There are several metal add-on processes that a shop can use to repair a worn or damaged component, but for this article we’ll review only the most common: welding, spray, and electroplating (tank and brush).
If asked to name one metal add-on repair process, most would immediately be able to identify welding. Welding is one of the oldest, and most well-known metal add-on repair processes. During the process, multiple beads of weld are applied next to and on top of each other until sufficient thicknesses are built up. Adhesion is very good, but depending on the application, deposit purity and oxide inclusions can be an issue. Welded surfaces will always require a final machining or grinding operation to achieve tight tolerances. Welding can also create stress in the base metal and may cause heat distortion. While this is typically not a problem on large components, distortion can be an issue on small cross-sectional areas.
Tank, or bath, electroplating is also well-known metal surface finishing process. While tank plating has many advantages, such as plating in batches and the ability to plate irregularly shaped parts, it also has serious disadvantages. All the parts to be repaired must be disassembled before plating (and ultimately reassembled upon completion). Also, if only a small area of the component needs plated, time-consuming masking is required. Many components in the marine, aerospace, and oil and gas industries are too large for standard tank sizes or are extremely difficult to mask – to ensure protection of adjacent areas. Finally, the system requires continuous chemical solution control and large, expensive equipment. While widely used, tank plating does not lend itself to the typical maintenance shop, therefore parts are typically sent out, rather than repaired in-house.
Brush plating, or selective plating, is a method of electroplating on a localized area without the use of an immersion tank. The SIFCO Process® equipment is portable, enabling technicians to plate parts in place with minimal disassembly, minimizing downtime. In a typical operation, the part is first masked to isolate the area to be plated and then a series of base material-specific preparatory steps are conducted to ensure an adherent deposit. The last step is the plating of the metal deposit to the desired thickness. Most brush plating applications are dependent upon the operator; but when properly executed, brush plating can provide precise build-ups, minimizing post machining or grinding. With the brush plating method, there is also no risk for thermal distortion, internal stresses or cracking from heat. Of all the metal-add processes, brush plating is the most precise, while providing the best adhesion.
Spray metallizing includes thermal spray applications such as flame spray, plasma spray, HVOF, and others. With all thermal spray applications metal rod, wire, or powder are melted and sprayed onto the component’s surface using a special gun. Using this technique, as much as one-eighth inch (3mm) of metal, intermetallic alloys, or metallic oxides or carbides can be deposited quickly and inexpensively. However, subsequent machining is typically needed when tight tolerances are involved. While the porous deposits may be advantageous for oil bearings, they are not suitable for many applications, especially when corrosion protection is a requirement. But, the biggest drawback of using thermal spray is its adhesion. Of the four metal add-ons, thermal spray has the poorest adhesion.
Overall, each type of metal add-on repair has its advantages and disadvantages, but if you are looking for an application to repair your component in place, and to size, you needn’t look further than brush plating.