HIGH TEMPERATURE

High Temperature

High Temperature

Manufacturers of jet engines machine the casings, compressor blades, and bladed disks, or blisks, of their products from the hardest alloys to one-thousandth of an inch tolerances. Precision cutting and milling titanium aluminides, Inconel 718, and Waspalloy, among others, is a painstaking process that places thermal and mechanical stresses on the part, and shortens the life of tooling. (Takehara 2007:47-58)

An alternative method, especially efficient in working complex parts out of very hard conductive metals, is electrochemical machining, or ECM, a noncontact technology that spares both tool and part from machining wear, and finishes some parts in half the time of conventional mechanical machining techniques.

The Sermatech Manufacturing Group, a leading user of ECM, is researching new applications for the technology, which the company uses principally on turbine components and various aircraft parts.

Meanwhile, electrochemical grinding, a related process also used to shape turbine parts, has established itself in other fields, including the precise machining of medical devices. (Brewer 2007:12-20)

Electrochemical machining was first proposed by the Russian scientists B.R. and N.I. Lazarenko in 1943, who theorized that electrolysis could be used in order to remove metal from a workpiece, a reverse of electrolytic coating, which adds material.

The basic components of ECM are the workpiece, the conductive tool, a recirculating electrolyte, and a power source. The part must be made of a conductive metal. The tool is typically made of copper, brass, or stainless steel, while the most commonly used electrolyte is a concentrated solution of inorganic salts, such as sodium chloride, and the direct current power source is low voltage and high amperage.

In the ECM process, the dc power source charges the workpiece positively and charges the tool negatively. As the machine slowly brings the tool and workpiece close together, perhaps to within 0.010 of an inch, the power and electrolyte flow are turned on. Electrons flow across the narrow gap from negative to positive, dissolving the workpiece into the shape as the tool advances into it. The recirculating electrolytic fluid carries away the dissolved material as a metal hydroxide.

This noncontact capability means the ECM tool does not have to be made of expensive alloys tougher than the workpiece, as would be necessary for mechanical machining. The process also reduces tool wear and minimizes scrap costs. ECM places less heat and mechanical stress on the workpiece than mechanical machining does; such stresses can damage a part's microstructure.

ECM can make a virtually finished part in one pass, regardless of the metal's hardness. The tool can be used over again to make many parts.

Among the drawbacks of ECM is its high tooling costs. The tool and the programming must be tailored to make the correct inverse part geometry. Power, up to 40,000 amps, also must be bussed into the workpiece. Most tooling components have to be made from copper alloys or stainless steel, to hold up to the saline electrolyte. Also, manufacturers of ECM systems must ensure that the salty electrolyte does not corrode the equipment or the ...