TWI Knowledge Summary

Arc spraying

by Richard Halldearn

Arc spraying is the highest productivity thermal spraying process. A DC electric arc is struck between two continuous consumable wire electrodes that form the spray material. Compressed gas (usually air) atomises the molten spray material into fine droplets and propels them towards the substrate. The process is simple to operate and can be used either manually or in an automated manner. It is possible to spray a wide range of metals, alloys and metal matrix composites in wire form. In addition, a limited range of cermet coatings (with tungsten carbide or other hard materials) can also be sprayed in cored wire form, where the hard ceramic phase is packed into a metal sheath as a fine powder. The table shows the typical performance of arc spraying compared with other thermal spraying processes.

Comparison of thermal spraying process and coating characteristics

A combination of high arc temperature and particle velocities greater than 100 m.sec ^-1 gives arc sprayed coatings superior bond strengths and lower porosity levels when compared with flame sprayed coatings. However, use of compressed air for droplet atomisation and propulsion gives rise to high coating oxide content.

Arc spraying process

Applications

Arc spraying has the highest deposition rate of the thermal spraying processes and can be used to spray large areas or large numbers of components on repetitive production line operations. Typical applications include:

• Spraying of large structures such as bridges and offshore fabrications with zinc and aluminium to give corrosion protection.
• Reclamation of engineering components such as journals, bearings and shafts with steel and bronze alloys.
• Spraying of electronic component housings with copper, zinc and aluminium to give conductive coatings which provide shielding from electromagnetic interference.

Camshaft bearing recovered by arc spraying with 13% Cr steel

Developments

Although arc spraying has been available for many years, the technology is still being developed. Notable improvements are systems which use an inert gas shroud. This gives a lower oxide content and improves deposit efficiency, offsetting the higher gas cost. Equipment suppliers have developed improved power sources that reduce dust and fume levels and give enhanced deposit efficiency and oxide content.

Coating quality

Surface preparation and the spraying parameters used with the process are key factors affecting coating quality. Surface preparation is important for coating adhesion and can affect the corrosion performance of the coating. It is influenced mainly by grit blast profile and surface contamination.

Spraying parameters are more likely to affect coating microstructure and also influence coating performance. Important parameters include gun to substrate orientation and distance, gas flow rates and powder feed rates. If any of these are changed, they may affect coating quality.

Adhesion of a thermally sprayed coating is mainly due to mechanical keying. However, this does not allow the bond strength to remain independent of the substrate material. There can be significant differences between the level of adhesion achieved on carbon steel and stainless steel for example. It may be due to the level of surface oxide or differences in thermal expansion. Such factors should be considered before applying the coating.

All thermal spray coatings contain a degree of internal stress. This stress increases as the coating gets thicker. Therefore, there is a limit to the thickness of coating that can be applied. In some cases, a thinner coating will have a higher bond strength.

Benefits

There are particular reasons why arc spraying may be selected over other surface engineering techniques and they are likely to include a combination of the following:

• The need for lower costs and higher spray rates
• Coating of large areas either by manual spraying or using automated equipment
• The required coating performance can tolerate some oxide and porosity, yet better adhesion and higher deposition rates are needed than can be achieved by using flame spraying.

Risks and precautions

As with all the thermal spraying processes there are particular health and safety issues that must be addressed before spraying:

Extraction of dust and fume - All thermal spraying processes produce dust and fume. Therefore, adequate extraction or ventilation to remove this dust and fume from the particular working environment should be provided. In addition, if the operator is exposed to this environment, then suitable breathing equipment should be considered.

Arc eye - Arc spraying produces large amounts of UV radiation and welding type eye protection is needed. The correct grade of welding screen must be used. Users should refer to EN 169 which specifies a range of permanent filter shades of gradually increasing optical density which limit exposure to radiation emitted by different processes at different currents. It must be stressed that shade numbers indicated in the standard and the corresponding current ranges are for guidance only.

The operator's own preference and the application should be taken into account when selecting the shade number for a particular task. Standard filter glasses are now marked with the CE mark showing they have been independently tested to meet the full requirements of the standard.

EN 379 defines requirements for the photosensitive variable density lenses that are now available. These can be used with complete confidence, as there are failsafe requirements in the standard such that even if the lens does not darken when the arc is struck, dazzle may occur but no permanent eye damage will result. The overriding benefit of such reactive lenses is the welder's ability to see and position the electrode correctly before striking the arc. This can greatly reduce arc initiation defects.

More information on arc eye and radiation hazards can be found in:

FAQ: What is arc-eye?

FAQ: Ultraviolet, visible and infrared radiation hazards

Fire or explosion of dust and fume - Build up of some metals in the form of dust can lead to fire and explosions. This is of concern for powders of aluminium and zinc. To avoid this, adequate provision for extraction and filtration of the spray dust, along with regular cleaning, should be provided. The dust should not be allowed to get damp as this can lead to evolution of hydrogen gas. Additionally, sources of ignition should be avoided such as static charges from rotating equipment, for example fans in the ducting.

Further information

A comprehensive Best Practice Guide on Thermal Spraying is available to TWI Industrial Members

For specific enquiries please e-mail TWI's Surfacing Section

You can use the Weldasearch literature database to supplement what you find in JoinIT.

References

EN169: 1992 Specification for filters for personal eye-protection equipment used in welding and similar operations.

EN 379: 1994 Specification for filters with switchable or dual luminous transmittance for personal eye-protectors used in welding and similar operations.