Effect of Powder Spraying Method on CO2 Shielded Arc Cladding Microstructure and Wear Resistance

Literature Overview

This literature investigates how different powder feeding and spraying techniques in CO2 gas-shielded arc welding (GMAW) affect the microstructure and wear resistance of the cladding layer. The study is particularly relevant to engineers working with flux-cored and powder-fed welding processes, where the manner of powder delivery to the arc significantly influences dilution, composition, and ultimately the properties of the deposited overlay. The research compares side-feed powder feeding, center-feed powder feeding, and submerged arc powder feeding configurations, evaluating their effects on carbide distribution, hardness uniformity, and sliding wear resistance.

Microstructural Analysis of Different Spraying Methods

The microstructure of the cladding layer is fundamentally governed by the cooling rate, which in turn is influenced by the thermal input and heat distribution pattern determined by the powder spraying method. The literature presents the following comparative microstructural findings:

The center-feed powder method produces the most uniform carbide distribution because the powder is introduced directly into the arc center, resulting in more complete melting and homogenization of the melt pool. This leads to a finer, more evenly distributed carbide network, which provides superior wear resistance under sliding abrasion conditions. The side-feed method, while simpler to implement, tends to produce localized areas of high and low carbide concentration due to uneven powder distribution in the arc zone.

Wear Resistance Testing and Results

Wear tests were conducted using a pin-on-disk apparatus under dry sliding conditions with SiC abrasive paper (600 grit) and with alumina slurry (15 μm particles in water). The results demonstrate clear differentiation among the three methods:

• Dry sliding wear: Center-feed powder achieved the lowest specific wear rate of 1.2 × 10⁻⁶ mm³/N·m, compared to 2.1 × 10⁻⁶ mm³/N·m for side-feed and 3.5 × 10⁻⁶ mm³/N·m for submerged arc powder feeding.
• Slurry erosion wear: Center-feed powder again performed best with a wear rate of 8.5 × 10⁻⁹ m³/N·m, while side-feed recorded 14.2 × 10⁻⁹ m³/N·m and submerged arc powder 22.1 × 10⁻⁹ m³/N·m.

The superior wear resistance of the center-feed method is attributed to the finer and more uniformly distributed carbides, which provide more consistent resistance to abrasive particle penetration and micro-ploughing. The literature also notes that the martensitic matrix in the center-feed and side-feed overlays provides additional support to the carbides, preventing their pull-out during sliding contact.

Process Parameters and Optimization

The literature provides detailed process windows for optimal results with each spraying method:

The critical finding is that the powder-to-wire ratio must be carefully controlled. For center-feed powder, a ratio of 0.3–0.5 (powder mass / wire mass) provides the best balance between composition control and deposition rate. Deviations beyond this range lead to either excessive dilution (low ratio) or incomplete powder melting (high ratio), both of which degrade overlay properties.

Key Defects and Their Mitigation

The primary defects associated with powder-fed CO2 GMAW cladding include:

1. Incomplete powder melting: When powder feed rate exceeds the arc's melting capacity, unmelted powder particles are entrapped in the weld. This creates weak spots and porosity. Mitigation: reduce powder feed rate or increase welding current to enhance arc power.
2. Excessive spatter: CO2 shielding gas at high flow rates can cause turbulence and spatter. Mitigation: use a mixture of 80% Ar + 20% CO2 to reduce spatter while maintaining adequate arc stability.
3. Cracking due to high carbon content: If the powder contains high carbon, the resulting overlay may be excessively brittle. Mitigation: control carbon content in the powder to below 2.5% and consider adding a low-carbon transition layer.
4. Non-uniform hardness: Caused by inconsistent powder feeding. Mitigation: use a calibrated powder feeder with constant pressure control and regular calibration checks.

Engineering Practice and Process Recommendations

In industrial applications, the center-feed powder method is recommended for applications requiring high wear resistance, such as pump impellers, valve seats, and crusher components. The side-feed method is more suitable for field repairs and large-scale surface cladding where equipment complexity must be minimized. The submerged arc powder method offers the highest deposition rate and is suitable for thick overlay builds but sacrifices some hardness and wear resistance due to slower cooling rates.

The literature also emphasizes the importance of multi-pass welding strategies. A typical sequence involves a first pass with a low-carbon transition filler to ensure good bonding, followed by two to three overlay passes with the wear-resistant powder. The interpass temperature should be maintained between 150–200°C to avoid excessive grain growth while preventing cold cracking.

Study Insights and Implications

This literature provides a clear framework for understanding how powder delivery geometry influences the metallurgy and performance of CO2 GMAW cladding layers. The finding that center-feed powder produces the finest and most uniform carbide distribution is particularly significant for engineers designing wear-resistant overlays. It suggests that investment in center-feed powder equipment, despite its higher initial cost, can yield substantial savings in maintenance and replacement costs for critical wear components. The study also reinforces the importance of process parameter optimization through systematic experimentation rather than empirical trial-and-error, as the interactions between powder feed rate, arc parameters, and resulting microstructure are complex and non-linear.