Study On The Process,Microstructure And Properties Of TiB₂ Reinforced S136 Tool Steel Fabricated By Selective Laser Melting

Selective Laser Melting（SLM）is one of the most competitive laser additive manufacturing technologies.Based on the principle of layer stacking,high-energy laser beam is applied to melt fine metal powder layer by layer without any fixture,which can directly manufacture metal parts with complex structures and diverse performances.In this paper,the micro-scale S136 powder and nano-scale TiB₂ powder were used as raw materials to prepare mixed powders with different TiB₂ content by mechanical ball milling,and TiB₂/S136 composite samples were successfully fabricated by SLM（TiB2=0,0.5,1.5,2.5wt.%）.The effects of process parameters and TiB₂ content on the phase composition,density,microstructure,microhardness,friction and wear,tensile properties and corrosion resistance of SLMed samples were investigated systematically,laying the theoretical and technological foundation for the application of SLM technology in the high quality injection mold manufacturing industry.The main work is as follows:（1）The TiB₂/S136 composite samples were formed by different SLM process parameters.When the laser body energy density was too low,the powder was not completely melted,and a large amount of residual pores were formed.When the laser body energy density was too high,there were microcracks affected by thermal stress.The process parameters after the optimized test are:laser power 280 W,scanning speed 700 mm/s,layer thickness 0.05 mm,scanning pitch 0.12 mm.Under the optimum process parameters,the formed sample surfaces possessed a few defects and a density of up to 97.3%,which was characterized by a finely distributed equiaxed grains.Its microhardness was as high as742.4±5.2 HV_0.1.Additionaly,its wear resistance was excellent,friction coefficient and wear rate were 0.6593 and 0.272×10^-4 mm³/Nm,and tensile strength reached 1051.3±20 MPa,and elongation was 5.84%.（2）Based on the optimized laser forming process parameters,the TiB₂/S136 composite samples with different TiB₂ content were separately fabricated by SLM.Compared with the SLM pure S136 samples,the samples possessed finer and uniform crystal grains,and TiB₂was uniformly distributed at the grain boundary of the matrix in a continuous ring structure when the content of TiB₂ was 0.5wt.%.Therefore,the microhardness increased to 761.5±4.8HV_0.1,the average friction coefficient and wear rate were 0.6489 and 0.152×10^-4 mm³/Nm,respectively,and the tensile strength was as high as 982.8±30 MPa,which was mainly attributed to grain refinement and grain boundary strengthening of the TiB₂ particles.However,with the increase of TiB₂ content,the surface defects（holes or cracks）of the SLM samples increased,leading to the density decreased significantly.Moreover,the TiB₂ ring structure was coarse and discontinuous,which made a negative effect on the mechanical properties of the samples.（3）Through electrochemical corrosion and immersion corrosion experiments,it could be found that the SLM 0.5wt.%TiB₂/S136 composite samples exhibited the best corrosion resistance,and the corrosion potential and corrosion current were-0.29V,0.42±0.05μA cm^-2,respectively.Due to the addition of TiB₂,a new passivation film TiO₂ formed in the etching liquid on the surface of the matrix was beneficial for preventing further dissolution of the matrix by the etching solution.With the content of TiB₂ gradually increasing,the interfaces between TiB₂ and the matrix increased,and more galvanic corrosion occurred at the grain boundary,which accelerated the dissolution of the matrix,resulting in weaker corrosion resistance.The above research results showed that by combing with the optimal process parameters and optimal TiB₂ content,the TiB₂/S136 mould steel with excellent mechanical properties,and corrosion resistance were fabricated by SLM,which could further meet the use requirements of high performance and complex molds.