Heat Transfer Model of MIG Stationary Welding During Spray Transfer

Literature Overview

The paper by Wu Chuansong, Wu Lin, and Chen Dinghua from Harbin Institute of Technology, published in the Transactions of the China Welding Institute (1991, Vol. 12, No. 3, pp. 189-194), presents a groundbreaking heat transfer model for MIG welding during spray transfer. This work represents a fundamental advancement in welding thermal modeling by incorporating the heat contribution of droplet transition into the weld pool, moving beyond the classical approach that treats MIG and TIG welding differently only through the arc power utilization coefficient. The research provides the theoretical foundation for numerical simulation of MIG weld penetration profiles, particularly the characteristic finger-shaped penetration observed in spray transfer welding.

Core Technical Contribution

The classical welding thermal theory treats the MIG welding process as fundamentally similar to TIG welding, with the primary difference being the selection of the arc power effective utilization coefficient (η). This paper introduces a new methodology that explicitly accounts for the heat carried by transferred droplets into the weld pool interior, represented as ΔH(r,z), in addition to the arc heat flux q(r) acting on the workpiece surface. Furthermore, the model incorporates the influence of fluid flow within the weld pool on the thermal process, providing a comprehensive coupled heat transfer and fluid dynamics framework.

Model Components

Physical Basis of the Model

The key physical insight of this work is that during spray transfer, each metal droplet carries significant thermal energy into the weld pool. This energy is deposited not at the surface but at the point where the droplet penetrates into the liquid pool, typically at a depth below the surface. The droplet carries both sensible heat (above the melting point) and latent heat of fusion, both of which contribute to the local thermal field. The position and magnitude of this internal heat source depend on the droplet velocity, size, and the fluid dynamics of the weld pool at the point of impact.

The finger-shaped penetration profile characteristic of spray transfer MIG welding is a direct consequence of this internal heat deposition. Unlike TIG welding where the heat is applied entirely at the surface and conducts downward, MIG spray transfer welding deposits heat both at the surface (from the arc) and below the surface (from the droplets), creating a deeper, narrower penetration channel that extends downward like a finger.

Engineering Practice Significance

Understanding the thermal field distribution within a MIG weld is essential for predicting weld geometry, residual stress patterns, and microstructural evolution. This model provides the theoretical framework for:

1. Weld penetration prediction: The finger-shaped penetration model allows engineers to predict the depth and width of penetration for given process parameters, enabling single-pass welding of thicker sections without undercuts or incomplete fusion.
2. Residual stress estimation: The thermal history derived from the model can be used to predict residual stress distributions, which is critical for distortion control in structural welding.
3. Microstructure prediction: The cooling rate at different locations within the weld can be calculated from the thermal model, enabling prediction of grain size, phase transformation, and mechanical properties.
4. Process optimization: The model provides a quantitative basis for optimizing process parameters to achieve desired weld geometry while minimizing defects.

Application to Welding Procedure Development

Study Insights and Reflections

This paper represents a fundamental contribution to welding thermal modeling that has influenced subsequent research in computational welding mechanics. The recognition that droplet heat contribution cannot be neglected in MIG welding thermal analysis was a paradigm shift that enabled more accurate numerical predictions of weld geometry and thermal history. The coupled heat transfer and fluid dynamics approach acknowledges that the weld pool is not a passive recipient of surface heat flux but an actively flowing body of molten metal that redistributes heat through convection.

The finger-shaped penetration prediction is particularly significant because it matches experimental observations that previous models could not reproduce. This validation establishes the credibility of the model and demonstrates that the droplet heat contribution is not merely a theoretical consideration but a physically significant phenomenon that must be included in any accurate MIG welding thermal model. In my experience with weld simulation work, the ability to predict the finger-shaped penetration profile is essential for designing single-pass welding procedures for thick-section structures.

The model also has implications for understanding weld defects. The concentrated thermal field at the root of the finger-shaped penetration creates steep thermal gradients that can promote solidification cracking, particularly in alloys with wide freezing ranges. Additionally, the fluid flow patterns predicted by the coupled model can explain the formation of lack of fusion defects at the sides of the penetration, where the thermal field may be insufficient to maintain a liquid state at the fusion boundary.

Reference Value and Outlook

This research established the theoretical foundation for modern computational welding analysis of MIG processes. The coupled heat transfer and fluid dynamics model developed in this paper has been extended and refined by subsequent researchers to include phase transformation, plastic deformation, and multi-pass welding analysis. The fundamental insight that droplet heat contribution must be explicitly modeled in MIG welding thermal analysis remains valid and is now incorporated into commercial welding simulation software.

For engineering practice, the model provides a quantitative basis for welding procedure optimization that goes beyond empirical trial-and-error approaches. By understanding the thermal field distribution within the weld, engineers can predict and control weld geometry, residual stresses, and microstructural evolution with greater confidence. The model also enables the development of advanced welding processes, such as hybrid laser-arc welding, where the interaction between multiple heat sources creates complex thermal fields that require sophisticated numerical analysis. Future developments in this area should focus on incorporating real-time process monitoring data into the thermal model, enabling adaptive control of welding parameters to maintain optimal thermal conditions throughout the weld length.