On the search for the origin of the bulge effect in high power laser beam welding

Abstract

The shape of the weld pool in laser beam welding plays a major role in understanding the dynamics of the melt and its solidification behavior. The aim of the present work was its experimental and numerical investigation. To visualize the geometry of the melt pool in the longitudinal section, a butt joint configuration of 15 mm thick structural steel and transparent quartz glass was used. The weld pool shape was recorded by means of a high-speed video camera and two thermal imaging cameras, a mid-wavelength infrared camera and a newly developed infrared camera working in the spectral range of 500 to 540 nm, making it perfectly suited for temperature measurements of molten materials. The observations show that the dimensions of the weld pool vary depending on the depth. The regions close to the surface form a teardrop-shaped weld pool. A bulge region and its temporal evolution were observed approximately in the middle of the depth of the weld pool. Additionally, a transient numerical simulation was performed until reaching a steady state to obtain the weld pool shape and to understand the formation mechanism of the observed bulging phenomena. A fixed keyhole with an experimentally obtained shape was used to represent the full-penetration laser beam welding process. The model considers the local temperature field, the effects of phase transition, thermocapillary convection, natural convection, and temperature-dependent material properties up to evaporation temperature. It was found that the Marangoni convection and the movement of the laser heat source are the dominant factors for the formation of the bulge region. A good correlation between the numerically calculated and the experimentally observed weld bead shapes and the time-temperature curves on the upper and bottom surface was found.

Numerical Analysis of the Partial Penetration High Power Laser Beam Welding of Thick Sheets at High Process Speeds

“…In order to gain more understanding about the keyhole and molten pool dynamics and more precisely how these correlate with the formed defects, several experimental attempts have been published in the literature. Monitoring and detecting systems, based on different measuring techniques, e.g., acoustic [18], infrared and imaging signals [19], high-speed imaging [20], etc., have been utilized for the collection of process data. By making use of the collected data, new insights into the process can be obtained.…”

Elucidation of the Bulging Effect by an Improved Ray‐Tracing Algorithm in Deep Penetration Wire Feed Laser Beam Welding and Its Influence on the Mixing Behavior

“…Similar behaviors can be seen in several different studies. [8,21] Consequently, this narrow region has a detrimental effect on the potential exchange of material between the upper part of the weld pool, where the filler wire is transferred to, and the base metal in the lower part. A sequence of such blocking effects by the separation of the different zones of the weld pool is shown in Figure 9b-e, leading to the bad mixing behavior here that was also found experimentally.…”

“…Under certain conditions, this region can also develop into a bulging region being characterized by an elongation of the solidification line. [8] From this analysis, it is straightforward to assume that such narrow regions in the weld pool are detrimental to the homogeneity of mixing, especially during WFLBW, as this can interrupt the downward element transport being inserted at the top side of the weld pool.Advances in numerical methods and in available computer power have paved the way for extensive numerical modeling for small-scale transient multiphysics processes like LBW.…”

High-power laser beam welding for thick section steels – new perspectives using electromagnetic systems

“…Nonetheless, they are useful even for more specific cases, e.g. in simulations considering equivalent heat source concepts coupling a stationary fluid flow simulation with a transient heat conduction model in autogenous laser beam welding [30, 31], or for the elucidation of the relationship between flow phenomena in the weld pool, the resulting weld geometry and the final mechanical resistance of the weld [32, 33]. Similar simplified models are in the same way well suited if one is interested in evaluating the resulting pressure differences in the weld bead during the application of electromagnetic fields as an inductive weld support system for a given magnetic flux density applied or to limit the parameter window for such application.…”