Hydride-dehydride Research Articles

Complex-shaped titanium components fabricated via metal injection molding using hydride-dehydride titanium powder

Metal injection molding (MIM) is a net-shape manufacturing method that lessens the costs of production and increases the affordability of titanium goods. A range of CP-Ti specimens was prepared to confirm the primary manufacturing parameters for titanium metal injection molding utilizing affordable hydride-dehydride (HDH) powders. Mixture of powder and polyformaldehyde-based binder was injection molded, debonded, and then sintered. The optimum powder loading is 55 vol%. The ideal conditions of catalytic debinding were attained by placing the specimens in the catalytic debinding system at 120 °C temperature. Samples with minimal deformation and low impurity content were obtained when thermal debinding was carried out at a modest heating rate of 1.0 K/min and a holding time of 1.5 h. All the sintered samples have a similar microstructure, that is, lamellar α + β structure. The sample exhibits a strength of 739 MPa and an elongation of 6.1 % at the optimum sintering conditions (sintering at 1250 °C for 2 h). The samples prepared by MIM have excellent wear resistance. This process provided a straightforward and cost-effective means of producing complex-shaped titanium components from HDH Ti powder.

Hydride–Dehydride Processes and Behaviors for Ductile Refractory Complex Concentrated Alloys

Hydride–Dehydride Processes and Behaviors for Ductile Refractory Complex Concentrated Alloys

Achieving synergy of strength and ductility in powder metallurgy commercially pure titanium by a unique oxygen scavenger

Excessive interstitial oxygen (O) contamination, usually causing a dramatic loss of ductility, remains a longstanding challenge for titanium (Ti) and its alloys. Here, we propose to solve this critical problem by adding a minor CaC2 oxygen-scavenger via a simple powder metallurgy (PM) pressureless sintering approach. It is found that the surface oxide layer of hydride-dehydride (HDH) Ti powder begins to dissolve into the Ti matrix between 700 °C and 800 °C during the PM sintering process. The incorporation of CaC2 can react with the surface oxide layer (617-676 °C) prior to its active dissolution and form micrometer-sized TiC and nano-sized CaTiO3 particles, significantly refining the α-Ti grains and creating clean and well-bonded interface with Ti matrix. Therefore, the unique oxygen-scavenging effect by CaC2 produces high strength and superior ductility for Ti alloy. Even with an initially high oxygen content (∼4000 ppm), the Ti-0.4 wt.%CaC2 sample still exhibits a high ultimate tensile strength of 621±25 MPa and superior elongation of 29.3±2.6%, respectively. These values correspond to an increase of 17.6% and 301.4% compared to the as-sintered commercially pure titanium (CP-Ti) properties and far exceed the ASTM standard B381 for Grade 4 wrought Ti alloy (550 MPa and 15%) with the same O content. This work offers a novel method to develop high-strength and superior-ductility Ti materials from much more affordable Ti powder.

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.

Gas bubble coalescence in laser directed energy deposition of irregular HDH titanium alloy powder feedstock

In this study, the synchrotron X-ray imaging technique was used to investigate the coalescence of gas bubbles during laser directed energy deposition (L-DED) of irregular hydride-dehydride (HDH) titanium alloy particles onto a titanium alloy substrate. The objective is to better understand the coalescing mechanism of gas bubbles during the L-DED process of unique feedstock materials. The coalescence frequency of gas bubbles is the percentage of bubbles merging and is dependent on collision frequency and coalescence efficiency. Forces in the melt pool flow such as Marangoni forces, buoyancy force, vaporization pressure, and acoustic waves, along with the number of generated bubbles, were the major factors for increasing the collision frequency between bubbles. In addition, the random forest model determined that the diameter of the smaller bubble during collision was the most significant factor impacting the efficiency of coalescence, which was equal to 48% when the smaller diameter was larger than a threshold of 76 μm and 3.5% and when the diameter was smaller than 76 μm. The results also showed that 6.5% of the formed bubbles in the melt pool led to coalescence. Overall, this work can help mitigate pores, verify simulation models, and promote irregular or recycled powder feedstock as effective replacement to atomized feedstock.

The hydride-dehydride process combined with plasma spheroidization: An alternative route for producing spherical metallic powders of U-6 wt.% Nb

In this work, we investigate the viability of the Hydride-Dehydride (HDH) process coupled with plasma spheroidization for producing U-6 wt.% Nb powders as an alternative method to atomization. The HDH process produces irregularly shaped powder with sub-micron U-Nb segregation and phase heterogeneity, and the subsequent spheroidization process re-homogenizes both the U-Nb composition and phase that is similar to atomized U-6 wt.% Nb powders. The surface oxide layer of the spheroidized powder is thinner than atomized U-6 wt.% Nb powders, ranging from 5 to 15 nm and display an isotropic microstructure, absent of large dendrites across the sphere as seen in atomized U-6 wt.% Nb. HDH also offers the advantage that oversized and undersized powders can be reprocessed. Combined, we demonstrate that HDH coupled with plasma spheroidization is a low-waste, alternative production route for spherical U-6 wt.% Nb powders.

Metal injection molding of high-performance Ti composite using hydride-dehydride (HDH) powder

Aiming at solving the interstitial oxygen contamination for titanium (Ti) parts fabricated by metal injection molding (MIM) technology, we innovatively introduced LaB6 oxygen scavenger to eliminate its adverse effect and produce a high-performance Ti material using inexpensive hydride-dehydride (HDH) Ti powders. Rheological behavior of feedstocks, decomposition behavior of binders, as-sintered microstructure and mechanical properties were systematically studied. The viscosity of feedstock F2 containing LaB6 powder meets the requirement of injection molding operation. The as-sintered 0.5 wt% LaB6/Ti composite has homogenous and fine microstructure with the average grain size of 65.6 μm. The in-situ TiB and oxygen-containing (La2O3 and La-Cl-O) reinforcements are formed by the reaction of LaB6 and Ti matrix, which are uniformly distributed in the composite. The generated TiC particle and Ti matrix exist an orientation relation of (111¯)TiC||(011¯0)Ti and [101]TiC||[0001¯]Ti. The LaB6/Ti composite possesses excellent mechanical properties, with the ultimate tensile strength of 632 MPa, yield strength of 548 MPa and elongation of 15 %. Attributed to the oxygen scavenger of LaB6, the elongation of the composite is greatly increased by 163 % compared with pure Ti. This work provides a valuable guidance for the development of low-cost and high-performance Ti parts by MIM technology.

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.

Sintering High Green Density Direct Powder Rolled Titanium Strips, in Argon Atmosphere

Presently, the majority of titanium powder metallurgy components produced are sintered under high vacuum due to the associated benefits of the vacuum atmosphere. However, high-vacuum sintering is a batch process, which limits daily production. A higher daily part production is achievable via a continuous sintering process, which uses argon gas to shield the part from air contamination. To date, there has been limited work published on argon gas sintering of titanium in short durations. This study investigated the properties of thin high green density titanium strips, which were sintered at the temperatures of 1100 °C, 1200 °C and 1300 °C for a duration of 30 min, 60 min and 90 min in argon. The strips were produced by rolling of −45 µm near ASTM (American Society for Testing and Materials) grade 3 hydride–dehydride commercially pure titanium powder. The density, hardness, tensile properties and microstructure of the sintered strips were assessed. It was found that near-full densities, between 96 and 99%, are attainable after 30–90 min of sintering. The optimum sintering temperature range was found to be 1100–1200 °C, as this produced the highest elongation of 4–5.5%. Sintering at 1300 °C resulted in lower elongation due to higher contaminant pick-up.

Investigation of Microstructure and Mechanical Properties for Ti-6Al-4V Alloy Parts Produced Using Non-Spherical Precursor Powder by Laser Powder Bed Fusion.

An unmodified, non-spherical, hydride-dehydride (HDH) Ti-6Al-4V powder having a substantial economic advantage over spherical, atomized Ti-6Al-4V alloy powder was used to fabricate a range of test components and aerospace-related products utilizing laser beam powder-bed fusion processing. The as-built products, utilizing optimized processing parameters, had a Rockwell-C scale (HRC) hardness of 44.6. Following heat treatments which included annealing at 704 °C, HIP at ~926 °C (average), and HIP + anneal, the HRC hardnesses were observed to be 43.9, 40.7, and 40.4, respectively. The corresponding tensile yield stress, UTS, and elongation for these heat treatments averaged 1.19 GPa, 1.22 GPa, 8.7%; 1.03 GPa, 1.08 GPa, 16.7%; 1.04 GPa, 1.09 GPa, 16.1%, respectively. The HIP yield strength and elongation of 1.03 GPa and 16.7% are comparable to the best commercial, wrought Ti-6Al-4V products. The corresponding HIP component microstructures consisted of elongated small grains (~125 microns diameter) containing fine, alpha/beta lamellae.

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.

Investigating the impact of a selective laser melting process on Ti6Al4V alloy hybrid powders with spherical and irregular shapes

ABSTRACT A hybrid powder of Ti6Al4V, which consists of 50 wt% plasma atomised (PA) spherical and 50 wt% hydride-dehydride (HDH) irregular-shaped powder with a flowability of 36.5 s, is projected as the raw materials in this research. In addition, Ti6Al4V (Gr.23) plasma atomised 100 wt% spherical powder was used as the reference powder with a flowability of 28 s to facilitate the tensile property discussion. It was found that the powder flowability is the crucial parameter governing the tensile properties of fabricated specimens. It was shown that the decrease in powder flowability (36.5 s) resulted in increases in susceptibility to the formation of a lack of fusion defects with dropped tensile properties. The microstructure of the fabricated Ti6Al4V alloy features the formation of prior β grain boundaries and an alpha prime (α′) martensite. Furthermore, no intermetallic aluminide phase (Ti3Al) precipitations in grain boundaries were detected through energy-dispersive X-ray spectroscopy analysis.

Partially biodegradable Ti-based composites for biomedical applications subjected to intense and cyclic loading

Abstract This work presents a titanium (Ti) - magnesium (Mg) composite as a novel material that is designed for application in biomedical devices that are subjected to intense and cyclic loading (such as dental implants). This study concentrates on how the mechanical properties, fatigue endurance, in-vitro corrosion response and cytotoxicity of TiMg composites depend on the use of high-purity regular-morphology plasma atomized (PA) Ti Grade 1 powders. It also compares the desired results with those obtained for composites fabricated using hydride–dehydride (HDH) Ti Grade 4 powders. The results showed that the as-processed Ti(PA)+17 vol% Mg composite exhibited an attractive combination of an ultimate tensile strength (UTS) of 450 ± 1 MPa, 0.2% strain offset yield stress (YS0.2) of 331 ± 5 MPa, and elongation (e) of 8.9 ± 1%), in addition to a reduced Young’s elastic modulus (E) of 88 ± 0.1 GPa. Moreover, the as-processed Ti(PA)+17 vol% Mg composite showed a reasonable fatigue life, while it reached a fatigue tensile stress of 260 MPa at 1.5·107 cycles. The advantageous mechanical properties of the Ti(PA)+17 vol% Mg composite, superior to those of Ti(HDH)+Mg counterparts, resulted from a specific ultrafine Ti grain structure. The corrosion behavior of the Ti(PA)+17 vol% Mg composite was systematically studied during immersion in a Hank’s balanced salt solution (HBSS) for up to 21 days. The elution of Mg enabled a further decrease of E down to 84.3 ± 0.3 GPa, while the tensile strengths remained unaffected, and e and fatigue stress decreased. Nevertheless, the mechanical performance of eluted Ti(PA)+17 vol% Mg remained appropriate for a specific application of dental implants with the addition of porosity induced osseointegration. Owing to a refined Mg grain structure, in-vitro corrosion tests confirmed a reduced elution rate of the Mg component from Ti(PA)+17 vol% Mg when compared with that of Ti(HDH)+Mg counterparts. The cytotoxicity assays demonstrated that as-extruded Ti(PA)+17 vol% Mg exhibited a nontoxic response upon culturing with resistant hAD-MSC cells, which was comparable with that of the commercial purity (CP) Ti Grade 4 reference material. For a highly sensitive L929 cell line, the metabolic activity of Ti(PA)+17 vol% Mg had to be increased by surface pretreatment of a TiMg sample. All results of this study indicate that the new Ti(PA)+17 vol% Mg composite is immensely promising for use in prosthodontic surgery.

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.

Selective laser melting of CP-Ti to overcome the low cost and high performance trade-off

In this study, commercially pure titanium (CP-Ti) parts were successfully fabricated by selective laser melting (SLM) using cost-effective hydride-dehydride (HDH) Ti powders for the first time modified by jet milling. Jet milling effectively improves the particle-shape sphericity, suppresses the impurity pick-up, and produces localized plastic deformation. The flowability of the jet-milled powders is tremendously improved to 29.7 s/50 g that satisfies the SLM processing well, while the oxygen content only increases by 0.02 wt.% (the raw oxygen level: 0.15 wt.%). The oxide layer in the powder surface is determined with the thickness of ∼8 nm and TiO being the predominant phase before and after jet milling. The SLM-made (SLMed) CP-Ti achieves dominant martensitic α’ phase with the fracture tensile strength up to 731.5 ± 5.7 MPa and elongation of 20.5 ± 1.1%, comparable with those using expensive atomized powders. Contrary to the conventional metallurgical mechanism for Ti which suffers the cost-performance dilemma, this work presents SLMed CP-Ti with excellent synergy of strength and ductility while using the cost-affordable HDH Ti powders.