Evaluation of Fatigue Strength Properties of Laser Welded Butt Joints of 1180 MPa Recycled Steel Plates

A laser welded butt joint was fabricated using 1180MPa recycled steel plate and a tensile test was performed. Two types of fatigue tests using different loading methods were also performed. The tensile properties of this test material were 1218 MPa tensile strength and 23.8% elongation, which were almost the same strength and ductility as the base material. There were no abnormalities in the welded parts, and the joints were properly joined. The 107 cycles fatigue limit of the base material (as pickled) was 450 MPa for both axial and bending loads at a stress ratio R=-1. In addition, at 107 cycles fatigue limit of the bending fatigue test at R=0 was 340 MPa, which overlapped with the modified Goodman line on the fatigue limit diagram. The fatigue strength of the laser welded butt joint was constant under axial and bending loads in the low life range of R=-1, but varied in the high life range. The fatigue fracture surface under bending load was present only on one side and was caused by a defect located approximately 100 μm away from the surface. For R=0, the fatigue properties were in good agreement under axial and bending loads. The fatigue limit was 250 MPa for both R=-1 and R=0 conditions. When the laser input side was set to the maximum stress side and the stress ratio was set to R = 0 on the plane bending fatigue testing machine, the fatigue limit was dramatically improved to 340 MPa, and the same fatigue strength as that of the base material after pickling was obtained.