CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Surface Roughening Effects on Babbitt Alloy MIG Cladding Interface
Literature Overview
The research by Que Mingxin, Bai Xingwang, Zhou Xiangman, and Zhang Haiou from University of South China, Three Gorges University, and Huazhong University of Science and Technology (2021), funded by the National Natural Science Foundation of China (51975270, 51705287, 51505210) and Hunan Provincial Natural Science Foundation (2019JJ40245), investigates how surface roughening of the substrate affects the interface microstructure and bond strength of Babbitt alloy deposited via MIG (MIG/MAG) cladding. Published in Surface Technology, this work addresses a practical engineering problem: how to improve the metallurgical bond between Babbitt alloy (a soft tin-based or lead-based bearing alloy) and steel substrates when using arc welding processes.
Core Technical Points
Babbitt Alloy Cladding Challenges
Babbitt alloys (typically Sn-based: Sn-Sb-Cu, or Pb-based: Pb-Sn-Sb) are widely used for bearing surfaces due to their excellent tribological properties — low friction coefficient, good embeddability, and conformability. However, their low melting point (230-260 °C for Sn-based, 280-310 °C for Pb-based) presents a fundamental challenge for arc welding cladding on steel substrates. The large thermal conductivity mismatch between steel and Babbitt alloy, combined with the risk of excessive substrate heating, makes achieving a sound metallurgical bond difficult.
Surface Roughening Mechanisms
The researchers investigated several surface roughening methods:
| Roughening Method | Surface Ra | Process Parameters | Effect on Bond |
|---|---|---|---|
| Shot peening | 25-50 μm | Al₂O₃ shot, 0.8-1.2 mm, 2-3 m/s | Moderate improvement |
| Sandblasting | 20-40 μm | SiC grit, 60-80 mesh | Moderate improvement |
| Grooving (mechanical) | 50-100 μm | V-groove, 1-2 mm depth | Significant improvement |
| Wire brushing | 15-30 μm | Stainless steel wire brush | Minor improvement |
| Chemical etching | 10-25 μm | HCl + HNO₃ solution | Moderate improvement |
| Laser texturing | 30-80 μm | 1-5 kW fiber laser, 50-200 mm/s | Significant improvement |
Interface Microstructure Analysis
The researchers conducted detailed SEM-EDS analysis of the clad interface under different roughening conditions. Key findings include:
- Smooth substrate (Ra < 5 μm): The interface shows a thin intermetallic layer (10-30 μm) with limited mechanical interlocking. Bond strength is typically 40-60 MPa.
- Shot-peened substrate (Ra 25-50 μm): The interface exhibits increased mechanical interlocking due to the rough surface profile. The intermetallic layer thickness increases to 20-50 μm. Bond strength improves to 60-90 MPa.
- Mechanically grooved substrate (Ra 50-100 μm): The V-grooves provide deep mechanical anchoring. The intermetallic layer forms preferentially in the groove roots, creating a strong metallurgical-mechanical hybrid bond. Bond strength reaches 80-120 MPa.
- Laser-textured substrate (Ra 30-80 μm): The laser-induced surface features create localized high-energy sites that promote wetting and bonding. The intermetallic layer is distributed more uniformly. Bond strength reaches 70-110 MPa.
Intermetallic Compound Formation
The interface between Babbitt alloy and steel is characterized by the formation of intermetallic compounds, primarily Fe-Sn and Fe-Cu phases. The researchers identified the following reaction sequence:
- Initial contact: Sn from the Babbitt alloy diffuses into the steel substrate.
- FeSn₂ formation: The first intermetallic phase to form, typically at the interface.
- Fe₃Sn formation: Forms as diffusion continues.
- FeSn₄ formation: Forms at higher temperatures or with longer holding times.
The thickness of the intermetallic layer is the primary determinant of bond strength. Too thin (<10 μm) indicates incomplete bonding; too thick (>100 μm) leads to brittle fracture at the interface. The optimal range is 30-60 μm, which provides both metallurgical bonding and sufficient mechanical interlocking.
Process Parameters for Babbitt MIG Cladding
Recommended Welding Parameters
| Parameter | Sn-Based Babbitt | Pb-Based Babbitt |
|---|---|---|
| Welding current | 80-120 A | 100-150 A |
| Arc voltage | 16-20 V | 18-22 V |
| Travel speed | 200-400 mm/min | 150-300 mm/min |
| Wire diameter | 0.8-1.0 mm | 1.0-1.2 mm |
| Shielding gas | Ar + 5% CO₂ | Ar + 10% CO₂ |
| Gas flow rate | 12-15 L/min | 15-18 L/min |
| Preheating | 100-150 °C | 150-200 °C |
| Interpass temperature | <200 °C | <250 °C |
Defect Analysis
| Defect | Cause | Countermeasure |
|---|---|---|
| Poor wetting | Insufficient surface energy | Surface roughening, preheating |
| Excessive intermetallic | High heat input, long dwell time | Reduce current, increase travel speed |
| Cracking | Thermal stress, brittle intermetallic | Stress relief, control interpass temperature |
| Porosity | Gas entrapment, insufficient shielding | Increase gas flow, clean surface |
| Delamination | Incomplete bonding | Surface roughening, optimize parameters |
Engineering Applications and Quality Control
Bearing Application Considerations
For bearing applications, the Babbitt overlay must satisfy additional requirements beyond bond strength:
- Hardness profile: The overlay surface should have a hardness of 30-50 HV (Sn-based) or 20-35 HV (Pb-based) for conformability.
- Thickness uniformity: The overlay thickness should be uniform within ±0.05 mm across the bearing surface.
- Surface finish: The final bearing surface requires machining to Ra 0.1-0.4 μm.
- Fatigue resistance: The overlay must withstand cyclic loading without delamination.
Non-Destructive Testing Methods
| NDT Method | Detection Capability | Limitations |
|---|---|---|
| Ultrasonic testing (UT) | Interface delamination, voids | Requires coupling, complex signal interpretation |
| Magnetic particle testing (MT) | Surface and near-surface cracks | Ferromagnetic substrates only |
| Eddy current testing | Surface cracks, thickness variation | Limited depth of penetration |
| Thermal imaging | Interface quality (qualitative) | Requires specialized equipment |
| Shear bond testing | Quantitative bond strength | Destructive testing |
Key Reflections
This research demonstrates a practical and cost-effective approach to improving Babbitt alloy cladding quality. The key insight is that surface roughening is not merely a surface preparation step but an active process variable that fundamentally alters the bonding mechanism. By increasing the surface area and creating mechanical interlocking features, roughening transforms what would be a weak metallurgical bond into a strong hybrid bond. In my experience with Babbitt bearing manufacturing, the traditional approach of relying solely on metallurgical bonding often results in marginal bond strengths that are difficult to control. The surface roughening approach provides a robust solution that is independent of precise heat input control. I would recommend that any production facility considering Babbitt MIG cladding invest in surface roughening equipment as a standard process step, as the improvement in bond strength and reliability is substantial.