Surface accuracy achieved by upsetting of preforms generated by laser rod end melting

Abstract

Cold forming generally allows the reproduction of parts with a high production rate and low waste of material. For this reason, many small parts for example of the body of a smartphone undergo at least one forming operation during their fabrication, e.g. screws, brackets or springs. However, when the size of forming products is reduced to the micro range, size effects appear and affect the forming process. One size effect leads to the fact that conventional multi stage upsetting processes cannot be scaled to micro range accordingly. As a consequence, the maximum achievable upset ratio decreases from 2.3 in macro range to values below 2 in micro range. This reduces the efficiency of the upsetting process significantly as more upsetting stages have to be carried out to reach the desired shape of the forming product. A very promising approach to reduce required forming stages is the laser rod end melting process which takes advantage of a size effect. Herein, the lower end of a rod is melted by the energy of a laser beam. The molten part of the rod automatically forms spherical due to surface tension. This droplet-shaped part is called “preform”. After total cooling, the preform is upset within a single stage cold forming operation to achieve the desired shape. In this paper, results of the upsetting operation with regard to surface accuracy of the forming product are presented. It is shown that surface characteristics with dimensions as small as 500 nm can be reasonably reproduced.