On the Mechanics of Sintering of Hardmetal Powders

QC251125

Abstract

Powder metallurgy is used in the manufacturing of cutting tool inserts to achieve the desired material properties. Tungsten carbide, WC, mixed with a metallic binder, such as Cobalt, Co, is a common cemented carbide used for cutting tool inserts. In large scale production, the powder is pressed and sintered. During the manufacturing process, the volume of the cutting tool significantly decreases. During the sintering process, very little can be done to change the final shape, and the shrinkage during sintering must be accounted for during pressing. This thesis will investigate the mechanical modeling of shrinkage that occurs in the sintering process.

The constitutive model at issue can capture the isothermal stage in the sintering process and can be adjusted to a specific powder. To increase its usability, a sensitivity analysis was performed in Paper A to determine if all parameters must be determined when using a new powder mixture. It was found that some of the parameters were more sensitive than others when optimizing parameters using experimental results. The less sensitive parameters could be constant, reducing the number of necessary experiments to determine all adjustable parameters. 

The different stages of sintering were also investigated in Paper A, where the model had difficulty describing the shrinkage both in the initial stage and in the liquid phase, where the Cobalt starts to melt. The latter was expected, since the constitutive model was explicitly developed for the solid stage of sintering.

To evaluate the model, all the experiments were performed on a specific WC-Co powder blend. For adherence to match the industrial process for cutting tools made from this powder, the debinding phase was included early in the sintering cycle. This was not done in the previous development of the sintering model. The influence of the debinding process was experimentally investigated in Paper B, where it was shown that the shrinkage, as well as the final microstructure, is influenced by the exclusion of the debinding stage. 

In Paper C, multiple dilatometer and sintering furnace experiments were performed to further develop the model. Complements to the initial stage and liquid phase were introduced, and the effect of changing the initial density, post-compaction, was investigated. Different sintering cycles were used to ensure the robustness of the constitutive parameters of the model. The deviatoric influence was tuned with bending experiments. 

To verify the model, Paper D compares simulations to experiments. This was done using two different press dies, where the same amount of powder was pressed to different heights. The powder blanks were sintered to different maximum temperatures and measured. The compaction and sintering process was simulated and compared to the experimental values, showing that the sintering model captured the process well. A sintering furnace was used, where one of the sintering cycles was representative of the industrial production of cutting tools.  

urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-373254