Hydride-dehydride Powders Research Articles

Fatigue behaviour of additive manufactured Nb-48Ti alloy parts from powders produced by Plasma Atomization (PA) and Hydride-Dehydride (HDH) process

Fatigue behaviour of bcc Nb-48 wt%Ti (Ti-36at%Nb) alloy parts processed by Laser Powder Bed Fusion (L-PBF) using as raw material two different types of powders, spherical powder produced by Plasma Atomization (PA) and irregularly shaped powder produced by Hydride-Dehydride (HDH) process, are compared. Laser power and scanning speed were selected to obtain rods with porosity below 0.5 vol%. All rods were produced using the same scanning strategy (chessboard with shift). The building platforms were heat treated for stress relieving at 650 °C for two hours before removing the rods using wire electro-discharge machining. The heat-traded rods were machined to obtain specimens for rotate bending fatigue tests, and S-N curves were plotted. Fatigue behaviour for PA and HDH sets were similar but with higher dispersed results for the specimens produced from the HDH powder. The fracture was nucleated in porosities near the machined sample surface, mostly on lack-of-fusion type pores. The fatigue cracks propagated in a “zigzag” mode with peaks and valleys parallel to the L-PBF building direction (BD) in both sets. One of the analysed samples, produced from HDH powder and with a lower fatigue life than average, showed a significantly higher number of pores, indicating the existence of a laser process parameter yet to be controlled.

Microstructure and corrosion behavior of differently heat-treated Ti-6Al-4V alloy processed by laser powder bed fusion of hydride-dehydride powder

This study investigates the use of hydride-dehydride non-spherical Ti-6Al-4V powders in laser powder bed fusion process and the effects of post-heat-treatments on additively manufactured parts. As-built parts show anisotropic microstructure with α′ martensite and some β phases. Post heat-treated parts exhibit α + β phases, with characteristics dependent on the heat treatment. Heat treatment below β-transus leads to homogenized grain structures with improved corrosion resistance. Electrochemical analysis reveals a very stable corrosion rate due to faster formation of a protective passive layer aided by the fine-structured β phase. X-ray photoelectron spectroscopy examines corrosion behavior and film growth mechanism in saline water.

CIP-FAST: assessing the production of complex geometry titanium components from powders by combining Cold Isostatic Pressing (CIP) and Field Assisted Sintering Technology (FAST)

ABSTRACT A novel, two-step, solid-state method to produce complex geometry titanium parts was investigated by combining Cold Isostatic Pressing (CIP) with Field Assisted Sintering Technology (FAST). Hydride-dehydride powders of commercially pure titanium and Ti-6Al-4V were CIP’ed into shaped compacts using silicone moulds, then further consolidated using FAST, with ZrO2 powder as a secondary pressing media. The final parts retained the complex features from the CIP moulds but were compressed in the pressing axis. Densities >99% were achieved, with optimised FAST processing parameters required for the different alloys. High hardness and fine equiaxed microstructures were observed at the edges of the parts, suggesting oxygen transfer from the ZrO2 pressing media had occurred, with more investigation needed to better understand and prevent this. Despite this, the CIP-FAST process route has been demonstrated to be a fast, low-cost and material-efficient option to produce a wide variety of complex titanium parts.

Enhanced mechanical properties by laser powder bed fusion using cost-effective hydride-dehydride titanium powders

The high cost of powder feedstock has emerged as a critical issue in additive manufacturing of titanium (Ti). In this work, pure Ti parts were successfully produced via laser powder bed fusion (LPBF) by employing cost-effective hydride-dehydride (HDH) powders modified through gas-soild fluidization. The microstructures and tensile properties were comprehensively investigated as a function of scanning speed. The pure Ti parts made under the optimized processing parameter exhibited an ultimate tensile strength of 826.1 MPa with a fracture strain of 22.3% at room temperature, superior to those made by the expensive atomized powders. The fatigue performance of the LPBF-made pure Ti using the HDH powders is comparable to those made by rolling. The excellent mechanical performance can be attributed to the effects of grain refining, texture evolution, dislocation recovery, and cyclic softening by dislocation cells and tangles generated. This work provides an effectively low-cost route to fabricate pure Ti by LPBF with excellent mechanical properties, which also serves as a useful guidance on the processing window setup using the HDH powder.

Fatigue performance of laser powder bed fusion hydride-dehydride Ti-6Al-4V powder

Hydride-dehydride (HDH) Ti-6Al-4V alloy with particle size distribution of 50–120 µm is laser powder bed fusion ( L -PBF) processed using optimum processing parameters and a near-fully dense structure with a density of 99.9 % is achieved. Microstructural observations and phase analyses indicate formation of columnar β grains with acicular α/α′ phases in as-built condition. The roughness of the as-fabricated samples is significant with an average roughness of R a = 15.71 ± 3.96 µm and a root mean square roughness of R rms = 108.4 ± 24.9 µm, however, both values are reduced to R a = 0.19 ± 0.04 µm and R rms = 4.9 ± 0.6 µm after mechanical grinding. Mechanical tests are carried out on as-fabricated specimens followed by stress relief treatment. All samples are tested to failure in fatigue, under fully-reversed tension-compression conditions of R = −1. The as-built samples failed from the surface with crack initiation mainly at micro-notches, whereas after mechanically grinding, crack initiation changed to subsurface defects such as pores. Minimizing surface roughness by mechanically grinding eliminates surface micro-notches which improves fatigue strength in the high cycle fatigue region. Fatigue notch factor calculations showed that the effect of surface roughness was significantly lower when HDH powder is used compared to standard spherical powder. X-ray diffraction analysis revealed an in-plane compressive stress, micro-strain and grain refinement on the surface of the mechanically ground samples. Fractography observations (macroscale) revealed a fully brittle fracture in the first stage of crack growth with a transition to a dominantly ductile fracture in the third stage of crack growth. On the other hand, at the micro scale, even the brittle fracture regions showed evidence of ductile fracture within the α′ martensite laths.

Laser-beam powder bed fusion of cost-effective non-spherical hydride-dehydride Ti-6Al-4V alloy

Hydride-dehydride (HDH) Ti-6Al-4V powders with non-spherical particle morphology are typically not used in laser-beam powder bed fusion (LB-PBF). Here, HDH powders with two size distributions of 50–120 µm (fine) and 75–175 µm (coarse) are compared for flowability, packing density, and resultant density of the LB-PBF manufactured parts. It is shown that a suitable laser power-velocity-hatch spacing combination can result in part production with a relative density of > 99.5% in LB-PBF of HDH Ti-6Al-4V powder. Size, morphology, and spatial distribution of pores are analyzed in 2D. The boundaries of the lack-of-fusion and keyhole porosity formation regimes are assessed and results showed parts with a relative density of > 99.5% could be LPBF processed at a build rate of 1.5–2 times of the nominal production rates in LPBF machines. The synchrotron x-ray high-speed imaging reveals the laser-powder interaction and potential porosity formation mechanism associated with HDH powder. It is found that lower powder packing density of coarse powder and high keyhole fluctuation result in higher fractions of porosity within builds during the LB-PBF process.

Laser powder bed fusion of a Nb-based refractory alloy: Microstructure and tensile properties

In this study, Nb521 (Nb–5W–2Mo–1Zr) parts were successfully fabricated by laser powder bed fusion (LPBF) using quasi-spherical hydride-dehydride (HDH) powders modified by jet milling. The relative density of the LPBF made (LPBFed) Nb521 parts is 98.8 ± 0.2%, which were subsequently subjected to hot isostatic pressing (HIP) to achieve almost full densification. Nanoscale ZrO2 precipitates were found in the LPBFed parts and those after HIP (HIPed), showing semi-coherent with the matrix. The precipitation behavior is strongly related to the melt pool solidification process during LPBF in terms of thermal distribution and solidification rate. The LPBFed Nb521 parts after HIP presented sound tensile properties with the fracture strength of 678.7 ± 1.1 MPa and elongation of 5.91 ± 0.32%. The precipitated nanoscale ZrO2 underwent plastic deformation after tension, exhibiting the stress-induced tetragonal-to-monoclinic phase transformation and shearing by dislocations. This work presents the LPBFed Nb521 alloy with promising performance by employing low-cost jet-milled HDH powders.

Study of printability and porosity formation in laser powder bed fusion built hydride-dehydride (HDH) Ti-6Al-4V

Laser powder bed fusion (L-PBF) is a dominant process in the fast-emerging additive manufacturing (AM) industry. Despite the many advantages that can be gained by adopting AM, sustained future growth in this industrial sector requires AM to overcome several barriers, most notably, the formation of in-part defects and the high production cost. Motivated by the growing interest of reducing feedstock cost, we studied the cost-efficient hydride-dehydride (HDH) Ti-6Al-4 V powder in L-PBF and show that the in-part porosity can be controlled to produce nearly fully dense (> 99.8% density) components by optimizing process parameters. The process map that was developed with the HDH powder offers a general guideline for the usage of non-spherical powder in powder bed AM. In this work, we investigated the size, morphology, and spatial distribution of the powder-induced porosity in 3D and quantified the interaction between the HDH powder layer and keyhole fluctuation in-situ by utilizing advanced synchrotron-based x-ray computed tomography and dynamic x-ray radiography. With the assistance of a Monte Carlo image-based analysis, we propose two porosity formation mechanisms and attribute them to the unique characteristics of the HDH powder bed, i.e., variable local packing and larger particle size.

The microstructure and mechanical properties of extra low interstitials (ELI) Ti-6Al-4V alloys manufactured from hydride–dehydride (HDH) powder

Ti alloys produced from HDH powder have suffered from high O/N interstitials content. In the present study, a whole well-protected process from Ti sponge to as-sintered materials is adopted, and thus the lowest O/N contents of as-sintered materials are 0.07 wt% and 0.018 wt%, within the specification of Grade F-23 (Ti-6Al-4V ELI) according to ASTM B988-13. The passivation law of powder and its compact has also been studied in detail. Subsequently, sintering experiment indicates fine ELI powder has an excellent sinterability. A relative density over 96% can be obtained by sintering at 1100 °C. Capsule-free HIP post-sintering is performed to achieve full density and obtain fine and near-equiaxed α grain at the same time. The as-HIPped sample has better comprehensive mechanical properties, with strength exceeding 970 MPa and elongation exceeding 16.4%.

Laser powder bed fusion parameters to produce high-density Ti–53%Nb alloy using irregularly shaped powder from hydride-dehydride (HDH) process

Laser powder bed fusion (LPBF) – also known as selective laser melting (SLM) – is a technology of additive manufacturing (AM) that offers benefits to the fabrication of implants. This approach can create customized and complex parts with low elastic modulus to reduce stress shielding. The use of irregularly shaped powder is not common due to its low flowability and low apparent density. However, its low cost arouses interest in the production of materials by this technology. This work discloses the processing window that allows the fabrication of Ti-53wt.%Nb alloy parts with high density using irregularly shaped powder from hydride–dehydride (HDH) process and analyzes the influence of the process parameters on the microstructure and hardness of the samples manufactured by LPBF. Energy densities (EV) from 16 to 317 J/mm3 were investigated. Experimental density measurements by the Archimedes' principle and pore area fraction were calculated and relating to the density estimated based on X-ray diffraction (XRD) results of the HDH powder. Vickers hardness showed strong correlation to the content of interstitial elements of samples made under different EV. This work proves that is possible to obtain samples with high density using HDH powder in LPBF and that the content of interstitial elements increased with the energy density, as well as the hardness of the alloy.

Use of Non-Spherical Hydride-Dehydride (HDH) Powder in Powder Bed Fusion Additive Manufacturing

There is a growing interest in using recycled materials and economically produced powder in additive manufacturing processes. State-of-the-art powder bed fusion additive manufacturing processes typically use spherical powder that are produced using an atomization process. However, using irregularly shaped Ti-6Al-4V powder from the Hydride-Dehydride (HDH) process is more economical because fewer processing steps are required and it can use recycled material as feedstock. In this work, the use of HDH powder in the electron beam additive manufacturing (EBAM) process is investigated. Deposition parameters for the HDH powder were developed, followed by a detailed study of as-built porosity and microstructure. The powder flow characteristics were also studied, and the as-built part porosity was compared to the parts built using spherical atomized powder. This work demonstrates the successful use of non-spherical HDH powder in the EBAM process.

FAST-forge of Titanium Alloy Swarf: A Solid-State Closed-Loop Recycling Approach for Aerospace Machining Waste

Titanium alloys have excellent properties, but components are very expensive due to the high levels of processing required, such as vacuum melting, multi-stage forging, and machining. As a result, forged titanium alloy components are largely exclusive to the aerospace industry, where a high strength-to-weight ratio, corrosion resistance, and excellent fatigue resistance are essential. However, a typical buy-to-fly ratio for such components is approximately 9:1, as much of the forged billet is machined to swarf. The quantity of waste titanium alloy swarf generated is increasing as aircraft orders, and the titanium components contained within them, are increasing. In this paper, waste swarf material has been recycled using the two-step solid-state FAST-forge process, which utilizes field assisted sintering technology (FAST) followed by hot forging. Cleaned Ti-6Al-4V swarf was fully consolidated using the FAST process at sub-transus and super-transus temperatures, followed by hot forging at sub-transus temperatures at different strain rates. It was demonstrated that swarf-derived Ti-6Al-4V FAST billets have equivalent hot forging flow behaviour and resultant microstructures when directly compared to equivalently processed conventional expensive hydride–dehydride powder, and previously reported Kroll-derived melt-wrought material. This demonstrates that titanium swarf is a good quality feedstock for downstream processing. Additionally, FAST-forge is a viable processing route for the closed-loop recycling of machining waste for next-generation components in vehicles and non-aerospace applications, which is game changing for the economics of titanium alloy components.

Influence of Powder Microstructure on the Microstructural Evolution of As-Sprayed and Heat Treated Cold-Sprayed Ti-6Al-4V Coatings

Cold spray repair of high-value titanium alloy components has gained considerable interest; however, the influence of deposition conditions on the resulting microstructure and properties is not well established. This work examines the influence of feedstock powder type on the microstructural evolution of cold spray-deposited Ti-6Al-4V powders following deposition and after low-temperature heat treatment. Plasma-atomized, gas-atomized, and hydride de-hydride Ti-6Al-4V powders were deposited on Ti-6Al-4V substrates using cold spray technology and subsequently annealed at 550 °C for 5 h. Powders and cold spray depositions were characterized using x-ray diffraction, optical microscopy, scanning electron microscopy, and electron backscatter diffraction. Atomized and hydride de-hydride powders were characterized by a martensitic alpha and equiaxed alpha microstructure, respectively. Phase analysis revealed hydride de-hydride powders to contain beta phase regions near alpha grain boundaries; however, beta phase was not observed in atomized powders. Atomized coatings retained their powder microstructure in particle interiors but demonstrated ultra-fine grain formation near particle boundaries, likely due to dynamic recrystallization. In contrast, hydride de-hydride powders showed a larger increase in microstrain after deposition, without ultra-fine grain formation. Heat treatment resulted in recovery and recrystallization for all coatings and, in the case of atomized coatings, resulted in beta phase precipitation in regions that experienced large plastic strains.

Microstructural investigation of porous titanium coatings, produced by thermal spraying techniques, using plasma atomization and hydride-dehydride powders, for orthopedic implants

The present study focuses on porous titanium (commercially pure Ti) coatings for future use on medical implants, manufactured by specific thermal spray method. Its objective is to investigate the possibility of Ti porosity to offer room for medication storage in addition to bone growth promotion. Three specimens for each different type of coating were produced by different sets of thermal spray process/feedstock powder, namely, (1) Flame Spray (FS) with spherical Plasma Atomization (PA) powder, (2) Atmospheric Plasma Spray (APS) with spherical PA powder and (3) APS with angular Hydride – Dehydride (HDH) powder. The specific types of specimens along with the two powders produced by the PA and HDH processes were characterized using various techniques, namely (a) Roughness measurements of the coated surfaces and thickness measurements of the coatings, which disclosed the highest values for the FS-PA coating, Ra = 11.19 ± 0.21 μm and average thickness of 281 μm, (b) Optical microscopy and SEM-EDS to study the surface morphology and microstructure of the coatings and discover any possible oxidation or other phases. The porosity percentage was measured and the pore size distribution was also estimated. The porosity percentages of the coatings ranged from 12% on the APS-PA coating to 34% on the FS-PA coating, (c) XRD analyses which showed varying titanium oxide formations for each type of specimen. These oxides may prove to be beneficial for the development of osseous tissue, (d) assessment of biological response of the coatings in vitro that resulted in satisfactory primary response in a physiological environment and the formation of calcium phosphate phases with Ca/P ratio of 1.61 and (e) Vickers microhardness measurements, which resulted in values that fluctuated from 786.6 for the FS-PA coating to 967.6 for the APS-HDH coating. Overall, the produced FS-PA coating is characterized by high porosity content and satisfactory bioactivity, in order to fulfill the objective of the project for biomolecule incorporation and drug storage.

Determination of roll compaction parameters required for high green density, defect free Ti-6Al-4V strips

Direct powder rolling (DPR) is an unconventional powder metallurgy technique for fabricating flat mill products. It offers significant cost reduction by using low processing temperatures and reduced fabrication steps compared to ingot metallurgy. Roll compaction is the first stage in the DPR process, hence proper selection of roll compaction parameters is vital for achieving high density defect free (no visible cracks, alligatoring or centre split) green compacts. Unsuitable roll compaction parameters can result in fragile low green density strips which could be difficult to handle during subsequent processing or high green density strips with defects. The focus of this study was therefore, to determine the range of roll compaction parameters required in producing high density, defect free strips. The Ti-6Al-4V strips studied were produced by mixing 150 μm titanium hydride de-hydride powder with a 45 μm 60Al40V master alloy. The powder mix was roll compacted at varying roll gaps and rolling speeds. The compacted strips were evaluated by measuring the green strip density and thickness. It was concluded that for the Ti-6Al-4V powder studied in this work, the preferred roll compaction parameters for achieving high density, defect-free green strips are 0.3 mm to 0.5 mm roll gap with roll speeds from 3 rpm to 10 rpm.

Ultrafine microstructures in eutectoid element bearing low-cost Ti-Fe alloys enabled by slow bainite formation

Powder metallurgy Ti-Fe alloys are of growing interest due to their low cost and good mechanical behaviour, which is comparable to that of the workhorse Ti-6Al-4V. Fe is normally added as alloying element to develop α+β alloys characterised by the typical lamellar structure. In this work, treating the alloy as metastable β-Ti, the slow bainitic transformation has been used to enable the formation of ultrafine α+β microstructures in a powder metallurgy Ti-5Fe alloy, achieving superior properties. The kinetics for the formation and decomposition of the phases was studied to achieve different microstructures. α-Ti is formed at temperatures as low as 400 °C in less than 24 h, while ω-Ti is stable up to 450 °C. In this case, the high oxygen typical of low cost PM alloys manufactured using hydride-dehydride powder made a positive contribution by stabilising α-Ti over ω-Ti in shorter times and at lower temperatures.

Influence of feedstock powder and cold spray processing parameters on microstructure and mechanical properties of Ti-6Al-4V cold spray depositions

A high pressure cold spray system was used to deposit three Ti-6Al-4V feedstock powders (i.e., hydride de-hydride, plasma atomized, and gas atomized) on Ti-6Al-4V substrates while varying gas temperature and nozzle length. Particle impact temperature and particle velocity were calculated using a 1-D axial model. The microstructure of the feedstock powders and the cold spray depositions were characterized via optical and scanning electron microscopy. The hardness of the as-received powders was determined using nanoindentation. To assess deposition quality, coatings were characterized in terms of porosity, microhardness, and adhesion strength. Results showed that hydride de-hydride powders were characterized by an equiaxed alpha grain structure with intergranular beta phase regions while atomized powders were characterized by martensitic α phase structures. Cold sprayed coatings revealed two distinct microstructures. Regions that experienced low/moderate plastic strain retained the as-received powder microstructure while regions that experienced significant plastic strain were characterized either by a featureless microstructure (atomized coatings) or the presence of fine, elongated beta precipitates (hydride de-hydride coatings). Depositions performed using a long nozzle resulted in the best deposition quality, with porosity as low as 0.3% and adhesion strengths >69MPa. While atomized powders resulted in comparatively higher quality coatings for all process conditions, hydride de-hydride coatings of excellent quality (average porosity≈0.6%, adhesion strength >65MPa) were achieved under optimal conditions. Thus, hydride de-hydride powders may hold promise as a cost effective alternative to atomized powders for Ti-6Al-4V cold spray depositions.

FAST-forge − A new cost-effective hybrid processing route for consolidating titanium powder into near net shape forged components

Reducing the high cost of titanium to a level where it can compete with currently used commodity metals offers opportunities to many industries to exploit its excellent combination of properties to improve performance or reduce weight. The key to decreasing cost is to reduce the number of processing steps to go from ore to component, as well as material wastage from excessive machining. This paper describes a new solid-state hybrid manufacturing route, termed by the authors as FAST-forge, for converting titanium alloy powder into components with wrought properties in two steps; utilising field assisted sintering technology (FAST) to produce a shaped preform billet that is finished to near net shape by a one-step precision hot forge. The route has been demonstrated at the laboratory scale using Ti-6Al–4V hydride-dehydride powder by producing fully consolidated, microstructurally homogeneous, double truncated cone specimens directly through FAST, which were then upset forged at a range of temperatures and strain rates. The microstructural evolution and forging behaviour of the Ti-6Al–4V after FAST consolidation is similar to conventional melt, multi-step forged product. Break up of primary α at high strains was observed at 950°C and 0.01s−1, 0.1s−1, and 1s−1. There is good agreement between finite element modelling of the hot forging and the experimental data, which will enable more complex shaped geometries to be produced via the proposed FAST-forge route in future. Such a route could be used to consolidate powder from a lower-cost alternative extraction method to become a disruptive technology that will enable a step-change in the economics of titanium components.

G0400305 機械的特性に及ぼすTiB強化Ti-6Al-4V合金のマトリックス粉末の粒径と形状の影響

It has been reported that the mechanical properties of titanium matrix composites depended on titanium particle shape and size. In this study, three types of Ti-6Al-4V matrix powders were used: hydride-dehydride (HDH) powders with particle sizes of <45 μm and <25 μm and gas-atomized (GA) powder with a particle diameter of 45 μm. TiB_2 powder with an average particle size of 1.81 μm was used as a reinforcement. TiB-reinforced Ti-6Al-4V alloys were prepared by SPS. Mechanical properties of composites composed of powders with different particle shape and sizes were investigated. As a result, the mechanical properties of the composites with HDH powders depended on the particle size. The TiB whiskers were uniformly distributed in the composites with smaller Ti particle. In the composites with HDH or GA of particle size of 45 μm, their mechanical properties depended on the particle size. The tensile strength and Vickers microhardness of the composite with HDH powder were higher than those of the composite with GA powder. On the other hand, the tensile strengths of the GA composites with the GA powder exhibited higher values at a high volume fraction of TiB.

Titanium compacts with controllable porosity by slip casting of binary powder mixtures

Titanium compacts with tailored porosity and pore size were fabricated by slip casting of binary powder mixtures, followed by debinding and vacuum sintering. Two different particles sizes (avg. 14μm & avg. 56μm) of pure titanium hydride–dehydride powder were used in six volume ratios (0:100, 20:80, 40:60, 60:40, 80:20 & 100:0). The packing density and sintering behaviour of the titanium compacts were characterised, in terms of porosity, pore size analysis, tensile properties and gas permeability. A theoretical model predicted that the green density reaches a maximum when the volume fraction of fine particles is 0.35. It was found that although experimental results showed similar behaviour there was no well-defined maximum green density. The sintered compacts showed that an increase in the volume fraction of fine powder particles reduced the porosity, permeability level and pore size, and increased the tensile properties. The relationship between permeability and porosity level was non-linear and this was caused by the differences in pore diameters in the compacts. The capillary tube model was used to discuss the relationship between the permeability, pore diameter and porosity. Using this information, a graded porosity compact was designed and fabricated by slip casting.