Precipitates Dissolution, Phase Transformation, and Re-precipitation-Induced Hardness Variation in 6082-T6 Alloy During MIG Welding and Subsequent Baking

Precipitation evolution-induced hardness variation in 6082-T6 alloy during various thermal histories of welding and subsequent paint baking was characterized systematically. Precipitation evolution in various subzones of the heat-affected zone (HAZ) from the weld toe to the base materials was precisely identified by transmission electron microscopy. Under heating processes of welding and baking with different temperature and time duration, the morphology evolution of precipitates comprised dissolution, coarsening in size, and phase transformation from β ″ to U 2, β ′ and β , etc. The mechanisms of the thermal softening and bake hardening of mechanical properties in HAZ are clarified. Quantitative calculation results showed that grain growth contributed little to the strength loss in HAZ, while precipitate evolution was responsible. Re-precipitating of Guinier–Preston zones and β ″ phase from the super-saturated solid solution produced by welding was found to make main contributions to the efficient hardness recovering of joints during post-weld paint baking.