Studies On Microwave Sintering Of WC-Co Hard Metals

In this work, WC-8Co alloys were successfully fabricated by using multi-mode microwave high-temperature sintering furnace. The evolutionary mechanisms of microstructures were studied at different sintering conditions and atmospheres. And the advantages and properties of microwave sintered samples were discussed compared with the conventional prepared ones. Based on the microwave properties of volumetric heating and high heating rate, the densification process and microstructure properties during the microwave sintering were researched. Besides, surface layer decarburization phenomena and mechanisms were studied. Densification mechanisms were discussed after analyzing the kinetic of WC grain coarsening. Local liquid phase sintering mechanism was proposed firstly (the densification and alloying occurred at hot spots preferentially). Finally, the temperature model was built according to the interaction of microwave and WC-Co compactions. The following important conclusions were made:(1) Be compared with the conventional heating method, the microwave sintering processing has no thermal inertia and the sintered compactions can respond the microwave power instantaneously. The time of microwave dewaxing and sintering are greatly shortened. The microwave sintered samples had the finer WC grains and more homogenous Co phase distribution. The whole microwave processing embodied the properties of energy saving and environmentally friendly.In addition, microwave sintered WC-Co alloys had the uniform hardness distribution and magnetism performance and better resistance to chemical and physical corrosions.(2) In multi-mode microwave furnace, the compact density has little effect on the heating rate. For pure materials of WC-Co composites, the WC green compacts had the highest heating rate. Moreover, the heating curve of the cobalt compacts showed the fluction at1100-1150℃. The absorbing ability of WC-8Co compacts was between the Co and WC compacts.The heating rate of the alloy at higher temperature was lower than compact. Besides, the WC-8Co alloy reprocessed in the microwave radiation was deformed.(3) W3Co3C3phase peaks appeared in the XRD pattern of WC-8Co compact, and W phase of WC powder after microwave radiation. WC grain coarsening was not significant in the microwave sintered fine and coarse grain cemented carbides. The sintering atmosphere had an important effect on the microstructures of microwave-sintered samples. The graphite vessel could prevent the pore exclusion.Pure N2and N2+H2atmosphere could accelerate the surface decarburization, but pure Ar atmosphere did not cause the decarburization. However, the heating property of Ar gas is not as stable as N2. A little CH4will lead to the precipitation of graphite phase. The decarburization became seriously after the compact samples exposed in the air.(4) Adding carbon black at the ball mill stage could reduce the decarburization, and improve the mechanical properties. Hardness and TRS of ultrafine microwave sintered WC-8Co samples were93.2HRA and3200MPa respectively when total carbon content was6.08wt%. The maximum TRS value was2640MPa of coarse WC-6Co samples when total carbon content was5.95wt%. The decarburization phenomenon can be reduced by enhancing the carbon potential in the atmosphere, and the TRS of ultrafine alloy could reach2053MPa. Moreover, a little CH4also can reduce the decarburization, and the TRS could reach2850MPa when CH4content was5vol%. In addition, the Ni additive could enhance the TRS when the content was0.8wt%.(5) By researching the densification of normal grain WC-8Co compact, it was found that the shrinkage of the sample was slow when temperature below1280℃, and increased sharply at1300℃until1420℃. Besides, the microstructures changed with the densification. Below1280℃, a lot of small powder still existed; all WC grains became truncated trigonal prism at1300℃and coarsened at higher temperatures. After calculated, apparent activation energy was obtained about84.48kcal/Mol, which was much lower than conventional preparation. The microstructure of microwave sintered WC-8Co alloy had the special WC distribution with small grains enclosed in grown grains, which was not observable in conventional prepared ones. The local liquid phase sintering mechanism could explain this unique microstructure. What’s more, the W-Cu and W-Ni-Fe alloy fabricated in the microwave radiation also can be found the same microstructure characteristics as in the WC-Co alloy. The heating rate was important to influence the microstructure of grain distribution. The faster the heating rate, the more significant the difference between the growth and small grain became.