High strength low alloy steel joint fabricated by laser welding with real-time high-frequency resistance heating

The balance between softening and hardening is still a great challenge for the laser welding of the high strength low alloy steel. For example, welding with low heat input was commonly used to avoid softening, but it would inversely harden the joint and introduce the undesired welding defects such as hydrogen-induced cracks. This work proposed a simple and robust processing technique, denominated High Frequency electric cooperated Laser Welding. Laser heat source was combined with high-frequency electric heat source to balance heat input distribution and reconcile the contradiction between softening and hardening in the welding of high strength low alloy steel. As a structural material sensitive to welding softening and hardening, S690QL steel was adopted to show the fundamental advancement of the new process. Benefitted from the skin effect and proximity effect of high-frequency current that improve the heat input distribution and the regulation of the high-frequency heat input proportion, the martensite content in the heat affected zone was greatly reduced, whilst the grain size was limited to a low value. Under the high-frequency heat input proportion of 47.1 %, lower bainite distributed throughout the heat affected zone. The softening and hardening issues of the joint therefore have been improved simultaneously: the tensile strength of the joint was almost equal to that of the base material, the impact energy of weld seam and the heat affected zone reached 77.6 J and 66.2 J respectively.