Arc and Droplet Transfer Behavior in Single-Pass Layered Surfacing

Literature Overview

The research by He Guanyu, Gu Yufen, Zhu Ming, and Shi Yu, published in Hot Working Technology in 2016, investigates the arc morphology and droplet transfer behavior during single-pass layered surfacing using the MIG (Metal Inert Gas) welding process. This study is particularly relevant to the field of additive manufacturing and rapid prototyping, where layered deposition of weld metal is used to build up components layer by layer. The authors established a MIG single-pass layered surfacing experimental system and used high-speed camera imaging to capture the arc morphology and droplet transfer behavior during both the first layer and subsequent layered welding.

Experimental Methodology

The experimental system consisted of a flat-characteristic welding power source, a MIG welding torch with controlled wire feed speed and travel speed, and a high-speed camera for recording the arc and droplet transfer events. The wire used was a standard stainless steel MIG wire, and the shielding gas was argon. The base plate was a carbon steel plate.

The key experimental parameters included:

Parameter	First Layer	Layered Welding
Welding current	Higher	Lower
Arc voltage	Higher	Lower
Travel speed	Standard	Standard
Wire feed speed	Higher	Lower
Shielding gas	Argon	Argon

The high-speed camera captured the arc morphology and droplet transfer events at a frame rate sufficient to resolve individual droplet detachment and transfer events. The images were analyzed to determine arc length, arc shape, droplet size, and droplet transfer frequency.

Arc Morphology and Arc Length Behavior

One of the most significant findings of this study was that the arc length during layered welding was noticeably shorter than during the first layer welding. This is counterintuitive, as one might expect the arc length to remain constant or increase due to the additional material being deposited.

The authors explained this phenomenon by analyzing the arc characteristic curve. During the first layer welding, the arc operates on the flat characteristic curve of the power source, and the arc length is determined by the balance between the arc voltage and the wire feed rate. However, during layered welding, the arc morphology changes due to the presence of the previously deposited weld bead, which alters the arc shape and the current density distribution.

The change in arc morphology causes a shift in the arc characteristic curve, which means that the arc self-regulation mechanism — the automatic adjustment of arc length to maintain stable welding — does not function as effectively during layered welding. This is because the flat characteristic power source is designed to self-regulate the arc length based on the assumption of a constant arc characteristic, but the changed arc morphology during layered welding violates this assumption.

As a result of the arc shortening, the dry length (the portion of the electrode wire that is not yet melted and is exposed to the arc) increases. The increased dry length has a larger voltage drop across it, which reduces the arc voltage. The combined effect of arc shortening and increased dry length voltage drop leads to a reduction in the overall arc voltage during layered welding.

Droplet Transfer Behavior

The droplet transfer behavior during layered welding was found to be more difficult compared to the first layer welding. This is attributed to the reduction in welding current during layered welding, which leads to a decrease in the total arc mechanical force (the electromagnetic and plasma forces that drive droplet detachment and transfer).

The arc mechanical force is composed of several components:

Electromagnetic force: Generated by the interaction between the current in the electrode and the magnetic field around the arc. This force acts to compress the arc and push droplets toward the weld pool.
Plasma force: Generated by the momentum of the ionized gas flowing from the arc to the weld pool. This force also contributes to droplet transfer.
Surface tension: Acts to hold the droplet on the electrode tip and resists detachment.

During layered welding, the reduced current leads to a decrease in both the electromagnetic force and the plasma force. This means that the total force available to drive droplet detachment is reduced, resulting in:

Lower droplet transfer frequency
Larger droplet size at detachment
Less stable droplet transfer

The larger droplet size is a consequence of the reduced detachment force — the droplet must grow larger before the force balance allows it to detach from the electrode tip. The lower transfer frequency means that fewer droplets are transferred per unit time, which affects the deposition rate and the weld bead geometry.

Process Stability and Deposition Quality

The changes in arc morphology and droplet transfer behavior during layered welding have significant implications for process stability and deposition quality. The shorter arc length and reduced arc voltage can lead to:

Reduced penetration depth
Changed weld bead geometry
Potential for increased spatter
Reduced deposition rate

The larger droplet size and lower transfer frequency can lead to:

Increased spatter due to incomplete droplet transfer
Uneven deposition with potential for porosity
Changed weld bead profile with wider and flatter beads
Reduced process stability and consistency

To mitigate these issues, the authors suggested that the welding parameters should be adjusted for layered welding to account for the changed arc and droplet transfer behavior. Specifically, the welding current should be increased to compensate for the reduced arc mechanical force, and the wire feed speed should be adjusted to maintain a stable arc length.

Engineering Practice Implications

In the context of additive manufacturing and rapid prototyping, understanding the arc and droplet transfer behavior during layered welding is critical for achieving consistent and high-quality deposits. The findings of this study provide valuable insights for process development and parameter optimization.

For industrial applications of layered surfacing, such as:

Building up thick surfacing layers on large components
Repair of worn or damaged surfaces
Manufacturing of complex-shaped components by layered deposition

The following process recommendations can be derived from this study:

Parameter adjustment: The welding current and voltage should be adjusted for each layer to account for the changed arc behavior. This may require the use of adaptive control systems that monitor the arc voltage and current and adjust the parameters in real time.
Wire feed speed control: The wire feed speed should be carefully controlled to maintain a stable arc length and consistent droplet transfer. Automatic wire feed speed control systems can help maintain the desired arc length despite the changing arc characteristics.
Shielding gas optimization: The shielding gas composition and flow rate should be optimized to provide adequate arc stability and protection during layered welding. Argon is generally preferred for MIG welding of stainless steel, but the flow rate may need to be adjusted for layered welding to account for the changed arc geometry.
Interlayer cleaning: The surface of each deposited layer should be cleaned before the next layer is deposited to ensure good metallurgical bonding and prevent contamination.
Thermal management: The heat input during layered welding should be carefully controlled to avoid excessive dilution and to maintain the desired microstructure in the deposited layers.

Study Insights and Reflections

This study provides a fundamental understanding of the physical mechanisms that govern arc and droplet transfer behavior during layered welding. The explanation of why the arc shortens during layered welding — due to the change in arc characteristic curve and the failure of the arc self-regulation mechanism — is a significant contribution to the understanding of welding physics.

The finding that droplet transfer becomes more difficult during layered welding due to the reduced arc mechanical force is particularly important for process development. It highlights the need for careful parameter optimization and possibly the use of advanced power sources with dynamic characteristics that can adapt to the changing arc conditions during layered welding.

The use of high-speed camera imaging to capture the arc and droplet transfer events is a powerful experimental technique that provides direct visualization of the welding process. This technique can be extended to other welding processes and applications to provide similar insights into the physical mechanisms governing the welding process.

Summary

The study provides a detailed analysis of the arc morphology and droplet transfer behavior during single-pass layered MIG surfacing. The key findings are that the arc shortens during layered welding due to changes in the arc characteristic curve, and that droplet transfer becomes more difficult due to the reduced arc mechanical force. These findings have important implications for process development and parameter optimization in layered surfacing applications, including additive manufacturing and rapid prototyping. The study underscores the importance of understanding the fundamental physical mechanisms of the welding process for achieving consistent and high-quality deposits in layered surfacing applications.