Precipitation Hardening at Elevated Temperatures above 400 °C and Subsequent Natural Age Hardening of Commercial Al–Si–Cu Alloy

The precipitation of intermetallic phases and the associated hardening by artificial aging treatments at elevated temperatures above 400 °C were systematically investigated in the commercially available AC2B alloy with a nominal composition of Al–6Si–3Cu (mass%). The natural age hardening of the artificially aged samples at various temperatures was also examined. A slight increase in hardness (approximately 5 HV) of the AC2B alloy was observed at an elevated temperature of 480 °C. The hardness change is attributed to the precipitation of metastable phases associated with the α-Al15(Fe, Mn)3Si2 phase containing a large amount of impurity elements (Fe and Mn). At a lower temperature of 400 °C, a slight artificial-age hardening appeared. Subsequently, the hardness decreased moderately. This phenomenon was attributed to the precipitation of stable θ-Al2Cu and Q-Al4Cu2Mg8Si6 phases and their coarsening after a long duration. The precipitation sequence was rationalized by thermodynamic calculations for the Al–Si–Cu–Fe–Mn–Mg system. The natural age-hardening behavior significantly varied depending on the prior artificial aging temperatures ranging from 400 °C to 500 °C. The natural age-hardening was found to strongly depend on the solute contents of Cu and Si in the Al matrix. This study provides fundamental insights into controlling the strength level of commercial Al–Si–Cu cast alloys with impurity elements using the cooling process after solution treatment at elevated temperatures above 400 °C.

Effect of Thermal Exposure Simulating Vapor Deposition on the Impact Behavior of Additively Manufactured AlSi10Mg Alloy

The present work focuses on the evolution of hardness and impact toughness after thermal exposure at high temperatures of the AlSi10Mg alloy produced by selective laser melting. The thermal exposure simulated the vapor deposition of coatings on aluminum alloys. The aim is to assess the possibility of combining the ageing step of heat treatments and the deposition treatment. The alloy was aged at 160 and 180 °C for up to 4 hours, both directly and after an innovative rapid solution treatment. Direct ageing had no significant effects on the microstructure, showing an almost constant hardness trend. These results accord with the impact properties, which showed a negligible difference in the impact toughness of the direct aged and the as-built samples. The same ageing treatments performed after rapid solution treatment induced age hardening in the alloy. The hardness values were lower by 38% than those of the directly aged samples. The innovative solution treatment positively affected impact toughness, which increased by 185% compared to the directly aged material. These results highlight that the ageing step can be integrated with the vapor deposition process. Moreover, the heat treatment is suitable for components requiring high impact strength after coating.

Internal Stress and Dislocation Interaction of Plate-Shaped Misfitting Precipitates in Aluminum Alloys

The fine misfit precipitates in age-hardenable aluminum alloys have important roles due to their excellent age-hardening ability, by their interaction with dislocations. The present study focused on the internal stress field of plate-shaped misfitting precipitates to evaluate their roles in dislocation overcoming the precipitates by means of micromechanics based on Green’s function method. The stress field of misfit precipitates on {001} and {111} habit planes were reproduced by homogeneous misfit strain (eigenstrain) of the precipitate (Eshelby inclusion method), and the dislocation motion vector on the primary slip plane was predicted by the force acted on the dislocation by the Peach–Koehler formula. According to simulation results, the dislocation interaction strongly depends on the stress field and geometry of misfit precipitates; repulsive and attractive forces are operated on the dislocations lying on the primary slip plane when the dislocation approaches the misfit precipitates. The hardening ability of different orientations of precipitation variants was discussed in terms of interaction force acted on the dislocation.