Study On Microstructure Control And Properties Of M2 High Speed Steel Formed By Laser Selective Melting

The high carbon and high alloy high-speed steel represented by M2 has the advantages of high hardness,good red hardness and strong wear resistance,and is one of the important materials for manufacturing cutting tools.Selective Laser Melting（SLM）technology can realize the rapid and integrated forming of complex metal structural parts,shorten the design and manufacturing cycle,and reduce the development cost,and is considered to be the most promising potential manufacturing method for new tools such as porous lightweight and complex internal flow channels.At present,SLM forming M2 high speed steel faces serious challenges,that is,the combination of high cooling rate(10⁶～10⁸ K s^-1)and high carbon content（0.8～0.9wt.%）promotes the formation of a large number of brittle martensitic structures,which adversely affects the comprehensive properties,especially the strong plasticity.In this regard,this paper first explored the law of influence of SLM forming process parameters on the defects,microstructure and properties of M2 high speed steel.On this basis,based on the online isothermal quenching method（that is,continue to hold heat above the martensitic phase transformation point after forming）,the forming bainite/martensitic structure was regulated.The effects of holding time on hardness,wear properties,tensile and compressive mechanical properties were studied.This study is expected to provide a new method and experimental basis for the regulation and optimization of microstructure properties of SLM formed high carbon steel.The effects of laser power and scanning speed on the density and microstructure of M2 high speed steel formed by SLM were studied.It was found that the laser power was 280 W,the scanning speed was 800 mm/s,the scanning distance was 0.05 mm,the powder thickness was 0.03 mm,and the sample surface had no obvious defects and the density reached the maximum（99.73%）when the substrate was preheated at 200℃.Under different process parameters,each formed sample is composed of a large amount of martensite,a small amount of bainite,residual austenite and MC carbides.The martensitic structure is cellular and the average grain size is less than 1μm.In addition,because the top region of the sample is in high temperature for a shorter time than the bottom,the transformation of the lower bainite structure is limited,so the lower bainite structure in the sample gradually decreases from bottom to top.The effect of holding time（30,60 and 90 min）on the microstructure of formed M2 high speed steel was studied.With the increase of holding time,the transformation of lower bainite structure is more sufficient and the structure is more uniform.When the holding time increased to 90 min,the lower bainite content increased from 5.2%to22.1%,and the grain size was less than 1μm with the increase of holding time,and no significant increase was observed.With the increase of holding time,carbide precipitation on the matrix gradually increased.The influence of energy density on hardness increases first and then decreases,and decreases after 266 J/mm³.With the increase of holding time,the microhardness of the sample increases first and then decreases,and the highest hardness is 845.8HV_0.3when holding for 30 min.At the same time,the wear performance is significantly improved under this parameter,and the wear rate is reduced to 1.03×10^-6 mm³N^-1m^-1,showing good wear resistance.With the increase of holding time,the tensile strength of the sample can be effectively improved.Compared with the sample without holding time,the tensile strength increases from 1121.9 MPa to 1447.8 MPa and the elongation increases from 0.7%to 2.4%.The maximum compressive strength of the sample after heat preservation is 4394 MPa,which is much higher than that of the sample without heat preservation.Compared with the uninsulated sample,the comprehensive properties of the sample can be effectively improved by on-line isothermal quenching treatment.