Surface Roughening Enhancement of Babbitt Alloy MIG Surfacing Bonding on Q235 Steel

Literature Overview

This paper by Que Mingxin et al. (2021), published in Surface Technology (Vol. 50, No. 10, pp. 239–245), investigates the effect of surface roughening treatments on the interface microstructure and bonding strength of SnSb11Cu6 Babbitt alloy deposited via MIG welding onto Q235 carbon steel substrates. The research addresses a longstanding challenge in overlay engineering: achieving reliable metallurgical and mechanical bonding between dissimilar materials with vastly different thermal expansion coefficients and melting points. Babbitt alloys, widely used in bearing applications due to their excellent anti-friction properties, notoriously exhibit poor wettability on ferrous substrates, leading to delamination under operational loads. The authors propose surface roughening as a practical solution and systematically evaluate its effectiveness.

Core Technical Findings

Surface Roughening Parameters and Design

The study designed two roughening geometries—straight grooves and grid grooves—with groove depths of 0.15 mm and 0.3 mm, and groove spacings of 1 mm and 1.5 mm. The parameter matrix was selected based on preliminary experiments to balance mechanical interlocking capacity against excessive surface area that might impede molten alloy flow. The roughening was achieved through mechanical machining prior to surfacing, a method readily applicable in industrial settings without requiring specialized equipment.

Bonding Strength Results

The most significant quantitative result is the improvement in interfacial bonding strength from 39.67 MPa (unroughened control) to 50.14 MPa (roughened samples), representing a 26.4% increase. This improvement is substantial for Babbitt alloy applications, where delamination typically initiates at interfacial stress concentrations under cyclic loading. The authors attribute this enhancement to three synergistic mechanisms: increased contact area, mechanical interlocking within grooves, and promotion of interfacial reactions at groove edges.

Interface Microstructure Analysis

XRD analysis confirmed the presence of the expected Babbitt alloy phases (β-Sn solid solution with Sb and Cu) in the overlay. SEM and EDS mapping revealed that at roughened interfaces, partial molten Babbitt alloy infiltrated into the groove cavities, creating a mechanical interlock that resists shear and tensile separation. At the groove edges, localized melting of the substrate steel occurred, facilitating interfacial reactions that produced thin intermetallic compounds. These intermetallics, while contributing to bonding, warrant monitoring as excessive growth could embrittle the interface.

Process Analysis and Engineering Implications

Wettability vs. Bonding Strength Trade-off

A critical finding is that roughening actually decreases the wetting angle (worsens wettability) of the Babbitt alloy on the substrate. This counterintuitive result is explained by the pinning effect: the three-phase line (solid-liquid-gas) is pinned at groove edges, preventing the molten alloy from spreading uniformly. Despite this reduced wettability, the net bonding strength increases because the mechanical interlocking and interfacial reaction effects outweigh the wettability penalty. This insight is vital for process optimization—engineers should not blindly pursue improved wettability but should consider the combined effect of all bonding mechanisms.

Grid vs. Straight Groove Comparison

Grid grooves provide larger contact area than straight grooves, which theoretically should yield higher bonding strength. However, the experimental results show that bonding strength differences among roughened samples are relatively small. The authors explain that grid grooves create more pinning points for the three-phase line, further degrading wettability to a degree that offsets the contact area advantage. This finding suggests that straight grooves with optimized depth and spacing may be more practical for industrial implementation, as they offer comparable bonding with simpler machining.

Practical Process Recommendations

For industrial application, the following process window is suggested based on the literature findings:

• Groove depth: 0.3 mm provides slightly better results than 0.15 mm due to deeper mechanical interlock
• Groove spacing: 1 mm is preferred over 1.5 mm to maximize contact area without excessive pinning
• Preheating: Moderate preheating of the substrate (100–150°C) can improve wettability without causing substrate deformation
• MIG parameters: Low travel speed and moderate current to ensure adequate heat input for groove edge melting without excessive dilution

Defect Analysis and Countermeasures

Common Failure Modes

FMEA Perspective

From a Failure Mode and Effects Analysis standpoint, the roughening process introduces a new potential failure mode: groove edge cracking during cooling due to differential thermal contraction between the Babbitt alloy and the steel substrate. The severity is moderate (potential for service failure under cyclic loading), the occurrence is low-to-moderate (depends on groove geometry and welding parameters), and detection difficulty is moderate (requires cross-sectional examination). The recommended preventive action is to use rounded groove bottom profiles rather than sharp V-grooves to reduce stress concentration factors.

Study Insights and Independent Reflection

This research demonstrates a pragmatic approach to solving the Babbitt alloy bonding problem—one that leverages mechanical surface preparation rather than complex metallurgical modifications. The finding that bonding strength can be improved by 26% through simple machining operations is highly valuable for maintenance engineers who need to repair bearing surfaces in the field. However, several questions remain unanswered: the long-term fatigue performance of roughened interfaces under cyclic bearing loads, the effect of roughening on corrosion resistance at the interface, and the scalability of the process to large-diameter bearing shells. Future work should also investigate the combined effect of surface roughening and intermediate bonding layers (such as nickel or cobalt-based coatings) to determine whether synergistic improvement is achievable. The methodology of systematically varying surface geometry parameters and correlating with bonding performance provides a template for similar investigations in other dissimilar metal overlay systems, including hardfacing of low-alloy steels onto austenitic substrates.