Al-Ce-Mn Solidification Phase Selection and Solid-State Phase Transformations

The design of Al alloys has become an important topic in Additive Manufacturing (AM). The adoption of Al alloys to AM has been difficult because traditional alloys are prone to processing related defects such as solidification cracking. The Al-10Si-Mg alloy was initially adopted because of its resistance to solidification cracking. However, the Al-10Si-Mg alloy has reduced tensile properties especially at high temperatures, where the silicon phase coarsens readily. Therefore, efforts have been made to design new Al alloys that can take advantage of the AM processing. The goal of new alloys is to optimize based on rapid solidification conditions, while being less prone to processing related defects. The Al-Ce, and higher order Al-Ce-X, systems have been adopted because of the low solubility and diffusivity of Ce through FCC Al. The challenge is metastable phases form as a function of the unique processing conditions. Metastable phases have been observed to have unique phase transformations during heat treatment. The goal is to understand the above observations.

In this work, an Al-10Ce-8Mn (wt.%) alloy is used to understand the metastable phase Al₂₀Mn₂Ce. The Al₂₀Mn₂Ce phase, surrounded by FCC Al has different decomposition pathways (at 400 C) depending on its thermal history within a single weld track. Initially, the research focuses on an AM Al-Ce-Mn part that demonstrates the change in decomposition pathways based upon the local solidification conditions, e.g., primary Al₂₀Mn₂Ce to eutectic (between FCC Al and Al₂₀Mn₂Ce) solidification. Following the AM parts, weld tracks are preformed to understand the role of solidification conditions that can lead to each decomposition pathway observed in the Al-Ce-Mn system. The interface response function model is used to understand phase selection as a function of solidification conditions. The overall goal of this work is to demonstrate how a single metastable phase in a weld track, specifically Al₂₀Mn₂Ce, may be manipulated by local solidification conditions, and how that affects the subsequent phase decomposition. The impact of this work is to understand the role of solidification conditions on the solid-state phase transformation to give insight into how the mechanical properties of a particular alloy can be controlled.