Pre-alloyed Powder Research Articles

Sintering Behavior and Mechanical Properties of Ti–TiBw Composites Prepared with Pre‐alloyed Ti–TiBw Composite Powders

Prealloyed powders present a promising alternative to traditional ball milling processes in powder metallurgy, addressing challenges such as uneven powder mixing and incomplete reinforcement reactions. This study investigates the preparation of Ti–TiBw composites using spark plasma sintering (SPS) at temperatures ranging from 950 to 1050 °C, utilizing Ti–TiBw prealloyed composite powders. The effects of sintering temperature on the microstructure and mechanical properties of Ti–TiBw composites are studied, comparing prealloyed composite powders with ball‐milled premixed powders. As a result, the composites exhibit excellent strength and ductility, with an ultimate tensile strength of 676 MPa at 1000 °C and an elongation of 20.42% at 1050 °C. Comparatively, Ti–TiBw composites prepared from premixed powders achieve densification at 1050 °C, 50 °C higher than those prepared from prealloyed powders.

Investigation of the Effect of Milling Time on Elemental Powders of Oxi‐Reduction Nickel and Hydrogenation–Dehydrogenation Titanium

Mechanical alloying (MA) is widely applied in the synthesis of blended elemental or prealloyed powders. This work evaluates the effect of milling time on elemental nickel and titanium powders produced by oxi‐reduction and the hydrogenation–dehydrogenation process, respectively. The powders are characterized by scanning electron microscopy, X‐ray diffraction, particle size, powder yield, and differential scanning calorimetry. It is observed that increasing the milling time promotes the formation of a structure composed of thin lamellae of nickel and titanium, which results in the beneficial effect of lowering the temperature for the formation of the intermetallic of the Ni–Ti system and in the powder yield achieved. The reduction of milling time in the MA process of NiTi alloys enhances technological efficiency by decreasing their overall processing time.

Role of Ti-6Al-4V addition in modulating crack sensitivity, solidification microstructure and mechanical properties of laser powder bed fused γ-TiAl alloys

Solidification cracking is still the primary challenge for laser powder bed fusion (LPBF) without high-temperature preheating of γ-TiAl alloys. Circumventing the peritectic reaction and promoting β-solidification by adding the β-stablizing elements have been proved to be an effective way to suppressing the crack formation. In this work, we attempted a more economic method to realize β-solidification via the addition of Ti-6Al-4V (TC4) pre-alloyed powder. The results showed that the TC4 addition successfully changed the solidification path from L→L+β→L+α→α→α+γ→α2+γ to L→L+β→β→α+β→α2+β/B2. As a result, the solidified microstructure was mainly comprised of primary α2 phase and small amount of retained β/B2 (8.5 %) phase distributed along the molten pool boundary. What's more important was that the TC4 addition led to a 44.6 % decrease in crack density. Contributions to low hot cracking sensitivity were found to mainly origin from the relatively lower cooling rate, higher fraction of low-angle grain boundaries (LAGBs) and smaller dendrite coalescence undercooling induced by the TC4 addition. Besides, the smaller temperature interval (10.63K) in the final solidification stage (0.90<fs < 0.94) for the TC4/TiAl alloy was also conducive to facilitate the liquid feeding and dendrite coalescence.

Mechanical energy absorption capacity of the porous Ti-6Al-4V alloy under quasi-static and dynamic compression

Porous materials are very efficient in absorbing mechanical energy in different applications. In the present study, porous materials based on the Ti-6(wt.%)Al-4V alloy were manufactured with the use of two different powder metallurgy methods: i) blended elemental powder approach using titanium hydride (TiH2) as well as V-Al master alloy powders and ii) using hydrogenated Ti-6-4 pre-alloyed powder. The powder compacts were sintered with additions of ammonium bicarbonate as a pore-holding removable agent. The emission of hydrogen from hydrogenated powders on vacuum sintering and the resulting shrinkage of powder particles permitted the control of the sintering process and creation of anticipated porous structures. Mechanical characteristics were evaluated under quasi-static and dynamic compressive loading conditions. Dynamic compression tests were performed using the direct impact Hopkinson pressure bar technique. All investigations aimed at characterizing the mechanical energy-absorbing ability of the obtained porous structures. The anticipated strength, plasticity, and energy-absorbing characteristics of porous Ti-6-4 material were evaluated, and the possibilities of their application were also discussed. Based on the obtained results, it was found that porous Ti-6-4 material produced with a blended elemental powder approach showed more promising energy absorption properties in comparison with pre-alloyed powder.

Effect of Ti/Ta ratio on the microstructure and mechanical properties of Hf20Mo15Nb25Ta20-xTi20+x refractory high-entropy alloys

The limited deformation capacity at room temperature remains a significant challenge in refractory high-entropy alloys (RHEAs). In this study, the fine-grained Hf20Mo15Nb25Ta20-xTi20+x (x = 0, 5, 10, 15) alloys were prepared by mechanical alloying and subsequent spark plasma sintering (SPS), based on precise component regulation. The impact of altering the Ti/Ta ratio (R, R = (20+x)/(20-x)) on the pre-alloyed powders and the as-sintered alloys were investigated. An increase in R-value led to larger powder particle sizes and lower powder yields, which was attributed to the improvement in the plasticity of the pre-alloyed powders. After SPS sintering, the as-sintered alloys were comprised of two BCC solid solution matrices and nanoscale precipitated phases. As the R-value increases, the stability of the microstructure decreases slightly, while the yield strength shows a slight improvement, and the plastic strain experiences a relatively significant enhancement. The as-sintered Hf20Mo15Nb25Ta20-xTi20+x alloys all exhibited an excellent balance of strength and plasticity. Theoretical calculations revealed that the yield strength of the alloys was the result of the combined effect of several strengthening mechanisms, predominantly substitutional solid solution strengthening. Furthermore, a systematic discussion was conducted on the reasons for the improvement in the plasticity of the as-sintered alloys. This study presents an effective approach to enhancing the room temperature deformation capacity of RHEAs, thereby facilitating their broader application.

Laser energy-dependent processability of non-equiatomic TiNbMoTaW high-entropy alloy through in-situ alloying of elemental feedstock powders by laser powder bed fusion

Pre-alloyed powder, which is primarily used in laser powder bed fusion (LPBF), has the disadvantages of requiring time and high manufacturing costs. To overcome these limitations, in-situ alloying, which mixes pure elemental powders and alloys them in real-time during the LPBF process, has attracted attention. In particular, manufacturing high entropy alloys (HEA) containing high-melting-point refractory elements through in-situ alloying presents considerable challenges. In this study, a non-equiatomic single body-centered cubic (BCC) solid-solution HEA was fabricated via in-situ alloying with Ti, Nb, Mo, Ta, and W powders through the LPBF process. Specifically, by applying a high volumetric energy density (VED), we successfully mitigated the segregation of constituent elements, leading to an enhanced crystallographic texture. Consequently, the reduction in the residual stress and high-angle grain boundary (HAGB) density progressed, contributing to an increased relative density. Thus, this study marks a pioneering endeavor for in-situ alloyed HEA fabrication via LPBF, illustrating the efficacy of in-situ alloying utilizing mixed powders.

Influence of carbon percentage on the wear and friction characteristics of ATOMET 4601 alloys in heavy-duty machinery

Heavy-duty machinery demands materials with strong wear resistance and good frictional properties, which conventional materials often lack. The knowledge of PM alloys’ friction and wear characteristics versus standard steel materials is limited. ATOMET 4601, a high-strength prealloyed powder with excellent compressibility, allows for parts with densities over 6.8 g/cm³. Carbon (0, 0.5, 1.0 wt.%) was added to enhance performance. These alloys, compacted to 75% of theoretical density and sintered at 1100 ± 10°C, were tested for wear and friction against EN31 steel. Results showed carbon improved tribological performance, with ATOMET 4601 + 1.0%C exhibiting the best wear resistance. Regression models and interaction plots indicated significant effects of load and speed on wear rate and coefficient of friction. Microstructural analysis revealed carbides and oxides in the ferrite matrix, with adhesive, abrasive, and oxidative wear as primary mechanisms.

Microstructure and tensile properties of Y2O3-dispersion strengthened CoCrFeNi high entropy alloys prepared via mechanical alloying using pre-alloyed powder

Nano-Y2O3-dispersion strengthened CoCrFeNi high entropy alloys are fabricated via mechanical alloying and spark plasma sintering using high purity elemental powders or pre-alloyed CoCrFeNi powder. All the alloys show an FCC matrix incorporated by small amount of BCC Cr-rich segregations, whose brittle nature is detrimental to the mechanical properties of the alloys. It has been found that using pre-alloyed powder significantly suppresses the formation of the Cr-rich phase, and its size and volume fraction can be further reduced by increasing the rotation speed of ball-milling during mechanical alloying. Besides, the grain refinement is also achieved under a higher rotation speed. Y2O3 nanoparticles with a number density of 1.8 × 1022 m−3 and an average diameter of 11.0 ± 7.3 nm are uniformly distributed in the alloy that produced from pre-alloyed powder under the rotation speed of 350 rpm during ball-milling. These Y2O3 nanoparticles share coherent interface with the FCC matrix, indicating the in-situ precipitation mechanism. Due to a good combination of grain boundary strengthening, dislocation strengthening and precipitation strengthening, this ODS high entropy alloy possesses a yield strength of 1281 MPa at room temperature.

Regulation of microstructure, phase transformation behavior, and enhanced high superelastic cycling stability in laser direct energy deposition NiTi shape memory alloys via aging treatment

The remarkable mechanical properties, fatigue resistance, wear and corrosion resistance, biocompatibility, shape memory effect, and superelasticity of NiTi shape memory alloys make them applied in diverse engineering areas, including satellite antennas, oil pipelines, and vascular stents. This paper focuses on the preparation of Ni50.8Ti49.2 alloy via laser directed energy deposition (LDED) utilizing pre-alloyed powder and the heat treatment of solid solution at 950 °C for 8 h followed by subsequent aging at 350 °C, 450 °C and 550 °C for 2 h. The influence of varying aging temperature on the microstructures, phase transformation behavior and superelastic cyclic stability of them was comprehensively summarized and discussed. The martensitic transformation temperatures of NiTi alloys after aging decrease as the aging temperature increases. Specifically, NiTi alloys aged at 450 °C for 2 h exhibit excellent superelasticity and high cyclic stability. After an initial loading-unloading cycle with an 8.079 % pre-strain, the alloy achieves 100 % complete recovery. Even after 10 cycles, the recoverable strain remains at approximately 7.374 %, with a recovery rate exceeding 90 %. These outstanding properties of high superelasticity and superior cyclic stability are significantly better than most other LDED NiTi alloys. This exceptional performance is primarily attributed to the synergistic effects of the R-phase and Ti3Ni4 precipitates on the shape memory alloy.

TiO2 nanofiber-derived in-situ Al2O3 particles reinforced TiAl matrix composites

In this study, Al2O3/TiAl composites were synthesized via powder metallurgy by incorporating TiO2 nanoparticles and nanofibers as oxygen sources into Ti-45Al-8Nb pre-alloy powders, followed by vacuum hot-pressing sintering to form in-situ Al2O3 particles as reinforcements. The addition of TiO2 nanofibers results in a better grain refinement effect and a more uniform distribution of Al2O3 particles within the composites. High-temperature tensile testing revealed that the composites prepared using TiO2 nanofibers exhibited slightly higher strengths and significantly improved ductility compared to those synthesized with TiO2 nanoparticles. This work not only introduces a novel additive for fabricating high-performance in-situ Al2O3/TiAl composites but also demonstrates a unique application of TiO2 nanofibers.

Decarburization and Gas Formation During Sintering of Alloyed PM Steel Components

AbstractThis study investigates the delubrication, reduction, and decarburization processes of powder metallurgical steel alloys (CrM, CrL, AHC, Mo85, SintD 35) and an unalloyed steel during sintering in a pure hydrogen atmosphere. Utilizing in-situ FTIR gas phase analysis, components with ethylenebisstearamide (EBS) as a lubricant are analyzed. EBS decomposition in steel components yields CO, CO2, H2O, and CH4, with dominant CH groups observed in the 230 °C to 480 °C range. In the temperature range between 750 °C and 850 °C, where CO formation is expected due to the reduction of surface iron oxides, CH4 is present instead, indicating that an “internal getter effect” also occurs in pre-alloyed powders. In addition, with high carbon activity, the reduction of internal iron oxides and the reduction of chromium oxides also trigger an internal getter effect. Depending on the carbon potential, these processes cause a considerable reduction in the carbon content of the powder metallurgical components. The study therefore shows that the decarburization of powder metallurgical components during the heat treatment phases prior to sintering in a 100 pct hydrogen atmosphere is less due to the mechanism of delubrication, but rather to mechanisms of carbothermal reduction.

Study on Crack Behavior of GH3230 Superalloy Fabricated via High-Throughput Additive Manufacturing.

This study utilized Fe, Co, Ni elemental powders alongside GH3230 pre-alloyed powder as raw materials, employing high-throughput additive manufacturing based on laser powder bed fusion in situ to alloying technology to fabricate the bulk samples library for GH3230 superalloy efficiently. A quantitative identification algorithm for detecting crack and hole defects in additive manufacturing samples was developed. The primary focus was to analyze the composition variations in specimens at varying Fe, Co, and Ni elemental compositions and their impact on crack formation. Experimental results demonstrated that increased laser power improved element distribution uniformity but it proved to be not significantly effective in reducing crack defects. Moreover, augmented Fe and Co alloying content could not eliminate these defects. However, elevated Ni content led to a decrease in the alloy's solidification cracking index and carbide reduction in solidification products. Notably, a significant reduction in cracks was observed when the Ni content of the alloy reached 63 wt.%, and these defects were nearly eliminated at 67 wt.% Ni content.

Nano-dispersion strengthened and twinning-mediated CoCrNi medium entropy alloy with excellent strength and ductility prepared by laser powder bed fusion

Laser powder bed fusion (LPBF) was explored to produce CoCrNi medium entropy alloys (MEAs) from pre-alloyed powder with a relatively high oxygen concentration (870 ppm). Microstructure, strength, and toughness were investigated, revealing the effect of oxide dispersion strengthening (ODS). LPBF CoCrNi MEAs had a porosity of <0.10 % and superior mechanical properties compared to CoCrNi-based MEAs reported in the literature. Notably, the CoCrNi MEAs denoted as S1 and S2, exhibited an excellent combination of yield strength (>735 MPa), a UTS > 1 GPa, and a total elongation of ∼ 50 %. High strength results from the high number density of nano chromium oxides (average particle size ∼20 nm), which were in situ formed during LPBF, contributing to a calculated dispersion strengthening of ∼220 MPa. The deformation mechanism was investigated by TEM characterization of the specimen after tensile testing, revealing a large number of nano-twins for S1, cellular structure and nano-twins formed for S3. Extremely low porosity and deformation twinning ensure steady plastic deformation in LPBF CoCrNi, enabling high ductility. The prospect of ODS via LPBF provides a pathway for the strengthening and toughening of medium to high entropy alloys (HEAs).

Additive Manufacturing of Interstitial Nitrogen‐Strengthened CoCrNi Medium Entropy Alloy

Equiatomic CoCrNi medium entropy alloy (MEA) is known for its excellent mechanical properties, oxidation resistance, and hydrogen embrittlement resistance. Interstitial elements like carbon and nitrogen further enhance these properties. However, there is limited research on interstitial solid solution‐strengthened alloys produced via powder bed fusion‐laser beam (PBF‐LB). This study investigates the role of nitrogen as an interstitial alloying element in CoCrNi MEA using PBF‐LB. Two alloy variants, one without nitrogen and one prealloyed with nitrogen, are analyzed through thermodynamic calculations and experimental validations. A minor nitrogen loss occurs in as‐printed samples compared to prealloyed powders, but the remaining nitrogen stays as an interstitial solid solution without forming detrimental secondary phases. This leads to improved mechanical properties, with yield and ultimate tensile strengths increasing from about 690 and 900 MPa in nitrogen‐free samples to over 750 and 1000 MPa in nitrogen‐containing samples while maintaining the high ductility of the nitrogen‐free counterpart in the as‐printed materials. Additionally, the nitrogen‐containing CoCrNi‐N MEA exhibits ≈35HV higher hardness than the nitrogen‐free variant in both as‐printed and heat‐treated states. These findings highlight the benefits of nitrogen addition in enhancing the mechanical properties of CoCrNi MEA produced by PBF‐LB.

Superior uniform deformation ability of intragranular nano TiB reinforced Ti-6.5Al–2Zr–Mo–V fabricated by hot isostatic pressing

In order to improve the uniform deformation ability of titanium matrix composites, the intragranular nano-TiB whiskers (TiBnw) reinforced Ti-6.5Al–2Zr–1Mo–1V (TA15) composite was designed and fabricated by hot isostatic pressing (HIP) at the temperature of 1000 °C. The TiBnw was successfully planted into the pre-alloyed powders before HIP. The composite sintered for 6h (HIP-6h) realized a complete densification process benefiting from a high sintering pressure of 150 MPa. While the 2h sintering process (HIP-2h) was insufficient to densify. The HIP-6h exhibited an unparalleled uniform elongation of 9.5 %, which was superior to the survey TA15 matrix titanium composites. The excellent uniform deformation ability was demonstrated to be associated with the equiaxed structure and the blocking and multiplication effect on dislocations from the intragranular TiBnw.

New processing routes for Zr-based ODS ferritic steels

In this work, Zr addition is proposed to refine the nanoparticle dispersion in an ODS RAF steel of composition Fe-14Cr-2W-0.3Zr-0.24Y (wt.%). Three batches of material are obtained using pre-alloyed atomized powder, where yttrium is directly introduced in the melt, and manufactured through three different processing routes. First route is based on the newly developed STARS route that aims to avoid subsequent mechanical alloying. The second route explores the impact of mechanical alloying in pre-oxidized powders. The third route uses mechanically alloyed powders without the pre-oxidation process. The ODS-powders were individually consolidated by hot isostatic pressing and later hot rolled. The obtained materials were characterized by small-angle neutron scattering (SANS) and X-ray absorption spectroscopy (XAS) techniques. SANS and XAS analysis point out the absence of oxide nanoparticles in the material based on the STAR route. SANS analysis confirms that the mechanically alloyed materials do exhibit the presence of nanoparticles. These are identified as Zr-O-rich nanoprecipitates by XAS and the calculated A-ratio by SANS is linked with the phase Y2Zr2O7. Their radii are in the range of 3–3.6 nm. XAS results show that mechanical alloying minimizes the initial differences regarding the oxidation state between the ODS powders with and without pre-oxidation.

Tribological and electrochemical behaviour study of porous TiNiCu alloys manufactured by the powder metallurgy process

ABSTRACT The experimental work aims to study the effect of the Copper (Cu) addition on the tribological and electrochemical behaviour of porous Ti50Ni50-x-Cux alloys. In the first step pure metal powders of titanium (Ti), nickel (Ni), and Cu were ball-milled during 8 h in a planetary ball mill, and then compacted under a 4 ton load to obtain pre-alloyed powders. The final step is sintering at 950°C for 7 h in a tubular furnace under a controlled atmosphere. X-ray diffraction measurements (XRD), scanning electron microscopy (SEM) analyses, microhardness measurements, friction tests and electrochemical were carried out to characterise the tribological and electrochemical performances of the elaborated specimens. DRX results reveal the formation of four major phases: TiNi, Ti2Ni, TiNi3 and TiNi0.8Cu0.2. Potentiodynamic polarisation tests show the positive effect of adding Cu on corrosion resistance, where the 6% Cu sample exhibits the best corrosion behaviour. Tribological results show a decrease in the coefficient of friction and wear rate with the addition of Cu, compared to the initial state, without Cu addition. The minimum coefficient of friction (0.585) was recorded for the 2% Cu sample, while the minimum wear rate (Ws = 1.06. E−4mm3/N/m) was achieved for the 4% Cu sample.

Exploration of a new AlCoCrNiNb high-entropy alloy: in situ alloying of a CoCrMo, M247, and Nb powder mixture via laser powder bed fusion

AbstractHigh-entropy alloys (HEAs) have compelling advantages, such as high strength and corrosion resistance, but they remain underexplored owing to the limited availability of certain prealloyed and elemental powders. In this study, an AlCoCrNiNb HEA was fabricated in situ via laser powder bed fusion (LPBF) using a powder mixture of commercial CoCrMo, Mar M247, and Nb elemental powders. The powder mixture was blended for 24 h using a horizontal blending machine, to obtain similar chemical compositions in the middle and top layers of the blended powder. However, local Nb aggregation was observed in the bottom layer owing to the particle-size effect. X-ray diffraction and chemical composition analyses revealed that the in situ alloyed AlCoCrNiNb HEA specimen obtained via LPBF was a homogeneous solid solution with a face-centered cubic structure. HEA exhibits a fine-grained morphology, and its maximum microhardness is approximately 970 HV. These characteristics are typical of rapid solidification and sluggish diffusion. These results underscore the effectiveness of using commercial prealloyed and elemental powders for fabricating AlCoCrNiNb HEAs through LPBF-based in situ alloying, thus advancing the development of HEAs. Thermodynamic calculations were performed to support these outcomes.

The Chemistry-Process-Structure Relationships of a Functionally Graded Ti-6Al-4V/Ti-1B Alloy Processed with Laser-Engineered Net Shaping Creates Borlite.

We quantify the chemistry-process-structure-property relationships of a Ti-6Al-4V alloy in which titanium-boron alloy (Ti-B) was added in a functionally graded assembly through a laser-engineered net shaping (LENS) process. The material gradient was made by pre-alloyed powder additions to form an in situ melt of the prescribed alloy concentration. The complex heterogeneous structures arising from the LENS thermal history are completely discussed for the first time, and we introduce a new term called "Borlite", a eutectic structure containing orthorhombic titanium monoboride (TiB) and titanium. The β-titanium grain size decreased nonlinearly until reaching the minimum when the boron weight fraction reached 0.25%. Similarly, the transformed α-titanium grain size decreased nonlinearly until reaching the minimum level, but the grain size was approximately 2 μm when the boron weight fraction reached 0.6%. Alternatively, the α-titanium grain size increased nonlinearly from 1 to 5 μm as a function of the aluminum concentration increasing from 0% to 6% aluminum by weight and vanadium increasing from 0% to 4% by weight. Finally, the cause-effect relationships related to the creation of unwanted porosity were quantified, which helps in further developing additively manufactured metal alloys.

In-situ alloying of maraging steel with enhanced mechanical properties and corrosion resistance by laser directed energy deposition

Maraging steels are known for their exceptional strength derived from the martensite matrix and nano-precipitate strengthening. However, the trade-off to the high strength is often a loss of ductility and minimal strain hardening. Introducing metastable austenite to the matrix to activate transformation-induced plasticity (TRIP) is an effective approach to achieve high strength and ductility synergy. Existing methods to introduce TRIP into additively manufactured maraging steels are limited by the compositions of pre-alloyed powder or additional heat treatment steps. In-situ alloying via the laser directed energy deposition (L-DED) process allows flexibility in tailoring the alloying composition to achieve an austenite–martensite microstructure. Herein, we develop a TRIP–maraging steel by in-situ alloying of M789 with 316L (4–8 wt%) during the L-DED process. The addition of 316L facilitates austenite reversion in solution-treated maraging steel. As a result, the TRIP–maraging steel exhibits a 76 % increase in uniform elongation at the expense of a minimal sacrifice of strength. Furthermore, the corrosion resistance of the TRIP–maraging steel is enhanced. This work showcases a pathway to developing superior alloys by in-situ alloying via additive manufacturing.