PSI - Issue 13

Anke Schmiedt et al. / Procedia Structural Integrity 13 (2018) 22–27

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A. Schmiedt et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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Fig. 5. Fracture surfaces of brazed specimens prior to (a) and after 6 weeks of pre-corrosion (b, c), after tensile (b) and fatigue (a, c) testing (SEM with BSE); Polished section of the pre-corroded brazed fatigue specimen within the circumferentially corroded region (d) and within the fatigue fracture area (e) with base material (BM), brazing seam (BS), and diffusion zone (DZ) (SEM with SE). (Schmiedt et al., 2018a) The pre-corrosion of the brazed specimens leads to a circumferentially corrosive attack in combination with localised effects that progressed in the area of the brazing seam, Fig. 5b. With ongoing condensate corrosion, the impact is more pronounced and the unaffected cross-section area is reduced . According to the polished section at the marked position, the corrosive attack is especially localised in the area of the diffusion zones, Fig. 5d. A diffusion of chromium from the base material to the brazing alloy, leading to a reduction of the chromium content within the diffusion zone, was reported from Colbus et al. (1974) and is expected to be the reason for the selective corrosion effects. Within the circumferentially corroded region, the crack propagated alternatingly through one of the two affected diffusion zones, Fig. 5d. The fracture, due to the quasi-static tensile loading of pre-corroded brazed specimens, occurred entirely within the light grey displayed gold-base filler metal (Fig. 5b). On the fracture surface of the fatigue specimen, a fatigue fracture area within the base material (Fig. 5e) can be distinguished from the final fracture area within the brazing seam (Fig. 5c). A crack initiation zone in the area of the corroded diffusion zones leads to numerous dark grey fatigue fracture areas on the left side of the BSE image. A transition from the forced fracture to the circumferentially corroded interface can be seen on the right side of the top-view, Fig. 5c. 4. Conclusions and outlook For brazed AISI 304L/BAu-4 joints, pre-corrosions acc. to VDA 230-214 lead to a pronounced corrosive attack, especially localised at the interfaces to the base material. Pitting effects will be prospectively described. After 6 weeks, a reduction of the ultimate tensile strength and the failure maximum stress in LIT down to 69% and 42% were determined. Strain, electrical, magnetic and temperature measurement techniques within fatigue tests are suitable to characterise the deformation and damage behaviour of brazed joints in various corrosion conditions. Strain values of the DIC system are in good agreement with the results obtained with mechanical extensometers, and a triggered DIC image acquisition allows to investigate local ratcheting fatigue effects at the brazing seam. Local strain concentrations are more pronounced with increasing tensile and fatigue stresses and are enhanced for pre-corroded joints. A decrease of the gauge length from 0.5 mm to 50 µm is planned for further studies. In addition, stress concentrations at the corrosion-dependent grooves and the effect of the gauge length on the yield strength will be evaluated. Furthermore, the corrosion fatigue properties of the brazed joints under superimposed loading will be investigated. Acknowledgements The financial support of German Research Foundation (DFG) is gratefully acknowledged (WA 1672/13-1, TI 343/96-1). References Chen, D., Sun, S., Dulieu-Barton, J.M., Li, Q., Wang, W., 2018. Int J Fatigue 110, 172. Colbus, J., Zimmermann, K.F., 1974. Gold Bull. 7, 42. Hahnenberger, F., Smaga, M., Eifler, D., 2014. Int J Fatigue 69, 36. Holländer, U., Weber, F., Möhwald, K., Maier, H. J., 2015. Welding and Cutting 14, 280. Koster, M., Kenel, C., Leinenbach, C., 2013. ICF13. Manka, M., Wojarski, L., Tillmann, W., Frieling, G., Myslicki, S., Walther, F., 2013. Loet 2013.

Nebel, T., Eifler, D., 2003. Sadhana 28, 187. Nikitin, I., Besel, M., 2008. Int. J. Fat. 30, 2044. Schmiedt, A., Jaquet, S., Manka, M., Tillmann, W., Walther, F., 2018a. Fatigue 2018. Schmiedt, A., Manka, M., Tillmann, T., Walther, F., 2018b. Weld World, 10.1007/s40194-018-0557-y. Smaga, M., Walther, F., Eifler, D., 2008. Mat Sci Eng A 483, 394.