The influence of post-welding heat treatment on the microstructure of electron beam welded joints from the Al–Zn–Mg–Cu alloy

The present work examined the correlation between post-welding heat treatment, the microstructure evolution, and microhardness of the electron beam welded joints from the Al–Zn–Mg–Cu alloy. The welds were investigated in the as-welded state, after post-welded direct aging, and full heat treatment (solution annealing, quenching, and aging). The microhardness measurements revealed a distinct drop for the as-welded state, from 190 HV0.1 for the base material (BM) to 150 HV0.1 in the fusion zone (FZ). Direct aging resulted in a moderate increase to 165 HV0.1 in the FZ. Following the application of the full heat treatment, a homogeneous microhardness distribution was achieved with an average value of 195 HV0.1. The second-phase precipitates were characterized using atomic resolution electron microscopy. For the BM of the initial weld and after direct aging, the majority of precipitates were equilibrium η, while for the sample after full heat treatment, GP zones and η′ precipitates were the most numerous. In the FZ, the occurrence of strengthening precipitates after welding was observed to be deficient. Additionally, the presence of segregation at the grain boundaries resulted in the ineffectiveness of direct aging in initiating precipitation processes. During solution annealing, the temperature was sufficient to induce the dissolution of alloying elements into the matrix. Subsequent aging resulted in precipitation processes of the strengthening η′ precipitates. Precipitate hardening was estimated as the primary strengthening factor of the Al–Zn–Mg–Cu alloy. Consequently, the weld exhibited the highest microhardness and homogeneity following full heat treatment, despite its significantly larger grain size.