Microstructure And Mechanical Properties Of Al-Mg-Sc-Zr Alloy Processed By Selective Laser Melting

Selective Laser Melting（SLM）is suitable for prototyping components rapidly with more design complexity,excellent quality and precise aluminum alloy parts.Traditional aluminum alloy formed by SLM is prone to cracking and the parts have poor performance accompanied by metallurgical defects.In this paper,the Al-Mg alloy modified by Sc/Zr microalloying elements was taken as the research subject.To understand the correlation relationship between the metallurgical defects,molten pool morphology,precipitation behavior of precipitates,microstructure evolution and mechanical properties of SLM forming medium Mg content,the Sc/Zr ratio with 2:1 of Al-4.67Mg-0.64Sc-0.3Zr-0.48Mn alloy system and laser energy density.Based on the effective regulation of microstructure and mechanical properties of SLM-fabricated Al-Mg-Sc-Zr alloy system by aging treatment,the role of aging parameters on microstructure and mechanical properties were studied.The main conclusions are summarized as following（1）The effects of laser emission mode and energy density on the metallurgical defects,texture analysis,molten pool morphology and mechanical properties were revealed.The optimized processing parameters were obtained.During the increasing of laser energy density,the unfused pores were transformed into metallurgical pores,along with the molten pool deepened and formed keyholes.However,the mixed crystal structure remained unchanged.A small amount of Al₃（Sc,Zr）primary phase is distributed at the boundary of the molten pool,while the rest of Sc/Zr atoms are solitarily dissolved in the matrix to form supersaturated solid solution.（2）The forming mechanism of mixed crystal structure and its effect on mechanical properties of SLM-fabricated samples were discussed.When VED is 52.08 J·mm^-3,the ultimate tensile strength（UTS）,0.2%yield strength（0.2%YS）and elongation（ε）of the specimens reach at 429.7MPa,363.5 MPa and 17.7%,and the microhardness is 115.4 HV.The strength and plasticity of the alloy are in harmony,and the micropore aggregation at the boundary between molten pool and scanning channel is the main deformation failure mode.（3）The influence of aging treatment on precipitation behavior of SLM samples and the strengthening mechanism of modified elements were explored.The aging hardening effect of the alloy system was significant,and the microhardness can reach up to 168.1 HV.The nano-sized L1₂-Al₃（Sc,Zr）particles were precipitated after aging,and the uniformity of microstructure refinement is improved.The 350°C/4 h is the best aging parameter,as the ultimate tensile strength,0.2%yield strength and elongation of samples reach at 570.9 MPa 552.7 MPa and 12.0%,respectively.This thesis consists of 47 figures,4 tables,84 references.