Page 39 - MetalForming-Oct-2018-issue

alkaline environments, because their surface oxide layers are stable at high pH. Aluminum readily forms a protec- tive oxide layer, but the oxides are only stable in a fairly narrow pH region on either side of neutral. This can present a problem when MWFs are kept at a pH above 9 to protect expensive steel tools, Kim says. At this high pH, alu- minum workpieces stain, and high- alkaline fluids can dissolve the protec- tive aluminum-oxide layer as fast as it can form, so MWFs require corrosion inhibitors.
Processes that generate fresh metal surfaces by machining or grinding away the oxide layer require the use of fluids that protect these surfaces from direct contact with tool surfaces or chips to prevent welding or adhe- sion, often referred to as “sticking.” Because stainless steels, aluminum and titanium form a pure oxide skin, the sticking effect is more pronounced for these metals than for steel or copper.
Mild corrosion leaves a yellow or gold stain on aluminum. This can occur when using the right type of MWF but for too long, resulting in depletion of the corrosion inhibitor. Using MWFs meant for ferrous metals can cause more severe corrosion that leaves a gray or black stain. Even the right MWF can stain an aluminum workpiece if certain additives (triazine biocides, for example) raise the pH of the fluid too high.
Semi-Solids? Solids? No Lubricant?
Not every metalforming operation uses a lubricant, McClure says. HSS containing boron (in the gigapascal- tensile-strength range), formed at 1202 to 1562 F, are quenched in the die to form very strong parts. Conventional lubricants cannot withstand these tem- peratures.
Magnesium often is machined dry, without a cutting fluid, because it does not need the cooling or the lubricating effects. MWFs are sometimes used in the more difficult machining opera- tions (e.g., deep-hole drilling) or work at high spindle speeds, but here the
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