Tailoring master alloys for liquid phase sintering: Effect of introducing oxidation-sensitive elements

Abstract

The use of low melting point master alloy (MA) powders contributes beneficially to sintering by increasing the distribution rate of alloying elements, enhancing homogenisation and sometimes also promoting densification. However, working with liquid phases poses important challenges like maintaining a proper dimensional control and minimising the effect of secondary porosity on the final performance of the steel. In this work, three different MA systems are compared: a low dissolutive Cu-based MA, and two systems with a higher degree of iron dissolution but different content in oxidation-sensitive elements. The combination of wetting experiments, step sintering tests and dilatometry studies show how the evolution of the microstructure, dimensional stability and overall densification are strongly affected by the characteristics of the liquid MA and in particular by its ability to dissolve the iron base particles, and by the amount of oxidationsensitive elements present in the composition of the MA powder.

Novel multiuse liquid phase promoter for PM components

“…In this sense, the dissolutive character of the promoted liquid phase plays a key role in the dimensional behavior of the steel, since highly dissolutive liquids provide significantly lower dimensional changes and it is possible to control the homogeneity of the microstructure and the final properties, by using fine and well-dispersed master alloy particles. 8 The designed alloy in the current research is dissolutive, which improves the dimensional behavior and the well-achieved dispersion of the admixed particles favors the homogeneity of the steel microstructure (see Figure 5).…”

“…This decision was taken due to the good wetting behavior of the liquid that can spread fast over the solid surface, after some instants in which dissolution of the solid takes place. These properties contribute positively when brazing porous materials, since the liquid phase can clog superficial porosity, so the penetration into the solids can be controlled and the problem of excessive infiltration because of capillarity effect can be overcome …”

“…The oxide layer naturally covering any metallic surface must be removed prior to the melting of the promoted liquid, so that it can wet and spread over the solid surface. Hence, the favorable wetting scenario with no presence of unreduced oxides depends on different factors, being necessary to define a wetting system, composed of the environmental conditions (temperature, atmosphere and pressure), the solubility between phases, the chemical composition of the present phases and the surface characteristics. The careful control of the whole liquid‐solid–atmosphere behavior is the key for a successful wetting performance, since it determines the change from an unfavorable wetting situation toward positive wetting conditions.…”

Application of thermal analysis techniques to study the oxidation/reduction phenomena during sintering of steels containing oxygen-sensitive alloying elements

“…The presence of oxides on the interface between the liquid master alloy and the iron base particles has an impact on the wetting behavior of the liquid, and this seems to affect its dimensional behavior [6]. For these reasons, it is necessary to adjust the composition of the master alloy, and/or the sintering processes, in order to minimize the concentration of oxides on the surface of the master alloy particles.…”

“…This route presents advantages like the possibility to maintain the compressibility of the base powder, or the flexibility in the selection of the final composition of the steel. But one of the most interesting advantages of using master alloys is the possibility to design the composition of the master alloy to promote the formation of a liquid phase that enhances the sintering mechanisms [6]. However, it is still not well known how the specific composition of the master alloy affects the oxidation/reduction phenomena and therefore how the processing conditions have to be adapted for these new materials.…”