Effect of tool traverse speed on joint line remnant and mechanical properties of friction stir welded 2195-T8 Al–Li alloy joints

Abstract

AA2195-T8 Al–Li alloy plates were welded by friction stir welding (FSW) at tool rotational speed of 1,000 rpm and tool traverse speeds (TS) of 100–400 mm·min−1 under three types of butting surface conditions, i.e., (1) without butting surface treatment, (2) butting surface milled, and (3) bead-on-plate welding. The effect of welding heat input and butting surface condition on joint line remnant (JLR) and mechanical properties of friction stir welded 2195-T8 Al–Li alloy was investigated comprehensively. In the stir zone of 2195-T8 FSW joints, there exists JLR composed of alumina-particle arrays and microcracks generated from the initial butting surface, and the morphology of JLR would evolve from smooth to serrate as TS increases. Moreover, as TS increases (i.e., the welding heat input decreases), JLR deteriorates the tensile strength of the 2195-T8 FSW joints, with joints prematurely fracturing along JLR. The fracture mode of 2195-T8 FSW joints was considered to be determined by the lower one between strength of JLR (S JLR) and strength of the lowest hardness zone (S LHZ), and JLR tends to be the fracture path at lower welding heat input. Furthermore, butting surface treatment (milling off oxide layer prior to welding) was found to be able to make the JLR in the 2195-T8 FSW joints less distinct and thus improve S JLR, while fracture along JLR could not be avoided.

A feasible operational parameter window for enhancement of welding speed in friction stir welding of 2195-T8 Al–Li alloy

“…The abnormal fracture phenomenon is more prevalent in the FSW Al-Li alloy joints with thin plates, which is attributed to the strong action from the tool shoulder. Yan et al [8] obtained defect-free joints in 2195-T8 alloy plates with 2 mm thickness, but the joints were fractured at the NZs during tensile tests, and similar results can also be found in some FSW joints of Al-Li alloys with medium thickness [12,14]. Chen et al [21] speculated that the abnormal fracture was related to the 'S' line originating from the fragmented oxide film at the plate butt surface.…”

“…It is clear that obtaining a defect-free FSW joint of Al-Li alloy with medium thickness is very challenging. Figure 2 summarises the welding performances of the present FSW joints and reported sound joints of 2195 and 2198 alloys with medium thickness [10][11][12][13][14][15][16][17][18], which shows a narrow zone under a blue dotted line for the sound FSW joints. It is concluded that the welding speeds of most sound joints were usually lower than 300 mm/min, and a relatively high welding speed of 400 mm/min (actually a moderate welding speed) can be accepted only at a moderate tool rotational speed of 800 rpm.…”

“…Due to the lack of a strong pinstirring effect, a distinct 'S' line was always observed in the SAZ. Though cracks really initiated from the 'S' line in some FSW joints, most joints did not fracture along the 'S' line [6,8,9,12,14,21]. Therefore, the 'S' line was not the intrinsic reason for the abnormal fracture in the NZ.…”

“…One of the remarkable features is that the welding window was significantly narrowed for the FSW of Al-Li alloys, compared to those of other precipitatestrengthened Al alloys. Table 1 summarises the welding parameters and tensile properties of some FSW joints for 2195 and 2198 Al-Li alloys [5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Though the tool rotational speed is in a wide range, the most important welding speed related to manufacturing efficiency is limited to a very narrow section.…”

“…Therefore, the precipitation evolution was not the key factor in determining the mechanical properties of the FSW Al-Li alloy joints. Most researchers have found that the FSW Al-Li alloy joints usually fractured at the nugget zone (NZ) rather than the lowest hardness zone (LHZ) even for defect-free joints, and the mechanical properties were decreased due to the abnormal fracture [6,9,12,14].…”