Heat Transfer Model for MIG Spot Welding with Jet Metal Transfer

Literature Overview

This foundational paper by Wu Chuansong, Wu Lin, and Chen Dinghua from Harbin Institute of Technology, published in Welding Journal in 1991, presents a novel heat transfer model for MIG welding with jet metal transfer. The work represents a significant departure from classical welding heat transfer theory, which traditionally treats MIG and TIG welding processes as fundamentally similar, differing only in the selection of the effective arc power utilization coefficient η. The authors challenge this simplification by incorporating the heat carried by transferred droplets into the molten pool interior, providing a more physically realistic representation of the MIG welding thermal process.

Theoretical Framework and Model Development

The classical welding heat process theory, dating back to the Rosenthal solutions and their extensions, considers only the arc heat flux q(r) acting on the workpiece surface. The distinction between MIG and TIG welding is captured solely through the arc power utilization coefficient η, which accounts for the fraction of arc power transferred to the workpiece. This approach neglects the additional heat input from metal droplets transitioning from the electrode wire into the molten pool.

The proposed model introduces an additional heat source term ΔH(r,z) representing the heat carried by transferred droplets into the interior of the molten pool. This volumetric heat source is distinct from the surface heat flux and accounts for the kinetic energy and thermal energy of the droplets as they impact the pool surface and penetrate into the liquid metal. The model also incorporates the fluid flow state within the molten pool, recognizing that convective transport significantly influences heat distribution.

Key Technical Achievements

The most significant achievement of this model is the numerical simulation of the finger-like (or needle-like) penetration depth characteristic of MIG welding with jet metal transfer. This morphology has long been observed experimentally but could not be reproduced by classical models that predict conical penetration for all arc welding processes. The finger-like shape arises from the combination of the concentrated surface arc heat flux and the additional interior heat input from high-velocity droplets in jet transfer mode.

The jet transfer mode of MIG welding is characterized by droplet transfer velocities exceeding the arc voltage-induced electromagnetic force, resulting in rapid droplet flight across the arc gap. These high-velocity droplets carry significant kinetic energy that is converted to thermal energy upon impact, creating a localized heat concentration beneath the pool surface that drives deep, narrow penetration.

Engineering Significance

For welding engineers, this model provides a more accurate predictive tool for weld geometry, particularly for penetration depth and shape. The ability to predict finger-like penetration enables better control of weld quality in applications where deep penetration is required, such as thick plate welding, pipe welding, and structural steel fabrication. The model also provides insight into the mechanisms governing weld pool stability and the transition between different metal transfer modes.

The coupled heat transfer and fluid dynamics approach has implications for understanding weld pool convection patterns, which influence solidification microstructure, segregation, and residual stress distribution. Engineers can use the model to predict how changes in welding parameters—current, voltage, wire feed speed, and travel speed—affect the penetration profile and, consequently, the weld quality.

Study Insights and Outlook

Published in 1991, this paper anticipated the development of computational welding mechanics that would become increasingly important in subsequent decades. The fundamental insight that droplet heat input is a significant contributor to MIG weld penetration—distinct from and additional to arc heat flux—remains valid and has been incorporated into modern finite element welding models. The model's ability to predict the finger-like penetration shape represents a qualitative breakthrough that validates the physical assumptions and provides confidence in its quantitative predictions.

For contemporary practice, this model serves as a theoretical foundation for the numerical simulation tools now used in welding process development and optimization. The principles established here—coupled heat transfer and fluid dynamics, volumetric heat source representation of droplet energy—have been extended and refined in subsequent research. Engineers developing new welding processes or optimizing existing ones should appreciate the physical basis provided by this work, as it explains why MIG welding achieves deeper penetration than TIG welding at comparable power levels and why the penetration morphology differs fundamentally between these two processes.