Study On Formability And Heat Treatment Of M2 High Speed Steel Produced By Additive Manufacturing

Tool and die are known as the "mother of modern industry",which shows the importance and indelible contribution of tool and die in modern industry.Tool and die steel is therefore the most important part of the mold,in which M2 high-speed steel is one of the most widely applied.In the traditional fabrication process,there exist serious segregation,coarse grains,complicated fabrication procedures,and poor formability.Laser additive manufacturing technology can make up for these problems,to certain extent.Therefore,this paper presents the study on laser additive manufacturing on M2 high-speed steel blocks,the determination of the best printing parameters for fabricating M2 high-speed steel blocks with good formability.Heat treatments were also performed on the fabricated parts,to determine the best heat treatment process parameters.Copper mold suction casting method was also applied to simulate the effect of cooling rate on solidification behavior,microstructure and mechanical properties of M2 high-speed steel samples.In this paper,M2 high-speed steel parts were fabricated using additive manufacturing Method.By applying appropriated pre-treatment,the problem of cracking of the sample during printing process was successfully solved.The printed parts were heat treated,and the microscopic analysis was conducted by using metallography,SEM,and other methods.Hardness was tested as an indication of mechanical property.Due to the high carbon content of M2 high-speed steel,the bottom of the printed parts cracked.By preheating the substrate to certain temperature,the problem of cracking was successfully solved,and M2 high-speed steel parts were printed without cracks.The optimal printing parameters are determined as: preheating substrate at 200°C,laser power P=2200W,scanning speed V=600mm/min.The formed part shows good morphology,uniform microstructure,and no obvious defects such as pores.The printed specimens have a maximum hardness of 63.8HRC at the top surface,57 HRC at the center,and 59 HRC at the bottom.Due to the uneven hardness it cannot be used directly and requires appropriate heat treatment.The best heat treatment process was determined as: quenching at1160°C for 30 minutes and tempering at 550°C for 2 hours.After heat treatment,a sample with uniform hardness is obtained,the hardness is maintained at 60.6HRC.Through the analysis of the printed M2 high-speed steel specimens,the cooling rate was found to have a significant effect on the microstructure of the printed specimens.At lower cooling rate,the microstructure is mainly dendritic austenite,with a small amount of eutectic phase between dendrites,and a small amount of martensite.With the decreasing cooling rate,the size of dendrite increases.Because the sample is mainly composed of austenite,the hardness is very low.The hardness increases slightly due to the formation of martensite.After tempering at 550 °C for 10 hours,the austenite structure did not show observable transformation,while the hardness slightly improved.After cryogenic treatment in liquid nitrogen for 2 hours,the stable austenite formed by rapid solidification also underwent martensitic transformation,forming a large number of fine lenticular martensite,which greatly increased the hardness.