Liquid Phases Tailored for Introducing Oxidation-Sensitive Elements through the Master Alloy Route

Abstract

Introducing alloying elements through Master Alloy (MA) additions provides the unique opportunity of designing their composition to enhance sintering by forming a liquid phase. However, working with liquid phases poses important challenges like maintaining a proper dimensional control and minimizing the effect of secondary porosity on the final performance of the steel.The critical parameters for designing low melting point compositions are analyzed in this work by combining the use of thermodynamic software tools, wetting angle/infiltration experiments, and advanced thermal analysis techniques. Due to their low ability to dissolve iron, Cu-based liquids present remarkable infiltration properties that provide homogeneous distribution of the alloying elements. Dissolutive liquids, on the other hand, tend to render more heterogeneous microstructures, rapidly solidifying in contact with the matrix. As a consequence of their lower infiltration capacity, dimensional changes upon liquid formation are significantly lowered. When using master alloys with high content in oxidation-sensitive alloying elements, the differences in oxygen affinity cause an oxygen transfer from the surface of the iron base particles to the surface of the master alloys. The change in the surface chemistry modifies the wetting capability of the liquid, and the dimensional stability becomes increasingly sensitive to the processing atmosphere. KEY WORDSmaster alloys, liquid phase sintering, oxygen-sensitive alloying elements, dimensional stability, wetting/infiltration IntroductionThe so called "master alloys" emerged in the very early seventies as a vehicle to introduce in low alloyed sintered steels elements with high oxygen affinity. The combination of these elements with others with lower sensitivity for oxygen -such as Fe -reduced the risk of oxidation during the early stages of sintering 1-5) .A master alloy could be defined as a powder with a high concentration of alloying elements that is designed to be mixed with a base iron powder in order to provide the desired final composition of the steel after the sintering process. Compared with the use of fully prealloyed steel grades, the master alloy route presents interesting advantages such as preserving the compressibility of the base powder, and yielding flexibility in the selection of the final composition. But probably one of the most interesting benefits of using master alloys is the fact that their composition can be specifically designed to promote the formation of a liquid phase that enhances the distribution of alloying elements and accelerates the sintering mechanisms.Several authors have considered the use of master alloys specifically as a means of activating sintering through the formation of a liquid phase with specific characteristics. For instance, Cu-based master alloys provide a liquid with the ability to distribute through the pore network and penetrate the iron grain boundaries, reducing the diffusion distances needed to homogeneously distribute other alloying elements [6][...

Chemical reactions during sintering of PM steel compacts as a function of the alloying route

“…Also here, reduction can be expected to be enhanced by dissolution of the elemental Si formed during reduction in the iron lattice, which however still requires higher temperatures than with admixed Cr and Mn, up to 1400°C being required to attain reasonably complete reduction. One way to avoid internal gettering at least to some degree is to sinter in H 2 [35,36] since in this case, reduction of the iron oxides occurs in a temperature that is so low that the getter elements are not too reactive, as shown for example, for Mn-Si master alloys [37]. For the alloy elements studied here, it can be seen that with Cr and Si addition, the low temperature m18 reduction peak at 400°C is quite pronounced.…”

“…One way to avoid internal gettering at least to some degree is to sinter in H 2 [35,36] since in this case, reduction of the iron oxides occurs in a temperature that is so low that the getter elements are not too reactive, as shown for example, for Mn–Si master alloys [37]. For the alloy elements studied here, it can be seen that with Cr and Si addition, the low temperature m18 reduction peak at 400°C is quite pronounced.…”

New Opportunities for Low Alloy Steels—Master Alloys for Liquid Phase Sintering

“…The implementation of scientific methodology to design liquid phases with specific properties [15,[44][45][46][47][48].…”

“…The use of master alloys in the development of sintered steels has an extra advantage when the composition is designed with a melting point under the sintering temperature [15]. During sintering, the master alloy melts and produces a liquid phase which enhances the diffusion processes and reduces the porosity, while at the same time, helps to distribute the alloying elements.…”

The Use of Transient Liquid Phases in Powder Metallurgy

“…In the former case, the two primary formation mechanisms are spontaneous melting and eutectic reaction, respectively [13]. For distribution of the liquid phase in the porous skeleton of the solid phase, "dissolutive" and "non-dissolutive" systems can be distinguished [14].…”