Computational Analysis of Laser Impact Welding Processes Using Eulerian Formulation

Over the past 70 years, impact welding has been used to join metallic substrates with the advantage of not exposing the materials involved to significant amounts of heat. This allows the direct joining of dissimilar or heat-sensitive alloys in ways not considered practical using fusion welding. Most impact welding techniques create a weld between large parts by applying a very large impulse to a flyer part within a short period, and cause the surfaces to collide at a particular range of angles. A number of analytical and, more recently, computational approaches have been developed to investigate the phenomena that occur at rapidly forming collision interfaces, as direct, in situ observation of such processes are experimentally challenging. This challenge is only compounded when the parts to be joined are very small, such as in laser impact welding, which welds flyer foils of 0.1 mm thickness or less to various geometries of substrates. As a prerequisite to success, impact welding requires the ablation of thin layers of material from both parts’ contact surfaces in a high velocity, high temperature jet. This process removes oxides and other contaminants, while indenting the surface asperities to promote mutual contact at high pressure. Extreme shear stresses occur within both the flyer and its target, temperatures increase dramatically at the collision point, and extreme plastic strains result from the concentration of shock stresses. Larger scale impact weld techniques, such as those involving the use of explosives, magnetic discharge, or the vaporization of an electrically conductive foil, are energetic enough such that meso- or micro-scale characteristics such the metallic grains’ microstructure, or the contact surface roughness, may be neglected when performing numerical or analytical investigations. However, the foils generally used for flyers in laser impact welding are thin enough such that these effects can become significant. Presented is a comprehensive analysis of laser impact welding using computational methodology based on the Eulerian finite element formulation. Frameworks including the inhomogeneous microstructure and measured rough surfaces of metallic foils are developed to predict the evolution of transient phenomena throughout the formation of laser impact welded joints, using material definitions appropriate for welds between dissimilar, as well as similar, alloys. Correlations are found between the weld morphologies numerically predicted and found experimentally; additionally, a basis to use the Eulerian formulation in laser impact welding modeling is established to help promote further development and adoption of the technique.