Development and application of metal crystal plasticity models

Abstract

Materials with HCP structures are not as widely used as other metals, partly owing to the difficulties that have been encountered in deforming them successfully. These metals display deformation behaviours called ‘twinning’ and ‘slip dislocations’. In simple terms, twinning refers to two crystals sharing the same lattice structure in a mirrored configuration. This induces strong anisotropic and asymmetric deformation behaviour. The crystal plasticity models developed by Hama’s team accurately predict the development of twinning when exposed to deformation stresses, meaning that simulations can be run to determine optimum forming processes considering the characteristic deformation behaviours of these metals. The models are also now being extended to cover other types of metals and take into account metal-forming time-dependency. Since face-centred cubic and body-centred cubic metals are much more commonly used than magnesium and titanium, partly owing to cost, the team is now developing models for these. The work has looked at time-independent deformation behaviour, but as Hama explains: ‘In industrial applications, it is well known that deformation behaviour changes according to forming speed.’ For instance, sheet metals that are stamped quickly into a shape will often exhibit spring-back, where the metal tries to regain its former shape. In contrast, when a metal is pressed slowly and held in position, the internal stresses can relax, and less spring-back is observed when the load is removed. Forming techniques are being developed to take these behaviours into account, and Hama explains: ‘We are planning to enhance our models to predict the effects of these new time-dependent forming techniques.’ The group foresee their models being used to create new metal alloys with required deformation behaviours, by calculating the crystal micro and mesoscale structures needed to achieve the desired macro properties. The team’s work looks set to save manufacturers time and money and increase the reliability and efficiency of the metal components of the future.