Laser Bead-on-Plate Welding and Overlap Seams for Increasing the Strength and Rigidity of High Strength Steel
Nowadays, high requirements are being placed on producing lighter automobiles with a higher strength. To achieve graded strength properties and to improve the rigidity of high strength thin steel sheets, the side effects of laser joining processes can be used. Local physical and geometrical effects which have previously only been observed as side effects can be purposefully used to increase the rigidity and strength of sheet metal structures. By using a focused laser beam with a diameter of several tenths of a millimetre, bead-on-plate and overlap welding seams have been produced. The energy needed to produce this kind of welding seam can be limited to a small area of the workpiece. In comparison to other procedures, the basic material characteristics are retained after welding, the main reason for this being localized heat input. The continual development of laser beam sources to provide higher output powers has extended their spectrum of use in the field of joining technologies. One aim of the research is to produce local physical and geometrical effects with two different laser systems, on the one hand, with an Nd:YAG laser with a maximum output power of 4 kW, and on the other hand with a Yb:YAG laser with an maximum output power of 3 kW. Bead-on-plate and overlap welding seams were produced to demonstrate that rigidity and strength can be increased in metal sheets. The investigations were carried out on two high strength steels H340LA with two different zinc coatings (D and ZE) and TRIP700. The sheets were tested using tensile tests, 3-point bending tests and fatigue strength tests. During the tests, metallographic analyses were carried out. Seven different specimens were investigated, one without bead-on-plate welding seams, and six with different welding geometries, expect the material H340LAZE which was investigated with three various welding geometries. To analyse the complex stress status, investigations of the residual stress and the fracture were done. Calculations of the seam volume were done to be able to calculate the tensile strength for linear bead-on-plate welding seams and the maximum force for bending overlap welding seams. The tensile and bending tests showed that higher forces were needed before failure occurred, e.g. bending the specimens. Furthermore, the investigations showed that the strength of the specimens with welding seams increased, in comparison to the specimens without welding seams. Another result of the experiments is that there is a dependency between the fatigue strength and the position of the seam relative to the direction of the testing force.
Heinz Palkowski and Kai-Michael Rudolph
M. Schimek et al., "Laser Bead-on-Plate Welding and Overlap Seams for Increasing the Strength and Rigidity of High Strength Steel", Advanced Materials Research, Vol. 137, pp. 161-190, 2010