PSI - Issue 68

Niklaas Becker et al. / Procedia Structural Integrity 68 (2025) 776–781 Niklaas Becker / Structural Integrity Procedia 00 (2024) 000–000

777

weight and can also be used for higher-strength aluminum alloys (Kwee et al., 2019). The probably best-known solid state process is friction stir welding (FSW), but since spot-type joints are often su ffi cient in structural engineering, friction stir spot welding (FSSW) or refill friction stir spot welding (refill FSSW) have gained some significant inter est. Refill FSSW has some advantages over FSSW, such as greater strength and better surface quality, mainly due to the removal of the exit hole (Fratini et al., 2007). For this reason, refill FSSW is also seen as a potential successor to resistance spot welding and riveting in automotive and aircraft industry (Allen and Arbegast, 2005). Since the struc tures in automotive and aircraft industry are dynamically loaded structures, particular attention is paid to the fatigue properties of the joint. It is known from the literature that residual stresses influence the fatigue properties of welded joints. For example, Pouget and Reynolds (2008) conducted fatigue crack propagation tests on FSW welds and came to the conclusion that the influence of residual stresses is greater than that of the microstructure. Ma et al. (2011) also investigated fatigue crack propagation in FSW seams and found that it was di ffi cult to initiate cracks in compres sively loaded areas. Becker et al. (2024) investigated crack propagation in refill FSSW and showed that it is possible to deflect the fatigue crack with the help of compressive residual stresses. The influence of residual stresses on the fatigue life of solid state joining processes has been illustrated. This study investigates the influence of the welding parameters on the resulting residual stress field. For this purpose, samples with di ff erent heat input were produced, where the temperature during the welding process was recorded. The resulting residual stresses and hardness, as an indicator of the strength, were analyzed.

2. Materials and methods

2.1. Material and welding

The tests were carried out with AA6082-T6 sheet material. The specimens, shown in Figure 1, had a size of 140x135 mm, with the longer dimension corresponding to the rolling direction and a thickness of 5 mm. All welds were performed with the Harms & Wende RPS100 at the Helmholtz-Zentrum Hereon (Germany) and executed as blind welds, i.e. in just one sheet. The welding tool is made of hotvar and has the following diameters: clamping ring 17 mm, shoulder 9 mm, probe 6 mm. The specimens were welded using the shoulder plunge variant with the constant welding parameters: plunge depth (Pd), rotational speed (Rs) and the varying parameter plunge time (Pt): 0.8 sec, 1.0 sec, 1.5 sec, 2.1 sec, 3.5 sec. The clamping force was set to 16 kN.

a)

Rolling direction

b)

0.5

2.5

5.0

10.0

Fig. 1. a) Specimen geometry with spot weld; where the solid line represents the location of the residual stress measurement, the dotted lines represent the location the hardness measurements and in b) the locations of the thermocouples are represented. All dimensions in [mm].

2.2. Temperature

In order to record the temperatures during the welding process, these were measured at four di ff erent positions: in the center and at 2.5 mm, 5.0 mm and 10.0 mm from the weld in center, see Figure 1 b). The temperature was measured 0.5 mm from backside of the sheet using a thermocouple (type K) with a frequency of 50 Hz.