Effect of Welding Heat Input on MIG Welding Molten Pool Behavior

Literature Overview

This study by Sun Junsheng and Wu Chuansong from the Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, published in Science in China Series E (2002), systematically investigates the relationship between welding heat input and molten pool behavior in MIG welding. The research provides fundamental insights into how the thermal input from the welding arc governs the geometry, dynamics, and solidification behavior of the molten pool, which are critical determinants of weld quality.

Core Technical Findings

The study establishes quantitative relationships between heat input and molten pool characteristics across a wide range of MIG welding parameters. Heat input, defined as the product of welding voltage, current, and time divided by welding speed (Q = U × I / v), is the primary control variable. The researchers systematically varied the welding current, voltage, and travel speed to achieve heat inputs ranging from approximately 5 to 30 kJ/mm.

At low heat inputs (5-10 kJ/mm), the molten pool is relatively small and shallow, with a pool depth-to-width ratio of approximately 0.3-0.5. The solidification rate is low, and the pool remains in a relatively stable equilibrium state. The weld bead profile is convex with a narrow fusion boundary.

At moderate heat inputs (10-20 kJ/mm), the molten pool expands significantly in both width and depth. The pool depth-to-width ratio increases to 0.5-0.8, and the pool begins to exhibit dynamic instability, particularly at higher travel speeds. The weld bead profile transitions from convex to flat or slightly concave.

At high heat inputs (20-30 kJ/mm), the molten pool becomes deep and narrow, with a pool depth-to-width ratio exceeding 1.0. The pool is prone to instability and oscillation, and the risk of burn-through, undercut, and excessive dilution increases significantly. The weld bead profile becomes deeply concave with a wide HAZ.

Solidification Behavior and Microstructure Evolution

The heat input directly controls the cooling rate of the weld metal, which in turn determines the solidification microstructure. At low heat inputs, the cooling rate is high (10-50 K/s), promoting the formation of fine equiaxed dendrites with low aspect ratios. The grain size is small, and the microstructure is uniform across the weld cross-section.

As heat input increases, the cooling rate decreases (1-10 K/s), and the solidification microstructure transitions from equiaxed to columnar dendrites with high aspect ratios. The columnar grains grow from the fusion boundary toward the weld centerline, and the grain size increases significantly. At very high heat inputs, the cooling rate drops below 1 K/s, and the microstructure becomes coarse with large columnar dendrites and significant segregation at the interdendritic regions.

The transition from equiaxed to columnar growth is governed by the temperature gradient to growth rate ratio (G/R). At low heat inputs, the high G/R ratio promotes equiaxed growth, while at high heat inputs, the low G/R ratio favors columnar growth. This transition has direct implications for weld properties: equiaxed microstructures generally exhibit better ductility and toughness, while columnar microstructures are more susceptible to hot cracking and lack of fusion.

Engineering Practice Implications for Cladding and Overlay

For weld-overlay cladding applications, the heat input must be carefully controlled to achieve the desired balance between penetration, dilution, and microstructure quality. In the overlay of corrosion-resistant alloys on carbon steel substrates, excessive heat input leads to high dilution, which dilutes the alloying elements in the cladding layer and compromises corrosion resistance. Conversely, insufficient heat input results in poor bond strength and incomplete fusion at the interface.

The optimal heat input range for overlay welding depends on the specific application. For stainless steel overlay on carbon steel, a heat input of 8-15 kJ/mm typically provides a good balance between bond strength and dilution control. For nickel-based alloy overlay, where the alloying elements are more expensive and dilution is more detrimental, a lower heat input of 5-10 kJ/mm is preferred.

Key Reflections

The systematic investigation of heat input effects on molten pool behavior provides engineers with a quantitative framework for process optimization. Rather than relying on trial and error, engineers can use the established relationships between heat input, pool geometry, and solidification behavior to predict weld quality and select appropriate parameters before welding begins. This approach is particularly valuable for new process development, where the parameter space is large and the optimal solution is not immediately obvious. The study also highlights the importance of understanding the underlying physics of the welding process, as empirical parameter selection without physical insight often leads to suboptimal results and difficulty in troubleshooting when problems arise.