Forming Safety Critical Panel Beams with Efficacious Graded Microstructures for Passenger Cars
In recent years, a hot stamping with cold die quenching process has been developed in producing high strength structural components made of quenchable steels, particularly for automotive safety components, such as side-door impact beams, bumper beams, side rail members and anti-intrusion beams. The process includes heating the quenchable steel blank to above the austenite temperature, giving sufficient time to allow full transformation to austenite, then transferring the workpiece to the press and forming the part quickly with a water-cooled die-set. The material is then held in the cold-die set for about 15 seconds to achieve rapid cooling and enable the austenite to transform to martensite and bainite. Thus the material can be formed with lower forming forces, achieving higher strength.
Based on the significant work which has been carried out within the group for the development of modelling techniques to predict microstructure evolution under the thermo-mechanical processing conditions of this process, the aim of this project is to develop techniques to enable panel parts to be formed with tailored microstructure distributions (and hence tailored mechanical property distributions) to achieve, for example, maximum stiffness, which translates into maximum vehicle energy absorption in a collision.
The research project, which started in Oct 2009, is cooperated with SAIC Motor Corp. The following objectives will be achieved at the end of the project:
- Develop modelling techniques to predict the microstructure distribution of formed parts. Particular concentration will be on Bainite and Martensite for a front bumper.
- Simulate impact for panels having distributed mechanical properties. Define the required mechanical property distribution for a front bumper for maximum vehicle impact energy absorption.
- Establish testing techniques to validate the modelling results.
- Develop new forming techniques so that tailored distributed microstructures of formed parts can be created and controlled.
- Develop new tooling concepts to enable the parts to be formed with the specified microstructure distribution.
Multiscale Modelling for Next Generation Steels
Porosity in the cast bloom may lead to reduced strength or inconsistency within the rolled product. Currently there is a lack of knowledge regarding the mechanisms of consolidation and there is a need to develop accurate modelling equations to predict the behaviour of a porous cast.
The aim of this project is to develop a robust multiscale modelling framework for the accurate prediction of central consolidation of as-cast products, taking into account physical length scale issues, porosity distribution, thermo-mechanical conditions, oxidation and diffusion bonding, i.e. healing of pores. A multi-scale viscoplastic framework and stochastic modelling will be adopted for modelling consolidation during both mechanical and multipass rolling. Further development of mechanical testing methods will also be addressed in this proposal, together with a more reliable method of characterising the porosity distribution of as-cast products.
Model materials will be created to characterise diffusion bonding features at different temperatures and stress-state conditions. The bonding strength and the change of mechanical properties for the materials as-cast and as hot rolled will be modelled and predicted.
Aluminium Foam Sandwich Panels for Lightweight Wing Structures
The main aim of the project is to work on the modelling and development (design and manufacture) of novel sandwich structures for new lightweight aircraft wing designs lacking ribs and stringers. The advantages of metallic foam cores relative to alternative core materials include:
- Can be made isotropic
- Can be manufactured with integral skins
- Can be open cell for very low density and reduced moisture trapping
- Can be readily made in curved shapes
- Can combine low density with good bending stiffness and strength
- Rapid, automated manufacturing
- Tool drop impacts lead to visible damage and core densification
The overarching objective of the work is to develop multi-scale computational models for predicting the performance and damage of aluminium foam core sandwiches under relevant loading scenarios (e.g. wing up/down bending, tool drop impact, etc.), and to compare the predictions to those of other sandwich structure designs, such as tied foam core (or conventional honeycomb and truss cores). Basic testing on aluminium foam cores (tension/compression and densification) and aluminium foam core sandwiches (four point bending) is to also be conducted.
Linkage of Intermediate Damage in Mechanics to Material and Fracture Models
The main aim of this project is to provide quantitative links between the material response in metallics and the material theories and models developed within QinetiQ. This involves an integrated series of tests to determine quantitative information concerning the level of damage (i.e. void growth, cracks, etc.) in sample materials. The QinetiQ models are used in the hydrocode simulations of high strain rate loading in a variety of applications ranging from shaped charge jet formation, blast interaction with structures and general impact with structures. There is great interest in improving the fidelity of these simulations to give a quantitative indication of localisation and failure on the response of a structure.
Multiscale Modelling of Delayed Hydride Cracking in Fuel Cladding Materials
This project is focused on the multiscale modelling of hydrogen embrittlement of fuel cladding materials in pressurized water reactors. Techniques ranging from ab initio methods such as density functional theory (DFT) and microscale methods such as two-dimensional discrete dislocation dynamics (DD) are employed to study the interaction between hydrogen and dislocations on the two length-scales, and how this affects the embrittlement process. A number of viable hydrogen related embrittlement mechanisms have been proposed, which includes the formation of brittle hydrides (DHC), and it is the aim of this project to understand better a number of these mechanisms, and how one might use the theories already available to develop the theory on DHC. Current work includes an ab initio study of hydrogen in strained and unstrained zirconium, the interaction between hydrogen atoms in zirconium, and also between hydrogen and defects. Additionally, work is being carried out using the DD simulation to understand the effects of the elastic field screening of dislocations by hydrogen atmospheres.
- Dr Daniel Balint
- Dr Mark R Wenman
- Prof Adrian Sutton
- EPSRC (Centre for Doctoral Training in Theory and Simulation of Materials)
Miss Jassel Majevadia
An Investigation of the Hot Forming Quench (HFQ) Process for AA6082 Aluminium Alloys
This research mainly looks into the mechanical properties and microstructure evolution during the thermal treatments of AL-alloys, particularly the Solution Heat Treatment (SHT) of 6xxx aluminum alloys during the novel Heat, Form & Quench (HFQ) sheet metal forming operation. Age hardening sheet aluminum alloys have recently gained a large amount of interest from the automotive industry, for applications such as light weight structural components. The material must be thermally treated to obtain a high strength comparable strength to competing materials such as steel. The main objective of this work is the design of innovative experimental procedures and the development of modelling tools to accurately investigate and describe the newly HFQ process for AA6082 aluminium alloys. The mechanical and microstructural properties ofAA6082 aluminium alloys are investigated.
Materials Modelling for Inertia Welding
Inertia welding is a solid state welding process, in which one component is stationary and the other is attached to a flywheel which is accelerated to a certain rotating speed and then released to provide an inertial force, while in the same time an axial pressure is applied, bringing the rotating component into contact with the stationary one and joining the two parts together with the heat generated by friction. Because of its localised weld and the ability to join dissimilar materials, it is finding use in aerospace industry (such as turbine wheel and shafts) which calls for the very best performance of engines.
During inertia welding, heat is built-up at the interface and dissipated mostly through the joining components and there is inevitably some heat affected zones (HAZ) near the weld interface in both components. Under applied pressure/load, the materials near the weld interface undergo severe deformation. This would lead to changes in microstructure and properties of the materials in this region. As part of the TSB project - PROcess MOdelling for Tomorrow's Engine (PROMOTE), a unified approach is employed to model these changes of nickel superalloy (RR1000) and steels (Super CMV and Aermet 100). A set of rate equations is developed, which simultaneously considers the evolution of microstructure and of dislocation density due to deformation, static and dynamic recovery and recrystallisation, as well as their effects on the flow stress. The material constants are optimised through the experimental data from thermomechanical testing using Gleeble machine.
These material models are then incorporated into FE code to simulate inertia welding process.
Modelling of Phase Transformation in Hot Stamping of Boron Steel
With the development of the automotive industry, car manufacturers have increasing concerns with car safety requirements and the relevant environmental impacts. Application of high strength light materials becomes favorable across the world. As a consequence, a new forming approach termed hot stamping and cold die quenching, which is particularly designed for producing critical safety components of passenger cars by using advanced high strength steels, was developed. This is a hybrid-forming process which enables the steel blanks to be formed and heat treated in a single operation. The material is formed at the softest state, and can be strengthened during quenching in cold dies. Therefore, best ductility, highest strength and minimized spring back can be achieved.
Theoretical study and modelling work is urgently required for this newly developed forming process. This research work fills in the gap by modeling the material behavior during the hot stamping and cold die quenching process. Mechanisms of phase transformation of boron steel have been studied; unified constitutive equations which can predict austenite, bainite and martensite phase transformations were introduced. Mechanical property of the material has also been investigated, and a visco-plastic damage model was developed. Finite element analysis has been conducted by implementing the developed models into ABAQUS via VUMAT. The simulation result agrees well with experimental test. A better understanding of the hot stamping and cold die quenching of boron steel has been achieved which is beneficial for both car manufactures and steel companies.
Multiscale Modelling for Multi-layered Surface Systems (MSS)
In order to combat surfaced related degradations, a large variety of different surface engineering systems, with thickness ranging from the nano-meter scale to the millimetre scale, have been developed in past decades. Typical examples, such as Fig.1, can be found in high-speed cutting tools, where multilayered surface coating systems, are used to enhance the wear and fatigue performances.
Current designs of such multilayered surface systems are largely based on empirical experiences using trial-and error method. Hence, the mechanical performance of such surface systems is not reliable and failure can not be predicted with sufficient accuracy.
The purpose of this project is to establish integrated, generic, robust multiscale materials modelling techniques for the design and performance prediction of multilayer surface systems (MSSs). In particular, it has the following objectives.
- Develop molecular dynamics techniques to model atom deposition processes and the atomic structure and interfaces to achieve optimal coating microstructures;
- Develop multiscale modelling and corresponding experimental techniques to determine nano and crystal behaviour of each layer of a surface coating and the macro behaviour of MSSs;
- Develop an integrated multiscale modelling approach to link molecular dynamics (nano), crystal plasticity (micro) and continuum mechanics;
- Develop modelling techniques and software systems for design, processes and applications of multiscale MSS;
- Develop modelling-based design methodology for optimised MSSs for high performance components aiming for improved lifetimes and reduced market lead time.
Constitutive Modelling and Simulation for Creep Age Forming Process
Creep Age Forming (CAF) is a new forming method based upon the stress relaxation or creep phenomenon that occurs during the artificial age-hardening of a metal in a vacuum autoclave, which has been recently developed for the manufacture of heat treatable aluminium alloy panel components, particularly, aircraft wing panels, with high accuracy and complex multiple curvature. Unlike other conventional forming methods, such as roll forming, brake forming, shot-peening or stretch forming, CAF components have lower forming stesses and residual stresses, which on the one hand decrease the possibility of processing crack, plastic instability in the forming process, and on the other hand improve the long-term performance of the components since it improves the resistance to both fatigue and stress corrosion cracking.
Significant research work has been carried out over the last decade and the applications have been expended. But there are still a lot of problems exist for better applications of such technique. One of the most important things is the prediction of springback. While the foremost thing referring to springback prediction is to establish a proper micro-macroscopical coupled constitutive equations for the component. Until now the interaction between stress relaxation and ageing precipitation still didn't be totally understood. Moreover a general micro-macroscopical coupled constitutive equations and relative database package, that is materail subroutine suitable for all materials undergoing CAF process will also be a challenge for the future.
An integrated system to carry out micro-macroscopical coupled simulation for the creep age forming of heat treatable metals using commercial FE codes still needs to be further improved. More attention should be paid on the microstructure analysis to set up the relationship among the forming conditions, microstructures and mechanical properties for the CAF process in the long run.
Representation of Virtual Microstructure for Polycrystalline Materials
My research relates to representation of virtual microstructure for polycrystalline materials to facilitate the micromechanical simulations, especially the CPFE (crystal plastic finite element) simulations.
A type of controlled Poisson Voronoi tessellation (CPVT) has been proposed and applied as a basic mechanism to generate the virtual microstructures in agreement with the designated grain distribution properties. Measurements from the quantitative metallurgy, such as including mean grain size and proportion of grains, are used to as control parameters to automatically produce the tessellation with expected regularities.
Based on the proposed CPVT mechanism, a series of research work have been conducting including 2-dimensional microstructure representation, cohesive layers formulation and junction partition and 3-dimensional microstructure representation, cohesive layers formulation and junction partition.
A software system is building to systematically implement the proposed methods and to help to formulate the virtual microstructure in the Finite Element computational environments, particularly Abaqus. Virtual microstructures obtained in our system can be directly imported into the Finite Element software.
CPFE simulations have been performing during all the research work. The effects of regularity on crystal plasticity are ready to be discussed and also the inter-grain behaviours are going to be studied.
Modelling of Superplastic Forming
Motivations of the Research
Superplastic forming (SPF) is being used increasingly by a range of UK and international industries, e.g. aerospace (Ti-alloy gas turbine components), high-performance automotive (Al- and Ti-alloys), architecture and defence (Fig.1&2).
Formability at lower temperatures and higher strain rates reduces cost and energy.
Although SPF is the end goal, it is first necessary to model phenomena (e.g. grain boundary sliding and recrystallisation) relevant to other material deformation modes present in industrial materials processing.
No comprehensive multi-scale framework exists for modelling superplastic deformation without prescription of a controlling mechanism. Natural selection of the active mechanism, for a range of materials, strain rates and temperatures, is the novel aim of this work.
Objectives of the Research
Develop new continuum rules for grain boundary sliding, decohesion and boundary/core interactions, informed by discrete dislocation (Fig.3) studies used to investigate the cooperation between dislocation core plasticity and grain boundary evolution and deformation.
Implement the new rules and validate against benchmark experiments of superplastic deformation. Study and simulate superplastic forming model problems of industrial relevance, such as superplastic forming of Ti-alloy sheet for aero-engine housing panels. Develop and disseminate the software package to industry partners.
Advanced Damage Modelling of Free Machining Steels
Steels are often machined after being hot-formed. Hot-forming requires steels that are ductile but this property makes the material difficult to machine. A compromise is made by adding alloying elements, such as Lead and increasingly Bismuth and Tellurium, the new steel alloys are known as free machining steels. The additions markedly increase the machinability but have a deleterious effect on the ductility. Damage in the final products requires that they be scrapped and re-melted, which is costly. The aim of this project is to develop a viscoplastic damage model for a variety of free machining steels.
Most hot forming operations are in large batches; therefore any small percentage of scrapped products can have a massive cost. Therefore, a model to accurately predict damage will be a useful aid in avoiding it. To produce a model the damage mechanisms have to be understood. The damage mechanisms change depending on the temperature, strain rate, microstructure and stress state.
It is possible to measure ductility relative to the changing temperature and strain rate using uniaxial tests. However, to measure the effects of stress state on damage a suitable representative characteristic, such as the stress triaxiality, can be used. The stress triaxiality is considered a significant factor in nucleation and growth of damage. A test piece has been designed and produced specifically to investigate this factor. By compressing the test piece and examining the deformed microstructure the relationship between damage and stress state can be understood.
Dr Michael Kaye
Modelling of Interfacial Problems at the Micro-scale
This project will concentrate on interfacial problems related to mechanical bonding/debonding and the relative movement and reorganization of boundaries between grains or between grains and inclusions.
The main aim of the research is to develop micro- and/or nano-scale modelling methods to study the mechanical behaviour of grain boundaries and inclusion-matrix material interfaces under different deformation conditions. This will include implementation of rules derived on first principles for defect/interface interactions, and subsequent studies to validate the behaviour in select model problems against existing experimental evidence. The ultimate aim is to develop fundamental models that describe grain boundary and interface behaviour and can be implemented in standard FE codes for larger multiscale simulations. In this way, specific applications of industrial relevance, such as the deformation mechanisms active under hot forming conditions, can be understood and damage nucleation and growth features can be predicted.
A New Process Development for Efficient Precision Gas Turbine Blade Forming
The performance of gas turbine (GT) blades plays a significant role in the aircraft industry. Dimensional accuracy has a great effect on GT blades forging. Distortion, which is defined as the deviation between designed shape and forged shape, has a crucial influence on the dimensional accuracy of GT blades. It is found that elasticity induced springback and thermal distortion are the two main causes contributing to the GT blade distortion. Elasticity induced springback cannot be reduced due to metal's elastic property. In order to reduce the effect of thermal distortion a new process is developed for the production of high precision blades.
Titanium alloy GT blades are usually forged at elevated temperatures. After forging, unloading and cooling, non-uniform thermal contraction causes thermal distortion in forged GT blades. Based on the experimental observation that thermal distortion is the most severe at the first several seconds during cooling, the GT blade is held between the two dies to minimise the thermal distortion. The determination of holding time is a key problem to be solved.
The main tasks are divided into experimentation and modelling. Heat transfer tests, thermal-mechanical compression tests and forming tests will be carried out. The tests enable the determination of heat transfer features, obtaining thermal-mechanical properties and allow evaluation of the thermal distortion of the blades. Heat transfer coefficients will be determined using reverse engineering techniques. A set of viscoplastic constitutive equations will be developed and determined and will be implemented into Deform for forming process.
Dr Qian Bai