| WC-Co based hardmetals are widely used in metal cutting. And how to achieve thecombination of higher hardness and better strength simultaneously has aroused lots ofattention. With the rapid development of nano-technology, this issue can be settled in a certainextent. More recently, in order to obtain hardmetals with ultrafine or nano-sized grain,research efforts have been directed towards the doping of grain growth inhibitors, which hasbeen proved to be a simple and effective method. In this dissertation, the history and researchstatus of hardmetals were reviewed, as well as the methods of nano composite powderpreparation, and the forming and sintering of hardmetals. Also, the advantages anddisadvantages were detailedly introduced for those methods mentioned above. Additionly, thegrain growth inhibitors were studied.WC-10Co based ultrafine hardmetals were prepared by DBDP assisted high energy ballmilling and subsequent carbonizing and sintering in one-step. The ball milling process andsintering process were studied. At the same time, the composition of WC-10Co hardmetalswere adjusted by doping different additives, and the influence of different additives onmicrostructure and mechanical properties was analysed detailedly. In addition, cuttingcomparative test was conducted through homemade and commercial hardmetal. And theirwear resistance and the wear mechanism were discussed.Research on the nano powder mixture added with grain growth inhibitor VC prepared byDBDP assisted milling showed that nano powders obtained by short milling time and highball to powder ratio have a better refinement and activation, together with a lower reactiontemperature. WC-10Co-0.9VC hardmetals obtained by the better DBDP milling process andsubsequent carbonizing and sintering in one-step at1MPa with different time. It has beenfound that hardmetals sintered with60min have low porosity and good mechanical properties.When grain growth inhibitors VC and Cr3C2were added to WC-10Co hardmetal, mostVC have a segregation on the interface of WC and Co phase, while a large number of Cr3C2would solid solution in the Co binder phase. Both of them can prevent the process of WCdissolved in Co liquid phase and segregated on WC grain surface. As a result, WC graingrowth can be suppressed, but the inhibitory effect of VC is more obvious. When0.9wt.% VC and0.3wt.%Cr3C2were added to WC-10Co hardmetals concurrently, hardmetalsprepared by carbonizing and sintering in one-step at vacuum and1390℃have an averagegrain size of200nm. And the relative density and Rockwell hardness of hardmetals are up to98.8%and92.0HRA respectively, with a a transverse rupture strength of2240MPa.Additionally, WC-10Co-0.7Si hardmetals with7wt.%Si were obtained by the same process.It has been found that hard and brittle W5Si3formed after sintering. Consequently, thehardness increased, but the relative and strength both decreased.WC-10Co-0.9VC hardmetals prepared by one-step sintering at vacuum and HIP have ahigher hardness and density. But with the graphite phase occured in the internal of thehardmetals, the strength decreased apparently. The cutting test was conducted with theas-prepared hardmetal cutting tool and YG8hardmetal cutting tool. The results showed thatno matter which hardmetal it is, it would gone through three stages including initial wear,normal wear and severe wear. WC-10Co-0.9VC hardmetal cutting tool has a lower amount offlank wear at the beginning of wear, owing to the.absence of graphite on its surface. With theincreasing of flank wear, the graphite phase appeared in the wear area. And the existence ofgraphite results in the reduction of the strength and acceleration of the adhesive wear;meanwhile, the WC particles located between the graphite phase can be easily peeled off andbecame hard particles distributed in the wear area, thus leading to the occurrence of abrasivewear. Under the effect of two kinds of wear, the flank wear loss increased rapidly, andmeanwhile generated a large number of cutting heat, and consequently, a adhesive layer of Fewas formed on the flank face. Although YG8cutting tool has a larger amount of flank wear, itpossesses a longer duration during the wear process, which indicates a high tool life. |
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