Hot Isostatic Pressing Research Articles

Hot Isostatic Pressing of Additive Specimens, Grown from IN718 Alloy Powder Obtained by Gas Atomization

Hot Isostatic Pressing of Additive Specimens, Grown from IN718 Alloy Powder Obtained by Gas Atomization

A Perspective on Powder Metallurgy and Additive Manufacturing of High‐Nitrogen Alloyed Stainless Steels

Incorporating nitrogen as an alloying element in stainless steels can significantly enhance their mechanical and chemical properties. However, the limited solubility of nitrogen, particularly in the liquid phase, presents challenges. This perspective article explores an innovative powder metallurgical approach to producing high‐nitrogen steels (HNS) by utilizing a mixture of stainless steel and Si3N4. This mixture undergoes hot isostatic pressing (HIP) followed by direct quenching, facilitating diffusion alloying, and solution annealing in a single step. The article also examines adapting this method to powder bed fusion of metals using a laser beam (PBF‐LB/M) to overcome nitrogen solubility limits, which currently constrain the achievable nitrogen content in PBF‐LB/M‐manufactured stainless steels. The approach aims to retain Si3N4 particles within the matrix during PBF‐LB/M to enrich the steel with nitrogen during subsequent HIP. However, laser interaction with Si3N4 can lead to nitrogen loss, prompting an alternative strategy: a shell–core structure based on a gas‐tight shell enclosing loose Si3N4 particles. These particles dissolve during HIP, enriching the matrix with nitrogen and enabling the production of HNS. This article highlights the potential for extending this approach to other stainless steel groups, broadening the possibilities for HNS production through both conventional HIP and PBF‐LB/M manufacturing routes.

Effect of Processing Parameters on Recrystallization During Hot Isostatic Pressing of Stellite-6 Fabricated Using Laser Powder Bed Fusion Technique

Crack-free Stellite-6 alloy was fabricated using the laser powder bed fusion technique equipped with a heating module as the first attempt. Single tracks were printed with a build plate heated to 400 °C to identify the processing window. Based on the melt pool dimensions, two combinations (sample A: 300 W/750 mm/s and sample B: 275 W/1000 mm/s) were identified to print the cubes. The as-printed microstructure comprised FCC-Co dendrites with M7C3 in the interdendritic region. W-rich M6C particles were found in the overlapping regions between the melt pools, matching the Scheil simulations. However, gas pores were observed due to the higher nitrogen and oxygen content of the feedstock requiring hot isostatic pressing (HIP) at 1250 °C and 150 MPa for 2 h. Sample A was partially recrystallized with slightly coarsened M7C3, while sample B underwent complete recrystallization followed by grain growth along with higher coarsening of the M7C3 after HIP. The varying recrystallization behavior can be attributed to the difference in residual stresses and grain aspect ratio in the as-built condition dictated by laser power and scanning speed. The microhardness after HIP was slightly higher than its wrought counterpart, indicating no severe impact of post-processing on the properties of Stellite-6 alloy.

Advancing laser powder bed fusion with non-spherical powder: Powder-process-structure-property relationships through experimental and analytical studies of fatigue performance

This study investigates the multifaceted interdependencies among powder characteristics (i.e., non-spherical morphology and particle size ranging 50-120 or 75-175µm), laser powder bed fusion (L-PBF) process condition (i.e., contouring), post-process treatments (i.e., hot isostatic pressing (HIP) and mechanical grinding) on the pore, microstructure, surface finish, and fatigue behavior of additively manufactured Ti-6Al-4V samples. Microstructure analysis shows a phase transformation α′ → α+β microstructure after HIP treatment (at 899±14 °C for 2h under the applied pressure of 1034±34bar) of the as-built Ti-6Al-4V parts. The findings from pore analysis using micro-computed tomography (μ-CT) show an increase in sub-surface pores when relatively smaller powders are L-PBF processed including contouring. Surface optical profilometry reveals a decrease in surface roughness when fine powder is L-PBF including contouring. Pore analysis conducted through μ-CT reveals that the presence of lack-of-fusion pores within the L-PBF processed coarse powder is more pronounced when compared to the fine powder. Furthermore, HIP treatment does not eliminate these pores. The fracture failure in as-printed parts occurs at the surface, while the combination of HIP and mechanical grinding alters crack initiation to subsurface pore defects. Fractography reveals that HIP and as-built samples followed the facet formation and pseudo-brittle fracture mechanisms, respectively. Fatigue life assessments, supported by statistical analysis, indicate that mechanical grinding and HIP significantly enhanced fatigue resistance, approaching the benchmarks set by wrought Ti-6Al-4V alloy. A fatigue prediction model which considers the surface roughness as a micro-notch has been used.

S1aUnnotched fatigue behavior of the lased powder bed fused and hot isostatic pressed Inconel 718 at 600 °C

The microstructure and mechanical properties of hot isostatically pressed (HIP) Inconel 718 additively manufactured using the laser beam powder bed fusion (LPBF) process were investigated at 600 °C, with emphasis on the high cycle fatigue behavior evaluated using the rotating bend fatigue tests, and elucidation of the role of γ” precipitate size on the fatigue resistance. Experimental results show that the unnotched fatigue strength (σf) of the peak aged alloy improves only by 7% after HIP, as compared to that of the non-HIP peak aged alloy, despite a significant improvement in the ductility due to HIP. Over-aging the HIP alloy, however, led to a further enhancement (by ∼14%) in σf. Fractographic analyses suggest that the fatigue crack initiation occurs from the intrusions and extrusions on the fracture surface of both versions of the aged alloy. Microstructural investigations reveal the presence of persistent slip bands (PSBs) with different morphologies in peak- and over-aged microstructures. A change in the deformation mechanism due to the size of precipitates was identified to be the reason for the formation of nano-grains (through dynamic recrystallization) within the PSBs of the alloy with a coarse γ” microstructure. In several {110} grains, ∼5 μm thick nanocrystalline grain zones formed on the specimen surface; they completely suppressed the formation of PSBs and enhanced the fatigue resistance of the HIP LPBF IN718 alloy in the over-aged condition.

Tensile properties of R512E coated Nb[sbnd]C103 manufactured using laser powder bed fusion (L-PBF)

C103 is a niobium based refractory alloy commonly used in the aerospace industry. To protect the metal from high temperature, oxidizing environments, C103 is often coated with an environmental barrier coatings. A common coating for this material is known as R512E, a disilicide slurry applied and cured with heat. This paper aims to determine the effects on mechanical properties of common post processing techniques used on C103 produced via laser powder bed fusion. Tensile specimens were produced in both horizontal and vertical orientations, post processed via hot isostatic pressing, coated, and further heat treatment to simulate a typical operating environment for the coated material. Tensile tests were then conducted at room temperature and compared to ASTM B654 standards for wrought C103. Strength decreased after HIP and coating but remained unchanged after further heat treat. Grain growth was evident after HIP and coat applications which is in direct correlation with decreasing strength and increasing ductility.

Grain boundary migration hysteresis during recrystallization of IN738LC alloy fabricated via laser powder bed fusion

The metastable microstructure formed during laser powder bed fusion (LPBF) significantly influences recrystallization behavior, which is crucial for flexible control of microstructure. To investigate these effects, various IN738LC alloys were fabricated by adjusting the volume energy density (VED) and then subjected to the same hot isostatic pressing (HIP) treatment. The microstructures before and after heat treatment were systematically analyzed using multi-scales characterization methods. The results indicate that the recrystallization fraction in HIP-treated samples decreased from 75% to 24% as the VED increased from 53 J/mm3 to 125 J/mm3. This reduction is primarily attributed to the increased grain boundary migration hysteresis caused by the higher density and larger diameter of MC-type carbide particles (i.e., MC particles). Subsequently, these HIP-treated samples were further processed using standard heat treatment (SHT) to evaluate their tensile properties at 23 °C and 900 °C. At 23 °C, the sample with the lowest recrystallization fraction (VED = 125 J/mm3) exhibited the best combination of strength and ductility, with an ultimate tensile strength (UTS) of 1422 MPa, yield strength (YS) of 932 MPa, and elongation of 21%. At 900 °C, the sample with a medium recrystallization fraction (VED = 75 J/mm3) demonstrated the best combination of strength and ductility, with an UTS of 626 MPa, YS of 420 MPa, and elongation of 15.5%.

Microstructure and characteristics of Zn and Mg-doped LiNbO3 materials for electrochromic devices

In this investigation, polycrystalline targets of lithium niobate (LN), zinc-doped lithium niobate (Zn:LN), and magnesium-doped lithium niobate (Mg:LN) were synthesized utilizing the Hot Isostatic Pressing (HIP) and Cold Isostatic Pressing (CIP) sintering technique. Subsequently, amorphous thin films of LN, Zn:LN, and Mg:LN and electrochromic devices were fabricated via radio frequency (RF) magnetron sputtering. Comprehensive characterization of the microstructure and optoelectronic properties of the targets, films and devices was conducted employing scanning electron microscopy, X-ray diffraction, spectrophotometry, atomic force microscopy, and an electrochemical workstation. Studies have shown that doping with zinc and magnesium enhances the sintering quality of ceramics, reducing lithium vacancies in polycrystalline lithium niobate ceramics. Zinc doping increases the resistance of lithium niobate, whereas magnesium doping introduces a secondary phase that decreases the resistance. The resistances of LN, Zn:LN, and Mg:LN polycrystalline ceramics are 8.35 GΩ, 9.80 GΩ, and 7.61 GΩ, respectively. The doping process refines the growth of the target and film particles, enhancing the microstructural quality. Notably, the ionic conductivity of the Mg:LN film escalates to 1.7 × 10−5 S/cm. Using doped lithium niobate thin films as electrolytes, the electrochromic devices showed a 38 % increase in coloration efficiency and a 46 % improvement in bleaching efficiency, significantly enhancing device performance.

Comparative Review of the Microstructural and Mechanical Properties of Ti-6Al-4V Fabricated via Wrought and Powder Metallurgy Processes

This review examines the microstructural and mechanical properties of a Ti-6Al-4V alloy produced by wrought processing and powder metallurgy (PM), specifically laser powder bed fusion (LPBF) and hot isostatic pressing. Wrought methods, such as forging and rolling, create equiaxed alpha (α) and beta (β) grain structures with balanced properties, which are ideal for fatigue resistance. In contrast, PM methods, particularly LPBF, often yield a martensitic α′ structure with high microhardness, enabling complex geometries but requiring post-processing to improve its properties and reduce stress. The study evaluated the effects of processing parameters on grain size, phase distribution, and material characteristics, guiding the choice of fabrication techniques for optimizing Ti-6Al-4V performance in aerospace, biomedical, and automotive applications. The analysis emphasizes tailored processing to meet advanced engineering demands.

Fabrication and characterization of LuAG: Er ceramics with high optical transmission

Highly transparent ceramics based on LuAG with erbium content ranging from 1 to 50 at% were synthesized from powders prepared via chemical precipitation in an aqueous solution. The impact of hot isostatic pressing on the optical properties of these ceramics was investigated. It was demonstrated that employing hot isostatic pressing enhances the optical transmittance of LuAG:Er ceramics. The influence of erbium concentration on the average grain size of the ceramics was also examined. Additionally, temperature-dependent thermal diffusivity profiles of LuAG:Er ceramics as a function of erbium content were analyzed. Erbium concentrations yielding the highest luminescence intensity in spectral ranges around ∼1.5–1.6 μm and ∼2.7 μm were determined.

Hot isostatic pressing of ball milled novel grade high entropy alloys at different sintering time and the investigation of their mechanical and corrosion resistance properties

Abstract With the discovery of high entropy alloys, new materials with superior properties have emerged. According to recent research, high-entropy alloys’ multi-component structure and mixing entropy have made them more prominent than other alloys. Because of their excellent chemical and mechanical properties—such as high hardness, high-temperature resistance, high wear resistance, chemical stability, and high corrosion resistance—high entropy alloys outperform other material types in various applications. A new grade of mechanically alloyed high entropy alloy (HEA) of composition 23Fe-21Cr-18Ni-20Ti-18Mn was consolidated by a hot isostatic pressing (HIP) at a temperature of 1000 °C, at different sintering time of 30, 60, and 90 min respectively. We have investigated the impact of sintering time on the microstructure, mechanical, corrosion, and wear-resistance properties. The x-ray diffraction (XRD) spectra of the 30 min HIPed HEA sample showed dominant s-FeCr phases and traces of γ-Fe, and the Ni-Ti phases. Whereas, the 90 min HIPed HEA samples showed more dominant Ni-Ti and traces of γ-Fe, and β-Mn phases. There is a phase transformation from BCC to HCP of consolidated HEA at increased holding time. The density of the samples increases from 5.882 to 6.327 g cm−3 and the porosity percentage decreases from 12.93 to 6.35% with the increase in the holding time. The Vickers microhardness value for 30, 60, and 90 min HIPed 23Fe-21Cr-18Ni-20Ti-18Mn HEA at 1000 °C was found to be 433, 513, and 793 HV respectively at an indentation load of 0.1 kgf. The consolidated HEA sample undergoes an abrasive and oxidative wear mechanism with ploughing and plastic deformation modes. The morphology of the wear debris was investigated using SEM. The 90 min sintered sample showed an excellent corrosion resistance due to the high rate of material densification and minimum surface flaws.

Inhomogeneous distribution of oxides in various 9Cr ODS alloys and their mechanical performance at room temperature and 650 °C

Oxide dispersion strengthened (ODS) steels have been considered as promising structural materials for fusion reactors. In fabricating ODS steels, homogenous distribution of nano-sized oxides is always expected. In this work, Ti and C were respectively added into the 9Cr alloy with 2 wt% Mn, and the effects of individual Ti addition on microstructure and mechanical performance of ODS alloys are studied. Regularly distributed oxides were revealed in the 9Cr alloys fabricated by hot isostatic pressing (HIP), irrespective of alloying compositions, which indicates the strong interaction between oxides and phase interface. Alloying elements within the matrix can be incorporated into these regularly distributed oxides. Oxide nanoparticles will affect the phase transformation behaviors and the microstructure of as-HIPed 9Cr alloys without oxides is different from that of 9Cr ODS alloys. By performing post-HIP heat treatments, the matrix conditions of 9Cr ODS alloys can be adjusted, and ferritic and martensitic 9Cr ODS alloys are respectively obtained. With tensile tests at room temperature and 650 °C, effects of oxides and matrix conditions on 9Cr alloys were evaluated. It is found that individual Ti addition has a softening effect on the 9Cr alloys, despite that whether oxides are added. Simultaneous addition of Ti and C can improve the strength of 9Cr alloys evidently. Poor ductility at 650 °C is a critical issue for 9Cr ODS alloys, while the 9Cr alloys without oxides have favorable combination of strength and ductility at both room temperature and 650 °C.

Use of aqueous polyvinyl alcohol in binder jetting of Inconel 718

Binders used in binder jetting often pose health and environmental risks during processing and post processing operations. The print-heads which are used to deposit binder selectively on the feedstock are prone to clogging, despite the trend of print-heads being highly customised to suit different kinds of binders. These factors often hide the advantages of binder jetting as an additive manufacturing process, especially its scalability and its faster printing rates in comparison to powder bed fusion methods. The work presented here takes a step back and focuses on the development of an aqueous, polyvinyl alcohol (PVA)-based liquid binder that is easy to manufacture and store, safe to handle, and can be reliably jetted to print parts. The feedstock considered was Inconel 718, a nickel-based super alloy which can be effectively processed by binder jetting without niobium segregation. PVA was added to the Inconel 718 powder in dry, granular form to manufacture a modified feedstock. The study also investigated the role of molecular weight of the PVA used, sintering environments and post-processing methods like hot isostatic pressing (HIP) on process responses like part densification, tensile strength, and hardness. Three different types of PVA were chosen which had molecular weights 10,000 g/mol (low molecular weight or LMW), 26,000 g/mol (medium molecular weight or MMW), and 84,000 g/mol (high molecular weight or HMW). The compatibility of the liquid, aqueous PVA-based binders with virgin Inconel 718 was examined by measuring the contact angle. The liquid, aqueous binder having MMW PVA reported better wetting with the Inconel 718 powder with a wetting angle of 26.6 which was lower than the wetting angle of 42.4°, seen in case of a commercial resin-based binder. The green strength reported by the MMW PVA liquid binder was 220 kPa which was higher than the other two PVA-based liquid binders. Green parts, upon successful printing, were sintered at 1260 °C. It was observed that a part printed using MMW PVA had a densification of 96.16% when sintered in 99.98% by volume argon gas, which increased to 98.96% after undergoing HIP. The same part reported a densification of 88.69% when sintered in a 95% by volume N2 and 5% by volume H2 gaseous environment, which was later attributed to the uptake of nitrogen by the chromium present in Inconel 718, which prevented necking between particles. Tensile specimens printed using MMW PVA, sintered in a 99.98% argon environment, showed the highest ultimate tensile strength of 220 MPa, which increased to 1010 MPa after the HIP process, which can be compared to commercially available Inconel 718.

A review of near‐net shape forming by hot isostatic pressure technology

AbstractNear‐net shaping hot isostatic pressing (NNS‐HIP) forming process has the advantage of high material utilization ratio and good uniformity of billet microstructure. It is widely used to produce critical parts for harsh service environments in special fields such as aerospace and energy and chemical industry. This paper introduces the process characteristics and production flow of hot isostatic near‐net forming, the representative materials of hot isostatic near‐net forming, the development and application at home and abroad, and the micro‐ and macromodels of numerical simulation powder densification by HIP near‐net forming. Finally, the challenges and development trends of this technology are prospected, and corresponding countermeasures are put forward.

Mn2+ doped aluminum-rich spinel transparent ceramic phosphor with higher performance for wide color-gamut backlight display

Due to the great potential of narrow-band green-emitting phosphors for wide color-gamut white light-emitting diodes (WLED) backlight displays, there is great interest in finding host materials with the desired luminescent properties and high quantum efficiency. Herein, a series of Mg0.98-xAl2(1+x/3)O4: 0.02Mn2+ (x = 0, 0.125, 0.25) powders were synthesized using high-temperature solid-phase reactions, followed by a combination of pressureless sintering and hot isostatic pressure sintering to fabricate transparent ceramic phosphors. The crystal structure refinement, NMR spectroscopy, and photoluminescence results show tetrahedral coordination of Mn2+ ions and an increase in cation vacancies and disorder with increasing x. These transparent ceramics exhibit narrow-band green emission, which shifts from 523 to 520 nm with increasing x. The Mg0.855Al2.08O4: 0.02Mn2+ ceramic sample has an in-line transmittance of over 80 % above 500 nm and internal quantum efficiencies as high as 96.2 %. The luminous intensity of the Mg0.855Al2.08O4: 0.02Mn2+ ceramic remains above 88.6 % at 425 K compared to room temperature, with a thermal conductivity of 12.4 W·m−1·K−1. A WLED device fabricated using the Mg0.855Al2.08O4: 0.02Mn2+ ceramic, K2SiF6:Mn4+ phosphors, and a blue chip show a wide color gamut of 126 % National Television System Committee standard. The results show that Mn2+-doped aluminum-rich spinel transparent ceramics have great potential for application in wide color gamut WLED backlight devices.

Numerical Studies of Influencing Factors on the Homogeneity of the Powder Mixture during the Powder Spreading Process of Powder Bed Fusion–Laser Beam/Metal

Recent studies have focused on the alloying of nitrogen (N) in high‐alloy stainless steels by powder bed fusion–laser beam/metal (PBF‐LB/M). However, N‐induced pore formation remains a challenge. To overcome this, a combined PBF‐LB/M and hot isostatic pressing (HIP) process is developed. AISI 304L stainless steel powder was mixed with silicon nitride (Si3N4) powder and processed by PBF‐LB/M, allowing partial retention of Si3N4. This helps to maintain sufficient N content while reducing pore formation. The microstructure is then homogenised by HIP. A major challenge of this process is to maintain the homogeneity of the deposited powder mixture on the powder bed to ensure a consistent N content in the final products. This study combines experimental and numerical methods to address this issue: scanning electron microscopy is used to characterize the microstructure and map the local N content, while various numerical studies based on the discrete element method are carried out to investigate the effects of layer thickness, substrate surface roughness, and powder properties on the intermediate product. The numerical approach effectively predicts the N content distribution and homogeneity on the powder bed. The models are validated with experimental data and optimal process conditions for PBF‐LB/M are identified.

Microstructure and Fatigue Behavior of PM-HIPed Ni-Based Superalloys and Martensitic Tool Steels: A Review

Hot isostatic pressing (HIP) is a near-net shape powder metallurgy (PM) technique, which has emerged as an efficient technique, offering precise control over the microstructure and properties of materials, particularly in high-performance alloys. This technology finds applications across a wide range of industries, such as aerospace, automotive, marine, oil and gas, medical, and tooling. This paper provides an overview of powder metallurgy and hot isostatic pressing, covering their principles, process parameters, and applications. Additionally, it conducts an analysis of PM-HIPed alloys, focusing on their microstructure and fatigue behavior to illustrate their potential in diverse engineering applications. Specifically, this paper focuses on nickel-based superalloys and martensitic tool steels. The diverse microstructural characteristics of these alloys provide valuable insights into the PM-HIP-induced fatigue defects and properties.

Review of Solid-State Consolidation Processing Techniques of ODS Steels (Hot Extrusion, Hot Isostatic Pressing, Spark Plasma Sintering, and Stir Friction Consolidation): Resulting Microstructures and Mechanical Properties

Review of Solid-State Consolidation Processing Techniques of ODS Steels (Hot Extrusion, Hot Isostatic Pressing, Spark Plasma Sintering, and Stir Friction Consolidation): Resulting Microstructures and Mechanical Properties

Effect of HIP on Creep Properties in the Single Crystal Superalloy

The Rene N5 is a second-generation single crystal superalloy containing 3wt% Re, commonly used in gas turbine blades for power generation at temperatures up to 1600°C due to its excellent creep life. Generally, single crystal (SX) and directionally solidified (DS) blades are not applied to hot isostatic pressing (HIP) because their porosities are lower than conventionally cast (CC) blades, and it causes recrystallization issue. However, in this study, HIP was chosen as a candidate to improve the mechanical properties of the second-generation single crystal superalloy Rene N5. HIP followed by ultra-rapid cooling rate of 660K/minute was applied in an attempt to address this issue. This approach was expected to enhance creep rupture properties through pore reduction and microstructural homogenization. Furthermore, by setting the HIP temperature and time identical to the solution treatment condition, the possibility of replacing solution treatment with HIP was discussed. As results of creep rupture tests, the rupture life was excellent in the order of specimens treated with HIP + solution + aging, solution + aging, and HIP + aging treatments. It is interesting that this is found to be primarily influenced by the size and shape of carbides and γ’ phases, with the effects of crystallographic orientation, shrinkage porosity, and eutectic phase being negligible.

Fatigue mechanisms at 450°C of a highly twined (>70%) and HIP-densified IN718 superalloy additively manufactured by laser beam powder bed fusion

Fatigue resistance at elevated temperatures is crucial for certifying aerospace structures additively manufactured by the laser beam powder bed fusion (PBF-LB) method with IN718 superalloy. This study employed a multi-step, supersolvus heat treatment process with hot isostatic pressing (HIP), called RHSA, to minimize pores and brittle phases. Stress intensity factor (△K) calculations using data from X-ray computed tomography and shape factors referencing finite element analysis (FEA) studies confirmed the suppression of △K below the threshold of conventional IN718 (∼5 MPa√m), shifting fatigue behavior to grain-structure-dominated. Despite a very high twin boundary (TB) fraction (>70%), fatigue tests at 450°C and R = 0.1 demonstrated low scatter. Slip trace analysis and high-resolution electron backscatter diffraction (EBSD) revealed that TB-induced strain concentration became prominent only at high △K, causing cracking at 45⁰ to the loading direction. The randomly oriented TBs with higher angles (60⁰) compared to high-angle grain boundaries (HAGBs) (30–40⁰) likely enhanced slip resistance and provided a net strengthening effect, which can explain the lower-than-average TB% along fracture paths. These insights suggest that a high TB fraction is not detrimental if fatigue stress is not excessive, alleviating concerns about annealing twins during defect minimization in AM IN718, allowing novel processes to improve fatigue resistance in PBF-LB IN718.