Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of Droplet-Free Arc Hot-Wire GTAW Self-Lubricating Wear-Resistant Cladding Layer
Literature Overview
This paper, authored by Tian Chunying and colleagues from Tianjin Sino-German Applied Technology University and Jiamusi University, published in Surface Technology (2020, Vol. 49, No. 7, pp. 199-206), investigates a novel cladding technology combining droplet-free arc hot-wire GTAW (Gas Tungsten Arc Welding) with a self-lubricating graphite-containing alloy. The study compares the droplet-free hot-wire GTAW method with conventional GMAW (Gas Metal Arc Welding) for depositing a graphite-phase-containing wear-resistant cladding layer on Q235 steel, evaluating friction-wear performance, microstructure, and elemental composition.
Technical Background and Innovation
The development of self-lubricating cladding layers represents an emerging area of surface engineering, driven by the need for friction reduction and wear resistance in applications where conventional lubrication is impractical or undesirable. The key innovation in this study is the use of droplet-free arc hot-wire GTAW, a welding process that combines:
- GTAW (TIG) arc: Provides a stable, low-splatter arc with precise heat input control.
- Hot-wire feeding: A heated filler wire is fed into the arc, providing additional heat input without increasing the arc current.
- Droplet-free transfer: The filler wire is melted gradually in the arc zone without forming discrete droplets, ensuring smooth, continuous metal transfer.
This combination achieves:
- Low base metal dilution: The GTAW arc provides concentrated heat input with minimal penetration into the base material.
- High deposition rate: The hot-wire provides additional heat input, increasing the melting rate of the filler wire.
- Preservation of alloy composition: The droplet-free transfer and low dilution minimize the loss of volatile alloying elements and the degradation of the self-lubricating graphite phase.
Filler Wire Design
The self-lubricating filler wire was designed as a flux-cored wire containing nickel-coated graphite powder. The wire design addresses the challenge of incorporating graphite (which has a low melting point of ~3600°C under inert atmosphere but sublimates at lower temperatures in oxidizing conditions) into a weldable alloy system:
| Component | Function | Key Consideration |
|---|---|---|
| Ni-coated graphite powder | Self-lubricating phase | Must survive welding thermal cycle |
| Fe-Cr-B matrix alloy | Wear-resistant base | Provides structural integrity |
| Flux coating | Arc stabilization and slag formation | Protects graphite from oxidation |
The nickel coating on the graphite particles serves a dual purpose: it improves the wettability of the graphite particles in the molten weld pool, and it provides a protective barrier against oxidation during the welding process.
Microstructural Analysis
Droplet-Free Arc Hot-Wire GTAW Cladding Layer
The cladding layer deposited by the droplet-free hot-wire GTAW method exhibits:
- Matrix microstructure: Fe-Cr-B wear-resistant matrix with a fine-grained structure.
- Graphite phase distribution: Large amounts of particulate graphite phase distributed along grain boundaries.
- Metallurgical bonding: Good bonding between the graphite particles and the metallic matrix, facilitated by the nickel coating.
- Dilution rate: Significantly lower than conventional GMAW, preserving the intended alloy composition.
Conventional GMAW Cladding Layer (Comparison)
The cladding layer deposited by conventional GMAW exhibits:
- Matrix microstructure: Similar Fe-Cr-B matrix but with coarser grain structure.
- Graphite phase distribution: Reduced graphite content due to higher dilution and thermal degradation.
- Metallurgical bonding: Less uniform bonding due to higher heat input and dilution.
- Dilution rate: Higher, leading to composition deviation from the intended design.
Friction-Wear Performance Evaluation
Pin-on-disk friction-wear testing revealed dramatic differences between the two cladding methods:
| Parameter | Droplet-Free Hot-Wire GTAW | Conventional GMAW |
|---|---|---|
| Initial friction coefficient | ~0.65 | ~1.4 |
| Friction coefficient trend | Decreases with time | Slightly increases with time |
| Wear surface morphology | Smooth, black, no ploughing grooves | Ploughing grooves, rough surface |
| Self-lubricating behavior | Confirmed | Not observed |
| Wear mechanism | Mild adhesive/abrasive wear | Severe abrasive/adhesive wear |
The low friction coefficient of ~0.65 for the droplet-free hot-wire GTAW cladding layer, which further decreases with friction time, confirms the self-lubricating function of the graphite phase. The smooth, black wear surface without ploughing grooves indicates that the graphite phase forms a protective transfer film on the counterface, reducing direct metal-to-metal contact.
The conventional GMAW cladding layer, with a friction coefficient of ~1.4 that slightly increases with time, shows no self-lubricating behavior, indicating that the graphite phase was either degraded or insufficiently distributed during the higher-heat-input GMAW process.
Process Parameter Optimization
The study identified the following process parameters as critical for successful self-lubricating cladding:
| Parameter | Recommended Range | Effect |
|---|---|---|
| Arc current | Moderate (GTAW range) | Controls heat input and dilution |
| Auxiliary wire current | Low | Minimizes graphite thermal degradation |
| Travel speed | Moderate | Balances deposition rate and dilution |
| Shielding gas flow | Adequate | Protects molten pool and graphite from oxidation |
| Wire feed rate | Matched to arc current | Ensures stable arc and consistent deposition |
The key finding is that selecting a low auxiliary wire current is critical for preserving the graphite phase integrity. Higher auxiliary current increases the thermal load on the filler wire, promoting graphite oxidation and sublimation before the graphite particles are incorporated into the weld pool.
Engineering Practice Implications
The droplet-free arc hot-wire GTAW technology offers significant advantages for self-lubricating cladding applications:
- Low dilution: Preserves the intended alloy composition, critical for maintaining the self-lubricating graphite phase.
- High deposition rate: The hot-wire provides additional heat input, enabling faster cladding without increasing arc current.
- Process stability: The droplet-free transfer ensures smooth, consistent metal deposition without the arc instability associated with droplet transfer.
- Field applicability: The GTAW-based approach is portable and suitable for field repair applications.
- Material versatility: The low heat input and low dilution make the process suitable for cladding a wide range of alloy systems, including those with volatile or thermally sensitive components.
Potential applications include:
- Self-lubricating bearing surfaces in pumps and compressors.
- Wear-resistant, low-friction surfaces in aerospace actuators.
- Sealing surfaces in valves operating under high pressure and temperature.
- Mold surfaces requiring both wear resistance and easy release.
Study Insights and Reflections
This paper demonstrates that the choice of welding process is as critical as the choice of filler material in achieving the desired cladding properties. The same filler wire composition, when deposited by conventional GMAW, fails to achieve the self-lubricating function due to the higher heat input and dilution, whereas the droplet-free hot-wire GTAW process preserves the graphite phase and achieves excellent self-lubricating wear resistance. This finding has broader implications for cladding technology in general: the welding process must be carefully matched to the filler material's thermal sensitivity and composition requirements.
The concept of combining a wear-resistant metallic matrix with a self-lubricating solid lubricant phase is a well-established approach in tribology, but the challenge of incorporating thermally sensitive lubricant phases into a cladding layer has historically limited the practical application of self-lubricating hardfacing alloys. The droplet-free hot-wire GTAW process provides a viable solution to this challenge, opening new possibilities for self-lubricating surface engineering in industrial applications. The reduction of the friction coefficient from ~1.4 to ~0.65 represents a nearly 50% reduction in friction, which translates directly to energy savings, reduced wear, and extended component life in friction-critical applications.