The Experts below are selected from a list of 282 Experts worldwide ranked by ideXlab platform

Randall M German - One of the best experts on this subject based on the ideXlab platform.

  • Liquid Phase Sintering of tungsten heavy alloys
    International Journal of Powder Metallurgy, 2020
    Co-Authors: Peizhen Lu, Randall M German, Brian Marx
    Abstract:

    Compact shape and changes in microstructure during Liquid Phase Sintering are affected by the initial pore structure and alloy composition. In this study, compacts with different pore characteristics were sintered under the same conditions to monitor the evolution of microstructure and compact distortion. Tungsten content was varied from 80w/o to 90w/o and the Ni:Cu ratio from 8:2 to 2:8 to investigate the rigidity effects of microstructure on densification and distortion. The results show that the higher the level of porosity and the larger the pore size, the smaller the distortion, for a given pore size and pore content, both a low Ni:Cu ratio and a high tungsten content hinder densification and distortion. A Sintering driving force model is established to evaluate the effect of porosity and composition on densification and distortion during Liquid Phase Sintering.

  • Establishment of the Scientific Underpinnings in Powder Injection Molding and Liquid Phase Sintering
    Journal of The Japan Society of Powder and Powder Metallurgy, 2020
    Co-Authors: Randall M German
    Abstract:

    Powder injection molding and Liquid Phase Sintering share common features with respect to the rheological response of the solid-Liquid-pore system. Injection molding of powders dates from the 1930s, and Liquid Phase Sintering of metallic powders dates from about that same time. Both processes share a strong sensitivity of viscosity to solid content and temperature, but there are differences. Accordingly, a model is introduced that includes factors as strain rate, grain size, solid content, and degree of grain bonding. In Liquid Phase Sintering there is solid solubility in the Liquid, which influences the time-dependent viscosity through grain bonding. Similarly, in powder injection molding the entanglement of long polymers leads to a time-dependent viscosity. Thus, both powder injection molding and Liquid Phase Sintering show similarities that support a model for the rheological response useful in computer simulations.

  • Coupled Densification—Shape Distortion Analysis of Liquid Phase Sintering Affected By Gravity
    Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2013
    Co-Authors: José A. Alvarado-contreras, Randall M German, Andrey L. Maximenko, Eugene A. Olevsky
    Abstract:

    The paper presents a model of nonlinear viscous behavior in the examination of gravity-induced distortion during Liquid Phase Sintering. The model uses a finite element formulation to describe densification, viscous flow, and the associated dimensional changes and component slumping during Sintering. The approach assumes Sintering is conducted under isothermal conditions with averaged material properties. The numerical results are compared with experimental data and with a mathematical model on the distortion during Liquid Phase Sintering of tungsten heavy alloys. The preliminary conclusions aim to provide better understanding of dimensional control during Liquid Phase Sintering.

  • Review: Liquid Phase Sintering
    Journal of Materials Science, 2009
    Co-Authors: Randall M German, Pavan Suri, Seong Jin Park
    Abstract:

    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.

  • Porosity effect on densification and shape distortion in Liquid Phase Sintering
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2001
    Co-Authors: Peizhen Lu, Xiaoping Xu, Wuwen Yi, Randall M German
    Abstract:

    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.

Guo-jun Zhang - One of the best experts on this subject based on the ideXlab platform.

John L. Johnson - One of the best experts on this subject based on the ideXlab platform.

  • Activated Liquid Phase Sintering of W–Cu and Mo–Cu
    International Journal of Refractory Metals & Hard Materials, 2015
    Co-Authors: John L. Johnson
    Abstract:

    Abstract A model for densification based on the master Sintering curve concept is applied to solid-state Sintering of W and Mo and to Liquid-Phase Sintering of W–10Cu and Mo–18Cu. The effects of small additions of Ni, Co, Fe, and Pd to W–10Cu and Mo–18Cu are investigated. Separate grain growth models for solid-state and Liquid-Phase Sintering are also applied to these systems. The model predictions are compared to experimental results to determine the effects of the activators on densification and grain growth mechanisms. Ni and Pd additions only slightly reduce the activation energy for densification, while Co and Fe have a much larger effect. The effects of Co and Fe additions on grain growth rate constants are not adequately described by consideration of their effects on the solubility of W and Mo in the Liquid Phase. The experimental results with Co and Fe additions are better described by a solid-state grain growth model. The effects of Co and Fe additions on densification and grain growth are consistent with enhanced diffusion through a segregated solid Phase during Liquid-Phase Sintering.

  • activated Liquid Phase Sintering of w cu and mo cu
    International Journal of Refractory Metals & Hard Materials, 2015
    Co-Authors: John L. Johnson
    Abstract:

    Abstract A model for densification based on the master Sintering curve concept is applied to solid-state Sintering of W and Mo and to Liquid-Phase Sintering of W–10Cu and Mo–18Cu. The effects of small additions of Ni, Co, Fe, and Pd to W–10Cu and Mo–18Cu are investigated. Separate grain growth models for solid-state and Liquid-Phase Sintering are also applied to these systems. The model predictions are compared to experimental results to determine the effects of the activators on densification and grain growth mechanisms. Ni and Pd additions only slightly reduce the activation energy for densification, while Co and Fe have a much larger effect. The effects of Co and Fe additions on grain growth rate constants are not adequately described by consideration of their effects on the solubility of W and Mo in the Liquid Phase. The experimental results with Co and Fe additions are better described by a solid-state grain growth model. The effects of Co and Fe additions on densification and grain growth are consistent with enhanced diffusion through a segregated solid Phase during Liquid-Phase Sintering.

  • Solid-state contributions to densification during Liquid-Phase Sintering
    Metallurgical and Materials Transactions B, 1996
    Co-Authors: John L. Johnson, Randall M German
    Abstract:

    Densification via Liquid-Phase Sintering generally requires transport of substantial amounts of dissolved solid through the Liquid. However, in composite systems, such as W-Cu, solid solubility in the Liquid is almost negligible, and densification is hindered by the low amount of total mass transport. In this case, solid-state Sintering of the skeletal solid structure in the presence of the Liquid is a significant densification mechanism. In this article, the relative contributions to densification of both Liquid and solid mass transport mechanisms are considered. A computer simulation is constructed to predict the densification behavior and concurrent microstructural development of LiquidPhase sintered composites for realistic heating cycles. Governing differential equations for densification are derived from idealized models of the microstructure, considering grain size, diffusion distance from vacancy source to sink, pore size, and pore morphology. Temperature-dependent terms, including the diffusivity, solubility, and surface energy, govern densification and microstructural parameters, such as the grain size, dihedral angle, and contiguity. Predictions for the sintered density, grain size, and contiguity are compared to experimental results for the W-Cu and W-Cu-Ni systems with approximately 20 vol pct Liquid. For W-Cu, which has almost no intersolubility, solid-state Sintering of W in the presence of Liquid Cu is the dominant densification mechanism. Nickel additions increase solid solubility in the Liquid and improve typical Liquid-Phase Sintering contributions to densification. Alternatively, high sintered densities can be achieved in the absence of solubility with a sufficiently small particle size due to the solid-state contribution.

  • Liquid-Phase Sintering under microgravity conditions
    JOM, 1995
    Co-Authors: Randall M German, Ronald G. Iacocca, John L. Johnson, Anish Upadhyaya
    Abstract:

    Liquid-Phase Sintering routinely is used to form dense composite structures. The technique is generally restricted to high solid contents to ensure rigidity of the compact to achieve net shaping. Experimental conditions present during microgravity processing have allowed Liquid-Phase Sintering over a wider range of Liquid-solid ratios than possible on Earth. Early results from experiments performed on the space shuttle} Columbia during 1994 are used to show the novel behavior associated with microgravity conditions.

Yan Mei Kan - One of the best experts on this subject based on the ideXlab platform.

Suk-joong L. Kang - One of the best experts on this subject based on the ideXlab platform.

  • BASIS OF Liquid Phase Sintering
    Sintering, 2020
    Co-Authors: Suk-joong L. Kang
    Abstract:

    Liquid Phase Sintering is a consolidation technique of powder compacts containing more than one component at a temperature above the solidus of the components and hence, in the presence of a Liquid. The microstructure change during Liquid Phase Sintering is fast because of fast material transport through the Liquid. The typical densification curve of Liquid Phase Sintering with Sintering time is similar to that of solid state Sintering. A considerable densification usually occurs in the solid state during heating to the Liquid Phase Sintering temperature; the initial microstructure of Liquid Phase Sintering is strongly affected by the solid state Sintering stage. When activated Sintering occurs, more than 90% relative density can be achieved during conventional heating to the Liquid Phase Sintering temperature. As a Liquid Phase forms during the heating of a powder mixture compact, Liquid flows into fine capillaries because of the capillary pressure difference between the fine and coarse channels between solid particles. The solid particles can be redistributed by this Liquid flow, and in Liquid Phase Sintering models, this phenomenon is referred to as particle rearrangement.

  • Liquid Phase Sintering
    Sintering of Advanced Materials, 2014
    Co-Authors: Suk-joong L. Kang
    Abstract:

    Abstract: This chapter describes the fundamentals of grain growth and densification, and related microstructural evolution during Liquid Phase Sintering (LPS). Two different types of grain growth behavior, stationary (conventionally called normal) and nonstationary in terms of the relative grain size distribution, and their theoretical treatments are described. Particular emphasis is placed on the prediction of nonstationary grain growth with the suggestion of general principles of microstructural evolution during LPS. Fundamental differences between the two densification mechanisms, contact flattening and pore filling, are described and their validities discussed. Model calculations of densification kinetics by pore filling theory are also provided.

  • Microstructure development during Liquid-Phase Sintering
    Zeitschrift Fur Metallkunde, 2005
    Co-Authors: Suk-joong L. Kang
    Abstract:

    Abstract Microstructure development during Liquid-Phase Sintering has been analysed using the pore-filling theory. The interdependence between sintered density and average grain size, i. e., the relative density–grain size trajectory has been described for various types of processing and Sintering parameters. The effects of Sintering temperature, initial porosity, average pore size, Liquid volume fraction, dihedral and wetting angle, and Sintering atmosphere pressure have been evaluated. A critical examination has also been made of the factors determining the trajectory shape and the implication of activation energy for densification. The predicted microstructure developments should demonstrate various possibilities of controlling microstructure during Liquid-Phase Sintering.

  • theoretical analysis of Liquid Phase Sintering pore filling theory
    Acta Materialia, 1998
    Co-Authors: Suk-joong L. Kang
    Abstract:

    Abstract Based on the pore filling model and previous calculations, a new Liquid-Phase Sintering theory has been developed for compacts containing isolated pores with size distribution. The relevance of the model has been critically examined in consideration of previous experimental and theoretical results. The developed pore filling theory considers both densification and grain growth during Sintering, in contrast to the classical Kingery theory which does not take grain growth into account. The effects of such various parameters as pore size distribution, pore and Liquid volume fraction, dihedral and wetting angle, particle size (scale), entrapped gas, etc., can be predicted. The present theory appears to describe well the microstructural development during Liquid-Phase Sintering in reflecting real phenomena.

  • The Effect of Mo Addition on the Liquid-Phase Sintering of W Heavy Alloy
    Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 1996
    Co-Authors: Hee-dong Park, Suk-joong L. Kang, Woon-hyung Baik, Duk-yong Yoon
    Abstract:

    The morphological and compositional changes of grains have been investigated in the initial stage of Liquid-Phase Sintering of W-Mo-Ni-Fe powder compacts. Both large (5.4-μm) and small (1.3-μm) W powders have been used to vary their time of dissolution in the Liquid matrix. When 8OW-10M0-7Ni-3Fe (wt pct) compacts of fine (about 1- to 2-μm) Mo, Ni, and Fe and coarse (5.4-μm) W powders are Liquid-Phase sintered at 1500 °C, the Mo powder and a fraction of the W powder rapidly dissolve in the Ni-Fe Liquid matrix. The W-Mo grains (containing small amounts of Ni and Fe) nucleate in the matrix and grow while the W particles slowly dissolve. In this transient initial stage of the Liquid-Phase Sintering, duplex structures of coarse W-Mo grains and fine W particles are obtained. As the W particles dissolve in the Liquid matrix during the Sintering, the W content in the precipitated solid Phase also increases. The dissolution of the small W particles is assessed to be driven partially by the coherency strain produced by Mo diffusion at the surface. During Sintering, the W particles continuously dissolve while the W-Mo grains grow. When the compacts are prepared from a fine (1.3-μm) W powder, the W grains dissolve more rapidly, in about 1 hour, and only W-Mo grains remain. These observations show that the morphological evolution of grains during Liquid-Phase Sintering can be strongly influenced by the chemical equilibrium process.