Research On Forming Process Of Selective Laser Melting Mg-Y-Sm-Zn-Zr Magnesium Alloy

Magnesium alloy has broad prospects in the field of lightweight for their as lightest metal structural material,but the poor high-temperature mechanical properties limit its industrial application.The research of new heat-resistant magnesium alloys and the development of near-net forming processes have been the research hotspots in recent years.Selective laser melting(SLM)is an additive manufacturing technology based on a layer-by-layer forming process.It can not only process complex shapes that are difficult to achieve by traditional processes,but also improve the microstructure when the material out of thermodynamic equilibrium under the action of rapid solidification.At present,there are few reports on the SLM process research of multi-component rare earth magnesium alloys.In this paper,the Mg-3.4Y-3.6Sm-2.6Zn-0.8Zr heat-resistant magnesium alloy studied by the research group was selected as the experimental material,and the alloy powder was prepared by the method of mechanical grinding.The SLM forming process of the alloy was explored from the process parameters(e.g.,laser power,scanning speed,powder layer thickness),and through optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM),microhardness and nanohardness testers and other equipment analyzed the formability,microstructure and microhardness of alloy SLM specimens.The main findings are as follows:The alloy powder meets the requirements of the SLM forming process was prepared.After the ball milling time at 6 h and the rotating speed at 400 rpm,the highest yield of powders with a particle size of less than 70?m was 97.34%,the morphology of the powder changes from irregular lamellar to granular,and the particle size ranged from 40?70?m,average particle size of 55?m.Process parameters affect SLM forming quality.When the laser power is 30 W,the sample has more serious?spheroidization?and void defects,and the more circular pores attributed to the recoil pressure generated by the evaporation of Mg element when the laser power increased to 50 W.With the increase of the laser power from 30W to 60 W,the magnesium content in the formed sample did not show serious element evaporation compared with the alloy nominal composition.Under the experimental conditions with the laser power of 40 W,the scanning speed of 300 mm/s,the thickness of the powder layer is 20?m and the hatch spacing of 80?m,the maximum relative density of the sample was 98.6%.Different from the as-cast microstructure,the microstructure of the SLM formed sample was mainly composed of Mg matrix,eutectic phase(Mg,Zn)₃(Y,Sm)and Y₂O₃,Mg₁₂(Y,Sm)Zn eutectic phase and lamellar LPSO structure were not produced.The morphology of the grains presented fine equiaxed grains with an average size of 1?3?m in the center of the scanning tracks,while the microstructure was columnar crystals in the overlapping area between the two laser spots.The average microhardness of SLM samples in the cross section was 105 HV and the vertical section reached 95 HV,both reaching over the microhardness of the as-cast specimen(81.5 HV).The nanoindentation experiment found that the nanohardness of the eutectic phase(Mg,Zn)₃(Y,Sm)is much higher than Mg matrix,thus significantly improving the microhardness of the SLM formed samples.The hardening mechanisms of the experimental alloy SLM formed samples were primarily manifested as grain boundary hardening and solid solution hardening.