Experimental And Numerical Study On Constrained Groove Pressing Of Sheet Metals

Improving the mechanical properties and strength-to-weight ratio of materials is an important way of realizing the lightweight design of products, energy saving and consumption reducing and meeting the urgent requirements of modern production. Constrained groove pressing (CGP) is a new severe plastic deformation technique with great potentials of development and engineering application. It has significant advantages of fabricating bulk ultra-fine grained (UFG) sheet metals with excellent properties. In recent years, CGP has been successfully applied to producing UFG sheets of various pure metals and alloys, and grain refinement and significant improvement of mechanical properties of materials have been achieved. However, relevant researches on CGP at home and abroad have just started on, and grain refinement mechanisms and deformation rules of this technique still need to be studied systematically and thoroughly. Thus, comprehensive investigations about the influence rules of die structure, process parameters and deformation conditions on CGP process to reveal its deformation mechanisms have important theoretical and practical significances for optimization of process procedure, improvement of process results and promotion of engineering application of the technique.In this paper, CGP of several face-centered cubic pure metals is studied systematically via numerical simulations and process experiments. The influence rules and mechanisms of pass number, die structure, pressing procedure and deformation rate on microstructure and mechanical properties of materials, effective strain distribution and forming load are investigated in detail. Meanwhile, the internal relations between microstructure and macro properties of materials are explored in depth, and the grain refinement mechanisms of CGP process are revealed. The main contents and conclusions in this work are as follows.(1) The finite element (FE) modeling technique based on ABAQUS is studied, and an elastic-plastic FE model for a multi-pass CGP is built. The flowing laws and deformation behaviors of materials during CGP as well as the characteristics of forming load of CGP dies are investigated numerically. The deformation characteristics of shear, undeformed and interface regions of CGP sample and their variation rules with pass number are analyzed in detail. Meanwhile, the interactions among different regions and their influence mechanisms on process results are revealed. The distribution characteristics of effective strain both inside and at the surface of CGP sample and the influence rules of pass number and friction coefficient on effective strain distribution of sample and die load are researched.(2) Multi-pass CGP experiments of pure aluminum, pure copper and pure nickel sheets are carried out at room temperature. UFG microstructure at sub-micron level is obtained successfully, and the mechanical properties of materials are significantly improved. Utilizing room-temperature uniaxial tensile tests, SEM analysis of fracture morphology, Vickers micro-hardness measurements, metallographic and TEM observations and XRD, the influence rules and mechanisms of pass number on grain refinement and mechanical property variation of materials are investigated. The internal relations between microstructure and mechanical properties of materials are explored. In addition, the grain refinement mechanisms of CGP process and the effect of stacking fault energy on grain refinement results are revealed.(3) The influence rules of process parameters and deformation conditions such as friction condition, pressing procedure and deformation rate on CGP results of pure aluminum are studied experimentally, which establishes a theoretical foundation for engineering application of CGP technique. According to the stress-strain states at different positions of sample, a new method of regional division is proposed. The surface residual stress state of CGP pure aluminum sheets and its formation mechanisms as well as the effect of lubrication on it are investigated. Additionally, the relieving results of residual tensile stress at the sample surface during artificial and natural aging are investigated.(4) A serious of annealing schemes are made, and annealing treatments of UFG pure aluminum sheets processed by different passes of CGP are conducted. The microstructure evolution of materials is analyzed by virtue of TEM and SEM/EBSD techniques, and the influence rules of pass number and annealing temperature on microstructure and mechanical properties of UFG pure aluminum fabricated by CGP are investigated. The optimum deformation and post-annealing scheme is obtained, and pure aluminum sheets with superior comprehensive properties are produced. Meanwhile, the phenomenon of annealing strengthening occurring during low temperature annealing of materials is studied, and a kinetic analysis is done about the grain growth process of UFG pure aluminum.(5) Through metallographic observation, fracture morphology analysis of tensile specimens and electrical resistivity measurements, the variation rules and internal mechanisms of deformation homogeneity of CGP pure copper are researched in detail. The differences between CGP and unconstrained groove pressing (UGP) in strain accumulation rate, deformation homogeneity, grain refinement and mechanical property improvement are revealed by finite element analysis and experimental study. Meanwhile, coupling the constraint of CGP dies, a material constitutive model (M-ETMB model) is built based on dislocation theory. The influence mechanisms of die constraint on process results of CGP/UGP are deeply revealed with the model.(6) Combining numerical simulations with process experiments, the influence rules and internal mechanisms of die structure such as groove width and angle on effective strain distribution characteristics, forming load, grain refinement and mechanical property improvement and texture evolution of CGP pure nickel are investigated, and the effects of die structure on deformation characteristics of sample are analyzed in detail. Based on theoretical analysis, the relations between die structure and steady forming load as well as the conditions of die structure for ensuring stable shear deformation of sample are obtained. In addition, the initiation and propagation mechanisms of surface micro-cracks of CGP sample are studied, and morphological characteristics of fracture surface of sample are analyzed. Accordingly, effective approaches to delay the formation of cracks are proposed.