Study On Selective Laser Melting Of High-strength Al-Mg-Sc-Zr Aluminum Alloy

Selective laser melting(SLM)has become the most widely used powder bed process for metal 3D printing due to its unique advantages including high precision,unlimited complexity,and excellent manufacturing performance.SLM has been widely used in aerospace,national defense and other fields.However,mature aluminum alloys for SLM are limited to AlSi10Mg whose mechanical property cannot meet the needs of high-strength structural parts,so the selective laser melting of high-strength aluminum alloy is crucial for the further development in these industries.In this paper,the following research is carried out on the selective laser melting of Al-Mg-Sc-Zr high-strength aluminum alloy.The effects of key process parameters such as laser power and scanning speed on the density,surface roughness,and microstructure of the SLMed Al-Mg-Sc-Zr alloy part were explored.A process window that meet the requirements for manufacturing and performance has been obtained.The structure of the SLMed alloy is a submicron-level fine equiaxed crystal at the boundary of the molten pool and a micron-level columnar crystal grown from the boundary to the inside.The main phase is the super-saturated α-Al phase in which Mg,Sc,and Zr elements are dissolved.The tensile strength rises with growing density and decreases with the increase of metallurgical defects.The optimized process parameters were obtained as laser power of 300W,scanning speed of 800mm/s,hatch distance of 0.15mm,layer thickness of 30μm,and interlayer rotation angle of 67°.In this manufacturing process,an Al-Mg-Sc-Zr alloy testpiece as built with a density of 99.84%,a tensile strength of 338 MPa,an elongation of 20%,and a Vickers hardness of 80 HV 0.5 was obtained.The effects of heat treatment parameters on the microstructure and properties of SLMed Al-Mg-Sc-Zr alloys were investigated,and optimized heat treatment parameters were obtained.The results show that heat treatment would break the equilibrium state of the supersaturated solid solution formed in SLM,after which the precipitation-enhancing phase would precipitate from the ℃-Al phase.At 500℃,the alloy rapidly precipitates strengthening phase,and then recovers and recrystallizes,which reduces the strength of the alloy.At 330℃,the alloy slowly precipitates strengthening phase,prolonged heat preservation had few effects on alloy properties.The optimized heat treatment parameter is aging at 330 ℃for 0.5 hour.Al-Mg-Sc-Zr alloy after aging treatment has a tensile strength of 488MPa,an elongation of 14%,and a hardness of 150HV0.5.The strengthening mechanism of SLMed Al-Mg-Sc-Zr alloy was discussed.During SLM and heat treatment,fine grain strengthening mechanism is combined with precipitation strengthening.Results of EBSD and TEM inspections show that the grain size of SLM formed Al-Mg-Sc-Zr alloy is between 0.5 and 5μm,while the grain size of traditional AlSi10Mg is between 5 and 30μm.The grain size of the former is 10 times finer,so the interfacial area of the crystal is greatly increased,thereby hindering the dislocation slip and increasing the strength of the Al-Mg-Sc-Zr alloy.After the heat treatment,a secondary phase Al3(ScxZr1-x)coherent with the matrix is precipitated,forming secondary particles with small and dispersed sizes of tens of nanometers.It also produces a large resistance to the dislocation slip,thereby strengthening the alloy drastically.