Capsule-free Hot Isostatic Pressing Research Articles

Consolidation of water-atomized chromium–nickel-alloyed powder metallurgy steel through novel processing routes

When processing powder metallurgy (PM) steels, the conventional press and sinter route can reach a relative density up to 95%, which is insufficient for applications when dynamic mechanical performance is critical. In this study, a novel route is demonstrated consisting of cold isostatic pressing (CIP) followed by sintering and capsule-free hot isostatic pressing (HIP), allowing to achieve full density PM steels. Water-atomized steel powder admixed with 2 wt.% Ni was subjected to CIP and followed by sintering in 90N2/10H2 atmosphere at 1120 and 1250°C, and in vacuum (10−2 mbar) at 1250 and 1350°C, respectively. At the highest explored CIP pressure of 600 MPa, the three high-temperature sintering runs at 1250°C in 90N2/10H2 atmosphere and vacuum, and 1350°C in vacuum resulted in relative density of ∼94% and closed surface pores. This condition with necessary closed porosity then allowed subsequent capsule-free HIP after sintering, resulting in full densification of the components.

Optical properties of Tb3+- and Cr3+-doped MgAl2O4 ceramics prepared by capsule- and carbon-free hot isostatic pressing

Transparent spinel with high real in-line transmittance (RIT up to 79%) was prepared from commercial spinel powder by capsule-free hot isostatic pressing (HIP). Optimal pre-sintering conditions were found in order to enhance the RIT values and doping with Cr3+ (0.01–0.04 at%) and Tb3+ (0.05–0.125 at%) secured photoluminescence. The incorporation of the dopants in tested range does not noticeably reduced the RIT. The Cr3+-doped MgAl2O4 shows efficient narrow red emission at about 685 nm (R-line) due to the 2Eg→4A2g transition of Cr3+ ions located in octahedral sites of D3d local symmetry with a strong crystal field. The Tb3+-doped MgAl2O4 ceramics have shown emissions from blue to red spectral range (5D4→7F3–6 transitions), with prominent green emission centered at 545 nm. The maximal emission intensity was achieved at 0.075 at% of Tb3+, Further increase of Tb3+ content led to the concentration quenching caused by Tb3+ ions segregation at the grain boundaries.

Grain growth during capsule-free hot isostatic pressing

This paper provides a new insight to the capsule free hot isostatic pressing (HIP) of alumina and yttria-stabilized zirconia ceramics. The first part of the paper focuses on the pre-sintering stage, namely the impact of conventional (2, 5, 10 °C/min) and rapid heating rate (100 °C/min) on pore closure and grain growth. It is shown that the rapid heating neither changes the pore closure behavior nor restricts the grain growth compared to conventional pre-sintering. The second part investigates the influence of temperature, dwell time, and applied pressure on the microstructure development during HIP. Our findings indicate the existence of a kinetic window, providing the optimal HIP conditions (1200–1250 °C, 200 MPa, 1–4 h for investigated powders) for production of almost fully dense compacts with only minimal grain growth. Our results also show that the applied pressure (up to 200 MPa) does not facilitate the grain boundary mobility of the investigated materials even at a higher temperature (1300 °C) at which the grain growth normally occurs. The potential of optimized capsule-free HIP was demonstrated by preparation of a transparent c-YSZ sample by a combination of rapid pre-sintering and optimized post-HIP.

Capsule-free hot isostatic pressing of sintered steel to full density using water atomised iron and Cr-alloyed powder consolidated by cold isostatic pressing

ABSTRACT Powder metallurgy (PM) steel with enhanced density is essential for meeting the demands on high strength and high-performance applications. In this study, cold isostatic pressing (CIP) is applied for consolidating water-atomised iron and Cr-alloyed steel powder to reach about 95% densification before sintering. Additionally, the effect of the lubricant and graphite addition on the compressibility of the different base powder types was evaluated. The maximum green densities of ≈7.55 g cm−3 for iron powder and ≈7.45 g cm−3 for Cr-alloyed powder were reached after CIP at 800 MPa. The sintering at 1250°C with added graphite allowed to achieve increased densification to ≈7.60 g cm−3 for the iron and ≈7.50 g cm−3 for Cr-alloyed grades, respectively. These levels were sufficient for surface pore closure, which allowed the application of the capsule-free hot isostatic pressing (HIP) after sintering to reach full density. This approach paves the way for manufacturing large-sized PM components.

A net-shape forming process of Ti–6Al–4V sphere joints

ABSTRACT Near-net shape forming of the hollow sphere joints (60 mm in diameter) used in large space truss structure of spacecraft was realised by powder injection molding and diffusion bonding. For metal injection molding (MIM) process, two binder systems based on polyoxymethylene (POM) and paraffin wax (PW) are comparatively studied. The powder loading, power law index, viscous flow activation energy and integrated rheological factors of feedstock were systematically studied. Except powder loading, The feedstock based on POM system has a smaller strain and temperature sensitivity (n = 0.368, E = 17.95 kJ mol−1) and better rheological properties (α = 6.3). The lowest interstitials contents of as-sintered samples are 0.05 wt-% for C, 0.28 wt-% for O and 0.046 wt-% for N, which meet the Grade 5 PM (Ti–6Al–4V) specifications according to ASTM B988-13. The tensile strength and ductility reach 978 MPa and 6.3%, respectively, after 1300 °C sintering and capsule-free hot isostatic pressing (HIP).

Influence of hot isostatic pressing on the properties of 316L stainless steel, Al-Mg-Sc-Zr alloy, titanium and Ti6Al4V cold spray deposits

In this paper an effect of capsule-free hot isostatic pressing (HIP) on the porosity and mechanical properties of different cold spray deposits was experimentally evaluated. The HIP post-treatment significantly decreased (more than 2 times) the porosity of 316L and pure titanium deposits. The densification effect of HIP on the porous Ti6Al4V deposit was significantly less pronounced. Mechanical properties of the deposits after HIP were also improved. In particular, the tensile strength of the stainless steel, pure titanium and Ti6AL4V increased from 30 MPa, 70 MPa and 20 MPa to 211 MPa, 480 MPA and 560 MPa correspondingly. Compressive strength of the Al-Mg-Sc-Zr alloy samples after HIP was also increased ~25%. Improvement of mechanical properties is explained by material diffusion and microstructure changes during HIP cycle.

Microstructure of Al–Mg-Sc-Zr alloy cold spray deposits after heat treatment and hot isostatic pressing

Cold spraying is a coating deposition and additive manufacturing method capable of producing aluminum alloy parts at a high production rate under ambient conditions. In this work, the cold spray was applied for the deposition of AlMgScZr alloy initially developed for laser bed powder fusion additive manufacturing. The low-porosity deposits were successfully produced. Different heat-treatments as well as capsule-free hot isostatic pressing (HIP) were performed to homogenize the structure. TEM analysis of the deposits showed that heat-treatment at 300°С (3 h) + 360°С (4 h) allowed to release the Al3(Scx,Zr1-x) hardening phase with mean grain size 7.6 nm. At the same time, the heat treatment had a limited influence on the material diffusion between the splats. Hot isostatic pressing conducted at 160 MPa and 300°С (3 h) + 360°С (4 h) was more efficient for enhancing of material diffusion between splats and also allowed to the material hardening by the release of Al3(Scx,Zr1-x) hardening phase with the mean grain size 3.1 nm. The compressive strength tests revealed significant improvement of ultimate compressive strength and the strain at failure of the deposits after heat treatment and HIP.

Microstructure and Mechanical Properties of Ti6Al4V Alloy Consolidated by Different Sintering Techniques

In this paper, we investigated the effect of the different sintering techniques including vacuum sintering, capsule-free hot isostatic pressing (HIP), and capsule HIP on the microstructure and mechanical properties of Ti6Al4V alloy. The obtained results indicated that full density Ti6Al4V alloy could be obtained by using capsule HIP technique. The alloy sintered by capsule HIP had the highest hardness (~405 HV) and compressive yield strength (~1056 MPa). It is interesting that the geometry has a significant influence on the relative density and mechanical properties of the alloy sintered by the capsule-free HIP. The relative density, hardness, and compressive yield strength rise from center to periphery of the specimen. This is attributed to the heating and pressing in the capsule-free, which are external, leading to the densification processes starting from the outside to the inner parts of the pressed specimen. Using theoretical prediction with Gibson and Ashby power law found that the yield strength of the alloy sintered by capsule-HIP technique is much lower than that of the calculated value due to the formation of the coarse lamellar microstructure of -Ti grains.

Influence of hot isostatic pressing on structure and properties of titanium cold-spray deposits

In this paper, an experimental study of the influence of capsule-free hot isostatic pressing (HIP) on the microstructure and tensile strength of cold-spray pure titanium deposits is considered. It is shown that hot isostatic pressing at 110 MPa and 1173 K significantly changes the microstructure of titanium cold-spray deposit. Total porosity was decreased from 4.3 to 2.2% due to elimination of small-scale porosity (pores with size less than 5 μm), whereas larger pores were not completely closed. Measured ultimate tensile strength (UTS) of as-sprayed and HIP-treated samples was 110 and 480 MPa, correspondingly. Increase of UTS is explained by material diffusion and microstructure changes during the HIP cycle. At the same time, ductility of the samples after HIP was only ~ 8% which is significantly lower than that for the bulk pure titanium.

Experimental and finite element simulation study of capsule-free hot isostatic pressing of sintered gears

A novel approach to reach full density in powder metallurgy (PM) components is demonstrated in this work. Water-atomised Mo-prealloyed steel powder is utilised for manufacturing cylindrical and gear samples through double pressing and double sintering (DPDS) process route. The effect of sample geometry and powder size fraction on densification is investigated and it is found that the DPDS route enables a density level of > 95% which is sufficient to eliminate the surface open pores. Reaching such high density is necessary, in order to perform capsule-free hot isostatic pressing (HIP). After HIP, full densification is achieved for the cylindrical samples and only near full density is realised for the gears resulting in neutral zone formation due to the density gradient. In order to predict the densification behaviour during the compaction, FEM simulations considering the gear geometry are performed for both the pressing stages and HIP. The simulation predicted a similar densification behaviour with the formation of the neutral zone. The proposed DPDS route with capsule-free HIP in combination with FEM simulation is demonstrated as a potential route for manufacturing full-density PM steel components, e.g. gears, suitable for high-performance applications.

Study of pore closure during pressure-less sintering of advanced oxide ceramics

Capsule-free hot isostatic pressing (HIP) represents a cost-effective variant to classical HIP allowing production of ceramics with top properties. For successful application of capsule-free HIP it is necessary to close all pores to prevent infiltration of the high-pressure gas into ceramics. This occurs when the sample completely transits from 2nd to 3rd stage of sintering, i.e. all open pores get closed at a density called critical density. A robust experimental study of this transition for several materials (alumina, magnesia-alumina spinel, tetragonal and cubic zirconia) has been carried out and compared with three theoretical models. From theoretical models it follows that critical density is a material parameter depending only on dihedral angle and being independent of particle size, green density and pore size distribution. It varies from 88.1% to 93.7% of theoretical density for studied materials and used models. Measured critical densities agree well with models by Beere and Carter & Glaeser for cubic systems (spinel and cubic zirconia). However, they do not match very well experimental data for alumina (hexagonal system) being in agreement with other open sources data. The sophisticated model by Svoboda et al. significantly underestimates the critical density for all systems. The reason of disagreement is analyzed in detail and several hypotheses explaining differences between the model and reality are proposed. It is recommended to use Beere’s and Carter’s & Glaeser’s models for prediction of critical density, while the model by Svoboda et al. characterizes rather the stage of pore closing initiation.

J0330202 多孔質チタン焼結体の性状に及ぼすカプセルフリーHIP処理の影響とその性状改善

Titanium and its alloys of low elastic modulus have been required as materials for the implants. Porous materials have been considered as a way to respond to the request, so the conflicting problem of mechanical strength and porosity has been challenged. By the way, production of porous materials by capsule-free hot isostatic pressing (HP) has been said to be able to control the porosity and mechanical strength simultaneously, hi this study, commercial titanium powders were compacted by several uniaxial loads (4.9-24.5 kN) in a die having a diameter of 15 mm, and the green compactions were sintered by a capsule-free HIP method under an argon gas atmosphere at a holding temperature of 1523 K, a holding time of 1 hour. Porosity of the sintered body was, however, not so high with an average relative density of 72 %. Results showed that characteristic improvement of porous titanium sintered compacts was expected by homogenization of the green compacts, optimization of the HIP pressure and so on.

J0320303 低温焼結により作製されたチタン多孔質体のカプセルフリーHIP処理とその評価([J032-03]ポリマー・セラミックス・メタル多孔質材料の開発と応用(3),材料力学部門)

The low elastic modulus titanium alloy used for implant has been investigated, and the conflicting problem of strength and porosity has been challenged. Manufacturing of porous material by capsule-free hot isostatic pressing (HIP) can be expected for simultaneous control of porosity and mechanical strength. In this study, commercial pure titanium powders were sieved below 150μm and 106μm, and the sieved powders were compacted by hydraulic hand operated pressing machine. Then, the compacts were sintered by electric furnace under atmospheric pressure argon gas at 823K, and finally capsule-free HIPed under high-pressure argon gas (150MPa) at 1523K. Results showed that the HIPed compacts maintained pores sufficiently, and the character was affected by compaction load. The aspect of pore forming was, however, considered to be different in the surface and the inside.

Superelastic Porous NiTi with Adjustable Porosities Synthesized by Powder Metallurgical Method

The effects of cold compaction pressure, TiH2, and NH4HCO3 on the physical and mechanical properties of porous NiTi fabricated by capsule-free-hot isostatic pressing (CF-HIP) were investigated. The porosities vary from 20 to 48% depending on the pressure. Porous NiTi with as great as 42% porosity and irregular pores can be prepared by the addition of TiH2, whereas 48% porosity and circular pores can be achieved by NH4HCO3 addition. All the porous NiTi fabricated by CF-HIP using these three methods exhibit superelastic behavior that closely matches human bones.

Mechanical behavior of cellular borosilicate glass with pressurized Ar-filled closed pores

High strength borosilicate foams were fabricated by melting glass powder under high-pressure argon gas and subsequent heat treatment of the glass bulk at atmospheric pressure. In the first step, borosilicate glass powder was melted at 1100°C for 1h by capsule-free hot isostatic pressing (HIPing) under a high gas pressure of 10–70MPa. Pressurized Ar-filled spherical pores were introduced into the glass, and argon atoms were dissolved in the glass network structure. The expansion of argon-filled pores and the release of the dissolved Ar gas resulted in the formation of pressurized Ar-filled closed pores by isothermal heat treatment at 800°C for 10min. A high porosity of up to 80% with a bimodal distribution of micro-size cells was obtained for the resultant cellular borosilicate glass. By increasing the total gas pressure from 10 to 70MPa, the compressive strength and the Young’s modulus were increased considerably from 15 to 52MPa and from 4.1 to 12.6GPa, respectively, which can be substantially attributed to the high collapse stress from the high enclosed gas pressure. The cellular glass with a high porosity showed a large failure strain under uniaxial compression.

Fabrication of Full-Density Mg-Ferrite/Fe-Ni Permalloy Nanocomposites with a High-Saturation Magnetization Density of 1 T

Dense nanocomposites consisting of spinel Mn-doped Mg-ferrite and permalloy, (Mg[Fe1 − xMnx]2O4(0 ≤ x ≤ 0.4)/53Fe–47Ni), have been fabricated from powder mixtures of MgO/α-Fe2O3/MnO and N2-atomized metal particles utilizing high-pressure cold isostatic press (CIP), pulsed electric current pressure sintering (PECPS), and capsule-free hot isostatic pressing (HIP). Metal particles were coated with the mixed oxide powders homogeneously under high shear compression in Ar. The coated powders were densified into relative densities of 83–85% by high pressure CIPing (1 GPa). They were then presintered at 823 K for 3 min under uniaxial pressure of 100 MPa in Ar using a PECPS. The pre-sintered bodies had relative densities of 93–95% composed of spinel ferrite and permalloy; the mixed metal oxides were changed into magnesium ferrites by heating briefly at low temperature. Then, HIP (1123 K/6 h/196 MPa-Ar) was adopted to densify the pre-sintered bodies into a nearly full density; furthermore, a sintering atmosphere control was performed using ferrite muffles to maintain both phases. Sintered materials consisting of granular metal particles (φ 8.5 μm) isolated and surrounded by a thin ferrite discontinuous layer (0.2–1.3 μm) with a relative density ≥99.5% revealed both a saturation magnetization density ≥1 T and moderate permeability values of 30–35 at 1 MHz.

Fabrication of TiN Particle-Dispersed Al2O3 Composites Utilizing High N2-Pressure SHS

<p>Fabrication of fine TiN particle-dispersed dense Al<sub>2</sub>O<sub>3</sub> composites with the compositions of Al<sub>2</sub>O<sub>3</sub>/TiN=100/0~90/10 vol% has been conducted from Al<sub>2</sub>O<sub>3</sub>/(Ti,TiN<sub>0.3</sub>) mixed powder compacts by capsule-free hot isostatic pressing (HIP) utilizing high-pressure N<sub>2</sub> SHS. Fine Ti powders (φ ~ 0.3 μm) with TiN<sub>0.3 </sub>phase were prepared by thermal decomposition of planetary ball-milled fine TiH<sub>2</sub> powders at 400°C (673 K) for 1 h in a vacuum, followed by heating in N<sub>2</sub> at 200 °C (473 K) for 2 h. The Al<sub>2</sub>O<sub>3</sub> powder compacts (relative densities of 57.2-57.8%) with homogeneously dispersed (Ti,TiN<sub>0.3</sub>) particles were prepared. The mixed powder compacts were hot isostatically pressed (HIPed) under the conditions of 1350°C (1623 K) at 7 MPa N<sub>2 </sub>for 1 h, followed by the heating at the same temperature for 2 h under 196 MPa-N<sub>2</sub>. At the first stage of heating [1350°C (1623K)/7MPa/1h], solid/gas reaction of SHS between (Ti,TiN<sub>0.3</sub>) and N<sub>2</sub> was introduced to form TiN and densification of the Al<sub>2</sub>O<sub>3</sub> powder compacts up to the relative density of 92-93% with closed pores was performed. And at the sequent second stage [1350 °C (1623K)/196MPa/2h], densification of the most of pre-sintered composites consisting of Al<sub>2</sub>O<sub>3</sub> and TiN reached higher relative densities than 98.5%. Dispersion of TiN particles (~φ 0.30 μm) in the composites suppressed the grain growth of Al<sub>2</sub>O<sub>3</sub> during HIP-sintering. Mechanical properties, such as bending strength (σ<sub>b</sub>), Vickers hardness (<em>H</em><sub>V</sub>), fracture toughness (<em>K</em><sub>1C</sub>), and electrical resistivity (ρ) of the composites were evaluated as a function of TiN content; the maximum values of σ<sub>b</sub>=640 MPa, <em>H</em><sub>V</sub>=19.5 GPa, and <em>K</em><sub>IC</sub>=4.5 MPa・m<sup>1/2 </sup>were obtained in the Al<sub>2</sub>O<sub>3</sub>/TiN=97/3~95/5 vol% composites. Among the composites, the lowest ρ value of 2.6×10<sup>3</sup> Ω・m was attained at Al<sub>2</sub>O<sub>3</sub>/TiN=90/10 vol% composite.</p>

Influence of gas pressure on the surface composition of capsule-free HIPed borosilicate glass

Borosilicate glasses have been fabricated by capsule-free hot isostatic pressing. The oxidation reaction of carbon which come from graphite crucible and interfacial processes between carbon and borosilicate glass were important for carbon-rich layer formation on the surface of borosilicate glass material. It led to the evolution of color of glass surface. Effects of total gas pressure on HIP phase diagrams were considered to evaluate the difference of surface layer composition and melting behavior of borosilicate glasses at 1100°C under capsule-free HIPing. It was shown that, below total gas pressure of 40 MPa, CO gas was the most stable substance during glass melting and then carbon-rich layers were formed on the borosilicate glasses surface by precipitation-diffusion process during cooling when keeping high pressure, which led to the surface color change from black to gray under different pressure. When gas pressure was higher 40 MPa, the carbon was stable substance and the surface color was white.

Hydrogen release from titanium hydride in foaming of orthopedic NiTi scaffolds

Titanium hydride powders are utilized to enhance the foaming process in the formation of orthopedic NiTi scaffolds during capsule-free hot isostatic pressing. In order to study the formation mechanism, the thermal behavior of titanium hydride and hydrogen release during the heating process are systematically investigated in air and argon and under vacuum by X-ray diffraction (XRD), thermal analysis, including thermogravimetric analysis and differential scanning calorimetry, energy dispersive X-ray spectroscopy, and transmission electron microscopy. Our experiments reveal that hydrogen is continuously released from titanium hydride as the temperature is gradually increased from 300 to 700°C. Hydrogen is released in two transitions: TiH1.924→TiH1.5/TiH1.7 between 300°C and 400°C and TiH1.5/TiH1.7→α-Ti between 400°C and 600°C. In the lower temperature range between 300°C and 550°C the rate of hydrogen release is slow, but the decomposition rate increases sharply above 550°C. The XRD patterns obtained in air and under vacuum indicate that the surface oxide layer can deter hydrogen release. The pressure change is monitored in real time and the amount of hydrogen released is affected by the processing temperature and holding time. Holding processes at 425°C, 480°C, 500°C, 550°C, and 600°C are found to significantly improve the porous structure in the NiTi scaffolds due to the stepwise release of hydrogen. NiTi scaffolds foamed by stepwise release of hydrogen are conducive to the attachment and proliferation of osteoblasts and the resulting pore size also favor in-growth of cells.

Internal friction of capsule-free HIPed porous alumina

Porous alumina were sintered by conventional sintering and capsule-free Hot Isostatic Pressing (HIPing), at temperatures between 800 and 1500 °C under pressures 0.1 MPa or 200 MPa for 1 h or 50 h. Young’s modulus and internal friction of samples were measured by resonance method. The results show that Young’s modulus is mainly dominated by porosity of material. Capsule-free HIPed porous materials have slightly higher Young’s modulus than conventionally sintered ones at the same porosity. Internal friction is governed by both porosity and specific surface area. Capsule-free HIPed porous alumina has lower internal friction coefficient than conventionally sintered ones at the similar porosity or at the similar specific surface area. Enhanced surface-self diffusion under high gas pressure reduces internal friction coefficient, and affects internal friction more than Young’s modulus.