Porosity control and properties improvement of Al-Cu alloys via solidification condition optimisation in wire and arc additive manufacturing

This study presents an innovative liquid-nitrogen cooling (LNC) strategy to address hydrogen porosity in Wire and Arc Additive Manufactured (WAAM) Al-Cu alloys, which negatively affects part properties. A coupled thermo-mechanical finite element model, calibrated with in-situ measurements, is used to analyse the thermal, mechanical and metallurgical evolutions of two single-walls fabricated with conventional gas cooling (CGC) and LNC, respectively. A hydrogen solute coupling model evaluates hydrogen supersaturation during solidification. The LNC strategy significantly reduces porosity by optimising the solidification process: (i) Grain size is reduced, lowering hydrogen concentration at the solid/liquid interface; (ii) The length and duration of the hydrogen supersaturation region are shortened due to higher temperature gradients; (iii) Enhanced Marangoni convection and reduced molten pool depth facilitate hydrogen bubble escape. Compared to the CGC part, the LNC part shows a 63.8% reduction in pore density and a 59.4% reduction in overall porosity, achieving a final porosity of 0.39%. This improves mechanical properties, with the LNC component displaying a yield strength of 100.3 MPa, ultimate tensile strength of 250.1 MPa and elongation to failure of 19.4%. Despite a slight increase in residual stresses, the LNC strategy prevents cracking in Al-Cu alloys with high cracking susceptibility.