A constitutive model for modeling of the deformation behavior in microforming with a consideration of grain boundary strengthening

Micro-manufacturing technology is now getting more and more important in industries due to product miniaturization. Microforming, as one of the micro-manufacturing processes, has attracted much attention in the past decade. In microforming, the traditional metal-forming theories are often crippled or rendered totally inefficient in analysis of micro-deformation behavior due to the size effect arising from the part geometry scaling down from macro- to micro-scale. To reveal its deformation behavior, the size effect needs to be considered and appropriate constitutive models to be developed. In this research, by employing the surface layer and composite models, each grain in the microforming workpiece is divided into two portions, viz., grain interior and grain boundary. The flow stress of grain is determined by the ‘law of mixtures’ based on the flow stresses of grain interior and boundary, and in such a way that a new constitutive model is developed with a consideration of grain boundary strengthening. This new constitutive model considers both the grain and geometry size effects simultaneously. Using this newly developed model, the micro-bulk upsetting and the micro-tensile test of sheet metal are numerically simulated by Finite Element Method (FEM). The grain and geometry size effect on the flow stress of workpiece is thus investigated. The model is further applied to the micro-extrusion of CuZn30 alloy with three different grain sizes. The validity of the model is thus verified through physical experiments and FEM simulations of three different micro-plastic deformation processes.