Composition Design,Process Preparation And Properties Of Al-Si-Mg Alloy For SLM

Al-Si-Mg alloy is a mature lightweight material with high strength,low density and excellent heat conductivity.Selective Laser Melting?SLM?technology can realize one-time near-net forming of complex metal parts.The formed parts have excellent mechanical properties,high dimensional accuracy and good surface quality.It has broad prospects in aerospace,medical devices and other fields.During SLM molding,laser interacts with alloy powder violently,and the powder melts and solidifies instantaneously.The melt cooling rate reaches 10³-10⁶ K/s,which can significantly increase the solid solubility of alloying elements and increase mechanical properties.The research of SLM forming Al-Si-Mg alloy started late.At present,the SLM forming Al-Si-Mg alloy is mainly based on the traditional casting alloy composition.The content of alloying elements is limited,which can not greatly improve the alloy properties and expand the application fields.Based on this,the new composition of Al_90.18Si_8.31Mg_1.5 alloy with high Mg content was designed on the basis of"cluster-plus-glue-atom"model in the traditional A357(Al_91.72-93.06Si_6.25-7.24Mg_0.5-0.78)alloy.To explore the influence of SLM forming process and heat treatment conditions on the structure,microstructure and properties of Al_90.18Si_8.31Mg_1.5 alloy.The details are as follows:In the rapid solidification process of alloy melt,the local short-range structure between melt and solid is hereditary and compatible.This local structure can be described by the"cluster-plus-glue-atom"model.Based on the crystal structure of Al-Si alloy solid solution?FCC?and Mg₂Si,the stable clusters between Si-Al and Si-Mg atoms can be obtained with the solute atom Si as the center:[Si-Al12]and[Si-Mg12Si4].The composition of Al-Si-Mg aluminium alloy can be designed by adding Al atom and adjusting the proportion of the two clusters appropriately.The composition of Al-Si-Mg aluminium alloy was{[Si-Al12]+0.02[Si-Mg₁₂Si₄]}Al₁=Al_90.18Si_8.31Mg_1.5?mass percentage?.The new alloy powders were prepared by gas atomization.After analysis,about 93%of the powders were distributed in the range of 20-60?m,with an average particle size of 35.2?m.The powders were mainly spherical and evenly distributed.Some of them had slight agglomeration and contained a small amount of hollow powders.Al_90.18Si_8.31Mg_1.5 alloy samples were prepared by SLM.The effects of laser power and scanning rate on the forming properties of the samples were studied.The results show that when the laser power is 200 W and the scanning speed is 1200 mm/s,the overall forming performance of the sample is better.At this time,the sample has high density,high hardness and fewer defects such as voids and spheroidization.The main phases in the microstructures of Al_90.18Si_8.31Mg_1.5 alloy prepared by SLM are network eutectic Si structure and island?-Al structure.According to the size and shape of Si phase and?-Al matrix structure,the molten pool can be divided into fine grained zone,coarse grained zone and heat affected zone from inside to outside.The tensile properties of the samples are better than those of the conventional cast alloys.At the same time,the mechanical properties of the samples prepared by SLM are anisotropic,and the transverse yield strength and elongation of the deposited samples are slightly higher than those of the longitudinal ones.The tensile strength and yield strength of longitudinal tensile specimens are 514-524 MPa and 326-357MPa,respectively,and the elongation is 6.2-6.5%.The effect of heat treatment process on the structure and mechanical properties of Al_90.18Si_8.31Mg_1.5 alloy samples prepared by SLM was studied.Results show that after300?heat preservation 0 to 3 h,with the extension of heat preservation time,the sample significantly lower hardness,compressive yield strength also showed a trend of decrease,plasticity increased significantly.The hardness of the sample increases first and then decreases rapidly with the increase of temperature after holding for 2 hours at 100-535?.The hardness value reaches its maximum at 150?.The variation trend of ultimate tensile strength and yield strength is similar to that of hardness.Hold for 0-48 hours at 150 C.With the prolongation of holding time,the hardness of the sample increases gradually.At 12 h,the hardness value reaches the maximum,¹69+2 Hv.After holding time exceeds 12 h,the hardness begins to decrease slowly.Tensile strength and yield strength of tensile specimens have similar trend with hardness.At 12 h,the maximum tensile strength and yield strength are 549±18 MPa and 408±17 MPa,respectively,and the elongation is about 3.5%.