| Metal Injection Molding (MIM) is an well-known hot manufacturing technology, as an advanced near net shape process with well recognized capability for efficiently producing complex multidimensional parts with high performance at low cost in powder metallurgy. Application of this technology to larger parts with thick section beyond 20 to 30 mm demanded by industrial market has now been limited due to the barrier in debinding and the high cost of the fine powder in MIM technology. A feasible process route has been outlined and experimented in this paper including injection molding coarse powders, study of thermal-debinding mechanism of larger parts, effect of particle size on densification and mechanical properties of the sintered products, the enhanced sintering of the coarser powder and corrosion resistance of coarse water-atomized MIM 316L and so on. A large amount of experimental and theoretical analyses were conducted by optical microscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and differential thermal analysis (DTA).The feasibility of the process of Injection Molding larger parts using coarse powders with mean particle size of 65.71 μ m,26.69 μ m,19.25 μ m was studied. The feedstocks made of the powder and a wax-based binder with powder loading 55vol% was injection molded. It proved that coarse powders can be injection molded successfully by optimizing molding parameters via flowability experiment on Archimedes spiral channel mould using orthogonal design methold. And the mechanical properties of 26.69 μ m, 19.25 μ m powders can reach the American MPIF standards after sintering in Argon for one hour. The properties of the 19.25 u m powder with σb=549MPa, σ0.2=210MPa, Φ=60.5%, 59.1HRB can meet the demanding of MIM316L structure well.Several height larger parts with 32 mm cylinder diameter was shaped by using 26.69 μ m and 19.25 μ m coarser powders. According to the theory of debinding critical thickness, three different thermal debinding routes were performed. The result shows the larger the particlesize, the larger parts can be more easily and better deboud : the maximum thickness of the debound parts is 20 mm for 26.69 u m and 15 mm for 19.26 u m under the same condition. It strongly proved that coarser powder can be use to resolve the problem of debinding barrier of large parts and high cost.Seven different powders with mean particle size of 65.71 > 26.69> 19.25, 9.95, 9.57> 7.84> 5.94 u m was metal injection molded to study the effect of particle size on densification and mechanical properties and its corrosion resistance. All the samples were sintered at 1350°C, 1360°C, 1375°C for one hour. It shows that the sintering temperature influences the densification and mechanical property and its corrosion resistance of coarser powders conspicuously and the finer powder show more densification and can get higher comprehensive properties than coarser powders. The mechanical properties of the six powders have reached the MPIF standard except water-atomized 65.71 u m powder. The 26.69 V m and 19.25 u m powders after sintering at 1360 °C has the best combination property of the mechanical property and corrosion resistance and the others at 1375 °C for one hour can get the best properties.Theoretical analyse and prediction of bimodal sintering model for improving the sintering property of coarse powder were performed. The 65.71 u m coarser powders was metal injection molding with 55 volume powder loading by adding 25 weight percent fine powder of PF-20R, the properties was greatly improved to 55 percent increase in tensile stress, 58.6 percent in yield stress, 70.3 percent in elongation than before at the cost of 22 percent increase, and with 50 percent increase in the elongationt than the finer powder of PF-20R. The experiment result was in good accordance with the theoretical model. So adding fine powders to coarser powders has been proved to be one feasible and economical way of enhancing sintering of coarser powders used by MIM large parts. |
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