Powder Injection Molding Of Tungsten Cemented Carbide

Tungsten cemented carbide is widely used as cutting tools and wear resistance materials for its excellent properties. Conventionally, tungsten cemented carbide is made by press/sinter technology which can not produce wares with high complexity. Nevertheless, the products are difficult to further machining, as restricts the application of the materials. Powder injection molding (PIM) is a net shape forming technology which is good at producing small products with complex shape of almost all powder materials. It will be an effective method to produce tungsten cemented carbide parts with complex shape, which will widen the application area of the materials. Based on the ideas, PIM technology of tungsten cemented carbide and some basal aspect relating are investigated in this paper.A new wax-based binder, which is provided with good fluidity, high strength and good shape retention, is developed. It consists of paraffin wax (PW) of 60 percent, liquid Paraffin wax (LPW) of 5 percent, high-density polyethylene (HOPE) of 10 percent, polypropylene (PP) of 10 percent, Ethylene propylene block copolymer (EPR) of 5 percent and stearic acid (SA) of 5 percent. The contact angle of tungsten cemented carbide and the binder is 3.76 . The viscosity of the binder and feedstock is 0.021 Pa .s and 51.52 Pa 's respectively. The feedstock is easy homogenized and a critical powder loading of 65vol.% is achieved. An ideal rheology is achieved by feedstock with a powder loading of 62vol.% and the fluidity exponent n and viscosity fluid energy E are 0.331 and 24.6kJ/mol respectively at 160 # for a shear rate of 1185s-1 .Influence of powder properties on PIM technology is studied using regression method Characteristics which influence evidently the properties of the feedstock and PIM technology is point out and the mathematics model is established to clarify their relationship. The relationship between powder characteristics(Dv, Sw, Ar) and powder packing density(ft) is formulized as ft=129.4-1.2Dv+0.72 0 -l7.9Sw-44.2Ar, where Dv is the average diameter of the particles, Sw is the aspect of the powder and Ar is the aspect ratio of the particle; Powder characteristics affect the fluidity of the powder (Mi) and rheology of the feedstock ( *) according to equation Mi=5.82-0.065Dv-0.28 o +1.47Ar and n =-70.9-0.55 o +91.5Ar-11.76Sw respectively, where o is the particle size deviation of the powder, which indicates the particle size distribution width of the powder. Powder characteristics affect injection parameters such as barrel temperature(Ti), injection pressure(Pp), injection holding time(th) according to equation Ti=1.48+0.995 o -11.6Sw + 112.45Ar , Pp= - 236.63-12.28 Sw+197.28Ar and th=-3.84+3.35Ar+0.02Dv-0.51Sw respectively.Influence of key injection molding process parameters on the quality of the as injection molded specimens is studied and the parameters are optimized using orthogonal arrays experimental design and statistical analysis method. The optimized injection parameters for rectangle specimens are as following: injection pressure 144MPa, injection temperature I65癈, injection speed 60% and die temperature 30癈. The optimized injection parameters for watchcase ring are as following: injection pressure 80MPa, injection temperature 155#, injection speed 60% and die temperature 25 #. Some interaction between injection parameters has great influence on the quality of the green parts.Kinetics of the thermal debinding process is studied and the results indicate that during the beginning and midterm, the control step is binder vapor diffusing throughthe binder liquid. The diffusing rate is 2.66 X10"6 cm2/s at 350 and the diffusing active energy is 30.4kJ/mol. After the low molecular weight components are almost removed from the specimens, decomposing of the co-mixture of HOPE, PP, EPR become the control step of thermal debinding. PIM parts of tungsten cemented carbide can be debinded by thermal debinding method in certain atmosphere. Ideal carbon control is achieved in 25%H2/75%N2 mixture atmosphere.Focusing on the influen...