Compositional sensitivity in additively manufactured aluminium alloy 2139
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
One of the biggest challenges when utilizing additive manufacturing (AM) for the fabrication of aluminium alloy structures is achieving wrought-equivalent properties. Wrought alloys that are precipitation strengthened typically require some form of mechanical processing (e.g., rolling or stretching) to achieve peak strength. The ability to produce net- or near-netshape structures is a key benefit to AM; mechanical post-processing is therefore generally undesirable. There are few alloys that can be heat treated to wrought-equivalent strength without the need for a mechanical processing step. Aluminium 2139, an Al-Cu-Mn-Mg-Ag composition, is an alloy that can be strengthened through heat treatment alone, making it ideally suited for use in AM processes. The challenge with this alloy, however, is the high volatility of Mg compared with Al and the other alloying constituents. This volatility is particularly important in AM, in which preferential vaporization from the molten pool is known to occur. Although there is only a small amount of Mg in the alloy, its presence is critical for achieving peak strength in the heat-treated condition. Small variations in Mg concentration due to preferential vaporization can reduce the strength of the deposited alloy. Because AM is an incremental process, there is also the potential for chemical and property gradients within a single part caused by changes in the processing conditions as the AM build progresses. This project explores the relationship between process settings and vaporization loss in additively manufactured aluminium 2139. A correlation is developed between Mg concentration and precipitate size and distribution. These relationships are linked to vaporization theory, and a predictive model is developed that can predict vaporization loss as a function of molten pool temperature. The results provide a control strategy that can be used to ensure chemical consistency in additively manufactured aluminium alloy 2139.