Arc and Droplet Transfer Behavior in Single-Pass Stacked Overlay Welding

Literature Overview

This paper by He Guanyu, Gu Yufen, Zhu Ming, and Shi Yu (2016), published in Hot Working Technology (Vol. 45, No. 19, pp. 176-178), investigates the arc morphology and droplet transfer behavior during single-pass stacked overlay welding using a MIG process with a flat-characteristic power source. The authors established a MIG single-pass stacked overlay welding experimental system and used high-speed camera imaging to capture the arc morphology and droplet transfer in both the first pass and subsequent stacked passes. This work is funded by the 973 Program, the Lanzhou University of Technology Red Willow Youth Distinguished Talent Program, and the State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals. The research is particularly relevant to engineers working on additive manufacturing, overlay welding for surface engineering, and automated welding process optimization.

Core Technical Findings

The key findings are:

1. During stacked overlay welding, the arc length is significantly shorter than during the first pass welding.
2. The arc shortening is caused by a change in arc morphology, which alters the arc characteristic curve, preventing the self-regulation phenomenon that normally maintains arc length stability.
3. The increase in stick-out (contact tip to workpiece distance) and the increased voltage drop across the stick-out contribute to a reduction in arc voltage.
4. The reduction in current during stacked welding leads to a decrease in total arc mechanical force, resulting in lower droplet transfer frequency and larger droplet size.

Arc and Droplet Transfer Parameters

Interpretation of Technical Mechanisms

Arc Length Shortening Mechanism

In conventional MIG welding with a flat-characteristic (constant current) power source, arc length self-regulation operates as follows: if the arc length increases, the arc resistance increases, the arc voltage rises, and the current decreases, which increases the droplet transfer frequency and causes the arc to shorten. Conversely, if the arc length decreases, the voltage drops, the current increases, and the droplet transfer frequency decreases, causing the arc to lengthen.

During stacked overlay welding, the previously deposited layer acts as a thermal mass that absorbs heat from the arc. This changes the arc morphology and the arc characteristic curve. The arc becomes more constricted, and the effective arc resistance decreases. With a flat-characteristic power source, the current does not increase as expected, and the self-regulation mechanism is disrupted. The result is a shorter, more intense arc that does not self-regulate in the conventional manner.

Droplet Transfer Difficulty

The reduction in current during stacked welding directly reduces the electromagnetic force (Lorentz force) acting on the molten droplet at the wire tip. The electromagnetic force is proportional to the square of the current, so even a modest reduction in current leads to a significant reduction in electromagnetic force. Additionally, the surface tension force holding the droplet at the wire tip remains relatively constant, creating a force imbalance that delays droplet detachment. The result is larger droplets that transfer at a lower frequency, which can lead to unstable weld bead geometry and increased spatter.

Stick-Out Effects

The stick-out (the distance from the contact tip to the workpiece surface) increases during stacked welding because the deposited layer raises the workpiece surface. The voltage drop across the stick-out is proportional to the stick-out length and the current, and this voltage drop subtracts from the arc voltage. With a flat-characteristic power source, the reduced arc voltage leads to a further reduction in current, compounding the problem.

Engineering Practice Integration

For engineers implementing single-pass stacked overlay welding processes, the following recommendations are based on the findings of this paper:

• Power source selection: A drooping (constant voltage) characteristic power source may be more suitable for stacked overlay welding, as it provides better arc length control when the arc characteristic changes.
• Stick-out management: The stick-out should be adjusted or compensated for as the overlay thickness increases. Automated stick-out control systems can maintain consistent arc characteristics throughout the build-up.
• Current compensation: The welding current should be increased during stacked passes to compensate for the reduced arc current and maintain adequate droplet transfer force.
• Wire feed speed adjustment: Increasing the wire feed speed during stacked passes can help maintain arc length stability and droplet transfer frequency.
• Process monitoring: High-speed imaging or arc voltage/current monitoring can be used to detect changes in arc behavior and trigger process adjustments.

Process Optimization Parameters

Key Questions and Reflections

The paper focuses on the physical mechanisms of arc and droplet behavior but does not address the resulting weld quality (porosity, fusion, dilution) in detail. In my engineering experience, the larger droplets and lower transfer frequency during stacked welding can lead to increased spatter and potential porosity if the shielding gas coverage is inadequate. The paper also does not discuss the effect of stacking multiple layers (beyond the first and second pass), which is common in thick overlay applications.

Another important consideration is the interaction between arc behavior and the thermal history of the previously deposited layer. The thermal conductivity and heat capacity of the deposited layer affect the arc temperature distribution and the cooling rate of the new weld, which in turn affects the microstructure and mechanical properties. This coupling between arc physics and metallurgy is not addressed in the paper but is critical for process optimization.

Study Insights and Implications

This paper provides valuable insight into the fundamental physics of arc and droplet behavior during stacked overlay welding, which is essential for process development and optimization. The understanding that arc self-regulation is disrupted during stacked welding has direct implications for power source selection and process parameter control. Engineers developing automated overlay welding systems should incorporate real-time arc monitoring and adaptive control to compensate for the changing arc characteristics as layers are deposited. The findings also highlight the importance of stick-out management, which is often overlooked in practical welding operations but can have a significant impact on weld quality and process stability.