Creation of ultra-fine grained (UFG) materials by incremental bulk metal forming

Mechanical properties of ultra-fine grained and nano-crystalline materials are characterized by high strength but low elongation at fracture. As known especially for load-bearing structural components a higher level of breaking elongation is desirable. Crack initiation in ultra-fine grained and nano-crystalline materials compared with unmodified materials is favourable however crack growth is faster. In order to improve the elongation at fracture for UFG materials a number of approaches exist, as for instance the creation of a bimodal structure or the combination of dissimilar areas of micro-structure configurations in different sections of a component. The present article focuses on the theoretical fundamentals of manufacturing of a hybrid component by incremental forming and presents first results of experimental tests. Incremental forming methods are evaluated with respect to the creation of ultra-fine grained material - with increased strength and lower propensity for crack initiation - in combination with a ductile core or ductile inner wall respectively. The forming method of spin extrusion is investigated and first positive experimental results are obtained. Due to the productivity of these methods in contrast with other severe plastic deformation methods a high relevance in industrial applications is to be expected.