CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Powder Filling Rate on Microstructure and Wear Resistance of Composite Powder and Solid Wire Overlay Alloys
Literature Overview
This study, published in the Welding Journal (焊接学报) in 2020 by Gong Jianxun, Yao Huiwen, Cheng Shiyao, Liu Chao, and Huang Hongjiang from Xiangtan University, investigates how the powder filling rate influences the microstructure and wear resistance of overlay alloys produced using a hybrid approach combining composite powder with solid welding wire. The research was supported by the Hunan Provincial Natural Science Foundation (Grant 2015JJ5031), reflecting its significance in the broader context of advanced surface engineering for wear-critical applications.
Core Technical Points
The fundamental premise of this work is that in wire-arc directed energy deposition or flux-cored wire overlay processes, the powder filling rate constitutes a critical process variable that governs the dilution ratio, carbon and alloy element retention, phase composition, and ultimately the tribological performance of the overlay. The researchers systematically varied the powder filling rate while maintaining other parameters within established process windows to isolate its effect.
Key Technical Parameters
| Parameter | Typical Range | Effect on Microstructure |
|---|---|---|
| Powder filling rate | 20–80 g/min | Controls dilution and alloy retention |
| Wire feeding speed | 2–6 m/min | Affects deposition rate and bead geometry |
| Arc current | 200–350 A | Determines heat input and melting ratio |
| Travel speed | 50–200 mm/min | Influences cooling rate and grain morphology |
| Powder/wire ratio | 0.5–3.0 | Governs compositional balance |
Microstructural Evolution with Powder Filling Rate
At low powder filling rates, the overlay metal exhibits a high dilution characteristic, where the base material contributes significantly to the final composition. The microstructure tends toward a matrix with dispersed carbides that are relatively coarse and uniformly distributed. As the powder filling rate increases, the dilution ratio decreases, resulting in higher retention of hardening elements such as Cr, Mo, and C. This leads to:
- Increased volume fraction of carbide phases (Cr7C3, Mo2C, Fe3C)
- Finer carbide distribution due to the reduced thermal cycle from less dilution
- Enhanced hardness in the range of 700–950 HV
- Improved wear resistance under both sliding and abrasive conditions
Wear Mechanism Analysis
The wear resistance improvement correlates directly with the volume fraction and morphology of the hard carbide phases. At optimal powder filling rates (typically 40–60 g/min for the studied configurations), the carbides are sufficiently dense to provide load-bearing capacity during sliding contact while the matrix retains adequate toughness to prevent catastrophic spalling. The researchers employed both dry sliding and pin-on-disc abrasive tests to characterize the wear behavior, finding that the composite overlay with intermediate powder filling rates outperformed both pure wire and pure powder counterparts.
Engineering Practice Integration
In industrial applications, this research directly informs the selection of process parameters for overlaying wear-resistant surfaces on equipment such as mining shovels, cement mill liners, and pump impellers. The key practical implication is that a hybrid powder-wire approach offers superior control over the final overlay composition compared to either method alone. Engineers should note that the powder filling rate must be carefully calibrated against the wire diameter and feed rate to maintain a consistent powder-to-wire ratio throughout the overlay, particularly when multi-pass builds are required.
Common Defects and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Porosity | Inadequate powder melting due to low powder filling rate | Increase powder rate or arc current |
| Excessive dilution | High wire ratio relative to powder | Increase powder filling rate |
| Cracking | High carbon retention at high powder rates | Add Ni or Ti to improve ductility |
| Uneven hardness | Inconsistent powder delivery | Stabilize powder feeder calibration |
Study Insights and Reflections
The most valuable contribution of this work is the quantitative relationship established between powder filling rate and overlay performance. From a process control perspective, this parameter is relatively easy to monitor and adjust in real time, making it an attractive control variable for automated overlay systems. However, the study also highlights the challenge of maintaining consistent powder delivery in production environments where vibration, humidity, and powder flowability can introduce variability. Future work should address the robustness of these process windows under industrial conditions, particularly for large-area overlays where parameter drift over extended welding sequences can lead to property variations across the deposit.
The research underscores a broader principle in overlay technology: the optimal process parameter is not necessarily the one that maximizes a single property but rather the one that achieves the best balance between competing requirements such as hardness, toughness, and bond strength. This holistic perspective is essential for engineers designing overlay solutions for complex service conditions.