Mechanical and corrosion behaviour of 3D printed aluminium bronzes produced by wire+arc additive manufacturing

Abstract

The emerging field of 3D printing has expanded to the fabrication of metallic components during the last decade. Among the most prominent applications is the production of aluminium bronze marine propellers by the Wire+Arc Additive Manufacturing (WAAM) method. This method incorporates a welding system attached to a robotic arm. The final product results after sequential bead depositions. The main assets of the method are the efficient material usage and the minimization of lead time, which both have a positive environmental and economic impact. 
The aim of this project is to evaluate the feasibility to produce 3D printed aluminium bronze (CMA and NAB alloys) blocks with the WAAM method and compare the mechanical and corrosion properties of the blocks with the market requirements. In order to achieve that, rectangular blocks were manufactured at the facilities of Delft University of Technology and at Rotterdam Additive Manufacturing Fieldlab (RAMLAB). Cross sectional areas were extracted and used for microstructural investigation and for hardness measurement. Subsequently, the blocks were machined to produce tensile and Charpy specimens along the built height. Finally, corrosion tests were performed, including open circuit potential measurements, polarization experiments and Scanning Kelvin Probe (SKP) tests.
The microstructural investigation revealed that the 3D printed CMA block consisted of a banded structure. The deposited layers consisted of two dominant phases, α and β, and a variety of precipitates. The Widmanstätten α phase nucleates at the grain boundaries mainly. The tempering promotes the growth of the α phase, making the grain boundaries more indistinguishable, while the β phase decomposes.
The mechanical testing results depicted that the hardness, the tensile strength and the absorption energy of the 3D printed blocks exceeded the specifications of the cast products, according to the ASTM standards. The built height direction is weaker than the welding direction; however, the deposition height plays no significant role in the mechanical properties. Samples were also tested after a heat treatment of 675 °C for 6 hours, as recommended in the literature. The result was a 25% reduction of the tensile yield strength and a 10% reduction of the ultimate tensile strength. However, the scatter in the measured values was reduced too. 
Regarding the corrosion results, the built height has little effect on the corrosion susceptibility, according to the polarization curves. The material exhibits a remarkable low corrosion rate, which justifies its use in marine applications. The Scanning Kelvin Probe (SKP) tests illustrated the beneficial aspect of the tempering heat treatment, which alleviates the large potential differences of adjacent deposited areas.
It can be concluded that the CMA alloys are tolerable to the oscillations of the production parameters, making them appealing to the additive manufacturing industry. The mechanical properties achieved, outmatch not only the specifications for the cast CMA products, but also the performance of similar 3D printed aluminium bronze structures, found in the literature.