Improvement of Fatigue Strength for Arc-Welded Joints of Ultra-High Strength Steel (UHSS) Sheets by Cold Spray Technique

This study aims to improve the fatigue strength of a welded joint of an ultra-high strength steel (UHSS) with a tensile strength grade of 980 MPa by cold spray deposition which causes stress concentration relief and compressive residual stress. Two types of coating: Fe/Al₂O₃ and Zn/Al₂O₃ coating were deposited on the joint by cold spraying using Fe/Al₂O₃ and Zn/Al₂O₃ mixtures respectively. Plane bending fatigue tests were conducted in order to investigate the fatigue performances of the joints. In both cases, the adhesive strengths between the substrates and the coatings were sufficient, so that fatigue cracks propagated inside the coatings rather than the interfaces. It is expected that the Al₂O₃ particles facilitate to generate a new surface through which the metals and the substrates combined.

The test results demonstrated that the Fe-deposited joint was improved in the fatigue strength at 3 million cycles from 275 MPa of as-welded joints to 450 MPa. However, the fatigue strength of Zn-coated joint was almost same as that of the as-welded one. It indicated that although the cold spray deposition has an effect on the fatigue strength of the joint, it is influenced by mechanical properties of the coatings.