Microstructures and Mechanical Properties of Hybrid, Additively Manufactured Ti6Al4V after Thermomechanical Processing

In the present study, we propose a hybrid manufacturing route to produce high-quality Ti6Al4V parts, combining additive powder laser directed energy deposition (L-DED) for manufacturing of preforms, with subsequent hot forging as a thermomechanical processing (TMP) step. After L-DED, the material was hot formed at two different temperatures (930 °C and 1070 °C) and subsequently heat-treated for stress relief annealing. Tensile tests were performed on small sub-samples, taking into account different sample orientations with respect to the L-DED build direction and resulting in very good tensile strengths and ductility properties, similar or superior to the forged material. The resulting microstructure consists of very fine grained, partially globularized alpha grains, with a mean diameter ~0.8–2.3 µm, within a beta phase matrix, constituting between 2 and 9% of the sample. After forging in the sub-beta transus temperature range, the typical L-DED microstructure was no longer discernible and the anisotropy in tensile properties, common in additive manufacturing (AM), was significantly reduced. However, forging in the super-beta transus temperature range resulted in remaining anisotropies in the mechanical properties as well as an inferior tensile strength and ductility of the material. It was shown, that by combining L-DED with thermomechanical processing in the sub-beta transus temperature range of Ti6Al4V, a suitable microstructure and desirable mechanical properties for many applications can be obtained, with the advantage of reducing the material waste.

Microstructural Study of Newly Designed Ti-6Al-1Fe Alloy through Deformation

Recently, iron (Fe) is introduced to substitute vanadium (V) in Ti-alloy. Therefore, new (α+β) titanium alloy, Ti-6Al-1Fe was designed through a complete replacement of V by Fe with major composition modifications of Ti-6Al-4V. This new alloy is believed could provide similar properties of Ti-6Al-4V through modification of its microstructures. Different heat treatments can lead to a diversity of microstructural permutations and combinations. Thus, it is very crucial to study in-depth understanding about the microstructure of Ti-6Al-1Fe. Results reveal that the microstructure of as-received alloy is a typical fine lamellar microstructure. The bi-modal microstructure can be obtained by hot rolling below beta-transus temperature (Tβ) followed by recrystallization treatment at 925°C. While cold rolling followed by recrystallization treatment at 925°C produce equiaxed microstructure.

Effect of Alloying Elements on Plastic Workability and Corrosion Behavior of Ti-X (X = 6 Co, 8 Cr, 4 Fe, 6 Mn, 10 Mo, and 36 Nb) Binary Alloys

The purpose of this study was to investigate the effect of the alloying elements on the plastic workability and corrosion behavior of Ti-X (wt.%) (X = 6 Co, 8 Cr, 4 Fe, 6 Mn, 10 Mo, and 36 Nb) binary alloys. The alloys with a molybdenum equivalence of 10 wt.% were fabricated by a vacuum arc re-melting process and were then homogenized at a temperature 20°C greater than the beta transus temperature for 14.4 ks. The plastic workability was investigated under uniaxial cold rolling, while the corrosion behavior was examined in Ringer’s solution at 37°C. Among the Ti-X alloys, the Ti-8 wt.% Cr and Ti-6 wt.% Mn alloys showed an outstanding plastic workability and corrosion resistance, respectively.

Kinetics of grain growth in Ti-2.7Al-5.7Fe-6Mo-6V alloy

The metastable (?Ti) alloy Ti-2.7Al-5.7Fe-6Mo-6V (wt%) was produced by semi-centrifugal casting of blended elemental powders. The phases were identified by X-ray diffraction (XRD), and overall composition was measured by X-ray fluorescence (XRF). The beta transus was determined by differential thermal analysis (DTA) and optical microscopy. The cast alloys were annealed at different temperatures under argon, up to 900oC, where they were in the solution-treated state, and the solution-treated alloys were aged between 400oC and 600oC. The kinetics of grain growth during heat treatment of the as-cast and solution-treated alloys was investigated by metallography, using the grain intercept method. Grain growth depended on whether the matrix was (?Ti) or (?Ti), and on the competing precipitate dissolution, or nucleation and growth processes. The as-cast alloy had a mean grain size of 19 ? 7?m, which increased to 63 ? 21?m after heat treating at 500?C for 2h. The alloy was duplex between 590?C and 800?C, and completely (?Ti) above 800?C. After solution treatment, the mean grain size was 40 ? 16 ?m, which was smaller than at the lower heat treatment temperatures. Following solution treatment, the mean grain size increased with increasing ageing temperature, up to 66 ? 22?m after 2h at 600?C. The growth exponents were lower than the 0.5 for normal grain growth in both cases, and there was an incubation period at 300?C and 400?C when the alloy was not solution-treated. Minimal grain growth was observed close to the beta transus.

The Effect of Processing Method on Microstructure and Mechanical Behavior of Ti-6Al-4V Plate Produced by Powder Metallurgy Technique

Three Ti-6Al-4V plate materials produced by powder metallurgy technique, included pre-alloyed hydride-dehydride (HDH) plate rolled to 75% reduction in thickness, and two blended elemental (BE) powder plates rolled to 75% and 87% reduction were evaluated. The objective of this study was to determine differences in microstructure and toughness between the pre-alloyed HDH and BE Ti-6Al-4V materials processed to the same product form. Heat treatments were performed below the beta transus temperature at 982, 871, 760, and 732°C (1800, 1600, 1400, and 1350°F) for 1, 2, and 4 hours in order to determine differences in heat treating response, and above the beta transus at 1076°C (1970°F) to determine the transformation temperature. The samples were evaluated by optical microscopy and scanning electron microscopy. Charpy impact testing was performed in order to determine differences in the energy absorbed during fracture. Pole figures (0002) of selected conditions were also performed in order to determine any differences in texture between the various conditions.

Contribution of Hydrogen Promoted Aluminium Partitioning to Tempered ά Martensite Embrittlement in Ti-6Al-4V

A thermohydrogen process promoting metastable phase decomposition (THP-MD) treatment was performed on wrought Ti-6Al-4V to determine the effects of microstructure evolution on tensile ductility. Tensile ductility was affected by the nature of phase and morphology evolution in which dissolved hydrogen played a key role. Hydrogen reduced the beta transus and stabilised more beta phase at aging/tempering temperature. A reduced beta transus in a similar heat treatment resulted in a bimodal morphology (in non-hydrogenated samples) or a fully acicular morphology (in hydrogenated samples). It also reduced the volume fraction of alpha at aging/tempering temperature which resulted in the extensive enrichment of reduced alpha with aluminium (Al) during tempering. The increased Al content in the reduced alpha promoted ordering of the HCP lattice to the brittle titanium aluminide (Ti3Al) phase. In addition to Ti3Al embrittlement, the acicular morphology of Ti-6Al-4V tempered hexagonal martensite (ά) offers limited resistance to crack propagation. The highest degree of embrittlement was observed in prior hydrogenated samples because of the combined effect of the acicular morphology and Ti3Al embrittlement.