CLADDING TECHNOLOGY SHANXI CO., LTDNon-Droplet Arc Hot-Wire GTAW Self-Lubricating Wear-Resistant Overlay Microstructure and Properties
Literature Overview and Research Background
This study focuses on the microstructure and properties of a self-lubricating wear-resistant overlay produced using non-droplet arc hot-wire gas tungsten arc welding (GTAW). Hot-wire GTAW is an advanced welding process that combines the advantages of conventional GTAW with a continuously fed wire electrode, enabling high deposition rates while maintaining excellent weld quality. The non-droplet arc variant eliminates droplet transfer, resulting in a more stable arc and reduced spatter. The concept of self-lubricating overlays is particularly attractive for applications where external lubrication is impractical, such as in mining equipment, aerospace components, and high-temperature machinery.
The self-lubricating mechanism typically involves the incorporation of solid lubricant phases such as graphite, MoS₂, or PTFE into the overlay matrix. In this study, the overlay is designed to incorporate a lubricating phase within a hard, wear-resistant matrix, creating a composite structure that provides both wear resistance and reduced friction.
Core Technical Points
Non-Droplet Arc Hot-Wire GTAW Process Characteristics
The non-droplet arc hot-wire GTAW process differs from conventional GTAW in several important ways. In conventional GTAW, the wire is fed into the arc and melts into droplets that transfer to the weld pool. In the hot-wire variant, the wire is preheated by the arc itself, which reduces the electrical power required and increases the deposition rate. The non-droplet arc configuration ensures that the wire melts directly at the contact point without forming free-flying droplets, resulting in a smoother weld surface and reduced porosity.
| Process Parameter | Typical Value | Effect on Overlay |
|---|---|---|
| Arc current | 150–250 A | Controls heat input and dilution |
| Wire feed rate | 5–12 m/min | Controls deposition rate |
| Travel speed | 150–400 mm/min | Controls heat input per unit length |
| Shielding gas | Ar or Ar/He mix | Controls arc stability and penetration |
| Wire diameter | 1.0–1.6 mm | Controls deposition rate and dilution |
| Preheat temperature | 50–150 °C | Reduces cracking risk |
The deposition rate of hot-wire GTAW can reach 3–5 kg/h, which is significantly higher than conventional GTAW (0.5–1.5 kg/h). This makes it suitable for building up thick overlays in a reasonable number of passes.
Microstructure of the Self-Lubricating Overlay
The overlay microstructure consists of a hard matrix phase and a dispersed lubricating phase. The matrix is typically a martensitic or austenitic stainless steel, which provides the structural strength and wear resistance. The lubricating phase is distributed as discrete particles or lamellae within the matrix, providing a solid lubrication mechanism during sliding contact.
The study reveals that the non-droplet arc process produces a finer microstructure compared to conventional GTAW, due to the more stable heat input and reduced dilution. The absence of droplet transfer results in a more uniform distribution of the lubricating phase throughout the overlay. This uniform distribution is critical for consistent self-lubricating performance across the entire overlay surface.
Mechanical and Tribological Properties
| Property | Test Method | Result | Comparison with Conventional GTAW |
|---|---|---|---|
| Hardness | HV10 | 420–480 HV | 10–15% higher |
| Tensile strength | ASTM E8 | 550–620 MPa | Comparable |
| Friction coefficient | Pin-on-disk | 0.15–0.25 | 40–60% lower |
| Wear rate | Pin-on-disk | 2.5×10⁻⁴ mm³/N·m | 50–70% lower |
| Bond strength | Peel test | 280–320 MPa | Comparable |
| Cracking resistance | Bending test | No cracks at 5% strain | Improved |
The self-lubricating overlay exhibits a friction coefficient of 0.15–0.25, which is significantly lower than conventional hardfacing alloys (0.4–0.6). This reduction in friction is attributed to the transfer of the lubricating phase to the counterface during sliding, forming a protective lubricating film. The wear rate is reduced by 50–70% compared to conventional overlays, demonstrating the effectiveness of the self-lubricating mechanism.
Standards and Process Analysis
For pressure vessel and piping applications, the qualification of hot-wire GTAW welding procedures must comply with NB/T 47014 or ASME IX. The non-droplet arc variant is not explicitly listed in these standards, which creates a qualification challenge. However, the process can be qualified as a variation of conventional GTAW, provided that the essential variables are properly documented and the procedure demonstrates consistent performance.
The key essential variables for hot-wire GTAW include:
- Preheat and interpass temperature
- Shielding gas type and flow rate
- Travel speed and wire feed rate ratio
- Electrode type and diameter
- Position of welding
The study recommends that the welding procedure be qualified with a dilution rate of 15–25% to ensure adequate bonding between the overlay and the base material while maintaining the self-lubricating properties of the overlay.
Defect Analysis and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Cracking | High cooling rate, hydrogen absorption | Increase preheat, use low-hydrogen wire |
| Porosity | Inadequate shielding, wire contamination | Increase gas flow, clean wire |
| Poor bond strength | Excessive dilution, base material contamination | Reduce heat input, clean base surface |
| Lubricant segregation | Uneven mixing, excessive heat | Optimize wire composition, control travel speed |
| Overlap (lack of fusion) | Low heat input, excessive travel speed | Increase current, reduce travel speed |
Engineering Practice Integration
The non-droplet arc hot-wire GTAW process is particularly suitable for repairing and refurbishing worn components in mining, cement, and power generation industries. The high deposition rate reduces repair time, while the self-lubricating property extends the service life of the overlay. For example, in a cement mill application, the use of self-lubricating overlays on grinding rollers can reduce lubrication maintenance by 80% while extending the overlay life by 40%.
One practical challenge is the compatibility of the self-lubricating overlay with the base material. The thermal expansion mismatch between the overlay and the base material can lead to cracking during cooldown. To mitigate this, the study recommends using a transition layer of austenitic stainless steel between the base material and the self-lubricating overlay. This transition layer absorbs the thermal mismatch and provides a more ductile interface.
Study Insights and Implications
This study demonstrates that the non-droplet arc hot-wire GTAW process is a viable and effective method for producing self-lubricating wear-resistant overlays. The key advantages are the high deposition rate, fine microstructure, and uniform distribution of the lubricating phase. Engineers should consider this process for applications where both wear resistance and self-lubrication are required, particularly in environments where external lubrication is impractical or where contamination from lubricants is unacceptable.