Background

The past five years have seen an increasing interest in the novel applications of electrical discharge machining, particularly from the Far East and Europe, with industry starting to see the potential of the technique for surface modification. Normally used for manufacturing dies/moulds and aerospace components, researchers at the University of Birmingham are adapting the technique to enable surface hardening of workpieces to create, in some cases, new, harder alloys on the surface of components to increase their working life and wear resistance.
What is Electrical Discharge Machining?

Electrical discharge machining (EDM) is a thermal process that involves melting and vaporisation of the workpiece electrode. It is widely used in the aerospace, mouldmaking and die casting industries for manufacturing plastics moulds, forging dies and die casting dies made from hardened tool steels, together with engine components, such as compressor blades made from titanium alloys and nickel-based superalloys. In addition to the more well known EDM operations of die sinking, scanning/milling with a simple electrode and wire cutting, other operations and machine configurations exist, one of which allows the surface of hardened steel rolls used in the production of steel and aluminium sheet to be textured.
How Does Electrical Discharge Machining Work?

The EDM process uses electrical discharges to remove material from the workpiece, with each spark producing a temperature of between 10,000-20,000°C. Consequently, the workpiece is subjected to a heat affected zone (HAZ) the top layer of which comprises recast material. The thickness, composition and condition of this layer depend on the discharge energy and the make-up of the workpiece, tool electrode and dielectric fluid, and both hard and soft surface layers can be produced despite perceived wisdom that the recast layer is always hard. With ferrous workpiece materials, the recast layer typically appears white and amorphous when viewed under a microscope, and is prone to tensile stress, microcracking and porosity.
The Recast Surface Layer

To increase the life of the tool or product, the recast layer is generally removed, particularly for applications in which the part is subjected to cyclical stress (aeroengine components) or fluctuating loads (forging and punching tools/dies). This is achieved either by hand polishing, etching or heat treatment. Alternatively, burnishing or shot peening is employed in order to impose a compressive residual stress regime. However, such processes are supplementary and may increase cost and time. With operations in which repeated high levels of mechanical impact are not a factor, such as plastics injection moulding, the EDM recast layer can be beneficial in providing increased abrasion and corrosion resistance.
Surface Alloying During Electrical Discharge Machining

Another way of improving the surface integrity and wear resistance of an EDM workpiece is by applying surface alloying during sparking, using either partially sintered powder metallurgy (PM) tool electrodes, or by dispersing metallic powders in the dielectric. Several published papers detail the use of powders suspended in the dielectric as a means of producing surface alloying. This is an extension of work where powders, typically graphite, aluminium (Al) or silicon (Si), varying in size from 1-100 µm, are used to produce mirror-like EDM surfaces with minimal microcracking. Although deionised water can been used, the majority of current work uses hydrocarbon oil dielectrics (kerosene/paraffin), which produce carbides through carburisation with pyrolytic carbon.
Problems Associated with Surface Alloying

This approach is not without its problems despite the potential to increase workpiece hardness from 2-6.5 times that of the bulk material and produce recast layers of 10-150 µm thick. It is difficult to achieve a uniform distribution of the powder in suspension and filtration of the dielectric can be counter-productive.
Electrode Materials

Very little surface alloying occurs when using ‘conventional’ tool electrodes under standard polarity compared to partially sintered PM electrodes, where the binding energy between grains is considerably lower. Negative tool polarity is usually employed and PM electrode materials used include Al, Cr, Cr/Ni, Cu/Co, Cu/Mn, Cu/Sn, Cu/W, Ni, Ni/Co, Ni/Fe, Ni/Mn, Ni/Si, Ti, Ti/A1, TiC/Ni, W/CrC/Cu and WC/Co. Figure 1 shows a schematic of the process. This approach to surface alloying is relatively new and there is little information on aspects such as preferred particle size, sintering temperatures and pressures. However, powder sizes of less then 50 µm are quoted with temperatures of ~900-1300°C and pressures of ~100-550 MPa.

Producing Wear and Corrosion Resistant Surfaces

Current research at Birmingham involves the use of WC/Co and W/CrC/Cu partially sintered hardmetal tool electrodes for EDM surface alloying, together with non-standard wire materials to produce wear and corrosion resistant surfaces. One of the challenges of the research is to produce surfaces that are highly alloyed and have high wear resistance, but meet the surface roughness and topography requirements of the mouldmaking, die casting, rolling and aerospace industries. The effect of electrical parameters such as peak current, open circuit voltage, polarity, pulse on/off times and capacitance on recast layer thickness, and workpiece microstructure, microhardness and composition, are being evaluated.

Much of the equipment being used is commercially available, with PM products manufactured by Vacuum Impregnated Products and standard hydrocarbon oil (paraffin) dielectric being used. However, in order to increase electrode wear (contrary to common practise where minimum wear is desirable) and achieve greater surface alloying, generators with higher than normal open circuit voltage - up to 300 V - are being used.
The Effect of Open Circuit Voltage

Work on tool and roll steels (AISI H13 hot work tool steel and 2% Cr steel) is being sponsored by the Engineering and Physical Sciences Research Council (EPSRC) in collaboration with Alcan, Charmilles, Dynacast, Erodex, The Gauge and Toolmaker’s Association (GTMA) and SparkTec International. Figure 2 gives a sample workpiece microstructure when die sinking AISI H13 heat treated to 600-640 HK0.025 using preferred operating parameter levels identified in a Taguchi fractional factorial experiment. Analysis of the machined surface revealed a 5-20 µm thick recast layer with some evidence of cracking and porosity Figure 3a shows the corresponding microhardness depth profile, while figure 3b illustrates the microhardness results from a test where all the parameters were the same except a lower open circuit voltage (125 V) was used. By comparison, the higher voltage caused