Reducing processing-induced residual stresses in SAE 4140 steels laser welded using modulated power emission

Abstract Residual stresses are a concern in weld beads since they increase the susceptibility to cracks and fatigue failure. Conventional methods to reduce these stresses are based on post-processing or the application of additional tools in the process, which increases the processing cost and time. In this context, this study aimed to measure the effect on residual stresses in welds of modulating the laser power emission applying two different wave shapes. The modulation method allows the energy delivery of the beam to be controlled by adding power steps, which changes the heat absorption and distribution by the molten pool. Square and step-shaped power modulation waves were used, through a YLS 10,000 fiber laser source (IPG Photonics) with a maximum power of 10 kW, in the welding of SAE 4140 steel of high hardenability. To measure residual stress fields in a specific critical point near the weld bead, the measuring technique combines the hole drilling method and speckle interferometry (instead of traditional strain gauges) for the identification of the displacements around the hole. The results showed that a reduction in the stress magnitudes was obtained for the step-shaped modulation wave, in the order of 77 MPa, which represents a reduction of 16% in comparison with the continuous wave with an increase of 37% in penetration depth, which is appreciable for applications in thicker plates.