Page 26 - MetalForming-Jun-2018-issue

Lubrication Advice
have become the popular choice for aluminum-stamping applications. Those oils, which contain chlorinated paraffins (CLPs), lead the way for their ability to stretch aluminum to its highest elongation point. Com- bined with good barrier and extreme- pressure (EP) protection, they allow the material to slip and flow during deep-draw processes. These compo- sitions also help reduce the impact of the aluminum-oxide layer on the surface of the substrate, which natu- rally adds some abrasiveness.
Stampers often form lighter-gauge aluminum parts using disappearing compounds as lubricants. These for- mulations find use when producing clean parts where the stamper seeks to eliminate post-stamping parts clean- ing. Traditional disappearing com- pounds use petroleum distillates as a carrier for a variety of boundary film lubricants. However, in addition to flammability concerns, these petrole- um distillates evaporate into the air and create volatile organic compounds ( VOCs).
New to the market, VOC-free disap- pearing compounds produce parts that can move directly to assembly or pack- aging without cleaning. For example, to manufacture the heat shield depicted in the photo on the previous page, the stamper changed from a petroleum- distillate vanishing product to a non- hazardous type, improving employee safety and the plant environment.
Renewable-Resource-Based Oils
Vegetable and many other types of renewable-resource-based soluble oils also have begun to make a targeted impact in the aluminum-forming mar- ket. In many cases they meet or exceed the performance of mineral-based sol- uble oils, while offering a non-petro- leum-oil environmental advantage. The seed-oil base forms a strong boundary film when adsorbed onto the alu- minum surface, preventing metal pick- up and allowing good sheetmetal flow.
Whether a mineral- or renewable- resource oil-based lubricant, new chlo- rine-free soluble oils now are pushing
Automotive-parts suppliers that typically deep-draw aluminum (shown here) using a chlorinated-paraffin straight oil can switch to a chlorine-free soluble oil and eliminate tearing and galling concerns while reducing lubricant use and cost.
the envelope on aluminum-stamping applications. As an example, consider that for several years, an automotive- parts supplier formed parts from 0.055- in.-thick Type 5754 aluminum, with a 4-in. draw depth, using a chlorinated- paraffin straight oil. By switching recently to a chlorine-free soluble oil, diluted 4:1 with water, the stamper eliminated sporadic tearing and galling while reducing lubricant use and cost. Furthermore, it eliminated all cleaning and application issues.
Now, don’t count out full-synthetic chemistry just yet. While older syn- thetics containing no petroleum or vegetable oil have lagged behind sol- uble oils, many newer products have risen to the occasion. Packed with newer polymer- and ester-additive technology, today’s full synthetics also prevent the pickup and drag normally experienced when forming aluminum.
Corrosion
As with most other substrates, com- patibility with the stamping lubricant is of great concern with aluminum, especially when diluting the lubricant with water. Although ferrous staining and corrosion aren’t of concern here, metalformers will prefer to avoid any
staining or pitting beyond the normal aluminum-oxide layer buildup.
Different alloys and finishes come with varying built-in corrosion resistance. For example, Types 5052 and 6061 aluminum offer excellent corrosion resistance and find use due to their good formability. How- ever, higher-strength alloys such as Types 7075 and 2024 offer average
or poor corrosion resistance.
When dealing with different alloys, certain lubricants are naturally passive while other lubricants must be buffered for pH and include additional corrosion inhibitors to prevent excessive attack on the substrate surface. Straight oils can offer very good corrosion inhibition due to the absence of water. Dry films also perform well, once any water car- rier has evaporated. The extent of cor- rosion from water-soluble lubricants can vary depending on the pH of the water and the lubricant, as well as water quality and the additives in the product. Higher alkalinity can destroy the aluminum-oxide protective layer, which leads most water-soluble prod- ucts to require corrosion inhibitors for
proper protection.
To minimize the propensity for
staining or corrosion in metalforming aluminum, metalformers should per- form quick stain tests on the substrates. To carry out these tests, mix the water- soluble lubricant to the desired con- centration, then cut an aluminum coupon and submerse it half-way into the diluted lubricant. After 24 hr. soak- ing at room temperature, inspect the coupon for staining and discoloration.
Post Processing
After forming, the presence of lubri- cant residue on the stamped aluminum parts can impact secondary processes such as cleaning, welding and painting. Depending on the severity of the form- ing, the remaining residue may be heavy or light, oily or sticky, thick or thin, and possibly pooled in corners or all over the part. Whatever the state, the stamper may have to plan for sec- ondary processing to remove the residue.
24 MetalForming/June 2018
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