UBC Theses and Dissertations
High Density supersolidus liquid phase sintering of steel powders Young, Erin
In this study, supersolidus liquid phase sintering (SLPS) of an industrially relevant Fe-C-Mo alloy powder was investigated with particular attention to the evolution of density and porosity. The material was produced by mixing graphite with a prealloyed Fe-Mo alloy. Differential Scanning Calorimetry (DSC) was employed to determine the solidus and liquidus temperatures of the alloy, and a high temperature experimental furnace was used to provide information on density evolution as a function of time and temperature. Specimens were sintered both in the solid state and in the presence of a supersolidus liquid phase, over a range of temperatures between 1000 and 1300°C, providing both densification data and metallographic specimens. Diffusion calculations and thermodynamic calculations using Thermocalc™ software were performed to complement the experimental data. On heating, several processes occur in the material prior to liquid formation, including delubrication of the material, phase transformation, carbon diffusion, and solid state sintering. It was determined that the carbon diffusion is rapid and the bulk concentration is homogeneous at temperatures well below those of first liquid formation. Following homogenization, some solid state sintering takes place, and while minimal densification occurs, the number of pores decreases markedly from the green state. At temperatures above the solidus where a liquid phase is present, it was found that densification occurs in two regimes. The first regime involves a fast rate of densification due to the capillary forces of the liquid, followed by a second, slower regime of densification that is governed by pore removal processes. The maximum amount of densification possible is determined by the fraction of liquid present, as defined by the sintering temperature. If the temperature is low, insufficient liquid is present and densification is extremely slow. If the temperature is too high, the liquid promotes distortion of the semi-solid sample; in addition, pores subject to sufficient internal gas pressure are capable of expansion in the weakened structure, actually increasing the volume of porosity. Samples were, however, successfully sintered to densities approaching theoretical in a narrow temperature window intermediate between these extremes. Recommendations for the development of robust sintering practices are offered.
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