Modelling of size effects in microforming process with consideration of grained heterogeneity

Abstract

Size effect is a special phenomenon in metal micro-forming process. As the deformation process is scale down to micro/mesoscale, the characteristics of single grain involved in the deformed region play a significant role on the material mechanical behaviours resulting in the invalidation of classical theories in microforming. This paper presents a newly developed material model in microscale on the basis of the grained heterogeneity (e.g. grain size, shape and deformability) and specimen dimension. Voronoi tessellation has been employed to describe the polycrystalline aggregate. The grain shape is controlled by the centroidal-voronoi algorithm to drive grains into steady state. Hardness of the grains obtained from Nano-indentation is used to identify the scatter of the grained deformability. Applying the new material model, the micro-compression test of pure copper is numerically simulated by finite element method (FEM). The influences of grain size and feature size on the deformation behaviours are discussed. The numerical simulation results are in good agreement with the experimental results in terms of the flow stress curves and profile of deformed parts. Based on the novel material model, a FE model of microcross wedge rolling is established and the obtained results show the strain of specimen core region increases with the magnification of grain size. © 2013 Elsevier B.V. All rights reserved. AbstractSize effect is a special phenomenon in metal micro-forming process. As the deformation process is scale down to micro/meso scale, the characteristics of single grain involved in the deformed region play a significant role on the material mechanical behaviours resulting in the invalidation of classical theories in microforming. This paper presents a newly developed material model in micro scale on the basis of the grained heterogeneity (e.g. grain size, shape and deformability) and specimen dimension. Voronoi tessellation has been employed to describe the polycrystalline aggregate. The grain shape is controlled by the centroidal-voronoi algorithm to drive grains into steady state. Hardness of the grains obtained from Nano-indentation is used to identify the scatter of the grained deformability. Applying the new material model, the micro-compression test of pure copper is numerically simulated by finite element method (FEM). The influences of grain size and feature size on the deformation behaviours are discussed. The numerical simulation results are in good agreement with the experimental results in terms of the flow stress curves and profile of deformed parts. Based on the novel material model, a FE model of micro cross wedge rolling is established and the obtained results show the strain of specimen core region increases with the magnification of grain size.

Influences of temperature and grain size on the material deformability in microforming process

“…grain size and temperature), a discrete distribution instead of a superposition of flow stress curves was observed due to the grain heterogeneity in micro scale. The scatter of flow stresses obtained from experiments reflects the inhomogeneous material behaviour in microforming [33,37]. Therefore, the scatter of flow stress was used to represent grain heterogeneity in this study.…”

“…In order to measure the surface asperity quantitatively, twelve generatrices uniformly spreading on the side surface of a cylindrical specimen were determined by the same method proposed by Lu et al [33]. After deformation, surface became rough as compared with the original sample surface.…”

A new micro scale FE model of crystalline materials in micro forming process

“…However, all the aforementioned models cannot explain the increasing data scatter from experiments and the localised deformation behaviour caused by the individual grain. To further reveal the size effects, the Voronoi models were developed [8][9][10]. The sample was divided into small subareas according to the Voronoi structures and the small subareas were assigned different mechanical properties.…”

Grain size effect of thickness/average grain size on mechanical behaviour, fracture mechanism and constitutive model for phosphor bronze foil

“…Chan et al [9] investigated the scatter effect of grain mechanical properties with micro-compression process and proposed a finite element model with consideration of grain size and the scatter effect of flow stress, which provided a basis for understanding and modelling of materials size effect in microforming process. Lu et al [10] enhanced this modeling by implanting Voronoi tessellation algorithm into pre-processor of finite element software. Moreover, Xu et al [11] studied the size effect on deformation behaviour and fracture feature by micro-blanking in terms of the ratios of different blanking clearances to grain size: C/D>1, C/D≈1 and C/D<1, and found out that the ultimate shearing strength reaches an extreme value when blanking clearance to grain size ratio is equal to 1.…”