Liquid phase sintering (LPS) is a process for forming high performance, multiple-phase components from powders. It involves sintering under conditions where solid grains coexist with a wetting liquid. Many variants of LPS are applied to a wide range of engineering materials. Example applications for this technology are found in automobile engine connecting rods and high-speed metal cutting inserts. Scientific advances in understanding LPS began in the 1950s. The resulting quantitative process models are now embedded in computer simulations to enable predictions of the sintered component dimensions, microstructure, and properties. However, there are remaining areas in need of research attention. This LPS review, based on over 2,500 publications, outlines what happens when mixed powders are heated to the LPS temperature, with a focus on the densification and microstructure evolution events.

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Review: liquid phase sintering

Sintering of ice is driven by the thermodynamic requirement to decrease surface energy. The structural morphology of ice in nature has many forms—from snowflakes to glaciers. These forms and their evolution depend critically on the balance between the thermodynamic and kinetic factors involved. Ice is a crystalline material so scientific understanding and approaches from more conventional materials can be applied to ice. The early models of solid state ice sintering are based on power law models originally developed in metallurgy. For pressure sintering of ice, these are based on work on hot isostatic pressing of metals and ceramics. Recent advances in recognizing the grain boundary groove geometry between sintering ice particles require models that use new approaches in materials science. The newer models of sintering in materials science are beginning to incorporate more realistic processing conditions and microstructural complexity, and so there is much to be gained from applying these to ice in the future. The vapour pressure of ice is high, which causes it to sublime readily. The main mechanism for isothermal sintering of ice particles is by vapour diffusion; however other transport mechanisms certainly contribute. Plastic deformation with power law creep combined with recrystallization become important mechanisms in sintering with external pressure. Modern experimental techniques, low temperature scanning electron microscopy and x-ray tomography, are providing new insights into the evolution of microstructures in ice. Sintering in the presence of a small volume fraction of the liquid phase causes much higher bond growth rates. This may be important in natural snow which contains impurities that form a liquid phase. Knowledge of ice microstructure and sintering is beneficial in understanding mechanical behaviour in ice friction and the stability of snow slopes prone to avalanches. (Some figures in this article are in colour only in the electronic version)

/pdf/sintering-and-microstructure-of-ice-a-review-1pnrh2fd8l.pdf

Sintering and microstructure of ice: a review

One shortcoming of metals and alloys used to fabricate various components of orthopaedic systems, such as the femoral stem of a total hip joint replacement and the tibial plate of a total knee joint replacement, is well-recognized. This is that the material modulus of elasticity (E′) is substantially larger than that of the contiguous cancellous bone, a consequence of which is stress shielding which, in turn, has been postulated to be implicated in a cascade of events that culminates in the principal life-limiting phenomenon of these systems, namely, aseptic loosening. Thus, over the years, a host of research programs have focused on the synthesis of metallic biomaterials whose E′ can be tailored to match that of cancellous bone. The present work is a review of the extant large volume of literature on these materials, which are called open-cell porous metals/alloys (or, sometimes, metal foams or cellular materials). As such, its range is wide, covering myriad aspects such as production methods, characterization studies, in vitro evaluations, and in vivo performance. The review also includes discussion of seven areas for future research, such as parametric studies of the influence of an assortment of process variables (such as the space holder material and the laser power in the space holder method and the laser-engineered net-shaping process, respectively) on various properties (notably, permeability, fatigue strength, and corrosion resistance) of a given porous metal/alloy, innovative methods of determining fatigue strength, and modeling of corrosion behavior.

Properties of open-cell porous metals and alloys for orthopaedic applications.

Tungsten heavy alloys (WHAs) belong to a group of two-phase composites, based on W-Ni-Cu and W-Ni-Fe alloys. Due to their combinations of high density, strength, and ductility, WHAs are used as radiation shields, vibration dampers, kinetic energy penetrators and heavy-duty electrical contacts. This paper presents recent progresses in processing, microstructure, and mechanical properties of WHAs. Various processing techniques for the fabrication of WHAs such as conventional powder metallurgy (PM), advent of powder injection molding (PIM), high-energy ball milling (MA), microwave sintering (MW), and spark-plasma sintering (SPS) are reviewed for alloys. This review reveals that key factors affecting the performance of WHAs are the microstructural factors such as tungsten and matrix composition, chemistry, shape, size and distributions of tungsten particles in matrix, and interface-bonding strength between the tungsten particle and matrix in addition to processing factors. SPS approach has a better performance than those of others, followed by extrusion process. Moreover, deformation behaviors of WHA penetrator and depleted uranium (DU) Ti alloy impacting at normal incidence both rigid and thick mild steel target are studied and modelled as elastic thermoviscoplastic. Height of the mushroomed region is smaller for and it forms sooner in each penetrator as compared to that for .

/pdf/recent-progress-in-processing-of-tungsten-heavy-alloys-58z33pfzij.pdf

Recent Progress in Processing of Tungsten Heavy Alloys

Book reviewSurface self-diffusion of metals: By G. Neumann and G. M. Neumann, Vol. 1 of Diffusion Monograph Series, F. H. Wöhlbier (ed.), published by Diffusion Information Centre, Bay Village, Ohio, 1972; 129 pp., 35 figs.

A compact gains strength in sintering through low-temperature interparticle bonding, followed by further strength contributions from high-temperature densification. On the other hand, thermal softening substantially reduces a compact's strength at high temperatures. Therefore, the in situ strength during sintering is determined by the competition among interparticle neck growth, densification, and thermal softening. Distortion in sintering occurs when the compact is weak. Most strength models for sintered materials are semi-empirical relations based on the sintered fractional density. These models do not include microstructure or sintering cycle parameters; thus, they do not provide guidelines for thermal cycle design to improve compact dimensional control. A strength evolution model is derived which combines sintering theories and microstructure parameters, including interparticle neck size, solid volume fraction, and particle coordination number. The model predicts sintered strength and when combined with thermal softening gives a good prediction of in situ strength. The validity of the model is verified by comparison to experimental data for sintered and in situ strength of bronze and steel powders.

Densification and strength evolution in solid-state sintering Part II Strength model

Porosity effects on distortion during liquid phase sintering were investigated experimentally. W–Ni–Fe alloys with compositions ranging from 78 to 93 wt% tungsten were liquid phase sintered at 1500°C for 30 min. Surprisingly, the results show that porosity has no appreciable effect on distortion. After sintering, similar distortion profiles were obtained for compacts with both high and low green porosities. On the other hand, distortion reduced with increasing solid:liquid ratio. No distortion occurred for samples with 93 wt% tungsten. Quenching experiments showed the onset of slumping was delayed until densification neared completion. In high liquid content samples, this means slumping occurred within a few minutes after melt formation, corresponding to liquid exudation. The results are in agreement with model predictions by German. The sintered microstructures were quantitatively measured for solid volume fraction and contiguity. No apparent effects of green porosity on microstructural evolution were observed.

The effect of porosity on distortion of liquid phase sintered tungsten heavy alloys

Prealloyed bronze (Cu-10Sn) powder and a mixed elemental steel (Fe-2Ni-0.9C) powder were evaluated for strength evolution during sintering. For the bronze powder, test samples were fabricated using a loose powder casting method, while the steel powder was formed by injection molding. In situ strength during sintering was measured using a bending fracture test. Primary focus was on measuring the effects of sintering temperature and time on in situ strength evolution. Sintering temperature had the most significant effect, but the strength underwent significant gains prior to densification. The results are explained by the competition among interparticler neck growth, densification, and thermal softening. Sinter strengthening is initially governed by interparticle bonding, followed by a contribution from densification at high temperatures. However, high temperatures also lead to significant strength degradation due to thermal softening. Densification is favored by the declining in situ strength associated with thermal softening at high temperatures.

Densification and strength evolution in solid-state sintering Part I Experimental investigation

Microstructure attributes such as solid volume fraction, dihedral angle, initial porosity, and initial pore size are important to densification and distortion control in liquid phase sintering (LPS). In our experiments, these parameters were varied to measure densification and distortion behavior during LPS using W–Ni–Cu alloys. Green compacts were formed using ethylene-bis-stearamide (EBS) as a pore-forming agent with the amount of polymer controlling the initial porosity. Different initial pore sizes were generated by varying the polymer particle size. Dihedral angle was varied by changing the Ni:Cu ratio in the alloys. Finally, the solid:liquid ratio was adjusted via the tungsten content. Dilatometry was employed to measure shrinkage and shrinkage rate during heating. Real time video imaging and quenching experiments isolated the sequence of densification and distortion events in LPS. Distortion was quantified using profiles determined with a coordinate measuring machine (CMM) to calculate a distortion parameter. Sintering results showed that at full densification, initial porosity and pore size have no observable effect on distortion for 80W–16Ni–4Cu alloy while distortion reduces with increasing solid volume fraction and dihedral angle. Microgravity experiments are proposed for further understanding of microstructure effects on densification and distortion in LPS.

Green microstructure effects on densification and distortion in liquid phase sintering

Densification and shape distortion are two important events associated with liquid phase sintering. In general, the initial porosity effects on both are ignored due to the rapid densification typical to liquid phase sintering. To completely describe the sintering process, porosity and pore size effects should be included in any sintering model. In this study, samples differing in initial porosity were sintered and quenched from various points in a sintering cycle to track the porosity effect on densification and distortion. Sinterability of the compact was defined as the ratio of sintering stress to compact strength to predict the densification and linked with sample composition and porosity. Porosity influences how the sample distorts during sintering. A high initial porosity sample is more likely to collapse along the longitudinal direction while a low initial porosity sample tends to spheroidize.

Porosity effect on densification and shape distortion in liquid phase sintering

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