Effect of particle size on densification of pure magnesium during spark plasma sintering

The effect of particles size ranges (<38 μm, 75–150 μm, 270–550 μm) of atomized magnesium powders on densification mechanisms during spark plasma sintering (SPS) process was investigated. The intrinsic driving force, local pressure and current of Mg powders with different particle sizes were analyzed by theoretical calculation. The results obviously indicate that the densification of pure magnesium can be improved by the reduction of particle size, suggesting the intrinsic driving force, local pressure and current intensity are enhanced significantly by a decrease in the particle size at the same sintering conditions, which can promote shrinkage of pores, formation of the sintering neck and mass transportation in the SPS process. Not only that, rapid densification is also interpreted in term of mechanical movement of particles, Joule heating effect and plastic deformation. However, the mechanical movement of the large particles is higher than that of small particles due to high punch displacement, and plastic deformation, detected by scanning electron microscopy, plays a main role in densification for large particles in the case during the sintering. Joule heating effect is the key factor for densification of small Mg particles, and high densification degree can be obtained by sintering small particles.     

Effect of green density and electric field direction on densification of YAG nano-powders by spark plasma sintering

Nanocrystalline YAG powders with 34-nm average particle size were cold isostatic pressed (CIP) between 10 and 240 MPa to form compacts with 43–52% green densities. Spark plasma sintering (SPS) was performed for 5 min at 1,400 °C and 90 MPa using clusters of three disc specimens packed in series with the electric field direction. The pore size and its volume fraction decreased with the increase in the CIP pressure. The final density varied between 86% and 99% and strongly depended on the specimen location in the series. Density, grain size, and pore size analyses showed that the specimens facing the electric current experienced higher SPS temperatures. Possible temperature gradient due to axial displacement of non-conducting oxide was too low to explain the observed effect. Effect of the green density on the final density was marginal.     

Moving finite-element mesh model for aiding spark plasma sintering in current control mode of pure ultrafine WC powder

The intricate bulk and contact multiphysics of spark plasma sintering (SPS) together with the involved non-linear materials’ response make the process optimization very difficult both experimentally and computationally. The present work proposes an integrated experimental/numerical methodology, which simultaneously permits the developed SPS model to be reliably tested against experiments and to self-consistently estimate the overall set of unknown SPS contact resistances. Unique features of the proposed methodology are: (a) simulations and experiments are conducted in current control mode (SPS-CCm); (b) the SPS model couples electrothermal and displacement fields; (c) the contact multiphysics at the sliding punch/die interface is modeled during powder sintering using a moving mesh/moving boundary technique; (d) calibration and validation procedures employ both graphite compact and conductive WC powder samples. The unknown contact resistances are estimated iteratively by minimizing the deviation between predictions and on-line measurements (i.e., voltage, die surface temperature, and punch displacement) for three imposed currents (i.e., 1,900, 2,100, 2,700 A) and 20 MPa applied pressure. An excellent agreement is found between model predictions and measurements. The results show that the SPS bulk and contact multiphysics can be accurately reproduced during densification of ultrafine binderless WC powder. The results can be used to benchmark contact resistances in SPS systems applicable to graphite and conductive (WC) powder samples. The SPS bulk and contact multiphysics phenomena arising during sintering of ultrafine binderless WC powders are finally discussed. A direct correlation between sintering microstructure, sintering temperature, and heating rate is established. The developed self-consistent SPS model can be effective used as an aiding tool to design optimum SPS experiments, predict sintering microstructure, or benchmark SPS system hardware or performances.     

Spark plasma sintering of ductile ceramic particles: study of LiF

Densification of cuboidal micrometer-sized lithium fluoride particles as ductile ceramic by spark plasma sintering (SPS) was investigated. Specimens were fabricated at different pressures and temperature conditions, ranging from 2 to 100 MPa at 500 °C and from 200 to 700 °C under 100 MPa of applied pressure, respectively. Dense specimens of 99 % relative density were fabricated by heating to 500 °C under constant pressure of 100 MPa. The densification showed first compaction by particle rearrangement, followed by plastic deformation via dislocation glide. Hot-pressing models were used to describe the densification by considering the temperature dependences of the yield stress, the strain hardening behavior and coefficients, and the pore size and shape dependences on the applied stress. A good agreement was found between the experimental and the theoretical densification curves. At low pressure of 2 MPa, the densification occurs by particle sliding, assisted by viscous flow at their surfaces, and local plastic deformation at the particle contacts, due to the intensified local stress. Finally, the micrometer-sized structural features and the contiguity achieved by plastic deformation at the start of spark plasma sintering (SPS) nullify any field effects in this model system at higher pressures; good agreement was obtained with expected conventional hot pressing.     

Superfast densification of nanocrystalline oxide powders by spark plasma sintering

Spark plasma sintering (SPS) is a newly discovered old technique which recently has been used for superfast densification of ceramic powders. Simultaneous application of pulsed high dc current densities and load is the necessary condition for rapid and full densification of ceramic powders by SPS. Commercial nanocrystalline magnesium oxide (nc-MgO) and yttrium aluminum garnet (nc-YAG) powders were densified to optical transparency using spark plasma sintering at distinctly different homologous temperatures (0.3 T _m for nc-MgO and 0.7 T _m for nc-YAG). The observed microstructure, density and grain size evolutions versus the SPS temperature were analyzed. The enhanced densification of the nc-MgO powder at the present SPS conditions was related to plastic deformation followed by diffusion processes. Densification of nc-YAG powder was described by the formation of viscous layer at the particle surfaces and corresponding densification by grain rotation and diffusion through the liquid phase. Densification by normal grain growth takes place at higher relative densities, regardless of the material.     

Microstructure evolution and densification behavior of TiC/316L composite powders during cold/warm die compaction and solid-state sintering: 3D particulate scale numerical modelling and experimental validation

To identify the microstructure evolution and densification behavior of TiC/316L composites in powder metallurgy (PM) process, 3D particulate scale numerical simulations were conducted to reproduce the cold/warm compaction and solid-state sintering of TiC/316L composite powders with corresponding physical experiments being carried out for model validation. The effects of compaction parameters and sintering temperature on the densification behavior of TiC/316L composite powders were systemically investigated. The particle deformation and morphology, stress/strain and microstructure evolutions, and grain size distribution in the whole process were characterized and compared to further illustrate the densification behavior and the underlying dynamics/mechanisms. The results show that compared with the cold compaction, the warm compaction can not only achieve higher relative density, smaller and more uniform equivalent stress, and weaker spring back effect, but also improve the friction condition among powder particles. The plastic deformation of 316L particles is the main densification mechanism during compaction. In the solid-state sintering of TiC/316L compacts, the densification is mainly indicated by shrinkage and vanishing of large residual pores along with the growth of the sintering necks, accompanied by the particle movement and growth along the boundary regions. Meanwhile, the particle displacement and grain size distribution are more uniform in the warm compacted TiC/316L component. Moreover, the equivalent (von Mises) stress in 316L particles is smaller than that in TiC particles.     

Densification and microstructural evolution of spark plasma sintered NiTi shape memory alloy

The effect of particle size and sintering temperature on the densification and microstructural characteristics of nickel-titanium shape memory alloy (NiTi-SMA) has been investigated using spark plasma sintering (SPS) process. The Ni and Ti elements in different particle sizes were alloyed in the composition of Ni_50.6Ti_49.4. The milled NiTi powders were consolidated using SPS process in a temperature range of 700–900?°C. The densification was characterized by plotting temperature, current and relative displacement of punch as a function of holding time. The results showed that a maximum relative density of ～98% can be achieved for NiTi-SMA with an average particle size of 10?µm at a sintering temperature of 900?°C. The microstructure of the sintered NiTi-SMA was examined using scanning electron microscope (SEM) and composition of NiTi alloy was analyzed using energy dispersive spectroscopy (EDS) analysis. The effect of sintering temperature on the microstructural evolution and transformation was also studied.     

In situ boron carbide–titanium diboride composites prepared by mechanical milling and subsequent Spark Plasma Sintering

Boron carbide–titanium diboride composites were synthesized and consolidated by Spark Plasma Sintering (SPS) of mechanically milled elemental powder mixtures. The phase and microstructure evolution of the composites during sintering in the 1,200–1,700 °C temperature range was studied. With increasing sintering temperature, the phase formation of the samples was completed well before full density was achieved. The distribution of titanium diboride in the sintered samples was significantly improved with increasing milling time of the Ti–B–C powder mixtures. A bulk composite material of nearly full density, fine uniform microstructure, and increased fracture toughness was obtained by SPS at 1,700 °C. The grain size of boron carbide and titanium diboride in this material was 5–7 and 1–2 μm, respectively.     

A comparative study of ZnS powders sintering by Hot Uniaxial Pressing (HUP) and Spark Plasma Sintering (SPS)

The sintering of two different polycrystalline zinc sulphide powders has been investigated by two different techniques. Conventional sintering technique (Hot Uniaxial Pressing, HUP) and the Spark Plasma Sintering (SPS) also known as electric field-assisted sintering technique (FAST) have been compared in terms of sintering parameters (temperature, pressure) and optical properties of the prepared ZnS ceramics. This study demonstrates the potentiality to sinter ZnS by SPS in very short times, at lower sintering temperatures, compared with HUP, with good densification (close to the theoretical density) and good optical transmission in the infrared range 8-12 μm.     

A methodology to investigate the intrinsic effect of the pulsed electric current during the spark plasma sintering of electrically conductive powders

Abstract

A novel methodology is proposed for investigating the effect of the pulsed electric current during the spark plasma sintering (SPS) of electrically conductive powders without potential misinterpretation of experimental results. First, ensemble configurations (geometry, size and material of the powder sample, die, plunger and spacers) are identified where the electric current is forced to flow only through either the sample or the die, so that the sample is heated either through the Joule effect or by thermal conduction, respectively. These ensemble configurations are selected using a recently proposed mathematical model of an SPS apparatus, which, once suitably modified, makes it possible to carry out detailed electrical and thermal analysis. Next, SPS experiments are conducted using the ensemble configurations theoretically identified. Using aluminum powders as a case study, we find that the temporal profiles of sample shrinkage, which indicate densification behavior, as well as the final density of the sample are clearly different when the electric current flows only through the sample or through the die containing it, whereas the temperature cycle and mechanical load are the same in both cases.     

Can the use of pulsed direct current induce oscillation in the applied pressure during spark plasma sintering?

The spark plasma sintering (SPS) process is known for its rapid densification of metals and ceramics. The mechanism behind this rapid densification has been discussed during the last few decades and is yet uncertain. During our SPS experiments we noticed oscillations in the applied pressure, related to a change in electric current. In this study, we investigated the effect of pulsed electrical current on the applied mechanical pressure and related changes in temperature. We eliminated the effect of sample shrinkage in the SPS setup and used a transparent quartz die allowing direct observation of the sample. We found that the use of pulsed direct electric current in our apparatus induces pressure oscillations with the amplitude depending on the current density. While sintering Ti samples we observed temperature oscillations resulting from pressure oscillations, which we attribute to magnetic forces generated within the SPS apparatus. The described current–pressure–temperature relations might increase understanding of the SPS process.     

Spark plasma sintering of UHTC powders obtained by self-propagating high-temperature synthesis

Fully dense ZrB₂–SiC and HfB₂–SiC ultra-high-temperature ceramics (UHTCs) composites are fabricated by first synthesizing via self-propagating high-temperature synthesis (SHS) the composite powders from B₄C, Si, and Zr or Hf reactants, and subsequently consolidating the product by spark plasma sintering (SPS) without the addition of any sintering aid. It was found that the SHS technique leads to the complete conversion of reactants to the desired products and the SPS allows for the full consolidation (>99.5% relative density) under the optimal operating conditions of 1800 °C/20 min/20 MPa and 1800 °C/30 min/20 MPa, for the cases of ZrB₂–SiC and HfB₂–SiC, respectively. Based on the results reported in this work, it can be stated that the combination of SHS and SPS methods represents a particularly rapid and convenient preparation route (lower sintering temperature and processing time) for UHTCs as compared to the techniques available in the literature for the fabrication of analogous products.     

The influence of oxygen and nitrogen contamination on the densification behavior of cryomilled copper powders during spark plasma sintering

It has been found difficult to fully densify some mechanically milled pure metal powders by spark plasma sintering (SPS). In this study, the densification behavior of cryomilled, nanostructured (NS) Cu powders during SPS was related to changes to the chemistry of the powders. The results showed that the presence of very small amounts of O and N in the powders, which were introduced during cryomilling and handling, significantly influenced the densification response. Moreover, reduction/removal of O/N via thermal annealing of the powders before SPS led to complete densification of the powders during subsequent SPS. The mechanisms responsible for this behavior were ascertained: O and N existed in the cryomilled powders in the form of thermally unstable compounds, and the subsequent thermal decomposition of these compounds during SPS generated the gaseous species, leading to porosity formation and incomplete densification; annealing of the powders before SPS removed the gases which resulted from thermal decomposition, thereby facilitating complete consolidation during subsequent SPS.     

Effect of Re on microstructural evolution and densification kinetics during spark plasma sintering of nanocrystalline W

In the present investigation, nanocrystalline W and W-xRe (x = 3, 5 wt.%) alloy powders were produced by mechanical milling/alloying using high energy ball milling. The nanocrystalline nature (∼50 nm) of these powders was validated by the Rietveld refinement of their respective X-Ray diffraction patterns. Subsequently, spark plasma sintering of the ball milled powders was carried out. It was observed that pure W was not able to densify completely (relative density of 93%) at a temperature of 1500 °C. However, the addition of 5 wt.% Re resulted in near complete densification (relative density of 97%) at the same sintering temperature. The enhanced densification of W-Re powders is mainly attributed to the ductilising effect of Re assisted by the nanocrystallinity of powders, and the application of pressure during sintering.     

Abnormal grain growth enhanced densification of liquid phase-sintered WC–Co in support of the pore filling theory

This study examines the effect of grain growth on densification during liquid phase sintering of compacts with faceted grains. Two kinds of WC powders with different sizes were used to produce WC–Co alloys. Large pores of ~5 μm size were generated in 95WC–5Co (wt%) using spherical Co particles of the same size. The overall sintering behavior was observed by measuring grain growth and densification as a function of sintering time at a sintering temperature of 1350 °C. When the WC powder was fine (0.4 μm), large pores disappeared upon filling of pores by liquid with the formation of abnormal grains. On the contrary, when the WC powder was large (4.2 μm), grain growth is not observed, and large pores remained intact even after a long period of sintering (24 h). These observations confirm that densification during final stage liquid phase sintering occurs via filling of pores by liquid as a result of grain growth. This finding is consistent with the model of densification predicted by the pore filling theory.     

Fabrication of nano-grained Ti–Nb–Zr biomaterials using spark plasma sintering

Nanostructured near-β Ti–20Nb–13Zr at % alloy with non-toxic elements and enhanced mechanical properties has been synthesized by spark plasma sintering (SPS) of nanocrystalline powders obtained by mechanical alloying. The consolidated bulk product was characterized by density measurements and Vickers hardness (HV), and X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) combined with energy-dispersive spectroscopy (EDX), and transmission electron microscopy (TEM) for structural details. The temperature during spark plasma sintering was varied between 800 and 1200 °C, while the heating rate and holding time of 100°K/min and 10 min were maintained constant in all the experiments. The effect of SPS temperature on the densification, microstructure, and HV was discussed. The results show that a nearly full density structure was obtained after SPS at 1200 °C. The microstructure of the obtained alloy is a duplex structure with the α-Ti (hcp) region having an average size of 70–140 nm, surrounding the β-Ti (bcc) matrix. The obtained alloy was chemically homogenized with a micro hardness value, HV of 660. The developed nanostructured Ti–20Nb–13Zr alloy is suggested for biomedical use as in implant material in dental and orthopedic applications.     

Can the use of pulsed direct current induce oscillation in the applied pressure during spark plasma sintering?

Abstract

The spark plasma sintering (SPS) process is known for its rapid densification of metals and ceramics. The mechanism behind this rapid densification has been discussed during the last few decades and is yet uncertain. During our SPS experiments we noticed oscillations in the applied pressure, related to a change in electric current. In this study, we investigated the effect of pulsed electrical current on the applied mechanical pressure and related changes in temperature. We eliminated the effect of sample shrinkage in the SPS setup and used a transparent quartz die allowing direct observation of the sample. We found that the use of pulsed direct electric current in our apparatus induces pressure oscillations with the amplitude depending on the current density. While sintering Ti samples we observed temperature oscillations resulting from pressure oscillations, which we attribute to magnetic forces generated within the SPS apparatus. The described current–pressure–temperature relations might increase understanding of the SPS process.     

Effect of Amount of Carbonyl Iron Powders on the Sintering Microstructure, and Mechanical Properties of Fe-4 Cu Alloy Synthesized Using Metal Injection Molding

An atomized iron powder used in conventional powder metallurgy, mixed with 4 wt.% Cu powders was injection molded with carbonyl iron powder and a sintering aid. The use of atomized iron powder can reduce cost, but decreases packing density and sintering rate. To improve the densification of atomized powders, 20-40 wt.% carbonyl iron powder was added for increasing packing density and promoting sintering. The sintered alloy was characterized by the bulk density, mechanical properties, and scanning electron microscope observations. The results of sintering for the samples added with 30 wt.% carbonyl powder show that the relative bulk density, hardness, tensile strength and elongation are up to 83.87%. HRF 92.2, 315.5 MPa and 4%, respectively. The proportion of carbonyl iron powders and sintering temperature were found to influence the relative bulk density and the mechanical properties of the specimens significantly.     

Densification mechanisms of spark plasma sintering: multi-step pressure dilatometry

The effects of electrical current and mechanical pressure on the densification of spherical copper powder during spark plasma sintering (SPS) are examined. A novel multi-step pressure dilatometry method is introduced to compare the constitutive behavior of the copper powder under nearly equivalent current-insulated and current-assisted SPS process conditions. The strain rate sensitivity agrees with that predicted for a dislocation climb-controlled creep densification mechanism for both processing setups. Accelerated densification rate leading to a higher final relative density is observed for the current-assisted SPS.     

粉末粒径对热压烧结碳化硼致密化及力学性能的影响

采用平均粒径分别为3.5 μm、1.5 μm和200 nm的碳化硼粉体为原料经1850℃热压烧结制备了碳化硼陶瓷, 研究了粉体粒径对陶瓷烧结致密化过程及其性能的影响。根据保温时间对线收缩率的影响及热压初期的塑性流动机理, 得出了不同粉体间烧结初期的激活能差。结果表明: 在相同工艺条件下, 随着粉体平均粒径的减小, 粉体的扩散激活能降低, 致密化初始温度降低, 而且完成塑性流动所需时间也会明显缩短, 致密化速率加快, 致密度增大; 碳化硼陶瓷的显微结构与力学性能亦随着粉体粒径的减小而改善; 1850℃保温1 h后, 平均粒径为200 nm的粉体制备的碳化硼陶瓷相对密度可达90.5%, 硬度为(17±1.8) GPa。