Haitao Xu

High-quality WC-Co composite powder is the prerequisite for achieving cemented carbides with superior mechanical properties. However, achieving homogeneous Co distribution on WC particles remains challenging due to the limited surface activity of WC and the high cost associated with noble-metal activation methods. Therefore, this study employed an electroless plating method based on non-noble-metal activation to prepare Co- coated WC composite powders and to clarify the effects of plating parameters on coating behavior, microstructure evolution, and mechanical properties of WC-Co cemented carbides. Results indicate that when plated with a lower reducing-agent concentration (15 g/L) or a lower temperature (70 °C), insufficient Co coating causes poor fracture toughness of the cemented carbides. Increasing the reducing-agent concentration to 25 g/L or the plating temperature to 80 °C promotes a more uniform Co distribution on WC particles, which suppresses WC grain coalescence during sintering. Under the optimized reducing-agent concentration of 25 g/L, the obtained WC-Co cemented carbide exhibits a homogeneous microstructure with an average WC grain size of 0.91 μm, a Vickers hardness of 2054.5 HV30, and a fracture toughness of 11.39 MPa·m^1/2. Excessive reducing-agent concentration or plating temperature deteriorates the Co coating uniformity, promoting Co aggregation and grain coarsening of the cemented carbides. This work demonstrates that precise control of electroless plating parameters enables the fabrication of high-quality WC-Co composite powders, providing a practical route for tailoring microstructure and optimizing mechanical performance of the cemented carbides.

The solid joining between the WC-8Co cemented carbide and alloy steels has great significance for their extensive applications. In this study, the WC-8Co and 40Cr steel were joined with the Ag-28Cu interlayer through the SPS method. The microstructure and mechanical properties of the joints obtained at three temperatures—740 °C, 760 °C, and 780 °C—were analyzed. The joining mechanism was studied, and the relationship between the microstructure and shear strength of the joints was also revealed. When processed at 740 °C, the poor bonding between the interlayer and the 40Cr substrates damaged the joint strength. Higher bonding temperature helped to eliminate the interfacial defects. The joint bonded at 760 °C consists mainly of Ag, Cu within the interlayer and Co-rich Fe(s,s) at the substrate/interlayer interfaces, without any defects. In such a case, the shear strength of the joints reached the maximum level of 236 MPa. However, the increased residual stresses at higher bonding temperatures (780 °C) spoiled the strength of the joints, resulting in the decreasing of the shear strength to 173 MPa. The study shed light on the fast joining of the WC-Co and alloy steels at relatively low temperatures.

The fabrication of WC-based cemented carbides faced challenges including inhomogeneous composition and grain coarsening. To solve these problems, WC–Co cemented carbides were fabricated via spark plasma sintering (SPS) using core–shell WC–Co powders prepared by an electroless plating method. The effects of sintering pressure and Co content on the microstructure and mechanical properties of the cemented carbides were investigated. The results showed that, with increasing sintering pressure, the relative density of the sintered samples was improved (98.4–99.6%) while the grains were coarsened (0.94–1.07 μm). The optimal properties (fracture toughness 11.11 MPa·m^1/2, and hardness 2100.3 HV30) were obtained when sintered with a pressure of 20 MPa. Grain coarsening at higher pressure (30 MPa) reduced the toughness of the cemented carbides. When the Co content was increased from 3 wt.% to 8 wt.%, fracture toughness was improved while the hardness of the cemented carbides was reduced, attributed to the intrinsic high toughness and low hardness of the Co phase. The WC–8 wt.% Co cemented carbides exhibited optimized synergic mechanical performance (hardness of 1874.2 HV30 and fracture toughness of 13.77 MPa·m^1/2). This work elucidated the relationship between the key sintering parameters (pressure and Co content) and the microstructure and mechanical properties of the cemented carbides. The achievements obtained provide a theoretical foundation for high-quality fabrication of the WC–Co cemented carbides.

In this work, the DD98M single crystal superalloys were brazed to themselves using a Pd-Si composite filler. The effect of brazing temperature and soaking time on the microstructure and mechanical behavior of the joints was studied. The microstructure and phase constitution in the joint were identified by the SEM and EDS analysis. The results indicated that the joint obtained was constituted by DD98M/zone 2/zone 1/zone 2/DD98M. The zone 1 was primarily made up of the Ni (Pd, Cr, Co) (s.s), Pd₄Si, Pd (Ni, Ti, Al) (s.s) and Pd-rich Ni (Pd, Cr, Co) (s.s), while the zone 2 consisted of the Ni (Pd, Cr, Co) (s.s) and Al₂Pd₅. During the brazing process, increasing the brazing temperature strengthened the fluidity of the liquid filler, which was favorable to eliminating the solidified pores in the brazing seam. Furthermore, a higher brazing temperature would cause the phases in the zones 1 and 2 to be coarsened remarkably. When setting the brazing temperature to 1060 °C, extending the soaking time made the amount of Pd (Ni, Ti, Al) (s.s) decrease, whereas the amount of Pd₄Si increased, because the peritectic reaction between the Pd (Ni, Ti, Al) (s.s) and remnant liquid filler was enhanced. Among the brazing process parameters under investigation, the maximum joint average shear strength obtained reached 338 MPa when the joint was brazed at 1060 °C for 30 min. A ductile fracture mode happened during the shear tests under a joining condition. The work performed can provide valuable data to design the single crystal superalloy brazed joint.