Effect Of Heat Treatment On Microstructure And Properties Of Al-Si-Mg Alloy By Selective Laser Melting

Al-Si alloy is one of the most important and widely used aluminum alloys in casting but is more difficult to manufacture complex-shaped parts such as thin-walled and closed-cavity by traditional casting,forging and other processing methods.Therefore,the ever-increasing demands for precision lightweight alloys parts in transportation,aerospace and other fields lead the development of selective laser melting(SLM)technology.The SLM technology has a fast cooling speed,the crystal grains of the aluminum alloy are finer,the mechanical properties of the material are improved,and the metal parts with complicated shapes can be obtained.The components formed by SLM can reach higher strength in the deposited state,but there are large residual stresses in the parts,deformation,warpage and other defects occurred during use.Heat treatment is an effective way to adjust the alloy structure and improve the mechanical properties of the alloy and eliminate the residual stress generated during rapid prototyping.In this paper,the changes of microstructure morphology,mechanical properties,secondary phase distribution,and residual stress of Al-Si-Mg alloy formed by SLM before and after heat treatment was analyzed.The results showed that compared with gravity casting,the Al-Si-Mg alloy formed by SLM was more likely to form fine grains,and the Si phase changed from coarse or needle-like to grid-like and was evenly distributed in the ?-Al matrix.Different molten pool structures were formed on XOY and XOZ plane,which were divided into coarse-grained zone,fine-grained zone,and heat-affected zone according to different grain characteristics in the molten pool.The cooling speed of the SLM forming process is fast,and it is easier to form supersaturated solid solution.The two strengthening mechanisms of fine grain strengthening and solid solution strengthening worked together.The comprehensive mechanical properties of the alloy were significantly improved.The tensile strength,yield strength and elongation of the alloys on XOY and XOZ plane were 430 MPa,376MPa,7.14% and 440 MPa,404MPa,5.88%.After T6 treatment,the homogeneous network-like Si phase in the alloy dissolved and fractured during the high-temperature solution treatment,and grew into a passivated round Si phase,the network-like structure disappeared and was evenly distributed on the ?-Al substrate.After 4h solution treatment of the alloy,the elongation of the alloy reached the maximum,18.87%,and the tensile strength and yield strength of the alloy decreased to211 MPa and 128 MPa.After aging treatment at different times,the aging time was extended,and the small-sized Si phase precipitated at the grain boundary grew.The alloy has the best mechanical properties when aging treated at 180? for 6h,the tensile strength,yield strength and elongation of the alloy on XOY plane were 297 MPa,268MPa,8.72%,and the residual stress value dropped to 138MPa;After direct artificial aging treatment of the alloy,the eutectic Si phase at the grain boundary dissolved and voids appeared inside.When artificial aging at 180? for 8h,the tensile strength,yield strength and elongation of the alloy on XOY plane were 411 MPa,393MPa and 4.84% respectively.At the same time,the yield strength of the alloy was the highest and the residual stress value dropped to 181 MPa.