Thermal Analysis and Microhardness Mapping in Hybrid Laser Welds in a Structural Steel

Abstract

Abstract : Hybrid welding, the combination of laser and arc welding, is being heavily investigated for potential applications in the fabrication of structural steel components. The hybrid process is an alternative to autogenous laser welding that requires good fit-up of the parts. With the hybrid process, the addition of filler metal alters the weld pass chemistry and fills any gaps that may occur. In some applications, the fit-up of parts is only part of the issue for laser welding. In some structural steel components, the chemistry of the base metal when autogenously laser welded can result in weldments (weld metal and heat-affected zone (HAZ)) that have high hardness values. This indirectly indicates that the weld metal is brittle and not suitable for certain applications. The hardness values of the weldment have been used as an acceptance criteria for certain industrial applications. The use of hybrid welding may address the hardness issue. The addition of filler metal through a gas metal arc (GMAW) based hybrid process decrease the cooling rate in the HAZ therefore improving mechanical properties. These improvements can often be detected by microhardness profiles. While traditional hardness profiles tabulate hardness in a few regions of the weld, hardness mapping better profiles the hardness trends in the weldment. This paper will present welding results for the hybrid structural welding of a structural carbon steel using a 4 kW Nd:YAG based hybrid laser system. The data will center on the thermal response of the steel to the welding process, on the hardness mapping of the weldments and how the heat input altered the hardness and the mechanical properties of the welds.