Study On Powder Reuse And Oxide Precipitation Behavior Of H13 Steel Manufactured Via Selective Electron Beam Melting

Due to its excellent red hardness,wear resistance,high hardenability and hot cracking resistance,H13 steel has been widely used to extrusion die,hot forging tools,and die casting dies.Selective electron beam melting(SEBM)technology can achieve "near-net forming" of complex parts through layer-by-layer stacking process,improve product quality,reduce production costs,and promote the transformation and upgrading of traditional industries.In this paper,taking H13 steel by SEBM as the entry point,the effects of powder raw materials and process parameters on the spreading behavior of powder bed,the evolution of properties during powder reuse and the precipitation behavior of oxides during rapid melting and solidification through discrete element method.The main conclusions are as follows:(1)The thickness of powder layer was the main factor affecting the spreading quality of powder bed.The density and uniformity of powder bed increase with the increase of powder layer thickness.The wide powder particle size distribution can effectively reduce the powder bed porosity,and the increase of the proportion of small particles leads to the deterioration of powder bed agglomeration.The relative packing density of the powder bed increases with the increase of the powder laying rate,while the porosity decreases first and then increases.(2)With the increase of reuse times,the D50(the cumulative distribution of particles is 50%of the particle size)increased from 57.29 μm to 65.66 μm,the dendrite morphology on the particle surface disappeared,the proportion of irregular particles increased,and the flowability deteriorated.The oxygen and nitrogen content of the powder increased from 149 and 237 ppm to 250 and 394 ppm,respectively.The oxygen element exists on the surface of the particles in three forms:metal oxide,metal hydroxide,and adsorbed water.In addition,the crystal spacing and crystallinity of the recovered powder increased.The micro-Vickers hardness of different batches of powder forming samples remained at about 540 HV0.2.(3)The as-built parts are mainly composed of α-Fe(BCC)and γ-Fe(FCC).During the solidification process,the element distribution is uniform and there is no obvious segregation.The microhardness of the as-built part is 53 9±10.1 HV0.2,the tensile strength is 1423.8± 33.7 MPa,and the elongation is 3.52±0.08%.At the end of solidification,the segregation of oxygen and titanium is the most serious,followed by Si,Al,and Mn.During the solidification process,Al2O3,Al2O3·TiO2,and MnO were first precipitated,followed by other oxides.The oxygen content is the main factor limiting the nucleation and growth of oxides such as Al2O3,SiO2,and MnO,and the content of Ti is the main factor limiting the precipitation of titanium-containing oxides.The rapid melting and solidification characteristics of additive manufacturing provide the possibility for the dispersion distribution of nano-scale oxides.The results show that the size of oxides at the end of solidification was less than 1 μm,and the size of titanic-containing oxides was in the nanometer scale(20～50 nm).