Study On The Microstructure And Properties Of S136 Tool Steel Fabricated By Selective Laser Melting

Mould is one of the most significant apparatus in modern manufacturing.Nowadays,China is already the biggest mould suppliers in the world,but there is still a gap in mould industries when compared with developed countries.And the development of related business is restricted.Therefore,it is urgent to accelerate mould industrial transformation and upgrading via advanced manufacturing technology.Selective laser melting?SLM?technique provides great advantages in the direct fabrication of fine and complex structural parts,making it particularly suitable for the formation of mould with conformal cooling channels and special-shaped nozzles.However,there are limited kinds of mould materials suitable for SLM.And few research works focus on the characteristics such as anti-corrosion properties which are important for mould injection process.Based on such problems,this paper investigated the SLM process paramenters and performance of S136 tool steel which has good properties but hasn`t studied deeply.The research includes process optimization,microstructure and properties analysis,heat treatment and material modified.The main details and conclusions are as follows:Process parameters of S136 tool steel were explored,and the effect of laser energy density on the morphology of the melt tracks was investigated.Finally,a suitable processing window was determined,and it was found that with a laser energy density of 0.25-0.28 J/mm,there was a good bonding between melt tracks and the revatively density of fabricated parts reached to 99%.The phase composition,microstructure,hardness and corrosion resistance of S136formed by SLM were studied and compared with the samples obtained by traditional processing methods.The reasons for the differences in the microstructure and properties of the two samples were analyzed.The results shows that the SLMed samples contained part of?-Fe and had a fine and uniform subgrain structure due to high cooling rate during the process.Benefitting from the fine and uniform structure,the hardness of SLMed samples was significantly higher than the castings ones.Due to their difference in the structure perpendicular to and parallel to the forming direction,the properties of the SLMed parts also exhibited anisotropy.Because there were contain?-Fe which can improve corrosion resistance in SLMed parts,and its microstructure was fine,the solid solution of the alloy was strengthened,so that the parts had better corrosion resistance than the castings.However,there are large corrosion pits in the area where defects and stress concentrated,and the surface of SLMed parts was seriously damaged by erosion.SLM prototype parts were tested and found that benefit from the conformal cooling channels structure,SLMed mould inserts can shorten the production cycle by 28%,while improving the quality of injection molding products.The quenching process of S136 mould steel fabricated by SLM was studied,and the influence of quenching process on microstructure,hardness and corrosion resistance of the sample was analyzed.After quenching at 980°C,1020°C,1050°C and 1100°C,all the subgrain boundaries in the original structure disappeared,and all the microstructures were transformed into martensite.As the heat treatment temperature increased,the carbides gradually dissolved,and the grain became larger.After quenching,the hardness of the specimen increased to 54.24 HRC,which was nearly 10%higher than that of the original SLM specimen.After quenching,as a result of the dissolve of subgrain and the evenly distribute of corrosion-resisting elements,the corrosion resistance had got greatly improved.Compared with the original state,the mass loss was reduced by nearly 97%.Taken together,the best quenching process parameter is 1050°C/1h.The microstructure and properties of S136/Cr alloy formed by SLM were studied,and the effects of Cr addition on microstructure and properties were analyzed.As results of the influence of unmelted Cr on heat transfer in molten pool,with the increase of Cr content,the content of martensite in the part increased firstly and then decreased.Until the addition amount increased to 10%,there were martensite in the parts only.Cr has the effect of grain refinement.With the increase of Cr content,the microstructure of the formed part became smaller,but the hardness decreased due to the defects such as cracks.Cr can preferentially combine with the O element to form a passivation film of Cr₂O₃,so the addition of Cr can improve the corrosion resistance of the sample;however,due to cracks,the corrosion resistance decreased when the Cr addition amount is too high.Taken together,the optimal additive amount is 4 wt.%.