First steps towards development and characterisation of a new healable aluminum-magnesium alloy processed by selective laser melting

The aim of this master thesis is to design and characterise a new healable Al-Mg alloy processed by selective laser melting (SLM). Using Scheil simulations, Al-Mg28 composition is selected in order to investigate a novel way of healing using the melting of the low melting point FCC Al/ Al3Mg2 eutectic phase. SLM powder feedstock of the desired Al-Mg28 composition is obtained by mechanically mixing pure Al and pure Mg powder. A final composition of 23.7 wt% of Mg is obtained due to Mg evaporation during process. The SLM process could not be optimised for this composition. Indeed, a maximum density of only 96.1 % is obtained for a sample exhibiting a high amount of hot cracks. It is observed that the Al-Mg28 samples are highly sensitive to hot cracking. Currently, it is believed that it is due to their brittleness. Indeed, their microstructure exhibits a high amount of eutectic phase composed mainly of brittle intermetallic Al3Mg2 and a high hardness value of 201 HV is obtained for these samples. An attempt to decrease hot cracking by grain refinement is made by adding 2 % of Zr to the initial composition. The addition is done by mechanically mixing pure Zr powder (mean diameter 28.43 μm) to the initial Al-Mg28 powder mix. Once more the SLM process could not be optimised and no improvement of density or reduction of hot cracking is observed. The microhardness of the Al-Mg28 + Zr2 samples is the same as the one of the Al-Mg28 samples. Only a few sub-micron Al3Zr grain refiner precipitates are observed while several large Al-Zr precipitates and unmelted Zr powder particles are observed throughout the microstructure. It allows to conclude that the way of adding Zr to the composition is unfitted to allow a significant grain refinement effect and an improvement of hot cracking resistance. Finally, four different potential healing treatments are tested on Al-Mg28 samples. The DSC heat treatment (slow cooling and heating, 15 minutes at 450 °C) shows a promising healing effect and triggers the melting of the eutectic phase. Tomography analysis shows a reduction of 0.37 % of porosity after healing treatment. A healing effect is observed for pores smaller than 10k μm3. However, a shape change of the sample and the creation of gas pores are also induced by the heat treatment.