A New Mechanism of Asperity Flattening in Sliding Contact—The Role of Tool Elastic Microwedge

Based on real-time observation of the workpiece surface in a series of Lo and Tsai’s (2002) compression-sliding experiment, it is found that the asperity contact area is much greater than that evaluated by the existing theorems such as the junction-growth theorem. With the aid of finite element analysis, it is verified that the tool sliding motion along with the minute elastic deformation (microwedge) of the tool surface around the asperity peaks increase the asperity contact area significantly even in a frictionless sliding. The microwedge induces a component of force along the sliding direction on the asperity. A combination of flattening and smearing effects can therefore aid in expanding the contact area. The greater the wedge angle, the stronger the propellent effect. An incremental model has also been developed to predict the evolution of contact area during sliding. The numerical simulation compares well with the experimental results. The new mechanism not only introduces an important tribological variable to forming processes, but also brings in a new concept of surface quality control for processes having a considerable sliding distance between workpiece and tool such as ironing, forging, and extrusion. New processes performing high relative sliding velocity can therefore be developed to ameliorate the brightness of products.