Multi-pass friction stir welding in alloy 7050-T7451: Effects on weld response variables and on weld properties

In some situations it may be necessary or desirable to perform a friction stir weld pass through material, which has already been friction stir welded. Examples include weld repair or crossing weld beads. In either case, one would like to know whether the control parameters used for welding require modification for passes through existing welded material and what is the effect of the multiple passes on the weld properties. To examine this issue, multiple weld passes (five) were performed in 6.4 mm thick 7050-T7451 plate at a tool rotation speed of 540 rpm and a welding speed of 6.77 mm s^-1. Results indicate that for this material and these conditions weld control parameters require no adjustment. The overall reduction from pass 1 to 5 in transverse tensile strength is 7%. In addition, weld metallurgy is only slightly changed in the heat affected zone (HAZ) due to overaging for each weld pass while there is no change in metallurgy in the nugget. Hardness in the HAZ reduces by 14 Vicker's hardness points from pass 1 to 5. The peak longitudinal residual stress is reduced with increasing number of passes.