SYSTEM AND PROCESS FOR PREFERENTIALLY LUBRICATING SHEET METAL FOR SUPERPLASTIC FORMING
Publication Date: 2005-Jun-17
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System and Process for Preferentially Lubricating Sheet Metal for Superplastic Forming
Superplastic forming (SPF) offers several advantages over conventional stamping techniques including increased forming strains, zero springback and low tooling costs. SPF is typically accomplished by blow-forming where a sheet blank is clamped in a die at superplastic temperatures and gas pressure is applied to one side. SPF is a net-thinning operation with no additional material flowing into the cavity, and although high temperature solid lubricants such as graphite or boron nitride are used, a relatively high sliding friction is generated at the contact interface of the sheet and die. Therefore, frictional effects must be managed to improve overall thickness uniformity and limit localized thinning.
Recent experimental work has shown that the absence of lubricant in some parts of the sheet can significantly improve formability. This results in either the ability to make parts with higher pressures and therefore faster forming times or to make parts that were not previously feasible with fully-lubricated sheets. For example, when forming over small radii material tends to thin excessively. Such thinning can be avoided by locally increasing the coefficient of friction in the area of the small radii such that the material sticks to the die. The simplest method of locally increasing the friction coefficient is to remove the lubricant from that area.
An extension of this idea is to use the preferential lubrication on the pre-forming step of a reverse gas pressure forming process. In this case, the sheet is first form...