The Microstructure,Defects,and Mechanical Properties Of The High-Pressure Die-Casting AE44 And AM60 Alloys

High-pressure die-casting(HPDC)magnesium alloys are suitable for use in weight reduction strategies for automotive application,which has resulted in large research interest in the world.Due to the complexity of this process,inhomogeneous microstructure and defects,such as porosity,cracks,and oxide inclusions,are observed in the HPDC Mg castings.It is well established that inhomogeneous microstructure and defects have significant impacts on the mechanical properties,especially the ductility.Despite of good flexibility and high productivity,the widespread usage of die-castings components is hindered by increased variability in ductility.Therefore,it is significant to investigate the relationship between the inhomogeneous microstructure,defects,and mechanical properties for Mg castings systematically.A solid understanding of their effects is essential for processing parameter optimization and mechanical properties improvement.In this study,the aim is to reveal the inhomogeneous microstructure,defects,and their impacts on the deformation behavior and ductility in HPDC AE44 and AM60 alloys.The through-thickness microstructure was examined and the strain field of a region of interest(ROI)was obtained through DIC measurement.The influences of microstructure feature,specifically defects and ultra-coarse externally solidified crystals(ESCs),on strain localization and ductility were analyzed.According to quantitative fractographic analysis,the main factors of ductility variation of AE44 and AM60 alloys were discussed.Besides,the influences of various pore characteristics on the strain to fracture were also investigated based on the X-ray tomography results of AM60 alloy.(1)The through-thickness grain-size variation showed a bimodal distribution in the studied alloys.The skins were mainly composed of fine equiaxed grains,while the core exhibited coarse ESCs,which surrounded by fine grains.Moreover,it should be highlighted that some ultra-coarse ESCs,which had diameters up to?600?m,appeared in the core of the AE44 alloy.The Al₁₁RE₃ phase was the predominant intermetallic phase,while Al_xRESi_2-x phase was the minor one in the AE44 alloy.And the?-Mg₁₇Al₁₂ was formed during cooling of the casting in AM60 alloy.Furthermore,a heterogeneous through-thickness distribution of intermetallic phases was also observed in both alloys;(2)In the studied alloys,both the elongations and ultimate tensile strengths showed a considerable scatter,whereas the yield strengths did not display a significant variation.However,the apparent gradient in microhardness mapping was also found in the thickness direction,which can be ascribed to the inhomogeneous distribution of the predominant intermetallic phase,i.e.,the Al₁₁RE₃(AE44 alloy)and?phase(AM60 alloy);(3)The through-thickness heterogeneous horizontal(?_xx)strain distribution can be observed at a nominal strain of 3%,i.e.,apparent strain localization appeared in the core.According to the high-resolution DIC and EBSD results,the strain localization in AE44alloy can be mainly caused by ultra-coarse ESCs with soft orientation and defects,specifically porosity.However,the porosity played a primary role in strain localization during deformation for AM60 alloy;(4)Not only defects but also the ultra-coarse ESCs had pronouncedly detrimental influences on the ductility in the AE44 alloy.Based on the CT data of fracture surface of the AM60 alloy,the strain to fracture strongly correlates with the area fraction of porosity in the fracture segment.Even though the location and volume of the large porosity could influence the fracture location,local porosity was still the dominant factor governing the fracture location and can be introduced as an effective criterion to estimate the fracture location;(5)The ductility of AM60 alloy was also a function of the highest local porosity in the gauge,i.e.,Y=14-4.39*X_HLPG.Therefore,the ductility of AM60 alloy can be predicted based on the highest local porosity in the gauge,which can be achieved before the tensile test.