| The purpose of this study is to understand deep penetration laser welding and the effect of laser welding parameters on the hourglass melt pool formation. A transient thermo-fluid-structural laser welding process is numerically computed using finite element techniques, and an evolution of the melt pool is continuously monitored for the measure of the weld shape and size with increasing a time step.;Deep penetration of melt is conducted by the use of high density laser heat energy; rapid evaporation, i.e., prompt phase change from solid to gas, is expected to generate a recoil pressure in the melt pool due to the enormous heat input source. The recoil pressure is considered as an important determinant to form a deep and narrow melt pool. Assuming that atmospheric and vaporized material pressures are balanced at the front of the laser beam, the evaporation of the melt leads to significant pressure work that drills down the melt to the opposite side of a base material when the material is heated over the boiling point.;Besides the recoil pressure, the surface tension of the molten material is also highly responsible for developing and widening the melt pool. The melt surface layer is often influenced by contractive forces of the molten material to minimize its surface free energy. Consequently, minimization of the energy has a substantial effect on the melt surface to stretch out its extent towards the not-yet-melted solid region. As a liquid droplet is often pulled into a spherical shape by the contractive force, the outer layer of the melt pool decreases the melt surface area for physical stability. As a result of minimizing the surface area and the surface free energy, the melt surface tends to have a flat layered shape. A contact angle at the liquid-solid interface therefore is assumed to be negligible in the laser welding study. Marangoni convection, the rate of heat loss from the melt to the ambient air, is induced by the temperature gradient of the melt surface layer; temperature dependent surface tension is conditionally applied on the layer when temperature rises over the melting point.;In the computation of melt dynamics, energy conservation and momentum equations are used to compute the effects of melt flow and the consequent thermo-fluid heat energy transfer. 2-dimensional governing equations from the Navier-Stokes, i.e., conservation of mass, linear momentum, and energy balance in fluid, are prepared to estimate how melt flow influences the rate of heat transfer and the distribution of temperature in a 2D domain. A mechanical analysis is followed by the thermo-fluid computation using a mechanism relevant to thermal expansion, and the stress distributions are investigated by the use of Von-Mises criterion.;The simulation results are compared with a set of experimental research which laser welds are made of low carbon steel. Assuming that melt flow has dominant influence on the formation of melt pool, a use of a different material and the properties is subject to yield similar results. The shape comparison of the welds describe that parameters relevant to any changes in the melt dynamics are of great importance on the formation of hourglass shaped melt pool during laser welding. |
