Zhuojin Pipe Fitting Co., LtdMicrostructure and Wear Resistance of Dissimilar Material Overlay Welds on Low Carbon Steel
Literature Overview
The research paper by Liu Zhengjun, Song Xingkui, and Tang Xingtao from Shenyang University of Technology, published in Transactions of the China Welding Institution in 2011 (Vol. 32, No. 4, pp. 99-102), investigates the microstructure and wear resistance of aluminum bronze alloy powder overlay welds deposited on low carbon steel using reversed-polarity weak plasma arc cladding. The study systematically examines the influence of welding parameters on the dilution ratio, microstructural evolution, hardness, and abrasive wear resistance of the cladding layer. This work is particularly relevant to engineers working on surface hardening of carbon steel components in abrasive service environments, where the selection of dissimilar overlay alloys and the control of dilution are critical success factors.
Process Configuration and Parameter Study
The plasma arc cladding was performed using reversed polarity (electrode negative) with a weak plasma arc configuration. The choice of reversed polarity is significant because it produces a deeper, narrower melt pool with lower dilution compared to straight polarity. The weak plasma arc configuration further reduces the heat input, minimizing thermal distortion of the base metal and limiting the dilution ratio.
| Parameter | Description |
|---|---|
| Cladding process | Plasma arc cladding (PAW) |
| Polarity | Reversed polarity (electrode negative) |
| Arc configuration | Weak plasma arc |
| Base material | Low carbon steel |
| Cladding material | Aluminum bronze alloy powder |
| Analytical methods | Metallography, SEM, XRD |
| Performance tests | Hardness measurement, wear loss measurement |
The parameter study varied the welding current, travel speed, and powder feed rate to establish the relationship between process parameters and cladding layer characteristics. The key finding is that the dilution ratio is primarily governed by the linear energy input (current × voltage / travel speed), with higher energy inputs producing higher dilution.
Microstructural Analysis
The cladding layer microstructure was characterized using optical metallography, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The analysis revealed the following key findings:
- Metallurgical bonding: The cladding layer is metallurgically bonded to the base metal with no interfacial defects such as cracks or lack of fusion. The fusion boundary shows a clean transition from the base metal microstructure to the cladding microstructure.
- Low dilution ratio: The reversed polarity weak plasma arc configuration achieves a low dilution ratio, meaning the cladding layer composition closely approximates the aluminum bronze powder composition. This is critical for maintaining the wear resistance properties inherent to the aluminum bronze alloy.
- Microstructure variation with parameters: The cladding microstructure varies significantly with welding parameters. When the cladding layer contains a dense α phase or when the α phase precipitates along grain boundaries in a network distribution, the wear resistance is significantly improved.
- Phase identification: XRD analysis confirmed the presence of α-phase (solid solution of aluminum in copper) and other secondary phases characteristic of aluminum bronze alloys. The relative proportions of these phases depend on the cooling rate and dilution ratio.
Wear Mechanism Analysis
The wear testing was conducted by friction against 45 steel (a medium carbon steel) under controlled conditions. The wear loss was measured as the weight loss of the cladding layer after a specified number of sliding cycles. The following wear mechanisms were identified:
| Wear Mechanism | Description | Influence on Wear Resistance |
|---|---|---|
| Adhesive wear | Material transfer between mating surfaces | Reduced by high hardness and low chemical affinity |
| Abrasive wear | Material removal by hard particles or asperities | Reduced by high hardness and hard second-phase particles |
| Fatigue wear | Surface cracking and spalling under cyclic loading | Reduced by fine-grained microstructure and ductile matrix |
| Oxidative wear | Oxide formation and removal at elevated temperatures | Reduced by oxide-resistant alloy composition |
The study concludes that the wear resistance of the cladding layer is a function of three interrelated factors: (1) the microstructure of the cladding alloy, (2) the hardness of the cladding layer, and (3) the characteristics of the mating surface (friction pair). The α phase in the aluminum bronze cladding layer provides a combination of hardness and ductility that resists both abrasive and adhesive wear mechanisms. When the α phase forms a network along grain boundaries, it provides additional crack-arresting capability that prevents fatigue wear failure.
Parameter Optimization for Maximum Wear Resistance
| Parameter Set | Dilution Ratio | Hardness (HV) | Wear Loss (mg) | Microstructural Feature |
|---|---|---|---|---|
| Low energy input | Low (<10%) | High | Low | Dense α phase |
| Medium energy input | Moderate (10-20%) | Moderate | Moderate | Mixed α and δ phases |
| High energy input | High (>20%) | Lower | Higher | Coarse grain structure |
The optimization of welding parameters for maximum wear resistance requires balancing the dilution ratio against the metallurgical quality of the cladding layer. Very low energy inputs produce low dilution but may result in incomplete melting of the powder particles, leading to unmelted powder inclusions. Very high energy inputs produce high dilution, which degrades the alloy composition and reduces wear resistance. The optimal parameter window is typically found at intermediate energy inputs where the dilution ratio is minimized while maintaining complete melting and sound metallurgical bonding.
Engineering Practice Considerations
For engineers applying aluminum bronze plasma arc cladding to low carbon steel components, the following practical considerations are important:
- Base metal preparation: The base metal surface must be cleaned and prepared to ensure sound metallurgical bonding. Any surface contamination can lead to porosity or lack of fusion at the fusion boundary.
- Powder quality: The aluminum bronze powder must be of consistent composition and particle size distribution. Powder oxidation during storage or handling can introduce oxide inclusions that degrade the cladding quality.
- Travel speed control: The travel speed must be maintained constant throughout the cladding process to ensure uniform heat input and consistent dilution. Variable travel speeds produce variable dilution and inconsistent microstructure.
- Post-weld inspection: The cladding layer should be inspected for hardness uniformity, dilution ratio, and surface quality. Any areas with unacceptable dilution or surface defects should be identified and repaired.
- Mating surface selection: The wear resistance of the cladding layer depends on the characteristics of the mating surface. Engineers should select mating materials and surface treatments that complement the aluminum bronze cladding layer for optimal wear performance.
Study Insights and Reflections
This work provides valuable guidance for the application of dissimilar material overlay welding in abrasive service environments. The systematic investigation of the relationship between welding parameters, dilution ratio, microstructure, and wear resistance establishes a rational basis for process optimization. The finding that the α phase network distribution significantly improves wear resistance is particularly important because it provides a microstructural design criterion for alloy selection and process parameter optimization.
The use of reversed polarity weak plasma arc for achieving low dilution in aluminum bronze cladding is a practical and effective approach. The plasma arc process offers superior dilution control compared to conventional arc welding processes such as GMAW or SMAW, making it the preferred method for dissimilar material cladding where composition control is critical. However, the plasma arc process requires specialized equipment and trained operators, which may limit its application in some industrial settings.
The wear mechanism analysis conducted in this study highlights the importance of considering the entire tribological system, not just the cladding layer in isolation. The interaction between the cladding layer and the mating surface determines the dominant wear mechanism and the resulting wear rate. Engineers designing overlay weld solutions for specific applications should always consider the complete friction pair system and optimize both surfaces for complementary wear behavior. This holistic approach to surface engineering is essential for achieving the maximum service life from overlay welded components in demanding service conditions.