Mitigation of Hot Cracking Susceptibility in Laser Welding of Transformation-Induced Plasticity Steels by Beam Shaping

Abstract

Hot cracking is a significant imperfection in laser welding of thin transformation induced plasticity (TRIP) steel sheets. This study aims to evaluate strategies to mitigate the hot cracking susceptibility through beam shaping, by performing free edge welding tests. TRIP steel sheets were welded with a laser beam with a constant laser power for various spot sizes and incident angles at different distances from the free edge. A high-speed camera and a welding camera are applied to film the behaviour of the plasma plume and the development of the molten fluid flow. Pre-attached thermocouples are utilized for in-situ transient temperature measurements. The weld pool dimensions and microstructures of the fusion zones are characterized by optical microscopy and semi-quantitative elemental analysis is conducted by scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS). Experiments showed that, as the heat input increases, the vaporization of material induces a recoil pressure and leads to keyhole formation, and the direction of maximum thermal gradient and the cooling rate determines the microstructure. At a fixed beam spot size and incident angle, an increased travel speed decreases the heat input, and results in a smaller bead width with less solidification shrinkage and thermal contraction. In the case of varying spot sizes with a fixed incident angle and travel speed, a small spot size of 0.2 mm refers to a high energy density, the concentrated beam creates an open keyhole that leads to energy loss, and the absorption rate of thermal energy is low. Such a condition generates a narrow bead width, which results in less shrinkage that mitigates the hot cracking susceptibility significantly. When the spot size increases, a wider bead is created that enhances the contraction and therefore higher tensile stresses, which causes the metal to be more prone to cracking. When the diameter of the spot increases to 1.2 mm, the thermal energy is so dispersed that the heat is not sufficient to evaporate significant amounts of metal and the operation mode switches from a keyhole mode to a conduction mode, which only partially penetrates the steel sheet. In the case of varying the incident angle with a fixed spot width and fixed travel speed, the inclined beam projects a prolonged elliptical impingement area and shifts the location of highest density according to the incident angle. A leading beam configuration, i.e. a small incident angle, acts as post-heating source of the molten liquid. For large incident angles of a trailing configuration, the beam impinges on the front wall of the keyhole and acts as a pre-heating source. A prolonged elliptical heated area increases the time for the back feeding liquid to heal the gaps between the columnar grains growing into the fusion zone and suppresses the hot cracking susceptibility. If the incident angle becomes too small (45°), the consequence is similar to a too dispersed spot size, i.e. partial penetration of the sheet. A trailing configurated beam can reflect on the front keyhole wall and enhances absorption, but it generates turbulent flow. When the angle comes too large (135°), the process is not entirely stable. In conclusion, for producing defect-free welds in industrial applications, it is recommended to utilize a small spot size that corresponds to less solidification shrinkage. Furthermore, a leading configuration is preferred, but the incident angle should not be too low, in order to achieve full penetration.