Weld Overlay of the Spiral Shaft of a Fibre Carbon Molding Machine

Literature Overview and Background

This literature reports on the weld overlay of the spiral shaft (helical screw) of a fibre carbon molding machine, a specialized extrusion component used in the processing of carbon fiber-reinforced polymer composites. The spiral shaft operates under extremely demanding conditions: high temperature (typically 200-400°C), high shear stress from the viscous flow of the composite material, and severe abrasive wear from the hard carbon fiber particles suspended in the polymer matrix. The combination of thermal, mechanical, and abrasive loading results in rapid degradation of the shaft surface, leading to loss of dimensional accuracy, reduced extrusion pressure, and ultimately machine failure. The weld overlay approach aims to extend shaft life by depositing a wear-resistant layer on the critical surfaces of the spiral flight and root.

Material and Process Selection

The selection of the overlay material was driven by the need to simultaneously resist abrasive wear from carbon fibers, thermal degradation at elevated operating temperatures, and corrosion from the polymer matrix at high temperature. Several candidate materials were evaluated, including hardfacing alloys based on chromium carbide, cobalt-based alloys, and nickel-based alloys. The final selection was a chromium-carbide-based hardfacing alloy deposited using plasma transferred arc (PTA) welding, chosen for its excellent abrasive wear resistance, thermal stability, and ability to form a smooth, dense surface layer.

PTA was selected over alternative processes such as flame spraying or HVOF (High Velocity Oxy-Fuel) spraying because it produces a metallurgically bonded overlay with minimal porosity, excellent surface quality, and controlled dilution. The dilution rate in PTA is typically 5-15%, which is significantly lower than in conventional arc welding processes, ensuring that the overlay composition remains close to the powder composition. This is critical for maintaining the target hardness and wear resistance properties.

Process Parameters and Microstructural Features

The PTA process parameters were optimized through systematic trial welding and metallographic evaluation. The plasma power was set at 6-10 kW, the arc current at 200-350 A, and the travel speed at 100-200 mm/min, depending on the powder feed rate and desired bead geometry. The powder feed rate was maintained at 150-300 g/min, and the shielding gas (argon) flow rate was 15-25 L/min to ensure complete exclusion of atmospheric contamination.

The resulting microstructure of the overlay layer consists of a matrix of solid solution strengthening (chromium and nickel dissolved in the iron base) with a high volume fraction of chromium carbides (Cr7C3 and Cr3C2) dispersed throughout. The carbide morphology is predominantly angular and acicular, which provides excellent resistance to abrasive wear by deflecting and fracturing abrasive particles. The grain structure is fine (approximately 10-20 μm), which contributes to both hardness and toughness. The absence of retained austenite in the overlay layer is advantageous for dimensional stability during thermal cycling in service.

Distortion Control and Geometric Accuracy

One of the most significant challenges in overlaying a spiral shaft is maintaining geometric accuracy, particularly the helical flight profile and the root clearance. Excessive heat input can cause local distortion of the spiral geometry, leading to interference with the barrel or uneven material flow. The following measures were implemented to control distortion:

1. Preheating the entire shaft to 200-300°C to reduce thermal gradients during welding.
2. Using a multi-pass approach with alternating bead directions to distribute heat input symmetrically.
3. Applying the overlay to the flight surface first, then to the root, with intermediate machining between stages to monitor dimensional changes.
4. Performing post-weld stress relief at 550-600°C to eliminate residual stresses that could cause delayed distortion.
5. Final machining of the spiral profile to restore dimensional accuracy within the original tolerance specification.

Wear Performance and Service Results

Field testing of the overlay-repaired spiral shaft demonstrated a 5-8 times improvement in service life compared to the uncoated original shaft. The overlay maintained its hardness and surface integrity throughout the extended service period, with only superficial wear marks visible after the equivalent of multiple original shaft replacements. The tribological testing confirmed that the chromium carbide overlay exhibited a wear rate approximately 10 times lower than the base steel under simulated carbon fiber abrasion conditions. The thermal stability of the overlay was verified through thermal cycling tests, which showed no significant hardness degradation after 100 cycles between room temperature and 400°C.

Study Insights and Implications

This literature highlights the critical role of process selection in determining overlay performance for high-temperature, high-abrasion applications. The selection of PTA over alternative processes was justified by the need for a dense, metallurgically bonded, oxide-free overlay with controlled dilution. The chromium carbide-based alloy proved to be an excellent choice for carbon fiber abrasion resistance, demonstrating that the selection of the overlay material must be guided by a clear understanding of the dominant wear mechanism. The distortion control measures are particularly instructive for engineers overlaying complex geometries, as they demonstrate that geometric accuracy can be maintained through systematic thermal management and process planning. The significant life extension achieved underscores the economic value of weld overlay as a maintenance and repair technology for specialized processing equipment.