Numerical Simulation And Experimental Research On Selective Laser Melting Of 7075 Aluminum Alloy

Selective laser melting（SLM）has attracted the attention and research of a large number of researchers because it can rapidly manufact metal parts with complex structure.However,the defects of 7075 aluminum alloy,such as holes and cracks,prone to generate during the SLM preparation process,which limits its application.In this paper,in order to provide theoretical support for optimizing the process interval and reducing sample cracking,thermal mechanical coupling field of 7075 aluminum alloy during the SLM preparation process are numerically simulated by using the finite element software ANSYS APDL 19.0,the variation on thermal behavior and stress distribution are analyzed,the effect of different process parameters on them are studied.The experimental verification is carried out.The types of defects of the samples are summarized,and the effect of different process parameters on the forming quality and microstructure of the samples are studied.The main research contents and results are as follows:（1）The temperature fields of one layer and one track,one layer and five tracks and two layers and two tracks are investigated.When the laser beam moves from point 1 to point 3,the cooling rate gradually increases.When the laser beam moves from the first track to the fifth track,the peak temperature at the center of weld pool first decreases and then increases,the cooling rate first increases and then decreases.When the laser beam moves from the first layer to the second layer,the cooling rate increases.The effect of different process parameters on the thermal behavior distribution is obtained.With the increase of laser power or the decrease of scan speed,the peak temperature at the center of weld pool gradually increases,and the existence time of liquid phase gradually increases.With the increase of laser power or scan speed,the cooling rate gradually increases.The weld pool size of actual sample is smaller than that of simulation.（2）The variation of stress field of one layer and three tracks in the process of preparation is studied.After laser beam has been scanned,the maximum surface stress is located in the formed solid area behind the weld pool.The effects of different process parameters on the residual stress distribution are obtained.With the increase of laser power or scan speed,the residual stress gradually increases.The maximum surface residual stress is located in the overlap zone of weld pool,and the stressσ_x along the scanning direction is greater than the stressσ_y along its normal direction.The residual stress of partition scan mode composed of short scan vector is less than that of non partition scan mode composed of long scan vector.（3）The types of defects in the samples are summarized,including low surface quality,holes and cracks.Holes include unfused metallurgical defect,keyhole and pore,cracks include solidification crack and liquation crack.The sample mainly containα-Al matrix,η’/η-Mg Zn₂phase and S-Al₂Cu Mg phase.The microstructure is columnar crystal which runs through multiple weld pools.The columnar crystal is composed of cellular structures,and there are dispersed fine particles between the cellular structures.（4）The effects of different process parameters on the manufacturing quality and microstructure of the samples are obtained.Compared with plane and strip,the sample with chessboard scan strategy has lower porosity and fewer cracks,but the size of its cellular structure is larger.With the increase of laser power,the number of unfused metallurgical defects gradually decrease and eventually disappear,the number of cracks gradually increases,the density of holes in the boundary of weld pool gradually decreases,the boundary characteristics of weld pool gradually become inconspicuous,the size of cellular structure in columnar crystal gradually increases,and the size of partial porosity defects between cellular structures also gradually increases.With the increase of scan speed,the number of keyholes decreases,the number of cracks gradually increases,the density of holes in the boundary of weld pool gradually increases,the boundary characteristics of weld pool gradually become obvious,the size of cellular structure in columnar crystal gradually decreases,and the size of partial porosity defects between cellular structures also gradually decreases.