TWI Industrial Member Report Summary 699/2000
Electrospark deposition (ESD) is a pulsed micro-welding process that is being used increasingly for small scale and precision repair of high value worn or mis-fabricated components. Although several commercial and experimental systems have been reported during the last 30 years based on a number of process variants, few notable papers have been published in the literature on this subject. Electrospark deposition is also known as spark hardening, electrospark toughening, electrospark alloying, pulsed fusion surfacing and pulsed electrode surfacing. Principal industrial applications to date include repair of defects in casting moulds and injection moulding tooling.
Based on short duration, high current pulses, the manual process imparts a low heat input to the substrate material, resulting in little or no modification of the substrate microstructure. Therefore, the process offers an advantage over fusion welding processes (including arc, laser and resistance welding) when repairing materials that are difficult to weld because of poor heat affected zone (HAZ) properties (eg liquation cracking, high hardness, low toughness). Components can be restored to their original dimensions, because with such low heat input the bulk substrate material remains near to ambient temperature, and thermal distortion, shrinkage and high residual stresses can be avoided. Moreover, the process generates a good metallurgical bond between the coating and the substrate, and hence also offers a technically superior alternative to thermal spraying, electroplating and vapour deposition process.
This report examines the potential of the electrospark deposition process for repair of a selection of substrate materials, including two alloys (a tool steel and a cast iron) that are generally considered to be difficult to weld, with a selection of wear and corrosion resistant consumables. Japanese equipment manufacturer, Technocoat Ltd, supplied an electrospark deposition unit, which has been commercialised recently in Europe, for this particular investigation.
The objective of this work was to evaluate the microstructure and properties of:
- Nickel alloy, cobalt alloy and tungsten carbide/cobalt coatings deposited by the ESD process.
- Tool steel, cast iron, stainless steel substrates subjected to the ESD process.