PSI - Issue 68

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

780

130

Heat affected zone

Heat affected zone

120

Stir zone

110

100

90

affected zone

affected zone

Thermo mechanically

Thermo mechanically

80

Pt=0.8 sec

Pt=1.0 sec

Hardness [HV0.2]

70

Pt=1.5 sec

Pt=2.1 sec

60

Pt=3.5 sec

50

15.0 12.5 10.0

7.5

5.0

2.5

0.0

2.5

5.0

7.5

10.0 12.5 15.0

Distance from weld center [mm]

Fig. 4. Hardness distribution related to plunge time.

strain is di ff erent. Therefore, the changes in residual stress profile can be associated with the di ff erent temperatures. Due to frictional heat during the welding process the material expands, resulting in compressive residual stresses outside the spot weld. If the material cools down after the welding process, tensile residual stresses arise due to the contraction of the material within the spot weld. Now it would seem obvious that the higher the temperature, the greater the contraction and therefore also the resulting residual stresses. However, at higher temperatures, the material is not able to withstand high stresses due to the temperature-dependent yield strength, leading to plastic deformation. This means that during the cooling process, a equilibrium between residual stresses and yield strength is only established below a certain temperature at which the material is able to withstand the residual stresses. Therefore, despite higher temperatures, no higher residual stresses can be observed at the spot weld, only the range in which the residual stresses act increases according to the temperature. Two temperature-dependent mechanisms have an influence on the hardness distribution. One is the precipitation characteristic and the other is the grain size. In the stir zone (SZ), the strength-increasing β ” precipitates are dissolved by the welding process and then form again through natural ageing. In the heat a ff ected zone (HAZ), the β ” precipitates are also dissolved by the welding process, but less strength-increasing β ’ precipitates are formed (Svensson et al., 2000). In addition, dynamic recrystallization occurs within the SZ during the welding process, which leads to a strength increase according to Hall-Pecht e ff ect (Hall, 1951; N. J. Petch, 1953). In the HAZ, on the other hand, grain growth occurs due to the elevated temperature. Overall, the hardness in the SZ is greater than the hardness in the HAZ due to the re-formation of β ” precipitates caused by natural aging, whereas only less β ” precipitates can form in the HAZ due to the prevailing β ’ precipitates. In addition, the strength-increasing Hall-Petch e ff ect occurs in the SZ, whereas grain growth occurs in the HAZ due to the elevated temperatures.

5. Conclusions

The experiments have shown that the temperature in the spot weld increases with increasing plunge time. However, this has no e ff ect on the absolute value of residual stresses within the spot weld. The situation is di ff erent for the hard ness, which decreases with increasing temperature. With both, residual stresses and hardness, it is equally noticeable that the area influenced by residual stresses and hardness increases with increasing temperature.