S136 Breathable Mould Steel Fabricated By Laser 3D Printing & Addictive Manufacturing

As one of the most widely used forming methods in metal laser 3D printing technology,selective laser melting?SLM?has developed and attracted wide attention in recent years on account of its remarkable characteristics of"free manufacturing",less processing procedures and short forming cycle.However,due to the limitation of laser spot diameter and raw powder particle size,SLM cannot directly prepare metallic foams with small micrometer-sized?<100?m?interconnected pores by preset pore in the CAD modeling stage.One of the important applications of this special porous metal is to solve the problem of gas trapping in the breathable mould steel in the injection mould industry.There are uniformly connected micrometer-sized pores in all directions,so trapped gas will be discharged smoothly from the pore of the breathable steel because the average molecular free distance is much smaller than the pore size.Hence it can improve the forming quality of plastics products.However,China's breathable steel mainly depends on imports,and the traditional method of preparing permeable steel faces the problems of high production cost and low production efficiency at present.In order to reduce the dependence of domestic market on the import of breathable mould steel and solve the problem that SLM cannot directly prepare porous materials with micrometer-sized interconnected pores structure through preset pore,S136 mould steel was selected as the research object,and chromium nitride?CrN_x?was used as foaming agent.S136 breathable mould steel with small micrometer-sized,interconnected pores?2-30?m?was successfully fabricated by creatively introducing foaming agent to additive manufacturing.The SLM forming quality and properties of pure S136 steel,including densification,microstructure,phase composition and mechanical properties,were investigated experimentally.The effects of different laser processing parameters and different CrN _x content on the pore structure characteristics?including porosity,pore size and pore morphology?of the as-printed breathable steel samples were investigated.The foaming mechanism was expounded.The permeability,mechanical properties and corrosion resistance of S136 breathable steel were measured and analyzed.The SLM processing parameters of pure S136 were studied.As-printed pure S136 steel has good SLM forming quality.When the laser power is 200 W and the scanning speed is 700 mm/s,the as-printed sample can reach the highest density of99.18%.After SLM,no new phase was formed.Cellular and columnar structure were found in the sample,with an average grain size of 1?m.The Vickers hardness of the specimens is 653.4-687.3 HV20/20,the maximum compressive strength is3.63 GPa,and the corresponding compressive strain is 28.20%.The pore structure characteristics of breathable steel prepared by SLM+foaming agent were studied.CrN_x and S136 steel powders were mixed by three-dimensional mechanical blender,and then SLM was used to form.The porosity of the as-printed sample could reach 24.36%in such condition:the optimum foaming agent content of 5%,laser power of 80 W and scanning speed of1000 mm/s.The pore on the surface of the sample presented a large number of irregular morphologies,and there were"mazed-shape"connected pore from top to bottom.The equivalent pore size distribution of 80%pores in the as-printed sample is 2-30?m,and all pores is less than 80?m.In addition,the foaming mechanism of pore formation is described.The properties of breathable steel prepared by SLM+foaming agent were studied.Gas fluid simulation and gas permeability test based on micro-CT have proved that the permeability of as-printed S136 breathable mould steel is9.432×10^-14 m².Its permeability is close to that of commercial PM-35 gas permeable steel(4.946×10^-13 m²).The mechanical properties and corrosion resistance of s-printed S136 breathable mould steel are better than those of commercial PM-35breathable steel.