Fracture occurs when the volume fraction of cracked particles (eutectic silicon and Fe-rich intermetallics combined) approximates 45 pct of the total particle volume fraction or when the number fraction of cracked particles is about 20 pct. The results are discussed in terms of Weibull statistics and existing models for dispersion hardening.
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