Study On Heat Treatment Strengthening And Microstructure Properties Of 7075 Aluminum Alloy Prepared By Laser Additive Manufacturing

Compared with traditional casting,forging and welding technology,laser additive manufacturing（LAM）technology has the advantages of short production cycle,high material utilization rate,and can shape complex structural parts with higher precision and lower surface roughness.However,there are also some problems in the actual production application,including:limited by the small space of equipment preparation,usually used for the manufacturing of small parts;multi-cycle effects based on the multilayer and multi-channel laser manufacturing process lead to the formation of uneven structure of molded parts,segregation,cracks and pores and other defects.7075 aluminum alloy has the advantages of low density,high specific strength,corrosion resistance,good electrical and thermal conductivity,cutting easily and so on.It is widely used in aerospace,transportation,automobile and electrical equipment manufacturing and other fields.However,the traditional method of preparing 7075 aluminum alloy has some problems,such as complex preparation process,low material utilization rate,low surface accuracy of forming parts and poor mechanical properties,which affect the popularization and application of this type of material.Laser additive manufacturing（LAM）technology can effectively overcome the shortcomings of traditional manufacturing process of aluminum alloy,and can obtain complex parts with less defects and high precision in a short time and cost.In this thesis,7075 aluminum alloy prepared by laser additive technology was selected as the research object.Through the design of different heat treatment processes,the microstructure and properties of the upper surface（X-Y surface）of the alloy material before and after heat treatment were analyzed by metallographic analysis,scanning electron microscopy（SEM）,energy dispersive spectroscopy（EDS）,X-ray diffraction（XRD）and transmission electron microscopy（TEM）analysis,as well as microhardness and electrochemical corrosion performance analysis.Based on the above research,the influence of different heat treatment processes on the microstructure,precipitate phase,microhardness and electrochemical corrosion property transformation of 7075 aluminum alloy prepared by laser additive was explored.The main research results were as follows:The original X-Z surface structure of 7075 aluminum alloy prepared by LAM has obvious inter-channel structure,and the X-Y surface structure is mainly composed ofα-Al solid solution and coarseθ（Al₂Cu）,S（Al₂Cu Mg）and T（Al₆Cu Mg₄）phases.At the same time,a large number of secondary phases containing Al,Mg and Cu are distributed in the grain boundary as dendrites.In addition,there are a large number of Mg₂Si particles and pores in the matrix.After heat treatment,the microstructure was significantly improved:1)After solid solution at 470℃for 1 h,a large number of coarse second phases dissolve into the matrix,and the dendrite characteristics basically disappear,but there are still some undissolved non-uniform second phases at the grain boundary.EDS analysis shows that at this time,the content of Cu in the matrix increases,and a small amount of Cu dissolves into the matrixα-Al after GP zone,and the structure cannot be transformed into theθphase through equilibrium because it is not aging treated.At the same time,a rod-shaped Al₃Ti phase precipitates in the grains;2)After 470℃×1h solid solution+120℃×24h single-stage aging treatment,the Al₃Ti phase dissolves,and the long strip second phase was precipitated at the grain boundary,andθ（Al₂Cu）phase was evenly and fine precipitated in the grain.At this time,the Cu content decreased,and the enriched Cu element in the matrix was precipitated out to form the strengthening phase after aging;3)After 470℃×1h solid solution+110℃×5h+150℃×14h two-stage aging treatment,the coarse second phase has completely dissolved into the matrix.At this time,the chain precipitated second phase structure at the grain boundary will block the dislocation movement,while the more uniform and fine dispersed second phases,θ（Al₂Cu）and S（Al₂Cu Mg）phases are precipitated inside the grain;4)After solid solution at 470℃×1h+natural aging for 180 days,the alloy matrix will also precipitate strengthening phase through the GP zone.However,due to insufficient phase transformation driving force in GP region at room temperature,it is difficult to precipitate fine second phase,so the morphology of precipitatedθ（Al₂Cu）phase is relatively coarse and uneven.The performance analysis of the alloy shows that the average hardness of the original7075 aluminum alloy prepared by LAM is the lowest on the X-Y surface,the average microhardness value is 70.7 HV_0.2,and the corrosion resistance is the worst,and the corrosion current density is 1.355×10^-5A/cm².After the heat treatment,the hardness and corrosion resistance of the alloy are significantly improved,and the microhardness and corrosion resistance of the alloy are in the best state by the solid solution+two stage aging treatment process.The microhardness of the alloy is between 146-160 HV_0.2 on the whole,which is 2times of the original state,and the corrosion current density is 5.623×10^-6A/cm².At this time,the fine dispersion phasesθ（Al₂Cu）and S（Al₂Cu Mg）precipitated by two-stage aging play a precipitation strengthening role,which effectively improves the properties of 7075 aluminum alloy.At the same time,the fine dispersion phasesθ（Al₂Cu）and S（Al₂Cu Mg）precipitated by two-stage aging play a precipitation strengthening role,which effectively improves the properties of 7075 aluminum alloy.Based on the above analysis,the microstructure and properties of 7075 aluminum alloy prepared by laser additive can be effectively improved by solution+two-stage aging heat treatment.