Microstructures And Mechanical Properties Of Zirconium-Containing Al-Zn-Mg-Cu Alloy Prepared By Selective Laser Melting

Selective laser melting(SLM)forming Al-Zn-Mg-Cu high-strength aluminum alloy is greatly restricted due to its hot tearing sensitivity.Composition optimization and microstructure controlling are main methods to avoid hot tearing in SLM process.In this study,ZrH₂ particles as the supply of Zrelement were firstly mixed up with Al-Zn-Mg-Cu alloy powders via a low-energy ball milling process,Then crack-free zirconium-containing Al-Zn-Mg-Cu alloy were prepared from the composite powders by SLM.In addition,a small amount of Si was added to reduce the porosity of the material,and crack-free zirconium-silicon-containing Al-Zn-Mg-Cu alloy were prepared.On this basis,the SLM process parameters were optimized and the mechanical properties of the material were improved by T6 heat treatment and hot isostatic pressing,which aiming to provide a technological reference for fabricating crack-free and high-performance Al-Zn-Mg-Cu high-strength aluminum alloy by SLM.The effects of ZrH₂ content on hot tearing,microstructures and mechanical properties of Al-Zn-Mg-Cu alloy formed by SLM were studied.During SLM process,L1₂-Al₃Zrphase can form in situ due to ZrH₂ addition,which play a key role in grain refinement,columnar to equiaxed crystal transformation and therefore crack elimination.With the increasing ZrH₂content,columnar grains as well as hot tearing gradually decrease,and hardness,strength and elongation gradually increase.When 1.5 wt%ZrH₂is added,the coarse columnar grains are completely substituted by fine equiaxed grains with an average grain size of 1.6?m,and the hot tearing also disappear.The effects of ZrH₂/Si dual phase addition on the hot cracking,porosity,microstructures and mechanical properties of Al-Zn-Mg-Cu alloy formed by SLM were explored.On the basis of adding 1 wt%ZrH₂,adding 1 wt%Si can eliminate the hot tearing in the Al-Zn-Mg-Cu alloy formed by SLM.With the addition of Si,hot tearing is eliminated,and density,hardness,tensile strength and elongation are all improved.The combined addtion of ZrH₂ and Si helps to eliminate hot tearing.The addition of ZrH₂ can refine the grains,inhibit the formation of hot tearing.The addition of Si induces the formation of eutectics with a low melting point which improves the fluidity and provides a large amount of liquid metal into the intergranular area in the later stage of solidification,backfill hot tearing.At the same time,the fluidity of the molten metal is improved to help fill the pores and further enhance the relative density of the material(>98%).The heat treatment process of zirconium-containing and zirconium-silicon-containing Al-Zn-Mg-Cu alloy were explored,and hot isostatic pressing was performed.The optimized heat treatment conditions for _Zirconium-containing and zirconium-silicon-containing Al-Zn-Mg-Cu alloy are(490?/1 h+120?/24 h)and(480?/1 h+120?/22 h),respectively.The mechanical properties of materials can be greatly improved through T6 heat treatment.The tensile strength,yield strength and elongation of zirconium-containing Al-Zn-Mg-Cu alloy after heat treatment increase to 556 MPa,495 MPa and 12.3%,respectively.The tensile strength,yield strength and elongation of zirconium-silicon-containing Al-Zn-Mg-Cu alloy increase to520 MPa and 460 MPa and 10.4%,respectively.After addition of Zrelement,the Al₃Zrphase in as-built sample remains stable during the aging process,and nano-Al₃Zrphase precipitates.The existence of these Al₃Zrparticles contribute to improving the mechanical properties.After hot isostatic pressing,pores in samples are almost completely eliminated.The tensile strength,yield strength and elongation of zirconium-containing Al-Zn-Mg-Cu alloy increase to 545 MPa,442 MPa and 14.5%,respectively.The tensile strength,yield strength and elongation of zirconium-silicon-containing Al-Zn-Mg-Cu alloy increase to 500 MPa,390 MPa and 12.2%,respectively.