Effect of cooling rate on solidification parameters and microstructure of A1-7Si-0.3Mg-0.15Fe alloy

The effects of cooling rate on the solidification parameters and microstructure of Al-7Si-0.3Mg-0.15 Fe alloy during solidification process were studied.To obtain different cooling rates,the step casting with five different thicknesses was used and the cooling rates and solidification parameters were determined by computer-aided thermal analysis method.The results show that at higher cooling rates,the primary α（Al） dendrite nucleation temperature,eutectic reaction temperature and solidus temperature shift to lower temperatures.Besides,with increasing cooling rate from 0.19 ℃/s up to 6.25 ℃/s,the secondary dendritic arm spacing decreases from 68 μm to 20 μm,and the primary dendritic volume fraction declines by approximately 5%.In addition,it reduces the length of Fe-bearing phase from 28 μm to 18 μm with a better uniform distribution.It is also found that high cooling rates make for modifying eutectic silicon into fibrous branched morphology,and decreasing block or lamella shape eutectic silicon.

Grain Size Distribution and Interfacial Heat Transfer Coefficient during Solidification of Magnesium Alloys Using High Pressure Die Casting Process

The objective of this study is to predict grain size and heat transfer coefficient at the metal-die interface during high pressure die casting process and solidification of the magnesium alloy AM60. Multiple runs of the commercial casting simulation package, ProCASTTM, were used to model the mold filling and solidification events employing a range of interfacial heat transfer coefficient values. The simulation results were used to estimate the centerline cooling curve at various locations through the casting. The centerline cooling curves, together with the die temperature and the thermodynamic properties of the alloy, were then used as inputs to compute the solution to the Stefan problem of a moving phase boundary, thereby providing the through-thickness cooling curves at each chosen location of the casting, Finally, the local cooling rate was used to calculate the resulting grain size via previously established relationships. The effects of die temperature, filling time and heat transfer coefficient on the grain structure in skin region and core region were quantitatively characterized. It was observed that the grain size of skin region strongly depends on above three factors whereas the grain size of core region shows dependence on the interracial heat transfer coefficient and thickness of the samples. The grain size distribution from surface to center was estimated from the relationship between grain size and the predicted cooling rate. The prediction of grain size matches well with experimental results. A comparison of the predicted and experimentally determined grain size profiles enables the determination of the apparent interracial heat transfer coefficient for different locations.

Crack-free and high-strength AA2024 alloy obtained by additive manufacturing with controlled columnar-equiaxed-transition

Laser powder bed fusion(LPBF)of high-strength Al alloys is challenging due to the formation of both hot and cold cracks.In the present work,highly dense and crack-free AA2024 samples could be additively manufactured via inoculation treatment of Zr-based metallic glass(MG)powders.The columnar grains in the LPBF-fabricated AA2024 alloy were significantly refined and almost completely transformed to the equiaxed grains with a bimodal grain size distribution consisting of ultrafine grains with a size smaller than 1μm and relatively coarser grains.The grain refinement can be associated with the formation of Al3Zr particles,serving as the heterogeneous nucleation sites for theα-Al matrix.Complete routes for columnar-equiaxed-transition(CET)have been revealed by tailoring the concentration of nucleation particles and solidification conditions.CET occurs both at the melt pool boundary due to the sufficiently high concentration of Al3Zr particles and at the topmost of the melt pool due to the heterogeneous nucleation driven by constitutional undercooling.Between these two regions,columnar grains epitaxially grow with orientations determined by the thermal gradient.The as-built Zr-based MG inoculated AA2024 specimens are robust in healing hot cracks due to a more tortuous propagation path of cracks for equiaxed grains.The as-fabricated AA2024/5%MG specimens exhibit a high ultimate tensile strength of 531 MPa due to crack elimination and grain refinement,surpassing most of the reported values for the LPBF-fabricated AA2024 alloy inoculated with other inoculated powders.The present work could provide a novel inoculation agent to fabricate high-strength Al alloys and the CET can be used to precisely control the grain morphology.