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As-cast microstructures: (a) very large grain structure produced by quench-casting; (b) coarse structure from high-temperature-pouring (725 ◦ C); (c) medium structure from medium-temperature-pouring (675 ◦ C), and fine-grained structures of dendritic (d), rosette-like (e) and globular (f) morphologies
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The deformation behaviors of A356 alloy with various initial grain structures, from a very coarse dendrite to a fine equiaxed structure, prepared at different pouring temperatures, from 625°C to 725°C, were investigated. Cylindrical specimens with 12 mm in diameter and 10 mm in height were compressed in the semisolid state to a height reduction of...
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... or elongated microstructures. In order to obtain a statistically valid result, at least 500 particles were counted in each specimen. For these measurements it was assumed that the boundary between solid and liquid in the semisolid state was defined by the boundary between -phase grains and areas of eutectic in the quenched microstructures. Fig. 1 shows the initial as-cast microstructures of cast- ings produced by the controlled-pouring method. The grain size and morphology varied significantly with the pour- ing conditions. Fig. 1(a) shows the microstructure from quench-casting in which a thin-walled steel tube was used as the mould and was quenched into water after melt pour- ...
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... between solid and liquid in the semisolid state was defined by the boundary between -phase grains and areas of eutectic in the quenched microstructures. Fig. 1 shows the initial as-cast microstructures of cast- ings produced by the controlled-pouring method. The grain size and morphology varied significantly with the pour- ing conditions. Fig. 1(a) shows the microstructure from quench-casting in which a thin-walled steel tube was used as the mould and was quenched into water after melt pour- ing. The rapid cooling condition resulted in a fine dendrite arm spacing, but the lack of mould chill resulted in large grains-about 1400 m. The microstructure is described as "dendritic and ...
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... mould and was quenched into water after melt pour- ing. The rapid cooling condition resulted in a fine dendrite arm spacing, but the lack of mould chill resulted in large grains-about 1400 m. The microstructure is described as "dendritic and very coarse-grained". Most of the eutectic phase was intragranular, distributed between the dendrite arms. Fig. 1(b-e) show the microstructures from permanent mould castings. Compared to quench-casting, the grain size was much smaller and the grain morphology was less branched, although dendrite arm spacings were similar. Fig. 1(b) shows the microstructure of the sample from high-temperature-pouring. It had a coarse-grained den- dritic structure with ...
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... as "dendritic and very coarse-grained". Most of the eutectic phase was intragranular, distributed between the dendrite arms. Fig. 1(b-e) show the microstructures from permanent mould castings. Compared to quench-casting, the grain size was much smaller and the grain morphology was less branched, although dendrite arm spacings were similar. Fig. 1(b) shows the microstructure of the sample from high-temperature-pouring. It had a coarse-grained den- dritic structure with grain size 900 m and dendritic morphology. Fig. 1(c) shows a medium-grained dendritic microstructure from a casting using a medium pouring temperature. The grain size was approximately 350 m. Low-temperature-pouring ...
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... permanent mould castings. Compared to quench-casting, the grain size was much smaller and the grain morphology was less branched, although dendrite arm spacings were similar. Fig. 1(b) shows the microstructure of the sample from high-temperature-pouring. It had a coarse-grained den- dritic structure with grain size 900 m and dendritic morphology. Fig. 1(c) shows a medium-grained dendritic microstructure from a casting using a medium pouring temperature. The grain size was approximately 350 m. Low-temperature-pouring resulted in fine grains, as shown in Fig. 1(d-f). The grain size was about 160-200 m. The grain morphology varied with the different solidifica- tion conditions from a ...
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... of the sample from high-temperature-pouring. It had a coarse-grained den- dritic structure with grain size 900 m and dendritic morphology. Fig. 1(c) shows a medium-grained dendritic microstructure from a casting using a medium pouring temperature. The grain size was approximately 350 m. Low-temperature-pouring resulted in fine grains, as shown in Fig. 1(d-f). The grain size was about 160-200 m. The grain morphology varied with the different solidifica- tion conditions from a dendritic structure ( Fig. 1(d)), to a rosette-like structure ( Fig. 1(e)), to a globular structure ( Fig. 1(f)). Most of the eutectic phase was intergranular. Table 1 summarises the microstructural characteristics ob- ...
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... a medium-grained dendritic microstructure from a casting using a medium pouring temperature. The grain size was approximately 350 m. Low-temperature-pouring resulted in fine grains, as shown in Fig. 1(d-f). The grain size was about 160-200 m. The grain morphology varied with the different solidifica- tion conditions from a dendritic structure ( Fig. 1(d)), to a rosette-like structure ( Fig. 1(e)), to a globular structure ( Fig. 1(f)). Most of the eutectic phase was intergranular. Table 1 summarises the microstructural characteristics ob- served. The materials made by low-temperature-pouring method have excellent microstructural homogeneity. In contrast, the electromagnetically stirred ...
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... from a casting using a medium pouring temperature. The grain size was approximately 350 m. Low-temperature-pouring resulted in fine grains, as shown in Fig. 1(d-f). The grain size was about 160-200 m. The grain morphology varied with the different solidifica- tion conditions from a dendritic structure ( Fig. 1(d)), to a rosette-like structure ( Fig. 1(e)), to a globular structure ( Fig. 1(f)). Most of the eutectic phase was intergranular. Table 1 summarises the microstructural characteristics ob- served. The materials made by low-temperature-pouring method have excellent microstructural homogeneity. In contrast, the electromagnetically stirred material has been reported to be ...
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... temperature. The grain size was approximately 350 m. Low-temperature-pouring resulted in fine grains, as shown in Fig. 1(d-f). The grain size was about 160-200 m. The grain morphology varied with the different solidifica- tion conditions from a dendritic structure ( Fig. 1(d)), to a rosette-like structure ( Fig. 1(e)), to a globular structure ( Fig. 1(f)). Most of the eutectic phase was intergranular. Table 1 summarises the microstructural characteristics ob- served. The materials made by low-temperature-pouring method have excellent microstructural homogeneity. In contrast, the electromagnetically stirred material has been reported to be nonuniform from the billet surface to centre ...
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Citations
... On the other hand, SSM processing has opened up a way to produce near-net-shape components of aluminium alloy with less energy consumption in comparison to the conventional casting process [21]. Although most studies are performed on aluminium-silicon alloy [1,14,[22][23][24][25][26][27], however, the use of wrought aluminium alloy in SSM processing has seen an increase for the past few years [12,[28][29][30][31][32]. A study conducted by Ahmad et. ...
... It indicated that lower pouring temperature was an important factor for the evolution of finer and more globular or spheroidal microstructures. Previous findings by several researchers also recorded the same output [23,33,39]. Moreover, pouring temperature is associated with the cooling rate during the gas-assisted DTM process. ...
Semisolid metal (SSM) processing is a forming process which occurs within solidus and liquidus temperatures, and it requires feedstock billets with spheroidal microstructures. In this paper, a novel gas-assisted direct thermal method (DTM) is proposed as a method to create feedstock billets with fine spheroidal microstructures. Effect of different combination parameters between pouring temperature and holding time during the gas-assisted DTM technique on the microstructure of aluminium alloy 7075 feedstock billets was investigated. Pouring of molten aluminium alloy 7075 was conducted at temperatures between 645 and 685 °C, while holding time was set between 20 and 60 s. The melt was cooled down within the semisolid temperature range in a cylindrical copper mould with the help of carbon dioxide (CO2) gas before quenching in room temperature water. Results revealed that the smallest primary phase grain size formed at the combination parameters of 645 °C pouring temperature and 20-s holding time. Furthermore, the same combination parameters also produced the highest circularity value. The addition of external CO2 gas surrounded the copper mould as a rapid cooling agent was found to have a significant improvement of 36.4% to the formation of smaller primary grain and spheroidal structure. It is concluded that the lowest pouring temperature and shortest holding time coupled with rapid cooling condition from the addition of external CO2 during DTM produced the finest and most globular structure of primary phase size.
... Sample 1 with a pouring temperature of 660 ° C with 20 s of holding time produced small grain size particles at 2547.87 µm 2 . According to the previous research, a lower pouring temperature could lead to the smaller primary and secondary phases in the grain and produce more globular microstructure [6,[18][19]. The pouring temperature which has been used in this work was just slightly above the liquidus temperature that provides fewer superheated to be extracted from the copper, lead to higher cooling rates. ...
The effect of different processing parameters consist of pouring temperature and holding time on the semisolid microstructure of Al6061 feedstock billet produced by direct thermal method has been investigated in the present paper. In this experimental works, molten aluminium alloy 6061 was poured into a thin cylindrical copper mould at a constant temperature of 660 °C and held at different holding times of 20 s, 40 s, and 60 s. After it reached the desired holding time, the copper mould was then quenched into room temperature water. The microstructure formation of feedstock billets was characterized after the feedstock billets were taken out from the mould. Results show that, due to the rapid cooling condition of the molten 6061 inside the copper mould, more globular microstructures were obtained. The sample produced with a pouring temperature of 660 °C and holding time of 20 s, globular microstructure were apparent. Simultaneously, the sample with a pouring temperature of 660 °C and a holding time of 20 s produced smaller grain size than the sample with a pouring temperature of 660 °C and a holding time of 40 s and 60 s. Based on the results, the globular microstructures were superior at a shorter holding time, which allowed them to be quenched faster into room temperature water. The results from this experimental works suggest that the DTM feedstock billet globular microstructure formation merely depended on the heat convection of the molten alloy out from the copper mould. The faster heat convection out from the molten alloy which retarded the formation of dendritic microstructure thus transformed it into a globular microstructure for semisolid metal processing.
... Nevertheless, the phenomenon aforementioned did not occur in the actual SSMF process. This was mainly because the mould cavity in the actual SSMF process was closed, which consequently avoided the outflow of the liquid phase [38,39]. ...
... Nevertheless, the phenomenon aforementioned did not occur in the actual SSMF process. This was mainly because the mould cavity in the actual SSMF process was closed, which consequently avoided the outflow of the liquid phase [38,39]. The macroscopic photo of the samples processed at the deformation temperature of 630 • C under different strain rates is shown in Figure 10b. ...
Al-Mg-Si based alloys are popular alloys used in the automotive industry. However, limited studies have been performed to investigate the microstructure, deformation characteristics, and deformation mechanism for the semi-solid 6063 alloys. In this study, the cold radial forging method and semi-solid isothermal treatment (SSIT) are proposed in the semi-solid isothermal compression (SSIC) process to fabricate high-quality semi-solid 6063 billets. The effects of deformation temperature, strain rate, and strain on the microstructure, deformation characteristics, and deformation mechanism of the SSIC of cold radial forged 6063 alloys were investigated experimentally. Constitutive equations were established based on the measured data in experiments to predict the flow stress. Results show that an average grain size in the range from 59.22 to 73.02 μm and an average shape factor in the range from 071 to 078 can be obtained in the microstructure after the cold radial forged 6063 alloys were treated with SSIT process. Four stages (i.e., sharp increase, decrease, steady state, and slow increase) were observed in the true stress- true strain curve. The correlation coefficient of the constitutive equation was obtained as 0.9796 while the average relative error was 5.01%. The deformation mechanism for SSIC of cold radial forged aluminum alloy 6063 mainly included four modes: The liquid phase flow, grain slide or grain rotation along with the liquid film, slide among solid grains, and the plastic deformation of solid grains.
... The proper combination of the pouring and holding time will greatly affect the formation of a globular microstructure within the billets. Previous research shows that smaller primary 3 and secondary phases within the microstructure were formed for lower pouring temperature which approaching the semisolid temperature [19]. ...
The microstructure variances of aluminium 6061 billet produce via direct thermal method with different pouring temperature and holding time are presented in this paper. The direct thermal method is one of the methods to create globular microstructure feedstock billet, which gives the material a thixotropic behaviour during semisolid metal processing. In this experimental work, a molten aluminium 6061 was poured into cylindrical copper mould before quenched in water at room temperature. The effect of pouring temperature of 660°C and 700°C and holding time of 20s and 60s were observed from the microstructure formation. The result shows that the combination of a pouring temperature of 660°C with a 20s holding time produces a finer near globular microstructure. These fines near globular microstructures gives a thixotropic behaviour improvement in better fluidity for a better flow during shaping. The pouring temperature just slightly above the liquidus temperature provides slower cooling rates from above to below the liquidus temperature. This process causes less superheat to be extracted by the cylindrical copper mould and gives a slow cooling rate action during the solidification stage that promotes the formation of further nuclei, which results in smaller grain size. The result also shows the combination of the lowest pouring temperature of 660°C with the lowest holding time of 20s produced the smallest grain size measured in area. However, the circularity and aspect ratio that indicates globular shape grain has a slight change in result which indicates that every feedstock billet has a near globular grain size. In conclusion, this work has shown that specific combination of pouring temperature and holding time has an effect on the microstructure formation of the feedstock billet produced by using direct thermal method.
... Different techniques have been innovated to obtain slurry with nondendritic structure whether by melt agitation which includes mechanical stirring, magnetohydrodynamic stirring, mechanical vibration and cooling slope (CS) methods or without melt agitation via spray casting, strain-induced melt activation, recrystallization and partial melting and grain refinement. [9][10][11][12] One of the most important obstacles that limit the industrial commercialization of the thixo-casting process is the running cost to produce feedstock with non-dendritic and globular microstructure. Therefore, some techniques were proposed for production of feedstock billets with globular structure over the recent years. ...
This work investigates the optimum pouring temperature during semisolid casting (rheo-casting) of Al–17%Si alloy (A390) to obtain the best combination of microstructure modification and wear resistance. Pouring semisolid slurry was done in both metallic and sand molds to observe the effect of cooling rate on the optimum pouring temperature. The molten metal/slurry was poured into the specified mold type through a cooling slope plate which was continuously water cooled. Different pouring temperatures: 670, 690, 710 and 730 °C, were applied, and their influence on the microstructure and alloy properties was analyzed. Based on the obtained results, the optimum pouring temperature for semisolid casting was decided for the two mold types. Conventional casting was conducted at the optimum temperature in both metallic and sand molds for comparison. It was observed that rheo-casting using the cooling plate is an effective process in microstructure modification of A390 alloy in terms of refining and redistributing the primary Si and fragmenting the coarse AlFeMnSi phase. The optimum temperatures to obtain small particle size of primary Si, uniform distribution and regular shape using the cooling plate technique were 690 °C and 710 °C for the metallic and sand molds, respectively. This refinement and homogenization of the microstructure enhanced the hardness and wear resistance of alloy A390.
... As mentioned in Ref. [21], the Melt was poured directly on the wall of crucible which rapidly cooled the melt and provided requisite undercooling for the nucleation. A low pouring temperature close to the liquidus produces a high degree of undercooling that reduces the critical radius of nuclei, which increases the effective number of nuclei [22][23][24][25][26]. ...
In this paper, two ways of microstructural characterization, optical microscopy (OM) and polarized light microscopy (PLM), were both employed to describe the microstructure of semisolid slurry prepared by swirling enthalpy equilibration device (SEED). The results show that PLM is more reliable and accurate than OM to describe the special morphology feature of semisolid slurry made by SEED process. Meanwhile, the effects of pouring temperature and mass of molten liquid on the primary α-Al particle size and morphology were also investigated using PLM. The quantitative metallographic results measured from PLM demonstrate that the grain size and morphology and their distribution are significantly affected by both pouring temperature and the mass of molten liquid. The grain size poured with 2.7 kg liquid decreases from 659 to 186 μm, and grain morphology transforms from dendrite to globular structure with pouring temperature reducing from 690 to 630 °C. The decreasing pouring temperature also promotes the distribution of spherical structure on the cross section. Meanwhile, the mass of molten liquid decreasing from 2.7 to 2.3 kg can decrease the grain size by maximum of 44% at high pouring temperature.
... In recent years, different methods, such as mechanical or ultrasonic vibration, magneto-hydrodynamic (MHD) stirring, stress-induced and melt-activated (SIMA), powder metallurgy process [12][13][14][15] and low temperature pouring and partial melting have been introduced to produce spherical microstructures. 16,17 Also, the cooling slope (CS) process is a simple method which can produce homogeneous globular microstructure without use of any complicated equipment. [18][19][20][21] Nourouzi et al. 22 have used a B-P algorithm to correlate a relationship between the process parameters (tilt angle of the CS, the pouring temperature and cooling length) and the grain size. ...
This work studied the semisolid (SSM) Al-A380 alloy formed by mechanical stirring under the circumstances of controlled atmosphere with argon gas. The A380 aluminium alloy produced due to this process can be used instead of the forging processes or die casting with better mechanical properties. In this paper, first, the effects of the solid fraction, the stirring speed and the mold temperature on the hardness and microstructure are studied using a mechanical stirrer. Then some tests such as XRD and ultrasonic test are done to investigate the effect of using the controlled atmosphere system. By optimizing the forming parameters and with the breaking of primary a phase dendrites, the microstructure will be changed to the spherical form, therefore the mechanical properties will be improved. The results showed that the hardness of the billets has increased significantly due to the uniform grains distribution at the condition of 580 ℃ stirring temperature, 300 r/min stirring speed and mold temperature of 100 ℃. Also, the process properly optimized with the controlled atmosphere. In this condition, the results illustrated that the mechanical properties of the alloy increased around 30%.
... In addition, Ran et al. [47] investigated the microstructural evolution of an Al-Si-Mg alloy during hot isostatic pressing (HIP), and found that some eutectic silicon phases precipitate at the sub-grain boundaries induced by HIP process. Ning et al. [48] found that theAl À Si À Mgalloys with fine globular structures have a low compression stress and uniformly deformed microstructures, while those with coarse dendrites show a very high compression stress and macro-structural breakage. ...
Hot compression experiments are performed to study the flow characteristics of a homogenized Sr-modified Al-Si-Mg alloy at the temperatures of 300–420 °C and strain rates of 0.01–10 s−1. It is found that the deformed grains and Si-containing dispersoids clusters in the matrix are elongated, and incomplete DRX is discovered on the elongated boundaries at high deformation temperatures. The non-uniform distribution of Si-containing dispersoids, soft dispersoid free zones (DFZs) and fragmentation of eutectic Si may cause the flow instability at high strain rates. The Si morphology in the homogenized state can be further modified by the subsequent thermo-mechanical deformation. Moreover, the flow stress first rises to the peak with the increase of strain because of strain hardening, and then slowly declines with the further straining owing to the occurrence of incomplete DRX and coarsening of Si-containing dispersoids. The high activation energy is closely associated with the dislocations pinning from Si-containing dispersoids. Additionally, taking the coupling impacts of temperature, strain and strain rate into consideration, an extended phenomenological model compensating softening effects is developed to depict the strain hardening behavior, as well as dynamic softening characteristic. The developed model is validated by the experimental results, and can be applied to accurately predict the flow stresses of the studied Al-Si-Mg alloy during high temperature deformation.
... al. [16], the energy needed to heat aluminium alloys for semisolid processing is 35% less than that required to heat the same aluminium alloys for casting process. Plenty of studies utilizing SSM processing were conducted on aluminium silicon alloy [1,12,[17][18][19][20][21][22]. It is due to the alloy has better flow ability, but fared poorly in mechanical properties as compared to wrought aluminium alloys. ...
Semisolid metal (SSM) processing, as a kind of new technology that exploits forming of alloys between solidus and liquidus temperatures, has attracted great attention from investigators for its thixotropic behaviour as well as having advantages in reducing porosity, macrosegregation, and forming forces during shaping process. Various techniques are employed to produce feedstock with fine globular microstructures, and direct thermal method is one of them. In this paper, the effect from different pouring temperatures and holding times using a direct thermal method on microstructure and hardness of aluminium alloy 6061 is presented. Molten aluminium alloy 6061 was poured into a cylindrical copper mould and cooled down to the semisolid temperature before being quenched in water at room temperature. The effect of different pouring temperatures of 660 °C, 680 °C, 700 °C, and holding time of 20 s, and 60 s on the microstructure of aluminium alloy 6061 were investigated. From the micrographs, it was found that the most globular structures were achieved at processing parameters of 660 °C pouring temperature and 60 s holding time. The highest density and hardness of the samples were found at the same processing parameters. It can be concluded that the most spheroidal microstructure, the highest density, and the hardness were recorded at lower pouring temperature and longer holding time.
... The decrease in pouring temperature increases the solid fraction within the melt, which reduces the chance of macrosegreegation and liquid entrapment. In addition, the decrease in pouring temperature up to a certain extent not only reduces the grain size but also improves the morphology from coarser dendritic to equiaxed one, which is in line with the earlier published literature [9]. Further decrease in pouring temperature leads to the premature solidification of the melt during flow along the slope, as observed in the trial experiments. ...
In the present work, effect of pouring temperature (650 C, 655 C, and 660 C) on semi-solid microstructure evolution of in-situ magnesium silicide (Mg 2 Si) reinforced aluminum (Al) alloy composite has been studied. The shear force exerted by the cooling slope during gravity driven flow of the melt facilitates the formation of near spherical primary Mg 2 Si and primary Al grains. Shear driven melt flow along the cooling slope and grain fragmentation have been identified as the responsible mechanisms for refinement of primary Mg 2 Si and Al grains with improved sphericity. Results show that, while flowing down the cooling slope, morphology of primary Mg 2 Si and primary Al transformed gradually from coarse dendritic to mixture of near spherical particles, rosettes, and degenerated dendrites. In terms of minimum grain size and maximum sphericity, 650 C has been identified as the ideal pouring temperature for the cooling slope semi-solid processing of present Al alloy composite. Formation of spheroidal grains with homogeneous distribution of reinforcing phase (Mg 2 Si) improves the iso-tropic property of the said composite, which is desirable in most of the engineering applications.
