Welding Digest

Arc thermal spray, also known as twin wire arc spray, is one of the older and simpler forms of thermal spray coating. Thermal spray technology has come a long way, from spraying basic metals and simple alloys to, now, spraying extremely hard ceramics and carbides. These advances don’t mean arc thermal spray is any less valuable. Thermal spray certainly isn’t the cure-all for everything, but each process has a specific fit in the coating world. Among many things, twin wire arc thermal spray is great for dimensional restoration, sacrificial corrosion coatings, custom alloy coatings, and bearing surfaces. Arc spraying is also fast and economical, which can translate into faster turnaround and better margins.

 1 — Dimensional Restoration The most basic use of arc thermal spray is dimensional restoration. Rotating parts often experience surface erosion from rotating inside sleeves or collars (Photo 1). This creates problems when the parts wear out of round and become unbalanced. Not only is this noisy but it can also decrease operating efficiencies and the vibration can cause damage to other machine components. Arc spray can be used to remanufacture the surface of the part, up to 1 4⁄ in., and restore it to OEM specifications.

2 — Sacrificial Corrosion Coatings Another application of arc spray is sacrificial corrosion coatings. Using aluminum and zinc-aluminum, arc spray can provide an anodic layer over steel. This layer can attract corrosion and keep the underlaying surface protected. Arc spray coatings are also porous so they can accept sealants more readily than a regular carbon or tool steel surface, so parts stay sealed longer.

3 — Custom Alloy Coatings One advantage of arc spray unique to this thermal spray process is how the spray material is fed during application. The gun used to apply this type of coating feeds two separate wires, charging them negative and positive and causing them to arc and melt. Compressed air then propels these molten droplets onto the surface of the part. Since arc thermal spray uses two wires, operators can create custom alloys by feeding two different material wires (Photo 2). For example, using carbon steel and bronze wires creates a more wear-resistant bearing surface coating than plain bronze. Operators can also combine a 316 stainless steel wire with a molybdenum wire to create an economically friendly anti-galling coating. Many combinations can be explored and engineered to fit the operating environment of the part.

4 — Bearing Surfaces The application of thermal spray to bearing surfaces using arc spray allows the use of many materials for softer bearing surfaces. Soft bearing surfaces are used to allow deformation so the bearing area can accommodate small amounts of misalignment. Arc spray can apply bronze, babbitt, brass, and aluminum bronze (Photo 3). Using thermal spray to apply these as coatings, as opposed to making the entire part from these materials, is not only more economical but has the added benefit of porosity. Like sealants, the porous surface more readily accepts lubricants and holds on to them longer. Arc spray bearing surfaces can also be removed and reapplied again and again when they get worn and no longer operate at peak performance.

Conclusion

 The purpose of any thermal spray coating is to keep your parts operating at peak performance longer, whether through preventative coatings or remanufacture. Arc spray offers many unique solutions and is one of the most affordable forms of thermal spray coating. So, whether you want to prevent corrosion or need to solve bearing area issues, arc thermal spray is an effective and profitable option.

This article was written by Lacey Reames (lreames@htscoatings.com) is proposal engineer and director of sales and marketing.