HAZ development and accelerated post-weld natural ageing in ultrasonic spot welding aluminium 6111-T4 automotive sheet

The effect of a paint bake treatment on joint performance in friction stir spot welding AA6111-T4 sheet using a pinless tool

“…The material can then be stored at room temperature for extended periods before use, without the final properties being as greatly affected by the pre-natural ageing time (the sheet used here was ~ 1 year old). This pre-treatment raised the parent sheet hardness above that of a directly solution treated material to ~ 83 Hv [10,[16][17][18][19]. As mentioned above, the peak temperature experienced in the weld centre would be expected to be over 400 C.…”

“…Furthermore, after artificial ageing little evidence was seen of minima at the edge of the weld areas in the hardness data. Such hardness minima typically occur at the HAZ/TMAZ border and are normally prominent in FSWs in heat treatable Al-alloys, where they give rise to a classical 'W-shaped' hardness profile, and [7,10]. A lack of hardness minima within the weld zone is an advantage from a joint performance point of view, because failure can become concentrated within such zones of reduced yield stress [11][12][13].…”

“…As mentioned above, the peak temperature experienced in the weld centre would be expected to be over 400 C. The softening behaviour seen immediately after welding in the hardness profiles can, therefore, be attributed to the dissolution of solute clusters and fine GPZs (Guinier-Preston zones) present in the T4* temper parent sheet [10], because the GP solvus temperature (~ 210 C) was greatly exceeded [20]. As further precipitation was largely suppressed, owing to the rapid welding cycle, this resulted in a supersaturated solid solution within the weld centre that could fully respond to post-weld ageing.…”

“…However, when fully re-solution treated at 560 °C for one hour the hardness level of the parent sheet fell to around ~55 Hv, which is 10 Hv below that of the weld zone. This ~ 10 Hv difference, between the hardness measured in the centre of the weld zone and the solutionised parent sheet, can be attributed to the increase in yield stress derived from the fine 2 to 5 µm diameter grain structure formed in the TMAZ (caused by the high level of plastic deformation experienced under the tool [7,10]). …”

“…When welding a heat-treatable alloy a factor that would be expected to influence joint performance in a pull-out failure is any loss of strength in the heat affected zone (HAZ), or thermomechanically affected zone (TMAZ) [7,10], caused by over-ageing during the welding cycle. To assess the significance of such an effect, hardness profiles were measured across the welds, before and after the simulated paint bake treatment, at a depth of 0.5 mm below the top surface.…”