TOOL SURFACE TOPOGRAPHIES FOR CONTROLLING FRICTION AND WEAR IN METAL-FORMING PROCESSES

A conceptual framework is introduced for the design of tool surface to pographies in bulk metal forming processes. The objective of the desig n is to control friction to desired levels while minimizing wear of th e workpiece and tool surfaces and adhesive metal transfer between the workpiece and tool. Central to the design framework are the tool/workp iece interface properties of lubricant retention and interface permeab ility. Lubricant retention refers to the capacity of an interface to r etain lubricant rather than freely channel it to the exterior of the t ool/workpiece conjunction. Permeability refers to the capacity to dist ribute lubricant to all areas within the conjunction. These properties lead to the concept of two-scale surface topography consisting of a f ine scale background of interconnected channels on which is superimpos ed an array of coarser-scale cavities. Control of friction and wear is achieved by designing the tool surface topographies at these two scal es to address the unique tribological conditions of specific bulk meta l forming processes. The coarser scab is designed to ensure adequate s upply of lubricant within the conjunction. The finer scale is designed to ensure adequate delivery of lubricant to all parts of the conjunct ion where nascent workpiece surface is being formed The design concept s are illustrated with results from laboratory experiments using the r olling process as an example, and comparing the performance of various roll surface topographies under similar processing conditions. A two- scale surface topography consisting of hemispherical cavities distribu ted across a background surface of finer scale, interconnected channel s was shown to reduce friction compared to a single-scale ground finis h, but nor as much as a single-scale coarse topography consisting of d ensely-packed cavities produced by an electrical discharge treatment O n the other hand, the smoother cross-sections of the cavities, especia lly when elongated in the direction of greatest relative motion, produ ced significantly less wear than either of the single-scale tool surfa ce treatments. It is concluded that two-scale engineering of tool surf ace topographies based upon the concepts of lubricant retention and in terface permeability can provide a broad basis for achieving desired l evels of interface friction while minimizing workpiece surface wear an d adhesive material transfer in many metal-forming processes.