Manufacturing of aircraft panels by friction stir lap welding of 7075-T6 stringers on 2024-T3 skin: process optimisation and mechanical properties

摘要

The present study reports on the effects of process parameters on weld quality of 1,5-mm 7075-T6 stringers lap-joined on 2,3-mm 2024-T3 skins. Advancing and retreating side locations on the joint configuration were alternated to determine optimal design arrangements. The effects of travel and rotation speeds on weld quality and defect generation were also investigated. Weld quality was assessed by optical microscopy and bending tests. It was found that: i) the increase of the welding speed or the decrease of the rotation speed resulted in a reduction of the hooking size and top plate thinning but did not eliminated them, ii) double pass welds by overlapping the advancing sides improved significantly the weld quality by overriding the hooking defect, iii) change of the rotational direction for a counter clockwise with a left-threaded pin eliminated the top plate thinning defect. In the second part of the investigation, FSW lap joints were produced using optimum conditions and underwent an extensive mechanical testing program. Moreover, different weld configurations were manufactured: discontinuous (with plunge-in entry and exit holes), continuous welds, single pass welds, double pass welds, and plugged discontinuous welds. The results were also compared with riveted lap joint of identical geometry mainly in terms of cyclic strength performance of FSW panels. It was found that i) the tensile strength of FSW joints approached that of the base material but with a significant reduction in the fatigue life, ii) the pin plunge and removal locations served as the key crack nucleation sites in specimens with discontinuous welds, and iii) double pass welds with overlapping advancing sides showed outstanding fatigue life and very good tensile properties. This research provided some valuable insight into both the fabrication and the application of FSW on stringer/skin lap joints.