Electrospark deposition: A new repair technique
Carole Reignier began at TWI in February 1999 as a project leader with the surface engineering team. After graduating in material science from INSA (National Institute of Applied Science in Lyon, France) she first took part in an exchange with Argonne National Laboratory at Chicago. Then, as a patent examiner, she was placed in charge of material relevant patents at the French National Patent Office in Paris for a year and a half.
A new electrospark deposition system for the repair of worn or mis-machined components is currently under investigation at TWI, Carole Reignier reports.
The system allows the restoration of damaged or over-machined components to original dimensions with the generation of little or no heat affected zone (HAZ). In addition, the wear resistance of the surface can be improved with the deposition of very thin coatings, eg tungsten carbide. Applications include repair of steel tools and dies, aluminium castings and other heat sensitive alloys ( Fig.1).
Fig.1 Electrospark deposition at TWI.
The process
The system comprises a capacitor-based power supply, an electrode holder (or applicator), and consumable electrodes. The consumable electrode is deposited onto the work piece by means of electric sparks, Fig.2, in a reverse manner to that of spark erosion. When the capacitor energy is released, the direct current generates a plasma arc at a temperature between 8000 and 25000°C between the tip of the electrode and the work piece. The plasma arc ionises the consumable and a small quantity of the molten electrode material is transferred onto the work piece and produces a strong bond with the substrate material.
Fig.2 The electrospark deposition process.
The period of the high-energy pulse is extremely short relative to the interval period, so very little heat is transferred to or accumulated within the substrate during each cycle. The low heat input to the substrate is claimed to result in little or no HAZ, distortion, pits, shrinkage or internal stress.
The electrospark deposition system uses three different applicators, with different power capacity and rotary or vibratory movement. The electrode rotates or vibrates to prevent the electrode from welding to the substrate. The electrode tip, which becomes red hot during deposition, is protected from oxidation by an argon shielding gas. In common with all arc-welding processes, the work piece is earthed. The operator keeps the applicator moving constantly over the surface and maintains the electrode in contact with the surface. Electrodes are either strips or rods, the rod varies in diameter from 1-3.2mm. The power supply current output and frequency are fully adjustable on the main unit, with additional remote fine tuning capabilities.
The system produces no toxic gas, liquid or unpleasant odour and noise. The operator should wear goggles to protect his or her eyes from sparks.
Heat treatment is not required before or after coating. The surface only needs to be free of grease, oil and dust, before coating. A good finish can be obtained quickly with sanding disks or emery paper. Figure 3 shows the finish quality that can be reached with a hand tool on chromium plated roll.
Fig.3 The middle section of a groove on a chrome-plated roll has been hand finished by files and oilstones. Courtesy of TechnoCoat.
Field of application
The electrospark deposition process was originally developed to repair small and shallow repairs to aluminium castings, tools and dies (
eg plastic injection moulding dies) that have been worn or damaged. Thin coatings can be deposited and the applicator is easy to handle with accuracy. For example, for plastic injection moulding tools, partitioning lines can be repaired and burr formation can then be avoided.
Electrospark deposition has many potential applications for repair, dimensional modification and wear prevention, including:
- Repair of weld defects, undercut, pinholes, small crack and, worn and chipped edges, cavities and grooves. Defects up to 2mm deep can be repaired.
- Dimensional modification, eg mould gates.
- Wear prevention with thin tungsten carbide coating (around 60 microns).
Electrospark deposition can be applied to bearings, blades, chucks, gears, mandrels, pumps, shafts, work-tools, die, valve seats and ejector pins. The power supply is portable and the applicator is light and easy to handle, and allows on-site repair work ( Fig.4).
Fig.4 Cross section of a tool steel sample coated with Stellite 6®.
The system can be used to coat most metallic components having reasonable electrical conductivity eg low and medium carbon steels, tool steels, stainless steel and copper alloys. Electrode materials tested to date and under development include:
- Nickel alloys
- Aluminium alloys
- Copper alloys
- Tungsten, titanium, chromium, and molybdenum carbides
- Nitrides
- Borides
- Ni and Co based cermets.
Properties and characteristics
A reasonably dense microstructure is observed, with less than 4% porosity and oxide, not dissimilar in appearance to a thermally sprayed coating,
Fig.5.
Fig.5 Repair of a damaged edge at TWI.
There is little evidence of HAZ in the substrate. A scanning electron microscope line mapping of a tool steel sample coated with nickel alloy, indicates a higher proportion of iron in a narrow zone of the coating adjacent with the substrate, Fig 6.
Fig.6 Line mapping of a tool steel substrate coated with nickel alloy.
This zone containing some elements from the substrate could explain the good bond strength of the coating to the substrate. Bend tests carried out at TWI have shown that 0.3mm thick nickel alloy coatings deposited onto 2.8mm thick mild steel strip by electrospark deposition do not crack or peel off when bent through 180°.
Conclusion
Initial trials at TWI have confirmed that the heat input generated by the electrospark deposition process system is low, and creates a small HAZ in the substrate. The electrospark deposition system appears to be suitable for repair welding sensitive alloys. Further work is now underway at TWI to increase the density of the coating and to apply the process to other welding sensitive alloys.
