Microstructure And Mechanical Properties Of Alsi7mg Alloy Fabricated By Selective Laser Melting

With the increasing demand for lightweight structure and functional optimization of aerospace parts,conventional manufacturing technologies are difficult to manufact parts with high precision and complex shapes.The emergence of Additive Manufacturing(AM)technology can effectively solve such problems.Selective laser melting(SLM)technology is one of the fastest-growing metal AM technologies in recent years.By melting metal powders and stacking them layer by layer,metal parts with a density close to 100% can be obtained.AlSi7 Mg alloy fabricated by SLM has a good matching of tensile properties and elongation,which is a potential material for aerospace parts with high reliability requirements.At present,there are few related studies on AlSi7 Mg alloy fabricated by SLM.The urgent task is to systematically investigate microstructure and mechanical properties of AlSi7 Mg alloy fabricated by SLM and explore the inherent relationship between microstructure and mechanical properties.In this paper,the influence of process parameters on forming quality of AlSi7 Mg alloy fabricated by SLM is studied,and samples with 99.8% relative density are manufactured.The internal relationship between the microstructure and mechanical properties and effect of heat treatments on the microstructure and mechanical properties are investigated.The main research contents and results are as follows:Firstly,the influence of process parameters during SLM process were studied.Research found that there were defects such as pores and un-melted powders inside specimens when scan speed and hatch spacing were too high or too low,which reduced relative density of specimens.Process parameters can produce specimens with relative density above 99.8% are as follows: laser power is 350 W,scan speed is 1 600 mm / s,hatch spacing is 0.13 mm,layer thickness is 30 ?m.Secondly,the microstructural morphology of AlSi7 Mg alloy fabricated by SLM were observed using optical microscope and electron microscope.It can be seen that the SEM image of AlSi7 Mg alloy fabricated by SLM exhibits reticular eutectic Si embedded in ?-Al matrix structure,where grains are small with size about 0.5 ?m.Meanwhile,the microstructure is nonuniform.There are three zones which could be separated at the overlap of adjacent scanning tracks.: fine-grain zone,coarse-grain zone and heat-affected zone.The microhardness value of the as-built samples is 110 HV,which is 27.9% higher than casting samples.The tensile strength and yield strength of horizontal samples can reach 435 MPa and 299 MPa and 14%,respectively,which have been significantly improved compared with casting specimens.Thirdly,the effects of annealing treatment on the microstructure and mechanical properties of AlSi7 Mg alloy fabricated by SLM were investigated.After annealing,network structure in as-built samples was destroyed,and the microstructure evolved into granular Si precipitates distributed uniformly in the Al matrix.After annealing at 350 ?/3 h,tensile strength and yield strength of horizontal samples decreased to 210.35 MPa and 152.01 MPa,respectively,and elongation increased significantly to 30.83%.Finally,the influence of solution and aging treatment were also studied.After solution and aging treatment,network structure was also destroyed and broke into coarse granular precipitated Si phases distributed in the Al matrix.In this study,535 ? / 3 h / WQ(waterquenching)+ 150 ? / 6 h / AC(air-cooling)solution and aging treatment is the best heat treatment.After this treatment,sample microhardness value can reach 120.18 HV,and tensile strength,yield strength and elongation are 349.27 MPa,309.67 MPa,and 17.12%,respectively,which have good matching tensile properties and elongation.