Hot Isostatic Pressing Sintering Research Articles

Enhancing mechanical properties and density of WC-Co with pressureless sintering via shell structure binder jetting additive manufacturing

The binder jetting (BJT) additive manufacturing (AM) process is suitable for producing complex WC-Co cemented carbide parts. To achieve near-full-density in the final parts, the hot isostatic pressing (HIP) sintering process is typically employed to enhance densification. However, HIP sintering requires high equipment specifications and is relatively more costly compared to conventional pressureless sintering. To improve the density of WC-Co parts printed by BJT AM under the pressureless sintering environment, the shell structure printing method was used in this study. The shell structure printing only jets binder to the model's outer profile, creating a shell that encapsulates the central powder, mitigates binder pyrolysis and residual effects on particle densification, and enhances post-sintering density. In this work, the effect of shell thickness, binder type, and printing layer thickness on the density and mechanical properties of the WC-12Co parts under pressureless sintering conditions were investigated. The results show that the density and mechanical properties of the WC-12Co parts significantly improved with the decreasing shell thickness. Specifically, the WC-12Co printed using shell structure achieves a relative density of over 98% and a transverse rupture strength (TRS) of 1954 MPa after pressureless sintering, a result comparable to BJT printed samples treated with HIP sintering. Moreover, the results indicate using a binder with lower residual carbon content and smaller printing layer thickness has a more positive effect on improving density and mechanical strength when employing the shell structure printing method. Microstructure morphology and phase analysis results indicate that the shell structure printing method does not affect the growth of WC grains in the shell and center regions, nor does it influence its phase composition. The shell structure printing method offers a new approach to the manufacturing of near-full-dense WC-Co materials using the BJT process.

Effect of CaO additive on the densification of MgO and MgO-YGAG:Ce ceramics

Highly transparent MgO ceramic was prepared by hot isostatic pressing (HIP) technique of vacuum-sintered MgO compact using CaO as the sintering additive. Densification process of MgO ceramic is significantly enhanced by doping 0.5 at% CaO at the vacuum-sintering temperature of 1100–1350 °C. By subsequent HIP sintering at 1650 °C, the relative density of 0.5% CaO-doped MgO ceramic vacuum-sintered at 1350 °C increases from 94.67% to 99.97%. It attains high in-line transmittance of 70.78% at 600 nm and 87.59% at 6000 nm, respectively. Meanwhile, the thermal conductivity reaches 53.2 W/(m∙K), demonstrating that CaO-doped MgO is a promising candidate material as the secondary phase of phosphor composite ceramics. Herein, 0.5% CaO-doped MgO-YGAG:Ce (Y2.82Gd0.18Al5O12:Ce) composite ceramic with stable binary-phase structure, high relative density of 99.1% and thermal conductivity of 32.6 W/(m∙K) was synthesized by vacuum-sintering at 1350 °C and the luminescence performance under blue laser excitation was evaluated.

Dependence of residual thermal stress in cermets on the sintering technique

The effects of representative sintering techniques on the residual thermal stress (RTS) in the as-prepared cermets were investigated using the WC-Co composites as an example. The real processing parameters of the spark plasma sintering (SPS) and sinter-hot isostatic pressing (Sinter-HIP) techniques were introduced to quantify RTS and its mechanisms. The real microstructures of cermets were used in the models for evaluation of RTS distributions in different phases. The macro-mesoscale coupled calculations indicated that during the cooling of both the SPS and Sinter-HIP processes, the WC phase of the sintered cermets has a state of compressive stress, while Co phase has a tensile stress state. The internal stress magnitude of both phases increases linearly with the decrease of temperature. There are larger temperature and displacement fields in the SPSed cermets compared to those prepared by Sinter-HIP. The stress accumulation in the SPS process is faster than that in the Sinter-HIP by 30%. The RTS in the SPSed cermets is highly concentrated at the acute dihedral angles of WC/Co interfaces, which tends to promote the face-centred cubic (fcc) to hexagonal close-packed (hcp) martensitic transformation of the Co phase.

Influence of Fabrication Method and Surface Modification of Alumina Ceramic on the Microstructure and Mechanical Properties of Ceramic-Elastomer Interpenetrating Phase Composites (IPCs).

The paper presents experimental results of the work conducted to improve the adhesion between alumina ceramics and urea-urethane elastomer in the interpenetrating phase composites (IPCs), in which these two phases are interpenetrating three-dimensionally and topologically throughout the microstructure. Measurements of the contact angle, surface roughness, and shear tests were used to evaluate the effectivity and select the quantity of a silane coupling agent and the ceramic fabrication method. The tests were conducted using samples of dense alumina ceramic obtained by three- or four-step methods. In the four-step process, hot isostatic pressing (HIP) was applied additionally. As a result of the coupling agent coat and HIP application, the ceramic substrate wettability by the elastomer was improved. The water contact angle was reduced from 80 to 60%. In the next step, porous ceramic preforms were fabricated using HIP sintering and a solution of silane coupling agent treated their surface. The composites were produced using vacuum-pressure infiltration of porous alumina ceramics by urea-urethane elastomer in liquid form. The influence of the coupling agent application on the microstructure and mechanical properties of the composites was estimated. The microstructure of the composites was identified using SEM microscopy and X-ray tomography. As a result of using the coupling agent, residual porosity decreased from 7 to 2%, and compressive strength, as well as stress at a plateau, increased by more than 20%, from 25 to 33 MPa and from 15 to 24 MPa, respectively, for the composites fabricated by infiltration ceramic preforms with 40% of porosity.

High-heat flux Cu-0.8Y alloys investigated by positron annihilation spectroscopy

This work studies the thermal stability of the microstructure and the evolution of the defects of two high-heat flux Cu-0.8 wt%Y alloys fabricated following two alternative powder metallurgy routes. One batch was produced by direct hot isostatic pressing (HIP) consolidation of Cu-0.8 wt%Y pre-alloyed atomized powders while an additional ball milling processing step was introduced before HIP sintering for the second alloy. The stability and recovery characteristics of the vacancy type defects in these alloys in the as-produced state and after processing by severe equal channel angular pressing to achieve a refine microstructure have been investigated by positron lifetime and coincidence Doppler broadening measurements in samples subjected to isochronal annealing from room temperature to 900 °C. Microhardness measurements and electron transmission microscopy analysis have also been performed to support the results obtained from the positron annihilation spectroscopy analysis techniques. The recovery curves of the positron lifetime and S-W plots show a recovery stage in agreement with the recovery stage V for Cu. However, a full recovery is not accomplished, and a stage that reverts the previous recovery takes place after annealing above ~600 °C, that leads to the formation of very stable defects at temperatures up to 900 °C, identified as vacancy aggregates and nanocavities. The characteristic shape of the coincidence Doppler broadening indicates that the dispersed Y-O particles in the Cu matrix appear to be responsible for stabilizing the vacancy aggregates and nanocavities for temperatures above 600–700 °C.

Evolution of Microstructure and Elements Distribution of Powder Metallurgy Borated Stainless Steel during Hot Isostatic Pressing

Prepared by powder metallurgy process incorporating atomization and hot isostatic pressing (HIP) sintering at six different temperatures from 600 to 1160 °C, borated stainless steel (BSS) containing boron content of 1.86 wt% was studied. The phase of BSS, relative density of different temperature, microstructure, elemental distribution, and mechanical properties were tested and analyzed. The phases of the alloy were calculated by the Thermo-Calc (2021a, Thermo-Calc Software, Solna, Sweden) and studied by quantitative X-ray diffraction phase analysis. The distributions of boron, chromium, and iron in grains of the alloy were analyzed by scanning electron microscopy and transmission electron microscope. The grain size distributions and average grain sizes were calculated for the boron-containing phases at 900, 1000, 1100, and 1160 °C, as well as the average grain size of the austenite phase at 700 and 1160 °C. After undergoing HIP sintering at 900, 1000, 1100, and 1160 °C, respectively, the tensile strength and ductility of the alloy were tested, and the fracture surfaces were analyzed. It was found that the alloy consisted of two phases (austenite and boron-containing phase) when HIP sintering temperature was higher than 900 °C, and the relative density of the prepared alloys was higher than 99% when HIP temperature was higher than 1000 °C. According to the boron-containing phase grain size distribution and microstructure analysis, the boron-containing phase precipitated both inside the austenite matrix and at the grain boundaries and its growth mechanism was divided into four steps. The tensile strength and elongation of alloy were up to 776 MPa and 19% respectively when the HIP sintering was at 1000 °C.

Fracture behavior of cemented carbides with CoNiFe medium entropy alloy binder

In this study, we prepared WC-CoNiFe and WC-Co with different grain sizes by sintering-hot isostatic pressing (SHIP) and evaluated their fracture behavior. The results showed that WC-CoNiFe has a higher fracture toughness (between 13.16 and 17.82 MPam1/2) than WC-Co (between12.08 and 16.21MPam1/2). Moreover, fracture toughness and fatigue crack growth (FCG) were closely related to the corresponding microstructural characteristics. The fracture toughness increased via ductile ligament bridging and crack deflection as the mean free path of the binder phase (λCo) increased, or the contiguity of the carbide phase (CWC) decreased. The fatigue fracture mode showed a transition from a cleavage-like brittle fracture to a ductile dimple-like fracture. The fatigue sensitivity of cemented carbides was positively related to the fracture toughness. As the fracture toughness increased, cemented carbides exhibited a transition from ceramic-like material to metal-like material. Considering two important controlling parameters of FCG behavior, the effect of maximum stress intensity factor (Kmax) on FCG was dominant compared to the stress intensity factor range (∆K). However, the difference in the relative influence of these two parameters diminished as the fracture toughness increased. Moreover, with the rise in Kmax value, we defined three stages of the FCG process: crack initiation, stable crack growth, and unstable crack growth. The results suggest that CoNiFe medium entropy alloy is a promising material for application as a binder phase of cemented carbides with high toughness.

A new quaternary Li0.19Al2.72O3.64N0.36 transparent ceramic with high hardness

A new quaternary Li0.19Al2.72O3.64N0.36 transparent ceramic with the Vickers hardness of 17.91 ± 0.2 GPa at a load of 9.8 N and the in-line transmittance of 83.4% at 3.5 µm was prepared by hot isostatic press sintering (HIP) after pressureless sintering. The 7Li and 27Al magic-angle spinning (MAS) NMR spectra combined with the XRD Rietveld refinement was adopted to analyze the crystal structure. It is revealed that all the lithium ions are located in octahedra and the cation vacancies are mainly concentrated in tetrahedra. Based on the bond valance models, the complicated relationship among composition, crystal structure and intrinsic hardness of the spinel-type transparent ceramic were disclosed. Compared with the γ-AlON, the higher hardness of Li0.19Al2.72O3.64N0.36 transparent ceramic is attributed to the coupling effect of enhanced hardness of bonds in octahedra and the slightly weakened hardness of bonds in tetrahedra.

Fabrication of Yb:Lu2O3 Transparent Ceramics by Hot Isostatic Pressing: the Effect of Yb Doping Concentrations

Highly transparent ytterbium doped lutctium oxide (Yb:Lu2O3) transparent ceramics with different Yb doping concentrations (0.5–10 at% Yb) were fabricated from commercial powders by vacuum sintering combined with hot isostatic pressing (HIP) method. The effects of Yb3+ doping concentrations on the microstructure, optical performance and the thermal conductivity of Yb:Lu2O3 transparent ceramics were investigated. The grain size of Yb:Lu2O3 ceramics first decreased with Yb doping concentration increasing and then kept nearly unchanged at high Yb doping concentrations. Under 1750°C HIP sintering for 2h, The in-line transmittance of the 5 at.% Yb:Lu2O3 ceramics (3 mm thick) exceeds 81.52% at 1100 nm. And it had the average grain size about 2μm. As the doping concentrations increase, the overall thermal conductivity decreases slightly. The fluorescence emission spectrum implied high possibility of realizing laser output at 1080 nm by pumping at 980 nm.

Comparison of a commercial powder and a powder produced from Ti-6Al-4V chips and their effects on compacts sintered by the sinter-HIP method

The present paper is related to the conversion of Ti-6Al-4V chips into powder and investigates the usability of the produced powder in powder metallurgy applications. In this regard, a disc-milling process was applied to Ti-6Al-4V chips and the obtained powder was subsequently compacted. The compacted samples were sintered by the sinter hot isostatic pressing (sinter-HIP) method at 1200°C under high vacuum, their mechanical properties and microstructure were investigated and compared with those of commercial powder compacts subjected to the same preparation processes. The results showed that the produced powder exhibits greater flowability and higher apparent density than the commercial powder. However, the sintered products prepared from the commercial powder exhibited a higher relative density, lower porosity, and, as a result, greater flexural strength compared with the sintered compacts prepared from the produced powder. In addition, transgranular fracture was greater in the sintered products of the commercial powder. The microstructural studies revealed that the sintered products made from both the commercial and the produced powders consisted of α- and β-phase but contained more α-phase. All of the examined properties were found to be substantially affected by the particle size of the powders.

Microstructure and properties of WC-17Co cermets prepared using different processing routes

WC-17Co cermets were prepared using three different processing routes. An as-ball milled powder mixture (BMPM) and pre-solid solution treatment powder (PSTP) were used as feedstock. Sinter-hot isostatic pressing (SHIP) and high-velocity oxy-fuel (HVOF) thermal spraying were used as consolidation methods. The microstructure and properties of the WC-17Co cermets prepared using the BMPM-SHIP, PSTP-SHIP, and PSTP-HVOF routes were characterized and systematically compared. Solid-solution strengthening of the Co binder phase accounted for the higher hardness and lower fracture toughness (KIC) of the cermets produced using the PSTP-SHIP and PSTP-HVOF routes compared with those of the BMPM-SHIP-produced cermet. Moreover, the SHIP method yielded WC grains featuring an equilibrium truncated triangular prism morphology, which were faceted compared to the round-edged WC grains prepared using HVOF. Owing to the formation of a dense protective tribofilm on its surface, the cermet prepared using the BMPM-SHIP route exhibited the lowest wear rate of 0.13 × 10−7 mm3/(Nm), while the wear rates of the cermets prepared using PSTP-SHIP and PSPT-HVOF were 0.24 × 10−7 and 0.96 × 10−7 mm3/(Nm), respectively.

Dual‐grained (Ti, W)C–Ni cermets by two‐step carbonization: Hot isotropic press sintering of NiTiW alloys and colloidal graphite

AbstractReactive sintering of crushed metals and carbon is advantageous for preparing toughened complete solid‐solution cermets (CSCs). Here, dual‐grained (Ti0.6, W0.4)C–18Ni cermets were prepared using a two‐step carbonization method. High‐energy milling of Ni–Ti–W–C (Ni(Ti0.6, W0.4)4C) mixture afforded NiTiW solid solution, which was then transformed into Ni6W6C by heating at 1250°C. Twinned (Ti, W)C platelets were formed by the carbonization of Ni2W4C during liquid sintering. W atoms that accumulated in the (111) planes of TiC promoted the {111} twin formation, leading to sandwich‐type (Ti, W)C platelets. Dense (Ti0.6, W0.4)C–18Ni cermets were obtained by hot isostatic press sintering in Ar with satisfactory Hv of 12.8 ± 0.2 GPa, high strength of 1960 ± 84 MPa and toughness of 15.80 ± 0.6 MPa·m1/2. A higher sintering temperature or graphite content accelerated the carbonization, resulting in coarser platelets with decreased aspect ratio. This study provides a new approach to modify the microstructure of CSCs and a method for preparing dual‐grained cermets.

Metrological characteristics of newly developed reference Vickers hardness blocks for the high hardness range

The paper describes the development and characterisation of metrological characteristics of reference Vickers hardness blocks for the high hardness range, development of the system for indentation analysis and interlaboratory comparison. The primary goal of the research was to develop the reference Vickers hardness block from nanostructured hardmetal with a hardness value of approximately 2000 HV1. Based on long-term and extensive research, reference Vickers hardness blocks with optimal metrological characteristics were selected and presented in the paper. Two blocks were consolidated using different powder metallurgy processes; sintering in a hydrogen atmosphere and a sinter hot isostatic pressing (sinter-HIP) process were described. The surfaces of the blocks were researched in terms of material and metrological characteristics and prepared to maintain the requirements according to EN ISO 6507-3:2005, concerning flatness, parallelism and surface roughness. In order to achieve more accurate and precise measurements of the indentations, the lengths of diagonals were measured using the 1D optical measurement system (1D OMS) developed by the National Laboratory for Length HMI/FSB-LPMD for high accuracy line scale calibration. Custom software was developed in Visual Studio 2012. To confirm the reliability of the hardness values determined by the 1D OMS and newly developed software for indentation analysis, the interlaboratory comparison was performed on the primary micro-hardness machine installed in the Physikalische-Technische Bundesanstalt (PTB), Germany. From the conducted research it was concluded that preliminary Vickers hardness blocks were developed successfully from nanostructured hardmetals. Higher hardness and lower non-uniformities over the test surface were achieved for fully dense blocks consolidated by a sinter-HIP process. The intercomparison confirmed the reliability of the measured results determined on the reference hardness standard and 1D OMS with newly developed software.

Microstructure and Hardness of Zirconia Toughened Alumina (ZTA) Ceramics by Hot Isostatic Press Sintering

Microstructure and Hardness of Zirconia Toughened Alumina (ZTA) Ceramics by Hot Isostatic Press Sintering

Microstructure and mechanical properties of WC–(Ti, M)(C, N)–Co cemented carbides with different nitrogen contents

Three kinds of (Ti, M)(C1−x, Nx) (M = Ta, Nb, W) quaternary solid-solution powders with various nitrogen contents were synthesized by carbothermal reduction–nitridation (CRN) process. Effect of nitrogen content on the microstructure and mechanical properties of WC–(Ti, M)(C1−x, Nx)–Co cemented carbides fabricated by sinter-hot isostatic pressing (HIP) were systematically investigated in this paper. The results show that the nitrogen content in the carbonitride raw powders strongly influences the morphology, the grain size and the compositions of the cemented carbides. All the cemented carbides with different nitrogen contents have a similar microstructural appearance: weak core–rim structure consisting of solid-solution phase embedded in WC–Co system. It is also observed that the carbonitride solid solution could somewhat reduce the WC grain growth, and the effectiveness of refining and the rim thickness are sensitively related to the nitrogen content of (Ti, M)(C1−x, Nx) solid solutions. In addition, with an increase in the nitrogen content, mechanical properties of these hard metals are enhanced, which were discussed in terms of the constituent, the microstructure and the solution behavior of the carbonitride solid solutions.

Hot isostatic pressing of transparent AlON ceramics with Y2O3/La2O3 additives

Highly transparent AlON ceramics were fabricated by hot isostatic pressing (HIP) of the sintered bodies composed of fine grains (∼20 μm). Sintering additives play an important role on the porosity and pore positions of the sintered AlON bodies, which can determine the final pore elimination during HIP sintering. Compared to single Y2O3 or La2O3 additive, the codoping of Y2O3–La2O3 additive is more effective to prepare sintered AlON body with small intergranular pores and lower porosity. Effects of additives on the phases, microstructures, as well as optical transmittance of the HIPed AlON ceramics were also extensively investigated.

HIP and pressureless sintering of transparent alumina shaped by magnetic field assisted slip casting

Alumina green bodies were shaped by slip casting under a strong magnetic field. Alumina ceramics were obtained by hot isostatic pressing (HIP) at 1275°C and pressureless sintering at above 1700°C, respectively. HIP-sintered alumina, with in-line transmittance of 52% at 600nm and 82% at 2μm, showed a sub-micrometer grain microstructure and a slight grain orientation. However, XRD results demonstrated the grains of pressureless sintered alumina sintered at >1700°C oriented along c-axis, and texture microstructure was observed by SEM. It indicated that magnetic field assisted slip casting produced “seeds” of grain orientation, and orientation degree that could achieve depended on the sintering temperature.

Study on the mechanical properties, microstructure and corrosion behaviors of nano-WC–Co–Ni–Fe hard materials through HIP and hot-press sintering processes

Abstract This study aims to explore a series of Hot Isostatic Pressing (HIP) and hot-press sintering processes in order to examine the effects on the mechanical properties, microstructures and corrosion behaviors of micro- and nano-WC materials. The experimental results showed that the transverse rupture strength (TRS) values of micro- and nano-WC increased to 1627.3 and 1842.7 MPa after 1250 °C, 125 MPa, 100 min HIP treatments, respectively. Meanwhile, the porosity decreased slightly. The corrosion test results also showed that HIP-treated micro- and nano-WC effectively improved corrosion resistance in a 3.5 wt% NaCl solution. In addition, the lowest porosity (0.21%), highest hardness (91.7 HRA) and highest K IC (14.7 MPa√m) values appeared in nano-WC after 1250 °C, 15 MPa, 1 h hot-press sintering. Moreover, the hot-press sintering procedure significantly inhibited the grain growth of the tungsten carbide materials.

Processing of CNTs Reinforced Al‐Based Nanocomposites Using Different Consolidation Techniques

In this work, the development of two types of Al‐based alloys with different concentrations of Si reinforced with MWCNT’s at 0.5–2.0 wt% is presented. Sonication of the CNT’s in ethyl alcohol was carried out for dispersion, and the mixtures were ball milled for 1, 3, and 5 hrs. SEM/EDS were used to study the morphology and the effects of changing milling parameters in addition to changes caused due to increasing concentration of the CNT’s. Furthermore, three sintering techniques, namely, Spark Plasma Sintering (SPS), Microwave Sintering (μWS), and Hot Isostatic Press Sintering (HIP) were employed to consolidate the ball milled powders at varying temperatures of 400, 450, and 500°C. It was found that SPS consolidated samples showed the most promising results amongst the three with the highest hardness values; around 100% densification, as well as the finest microstructure. On the other hand, microwave sintered samples showed the least appealing results, this could be attributed to the poor temperature distribution and the pressureless nature of the technique. A sintering temperature of 500°C was found to be the most suitable for these types of alloys.

ChemInform Abstract: Preparation of Zirconia-Toughened Bioactive Glass-Ceramic Composite by Sinter-Hot Isostatic Pressing.

High-strength bioactive ceramics, MgO-CaO-SiO 2 -P 2 O 5 glass-ceramic composites toughened by zirconia, were prepared by sinter-hot isostatic pressing (sinter-HIPing) to achieve easier mass production and higher reliability. Raw materials for preparing a dense presintered body were investigated to obtain almost complete densification by sinter-HIPing. It was found that the densification of a presintered body was influenced by residual glassy phases in the crystallized glass particles. By using a controlled crystallized glass powder and a zirconia powder as raw materials, a presintered body with a relative density higher than 94% was prepared, and then it was densified to near its theoretical density by sinter HIPing. This bioceramic exhibited an extremely high bending strength of 400 to 1000 MPa and fracture toughness of 3 to 5 MPa.m 1/2 for 30 to 80 vol% of zirconia