PSI - Issue 68

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

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the stresses first decrease, reach their minimum just before the spot weld, and then increase significantly, leading to a similar tensile stress value as in y-direction. The longer the plunge time, i.e. the higher the temperature, the larger the area influenced by residual stresses. Accordingly to the obtained residual stress distributions, the plunge time did not significantly influence the absolute value of these stresses, however, the region of residual stresses near the spot weld expands significantly.

Pt = 0.8 sec

Pt = 1.0 sec

200

200

xx

xx

150

150

yy

yy

100

100

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0

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Residual stress [MPa]

Residual stress [MPa]

100

100

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0

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0

20

40

Distance from weld center [mm]

Distance from weld center [mm]

Pt = 1.5 sec

Pt = 2.1 sec

200

200

xx

xx

150

150

yy

yy

100

100

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Residual stress [MPa]

Residual stress [MPa]

100

100

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Distance from weld center [mm]

Distance from weld center [mm]

Pt = 3.5 sec

200

xx

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yy

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Residual stress [MPa]

100

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Distance from weld center [mm]

Fig. 3. Residual stresses σ xx and σ yy related to plunge time.

The obtained hardness distribution is shown in Figure 4, which is typical for a solid-state processed precipitation hardening aluminum alloy (Threadgill et al., 2009). The maximum hardness is found in the base material and drops rapidly towards the spot weld. The minimum hardness is located in the HAZ, while in the SZ an intermediate hardness can be expected. In contrast to the residual stresses, a trend in the hardness values can seen in terms of welding parameters. The hardness in the spot weld decreases with increasing plunge time, i.e. higher temperature in the spot weld. In addition, as with the residual stresses, the a ff ected area grows with increasing plunge time.

4. Discussion

The residual stresses that occur are caused by the thermomechanical processing of the refill FSSW process. It is assumed that the mechanical contribution remains the same despite the di ff erent plunge times, even if the experienced