Powder metallurgical titanium alloys (TiNi and Ti-6Al-4V): injection moulding, press-and-sinter, and hot pressing

Abstract

Titanium and its alloys are materials of great interest because of their unique combination of low density, high specific strength, biocompatibility, and excellent corrosion resistance. However, the high production cost of titanium components limits their applications. Powder metallurgy (PM), especially near-net-shape technologies, offers a promise to reduce the manufacturing cost of titanium products. Among PM manufacturing techniques, metal injection moulding (MIM) has the ability to produce articles with complicated geometry; conventional press-and-sinter is a simple, cost-effective and most common powder metallurgical route; and hot pressing is a fast process in which elevated temperature and compressive stress are simultaneously applied to the powders, leading to the fully dense resultant product. Nevertheless, the titanium and its alloys MIM (Ti-MIM) technology has not seen significant breakthroughs because these materials are easily contaminated by the binders during debinding and sintering. The press-and-sinter routes are able to achieve > 97 % of theoretical density in Ti-6Al-4V alloys; however, when applied to other titanium alloys, for example Ti-Ni alloys, the commonly obtainable sintered density is low, with the presence of unwanted secondary intermetallic phases. Hot pressing, despite its ability of achieving full density, requires relatively complicated equipment, which increases the cost and energy. This research explores the above three PM techniques with emphasis on Ti-MIM and press-and-sinter. In selecting titanium alloys, a special interest is in TiNi – a typical shape memory alloy (SMA) because of its exceptional properties. Due to the increasing interest in using titanium hydride (TiH2) powder in recent years, this project extensively investigated the use of TiH2. The binder development and understanding how TiH2 affects sintering are our major research focuses. In this project, we are interested in fabrication and evaluation of porous TiNi alloys. Only does Chapter 10 deal with full densification by hot pressing. All these chapters together are to provide a big picture to the current powder metallurgical research in titanium and its alloys. X-ray diffraction (XRD), scanning electron microscope (SEM) either ex situ or in situ, thermal analysis and mechanical tests have been extensively used to investigate the three PM techniques with an attempt of establishing relationship among processing parameters, microstructure and mechanical properties. The main findings can be summarised below. (1) A water-soluble polyethylene glycol (PEG) based binder system was developed for formulating Ti-6Al-4V and pre-alloyed TiNi feedstocks. In relation to the PEG debinding mechanism, the work presented in this thesis corrected a misunderstanding commonly reported in the literature. A mathematical model to elucidate the water debinding of PEG was proposed and in this new debinding model, PEG does not form a gel. (2) Porous TiNi alloys were compared using different MIM feedstocks. The feedstocks were formulated either with the PEG-based or an agar-based binder system and elemental powders. Two batches of powder mixture were prepared and compared: Ni/Ti and Ni/TiH2. After being sintered at 1100 °C for 2 h, the TiNi samples demonstrated an average open porosity ca. 40 %. The as-sintered TiNi samples revealed multiple phases: B2 TiNi phase along with B19‟ TiNi, Ti-rich, Ni-rich phases, oxides and carbides. The effect of TiH2 powder and binder on the pore characteristics and the resultant mechanical properties are discussed. It is found that the use of TiH2 powder in the feedstock not only promotes densification but also enhances chemical homogenisation in the sintered samples. The sintered samples made from the Ni/TiH2 feedstock exhibited a lower porosity, smaller pore size and higher fracture strength, as compared to those made from the Ni/Ti feedstock. The sintered porous TiNi alloys made from the agar-based feedstock demonstrated a one-way shape memory effect. (3) The investigation was made into effect of backbone polymers in the binder system by comparing a newly developed polymer „Q‟ with polymethyl methacrylate (PMMA). It is found that the decomposition temperature of Q is almost 100 °C lower than that of PMMA, a significant achievement in Ti-MIM. The resulting oxygen pick-up in the sintered Ti-6Al-4V alloys made from the Q-feedstock was significantly lower than that from the PMMA-feedstock. As expected, better ductility was observed in the sintered Ti-6Al-4V alloys prepared from the Q-feedstock compared with the PMMA-feedstock. (4) Porous TiNi alloys were also fabricated by a traditional press-and-sinter process. Again two batches of powder mixture were prepared and compared: Ni/Ti and Ni/TiH2. The powder mixtures were pressed under different compaction pressures and subsequently sintered in vacuum at three different temperatures (i.e., 1000, 1100 and 1200 C) for 2 h. A post-sintering treatment was carried out at 1000 C for 6 h. An open porosity from 10.2 % to 33.8 % was observed with the largest pore size ranging from 3.5 to 27.4 μm. In comparison with the Ni/Ti sintered samples, the samples sintered from the Ni/TiH2 mixture exhibited a higher porosity, smaller pore size, higher fracture strength, smaller cyclic residual strain, higher recovery pseudoelastic strain and lower secant modulus. (5) To minimise the effect of particle size of TiH2 powder on densification, Ti and TiH2 with similar particle size were used in the press-and-sinter route. An in situ observation of TiNi powder sintering from TiH2 powder was presented, for the first time, using an environmental scanning electron microscope (ESEM). It is found that hydrogen release during dehydrogenation significantly affected the sintering behaviour and resultant microstructure. In comparison to the blended Ni/Ti powders, dehydrogenation occurring in the Ni/TiH2 blend led to a higher porosity, less densification and a lower degree of chemical homogenisation after being sintered at relatively low temperatures, e.g., 900 °C. However, with increasing the sintering temperature (i.e., over 942 °C) the sintered Ni/TiH2 blend showed better chemical homogenisation, although it exhibited even a higher porosity, lower densification, larger pore size, lower fracture strength, lower close-to-overall porosity ratio and lower Young‟s modulus as compared with the Ni/Ti sintered sample. (6) A fully dense TiNi alloy was produced by hot pressing of elemental powder mixture (Ni/Ti) with two different compositions at 1200 °C for 2 h under 25 MPa flowing argon. Despite full sintered density being achieved, the hot-pressed TiNi alloys still attained multiple phases: B2 TiNi phase along with B19‟ TiNi, NiTi2 and Ni3Ti phases. Post sintering at 1000 °C for 6 h did not result in a single B2 TiNi phase. Even worse, the post-sintering treatment led to a higher porosity and inferior mechanical property compared with the hot-pressed samples without the post-sintering treatment. In summary, powder metallurgical TiNi and Ti-6Al-4V alloys have been investigated in this study, with focus on three specific techniques – MIM, press-and-sinter and hot pressing. Porous TiNi alloys are readily obtained with the feasibility of controlling porosity. However, it was difficult, if not impossible, to obtain single B2 phase TiNi SMA through powder metallurgical routes. The binder developed in this study has a potential to be transferred to industry. This project also opens some more questions for future investigations.