Improved Surface Quality Via Electrochemical Machining
Surface quality is an important characteristic of manufactured parts that can include surface roughness, metallurgical condition of the surface layer, or burrs.
Surface roughness on wear surfaces create friction, thereby negatively impacting the longevity or sliding performance of the part. For example, the sliding surfaces on a surgical tool can have a significant impact on the “feel” of the device in the hands of a surgeon and could impede their ability to sense tissue interactions. On fluid flow surfaces, especially where fluid velocities are high, this same roughness can lead to detrimental drag or turbulence.
Turbomachinery is a prime example where smooth surfaces are critical to achieving high efficiencies given high fluid Mach numbers. Rough surfaces can also present pits or notches, allowing cracks to form more easily and impacting the fatigue life of the part while also contributing to decreased corrosion. Additively manufactured parts can have particular challenges with pit defects which can dramatically reduce service life and must be removed for many high stress applications.
The metallurgical quality of the surface layer can also have an important impact on part performance. Many thermal machining processes, like electrical discharge machining (EDM) and laser cutting, can produce surface defects.
Recast layers, made from the ejected material that has re-solidified on the wall of the cut surface, have a different metallurgical composition than the base metal and can often be a source of micro-cracks (Figure 1). Recast layers are a common problem with thermal techniques and are often not allowed in critical aerospace applications.
Heat affected zones can also be problematic and although the alloy composition is often the same as the parent metal, it may have been through a thermal cycle that has affected its mechanical properties.
In contact-based processes, burrs are a particular problem. These slivers of metals are the material that has been pushed out of the way without actually being cut. Burrs form almost exclusively from cutting processes like CNC milling or turning. These burrs can sometimes be easily spotted but still require a secondary process or extra inspection to remove them fully – leading to additional manufacturing costs.
However, in some cases, burrs can be compacted against the workpiece from the machining process and hidden or smoothed during a finishing process, making them almost impossible to see (Figure 2). These hidden burrs may only be removed during operation/contact of the device and can present a serious challenge for many applications such medical implants or surgeries where stray metal may produce an undesirable response or interaction with the body.
Traditional cutting processes, therefore, are a poor fit for industries with a very low margin of error, such as medical, aerospace and energy.
In short, surface imperfections are problematic in many applications.
PECM: Neither Contact- nor Heat-Based
Pulsed electrochemical machining (PECM), on the other hand, is a non-contact, non-thermal machining process. In this method, a gap is maintained between the tool and the part. PECM uses a current and an electrolyte solution to machine the workpieces. Waste materials remain at a much cooler temperature and are immediately washed away by the electrolyte solution. Surface irregularities are thereby limited, with no burrs at all. No heat affected zones (HAZ) are created, no deposits such as recast layers, and no corresponding microcracks.
PECM also allows for the precise repeatability of the part. And, while a recast layer from a thermal process can also be removed by electropolishing or grit blasting, PECM allows the machining and finishing to be completed in a single step.
With PECM, pristine surfaces become much more practical, expanding opportunities in applications that must maximize both flow and durability. Voxel aims to provide this level of quality in combination with feasible pricing, high repeatability, and a commitment to exceeding customer expectations. Contact us to learn more about PECM and how we can be of service to you.
*Rhys Gilmore, Julian Lohser. (June 2017). Characterization of the 15-5 Stainless Steel Electric Discharge Machining Recast Layer. Retrieved July 16, 2020, from https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1174&context=matesp