Laser welding with beam wobbling was employed to weld X70 pipeline steel. The influence of beam wobbling on weld surface morphology, fusion zone microstructure, and microhardness was investigated. The formation of spatter was suppressed by beam wobbling using a beam with higher energy density. At a welding speed of 1.0 m min⁻¹, the weld metal produced with a circular laser wobble pattern contained more martensite but less ferrite, compared to that of static spot welded joint. The presence of more martensite led to the increase in fusion zone hardness. A softened region was found in the inter-critical heat-affected zone. The hardness of the softened region could be improved by increasing the welding speed (from 1.0 to 1.5 m min⁻¹).

Laser welding was performed using different filler wires, ER70S steel, commercially pure iron, and pure nickel filler, in the context of welding X80 pipeline steel to assess the microstructure and mechanical properties of the weld metal. Introducing an ER70S wire promoted acicular ferrite formation in the fusion zone, compared to a bainitic microstructure in an autogenous laser weld. The use of pure iron wire was considered as a potential strategy for reducing hardenability, as it led to the dilution of alloying elements in the fusion zone, increasing ferrite content and reducing weld metal hardness to a level compliant with API pipeline standards. The addition of pure nickel wire was used to reveal the degree of weld metal mixing imposed by the laser (thus providing an unambiguous tracer element) when it is combined with filler material dilution in the fusion zone, revealing that the upper region contained 38% wire material and the lower region only 12%. This accounts for the differences observed between the upper versus lower portions of the weld metal when other wires are used, and the use of hardness mapping and micro-indentation demonstrates the correlation between the variations in mechanical properties and microstructural differences introduced by incomplete mixing of the filler wire elements.