Research On The Forming Defects Of Selective Laser Melted Mg-Y-Sm-Zn-Zr Alloy

Magnesium alloys are regarded as the lightest structural metal materials with excellent development potential and are widely used in aerospace,military,medical and other fields.The defects of magnesium alloy parts prepared by traditional processing methods limit its development.Developing new near-net forming technology can effectively meet the high-end applications of magnesium alloys.Selective laser melting(SLM)technology uses a high-energy laser beam as a heat source to rapidly solidify and cool metal powder with computer assistance to obtain complex net-shaped parts.However,there is insufficient research on the defect formation mechanism of magnesium alloy SLM technology.In this paper,Mg-3.4Y-3.6Sm-2.6Zn-0.8Zr alloy powder was selected as the experimental raw material,and the defect formation mechanism of the alloy was systematically studied from the perspectives of process parameters and scanning strategies.The formability and mechanical properties of magnesium alloy SLM samples were analysed using a scanning electron microscope(SEM),X-ray scanner(XCT),microhardness tester and universal mechanical testing machine.The main research conclusions are as follows:The scanning strategy directly changes the laser scanning path,resulting in differences in the thermal history and thermal cycle of the sample.The spheroidized particles produced by the small partition method lead to insufficient metallurgical bonding.The preferential solidification behavior of the droplets in the large partition method causes pores and powder accumulation at the partition boundary.Short scan vectors cause gas pore and keyhole defects due to increased recoil pressure in the molten pool.The filling of the lack of fusion defects and the connection of gas pore and keyhole defects in adjacent molten pools will form irregular pore defects.The layer rotation angles of 0°and 90°are beneficial to the release of gas pores and keyholes,but lead to the accumulation of pore defects layer by layer.The layer rotation angles of 37°and 67°suppress the accumulation of pore defects.The process parameters directly affect the laser energy density,resulting in different heat input absorbed by the powder.The single pass formability is excellent when the laser power is 60～80 W and the scanning speed is 300～700 mm/s.When the scanning distance is 80μm and the overlap rate is 41%,the adjacent melt tracks are closely connected,there are no spherical particles or pore defects on the surface of the melt tracks,and the surface of the block has high relative density and low roughness.The laser power and scanning speed directly cause differences in energy density,resulting in different defects in the sample.When the energy density is lower than 83.3J/mm~3,the metal powder cannot be completely melted to produce a lack of fusion defects,and the high-viscosity metal liquid will hinder the filling of pores.When the energy density is higher than 166.7 J/mm~3,the spheroidization phenomenon occurs,and the increase of recoil pressure leads to the generation of splash particles.The joint action of spheroidization and splash particles leads to decreased interlayer metallurgical bonding strength.When the energy density is 125 J/mm~3 and the scanning strategy is the island size4×4 mm and the layer rotation angle of 67°,the sample performance is the best:the average microhardness is 95.64 HV,the compressive yield strength and ultimate strength reach 301.8±5 MPa and 467.9±5 MPa.