Microstructural and mechanical characterisation of laser-welded lap joints with linear and circular beads in thin low carbon steel sheets

• Microstructure and strength of laser-welded lap joints with linear and circular bead.
• Different micro-hardness values and microstructures in different specimen zones.
• In tensile tests, all specimens fracture in base metal far away from weld zone.
• Circular welds have lower rotation angle of weld bead compared to linear weld.
• Theoretical model to estimate weld strength from material properties and geometry.

This paper presents a microstructural and mechanical characterisation of laser-welded lap joints in low carbon steel thin sheets. Different combinations of steel types (DC05, S355MC) and thickness values are used to assemble welded specimens with linear and circular weld bead. Metallurgical observations and micro-hardness tests are used to characterise the weld microstructure. Mechanical response in tensile test is then used to evaluate the static strength, rotation angle of weld bead and failure mode of welded specimens. Lap-joints with circular weld showed a lower rotation angle compared to linear welds. The fracture in all tested specimens occurred at the base metal, far away from the weld. A simplified mechanical model is finally proposed to derive theoretical formulae for estimating the tensile strength of welded joints as a function of material properties and weld geometry. The analytical results are in good agreement with experimental findings and they estimate an increased strength for circular welds, compared to linear weld with same lateral width. A design chart is also derived to allow a design of laser-welded joints with virtually equal strength of base metal and weld zone.