Chemical machining

Chemical machining is a technique that removes material through the interaction between chemicals and a metallic workpiece. Electrochemical machining (also widely referred to as ECM) offers an alternative to electro-discharge machining (EDM) and is suitable for cutting a variety of materials including very hard alloys.

Compared to EDM, there are several practical advantages. The surface finish possible with ECM is typically better than with EDM: ECM can produce mirror-finished parts. EDM tools suffer wear through use, thus losing precision over time; this affects both die-sinking and wire EDM. Die-sinking is what is commonly called ‘spark erosion’, where a formed electrode is plunged into a workpiece. The wear on the EDM tool is one reason why it is necessary for the wire (which is the ‘tool’ in wire EDM) to be continuously fed during machining. ECM suffers from no such tool wear and so, provided that the machine control is repeatable, there should be no loss of accuracy.

However, the main objection to EDM machining has traditionally been the fact that a layer known as recast is formed on the surface of the part being machined. As the name suggests, recast is a layer that has been melted and then solidified, and is essentially a very thin layer of cast material on the surface of the part. This can lead to a surface having a lower fatigue limit than the underlying material, and has led to EDM being shunned to a large extent in the past. However, recent advances in the technique have reduced recast layer thickness significantly, and there are finishing techniques that can reduce this.

By contrast, ECM offers the chance to machine without the recast layer being formed. Compared with EDM, which removes material via arcing through a non-conducting fluid (the reason why the process is often called spark erosion), ECM does not machine by arcing. The fluid itself is one of the electrodes, as it is an electrolyte (conductive fluid). ECM is sometimes described as the reverse of plating: where electroplating deposits metal from the fluid onto the surface of the component, ECM takes material from the surface of the component into solution.

However, despite the fact that no recast layer is formed, there are problems associated with ECM that can reduce a component’s fatigue strength. The first is one that you might have considered when I described the process as being similar in concept to electroplating, since one of the dangers of electroplating is hydrogen embrittlement. Hydrogen is absorbed into the surface of a part and can form hydrides that render the surface brittle. In the ECM process, the hydrogen is evolved at the surface of the tool rather than the workpiece. However, owing to the proximity of the tool to the workpiece, the hydrogen formed can easily be absorbed by the workpiece.

Titanium alloys are one type of material that can be seriously affected, owing to the strong tendency of titanium to form a brittle hydride. Another problem is intergranular attack, which happens because the different elements within the workpiece material are taken into solution at different rates. This can lead to material at the grain boundaries being taken into solution much faster than the grains themselves, essentially forming micro-cracks at the surface of the workpiece in between grains. Nickel alloys are noted as being particularly affected among heat-resistant materials*.

So, the moral here is to consider the advantages and disadvantages of electrochemical machining carefully before using the process.

* Kozak, J., “The Effect of Electrochemical Machining on the Fatigue Strength of Heat Resistance Alloys”, Fatigue of Aircraft Structures, vol. 1, 2011

Written by Wayne Ward