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Investigation and modelling of friction stir welded copper canisters
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. (Tillämpad materialteknologi)
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. , p. 62
Keywords [en]
Friction Stir Welding (FSW), Copper, Welding, Finite Element Method (FEM), SKB, Material flow
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-11999ISBN: 978-91-7415-568-6 (print)OAI: oai:DiVA.org:kth-11999DiVA, id: diva2:293619
Public defence
2010-03-04, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC20100719Available from: 2010-02-15 Created: 2010-02-12 Last updated: 2010-07-19Bibliographically approved
List of papers
1. Microstructure development in copper welded by the FSW-Process
Open this publication in new window or tab >>Microstructure development in copper welded by the FSW-Process
2004 (English)In: SCIENTIFIC BASIS FOR NUCLEAR WASTE MANAGEMENT XXVII / [ed] Oversby VM, Werme LO, Warrendale: Materials Research Society , 2004, Vol. 807, p. 483-488Conference paper, Published paper (Refereed)
Abstract [en]

To ensure safe storage of nuclear fuel waste, copper canisters are proposed as corrosion barrier. One alternative for sealing the copper canisters is Friction Stir Welding (FSW). During the joining process friction heat and mechanical deformation appear between the rotating tool and the material being welded. Liquid metal will not form, since this is a solid state welding process. Three distinct microstructural zones are developed namely the nugget, the thermomechanically affected zone (TMAZ) and heat-affected zone (HAZ). The nugget is in the centre of the weld, where the pin is located and where severe plastic deformation occurs that leads to recrystallisation. Surrounding the nugget, the TMAZ is only partially recrystallised, due to lower temperature increase and deformation compared to the nugget. The third zone, HAZ, surrounds the TMAZ. The initial nugget can have a classic round aluminium nugget image, when the welding conditions are cold, but the steady state nugget, is wider near the shoulder and shorter in the weld root.

Place, publisher, year, edition, pages
Warrendale: Materials Research Society, 2004
Series
Materials Research Society Symposium Proceedings, ISSN 0272-9172 ; 807
Keywords
Materials Science, Multidisciplinary
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-12008 (URN)000225038400079 ()2-s2.0-12844266096 (Scopus ID)1-55899-752-0 (ISBN)
Conference
27th Symposium on Scientific Basis for Nuclear Waste Management Kalmar, SWEDEN, JUN 15-19, 2003
Note
QC 20100716Available from: 2010-02-15 Created: 2010-02-15 Last updated: 2011-10-31Bibliographically approved
2. Microstructure and temperature development in copper welded by the FSW-process
Open this publication in new window or tab >>Microstructure and temperature development in copper welded by the FSW-process
2003 (English)In: 4th International Symposium on FSW, Park City, USA, April, 2003, 2003Conference paper, Published paper (Refereed)
Abstract [en]

The use of Copper canisters with cast iron inserts is one of the proposed methods for long time deposition of nuclear fuel waste. The joining of the lid and possibly the base of the canister can be performed with Friction Stir Welding (FSW). A thorough understanding of the microstructure development in these welds is of major importance, since defects must be avoided. The microstructure and hardness profiles were investigated for two FSW conditions, at the beginning of the weld when there are essentially cold conditions, and when the steady state is reached. In cold welds the features of the nugget are very similar to that observed when FSW-joining aluminium. However, when the welding conditions reach the steady state, the nugget gets wider and the hardness is lower than in the cold weld.

Electron Back Scatter Diffraction (EBSD) visualisation has been used to determine the grain size distribution of, twins and misorientation within grains. The orientation maps show a fine uniform equiaxed grain structure. There is no major misorientation within the grains in the nugget or at the root, but 25 mm from the weld centre the misorientation is large. This confirms that this area lies within the Thermal Mechanical Affected Zone (TMAZ) and is partially recrystallised.  The root has many more annealing twins than the nugget. This indicates that the nugget has experienced deformation after recrystallisation. A model has been used to correlate heat flow to the grain size and the hardness distributions.

Keywords
friction stir welding, microstructure, defect, void, copper, process parameter
National Category
Materials Engineering Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-12010 (URN)
Note
QC 20100716Available from: 2010-02-16 Created: 2010-02-16 Last updated: 2010-12-07Bibliographically approved
3. Finite element modelling of temperature distribution in friction stir welding process and its influence on distortion of copper canisters
Open this publication in new window or tab >>Finite element modelling of temperature distribution in friction stir welding process and its influence on distortion of copper canisters
2004 (English)In: SCIENTIFIC BASIS FOR NUCLEAR WASTE MANAGEMENT XXVIII, 2004, Vol. 824, p. 57-62Conference paper, Published paper (Refereed)
Abstract [en]

In an effort to enhance safety for long time disposal of waste nuclear fuel, friction stir welding has been developed as one alternative to seal copper canisters. To avoid the formation of voids and cracks during the welding process, an understanding of the heat and material flow and thereby the evolution of the microstructure, is of great importance. Finite element modelling has been used to simulate the heat and material flow as well as thermal expansion during the friction stir welding process. A model involving heat transfer, material flow, and continuum mechanics has been developed. The steady state solutions have been compared with experimental temperature observations as well as analytical solutions, showing good agreement. Temperature distribution is affected by the welding speed. For a given reference point perpendicular to the welding direction, a lower welding speed corresponds to a higher peak temperature. The plunging position of welding tool influences the temperature distribution and therefore the displacement distribution of the weldment.

Series
MATERIALS RESEARCH SOCIETY SYMPOSIUM PROCEEDINGS, ISSN 0272-9172 ; 824
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-12011 (URN)000224543500009 ()2-s2.0-14944346646 (Scopus ID)1-55899-774-1 (ISBN)
Conference
28th Symposium on the Scientific Basis for Nuclear Waste Management held at the 2004 MRS Spring Meeting San Francisco, CA, APR 13-16, 2004
Note
QC 20100719Available from: 2010-02-16 Created: 2010-02-16 Last updated: 2011-10-31Bibliographically approved
4. Finite Element Modelling of Friction Stir Welding on Copper Canister
Open this publication in new window or tab >>Finite Element Modelling of Friction Stir Welding on Copper Canister
2004 (English)In: 5th International Friction Stir Welding symposium, 2004Conference paper, Published paper (Refereed)
Abstract [en]

In an effort to enhance safety for long time deposit of waste nuclear fuel, friction stir welding has been tentatively used to seal copper canisters. To avoid the formation of voids and cracks during the welding process, and to understand the heat and material flow as well as the evolution of the microstructure, are of great importance. Finite element modelling has been used to simulate the friction stir welding process.

A model involving heat transfer, material flow, and continuum mechanics has been developed. The steady state solutions have been compared with experimental temperature observations as well as analytical solutions, showing good agreement. Temperature distribution is affected by the welding speed. For a given reference point perpendicular to the welding direction, a lower welding speed corresponds to a higher peak temperature. The plunging position of welding tool influences the temperature distribution and therefore also the thermal distortion of the weldment.

Keywords
friction stir welding, FSW, copper, finite element modelling
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-12012 (URN)
Conference
5th International Friction Stir Welding symposium, Metz, France, 14-16 September, 2004
Note
QC 20100719Available from: 2010-02-16 Created: 2010-02-16 Last updated: 2010-12-07Bibliographically approved
5. Material Flow during Friction Stir Welding of Copper
Open this publication in new window or tab >>Material Flow during Friction Stir Welding of Copper
2008 (English)In: 7th international Friction Stir Welding symposium: Awaji Island, Japan, 20-22 May 2008, 2008Conference paper, Published 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.

Keywords
friction stir welding; material flow; viscosity; heat transfer; simulation, FEM
Identifiers
urn:nbn:se:kth:diva-12013 (URN)
Note
QC20100719Available from: 2010-02-16 Created: 2010-02-16 Last updated: 2010-07-19Bibliographically approved
6. Flow modelling of friction stir welding using brass markers in copper
Open this publication in new window or tab >>Flow modelling of friction stir welding using brass markers in copper
(English)In: Science and technology of welding and joining, ISSN 1362-1718, E-ISSN 1743-2936Article in journal (Other academic) Submitted
Abstract [en]

This paper describes the behaviour of the material flow in 50 mm thick friction stir welded phosphorous-alloyed copper with both experiments and modelling work. In Sweden the friction stir welding is going to be used for sealing copper canisters for storage of nuclear waste from fuel elements. An understanding of the material flow during the process is essential in order to predict the weld quality and thereby the maintenance of the weld. A marker technique is used which involves inserting dissimilar material into the weld zone before joining. The material flow can then be examined. In these experiments Ø 9 brass rods are used. After welding, the copper was sliced, etched and examined by optical microscopy. The results show that the weld tool pushes the brass as far as 37 mm backwards from the inserted location, which is little more than one pin diameter. This brass is first seen on the advancing side. Most brass is found behind its inserted location, on both sides of the centre line. The material movement underneath the pin in the weld root is insignificant. The pin area has a nugget with a fine grain structure, but also a part near the shoulder with coarser grains. A finite element model is developed in order to describe the temperature and material flow. The model and the examined welds were compared showing a satisfactory agreement for most features.

Keywords
friction stir welding, viscous flow, marker, finite element method
Identifiers
urn:nbn:se:kth:diva-12016 (URN)
Note
QS 20120328Available from: 2010-02-16 Created: 2010-02-16 Last updated: 2017-12-12Bibliographically approved
7. Influence of tool geometry on the material flow during friction stir welding of copper
Open this publication in new window or tab >>Influence of tool geometry on the material flow during friction stir welding of copper
(English)In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774Article in journal (Other academic) Submitted
Abstract [en]

The understanding of the material flow during friction stir welding of thick sections of copper is of great interest. Sweden plans to use this method to seal 50 mm thick copper canisters which will contain spent nuclear fuel. In this study two different weld tools are investigated. The first experiment uses a pin length of 52 mm and the second a pin length of 30 mm. The shoulder diameter, process parameters and materials are the same for both experiments. Experiments as well as FEM modelling work are reported. The experimental part includes a marker inserting technique using brass rod, which allows the material flow to be examined. After welding, the weld line is sliced, etched and examined by optical microscope. The experiments show more brass in the weld with shorter pin length and the nugget zone has different appearances in the two cases. A finite element model is applied to describe the temperature, material flow, dynamic viscosity, strain rate and shear stress in the welds. The outcome of the model and the feature of the experimental welds are compared showing a satisfactory result. The size of the thermomechanically affected zone is predicted by this model.

Keywords
friction stir welding, copper, marker, viscosity, FEM simulation
Identifiers
urn:nbn:se:kth:diva-12017 (URN)
Note
QS 20120328Available from: 2010-02-16 Created: 2010-02-16 Last updated: 2017-12-12Bibliographically approved

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