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It is known that one of the main concerns associated with the conventional welding of precipitation-strengthened Al alloys is the formation of softening regions, resulting in the deterioration of mechanical properties. In this study, we show that linear friction welding (LFW) can completely suppress softening regions in precipitation-strengthened AA6061-T6 alloy by introducing a large shear strain and by controlling the interfacial temperature. We found that the LFW process resulted in an extremely low interfacial temperature; it decreased as the applied pressure increased from 50 to 240 MPa. This approach can essentially suppress both softening and hardening regions, leading to uniform hardness distribution in Al joints. The high-pressure LFW process demonstrated here can thus provide an innovated guidance to obtain high-performance Al alloy joints and be extended to other precipitation-strengthened Al alloys, which undergo high-temperature softening.

Linear friction welding (LFW) of AA1050 and AA5052 was performed at various applied pressures. The strengthening contributions of the joints were examined to investigate the relationship between the microstructure and mechanical properties. The dominant strengthening mechanisms at the weld centre of the AA1050 joint were both the grain-boundary strengthening and dislocation strengthening. While the weld centre in the AA5052 joint was dominated by the grain-boundary strengthening since the dynamic recrystallisation (DRX) during LFW was promoted due to the relatively low stacking fault energy (SFE). On the other hand, the strengthening mechanism in the regions away from the weld centre in the AA1050 and AA5052 joints was mainly dominated by the dislocation strengthening due to increased dislocation introduced during LFW.

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