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Material Flow during Friction Stir Welding of Copper
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.ORCID iD: 0000-0002-8494-3983
2008 (English)In: 7th international Friction Stir Welding symposium: Awaji Island, Japan, 20-22 May 2008, 2008Conference paper (Refereed)
Abstract [en]

SKB (Swedish Nuclear Fuel and Waste Management Co) intends to use the FSW method to seal copper canisters for nuclear fuel waste. The understanding of the material flow in this process is essential in order to obtain good weld quality. Material and temperature flow in 50 mm thick friction stir welded (FSW) copper has been investigated in this study. The main experiments have been performed at SKB’s canister laboratory in Oskarshamn. By inserting dissimilar material into the weld zone before welding and then join the material, the flow can be analyzed. Ø4 mm brass rods were used. After welding, the weld line was sliced, etched and examined by optical microscope. A three-dimensional finite element model was used to describe mass and momentum transport as well as heat transfer. The shoulder moved the first brass to its new position after welding. Independent of where the rod was inserted, the brass was first observed in the area where the tool has passed at the advancing side close to the uppe rsurface. Most of the brass was seen in front of the position of the inserted rod, i.e. the brass had moved backwards. After the inserted rod, no brass was observed at the retreating side, but some was found at the advancing side. The pin moved the brass from the root of the weld upward towards the shoulder both on the advancing and retreating sides. The flow around the pin never appeared more than a few mm below the pin, or more than 6 and 8 mm outside the pin on the advancing and retreating sides, respectively. The model and the examined welds were compared showing a satisfactory agreement except for some of the vertical flow.

In the presence of brass, a distinct nugget, often with onion rings, could be observed. The material velocity was highest near the weld tool, but at the advancing side there was a clear drop in velocity a few mm from the pin. This drop could cause void formation if the welding parameters are not chosen properly. The retreating side has a higher temperature and lowerhardness compared to the advancing side in this weld. One of the reasons for the higher temperature is that the component width is less at the retreating side than at the advancing side. Consequently generated heat is concentrated to a smaller area at the retreating side.

Place, publisher, year, edition, pages
Keyword [en]
friction stir welding; material flow; viscosity; heat transfer; simulation, FEM
URN: urn:nbn:se:kth:diva-12013OAI: diva2:294148
QC20100719Available from: 2010-02-16 Created: 2010-02-16 Last updated: 2010-07-19Bibliographically approved
In thesis
1. Investigation and modelling of friction stir welded copper canisters
Open this publication in new window or tab >>Investigation and modelling of friction stir welded copper canisters
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This work has been focused on characterisation of FSW joints, and modelling of the process, both analytically and numerically. The Swedish model for final deposit of nuclear fuel waste is based on copper canisters as a corrosion barrier with an inner pressure holding insert of cast iron. Friction Stir Welding (FSW) is the method to seal the copper canister, a technique invented by The Welding Institute (TWI).

The first simulations were based on Rosenthal’s analytical medium plate model. The model is simple to use, but has limitations. Finite element models (FEM) were developed, initially with a two-dimensional geometry. Due to the requirements of describing both the heat flow and the tool movement, three-dimensional models were developed. These models take into account heat transfer, material flow, and continuum mechanics. The geometries of the models are based on the simulation experiments carried out at TWI and at Swedish Nuclear Fuel Waste and Management Co (SKB). Temperature distribution, material flow and their effects on the thermal expansion were predicted for a full-scale canister and lid. The steady state solutions have been compared with temperature measurements, showing good agreement.

In order to understand the material flow during welding a marker technique is used, which involves inserting dissimilar material into the weld zone before joining. Different materials are tested showing that brass rods are the most suitable material in these welds. After welding, the weld line is sliced, etched and examined by optical microscope. To understand the material flow further, and in the future predict the flow, a FEM is developed. This model and the etched samples are compared showing similar features. Furthermore, by using this model the area that is recrystallised can be predicted. The predicted area and the grain size and hardness profile agree well.

Microstructure and hardness profiles have been investigated by optical microscopy, Scanning Electron Microscopy (SEM), Electron Back Scatter Diffraction (EBSD) and Rockwell hardness measurements. EBSD visualisation has been used to determine the grain size distribution and the appearance of twins and misorientation within grains. The orientation maps show a fine uniform equiaxed grain structure. The root of the weld exhibits the smallest grains and many annealing twins. The appearance of the nugget and the grain size depends on the position of the weld. A large difference can be seen both in hardness and grain size between the start of the weld and when the steady state is reached.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. 62 p.
Friction Stir Welding (FSW), Copper, Welding, Finite Element Method (FEM), SKB, Material flow
National Category
Materials Engineering
urn:nbn:se:kth:diva-11999 (URN)978-91-7415-568-6 (ISBN)
Public defence
2010-03-04, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
QC20100719Available from: 2010-02-15 Created: 2010-02-12 Last updated: 2010-07-19Bibliographically approved

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