Motorcycle Electric Starter Disc Gear Pulsed TIG Underwater Welding Process

Literature Overview

The paper by He Wenkun from the Ziyang Steel Pipe Plant of Sichuan Petroleum Administration Bureau, published in Welding Technology in 1998, addresses a specific manufacturing challenge: the welding of disc gear assemblies used in motorcycle electric starter mechanisms. The component consists of a disc gear and a sleeve, both made of 20# low carbon steel, with a carbonitriding and quenching heat treatment applied before welding. The unique structural constraints require the outer diameter of the sleeve to be machined before welding, with heat treatment preceding the welding operation, creating significant welding difficulties.

Technical Challenge Analysis

The fundamental difficulty in this application stems from the combination of several factors. First, the pre-hardened material (after carbonitriding and quenching) creates a hardened base metal that is susceptible to cracking during welding due to hydrogen-induced cracking and thermal stress. Second, the geometric constraints of the assembly—where the outer diameter must be machined before welding—limit the welding access and make post-weld machining of the complete assembly impossible. Third, the small size and thin section thickness of the components require precise heat input control to avoid distortion and maintain dimensional tolerances.

The application of pulsed TIG welding addresses these challenges effectively. The pulsed mode allows precise control of the average heat input while maintaining peak currents high enough for good penetration. The duty cycle of the pulse can be adjusted to control the heat affected zone width, which is critical for minimizing distortion in thin-walled components.

Process Development Approach

The underwater welding aspect of this process is particularly noteworthy. Welding in an aqueous environment provides several advantages for this application. The water acts as a heat sink, significantly reducing the heat affected zone and minimizing distortion. The cooling effect also reduces the risk of hydrogen-induced cracking by lowering the temperature of the base metal during welding. Additionally, the water environment helps contain any potential spatter and provides a stable arc atmosphere.

The selection of pulsed TIG welding for this application reflects careful consideration of the competing requirements. The pulse parameters—peak current, base current, pulse frequency, and duty cycle—must be carefully optimized to achieve full penetration while minimizing the heat input. The pre-weld preparation, including the removal of the hardened layer at the weld joint, is essential to prevent cracking in the heat affected zone.

Engineering Practice Considerations

In practice, the success of this welding process depends on several critical factors. The pre-weld cleaning and preparation of the joint surfaces is essential, particularly the removal of any residual hardened layer that could act as a crack initiation site. The fit-up tolerances must be tight to ensure consistent weld geometry and avoid excessive root gap that could lead to burn-through. The welding sequence must be planned to minimize residual stress and distortion, potentially using multiple short welds rather than continuous welding.

Post-weld stress relief treatment may be required to reduce residual stresses and prevent delayed cracking. The dimensional accuracy of the machined outer diameter must be verified after welding to ensure that distortion has not exceeded acceptable limits.

Study Reflections and Implications

This paper demonstrates the creative application of welding technology to solve specific manufacturing challenges in the motorcycle industry. The combination of pulsed TIG welding with underwater welding represents an innovative approach to managing the competing requirements of penetration, distortion control, and crack avoidance in pre-hardened thin-walled components. For engineers working on similar applications involving pre-hardened steel components with dimensional constraints, this paper provides valuable insights into process selection and parameter optimization strategies.

The work highlights the importance of integrating welding technology with the overall manufacturing sequence. Understanding how welding interacts with preceding operations such as heat treatment and machining is essential for developing successful production processes.