Towards hybrid laser-arc based directed energy deposition: Understanding bead formation through mathematical modeling for additive manufacturing

Abstract

This study highlights three phenomena in directed energy deposition using hybrid laser-arc: bead formation, change in the bead shape because of laser-arc interaction, and formation of defects. Unlike the arc-based deposition, the shape of the hybrid laser-arc bead differs from the pre-conceived parabolic or cosine form, which necessitates a new bead model. The laser-induced flow and preheating of wire electrode widen the molten pool width and cause a depression of the top surface of the deposited bead. Consequently, the fixed critical offset distance for multi-pass deposition (e.g., 0.66 or 0.73 times of the bead width, based on the flat or tangent overlapping model for arc-based deposition) does not yield the optimal flat surface for the hybrid laser-arc deposition. In this work, first, a parametric study assesses bead formation and defect phenomenon for high-strength steel as a candidate material, followed by an evaluation of the existing bead models for the appropriateness of the shape of defect-free hybrid laser-arc beads. Results indicate that a parabola composited with a unimodal beta distribution - that accounts for the laser influence (power and laser-arc distance) on the bead shape - is more responsive to the bead shape variation and is substantially more accurate than the existing models. Increasing laser power and laser-arc distance significantly changes the bead shape and reduces the critical offset distance. The traverse speed is the most critical factor followed by welding current leading to defect-free bead deposition. It is envisioned that the fundamental understanding developed as a result of the present investigation will provide a direction for hybrid laser-arc based additive manufacturing of several material classes, many of which are not weldable and therefore not additively manufactured by arc-based deposition.