Weapons systems components are frequently damaged in service by corrosion, impact, or wear and must be repaired in the field or shipped back to a repair depot. Where a localized repair is possible, it is usually done by brush plating of nickel and/or chromium. Where the component cannot be repaired in the damaged area alone, the entire surface and any coating on it must be removed and rebuilt, usually by hard chrome plating alone or a combination of sulfamate nickel and chrome plate for dimensional restoration. Where chrome plate is damaged, the only recourse for a permanent repair is to strip and replate. All of these processes create hazardous waste and expose personnel to toxic materials. In addition, there are many components that suffer significant damage for which there is neither a field- nor depot-level repair currently available. At present, these components must be condemned, resulting in costs associated with replacement and disposal. The objective of this project was to demonstrate electrospark deposition (ESD) as a technically feasible and commercially viable production-scale process for localized repair of weapons systems components and to transition ESD repairs for use on Department of Defense components for gas turbine engines (GTE), vehicles, and ships.
ESD is a micro-welding process capable of depositing metal alloy or ceramic/metal coatings onto any electrically conductive material. It is a rapid solidification process, producing nanostructural coatings that are metallurgically bonded to the substrate. The electrodes can be made in different sizes and geometries, resulting in the ability to perform localized or complex-geometry repair on base metal alloys or on most types of coatings. The equipment is portable, making it suitable for field repair applications. In this project, two demonstration sites were identified—Oklahoma City Air Logistics Center (OC-ALC) and Anniston Army Depot (ANAD). ESD units were acquired and placed in each of these facilities, and candidate components meeting the above criteria were identified. In addition, Naval Surface Warfare Center (NSWC) Carderock identified Navy ship components and used an existing in-house ESD unit to demonstrate repairs. Because the potential usage was so broad, a joint test protocol (JTP) was developed only for GTE applications to qualify ESD for localized repair by depositing the same alloy material from which the component is manufactured and for localized repair of chrome plating. In addition, repair procedures were developed for specific candidate components.
Various process and equipment developments were made, including process optimization, robotic coating application, and incorporation of ultrasonic impact treatment (UIT) to impart compressive stress to the ESD coating. The latter was found to improve fatigue and hardness. For the JTP, test coupons were prepared from Inconel 718 with specified defects that were then repaired using ESD application of the same material. Tensile, corrosion, and wear properties of the ESD-repaired areas were the same as the base material. The fatigue performance of ESD-repaired specimens was between that of undamaged specimens and damaged specimens that were not repaired. However, if UIT also was utilized, the full fatigue performance was restored. Acceptable repair of chrome plating on Inconel 718 coupons was demonstrated. ESD repair procedures were developed and documented for three GTE components currently overhauled at OC-ALC—TF33 10-12 stator segment, TF39 shaft, and TF33 #5 bearing housing. In addition, ANAD, working in conjunction with the Army Research Laboratory, developed, qualified, and implemented an ESD repair for the M1A1 Abrams Tank gun barrel cradle. NSWC Carderock developed an ESD procedure for repairing localized damage in submarine steering and diving control rods.
Successful qualification of ESD as a localized repair technology at both the depot- and field- level will result in a significant reduction in maintenance costs and waste generation. Fewer spare parts will have to be retained and turnaround times for returning parts to service will be greatly reduced, leading to an enhancement of overall readiness of weapons systems and a reduction in life-cycle costs. A detailed cost/benefit analysis on the three GTE components showed substantial savings using ESD if the areas to be repaired are small and localized, but no savings when the areas became too large. ESD is now being used to recover approximately 12 M1A1 Abrams Tank gun barrel cradles per year at an annual savings of around $300,000. (Project Completed - 2006)