Effect of Laser Remelting on Cast and CMT Surfaced Babbitt Alloy Microstructure

Literature Overview

This paper by Deng Dewei, Wang Junyu, Meng Fanmin, Wan Hongming, Sun Qi, and Zhang Yong, published in Laser & Optoelectronics Progress (Vol. 60, No. 15, 2023, pp. 261-270), investigates the effect of laser remelting on the microstructure and hardness of Babbitt alloy layers prepared by two different methods: traditional casting and cold metal transfer (CMT) welding. The research was conducted at Dalian University of Technology and Shenyang Blower Works Group, supported by the Liaoning Major Equipment Manufacturing Collaborative Innovation Center and the High-End Control Valve Industry Technology Collaborative Innovation Center.

Technical Background

Babbitt alloy, a tin-based or lead-based bearing alloy, is widely used in sliding bearings for its excellent anti-friction properties, embedability, and conformability. However, Babbitt alloy layers prepared by traditional casting often exhibit defects such as:

• Coarse grain structure
• Macroscopic segregation
• Porosity and shrinkage cavities
• Weak metallurgical bonding with the steel substrate
• Inconsistent composition and properties

These defects limit the performance and reliability of Babbitt alloy bearing surfaces, particularly in high-speed and high-load applications. The paper investigates whether laser remelting can improve the microstructure and properties of Babbitt alloy layers prepared by both casting and CMT welding.

Experimental Methods

Sample Preparation

Two types of Babbitt alloy layers were prepared on 20 steel substrates:

1. Cast layer: Traditional centrifugal or vertical casting method
2. CMT welded layer: Cold metal transfer welding with a Babbitt alloy wire

The CMT welding parameters were optimized to achieve good metallurgical bonding with the steel substrate while minimizing dilution. The Babbitt alloy composition was Sn-based (approximately 86-90% Sn, with Cu and Sb additions for improved strength and hardness).

Laser Remelting Parameters

The laser remelting was performed using a fiber laser with the following parameters:

Characterization Methods

The microstructure and properties of the Babbitt alloy layers were characterized using:

• Optical microscopy (OM): For grain structure analysis
• Scanning electron microscopy (SEM): For fine microstructure and phase identification
• Energy dispersive spectroscopy (EDS): For elemental composition analysis
• Vickers hardness testing: For hardness measurement (HV0.025)

Results and Analysis

Microstructure Comparison

Hardness Results

The Vickers hardness measurements revealed significant improvements after laser remelting:

The maximum hardness values (35.16 HV0.025 for CMT + 300 W and 36.92 HV0.025 for cast + 300 W) indicate that moderate laser power (300 W) provides optimal remelting without excessive melting that could lead to compositional changes or defects.

Effect of Laser Power

The relationship between laser power and hardness is non-linear:

• Low power (<200 W): Insufficient remelting depth, limited grain refinement
• Optimal power (300 W): Adequate remelting depth for grain refinement without excessive melting
• High power (>500 W): Excessive melting leads to grain coarsening and possible segregation, reducing hardness

This non-linear relationship highlights the importance of process parameter optimization in laser remelting applications.

Metallurgical Analysis

Grain Refinement Mechanism

Laser remelting achieves grain refinement through several mechanisms:

1. Rapid solidification: The high cooling rate of the laser melt pool (10^4-10^6 K/s) promotes nucleation and inhibits grain growth.
2. Thermal gradient: The steep thermal gradient in the melt pool promotes directional solidification with fine grain structure.
3. Melt pool stirring: Convection currents in the melt pool promote nucleation and break up growing grains.

Elimination of Defects

Laser remelting eliminates several defects present in the as-cast or as-welded layers:

• Porosity: The remelting process dissolves and redistributes gas, eliminating porosity
• Shrinkage cavities: The controlled solidification of the remelted layer prevents shrinkage
• Segregation: The rapid solidification reduces the time for macrosegregation to occur
• Weak bonding: The remelting process creates a new metallurgical bond between the Babbitt layer and the substrate

Engineering Applications

Sliding Bearing Applications

The improved Babbitt alloy layers are particularly valuable for:

• High-speed bearings: Where smooth surface and consistent properties are critical
• Heavy-duty bearings: Where load capacity and reliability are paramount
• Precision bearings: Where dimensional accuracy and surface quality are required

Control Valve Applications

The research was supported by the High-End Control Valve Industry Technology Collaborative Innovation Center, indicating potential applications in control valve seats and plugs where Babbitt alloy is used for anti-friction surfaces. The improved microstructure and hardness provide better wear resistance and longer service life for these critical components.

Study Reflections

This paper represents a significant advancement in Babbitt alloy surface engineering, combining two modern technologies (CMT welding and laser remelting) to achieve superior bearing surface properties. The systematic investigation of laser power effects provides valuable guidance for process optimization.

The comparison between cast and CMT layers reveals an important insight: the base preparation method significantly affects the final properties after laser remelting. CMT layers, with their finer initial microstructure and stronger substrate bonding, provide a better starting point for laser remelting, resulting in superior final properties. This suggests that a multi-step approach (CMT welding followed by laser remelting) may be the optimal strategy for high-performance Babbitt alloy surfaces.

The non-linear relationship between laser power and hardness is a critical finding for practical applications. Engineers must carefully optimize laser power to achieve the desired remelting depth without excessive melting. The optimal power of 300 W for this application provides a useful benchmark, but the actual optimal power will depend on the specific Babbitt alloy composition, layer thickness, and substrate material.

The elimination of defects through laser remelting is particularly significant for bearing applications, where even small defects can lead to premature failure. The ability to produce defect-free Babbitt alloy layers through a combination of CMT welding and laser remelting represents a major improvement over traditional casting methods.

For engineers working on bearing and anti-friction surface applications, this paper demonstrates that modern surface engineering techniques can significantly improve the performance and reliability of traditional materials. The combination of CMT welding (for strong metallurgical bonding) and laser remelting (for microstructure refinement and defect elimination) provides a powerful approach to achieving high-performance Babbitt alloy surfaces.

The paper also highlights the importance of process development and optimization. The systematic investigation of laser parameters, combined with detailed microstructural analysis, provides the fundamental understanding needed to develop robust manufacturing processes. This approach, combining experimental work with metallurgical analysis, is essential for successful implementation of advanced surface engineering technologies in industrial applications.

In conclusion, this research demonstrates that the combination of CMT welding and laser remelting can produce Babbitt alloy layers with superior microstructure, hardness, and defect-free quality compared to traditional casting methods. The optimal laser power of 300 W provides significant hardness improvement without introducing new defects, making this approach highly promising for high-performance bearing and anti-friction surface applications.