In-situ alloying of Al-Cu-Si using dual-wire plasma additive manufacturing

Description

In a basic research work, Kim et al. [1] showed the influence of different chemical compositions based on the elements Al, Cu and Si on the microstructure and the basic mechanical properties. In [2] it was shown that the addition of some minor amounts of Ni makes this type of alloy a good candidate for high temperature application. However, all these and more activities have been performed on the basis of laboratory-scale casting processes, which are difficult to transfer to actual applications due to their high cooling rates. The aim of the present study is the additive manufacturing of an Al-Cu-Si system using dual-wire plasma welding. Key elements for realisation included the appropriate selection of filler materials, programming of the welding system, determination of suitable welding parameters with special focus on the wire feed rates for the two different materials, and fabrication of appropriate specimens to conduct material characterization. By applying plasma alternating current welding to the aluminium base material using AlSi5 and CuAl8 filler wires, the novel Al-Cu-Si system was ultimately fabricated. To enable suitable characterization both metallurgically and mechanically, walls were built by three tracks layer by layer from which samples were extracted. A maximum of five layers were deposited, yielding in a 30 x 20 mm cross section wall with a length of 100 mm. Specific samples were mechanically cut and analysed using static tensile testing, Charpy impact testing, hardness testing, and metallography using light and scanning electron microscopy. Selected samples were also characterised regarding the prevalent phases and their phase fractions using X-ray diffraction experiments at a synchrotron radiation source.
A chemical mass composition of 78.3% Al, 14.2% Cu, 5.2% Si and 2.3% Mg of the build was achieved in a first trial. However, to improve process stability and hardness a modification was applied in a next step, resulting in the evaluation being limited to metallography and hardness testing for a mass composition of 57.2% Al, 39.8% Cu and 3.0% Si. Examination with light and scanning electron microscopy revealed the presence of pores and residues from improperly melted CuAl8 wire throughout the microstructure. While the pores were attributed to the high heat in the system and partial evaporation of individual elements, the residues resulted from poor melting of the CuAl8 wire during the welding process.
The melting of the CuAl8 wire and thus process stability could be optimised in the third experimental series, using the hot wire option, resulting in a technologically sound and highly homogeneous microstructure. However, this also increased the copper content in the system yielding in a nominal chemical mass composition of 57.2% Al, 39.8% Cu and 3.0% Si, leading to a significant embrittlement. The cause for the loss of ductility and the increase in hardness is the changed chemical composition, resulting in primary Al2Cu dendrites in series 3 instead of the primary αalpha-Al dendrites present in series 1. Whereas the first experimental series exhibited a peak hardness of approximately 170 HV, this value increased to 280 HV in the third series. The number, size, and distribution of pores could be reduced due to better heat control, which was visually confirmed under the light microscope. Due to the material's pronounced brittleness, the fabrication of Charpy impact and tensile test specimens proved to be very difficult, with the production of tensile specimens being impossible. The brittle behaviour of the material is also evident in the Charpy impact tests, which are in the range of about 2J.
The objective of producing initial Al-Cu-Si walls using dual-wire plasma processing was achieved. While the realised microstructure forms a good basis for further understanding, the mechanical results are not satisfactory. For further development, the main focus should be on the process parameters to reduce the copper content and to increase toughness.

Period	12 Mar 2024 → 14 Mar 2024
Event title	IIW Intermediate Meeting
Event type	Conference
Location	Seoul, UnknownShow on map