Abstract:
This technical assessment evaluates the thermal spray methodologies utilized at the Metallisation production facility in Dudley, UK. The analysis covers the mechanical and thermodynamic principles of three primary coating processes: Combustion Wire Thermal Spray (Flame Spray), Twin-Wire Arc Spray (TWAS), and High-Velocity Oxygen Fuel (HVOF) surfacing. The documentation explores the versatility of these processes for corrosion mitigation (anodic protection using Zinc and Aluminum), dimensional restoration of worn components (using high-carbon steels), and the application of cermet hard-facings (Tungsten Carbide-Cobalt-Chrome) for extreme wear resistance. Key technical highlights include the management of substrate thermal loading, the necessity of mechanical bonding via surface profile preparation, and the implementation of automated High-Velocity ID (Internal Diameter) lancing systems for specialized bore applications.
Technical Process Summary:
0:27 Overview of Thermal Spray: Metallisation operates as a 100-year-old entity specializing in the development of equipment for flame, arc, plasma, and HVOF processes. The core utility spans corrosion prevention, engineering repair (MRO), and decorative surfacing.
1:27 Combustion Wire Spray (Flame Spray): Demonstrates the Mark 73 system. It utilizes an oxy-propane heat source to melt 1.6mm metallic wire (e.g., Aluminum Bronze, Copper, Brass). An air motor drives the wire feed mechanically, while compressed air atomizes and propels the molten material.
3:25 Bonding Requirements: Successful adhesion requires rigorous substrate preparation, including degreasing and grit blasting to create a high surface roughness. Bonding occurs primarily through mechanical interlocking at the atomic/microscopic level.
5:02 Coating Metrology: Use of a Dry Film Thickness (DFT) gauge confirms typical single-pass thicknesses of approximately 80β86 microns (~3 mils). The process allows for indefinite thickness buildup through successive layering.
12:12 Twin-Wire Arc Spray (TWAS): Employs an electric arc between two oppositely charged metallic wires (2.3mm diameter). This process requires high-volume compressed air (55 CFM) for atomization. It utilizes a "push-pull" drive system involving a rotating flexible cable to ensure consistent wire feed without surging.
17:42 Substrate Thermal Loading: TWAS and Flame Spray are demonstrated as "cold" processes relative to the substrate. Molten zinc is successfully applied to combustible materials (paper business cards and wood) without ignition, indicating low heat transfer to the base material.
18:45 High-Velocity Oxygen Fuel (HVOF): Features a kerosene and oxygen combustion chamber (effectively a micro-rocket engine) that propels powdered feedstock (e.g., Tungsten Carbide Cobalt Chrome) at supersonic speeds. This creates "shock diamonds" in the exhaust plume.
21:30 HVOF Hard-Facing: HVOF is identified as a primary alternative to hard chrome plating due to environmental regulations. It provides extreme hardness for slideways and hydraulic components. Material deposition efficiency is approximately 50%, with powders applied at roughly 70g per minute.
22:54 Wear Resistance Testing: Comparative testing shows a standard hacksaw cannot penetrate the HVOF-applied Tungsten Carbide coating, whereas it easily cuts the underlying mild steel substrate.
24:07 Dimensional Restoration: Demonstration of rebuilding a worn shaft. High-carbon steel (0.8% C) is sprayed to increase the shaft diameter from 40.97mm to 42.53mm, allowing the component to be machined back to original specifications.
26:08 Advanced ID Coating (Halo Jet): Introduction of a specialized ID lance system for internal bores. This hybrid system utilizes both hydrogen and kerosene fuels with an argon carrier gas to apply high-performance engineering coatings within narrow internal diameters.