Projects per year

### Abstract

This thesis investigates how to apply optimization methods to numerical models of a
friction stir welding process. The work is intended as a proof-of-concept using different
methods that are applicable to models of high complexity, possibly with high
computational cost, and without the possibility for efficient gradient calculation. Thus,
the focus is on surrogate optimization methods with the aim of reducing the number of
expensive function evaluations, by using a low-fidelity model together with the highfidelity
model to be optimized. The methods used here do not require the user to supply
gradient information of the high-fidelity model.
The optimization schemes are applied to stationary thermal models of differing
complexity of the friction stir welding process. The optimization problems considered
are based on optimizing the temperature field in the workpiece by finding optimal
translational speed and rotational speed of the tool. Besides the deterministic problem
a robust optimization problem is considered in which the effects of uncertain material
and optimization parameters are taken into account. The objective is to obtain a
desired mean response while reducing the standard deviation of the response. Also an
optimization problem based on a microstructure model is solved, allowing the hardness
distribution in the plate to be optimized. The use of purely thermal models represents
a simplification of the real process; nonetheless, it shows the applicability of the optimization
methods considered and forms the basis for optimization of more detailed
models. Surrogate models of varying complexity, and similarity with the true model,
are applied and the effect on the optimization results is discussed.
Furthermore, the thesis contributes to the modelling of the heat transfer between
the workpiece and the backingplate by solving an inverse modelling problem in which
experimental data and a numerical model are used for determining the contact heat
transfer coefficient. Different parametrizations of the spatial distribution of the heat
transfer coefficient are studied and discussed, and the optimization problem is formulated
as a minimization of the difference between measured and calculated temperatures.
The magnitude and distribution of the heat transfer coefficient is determined for
the available data.

Original language | English |
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Number of pages | 110 |
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Publication status | Published - Feb 2010 |

### Keywords

- Friction stir welding
- Optimization

## Projects

- 1 Finished

## Tool Optimization for Welding Processes

Larsen, A. A., Stolpe, M., Hattel, J. H., Sigmund, O., Lindgren, L., Duysinx, P. & Lund, E.

15/06/2006 → 10/02/2010

Project: PhD

## Cite this

Larsen, A. A. (2010).

*Process optimization of friction stir welding based on thermal models*.