Quality Scan: Improve EDM/Laser Part Quality
Most often recognized as a brightening process used on stainless, electropolishing is often overlooked as a way to improve metal fatigue, eliminate contamination, reduce mold polishing costs, or shape fine edges.
A controlled metal-removal process that uses a combination of chemicals and electrical current to dissolve surface metal, electropolishing is nonmechanical, in that the metalworking action takes place without any metal-to-tool action. A fixtured part is lowered into a specialized bath, and the chemicals flow around all external (and many internal) surfaces. Once electricity is applied, the exposed metal surfaces begin to dissolve.
Ten years ago, 95% of electropolishing was performed on stainless. Today, the amount of tool steel, beryllium copper, molybdenum, nickel/iron, and other metals processed is growing. A good amount of that growth has occurred in the EDM/laser corner of metalworking.
The single largest benefit to laser/EDM components is removal of recast layer. Re-deposition of melted or vaporized metal ions has long been a problem due to contamination, metal fatigue, or metal-performance reasons. Recast is very brittle, rough material that is hard to remove, and at worst it prevents laser/EDM from grabbing a larger share of the metalworking market.
Most in the industry focus on the prevention or minimization of recast. Many laser/EDM shops are employing new gasenclosed cutting environments, reducing cutting rates, and trying exotic new cutting media to reduce or eliminate recast.The key disadvantages of all these methods are increased operating costs and/or lowered production rates. The good news is reduced recast—the bad news is higher product costs.
Rather than reduce cutting rates, some operators are finding that it's better to maximize production rates, and then dissolve the recast using electropolishing as a secondary operation.
There are five key benefits to using electropolishing on laser/EDM parts. It will:
- Dissolve burrs/radius edges. When properly applied, electropolishing dissolves metal from the entire part, but at a faster rate on edges and corners. Sharp edges are reduced, and in most cases eliminated. Because the process is electrochemical, it generates no part damage or adverse stresses in the metal part.
- Decontaminate metal surfaces. By carefully dissolving surface metal, imbedded or attached contamination is dissolved as well. Recast and related oxides and lubricants, imbedded abrasives from lapping/grinding, rouge from buffing operations, and most chemicals and reactive salts are removed.The metal surface is returned to a full native condition.
- Smooth and reduce friction. In most applications, dissolving surface metal reduces surface friction, reduces buildup of product, and improves plating adhesion, especially plated precious metals. Microscopic high points dissolve at a fast rate, leveling the surface. As a general rule, Ra values are cut by 50%, within a range of 12–32 Ra.
- Improve fatigue life. On fragile or high-cycle metal applications, the cutting and recast of metal often adversely affects metal fatigue. Most issues are related to the heat-affected zone near the cutting area, causing localized metal fracture or brittleness. This skin of rough or damaged metal creates unique stresses and focal points for failure. Parts that endure very high stress or high cycle movement can and do fail as a result. By removing that metal zone and smoothing surfaces, a wide range of metals can see extended lives of 5–25 times from as-cut condition.
- Reduce mold-finishing costs. Recast is thin but very tough. In many mold applications, tooling specialists find that a lot of time and expense is dedicated to cutting that tough layer from complex surfaces. Electropolishing can be a cost-effective way to dissolve recast, saving tool and mold polishers valuable time. While in some cases the resultant finish can eliminate all hand polishing, in most cases, the savings are more in the range of 20–30%.
As mentioned earlier, major advancements have been made both in the control of the process and the range of metals that can be treated. Generally, the process can be controlled to ±0.0001" (3 µm) per surface. Larger parts or ones that require greater amounts of stock removal may see a larger tolerance range. Most conductive metals are candidates for the process, including many proprietary metals and fused bimetals.
In almost all cases, the metal comes out with an appearance of being plated. That bright surface now becomes a mere side effect in making metal parts perform better.
This article was first published in the October 2007 edition of Manufacturing Engineering magazine.