Laser-Like Surfacing Repair of Diesel Engine Shoulder Seal Surface

Literature Overview

This paper by Sun Xiaofeng and colleagues from the Academy of Armored Force Engineering, published in China Surface Engineering (Vol. 28, No. 1, 2015), addresses the repair of corroded shoulder seal surfaces on old diesel engines using a laser-like welding technique. The study employs HS121 nickel-based alloy wire as the repair material and characterizes the repair layer through optical microscopy, X-ray stress measurement (XPS), electrochemical workstation testing, high-frequency reciprocating friction wear testing, SEM, and laser 3D profilometry. The research is motivated by the practical need to extend the service life of diesel engines in military and civilian applications where replacement of entire engines is not economically feasible.

Core Technical Findings

The laser-like surfacing repair layer achieves metallurgical bonding with the base material without obvious welding defects. The residual stress on the repair layer surface is only 16% of that produced by conventional argon arc welding. The corrosion rate is 19.88% of the base material, and the wear resistance is 2.66 times that of the base material. The self-corrosion potential of the repair layer is more positive than the engine base material, and the self-corrosion current density is approximately one-half that of the base material.

Comparative Performance: Laser-Like Surfacing vs. Conventional Methods

Process Analysis: Laser-Like Welding Technology

Laser-like welding, also known as near-laser welding or quasi-laser welding, is a welding technique that uses a high-energy-density light source to achieve welding characteristics similar to laser welding but with lower equipment cost. The technique typically employs a high-power arc source with a focused arc nozzle to create a narrow, deep weld pool with a high aspect ratio, mimicking the keyhole effect of laser welding. The high energy density and narrow weld pool result in lower heat input, faster cooling rates, and reduced dilution compared to conventional arc welding processes.

Advantages for Surfacing Repair

For the repair of diesel engine shoulder seal surfaces, the laser-like welding technique offers several advantages:

1. Low heat input: The narrow weld pool and high energy density result in lower total heat input, which minimizes the heat-affected zone (HAZ) and reduces thermal distortion of the engine component. This is critical for precision engine components where dimensional accuracy is essential.
2. Low dilution: The keyhole-like weld pool allows the surfacing material to be deposited with minimal dilution from the base metal, ensuring that the repair layer retains the desired composition and properties of the HS121 nickel-based alloy.
3. Low residual stress: The rapid cooling and low heat input result in lower residual stresses in the repair layer, which reduces the risk of cracking and improves the fatigue life of the repair.
4. Metallurgical bonding: Despite the low heat input, the laser-like welding technique achieves complete metallurgical bonding between the repair layer and the base material, ensuring the integrity of the repair.

Microstructural and Property Analysis

Microstructure

The laser-like surfacing repair layer exhibits a fine, columnar microstructure typical of rapid solidification. The HS121 nickel-based alloy is a solid-solution-strengthened alloy with excellent corrosion resistance and wear resistance. The fine microstructure resulting from the laser-like welding process provides a high density of grain boundaries, which can act as barriers to crack propagation and dislocation motion, contributing to the improved mechanical properties.

Residual Stress

The residual stress on the repair layer surface is only 16% of that produced by conventional argon arc welding. This is attributed to the lower heat input and faster cooling rate of the laser-like welding process. The lower residual stress is beneficial for the fatigue performance of the repair, as high tensile residual stresses can initiate and propagate fatigue cracks.

Corrosion Resistance

The corrosion rate of the repair layer is 19.88% of the base material, indicating a significant improvement in corrosion resistance. The self-corrosion potential is more positive than the base material, and the self-corrosion current density is approximately one-half that of the base material. These electrochemical measurements confirm that the HS121 nickel-based alloy provides excellent corrosion protection for the engine component. The more positive self-corrosion potential indicates that the repair layer is thermodynamically more stable in the corrosive environment, while the lower self-corrosion current density indicates a slower corrosion rate.

Wear Resistance

The wear resistance of the repair layer is 2.66 times that of the base material, as measured by high-frequency reciprocating friction wear testing. The HS121 nickel-based alloy is known for its excellent wear resistance due to its high hardness and good toughness. The laser-like welding process preserves the alloy composition and produces a fine microstructure that further enhances the wear resistance.

Engineering Practice Implications

The repair of diesel engine shoulder seal surfaces is a common maintenance task in both military and civilian applications. The shoulder seal surface is subjected to high pressure, high temperature, and corrosive environments, which can lead to corrosion damage and seal failure. Conventional repair methods, such as argon arc welding, often result in high residual stresses, significant HAZ, and poor corrosion and wear resistance. The laser-like welding technique offers a superior alternative that addresses these limitations.

The practical implementation of laser-like welding for engine repair requires:

• Surface preparation: The corroded area must be thoroughly cleaned and prepared to ensure good metallurgical bonding. This may involve grinding, chemical cleaning, or blasting.
• Process parameter optimization: The laser-like welding parameters, including power, travel speed, and wire feed rate, must be optimized for the specific component geometry and material.
• Quality control: The repair layer must be inspected for defects, residual stress, and corrosion and wear performance before the component is returned to service.

Key Questions and Reflections

A question that arises is whether the laser-like welding technique can be extended to other engine components and repair applications. The shoulder seal surface is a specific application, but the same technique could potentially be used for repairing cylinder bores, valve seats, and other critical engine components. The key is to optimize the process parameters for each specific application.

Another consideration is the long-term performance of the repair under real-world conditions. The laboratory tests provide valuable data, but the repair must be validated under actual engine operating conditions, which include thermal cycling, vibration, and exposure to various fuels and lubricants.

Study Insights and Outlook

This research demonstrates the effectiveness of laser-like welding for the repair of diesel engine components. The combination of low residual stress, excellent corrosion resistance, and superior wear resistance makes this technique a valuable tool for extending the service life of diesel engines. The use of HS121 nickel-based alloy as the repair material is well-suited to the demanding service conditions of diesel engines. The study also highlights the importance of process selection in repair applications—choosing the right welding process can make the difference between a reliable repair and a premature failure. Future research should focus on expanding the application of laser-like welding to a wider range of engine components and developing standardized repair procedures and quality control protocols.