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6398 |
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Thursday, June 11, 2020, Hall D 11:00 AM Automotive Industry |
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Improvement of fatigue strength of arc-welded joints of ultra-high strength steel (UHSS) sheets by cold spray technique |
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Naoki Yamaguchi* / JFE Steel Corporation, Japan K. Ogawa / Tohoku University, Japan Tsuyoshi Shiozaki/ JFE Steel Corporation, Japan Yoshikiyo Tamai/ JFE Steel Corporation, Japan Kanji Okuda/ Tohoku University, Japan Yuji Ichikawa/ Tohoku University, Japan |
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Application of ultra-high strength steels (UHSS) has been studied to reduce auto body weight. Most chassis parts are assembled by arc welding, and the major obstacle to application of UHSS is the fatigue strength of the arc welded joints. The key factors for controlling the fatigue strength of welded joints are stress concentration, residual stresses and microstructures at the weld toe. The aim of this study is to improve the fatigue strength of welded joints of UHSS with a tensile strength grade of 980 MPa by a cold spray technique which relieves stress concentrations and generates compressive residual stress. Two types of coatings, Fe/Al2O3 and Zn/Al2O3, were deposited on the joint by cold spraying using Fe/Al2O3 and Zn/Al2O3 mixtures, respectively. Stress relief heat treatment was applied to some Fe/Al2O3 coated specimens to investigate the effect of residual stress on fatigue strength. Plane bending fatigue tests (R=0) were conducted to investigate the fatigue performance of the joints. The adhesive strength between the substrates and the coatings was sufficient in both coatings, so that fatigue cracks propagated inside the coatings rather than at the interface between the coatings and substrates. The Al2O3 particles in these coatings are expected to facilitate the generation of a new surface through which the metals and the substrates combine. The test results demonstrated that the fatigue strength of the Fe/Al2O3 coated joint at 3 × 106 cycles improved from 275 MPa in the as-welded joints to 450 MPa. The fatigue strength of the heat-treated specimens was 350 MPa. This indicates that both stress concentration relief and compressive residual stress influence fatigue strength. However, the fatigue strength of the Zn/Al2O3 coated joint was almost the same as that of the as-welded one. This suggests that the hardness of the coatings can also influence fatigue strength. |