Porosity control and properties improvement of Al-Cu alloys via solidification condition optimisation in wire and arc additive manufacturing
Zhennan Wang, Xufei Lu, Xin Lin, Zhiwei Hao, Chenghui Hu, Yufeng Zhang, Haiou Yang, Xinghua Wang
Abstract
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.