Under the skin of EDM in the aerospace industry

Essentially every commercial, scientific, and military aeronautical and aerospace piece of hardware has used parts manufactured in part, or completely, using the EDM process. For decades it has been used to manufacture aerospace parts including engine, fuel system, and landing-gear components, as well as other high-stress, high-temperature parts. However, the surface integrity, and therefore safety, of EDM-machined aerospace components has been questioned. Here, specialist EDM equipment supplier, WMT, highlights how the surface integrity of EDM components is no longer such a major concern.

During the 1960s, EDM machines were manual sink EDMs that used relaxation generators with simple RC (resistor-capacitor) circuits. Copper electrodes were used as the early sink EDM machines were designed around this low-cost, electrically conductive material. These machines were not so accurate and slow. Though slow they could scrap a part in the blink of an eye, but it was the only technology capable of machining some complex geometries and the new exotic alloy materials being developed for aerospace use.

The next decade witnessed the introduction of numerically controlled sink and wire EDMs. These machines used high-speed switching transistors in place of the RC circuits to generate AC instead of DC sparks. Brass wire was used on wire EDMs and graphite electrodes used on sinker machines. Of course, these machines were much faster than their predecessors but had a significant drawback.

Surface cracking begin to appear
As the aerospace industry discovered, the EDM process damaged the surface of the components being machined. This damage was a result of the heat generated by the EDM process, and consisted of the recast layer, or white layer, and an annealed Heat Affected Zone (HAZ), which lay directly below the hard recast layer. The recast layer is made up of molten metal particles that have been re-deposited onto the surface of the workpiece. “Both the HAZ and recast layer could also contain micro-cracks that could cause stress failures of safety critical components. These concerns resulted in aerospace manufacturers either developing or revising specifications on the use of EDM to manufacture components,” explains WMT managing director, Ian Holbeche.

At this time, the HAZ and recast layers could be 0.1 to 0.25 mm thick. One industry standard required that one and a half times the HAZ/recast area be removed by conventional machining or chemical etching. This requirement added an extra step in the manufacturing process, increasing delivery times, as well as adding extra costs related to more processes and waste disposal.

Ian Holbeche says: “Many have referred to EDM in aerospace as a ‘necessary evil’ as the process is required to make many of the components used in aircraft because of the intricate shapes, tough alloys, and very tight tolerances involved. However, it also recognised the dangers of the damaged surfaces resulting from the process. The ‘burnt’ or carbonised material that comprises the recast layer could flake off of EDM-machined components during operation, producing potentially damaging contamination within the assembly housing of these components. If an EDM machined part requires anodizing after machining, the recast layer could interfere with the coating process. Similarly, the HAZ left behind by the EDM process was softer than the underlying material. This annealed zone could weaken prematurely and cause the material to develop stress fractures that could lead to anything from a minor malfunction to a catastrophic failure.”

During the 80s and 90s, EDM generators continued to be refined. More advanced circuits were used to filter out noise, control spark generation, monitor spark gaps, and automatically make continuous adjustments to the burn conditions. These refinements resulted in a more stable, predictable, and safe process. Newer high-tech generators produced less recast layer, a smaller HAZ, and were less likely to compromise the surface integrity of the components. Improvements to wire, graphite, and dielectric oils also contributed to a safer burn for those industries concerned with recast, HAZ, and micro-cracking.

Even though EDM technology had improved the aerospace industry was still using antiquated EDM machining specifications, based on machine technology from the 1970s. Modern EDM machines have significantly evolved from their predecessors. Today, detailed testing has shown EDM machines leave no measurable HAZ and produce recast layers of less than 0.01 mm. Micro-cracks are almost non-existent. These machines can produce components with finishes measuring 0.5 micron Ry, tolerances in the sub-micron range, and leave the surface of any part virtually undamaged.
“Manufacturing demands are driving EDM machine manufacturers to develop machines that will maintain extreme accuracy while completing jobs faster,” Ian Holbeche states. “Wire diameters have been reduced to just 0.02 mm, and fine-hole drilling machines can produce clean, accurate holes measuring just 11 micron in diameter.”

AE for wire
The application of EDM as a process requires electrolysis, the production of chemical changes by the passage of an electrical current through an electrolyte (a non-metallic electrical conductor through which current is carried by the movement of agitated ions).
During wire EDM operation stray energy in the dielectric fluid, produced by the cutting process itself, interacts with contaminants in the flushing fluid to disrupt the surface of the workpiece. The major result of this process in all materials is an increased heat-affected zone, or white layer, on the surface. Depending upon the workpiece material being cut, the visible results of this action will vary.

The current-carrying EDM wire commonly discharges particles as well as produces the cutting action on the workpiece. The stray current, once thought inevitable, causes detrimental surface effects such as bluing of titanium; cobalt binder depletion of carbide; anodic oxidation of aluminium; rusting of ferrous materials, and eventual micro-cracking of all materials.

This last effect had prohibited increased use of wire EDM in medical, aerospace and ordnance applications because that condition would render parts either unsafe or inoperable to the specifications required.

So, the challenge facing EDM builders was to engineer a power supply that would minimise, or even eliminate, the interaction of the stray current and contaminants on the workpiece surface. Various builders have taken various routes to solve this with anti-electrolysis (AE) generators. ONA offers its Easycut generator, which provides a cut that is 100 per cent free from electrolyte corrosion, without affecting the speed of 450 mm2/min with 0.33 mm diameter wire, while also preserving the surface integrity of the material being cut, with a surface finish of 0.2 micron Ra – 6 VDI. Featuring new technology, Excetek’s range of wire EDM machine tools are equipped with its EF Electrolysis Free AC generator that has RTS (real-time sparking). With an extremely fast response time RTS provides feedback to negate ineffective discharges by automatically adjusting cutting conditions to improve machining efficiency.

Ian Holbeche concludes: “The aerospace and other leading industry sectors have recognised that EDM is a valuable, viable process to manufacture components. However, early EDM machines compromised the surface integrity of these components. Therefore, secondary machining operations, including grinding, milling, or chemical etching, were required to remove recast, HAZ, and micro-cracks. Industry testing indicates today’s technologically advanced machines do not damage the surface of the material as in the past. As a result, manufacturers may now be able to use EDM to manufacture more components, possibly eliminating secondary machining operations, reducing costs and decreasing delivery times.”