A powder metallurgy method for manufacturing Ti-6Al-4V with wrought-like microstructures and mechanical properties via hydrogen sintering and phase transformation (HSPT)

Abstract

a b s t r a c tIn this paper, the feasibility of producing Ti-6Al-4V with a range of wrought-like microstructures and mechanical properties via a novel powder metallurgy (PM) process with simple heat treatments is investigated. The wrought-like PM Ti-6Al-4V is obtained by sintering blended elemental (BE) powder in a hydrogen atmosphere, followed by conventional Ti-6Al-4V heat treating profiles. This is achieved without resorting to energy-intensive thermomechanical processing (TMP) that is compulsory for producing high performance titanium alloys via wrought processing or traditional powder metallurgy.Currently, wrought processing is the state of the art for producing Ti alloys with microstructures and mechanical properties necessary for critical applications. However, wrought Ti is prohibitively expensive for use in most commercial applications, due to the multi-step, energy-intensive, thermomechanical processing (TMP) routes used. Additionally, many Ti components have high buy-to-fly ratios. Because of this, powder metallurgy (PM) has long been sought as a means to reduce the cost of Ti alloys, owing to its near-net-shape (NNS) capabilities [1]. However, low-cost PM Ti typically has relatively poor mechanical properties. Additional processes can be incorporated into traditional PM to improve mechanical properties, such as pre-alloying the powder, pressure-assisted sintering, and thermomechanical work. However, these processes significantly increase cost, thereby offsetting the economic benefits of using PM [2]. Therefore, the key for any successful alternative to wrought processing is to maximize the performance-to-cost ratio of Ti alloys by producing wrought-like microstructures and mechanical properties using low-energy materials and processing routes.Specifically to PM Ti, the key to achieve mechanical properties that can match that of wrought Ti while maintaining the low-cost advantage is to find a way to produce wrought-like microstructures without using pressure-assisted sintering processes or relying on TMP, which is energy-intensive and incompatible with the NNS nature of PM. This objective can be achieved only by using low-cost powder, blended elemental alloying, near-net-shape compaction, and pressure-less sintering processes that can deliver the desired microstructures in the as-sintered or heat-treated state. Achieving this goal would be a breakthrough that could transform the Ti industry.Hydrogen sintering and phase transformation (HSPT) is a new, low-cost, blended elemental (BE), press and sinter process for producing titanium alloys from hydrogenated Ti powder. The current authors have previously demonstrated the ability of HSPT to produce Ti-6Al-4V with over 99% relative density, an ultrafine grain (UFG) microstructure, and extraordinary mechanical properties in the as-sintered state [3][4][5][6][7]. Using hydrogenated Ti powder has been shown in the literature to have several benefits as a PM Ti feedstock, including improved densification behaviour [8]. However, the key innovation of HSPT is th...

Powder metallurgy of titanium – past, present, and future

“…Since 2011, a process called HSPT has been developed by Fang, Sun, and Paramore et al [4,172,174,[202][203][204][205][206][207][208][209][210][211][212][213] HSPT is a multi-step pressureless sintering process, in which Ti hydride or partially hydrogenated Ti powder is blended with master-alloy or elemental powders and sintered under a dynamically controlled hydrogen atmosphere. After sintering, residual hydrogen is removed by annealing under vacuum or inert gas ( Figure 21).…”

“…Conventional cold compaction and pressureless sintering ('press and sinter') . Only low-cost and near-net-shape compatible processing steps (no TMP such as forging and extruding) Figure 22 shows the microstructure of Ti-6Al-4V produced via HSPT with and without subsequent heat treatments, as detailed in the literature [208]. An SEM micrograph of the as-sintered microstructure produced via HSPT is shown in Figure 22(a), while Figure 22(b) is an optical micrograph of the same material.…”

“…Owing to the microstructures, HSPT Ti-6Al-4V has been shown to have wrought-like mechanical properties as well [174,208,213]. As-sintered UFG HSPT Ti-6Al-4V typically has strength exceeding 1 GPa and ductility exceeding 15%EL.…”

“…As mentioned in section 'Sintering of TiH 2 in H 2 ', the as-sintered ultrafine grain microstructure produced by the HSPT process may be further modified via simple heat treatments to produce globularised and bi-modal microstructures [208,213]. The samples with microstructures that result from these heat treatments were tested for fatigue performance by Paramore et al [174,213].…”

Metallurgical and Machinability Characteristics of Wrought and Selective Laser Melted Ti-6Al-4V

“…However wrought components are expensive for use in most applications, due to the multistep, energy intensive, thermomechanical processing routes used. Because of this, powder metallurgy and additive manufacturing have long been sought as a means to reduce production costs of Ti alloys, owing to additive manufacturing technologies near net shape capabilities and other advantageous features [2]. Additive manufacturing has gained attention among researchers and manufacturing industries because of its advantageous capabilities such as freedom of design, on-demand manufacturing, reduced raw material wastage, and low energy consumption [3].…”