An investigation of macro-textured tool surfaces for hot stamping of aluminium alloys

Abstract

The objective of this study is to investigate a novel tooling technique, macro-textured tool surfaces, which is used for hot stamping aluminium alloys. Ribbed texture features were designed on the blankholders of two typical forming techniques, top-hat part drawing and cylindrical deep drawing. Effects of textures on flange material deformation and draw-abilities were investigated experimentally, numerically and analytically. Comprehensive analytical buckling models, to enable the prediction of flange wrinkling using different tool designs and process conditions, have been developed for both cold and hot stamping conditions. These models can provide an efficient and robust guide for tool designers for the development of stamping dies with textured features. The research work is concerned with three aspects: experimentations of top-hat part and cylindrical deep drawing, numerical analysis and establishment of analytical buckling models. Experimentations were carried out through two forming techniques. A top-hat part drawing, which involves essentially plane strain and a deep drawn cylinder which in the flange undergoes strain in three dimensions. A series of hot draw-ability tests utilising these two techniques were performed for investigating effects of textured features on hot draw-ability and material deformation. In addition, flange wrinkling tests using deep drawing technique were also performed to determine wrinkling occurrences using different texture designs and process conditions. Finite element (FE) models of above forming techniques were established. For cold stamping, a dislocation-driven material model was implemented via user-defined subroutine. The developed FE models were validated by corresponding experimental results. Further numerical simulations have been performed to investigate effects of tool textures and process variables on material deformation and hot draw-ability. Temperature fields of test-piece and tool surfaces were identified numerically and utilised to analyse the improved hot draw-ability. Furthermore, the flange wrinkling phenomena determined by numerical simulations were used to verify the analytical buckling models also. A range of buckling models for the flange wrinkling phenomena were developed based on different geometry assumptions of the unsupported flange material using macro-textured designs. Firstly, analytical buckling models, based on beam, plate and shell geometry assumptions, were developed in cold stamping condition. These models were validated by corresponding experimental and numerical results. Relationships between texture dimensions, process variables and wrinkling occurrence were characterised. Then, a buckling model for hot stamping was developed. In this model, a viscoplastic continuum damage mechanics (CDM) based material model was utilised to reflect the strength responses of aluminium alloys at elevated temperatures. The non-isothermal feature of hot stamping process was modelled. The high temperature bucking model was validated by corresponding experimental results. Correlations between process variables, temperature and forming speed, and wrinkling occurrence were obtained.