Multiscale Modelling of Micro/Nano-Scale Structure and Damage Evolution in Superplastically Deforming Materials

Lead Research Organisation: Imperial College London
Department Name: Dept of Mechanical Engineering

Abstract

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. SPF is carried out by several UK wealth-generating companies (e.g. Doncasters, Aeromet, Superform) for a range of customers (including Airbus, Aston Martin, Boeing, Bombardier, BAE Systems, Ford Motor, GKN Lockheed, Goodrich, Raytheon, Siemens). Increasing use of SPF correlates roughly with availability of suitable materials capable of superplasticity, and with improvements in cost, speed and energy consumption of the SPF process. It is evident from literature of the last decade that a new generation of superplastic materials are emerging (e.g. high strain rate ceramics, metal matrix composites). New materials will lead to new applications, such as superplastic forming of complex-shaped ceramic armour plating. Furthermore, availability of new low cost structural alloys (e.g. Al-alloys) suitable for superplastic forming will lead to increased use of SPF in making complex shapes and replacing multi-part assemblies with single contiguous parts for increased structural integrity. Formability at lower temperatures reduces cost and energy consumption considerably, making SPF viable in industries with small profit margins and environmental restrictions (e.g. consumer automotive). Although modelling superplasticity is the end goal of this project, it is first necessary to model phenomena (e.g. grain growth, grain shape change and recrystallisation) common to a host of other material deformation modes relevant to many other industrial materials processing methods. UK metals forming industries (e.g. Doncasters, Corus) are increasingly interested in developing accurate models of their forming processes. Thus, the proposed work is apt and timely for advancement of a large and diverse sector of UK industry. Although superplasticity has been studied experimentally for over 80 years, there is not yet a comprehensive understanding of the physical processes of this important material phenomenon. Past modelling efforts have been hindered by there being no unique superplastic flow process. Rather, many small-scale (from atomic to micro) mechanisms combine with relative strengths that depend principally on grain size, temperature and strain rate to produce superplastic flow. It is essential for the further development of superplastic forming as a viable manufacturing method that a new modelling framework be developed in order to better understand the relation between the microstructure and mechanisms of superplasticity. This would enable materials to be thermomechanically processed with efficacious microstructures for lowering the process temperature and increasing the strain rate (and thereby reducing cost and increasing throughput) of superplastic forming applications in industries that exploit this phenomenon for forming metals and ceramics in contiguous complex shapes (e.g. aerospace, automotive, armour). This would be possible if low temperature mechanisms could be accessed via suitable changes to the microstructure, e.g. grain refinement, grain boundary/dislocation pinning or formation of vacancy/impurity complexes. Although the focus of the proposed work is modelling superplastic deformation, the impact will be far greater; a product of this effort will be a new multiscale modelling finite element package made of commercial software and custom subroutines. This will be the foundation of further studies into how deformation mechanisms at the micro and nano scales dictate overall material behaviour (e.g. fatigue, strain hardening and processes such as hot forming, forging, rolling, extrusion, drawing and machining).
 
Description This work is the basis of a fundamental breakthrough in virtual grain structure generation and has led to 7 peer-reviewed papers; it has also led to the development of the software VGRAIN implementing the algorithms. We are now able to link uniquely easily measured parameters (e.g. from micrographs) with the parameters that mathematically characterise a grain distribution, and generate arbitrary grain structures with cohesive grain boundaries automatically to capture both decohesion and sliding at grain boundaries for simulation of superplasticity (and subsequently other metal forming problems) using the crystal plasticity finite element method. We also developed a concurrent coupling method between the discrete dislocation plasticity method and the crystal plasticity finite element method for the work in this grant, but which is now also in use for solving problems on indentation, bicrystal tension and fretting fatigue.
Exploitation Route The findings led directly to the development of the open source VGRAIN software, which is now in use internationally. With this software being used currently by other research groups, it is clear the findings will be taken forward in that way. We will also bid for further funding to add features to VGRAIN. Further development of the coupling of discrete dislocation plasticity to crystal plasticity, and modelling polycrystalline grain structures using the extended finite element method, are ongoing.
Sectors Aerospace, Defence and Marine,Construction,Energy,Environment,Manufacturing, including Industrial Biotechology,Security and Diplomacy,Transport
URL http://goo.gl/aCGzar
 
Description Hussein Rappel Joint PhD Studentship 
Organisation University of Luxembourg
Country Luxembourg, Grand Duchy of 
Sector Academic/University 
PI Contribution Our work on grant EP/H007245/1 in representing grain structures led directly to discussions with Professor Stéphane Bordas, now at University of Luxembourg but at Cardiff University at that time, on extending our work on representing grain structures using level sets to establish a crystal plasticity extended finite element framework.
Collaborator Contribution We are contributing expertise on crystal plasticity and mathematical representation of grain boundary networks.
Impact There are no outputs yet since this collaboration began only recently (09/14), via funding from the Luxembourg government for a joint PhD studentship, although the collaboration had been planned over a number of years.
Start Year 2014
 
Description Morad Karimpour PhD Studentship 
Organisation Tata Steel Europe
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Private 
PI Contribution We carried out research, via the PhD studentship partially funded by Corus (now Tata Steel), which was linked to the grant EP/H007245/1 as detailed in the proposal. We solved the problems of virtual grain structure generation as explained in the outputs/outcomes section below, which led to the development of VGRAIN software.
Collaborator Contribution The partner (Dr Didier Farrugia at Tata Steel, then Corus) provided guidance on metallurgical aspects of the problem, and application of the modelling techniques to engineering practice at Tata Steel in the Long Product Rolling department. He provided his time and oversight via meetings, and also materials contributed to the project.
Impact This collaboration led to 4 peer-reviewed journal publications, and a new method for representing grain boundary networks with control of the regularity linked uniquely to easily measured metallographic parameters (average, maximum and minimum grain size). The method is the Controlled Poisson Voronoi Tessellation with cohesive grain boundaries, implemented in the open source software VGRAIN. The software VGRAIN is now used internationally (Atomic Weapons Establishment, Argonne National Laboratory, Tufts University, by other groups at Imperial College London, etc.).
Start Year 2009
 
Title VGRAIN (Virtual Grain Structure Generation Software) 
Description This software implements the Controlled Poisson Voronoi Tessellation (developed by the authors in associated publications) for generating virtual grain structures with regularity control, in 2D and 3D, and with automatic generation of cohesive grain boundaries and definition of the grain orientation distribution, for use in e.g. Crystal Plasticity Finite Element simulations. 
Type Of Technology Software 
Year Produced 2012 
Open Source License? Yes  
Impact This software provides the first capability to link via a unique mathematical mapping (as developed in associated publications) the regularity of a grain structure generated by Voronoi tessellation and three simple, measurable parameters: maximum, minimum and average grain size; this is the so-called Controlled Poisson Voronoi tessellation. Furthermore, the software can automatically generate cohesive zones along grain boundaries for the first time, including at multiple junctions (as developed in associated publications). 
URL http://goo.gl/NOnpmf
 
Description EUROMECH 505 Talk: A Coupled Approach to Model Plastic Flow using Dislocation Dynamics and Crystal Plasticity Finite Element Modelling 
Form Of Engagement Activity Scientific meeting (conference/symposium etc.)
Part Of Official Scheme? No
Type Of Presentation keynote/invited speaker
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact This talk led to interest and questions about the procedure we developed for coupling discrete dislocation plasticity models to crystal plasticity finite element models, and led directly to a current collaboration (see below).

The talk led to discussions on the research topic with Prof Fionn Dunne (in the audience), then at University of Oxford, which led to ongoing collaborations with Prof Dunne, now at Imperial College London and a jointly supervised PhD student working in this area.
Year(s) Of Engagement Activity 2010
URL http://www.euromech.org/colloquia/2010/505
 
Description IWCMM 21 Talk: A Controlled Poisson Voronoi Tessellation for Grain and Cohesive Boundary Generation Applied to Crystal Plasticity Analysis 
Form Of Engagement Activity Scientific meeting (conference/symposium etc.)
Part Of Official Scheme? No
Type Of Presentation paper presentation
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact The talk led to many questions and queries, after the talk and at subsequent intermissions and meals. This led to direct queries about the junction partitioning method developed for generating cohesive grain boundaries (see below).

A direct query was made about the junction partitioning algorithm for use by another researcher, and it is believed that this led to some of the subsequent citations of the paper presented at this conference, and an associated journal paper where the details of the cohesive grain boundary generation method were described. This conference led directly to use of the algorithms by other researchers and wide exposure of the new methods.
Year(s) Of Engagement Activity 2011
URL http://www2.ul.ie/web/WWW/Faculties/Science_&_Engineering/Research/Research_Institutes/MSSI/Research...
 
Description Mesomechanics 2009 Talk: A Virtual Crystal Plasticity Simulation Tool for Micro-Forming 
Form Of Engagement Activity Scientific meeting (conference/symposium etc.)
Part Of Official Scheme? No
Type Of Presentation keynote/invited speaker
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact The talk generated considerable interest, and later requests to use the open source software VGRAIN implementing the research.

Use of the software VGRAIN by other researchers.
Year(s) Of Engagement Activity 2009
URL http://www.eng.ox.ac.uk/elasticity/meso2009/committees.htm
 
Description Metal Forming 2010 Talk: Size Effects in Micro-Forming 
Form Of Engagement Activity Scientific meeting (conference/symposium etc.)
Part Of Official Scheme? No
Type Of Presentation paper presentation
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact The talk generated considerable interest, and later requests to use the open source software VGRAIN implementing the research.

Use of the software VGRAIN by other researchers.
Year(s) Of Engagement Activity 2010
URL http://plast.pse.tut.ac.jp/metalforming/
 
Description University of Cambridge Seminar: Grains, Interfaces and Size effects in Metal Forming 
Form Of Engagement Activity Scientific meeting (conference/symposium etc.)
Part Of Official Scheme? No
Type Of Presentation keynote/invited speaker
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact There was a stimulating discussion on the work before and after the seminar, which was well received.

New results and methods were disseminated to a key audience working in the field of the research.
Year(s) Of Engagement Activity 2011
URL http://talks.cam.ac.uk/show/archive/10139