Research On Forming And Property Control Of High Zn-content Aluminum Alloy By Selective Laser Melting

"Lightweight"is an important development direction for future aerospace,rail transportation and other fields,and the characteristics of abundant reserves,low density and high specific strength of aluminum alloy perfectly fit the demand of"light weight"and have a broad application prospect.The traditional processing method is difficult to prepare complex and micro structure parts,and the manufacturing cycle is long.As an advanced additive manufacturing method,selective laser melting（SLM）has the characteristics of free forming and high precision forming,which provides a feasible scheme for one-time net forming of complex precision components.However,related studies show that the properties of aluminum alloys prepared by SLM are poor,which usually can not reach the strength level of forged aluminum alloys,and can not adapt to the complex service environment in the background of aerospace.In the research of casting,spray deposition and other fields,ultra-high strength aluminum alloy was prepared by increasing Zn content.Based on this,a kind of high Zn aluminum alloy powder was designed,which is suitable for SLM one-time forming of high strength aluminum alloy.At the same time,in order to suppress the hot cracking tendency caused by high Zn content,a certain amount of Zr was added simultaneously to prepare aluminum alloy powder containing 14%Zn and 2%Zr,The forming quality,microstructure,properties and strengthening mechanism of the samples were studied.Firstly,the influence of process parameters on the surface quality and cross-section morphology of single pass samples is studied.With the change of laser power and scanning speed,the penetration and width of the weld increase asynchronously with the increase of energy input,resulting in three kinds of weld characteristics:heat conduction mode,transition mode and keyhole mode.Combined with the simplified weld overlap model,it is judged that the samples with good quality and properties can be obtained under appropriate negative lap.The density of the bulk sample increases rapidly with the increase of laser energy density in the range of 0～100 J/mm³,and reaches 95%at about 100 J/mm³;At 180 W-400 mm/s,the energy density is 136.4J/mm³,the highest density is 96.9%.The main defects at low energy density are incomplete fusion pores caused by insufficient heat input,while the main defects at high energy density are vapor pores formed by Zn burning.Secondly,the microstructure and mechanical properties of SLM samples were analyzed.With the increase of energy density,the coarse columnar grains decrease and the proportion of fine equiaxed grains increases.At the same time,the contact of the fine grains makes the boundary of the macro molten pool blurred gradually.The equiaxed grains are more evenly distributed,and the columnar grains are mainly<001>and<101>oriented.The burning loss rate of Zn and Mg increases with the increase of laser power,and the burning loss rate of Zn is higher than that of Mg.Al₃Zr phase is precipitated in the grain and mainly distributed on the grain boundaryη（Mg Zn₂）phase.The results show that the mechanical properties of the sample formed at 180 W-400mm/s are the best,the average microhardness of the bottom surface is 170.56 HV,the maximum tensile strength is 573 MPa,and the maximum elongation is 11.6%.Finally,the effect of aging treatment on the microstructure and properties of the samples was studied.During the aging treatment at 120℃,there is no significant change in the grain size,and a large number of flakes precipitate in the grainsη’Phase.The average microhardness of the upper surface of the sample reaches 181.7 HV after aging for 21 h,the tensile strength first decreases and then increases,and the maximum total elongation decreases continuously.After aging for 21 h and 24 h,the tensile strength of the sample is 597 MPa and 605 MPa,and the total elongation is 6.7%and5.3%respectively.The main strengthening mechanisms of SLM formed samples are solution strengthening and fine grain strengthening.After aging treatment,the solution atoms decrease,so the fine grain strengthening and second phase strengthening become the main strengthening factors.