James D. Paramore

Powder metallurgy of titanium – past, present, and future

ABSTRACT Powder metallurgy (PM) of titanium is a potentially cost-effective alternative to conventional wrought titanium. This article examines both traditional and emerging technologies, including the production of powder, and the sintering, microstructure, and mechanical properties of PM Ti. The production methods of powder are classified into two categories: (1) powder that is produced as the product of extractive metallurgy processes, and (2) powder that is made from Ti sponge, ingot, mill products, or scrap. A new hydrogen-assisted magnesium reduction (HAMR) process is also discussed. The mechanical properties of Ti-6Al-4V produced using various PM processes are analyzed based on their dependence on unique microstructural features, oxygen content, porosity, and grain size. In particular, the fatigue properties of PM Ti-6Al-4V are examined as functions of microstructure. A hydrogen-enabled approach for microstructural engineering that can be used to produce PM Ti with wrought-like microstructure and properties is also presented. Abbreviations: AM: additive manufacturing; ARC: Albany Research Center; BE: blended elemental; BUS: broken-up structure; CCGA: close-coupled gas atomisation; CHIP: CIP-sinter-HIP; CIP: cold isostatic pressing; CP-Ti: commercially pure Ti; DRTS: direct reduction of Ti-slag; CSIR: Council for Scientific and Industrial Research (South Africa); CSIRO: Commonwealth Scientific and Industrial Research Organization (Australia); EIGA: electrode induction gas atomisation; EMR: electronically mediated reduction; FFC: Fray, Farthing, and Chen; GA: gas atomisation; GIF: gaseous isostatic forging; GSD: granulation-sintering-deoxygenation; HAMR: hydrogen-assisted magnesium reduction; HDH: hydride–dehydride; HIP: hot isostatic pressing; HSPT: hydrogen sintering and phase transformation; MA: master alloy; MER: Materials & Electrochemical Research Corporation (US); MHR: metal hydride reduction; MIM: metal injection molding; OM: optical microscope; OS: Ono and Suzuki; PA: pre-alloyed; P/C: performance to cost ratio; PIF: pneumatic isostatic forging; PM: powder metallurgy; PREP: plasma rotating electrode process; PP: post-processing; PS: press and sinter; QIT: Quebec Iron & Titane, Inc. (Canada); SEM: scanning electron microscope; SPS: spark plasma sintering; SOM: solid oxide membrane; THP: thermohydrogen processing; TMP: thermomechanical processing; UFG: ultrafine grain; UGS: upgraded titanium slag; UTS: ultimate tensile strength; USTB: University of Science and Technology Beijing (China); VA: vacuum atomisation; VHP: vacuum hot pressing; WP: wrought process; YS: yield strength

Hydrogen-enabled microstructure and fatigue strength engineering of titanium alloys

Traditionally, titanium alloys with satisfactory mechanical properties can only be produced via energy-intensive and costly wrought processes, while titanium alloys produced using low-cost powder metallurgy methods consistently result in inferior mechanical properties, especially low fatigue strength. Herein, we demonstrate a new microstructural engineering approach for producing low-cost titanium alloys with exceptional fatigue strength via the hydrogen sintering and phase transformation (HSPT) process. The high fatigue strength presented in this work is achieved by creating wrought-like microstructures without resorting to wrought processing. This is accomplished by generating an ultrafine-grained as-sintered microstructure through hydrogen-enabled phase transformations, facilitating the subsequent creation of fatigue-resistant microstructures via simple heat treatments. The exceptional strength, ductility, and fatigue performance reported in this paper are a breakthrough in the field of low-cost titanium processing.

Hydrogen sintering of titanium and its alloys

Hydrogen sintering and phase transformation (HSPT) is a press and sinter process used to produce titanium alloys with engineered microstructures in the as-sintered state. During HSPT, titanium alloys are sintered under a dynamically controlled hydrogen partial pressure. HSPT utilizes phase transformations in the Ti–H system to refine the microstructure during sintering. Therefore, this process is capable of producing titanium alloys with strength and ductility in the as-sintered state that exceed ASTM standards for wrought titanium. HSPT circumvents the energy-intensive thermomechanical work that is compulsory for other titanium production technologies without sacrificing mechanical properties. Therefore, the performance-to-cost ratio of titanium alloys produced by this method should be greatly increased versus wrought processing and traditional PM. It has been projected that HSPT offers about an 80% energy savings per ton of titanium alloy produced versus wrought processing.

Mechanical Behavior of Additively Manufactured Ti-6Al-4V Following a New Heat Treatment

Hydrogen sintering and phase transformation is a Ti-6Al-4V heat treatment process capable of normalizing the microstructure of bulk parts. The mechanical properties of these processed parts are comparable to that of wrought Ti-6Al-4V, which makes it attractive for use in manufacturing areas where the resulting microstructure is difficult to control, like powder metallurgy and additive manufacturing. To investigate the application space of this heat treatment, Ti-6Al-4V parts were printed from three different additive methods (DMLS, EBM, and cold spray) and their tensile properties were evaluated in both the as-printed and heat treated states. Due to the size of the printed parts, millimeter scale tensile specimens were used and care must be taken to ensure the reduced sample size still produces reliable results. Preliminary tension tests show that the heat treatment process normalizes the microstructure, closes porosity, and improves ductility.

Powder Metallurgy Ti-6Al-4V Alloy with Wrought-Like Microstructure and Mechanical Properties by Hydrogen Sintering

Hydrogen sintering phase transformation (HSPT) is a low-cost, blended elemental, press and sinter powder metallurgy process. During HSPT, compacts of TiH2 powder are sintered in dynamically controlled partial pressures of hydrogen followed by a vacuum anneal (dehydrogenation). The use of hydrogen in the sintering atmosphere allows phase transformations in the Ti – H system to create an ultra-fine lamellar microstructure in the as-sintered state with mechanical properties that exceed ASTM standards. Additionally, the fine lamellar structure allows for secondary heat treatments to produce wrought-like microstructures. The removal of hydrogen in the dehydrogenation step is critical to prevent hydrogen embrittlement. The kinetics of dehydrogenation are discussed, in which a model for the concentration profile and an empirical equation for maximum hydrogen concentration as a function of time and size are developed.