Effect of α + β region sintering on the mechanical properties of injection molded Ti-6Al-4V compacts

Abstract

IntroductionThe titanium alloys have superior advantages such as high specific strength, high corrosion resistance and heat resistance. They are used in medical, aerospace and other fields (Niinomi, 2001). However, their poor workability involves the high cost by removal processing. According to the powder metallurgy, titanium alloy parts can be produced in the near net shape, which is effective in reducing the processing cost.Among many types of powder metallurgy, metal injection molding (MIM) process is an advantageous technique for the production of complex shaped parts. In addition, it is possible to manufacture metal parts with higher mechanical properties than that of the press sintered material because of high density using fine powder. Injection molded Ti-6Al-4V alloy compacts have been studied extensively for applications of MIM process (Noda et al., 2011)(Ferri et al., 2009)(Ferri et al., 2010 . Generally, injection molded Ti-6Al-4V compacts show sufficient static mechanical properties (tensile strength and ductility) comparable to that of the wrought material. The fatigue strength, however, is significantly lower than that of wrought material. One of the reasons for low endurance is the existence of residual pores, but the principal factor is the coarse lamellar microstructure in normal sintered compacts. The microstructures of the wrought material and the MIM sintered compacts are shown in Fig.1, respectively. The equiaxed fine microstructure of the wrought material is produced by hot plastic working in the α + β temperature region, which brings about high strength-elongation balance (Matthew and Donachie, 1988) and high fatigue strength. On the other hand, so-called lamellar microstructure, in which α phase is precipitated in a layered manner in β phase, was observed in Effect of α + β region sintering on the mechanical properties of injection molded Ti-6Al-4V compacts ， Kentaro KUDO* Kazunari SHINAGAWA** and Hideshi MIURA*** AbstractInjection molded Ti-6Al-4V compacts have normally a coarse lamellar microstructure, because it is sintered above the  transus temperature and it is cooled slowly in furnace. However, static mechanical properties such as tensile strength and ductility are equivalent to those of wrought materials, but they have the disadvantage that the fatigue strength is significantly lower than wrought materials. Therefore, improvement of fatigue strength is needed for injection molded Ti alloy compacts. In this study, injection molded Ti-6Al-4V compacts having equiaxed microstructure were prepared by sintering at α + β region temperature which is very lower than normal sintering temperature. When sintering is carried out in α + β region, sinterability is remarkably poor. Therefore, long-time sintering is required. Since the grain growth of the prior β grains was suppressed by the pinning effect of α grains and pores, it was possible to obtain the significantly fine grains as compared with the β region sintered compacts. The α + β region sintered compacts showed high strength, high e...

A Review on Material Extrusion Additive Manufacturing of Metal and How It Compares with Metal Injection Moulding

“…The backbone polymer is a higher molecular weight polymer with usually up to 50 vol.% in the binder system. Several grades of polymer can be utilised, as presented in Table 2, such as PA [158], PP [159], PE [161] and EVA [163,164]. The role of the backbone polymer is to provide the strength and to maintain the shape of parts after debinding before sintering.…”

“…The type and fraction of the binders can also be varied depending on the required properties. The acrylic resin and cyclohexyl methacrylate (CHMA) [159] or Atactic Polypropylene (APP) polymers [163] can be used as a part of the binder. The desirable properties of the binder are having a low softening temperature, rapid solidification, good adhesion with powder, non-reactive with the metal powder, low viscosity at the moulding temperature, easy to be debound, having high strength and stiffness at room temperature and, last but not least, safe and environmentally acceptable [149,166,167].…”

“…It is noted that Ti-6Al-4V alloy is typically sintered in the β phase region. Interestingly, low sintering temperature with an extended sintering time can also be applied to Ti-6Al-4V in MIM (as presented in Table 5 as 980 • C up to 96 h) to refine grain size [157,159,161,163]. This low sintering temperature within the α + β phase region for an extended time was proven to increase fatigue strength of MIM Ti-6Al-4V.…”

“…The higher sintering temperature and longer sintering time provide the highest relative sintered density [149]. The effect of sintering conditions on the relative sintered density of 17-4PH [8,49,64,92,110], 316L [9,41,45,52,53,56,57,66,68,70,72,75,81,98,108] and Ti-6Al-4V [78,79] alloys fabricated by metal MEX, MIM [134,157,[159][160][161]163,164,170,171] and the corresponding MIM international standards [169,172] are graphically compared in Figures 19-21, respectively. Figure 19 shows that most relative sintered densities of MEX 17-4PH alloy, which are in the range of 97% to 99%, meet the minimum value of the MPIF standard 35 for MIM materials (>96.7%).…”

“…Mechanical properties beyond tensile testing still need to be extensively studied, such as bending, compression and especially dynamic properties. The remaining porosity and the large grain size due to the nature of the printing and sintering processes are known to affect the dynamic properties [163,170,[203][204][205]. A very limited number of studies have examined the fatigue properties of metal MEX.…”