Over the past several decades, thermal spraying has evolved from being a cost-effective repair of worn components and machine parts to a high-technology process providing improved part performance and longer life to OEM components. One of the many areas in which thermal spray has established itself is a low-cost hard-facing alternative to weld cladding and chrome plating.
VRC Cold Spray technology can in many cases surpass thermal spray to make industrial repairs that meet “as good as new” requirements or often even surpass them. Cold spray can also be repaired with cold spray, with no known limit to the number of repairs. Contact us to find out how VRC Cold Spray technology can benefit your metal repairs.
We’re writing today’s blog to briefly introduce the thermal spray process’s characteristics and discuss the disadvantages of thermal spray for metal repairs.
What Is Thermal Spray?
Thermal spraying, much like weld cladding or chrome plating, is a metal coating process where a coating is applied to a substrate by melting the coating material and spraying it to the substrate (part surface). It then solidifies and bonds to the part surface, forming a stable and hard-wearing coating. There are some advantages (along with some disadvantages) to thermal spray coatings, detailed below.
During the thermal spray process, wire or powder is flame- or electricity-melted and “sprayed” onto the workpiece. The thermal spray torch makes successive passes across the workpiece to produce the coating. Thermal spraying has its advantages, as well as its disadvantages.
Some of the benefits of thermal spray for metal repairs include:
- Reduced Cost. Instead of making the whole part out of expensive material, a high-performance material is sprayed onto a low-cost substrate (base material).
- Thickness Range. Coatings can be 0.001 of an inch to more than 1 inch thick, depending on the materials used.
- Versatility. You can spray almost any metal.
In the flame spraying process, oxygen and fuel gas, such as acetylene or propane, are fed into a torch and ignited, creating an intense flame. Either metal powder or metal wire is injected into the flame, where it melts and is then sprayed onto the substrate.
The flame spraying process requires very little equipment. It can be readily performed in the factory or on-site. The process is relatively inexpensive and helpful for the application of metal alloys.
During the thermal spray process known as “arc spraying,” two wires are energized with DC current, continuously fed into the torch and joined close together in the nozzle. The electrical load on the wires causes arcing and the tips melt.
A carrier gas, such as nitrogen picks up the molten material and transports it to the workpiece.
Arc spraying is also relatively inexpensive, easy to learn, portable, and reasonably simple to maintain. Low particle velocities enable high maximum coating thickness for a given material. Recent advancements in nozzle and torch configurations are providing greater control over coating quality and the spray pattern.
With the right equipment, it’s possible to produce an elongated spray pattern or spray components with minimal internal diameters. As far as its shortcomings, arc spraying is limited to electrically conductive solid wires and cored wires.
One may consider the plasma spray process to be the most versatile of all the thermal spray processes. During operation, gases such as argon, helium, hydrogen, or nitrogen pass through a torch.
An electric arc disassociates and ionizes the gas. Once out of the nozzle, the atomic components recombine, producing a tremendous amount of heat. A technician injects powder into the flame, melts, and accelerated it to the workpiece.
Plasma spraying was initially developed to spray ceramics and is still the premier process for applying them. You can also spray metals and plastics with this technique.
High-Velocity Oxyfuel (HVOF) Spraying
The high-velocity oxyfuel (HVOF) process, invented only 20 years ago, has expanded the thermal spraying application into areas that were once unattainable.
During HVOF spraying, a combination of process gases, such as air, hydrogen, kerosene, oxygen, or propylene, is injected into the torch’s combustion chamber at high pressure together with oxygen gas and ignited. The gas velocities then achieve supersonic speeds. The metal powder is injected into the flame and also accelerated to supersonic speeds.
The HVOF process is the preferred technique for spraying wear-resistant carbides and is also suitable for applying wear- or corrosion-resistant alloys like Hastelloy, Triballoy, and Inconel®. These properties have enabled HVOF sprayed coatings to become an attractive alternative to cladding and chrome plating.
Thermal Spray Applications
Properly applied thermal spray coatings have many uses and advantages.
Various materials can be deposited as coatings, including metals, metal alloys, oxide, and non-oxide ceramics, plastics, cermets, and composite structures comprised of metals, ceramics, and plastics be applied using thermal spray.
Many components benefit from thermal spraying, either as part of the original manufacturing process, as part of a fabrication process, or as a reclamation or re-engineering technique.
A few typical applications are:
- Reclamation or re-engineering of a wide range of rotating and moving parts from machines of all kinds, including:
- Chemical plant or food machinery
- Electric motors
- Landing gear
- Machine tools
- Mining and quarrying machinery
- Papermaking machines
- Power generation and aerospace turbine repair
- Printing presses
- Road and rail vehicles
- … And virtually any equipment subject to wear, erosion, or corrosion
- New components can include:
- Aerospace combustion chambers
- Fluid seals
- Gate and ball valves
- Hydraulic piston rods
- Print rollers
- Rock drilling bits and tools for the oil and gas or petrochemicals industry
- Turbine blades
Thermal spray helps many components and machines operate in adverse environments where erosion, wear, corrosion, or heat conspire to reduce component life.
Disadvantages of Thermal Spray For Metal Repairs
We’ve already covered a few of the advantages of thermal spray.
Some of the disadvantages of thermal spray include:
- The bond mechanism between the coating and workpiece is primarily mechanical, not metallurgical.
- Thermal spraying is a line-of-sight process.
- The coatings are considerably weaker in tension than in compression.
- The coatings have tensile residual stresses because of the shrinking that occurs during solidification.
- The coatings have poor resistance to pinpoint loading.
- Thermal spraying disguises the substrate. As thermal spray coatings are so efficient, it is often impossible to tell the substrate’s origin.
- The technician cannot precisely evaluate effectiveness. Once the thermal spray coating is on, it is often difficult to tell exactly how well the coating has been applied.
- Thermal spray can involve a costly set-up. Some of the thermal spray coatings methods involve expensive apparatuses, resulting in a high initial set-up cost.
How To Get Around The Disadvantages Of Thermal Spray
Whether you’re creating a new feature, adding wear- or corrosion-resistance, or repairing more significant metal components, the cold spray professionals at VRC Metal Systems can help you get started with the right solution.
The advantages to utilizing VRC Cold Spray (instead of settling for the disadvantages of thermal spray) for metal repairs include:
- Fewer steps from the damage assessment to the completion of the repair
- Virtual elimination of heat-affected zones
- Much less alloy damage from welding or thermal spray repairs
- Increase the lifespan of metal components
- Repairs can often proceed without removing the broken portion from a larger piece of equipment. There is no need to disassemble whole systems or wait for components to come back from the repair facility
- Cold spray coatings achieve metallurgical bonds when applied to compatible materials.
- Cold spray is a solid-state self-peening process that gives coatings with compressive residual stresses.
- Cold spray coatings also have high cohesive strength, achieving wrought properties in some cases.
- The mobile VRC Cold Spray system saves significant human-hours in labor costs
- Cold spray can even assist with dimensional restorations
- Previous cold spray repairs that have experienced further damage can be re-repaired using cold spray
Unlike thermal spray, VRC Cold Spray technology can make industrial repairs “as good as” or sometimes even “better than new” requirements. Contact us to find out how VRC Cold Spray technology can benefit your metal repairs.