Microstructure And Mechanical Properties Of Zr/Ti Modified Al-Cu-Mg Alloy Processed By Selective Laser Melting

Al-Cu-Mg alloys parts prepared by Selective Laser Melting（SLM）technology can meet the requirements of light-weight and complex structural parts in aerospace field.However,Al-Cu-Mg alloys exhibit high cracking tendency at a high cooling rate condition because of their wide freezing ranges.Grain refinement is one of the principal methods to eliminate hot-tearing in SLMed Al-Cu-Mg alloys.In this work,ZrH₂,TiH₂and Al₃Zr were used to modify Al-Cu-Mg alloy powders via low-energy ball milling.The ZrH₂ content and SLM processing parameters were optimized to improve the mechanical properties of as-built Al-Cu-Mg alloys.The grain refinement mechanism and strengthening mechanism in in-situ-Al₃Zr/Al-Cu-Mg alloys were disscused.Microstructures and mechanical properties of as-built Al-Cu-Mg alloys with in-situ formed Al₃Ti and Al₃Zr were studied.Effects of Al₃Zr adding methods on the microstructures and mechanical properties of as-built samples were compared.Heat treatment and hot isostatic pressing（HIP）technology were used to improve the mechanical properties of SLMed in-situ-Al₃Zr/Al-Cu-Mg alloys.The aim in this work is to provide technological references for the research and development of SLMed Al-Cu-Mg alloys.The ZrH₂ content in Al-Cu-Mg composite powders is 1.0wt.%,1.5wt.%and 2.0wt.%.The microstructures and mechanical properties of as-built in-situ-Al₃Zr/Al-Cu-Mg alloys under different processing parameters were analyzed.The best amount of ZrH₂in composite powders is 1.5wt.%（0.45at.%）,and the addition of ZrH₂ expands the processing window of SLMed Al-Cu-Mg alloys.When the laser energy density is 370 J/mm³,the forming quality of in-situ-Al₃Zr/Al-Cu-Mg alloys reach to the highest,and the relative density is 97.2%.The average grain size of 1.28mm,the ultimate strength of 369±9 MPa and the elongation of 12.4±0.6%are achieved for in-situ-Al₃Zr/Al-Cu-Mg alloys.The grain refinement of samples results from the metastable phase of Al₃Zr（L1₂-Al₃Zr）produced by the reaction between ZrH₂ and Al melt.L1₂-Al₃Zr can acts as the nucleus of heterogenous nucleation and can further refine the grains by hindering grain growth.Grain refinement strength dominates the improvement of mechanical properties of in-situ-Al₃Zr/Al-Cu-Mg alloy without hot-tearing.L12-Al₃Ti and L12-Al₃Zr nucleating agents were in-situ formed by TiH₂ and ZrH₂,respectively.Al₃Zr particles were synthesized to add into Al-Cu-Mg alloys directly.As-built Al-Cu-Mg alloys with different nucleating agents were prepared by SLM.When the laser energy density is 370 J/mm³,the average grain size,tensile strength and elongation of the samples with in-situ formed Al₃Ti are 4.85mm,257±3 MPa,and 3.8±0.1%,respectively,and those of the samples with synthesized Al₃Zr are 1.88μm,358±2 MPa and 8.2±0.4%,respectively.The grain refinement of ZrH₂ is stronger than that of TiH₂.The average grain size of samples with in-situ formed Al₃Ti is 2.8 times larger than that of samples with in-situ formed Al₃Zr,because L1₂-Al₃Zr is easier to generate compared with L1₂-Al₃Ti.In-situ formed Al₃Zr is more conducive to refining grains and improving the plasticity and processing efficiency of SLMed Al-Cu-Mg alloys.The elongation of the sample with synthesized Al₃Zr is 33.8%lower than that of the sample with in-situ formed Al₃Zr.The nucleation particles produced from in-situ reaction are all L1₂-Al₃Zr with a better nucleation ability.High laser energy density is unnecessary for in-situ forming Al₃Zr to achieve the dispersion of grain refiner.In-situ-Al₃Zr/Al-Cu-Mg alloy samples after T6 heat treatment and HIP treatment were investigated,and heat treatment processing parameters were optimized.The relative density of HIPed samples can reach 99.2%.After solution 1 h at 540°C and aging 10 h at150°C,the elongation of HIPed-T6 samples exceeds 15%.The tensile strength and elongation of HIPed-T6 samples can reach 491±13 MPa and 18.7±2.8%respectively at the laser energy density of370 J/mm³.Microstructures of T6,HIPed and HIPed-T6 samples are similar.Some L1₂-Al₃Zr dissolves in aluminum matrix or undergoes equilibrium phase transition,and a needle/rod-shaped equilibrium phase of Al₃Zr(D0₂₃-Al₃Zr)precipitates at the grain boundary.However,coarse S（Al₂Cu Mg）phase appears in HIPed samples,which can be hardly eliminated completely after heat treatment.