Powder Metallurgy

Astroloy is a nickel-based alloy with a high-volume fraction of γ′ phase, which is widely used in high-temperature applications due to its excellent strength, corrosion, and oxidation resistance. However, traditional manufacturing methods are not suitable for this alloy, and powder Near Net Shape Hot Isostatic Pressing (NNS-HIP) has emerged as the optimal manufacturing process. Despite this, uncertainties remain regarding the effect of process parameters on microstructural evolution and properties. Previous studies established correlations between microstructural features like γ/γ′, carbides type, and volume fraction and prior particle boundaries and mechanical properties for conventionally HIPed and heat-treated (HTed) samples. This study compares those findings with quenched HIPed and HTed samples. Thermo-Calc calculations, related to the stability and composition of phases, were also correlated with experimental results. The HIP-quenched microstructures exhibited higher material strength compared to conventionally HIPed and HTed samples. Besides, an increase in the HIP temperature by 30°C significantly influenced the elimination of primary γ′ at the grain boundaries and the dissolution of carbides.

C-18150 is a precipitation hardening copper alloy that has increasingly gained interest in additive manufacturing (AM) for applications in space, aerospace, electronics and automotive due to its excellent combination of electrical and thermal conductivity, thermal softening resistance at elevated temperatures, moderate strength, and high ductility. In this work, the effect of post-processing thermal treatments on the tensile properties of C-18150, fabricated by laser powder-bed-fusion (L-PBF), using high productivity printing parameters, was investigated. For this purpose, the reference as built and aged (480°C, 5 h) samples were compared with those of post-processed, prior to final ageing, under different conditions: (i) solution annealing (980 °C, 3 h), (ii) hot isostatic pressing (HIP) (950 °C, 150 MPa), followed by solution annealing, and (iii) HIP (980 °C, 150 MPa) followed by uniform rapid quenching aimed at eliminating the solution annealing step. Tensile behaviour was discussed in view of microstructural evolution and defects (e.g. porosity) in relation to the post-processing thermal treatments.

To prepare low-oxygen, fine-grained 420 stainless steel powder with uniform particle size distribution suitable for metal injection molding (MIM) of small-modulus gears, this study employed a combined water–gas atomization method. The systematic investigation examined the influence of water atomization pressure (90–120 MPa) on key powder properties, including particle size distribution, microstructure, oxygen content, flowability and tapped density. Additionally, the study explored the enhancement of powder storage stability through pre-oxidation treatment. Results indicate that increasing water atomization pressure reduced the median particle size (D₅₀) from 12.10 μm to 8.9 μm, narrowing the particle size distribution. The overall sphericity of the powder is good, remaining at a moderate level under different pressures, while oxygen content decreased from 0.35% to 0.17%. Powders prepared at 115 MPa exhibited optimal comprehensive properties: uniform particle size distribution, sphericity of 0.91, oxygen content of 0.17% and tapped density of 4.53 g/cm³. Following pre-oxidation treatment at 200 °C and 300 °C, the oxidation rate during storage was significantly reduced, with oxygen content ultimately stabilizing around 0.26%. This study provides theoretical foundations and process references for the preparation of high-performance stainless steel powders for MIM applications.

Particle size control in gas atomization is essential for powder metallurgy processing, yet reliable prediction remains difficult due to the coupled influence of melt properties, gas dynamics and atomizer geometry. In this work, a dimensionless empirical model was developed to predict the median particle size (D₅₀) of gas-atomized powders. Twenty-six variables were reduced using Buckingham π analysis into three groups: melt parameter (Mp), gas parameter (Gp) and design parameter (Dp), which were correlated with dimensionless particle diameter. Copper powder was produced using a pilot-scale close-coupled atomizer under varying gas pressure, superheat temperature and nozzle configuration. A 3D map was plotted along with the 2D contour to locate optimum conditions for the desired powder particle size. An approximate generalised model has been established to resolve and identify the role of the influencing variables. The approach provides a practical tool for selecting atomization conditions to obtain the desired powder size.

Low-pressure powder injection moulding (LPIM) has advanced through developments in feedstock formulation, mould-filling simulation, thermal wick-debinding, and sintering of irregular metallic powders. This review focuses specifically on the wax-based, backbone-free branch of LPIM, in which thermal wick-debinding is the distinctive binder-removal route, as this is where the most recent quantitative progress has occurred. It synthesizes these developments and identifies the gaps that define the next phase of LPIM research. Wax-based binder formulations have been developed for stainless steels, iron, titanium, and Inconel 718, with solid loadings up to 62 vol.% and viscosities below 20 Pa·s. Numerical simulation has evolved from qualitative flow-pattern matching to quantitative in-cavity pressure validation, and current models reproduce experimental pressures with errors of 30%–64% across complex geometries. Thermal wick-debinding, the distinctive debinding route for backbone-free LPIM feedstocks, has been placed on a quantitative footing through a coupled experimental and COMSOL-based study. Sintering studies of irregular water-atomized iron powders, matched to optimized debinding and sintering schedules, have achieved densities and tensile properties comparable to those obtained with spherical gas-atomized feedstocks, supporting a lower-cost route to structural LPIM components. The review concludes with five research opportunities: closing the simulation–experiment gap; systematically studying the green-density–sintered-density relationship that distinguishes LPIM from HPIM, including dimensional accuracy and shrinkage anisotropy; extending to reactive and oxidation-sensitive alloys; integrating with material extrusion additive manufacturing, including SLA-printed and sacrificial mould strategies; and developing sustainable wax recovery, biosourced binders, and reduced-energy debinding routes.

High-speed steels are commonly used for cutting tools requiring a high impact toughness and fatigue resistance. Both properties are significantly enhanced by employing the powder metallurgical (PM) production route using HIP as consolidation technique. However, the inherent oxides of the used powder may reduce the fatigue limit. Hence, subsequent forging is commonly required, which in turn precludes the possibility of near net shape production by PM. The purpose of this study is to investigate the efficiency of an isothermal holding stage that is introduced prior to the application of pressure to allow oxide size reduction during the HIP process and thereby increasing fatigue limit. The fatigue limit is assessed by accelerated fatigue testing, allowing its determination with a single specimen, which is particularly crucial for HIP processes with limited capsule volume. Good agreement is observed with results from conventional fatigue testing based on the staircase method.

While components produced by melt metallurgy have long been subject to established rule sets for proof of strength, a comparable guideline has now been developed for press-and-sinter powder metallurgical components, taking into account their characteristic residual porosity. This paper presents a method that enables the generation of a synthetic S–N curve using hardness, density, and local stress parameters as input parameters. The development of this method required a comprehensive literature review, gathering data on fatigue strength parameters in relation to density, hardness, as well as specimen geometry and load information. The compiled data was utilized to deduce relationships and formulate an algorithm capable of estimating fatigue strength associated with a 50% failure probability. Moreover, the method can supply information regarding the S–N curve’s trajectory within the finite life region. Coupled with results on statistical scatter derived from the literature, comprehensive synthetic S–N curves for sintered steels can be constructed.

The masteralloy (MA) approach is an increasingly attractive alloying route for introducing elements into sintered steels. This holds in particular for elements such as Cr, Mn and Si, which are oxygen-sensitive but cheap and non-strategic. In the present study, MAs containing these elements were added to plain Fe base powder, and sintering was performed at 1150 and 1180°C, respectively, with subsequent sinter hardening. It showed that compared to the commercial grade Distaloy DH-1, the MA grades benefited from higher sintering temperature as well as from lower carbon content of 0.6% nominal. At 0.75%C, dissolution esp. of Cr containing MAs is retarded by formation of carbides. Compared to the reference material, the MA variants did not result in fully martensitic microstructures, which however may not be mandatory in service since the sintering contacts are preferentially strengthened. In general, the MA approach was found to require less and cheaper alloy elements to attain attractive hardness-toughness combinations.

Detection of a raised argon level in PM HIP material is a critical quality control. Because of the way argon is distributed, detection of a low content of argon is required, especially in PM HIP material made from nitrogen-atomized powder. In the first section of this paper, a study on the distribution of argon is described. Secondly, the published standard (EN ISO 5842:2023) requires the use of an argon-containing reference material. Such a material was produced and is now available in large quantities thanks to a number of research partners and collaborating organizations. The method used to produce this reference material, as well as its stability, uniformity, and variability in terms of reported content, is described in the second part of this paper. This constitutes the last piece of the puzzle, which enables robust and standardized quality control of PM HIP material.

This study investigates the effects of vacuum and thermal degassing on powder-filled capsules of super duplex stainless steel (UNS S32505) prior to hot isostatic pressing (HIP). Degassing was performed at pressures from atmospheric to 0.01 Pa and at 300°C or 600°C. After HIP and solution treatment, tensile strength, hardness, Charpy impact toughness, microstructure, and gas content were evaluated. All samples met fully the ASTM A988/A988M specificantions. These include chemical composition limits, the α–γ phase balance of 45–50% ferrite with no intermetallics. Oxygen slightly decreased with better vacuum, while nitrogen remained largely unchanged. Tensile and hardness properties were unaffected, but Charpy impact toughness improved with better vacuum, rising from 79 J (no degassing) to 139 J (highest vacuum). SEM–EDS showed oxide inclusions as the primary factor controlling toughness, with higher inclusion density promoting crack initiation. These results highlight the importance of vacuum degassing in minimizing oxide inclusions and enhancing fracture toughness in HIPed SDSS components.

In order to investigate phase transformations in a hot isostatic press (HIP) with integrated rapid cooling in situ, the electrical conductivity of PM HS 3-3-4 (1.3377) was measured as a function of temperature in the HIP process. This approach leverages the fact that phase changes result in distinct variations in electrical resistance. A four-wire measurement was conducted on a specifically designed PM HS 3-3-4 sheet metal sample. The experimental setup was integrated into the HIP system to determine the martensite start (M_s) temperature. HIP cycles were performed at an austenitizing temperature of 1000°C under isostatic pressure of 100 MPa. For comparison, unpressurized heat treatments following an identical temperature profile were conducted using a dilatometer to measure the M_s temperature. It was shown that the measurement setup with the newly developed sample geometry can be successfully used to determine phase transformations in the HIP. The comparison of the M_s temperatures of the samples heat treated under isostatic pressure and without pressure showed that the austenite phase is stabilized by the pressure during quenching, which leads to a shift of the M_s temperature to lower values.

This study addresses the strength-ductility trade-off in selective laser melting fabricated TC4 titanium alloy caused by the formation of a nonequilibrium martensitic α′-phase during rapid solidification. Specimens were solution-treated at 950 to 1000 °C, followed by water quenching, and aged at 500 or 550 °C. Microstructure was analysed using deep learning for α-phase quantification. Mechanical and corrosion properties were assessed through tensile tests, Vickers hardness measurements, and electrochemical testing in 3.5 wt% NaCl solution. Results demonstrate that the 990 °C solution treatment followed by 500 °C ageing produced a high fraction of 86.2% equiaxed primary α-phase. This refined structure led to a remarkable 126% improvement in elongation, increasing from 4.9% in the as-fabricated state to 11.1%, while maintaining high tensile strength of 870 MPa and hardness of 312 HV. Corrosion resistance was also significantly enhanced, evidenced by a 68 percent reduction in corrosion current density, along with increases of 0.25 volts in pitting potential and 0.32 volts in breakdown potential of the passive film. In contrast, increasing the solution temperature to 1000 °C reduced the primary α-phase fraction to 80.4% and resulted in inferior mechanical and corrosion performance, confirming 990 °C as the optimal solution temperature. This work demonstrates that solution treatment and ageing heat treatment effectively optimise the microstructure of selective laser melting processed TC4 alloy, achieving simultaneous improvement in both ductility and corrosion resistance without strength compromise. The strategy provides a viable processing route for manufacturing critical components in aerospace and biomedical applications that demand complex geometries, superior damage tolerance, and excellent corrosion resistance.

Piston-based material extrusion (PEX) is an additive manufacturing technology that enables the processing of metal injection moulding (MIM) feedstock to form green parts. The extrusion process does not require moulds and hence is a competitive alternative for the shaping of MIM process routes, offering short lead times and cost-efficiency for lower lot sizes, while integrating into the remaining process chain and equipment. This paper focuses on the microstructure and properties of Ti-6Al-4V fabricated by PEX and followed by a subsequent hot isostatic pressing (HIP) step for parts requiring superior density and mechanical properties. The HIP process is performed using standard as well as high-purity conditions (Quintus Purus^®). After sintering, a globular, mostly α microstructure is observed. A subsequent HIP followed by rapid quenching transforms the microstructure to a bi-modal state. A similar effect on microstructure is observed for the Quintus Purus^® HIP; however, the formation of α-case on the part surface is avoided. While the mean density of the sintered parts is 97.2%, HIP leads to an increase to 98.9–99.3% with the remnant porosity mainly stemming from open, connected pores. Although the sintered specimens already fulfil the industrial requirements, the Quintus Purus^® HIP leads to a slight increase in yield and ultimate tensile strength at minimal reduction of elongation. The standard HIP shows the same but slightly more pronounced trend. The results show that by a careful design of the specific HIP conditions, the microstructure and properties can be tailored for a specific application.

The incorporation of nitrogen as an alloying element in stainless steels can significantly improve both mechanical properties and corrosion resistance. The conventional production of high nitrogen-alloyed steels (HNS) is hampered by the limited solubility of nitrogen in the steel melt. This study investigates a new powder metallurgical process to produce HNS by mixing stainless steel powder with Si₃N₄ powder, followed by hot isostatic pressing (HIP) of the mixture, in which the Si₃N₄ particles dissolve in the solid austenitic phase, enriching the matrix with nitrogen and exploiting the higher solubility of nitrogen in the solid phase compared to the liquid phase. The innovative aspect of this approach is the use of HIP with integrated gas quenching, which allows simultaneous powder consolidation and heat treatment. This method exploits the austenite-stabilising effect of isostatic pressure, which increases the solubility of nitrogen in the austenitic phase, to achieve previously unattainable nitrogen-supersaturated matrices. The amount of Si₃N₄ to be added is determined based on CALPHAD calculations considering both pressure and temperature during HIP. The study evaluates the potential for producing HNS using two different base alloys: X2CrNi18-9 and X1Cr20. The X2CrNi18-9 steel produces a highly nitrogen-supersaturated austenitic state. The X1Cr20 alloy produces an almost fully austenitic condition in which only nitrogen is the austenite stabilising element. Micro-structural analysis is performed by secondary electron microscopy, electron backscatter diffraction and energy dispersive spectroscopy.

This work explores the impact of the microstructure on the corrosion resistance and the mechanical behaviour of S32505, a super duplex stainless steel (SDSS) produced by powder metallurgy hot isostatic pressing (PM-HIP). The results demonstrate that by the appropriate selection of hot isostatic pressing (HIP) and annealing conditions, improved mechanical and corrosion behaviour are achieved, compared to those of an equivalent forged SDSS (S32750). The fine and isotropic microstructure of PM-HIP SDSS, free of inclusions, σ phase or Cr₂N precipitates explain their outstanding impact behaviour and their mechanical strength and ductility more than 15% above the values obtained with the forged SDSS. Despite having a pitting resistance equivalent number lower than forged SDSS, the PM-HIP SDSS grade shows similar or higher pitting and crevice corrosion resistance. The feasibility of obtaining PM-HIP SDSS with suitable microstructure and properties, integrating solution annealing and quenching in an ultra-rapid quenching HIP unit, has also been demonstrated.

Hot isostatic pressing (HIP) is an effective forming method for producing bimetal parts in steel and titanium alloys with large contact areas and complex joining surfaces. In this study, a Cu/Nb bimetallic layer was employed as an intermediate layer to prevent the formation of intermetallic compounds at the steel–titanium alloy interface. The influence of HIP temperature on the microstructure, phases, elemental distribution and shear properties of the DT300/Cu/Nb/TC11 interface was investigated, and the deformation behaviour of the composite samples was elucidated through finite-element simulation. The results showed that as the HIP temperature increased from 900°C to 1000°C, the number of pore defects at the interface decreased, and the composite's shear strength increased from 171 to 215 MPa. The finite element simulation of the composite sample's shear deformation indicated that the properties of the Cu layer primarily influenced the composite sample's mechanical behaviour.

The near-net shape formation of high-volume fraction SiCp/Al composites via semi-solid hot isostatic pressing (HIP) was proposed as a potential solution to the challenges posed by the integration of the composite preparation and processing. The effects of the HIP process with solid-state holding followed by semi-solid heating, hereafter referred to as the solid-to-semisolid HIP (S-SS HIP) process on the microstructure and mechanical properties of the composite were investigated. The formation of the α-(AlSiMnFe) phase was observed in the composites produced by semi-solid HIP, and the MgAl₂O₄ phase precipitated at the interface between the SIC and 6061Al. The S-SS HIP process facilitated strain-energy storage and the formation of subgrains in the matrix. After T6 heat treatment, the average grain size of the S-SS HIP-T6 sample was significantly reduced. The UTS of the S-SS HIP-T6 sample increased by 5.1%, reaching 517.7 MPa, compared with the HIP-T6 sample produced without the S-SS HIP step.

One nickel-based superalloy and austenitic stainless steel were joined together using a technique that didn't require any capsule or welding. The trial was performed with a low-pressure furnace and in a Hot Isostatic Press. The Press, a Quintus QIH 15 L, was coupled with the uniform rapid cooling module to join the two metals and deliver a suitable heat treatment for both alloys. In contrast to the low-pressure furnace, the application of the hot isostatic pressing effectively halved the processing time required for the joining procedure. This research highlights the critical roles of time and temperature in achieving a successful bond between the two dissimilar metals. The interface between the two metals was studied in depth with optical and electronic microscopy and Energy dispersivex-ray spectroscopy (EDS) to assess better the solid-state diffusion of the involved elements.