Showing papers by "Jianguo Lin published in 2010"

Development of a VGRAIN system for CPFE analysis in micro-forming applications

Abstract: To aid crystal-plasticity finite-element (CPFE) simulation for the forming of micro-components, a VGRAIN system has been developed to generate the microstructure of materials. Based on Voronoi tessellation and probability theory, grains and grain boundaries for micro-materials can be generated. A gamma-distribution function with three physical parameters of a material, which are the average, minimum, and maximum grain sizes, is used to describe the grain-size distribution of the material and a numerical method has been developed to link the gamma-distribution variables and the physical parameters of the material. The distribution of generated virtual gains using the developed VGRAIN system is compared with that of microstructural-examination data for a number of engineering materials, close agreements being obtained for the cases studied. Grain orientations, which are defined using two angles, related to the global coordinate system, have been assigned in the VGRAIN system according to probability theory. The crystal orientations for the virgin and deformed materials are represented using pole figures, so that the grain orientation before and after deformation can be compared and analyzed. The generated virtual microstructure, with grain-orientation information, is then input the commercial FE solver, ABAQUS/CAE, for further pre-processing for CPFE micro-forming simulation. A crystal-viscoplasticity material model for face-centered cubic metals is implemented in the FE code through the used-defined sub-routine, VUMAT/UMAT. CPFE analyses have been carried out to investigate the grain-size and orientation effects on the distortion of formed micro-pins.

Modelling of localised thinning features in the hydroforming of micro-tubes using the crystal-plasticity FE method

Abstract: The wall thickness of hydro-formed micro-tubes is not uniform due to the low ratio of the wall thickness and the grain size. The features of localised thinning are different from those of the traditional hydroforming process, and this is related directly to the ratio of the wall thickness and the grain size of the material, and also to the amount of deformation. Macro-mechanics finite element (FE) modelling cannot be used to simulate such effects encountered in micro-tube hydroforming processes. In this paper, a simplified plane-strain crystal-plasticity finite element (CPFE)-based modelling technique has been developed and used to capture the localised thinning features in the hydroforming of micro-tubes. The grain structures within the tube workpiece, and their distributions and orientations, are generated automatically using the developed VGRAIN system. A set of crystal-viscoplasticity models is implemented in ABAQUS/Explicit FE code through the user-defined sub-routine, VUMAT. Single-crystal and multi-crystal structures have been studied, and the localised thinning has been analysed for different microstructures of the material using the CPFE modelling technique. It is confirmed from the analysis that the localised thinning in the hydroforming of micro-tubes is affected significantly by the microstructure and grain orientations of the material.

An investigation of springback scatter in forming ultra-thin metal-sheet channel parts using crystal plasticity FE analysis

Abstract: Springback is a difficult problem to solve in sheet-metal forming. It is more difficult in the forming of ultra-thin metal sheet (UTMS) parts. This is not only due to the lesser plastic deformation in the pure bending of UTMS but also due to their variations in microstructure. This research concentrates mainly on the understanding of the effects of grains, their sizes, and distributions on the amount of springback and to identify at which conditions the crystal plasticity finite element (CPFE) modeling technique has to be used for the process simulation. A simplified 2D FE analysis coupled with a crystal viscoplasticity model has been used in this study. Grains, their distributions, and their orientations are generated automatically based on probability theories using the developed VGRAIN system. For the same control parameters, average, maximum, minimum grain sizes, and distribution patterns were in either regular or non-regular distribution. Four microstructures of the UTMS are generated based on the gamma distribution. Up to six grain orientations are assigned randomly to each grain structure. Thus, a large number of FE analyses have been carried out based on the variation of grain structures and orientations. These are used to investigate the effect of grain size and orientation on the scatter of springback values in the forming of UTMS channel parts.

Method of forming a component of complex shape from sheet material

Abstract: Method of forming a component of complex shape from an Al -alloy sheet or a Mg- alloy sheet. The method comprises the steps of: a) heating the sheet to a temperature below the solution heat treatment (SHT) temperature for the alloy; b) forming the heated sheet between heated dies into or towards the complex shape; c) heating the sheet to at least its SHT temperature and substantially maintaining that temperature until SHT has been completed; and d) quenching the solution heat treated sheet between cold dies and at the same time completing the forming into the complex shape or maintaining that shape.

Hybrid forming processes for production of lightweight high strength automotive panel parts

Abstract: The research concentrates on a heat treatable AA 6082 aluminium alloy. A set of unified constitutive equations has been developed and determined from experimental data. In addition to modelling viscoplastic flow of the material at different temperatures, the equations contain other two specific features. One is to predict the failure of the material under various deformation conditions based on continuum damage mechanics theories. The other is to model the precipitation formation and growth under straining and aging conditions; thus, the strength distribution of formed parts can be predicted via process modelling. The determined unified constitutive equations are then implemented into the commercial finite element code ABAQUS/Explicit via the user defined subroutine, VUMAT. A finite element process simulation model and numerical procedures are established for the modelling of a hot stamping and cold die quenching processes for a spherical part with a central hole. To validate the simulation result...

Point-based approach to enhance the quality of geometric data for CAE applications

Abstract: Commercial CAD surface modelling software is based almost exclusively on Bezier, B-spline and NURBS representations. These methods offer simple interactive shape modification and computationally efficient interrogations, but have some serious practical limitations. The root cause of all these problems is that the parametric polynomial methods are not geometry-based, and the trouble is compounded by the master geometry philosophy which treats the CAD model as if it were absolutely correct. An alternative purely geometric approach to surface modelling has been established, where the surface shape is characterised by a grid of points lying on the surface, but it is recognised that this approach would require major changes in existing CAE systems. This application paper explains the rationale to develop algorithms, which would plug into existing CAE software systems, so that the user could benefit from the point-based construction without disturbing the established mode of operation. The new point-based approach has achieved significant success in ensuring that form tools produce panels within a specified tolerance.

Review of Materials and Process Modeling Techniques for Creep Age Forming

Abstract: The Creep Age Forming (CAF) technique has been recently developed and used to manufacture aircraft and aerospace panel components with improved mechanical properties, increased shape stability and reduced fabrication cost. Precipitation hardening, stress relaxation and creep phenomena take place synchronously in the forming process. Significant research work has been carried out over the last two decades and the applications have been expanded. This paper reviews the most recent development of materials and process modeling techniques for this new forming process. Some challenges for future modeling investigation about the novel process are also discussed.

Modelling Thermomechanical Behaviour Of Cr-Mo-V Steel

Abstract: This paper presents a mechanism-based approach for modelling the thermomechanical behaviour of a Cr-Mo-V steel. A set of unified viscoplastic constitutive equations were employed to model dislocation density, recrystallisation and grain size during deformation. The evolution of dislocation density accounts for the build-up of dislocations due to plastic strain, the static and dynamic recovery and the effect of recrystallisation. Recrystallisation occurs when a critical dislocation density is reached after an incubation time, and grain size becomes smaller after such event. Gleeble compression tests were used to obtain Stress-strain curves and evaluate the microstructural evolution at different temperature and strain rate, and the material constants for the model were determined from the experimental data.