EFFECT OF OPTICAL PARAMETERS ON FIBER LASER WELDING OF ULTRA HIGH STRENGTH STEELS AND WELD MECHANICAL PROPERTIES AT SUB-ZERO TEMPERATURES

摘要

The recently developed direct-quenched Ultra High Strength Steels have a combination of high tensile strength and toughness properties at sub-zero temperatures down to -80°C. The low carbon content of the material is beneficial for weldability. Modern high power lasers provide an excellent opportunity to modify the focal point diameter by changing the feeding fiber diameter, collimation and focal length and thus modify the beam intensity and heat input in actual welding process. In this study, butt joints of Optim 960 QC direct-quenched Ultra High Strength Steel (UHSS) with a thickness of 8mm were welded with 10 kW fiber laser with various different optical set ups in order to evaluate the characteristics of the joints within the range of low to high heat inputs possible for this welding process. The mechanical properties of the joints were studied by subjecting the specimens to a number of destructive tests of mechanical properties. The weld metal hardness and tensile strength was tested. It was found that high quality butt joints with superior tensile strength could be obtained. However, having a high level of all these properties in the joint narrows the process parameters' window, and the heat input needs to be strictly controlled.