Study On Microstructure And Properties Of A356 With Trace Rare Earth Element Sc/Y

Al-Si-Mg casting alloys have excellent machinability.A356 aluminum alloys are widely used in aerospace and automotive industries due to their good wear resistance and fatigue performance.However,with the challenges brought by the energy crisis and environmental pollution,the lightweight of automobiles can reduce the curb weight to improve the power of the automobile,reduce pollution and fuel consumption.For the refiner strengthening of A356 alloy,many scientific researchers have set the amount of refiner added between 0.1%-1%.Under the concept of microalloying,the addition content of（Sc+Y）was controlled not to exceed 0.05%,and the effect of refiner on the strengthening of A356 alloy was explored in this work.Nine alloys with different addition of Sc and Y were designed with 0%Sc,0.01%Sc,0.02%Sc,0.03%Sc,0.04%Sc,0.05%Sc,0.02%Sc+0.01%Y,0.02%Sc+0.02%Y,0.02%Sc+0.03%Y.The effect of Sc and Y additions on the microstructure and mechanical properties of A356 metallic tensile specimens in the present work was studied by solidification temperature analysis of aluminum alloy,optical microscopy（OM）,scanning electron microscopy（SEM）,tensile tests and differential scanning calorimetry（DSC）.Combined with the research needs,the first-principles calculation method in the Materials Studio software was used to explore the structural stability of the crystal structure under the absence of Y atoms and the orderly doping of Sc atoms.The main results are as follows:（1）The alloy containing both Sc and Y exhibited better mechanical properties and higher solidification supercooling comparing with Sc-free A356 alloy.The supercooling was enlarged by 14.2℃（Determined by solidifying temperature curves）and 3.4℃（Determined by DSC curves）,correspondingly.The yield strength（YS）,ultimate tensile strength（UTS）,elongation（El%）and vickers hardness（HV）were increased by 108.5%,92.3%,83.6%and 28.8%when 0.02%Sc and 0.02%Y were added.The trend of the curve of vickers hardness was consistent with that of the curve of SDAS^-1 with different addition of Sc and Y.In the microalloying direction（≤0.05%）,the addition of0.02%Sc+0.02%Y was the best in grain refinement,and the tensile fractures had typical ductile fracture characteristics.（2）After T6（solid solution（530℃×4h）,aging（150℃×4h））heat treatment,the A356 metallic tensile specimen added with 0.02%Sc+0.02%Y in the nine groups have better mechanical properties.The yield strength,tensile strength,and microhardness are235 MPa,293 MPa,and 161.34 HV,respectively.The shape of Si phase evolved from coarse flake or irregular structure to fine particle,and this change played the role of dispersion strengthening.Most of the Si phase existed at theα_Al grain boundary,and a small part is precipitated along the Si phase in the form of Mg₂Si phase.The DSC curves of different cooling rates show that a higher cooling rate（20℃/min）than10℃/min and 5℃/min delayed the characteristic temperature of the phase transition reaction Liquid→α_Al.（3）The constructed crystal structure model with point defects was geometrically optimized to explore the basic properties of different crystal structures.The more Sc atoms were doped,the C_ij value of the structure gradually decreased.There were hybrid covalent effects of Al-p orbital electrons,Y-d orbital electrons and Sc-d orbital electrons in the PDOS of different crystal structure models.The density of states values at the bond peak positions of-27.74 e V and-49.05 e V were related to the number of Sc atoms,and which increased exponentially with the increase of the number of Sc atoms.