Selective Laser Melting Of ZrH₂/AA6060 Composite Powders: Process,microstructures And Mechanical Properties

6XXX series aluminum alloy belongs to Al-Mg-Si alloy and is a heat treatable reinforced aluminum alloy.It has excellent weldability and corrosion resistance,high specific strength,beautiful appearance after surface coloring,and no stress corrosion cracking tendency.However,due to the relative broad freezing range of 6XXX series aluminum alloy,defects such as hot cracks and holes are prone to form during selective laser melting process.Up to now few studies have been reported on 6XXX series aluminum alloy and low mechanical properties are obtained for SLMed parts.The addition of Zr element can significantly improve the formability of aluminum alloys during SLM process,refine the grains and inhibit hot cracks.Therefore,ZrH₂/AA6060 composite powders were prepared by low-energy ball milling.Processing parameters,microstructures and mechanical properties of ZrH₂/AA6060 composite powders formed by selective laser melting were studied,which aims to provide technological guidance for the production of 6XXX series aluminum alloy parts with high densities and good properties by SLM.ZrH₂/AA6060 composite powders with submicron ZrH₂ uniformly dispersed on aluminum alloy matrix surface were prepared by low-energy ball milling at 125r/min-3h.The sphericity and oxygen content of powders have a little change after ball milling.During the SLM process,the addition of different contents of ZrH₂ can increase the absorption rate of the laser energy by the matrix powders and significantly increase the density of samples.With increasing ZrH₂content,the cracks in samples reduce,and the sensitivity of relative density to processing parameters weakens.When 1wt.%ZrH₂ was added,hot cracks in 6060 aluminum alloy formed by SLM disappear.The effects of SLM processing parameters on density,surface morphology and roughness of 1wt.%ZrH₂/AA6060 composite powders were investigated.Results show that with increasing laser power,pores in samples reduce.The density increases,and the surface roughness reduces.When the laser power is 300W,the density of samples reaches more than96%.Meanwhile,with the increase of scanning speed,the density of samples increases first and then decreases,and the surface roughness increases gradually.When the scanning speed is700mm/s,the comprehensive performance of alloy samples is the best,and the density is about98%,the surface roughness is only 22.72μm.Preheating temperature has a great influence on the density and surface morphology of samples.Increasing the preheating temperature can reduce the pores in samples and inhibit the surface spheroidization,and enhance the density of samples.Microstructures,mechanical properties and fracture mechanism of SLMed AA6060 and AA6060+Zr alloy were compared and analyzed.Compared with AA6060 by SLM without adding ZrH₂,after adding 1wt.%ZrH₂,ZrH₂ reacts with aluminum matrix to form Al₃Zr during SLM,which significantly refines the grain microstructure.The grain microstructure of the XOZ surface of samples changes from coarse columnar to columnar with small length and width and fine equiaxed grains.The fine grains make the<001>fiber texture in samples significantly reduce,and show no obvious preferred orientation.The grain microstructure of the XOY surface of samples transforms from coarse grains to fine equiaxed grains with different sizes.Meanwhile,after adding 1wt.%ZrH₂,the grain refinement effect significantly inhibits the occurrence of hot cracks on XOY and XOZ surfaces of samples.In addition,the mechanical properties of samples are significantly improved by adding 1wt.%ZrH₂.The fracture mechanism of samples changs from mixed fracture to ductile fracture.The heat treatment process of AA6060+Zr aluminum alloy prepared by SLM was studied.Results show that the optimum heat treatment parameters are solution at 530℃for 1 h and aging at 175℃for 8 h.After heat treatment,the microstructures of samples on XOZ surfaces still consist of columnar grains with small aspect ratio and fine equiaxed grains with no obvious change.The yield platform of samples disappears,and its tensile strength and hardness increase,but the yield strength and ductility decrease.