Qin, Zhenyuan
(2025)
Investigation of thermo-mechanical behaviour in incremental sheet forming.
PhD thesis, University of Nottingham.
Abstract
Incremental sheet forming (ISF) is an emerging flexible sheet forming process that has shown considerable advantages in manufacturing small batch or customised sheet products due to its high adaptability and reduced tooling costs. The principle of ISF is to use a computer numerical control (CNC) milling machine or a robotic system to control a hemispheric tool moving along a predefined tool path to progressively form the blank sheet into the desired shape. Conventional ISF is usually conducted at room temperature and hence exhibits limitations in manufacturing of hard-to-deform materials, e.g., magnesium and titanium alloys due to poor ductility at room temperature. To overcome this difficulty, heat-assisted ISF has been developed to further improve forming limit of hard-to-deform materials. Based on the heating source used, heat-assisted ISF can be divided into two categories, one is heat-assisted ISF by adopting an external heat source including electrical, laser, induction and hot air heating, while the other is so-called frictional stir incremental forming (FSIF), where the heating is generated by tool-workpiece interaction and frictional heat largely due to the ISF tool rotation.
FSIF has drawn considerable attention from the research community due to its low heating costs and high flexibility. However, the thermo-mechanical behaviour behind FSIF is still less understood, e.g., the tool-workpiece interfacial friction, heat generation, partition and transfer behaviour, the effect of process parameters on thermal response, and material deformation and fracture behaviour at varying forming temperatures.
This thesis investigates the thermo-mechanical behaviour in FSIF using theoretical modelling, experimental testing and finite element (FE) simulation.
Based on the PhD work, a novel theoretical thermal model is developed to correlate the relationship between friction-induced heat generation and material thermal response. The results indicate that the new theoretical model can capture the temperature distribution and variation under different processing conditions, and the results show a good agreement with the FE simulation results. A new tool path-defined straight groove test combined with mechanical and thermal detection is proposed to determine the coefficient of friction (COF) and heat partition coefficient (HPC) of aluminium alloy (AA1050) and commercially pure titanium Grade 1 (CP Ti Grade 1) sheets. The experimental and numerical results show that the determined COF and HPC values are sufficiently accurate. The interaction between friction force and thermal effect is observed by this testing method. The presented testing method and theoretical model provide an insight into the determination of the thermal-relevant parameters (COF and HPC), and the quantification of effect of friction-induced heat generation on the thermal response of AA1050 and CP Ti Grade 1.
A fully coupled thermo-mechanical FE model is developed to investigate the thermo-mechanical behaviour of AA1050 in FSIF process for the first time. Experimental testing and FE simulation are carried out to evaluate the effect of ISF process parameters on the forming temperatures and forces and to understand the heat generation mechanism. The results show a good agreement between ISF testing and FE simulation for truncated cone and pyramid parts under different process conditions. The effect of spindle speed on the formability is investigated and the forming limit under different stress states is examined. By using the developed FE model, the effect of process parameters on the friction heat and plastic work in FSIF are quantified, and the presented FE model provides an effective means to differentiate the contribution of three influential factors, i.e. the spindle speed, feed rate and plastic deformation to the total heat generation. The FE simulation results demonstrate that the spindle speed induced friction heat accounts for 95.19% of the total heat generation during FSIF process of the truncated cone part and 95.64% of the truncated pyramid part under the studied process conditions, which indicates that the spindle speed plays a predominant role in temperature increase while feed rate and plastic work have less effect on the total heat generation.
An extended micromechanics-based model with consideration of the effect of temperature due to the spindle speed induced friction heat on damage accumulation in ISF is developed to characterise ductile fracture behaviour of AA1050 at varying temperatures in ISF process under different strain states. The correlation between void evolution and damage accumulation at different temperatures is revealed, and the void volume fraction (VVF) at different deformation stages is determined by conducting microscopic in-situ tensile test. By introducing a new temperature-dependent VVF function and by determining appropriate Gurson-Tvergaard-Needleman (GTN) damage parameters, the ductile fracture at different temperatures is evaluated using FE simulation and validated by ISF experimental testing. The results show that the proposed temperature-dependent VVF function in GTN modelling is capable of predicting the ductile fracture of both the set temperature in in-situ tensile tests and the varying temperature conditions in ISF processes.
| Item Type: |
Thesis (University of Nottingham only)
(PhD)
|
| Supervisors: |
Ou, Hengan |
| Keywords: |
Flexible sheet forming process; Thermal modeling; Friction-induced heat generation; Material thermal response |
| Subjects: |
T Technology > TS Manufactures |
| Faculties/Schools: |
UK Campuses > Faculty of Engineering
UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering |
| Item ID: |
80742 |
| Depositing User: |
Qin, Zhenyuan
|
| Date Deposited: |
29 Jul 2025 04:40 |
| Last Modified: |
29 Jul 2025 04:40 |
| URI: |
https://eprints.nottingham.ac.uk/id/eprint/80742 |
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