Study On Microstructure And Mechanical Properties Of Laser Additive Manufactured And Heat Treated CoCrW Alloy

Stellite system is cobalt-based superalloys,which have been widely used in industry,owing its good mechanical properties,such as high hardness,good wear,and oxidation resistance at elevated temperature.Casting is the conventional manufacturing method of Stellite alloys,which limited the shape,size and application of the Stellite alloy parts.In addition,the post processing of the Stellite alloy parts is very difficult for the high hardness of Stellite alloys.In this study,laser additive manufacturing（LAM）technology was used to produce Stellite alloy parts,which can reduce the post process,manufacturing cycle and cost and make it easy to prepare the parts with complex shape.The sample was produced by LAM technology that combined with high power fiber laser and coaxial powder delivery system.The powders used in LAM were Stellite 12 powder.Through the comparison of a variety of process parameters,the final selected process parameters were as below: power,scan speed,powder flow rate,protective gas flow rate,carrier gas flow rate and overlapping rate were 3000 W,8 mm/s,18 g/min,15 L/min,5 L/min and 45%,respectively.The results show that,on the condition of optimizing the process parameters,the specimens produced by LAM technology have a compact structure,without visible cracks and pores.The results show that cobalt based cobalt matrix dendrites and inter-dendritic eutectic phases were the primary microstructure of Stellite 12 alloy.The crystal structure of cobalt is Face Center Cubic（fcc）,M23C6 is the primary carbides,and the eutectic phases,which were lamellar,were consist of cobalt and carbides.Heat treatment can modify the microstructure of this alloy,especially the carbides.Most of the carbides have been dissolved into the cobalt matrix after solution treatment,however,the carbides are not completely dissolved,and there are a part of blocky carbides that rich in Cr.Aging treatment produced a high number of carbides precipitating throughout the cobalt matrix,the shape of carbides change from lamellar to blocky,M23C6 that rich in Cr and M6 C that rich in W are the primary carbides.A large quantity of blocky carbides and globular and rod-like carbides were formed in the sample after solution plus aging treatment,the higher the temperature of aging treatment,the larger size of precipitate carbides.The microhardness of untreated sample is 518.08 HV0.3,the hardness of sample after aging and solution treatment are 591.02 HV0.3 and 282.20 HV0.3,respectively.The wear resistance of untreated samples at room temperature（RT）and 600℃ is better than that at 200 and 400℃.The weight losses of untreated samples at RT and 600℃（1.1 mg,0.9 mg）after wear testing are one quarter of that at 200 and 400℃（3.9 mg,3.4 mg）.The sample that underwent solution plus aging treatment has a superior wear resistance,the weight losses of these samples at RT and 600℃ after wear testing are 0.4 mg and 0.5 mg,respectively.After 48 h continuous oxidation experiments at 1000℃,the Stellite 12 alloy show good oxidation resistance,the mass gain of Stellite 12 alloy is 2.632 mg/cm²,and the mass gain rate is 0.552 g/m² ×h.The main reaction products in outer oxide scale are Cr₂O₃、CoCr₂O₄,and the inner oxide layer consists of SiO₂.