Optimization of the strength vs. conductivity trade-off in an aluminium alloy designed for laser powder bed fusion

The 6061 Al alloy in its T6 conditions is often considered a good candidate for applications requiring a good balance between strength and thermal conductivity. However, this alloy is often very difficult to process using laser powder bed fusion (PBF-LB) because of the development of hot cracks during fabrication. Here, we show that adding 2.3 wt% of Zr to the 6061 heritage alloy makes it processable by PBF-LB (suppression of hot cracks). Hot crack mitigation is attributed to grain refinement. However, the addition of 2.3 wt% of Zr greatly affects the microstructure and thus the mechanical and electrical/thermal properties of the Zr-modified 6061 alloy. Consequently, there is a need to design heat treatments to achieve a trade-off between yield strength and thermal conductivity. In this work, we designed two heat treatment sequences aiming at achieving such a trade-off: an adapted T6 sequence (550 °C/30 min + 180 °C/4 h) and direct ageing at 400 °C/4 h. On the basis of a multiscale microstructural study using optical microscopy, X-ray diffraction, and electron microscopy, we clarify the evolution of the microstructure induced by the designed heat treatments. The mechanical properties (hardness, tensile behavior) and thermal conductivity derived from electrical conductivity measurements are then discussed in light of the microstructural evolutions. The as-fabricated Zr-modified 6061 alloy shows a higher yield strength (370 MPa) than the heritage 6061 alloy in its T6 condition (260 MPa) but its thermal conductivity is found to be much lower (98 vs. 173 W/m.K). The two heat treatment sequences designed in this work enable the mechanical properties of the heritage 6061 alloy to be outperformed (yield strength of 350 and 460 MPa for the T6 and direct ageing heat treatment respectively) while maintaining an acceptable level of thermal conductivity (150 and 170 W/m.K for the T6 and direct ageing heat treatment respectively). [Display omitted]