Silicon Carbide Modified Metal Matrix Composite Coatings Based On Plasma Transfer Arc Welding

Dies used in hot forging are characterized by extremely high temperature at the interface, high local pressures and large metal to metal sliding. Therefore, the wear resistance at high temperature is one of the key factors to the die life. As an ideal wear-resistance material, particulate reinforced metal matrix composites(PRMMCs) show excellent potential in the field of hot forging die strengthening due to the combination of strengthening particle and toughening metal. The efficacy of die surface strengthening and recondition by surface engineering is always one of the hot issues in forging industry. However, there are so many technical difficulties in fabrication of PRMMCs by high energy beam of laser or plasma, i.e. the oxygenolysis of particle in the beam, the poor wettabillity between particle and metal matrix, and the unachievable goal of the dispersed distribution of the particle in the matrix. Silicon carbide(SiC) is commonly used as an ideal renforcing material due to its superior mechanical and chemical properties. Therefore, the research on SiC modified MMCCs based on plasma transfer arc welding(PTA) has important value in practical application and theoretical exploration.In this dissertation, SiC modified MMCCs is obtained by combination of PTA and plasma melt injection(PMI) method(PTA+PMI). This method is learning from the mature laser melt injection technology. In addition, the infusible SiC is coated with electroless nickel coating(ENC) to prevent burning loss during PTA+PMI. Subsequently, the nickel coated SiC(Ni/SiC) is used as dispersed strengthening phase to modify the nickel and cobalt matrix superalloys. The optimum bath solution and process parameters were obtained by exploring the effect of bath parameters on plating rate and bath effiency. With the guide of electroless plating mechanism for the particle, the optimum bath is expanded to coat other particles. The action mechanism of SiC in metal pool and the technical efficacy of electroless nickel coating in PTA+PMI are revealed through the study on microstructure of SiC modified MMCCs. The wear behavior of the modified MMCCs at elevated temperature was evaluated based on the simulative service conditions of hot forging at laboratory. Furthermore, the thermal fatigue property of the as-received MMCCs is checked using thermal shock test. The main contents and results are listed as follows.A high efficiency and stable electroless nickel bath solution with relative maximum nickel plating rate of 11.28% was optimized through Taguchi experimental design method, which is suitable for particle electroless nickel plating. The optimum bath could repetitively plate nickel for four to five times without replenishing of nickel salt and reducing agent. A continuous and uniform coating with the thickness of 6.5 ?m was obtained through repetitive plating for one time. The nickel deposition is initiated from the Pd activated surface by the rapid Ni chemical replacement. Subsequently, the electrochemical nucleation reaction can also occur as well as the self-catalytic reaction. These mechanisms are responsible for the nicke coating growth. With the progress of the time, a continuous and thick nickel coating could be obtained. The optimum bath has also a good effect in electroless plating nickel on WC and Mo particle.The deposition of ENC on SiC is found to be very effective in retarding the oxygenolysis of SiC during the PTA+PMI process. Meanwhile, the improved wettability of SiC by the ENC decreased the necessary minimal vertical velocity of SiC, which consequently promoted the impregnation of SiC resulting in a well modification on melt pool in PTA+PMI process. The injected Ni/SiC reacted with the Ni, Cr and Fe in the nickel matrix alloy melt pool resulting in formation of in situ M7C3(M=Cr, Fe) carbides and eutectic nickel silicide(Ni3Si). The dynamic movement of the injected Ni/SiC in melt pool is responsible for the coating system with many homogeneous Cr-rich M7C3 phases. The initial in situ carbides still acted as nucleation sites, their size was increased substantially. Consequently, large quantities of granular M7C3 phases were distributed uniformly in the coating. This strengthening mechanism is called dynamic in situ raction. Due to the buoyancy and resistance from the melt pool, the SiC concentration decreases with the increase of melt pool depth resulting in a graded distribution of the carbides in the coating, i.e. the size of carbides is larger in the top layer than that of bottom layer.In SiC modified nickel matrix composite coating, the microhardness increased with the value of 380 HV0.5. In addition, the stiffness and wear resistance properties were enhanced. Micro-abrasion as the dominant wear mechanism occurred in room wear test while the adhesive wear was avoided. Abrasion and mild adhesive wear as the main mechanisms occurred at elevated temperature wear. The eutectic Ni3 Si was the weak phase during high temperature wear test. Meanwhile, the modified coating showed excellent oxidation resistance due to the large amount of Si in the coating resulting from the decomposition of SiC. The thermal shock results showed that the thermal fatigue resistance of the modified coating deteriorated due to the brittle crack of M7C3 carbides.Based on the modified mechanism of SiC in nickel matrix coating, SiC modified Stellite 6 composite coating(S6) was fabricated. Microstructural results indicated that the same mechanism of dynamic in situ reaction occurred in S6, leading to a coating consisted of a mixture of Cr-rich M7C3 carbides and eutectic silicides(Co2Si, WSi2). The microhardness, elasticity modulus, oxidation resistance and high temperature wear resistance of S6 are enhanced. Unfortunately, the deterioration of thermal fatigue resistance resulting from the brittle fracture of the S6 occurred.The high temperature wear behavior results based on simulative service conditions of hot forging in laboratory showed that the modified nickel and Cobalt composite coating owned excellent abrasion wear and adhesive wear resistance. In addition, the frictional coefficient of the two kind of modified coatings is significantly decreased in comparison with 4Cr5MoSiV1 substrate steel as an indication of good anti-friction effect. These modified coatings could improve the life of hot forging die by using in the die where plastic deformation, oxidation and wear resistance is need while the heat fatigue resistance is not strict.