Showing papers by "Jianguo Lin published in 2012"

Investigation of deformation and failure features in hot stamping of AA6082: Experimentation and modelling

Abstract: This paper introduces a set of coupled viscoplastic constitutive equations for deformation and damage in hot stamping and cold die quenching of AA6082 panel parts The equation set can be used to predict viscoplastic flow and plasticity-induced damage of AA6082 under hot forming conditions Deformation and damage depend upon a coupled set of evolving internal state variables, eg dislocation density, which in turn is affected by thermally activated and deformation-dependent recrystallisation and recovery A phenomenological description of damage is derived based on the expected physical scaling with temperature, strain and strain rate The resulting equations were implemented in the commercial software ABAQUS via the user-defined subroutine VUMAT for carrying out forming process simulations An experimental programme was designed, and specialised testing facilities developed for calibrating and validating the FE process modelling results A good agreement between the process simulation and the experimental results has been achieved This confirms that the physical dependencies in the constitutive equations are correctly formed, and that the equations and FE model can be calibrated and used for hot stamping of AA6082 panel parts Further, forming process optimisation was carried out using the model to identify the optimal forming parameters for a basic panel part with a circular hole in the middle The study concludes with a discussion of the potential impact of the constitutive model, experimental characterisation and modelling results on AA6082 panel parts manufacture

An efficient closed-form method for determining interfacial heat transfer coefficient in metal forming

Abstract: During hot/warm metal forming processes, interfacial heat transfer between work-piece and dies has an important effect on the temperature distribution, microstructure evolution and mechanical properties of formed parts. An efficient numerical model has been developed for the determination of interfacial heat transfer coefficient (IHTC) at different forming and contact conditions. Based on FE heat transfer analysis for a typical tester, a 1D heat transfer model is proposed. A numerical procedure has been developed and implemented for determining the surface temperatures of work-piece and die, the contact heat flux, and the IHTC. The numerical model has been validated by the comparison of experimental, numerical and finite element results. For gas turbine blade hot forging process, the effects of pressure, glaze thickness and surface roughness on the IHTC of Ti-6Al-4V work-piece and H13 steel die were investigated. The application results indicated that this numerical method can provide a reliable means of predicting the IHTC in hot metal forming conditions.

Microstructure evolution in metal forming processes

Abstract: Part 1 General principles: Understanding and controlling microstructural evolution in metal forming: an overview Techniques for modelling microstructure in metal forming processes Modelling techniques for optimising metal forming processes Recrystallisation and grain growth in hot working of steels Severe plastic deformation for grain refinement and enhancement of properties Part 2 Microstructure evolution in the processing of steel: Modelling phase transformations in steel Determining unified constitutive equations for modelling hot forming of steel Modelling phase transformations in hot stamping and cold die quenching of steels Modelling microstructure evolution and work hardening in conventional and ultrafine-grained microalloyed steels Part 3 Microstructure evolution in the processing of other metals: Microstructure control in creep-age forming of aluminium panels Microstructure control in processing nickel, titanium and other special alloys

Effect of cross wedge rolling on the microstructure of GH4169 alloy

Abstract: The metal microstructure during the hot forming process has a significant effect on the mechanical properties of final products. To study the microstructural evolution of the cross wedge rolling (CWR) process, the microstructural model of GH4169 alloy was programmed into the user subroutine of DEFORM-3D by FORTRAN. Then, a coupled thermo-mechanical and microstructural simulation was performed under different conditions of CWR, such as area reduction, rolling temperature, and roll speed. Comparing experimental data with simulation results, the difference in average grain size is from 11.2% to 33.4% so it is verified that the microstructural model of GH4169 alloy is reliable and accurate. The fine grain of about 12–15 μm could be obtained by the CWR process, and the grain distribution is very homogeneous. For the symmetry plane, increasing the area reduction is helpful to refine the grain and the value should be around 61%. Moreover, when the rolling temperature changes from 1000 to 1100°C and the roll speed from 6 to 10 r·min−1, the grain size of the rolled piece decreases first and then increases. The temperature may be better to choose the value around 1050°C and the speed less than 10 r·min−1.

Experimental and numerical investigation of localized thinning in hydroforming of micro-tubes

Abstract: An experimental program has been carried out for hydroforming of stainless steel micro-tubes. Under careful control, it was found that failure takes place randomly, which is significantly different from observations of failure in hydroforming of macro-tubes, where failure loads and locations are predictable. This occurs because wall thinning of micro-tubes in forming processes is non-uniform, i.e. localized necking takes place randomly. To investigate the localized thinning mechanism, an integrated crystal plasticity finite element (CPFE) modeling system has been developed. In this paper, a simplified plane strain CPFE model is presented and used to investigate the localized thinning and failure features in hydroforming of micro-tubes. The crystal plasticity equations were implemented in the ABAQUS/Explicit FE code through a user-defined material subroutine, VUMAT. Single crystal, three-grain and polycrystal FE models were generated to study the localized thinning/necking mechanism and the effect of differing adjacent grain orientations, as well as the number of grains across the smallest specimen dimension, on the necking features. It has been confirmed from the analyses that the localized thinning observed in hydroforming of micro-tubes is significantly affected by the microstructure and grain orientations of the material.

Investigation of Material Flow in Forging Bi-metal Components

Abstract: High performance components, such as poppet valves, connecting rods, and pistons are manufactured from materials that pro- vide the necessary performance characteristics to withstand tough operating conditions. These materials usually possess high values of yield strength, Young's modulus, fatigue strength, and maintain them over a range of temperatures. Low density is also useful in compo- nents subjected to rapid speed changes. However, high performance materials, such as titanium alloys and stainless steels usually have a high density or are too costly to be used extensively. In many applications, high performance properties are required on particular surfaces or contact points, and are not necessary throughout the entire body of the component. One way of achieving this is to make components with more than one strategically located material. For practical reasons, normally the maximum number of alloys would be two. The work described in this paper is a preliminary investigation of the use of forging to produce bi-metallic components, through the production of a spur gear form. The experimental and simulation work described in this paper, provides knowledge of the effects of process variables on the outcome of forging bimetallic gears.

An Investigation, Using Standard Experimental Techniques, to Determine FLCs at Elevated Temperature for Aluminium Alloys

Abstract: An experimental procedure has been developed for the determination of FLCs at elevated temperatures. The GOM ARGUS system was employed for measuring surface strain based on pre-applied grids (pattern), and limit strains were determined according to the ISO 12004-2:2008 standard. Forming limit curves (FLCs) have been determined for AA5754 under warm forming conditions in an isothermal environment. The tests were carried out at various temperatures up to 300C and forming speeds ranging from 5 – 300 mm s. Results reveal the significant effect of both temperature and forming speed on FLCs of AA5754. Formability increases with increasing temperature above 200C. Formability also increases with decreasing speed. The presented FLC results show that the best formability exists at low forming speed and the high temperature end of the warm forming range.

Cohesive zone representation and junction partitioning for crystal plasticity analyses

Abstract: SUMMARY A novel scheme is presented for incorporating finite thickness cohesive interfaces in virtual grain structures for crystal plasticity finite element (CPFE) analyses of intergranular crack initiation and propagation. A Voronoi tessellation model is used to define the virtual grain structure, with automatically generated nonzero thickness cohesive zones (CZs) representing the grain boundaries and multiple junctions. An efficient grain boundary offsetting algorithm is presented, and issues related to automatically partitioning multiple junctions are discussed. Two feasible junction partitioning schemes are presented, the second of which has the advantage of partitioning junctions using uniform quadrilateral elements and naturally defining their normal and tangential directions. For the second scheme, a rule-based method is presented that carries out the preliminary meshing of CZ junctions, including data representation, edge event processing, and cut and trim operations. A virtual grain structure modelling system, VGRAIN, is introduced to implement the proposed CZ junction partitioning method and directly generate meshed virtual grain structures with CZ grain boundaries for CPFE studies. To demonstrate the proposed junction partitioning and CZ representation schemes, two finite strain CPFE simulations are presented for plane strain uniaxial tension and three-point bending, demonstrating large-scale crack initiation and propagation under shear and opening modes. Copyright © 2012 John Wiley & Sons, Ltd.

Quantifying damage accumulation during the hot deformation of free-cutting steels using ultra-fast synchrotron tomography

Abstract: Many different approaches have been proposed to simulate the nucleation and evolution of damage during the hot forming of steels. However, there is a lack of time-resolved, three dimensional quantification of the evolution of damage, a requirement for validation of the kinetic and morphological model predictions. One very significant industrial case is the hot forming of free-cutting steels (FCS), where small additions of heavy metal inclusions are added to enhance the machinability and surface quality of the steel. In this paper, we present the in situ ultra-fast synchrotron X-ray tomographic observations and quantification of FCS during hot deformation, including measurement of applied load. This allowed the correlation of strength to the different stages of the cracking process. The results are augmented with high spatial resolution interrupted studies. The interrupted tomographs enabled the quantification of the volume fraction, equivalent diameter, spatial distribution and orientation of inclusions and damage at various strain levels. The combination of these two studies provides a benchmark experiment for the validation of physically-based finite element models, both directly, and via constitutive equations for the time/temperature dependent effects of dislocation density, damage, strain rate and temperature.

The Influence of Process Parameters during Hot Stamping of AA6111 Aluminum Alloy Sheet

Abstract: The AA6111 aluminum alloy sheet is widely used in auto-body manufacture. It can make use of good plasticity under high temperature to form more complex parts by using the hot stamping. The influence of process parameters in hot stamping of AA6111 aluminum alloy sheet is investigated through numerical simulation in this paper, including blank holding force (BHF), friction coefficient, stamping velocity and initial forming temperature. Finally forming defects of numerical simulation are verified through the hot stamping experiments. The results show that it can effectively avoid wrinkling and fracture by controlling the BHF, good lubricant is in favor of forming and numerical simulation can accurately predict forming defects to guide the production.

Development of Constitutive Model of EA4T High-Speed Train Shaft Steel Based on Internal-State-Variable Method

Abstract: The internal state variable method may be the best tool，offering researcher the model framework，certainly based on physical mechanism. A constitutive equation model framework，reported in this study， has been proposed to predict the distribution of stress of EA4T steel. The constants ，arising in this model ，are determined using an evolutionary programming (EP) optimization technique . A good agreement between the computational and experimental results was found.

2D FE Micromechanics Modelling of Honeycomb Core Sandwich Panels

Abstract: A repeating unit cell 2D finite element modelling procedure has been established to model the mechanical behaviour of honeycomb core sandwich panels (e.g. Young’s modulus, energy absorbed, etc.). Periodic boundary conditions have been implemented within the model to simulate an infinitely long sandwich panel. An analytical solution using Timoshenko beam theory has been developed to predict the Young’s modulus of the honeycomb core, and this has been compared with the FE model results; it is found that there is good agreement between the two values. The FE model can shed light on the mechanics of more complex 3D metal foams.

Investigation of FE model size definition for surface coating application

Abstract: An efficient prediction mechanical performance of coating structures has been a constant concern since the dawn of surface engineering However, predictive models presented by initial research are normally based on traditional solid mechanics, and thus cannot predict coating performance accurately Also, the high computational costs that originate from the exclusive structure of surface coating systems (a big difference in the order of coating and substrate) are not well addressed by these models To fill the needs for accurate prediction and low computational costs, a multi-axial continuum damage mechanics (CDM)-based constitutive model is introduced for the investigation of the load bearing capacity and fracture properties of coatings Material parameters within the proposed constitutive model are determined for a typical coating (TiN) and substrate (Cu) system An efficient numerical subroutine is developed to implement the determined constitutive model into the commercial FE solver, ABAQUS, through the user-defined subroutine, VUMAT By changing the geometrical sizes of FE models, a series of computations are carried out to investigate (1) loading features, (2) stress distributions, and (3) failure features of the coating system The results show that there is a critical displacement corresponding to each FE model size, and only if the applied normal loading displacement is smaller than the critical displacement, a reasonable prediction can be achieved Finally, a 3D map of the critical displacement is generated to provide guidance for users to determine an FE model with suitable geometrical size for surface coating simulations This paper presents an effective modelling approach for the prediction of mechanical performance of surface coatings

Comparison of Thinning Features of a Hot and Cold Stamped Aluminium B-Pillar Model for Passenger Cars

Abstract: To overcome low formability and springback problems in forming aluminium panel parts, a novel process of hot stamping is introduced in this paper. It is good plasticity and low deformation resistance when aluminum alloys under high temperature, which makes it possible to form complex parts such as B pillars. Finite element method is used to analysis the thinning regions in this part, it can be acquired that the quality of the part in hot stamping is better than in cold stamping.