Control of Arc Starting Quality in PC-MIG Automatic Welding

Literature Overview

This paper by Wang Zhengcheng, Zhu Yurong, and Qian Juying from Northwestern Polytechnical University, published in Welding (1989, No. 3, pp. 1-5), investigates the control of arc starting quality in pulse current MIG (PC-MIG) automatic welding. The study examines the effects of slow wire feed arc starting parameters, pulse energy parameters, and local resistance values on the wire extension length on arc ignition characteristics and weld bead formation quality in the arc starting section. The research provides guidance for production applications of PC-MIG welding.

Core Technical Findings

The study systematically investigated the following parameters affecting arc starting quality in PC-MIG welding:

The study found that these parameters significantly influence the arc ignition characteristics and the weld bead formation quality in the arc starting section, with direct implications for production quality.

Interpretation of Technical Points

The arc starting process in MIG welding is a critical phase that determines the initial weld pool formation and the quality of the weld bead at the start of the weld. In PC-MIG welding, the pulsed current regime introduces additional complexity to the arc starting process because the current alternates between a low background current and a high pulse current. The transition from the arc starting phase to the steady-state pulsed welding regime must be carefully controlled to ensure consistent weld quality.

The slow wire feed arc starting technique involves feeding the wire at a reduced speed during the initial arc ignition phase. This technique allows the wire to preheat and form a molten tip before the full welding current is applied, facilitating arc ignition and reducing the risk of wire sticking or arc instability. The slow wire feed speed must be carefully controlled to balance the wire preheating effect against the risk of excessive wire extension and arc length variation.

The pulse energy parameters—pulse current magnitude and pulse duration—are critical for maintaining arc stability during the transition from the arc starting phase to the steady-state welding regime. The pulse current provides the electromagnetic force necessary to detach droplets from the wire tip, while the pulse duration determines the amount of heat input per pulse. During arc starting, the pulse parameters may need to be adjusted to compensate for the varying arc length and wire extension that occur during the ignition phase.

The local resistance on the wire extension length is a subtle but important parameter that affects the arc resistance and ignition characteristics. As the wire is fed through the contact tip, the wire extension length (the distance from the contact tip to the arc) varies due to the wire melting rate and the feed speed. This variation in wire extension length creates a local resistance that affects the total circuit resistance and the arc voltage. The study's investigation of this parameter highlights the importance of wire geometry control in achieving consistent arc starting quality.

Process and Standards Analysis

The control of arc starting quality is particularly important in automatic welding applications where the weld is produced without manual intervention. In accordance with welding procedure qualification standards such as AWS D1.1 (Structural welding code for steel) and ISO 15614 (Procedure qualification for fusion welding), the welding procedure must demonstrate the ability to produce consistent weld quality throughout the entire weld length, including the arc starting and ending sections.

The arc starting section is often a critical area for defect formation because the weld pool is not yet fully established, and the thermal input is lower than in the steady-state welding regime. Common defects in the arc starting section include incomplete penetration, lack of fusion, and excessive reinforcement. The parameters investigated in this study—slow wire feed speed, pulse energy parameters, and wire extension resistance—are directly relevant to preventing these defects.

The PC-MIG process, with its pulsed current regime, offers advantages over conventional constant-current MIG welding in terms of heat input control and spatter reduction. However, the pulsed current regime also introduces additional control challenges during arc starting because the current waveform must be precisely controlled to maintain arc stability. The study's investigation of pulse energy parameters provides guidance for optimizing the current waveform during the arc starting phase.

Integration with Engineering Practice

In automatic welding production environments, the arc starting quality directly affects the overall weld quality and the acceptance rate of the finished product. Defects in the arc starting section may require rework or rejection, which increases production costs and reduces manufacturing efficiency. The parameters investigated in this study provide practical guidance for optimizing the arc starting process in PC-MIG automatic welding.

The slow wire feed arc starting technique can be implemented in welding control systems by programming the wire feed motor to operate at a reduced speed during the initial arc ignition phase. The transition from the slow wire feed to the full welding speed must be synchronized with the arc ignition and the establishment of the weld pool. This synchronization can be achieved through current and voltage monitoring, which detects the arc ignition event and triggers the transition to full welding parameters.

The pulse energy parameters can be optimized through welding procedure qualification trials that systematically vary the pulse current magnitude and duration while monitoring the arc starting quality and weld bead formation. The optimal pulse parameters for arc starting may differ from those used in the steady-state welding regime, and the welding control system must be capable of switching between the two parameter sets.

The wire extension resistance can be controlled by maintaining a consistent wire extension length through precise control of the wire feed speed and the contact tip position. Any variation in wire extension length due to wire melting rate fluctuations or feed speed variations must be compensated for by adjusting the arc voltage or the pulse parameters to maintain arc stability.

Key Questions and Reflections

A critical question is the robustness of the arc starting control strategy under varying production conditions. In a production environment, factors such as wire diameter variations, contact tip wear, and shielding gas flow rate fluctuations can affect the arc starting quality. The control strategy must be robust enough to compensate for these variations and maintain consistent arc starting quality throughout the production run.

Another consideration is the integration of the arc starting control parameters with the overall welding control system. The welding control system must be capable of monitoring the arc starting event, switching between the arc starting parameters and the steady-state welding parameters, and detecting any abnormalities in the arc starting process. The system must also be capable of logging the arc starting parameters for quality traceability and process improvement.

The study also raises the question of whether the arc starting control parameters can be optimized for different welding positions and joint configurations. The arc starting quality may be affected by the welding position (flat, horizontal, vertical, overhead) and the joint configuration (butt, fillet, lap, tee), and the control parameters may need to be adjusted accordingly.

Study Insights and Implications

This paper provides valuable insights into the control of arc starting quality in PC-MIG automatic welding, a critical aspect of welding process control that directly affects weld quality and production efficiency. The systematic investigation of slow wire feed speed, pulse energy parameters, and wire extension resistance provides practical guidance for optimizing the arc starting process in production environments. The study demonstrates the importance of understanding the fundamental physics of arc starting—wire preheating, arc resistance, and current waveform dynamics—in developing effective control strategies. For engineers working in automatic welding production, this paper highlights the need for careful attention to the arc starting phase, which is often overlooked in favor of optimizing the steady-state welding parameters. The findings also underscore the importance of integrating arc starting control into the overall welding control system, ensuring that the arc starting quality is consistently maintained throughout the production run. The practical engineering approach demonstrated in this paper—systematically investigating the effects of individual parameters on arc starting quality—is a model for the development of robust welding process control strategies.