Research On Additive Manufacture Processing And The Microstructure Of Cemented Carbides

Due to its excellent properties of hardness,wear resistance,strength and toughness,heat resistance,and corrosion resistance and so on,cemented carbide is playing the important roles in national defense,manufacturing,geological exploration,and other fields.The traditional manufacturing technology for cemented carbide has significant difficulties in producing carbide tools with complex shapes and internal structures,which greatly restricts the applications of cemented carbide.The additive manufacturing,especially selective laser melting(SLM),which is booming in recent years,has the great potential to overcome the above mentioned obstacles of traditional manufacturing technology for cemented carbide.The current research on additive manufacturing of cemented carbide,however,is quite limited worldwide.This study investigated the effect of cemented carbide powder properties and SLM processing parameters on the mechanisms of densification,microstructure development,phase transformation,and mechanical properties for SLM processed cemented carbide.A systematic densification model is proposed for SLM processed cemented carbide.Furthermore,a systematic study of post-SLM heat treatment has been made subsequently.It is found that:(1)The morphology of cemented carbide powders has a critical influence on the densification of SLM processed carbide.Spherical carbide powder results in the higher densities,while the irregular shaped powder leads to more voids,porosity,and micro-cracks.(2)The powder-bed thickness directly determines the volume density of energy for SLM processing.Compared with the powder-bed thickness of 60?m,30?m powder-bed thickness results in a significantly higher density of SLM processed cemented carbide.(3)Through the comprehensive surface response experimental design,the correlation between SLM parameters and the relative density of WC-20Co,?,can be expressed as:?=93.46-0.4P-2.11S-0.35V-1.47PS-0.19PV-1.08SV-2.79 P~2-1.41S~2+0.83V~2where P is the laser power(W),S is the scanning speed(mm/s),and V is the hatch spacing(mm).The optimized SLM processing parameters per this model are laser powder 412.72 W,laser scanning speed 500 mm/s,and hatch spacing 0.07 mm.Experimental results showed that the highest density of SLM processed WC-20Co carbide was 96.33%.(4)The microstructure of SLM processed cemented carbide is obviously different from that of conventional liquid-phase sintered cemented carbide.Quick and uneven WC grain growth happened during SLM processing.The grain size distribution is obviously wider than that of the original WC powder,and apparent decomposition of WC leads to the formation of eta phase.In the vertical cross-section parallel to the laser beam,a layered structure of alternating coarse and fine WC segregation has been observed.While in the horizontal cross-section perpendicular to the laser beam,no layered structure was observed although both coarse and fine WC were mixed together.The hardness test results of the vertical cross-section and the horizontal cross-section are similar and no obvious anisotropy is observed.(5)The heat treatment of SLM processed carbide shows that further liquid phase sintering can improve the density and hardness of cemented carbide.The heat treatment resulted in the significant WC grain growth,but did not eliminate the eta phase as well as the anisotropy of the microstructure.(6)This research has carried out basic theoretical and technological exploration for the direct SLM processing of cemented carbide.The results provide both the necessary technical accumulation and the processing support for the quick development of cemented carbide made by SLM process.