Discharge Mechanism Analysis of Electric Spark Welding Under Different Electrode Motion Forms

Literature Overview

This paper by Chen Junchao, Han Hongbiao, Wang Zhonghao, and Wang Shun, published in Surface Technology (2021, Vol. 50, No. 6, pp. 281-287), investigates the discharge mechanisms of electric spark welding (ESW) under different electrode motion forms. The work was conducted at Henan University of Science and Technology and supported by the National Natural Science Foundation of China (51375146) and the Henan Provincial Key Research Project (17A460012). Electric spark welding, also known as electric spark surfacing or spark deposition, is a rapidly emerging additive manufacturing technique that offers high deposition efficiency and low heat input, making it attractive for surface engineering applications in the piping and equipment industry.

Electrode Motion Forms and Contact States

The study compares two electrode motion forms: rotating electrode and ultrasonic vibration electrode. The fundamental difference lies in the relative motion between the electrode and the substrate, which directly affects the contact state at the discharge point.

The rotating electrode moves tangentially relative to the substrate, creating a consistent contact geometry at the discharge point. The contact gap remains relatively stable, and the contact resistance is low due to the continuous sliding contact. In contrast, the ultrasonic vibration electrode oscillates vertically at ultrasonic frequencies, causing rapid and large variations in the contact gap and contact area. This results in higher and more variable contact resistance at the discharge point.

Discharge Type Classification

The discharge types identified in the study reveal important differences between the two electrode motion forms. The rotating electrode exhibits three discharge types: short-circuit discharge, gap discharge, and one type of mixed discharge. The ultrasonic vibration electrode, on the other hand, exhibits nine discharge types: short-circuit discharge, gap discharge, and six types of mixed discharge.

This difference in discharge type diversity has significant implications for the welding process. The rotating electrode's simpler discharge behavior results in more consistent arc characteristics and more predictable heat input. The ultrasonic vibration electrode's complex discharge behavior introduces greater variability in the welding process, which can be either advantageous or detrimental depending on the application requirements.

The short-circuit discharge occurs when the electrode and substrate are in direct contact, resulting in high current flow through the contact point. The gap discharge occurs when a small gap exists between the electrode and substrate, with current flowing through the ionized gas in the gap. Mixed discharge types involve transitions between short-circuit and gap conditions during a single discharge cycle.

Deposition Efficiency and Transfer Coefficient

The quantitative results of the discharge parameter analysis are presented in the following table:

The transfer coefficient, which represents the ratio of electrode material transferred to the substrate to the total electrode material consumed, is significantly higher for the ultrasonic vibration electrode (92%) compared to the rotating electrode (79%). This indicates that the ultrasonic vibration electrode achieves more efficient material transfer, with less material loss to the atmosphere or spatter.

The lower currents for the ultrasonic vibration electrode are a direct consequence of the higher contact resistance and larger contact gap variations. The rapid opening and closing of the contact gap during ultrasonic vibration limits the duration and magnitude of the discharge current, resulting in smaller average currents but higher transfer efficiency.

Engineering Practice Implications

For engineering applications in the piping and equipment industry, the choice between rotating and ultrasonic vibration electrodes depends on the specific requirements of the repair or surfacing operation. The rotating electrode is suitable for applications requiring high deposition rates and consistent heat input, such as large-area surface hardening of wear parts. The ultrasonic vibration electrode is more appropriate for applications requiring high material utilization efficiency and precise control of the deposited layer thickness, such as repair of precision components or application of thin overlay layers.

The higher transfer coefficient of the ultrasonic vibration electrode (92% vs. 79%) is particularly important for applications involving expensive alloy materials, such as nickel-based alloys or cobalt-based alloys used in corrosion-resistant and high-temperature applications. The 13 percentage point improvement in material utilization can result in significant cost savings for large-scale operations.

However, the complex discharge behavior of the ultrasonic vibration electrode requires more sophisticated process control. The variability in discharge types and current waveforms may lead to inconsistent deposited layer properties if not properly controlled. Process monitoring systems capable of real-time detection and adjustment of discharge parameters would be beneficial for maintaining consistent quality.

Study Insights and Reflections

This paper provides valuable insights into the fundamental mechanisms governing electric spark welding, which is a relatively new technology in the additive manufacturing landscape. The systematic comparison of discharge types, current waveforms, and transfer coefficients under different electrode motion forms offers a solid theoretical foundation for process optimization and equipment development.

One of the most important findings is the direct relationship between electrode motion form and discharge behavior. The tangential rotation of the electrode creates a stable contact geometry with low resistance and small gap variations, leading to higher currents and simpler discharge types. The vertical ultrasonic vibration creates a dynamic contact geometry with high resistance and large gap variations, leading to lower currents but more diverse discharge types and higher transfer efficiency.

The research also highlights the importance of understanding the contact mechanics at the electrode-substrate interface. The contact state, including contact gap, contact area, and contact resistance, is the primary determinant of discharge behavior. Any future development of electric spark welding technology should focus on precise control of these contact parameters to achieve desired deposition characteristics.

For the piping and equipment industry, electric spark welding offers a promising alternative to conventional arc welding for surface repair and hardfacing applications. The low heat input reduces the risk of distortion and residual stress, which is particularly important for thick-walled components and large-diameter pipes. The high deposition efficiency makes it suitable for rapid repair of worn or damaged surfaces in the field.

In conclusion, this paper establishes a clear correlation between electrode motion form and discharge mechanism in electric spark welding, providing a theoretical basis for selecting the appropriate electrode motion form for specific applications. The findings on transfer coefficient and discharge type diversity will guide future process development and equipment design for industrial applications.