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  • 1.
    Ericsson, Mats
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Fatigue of friction stir welded T-joints2005In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452Article in journal (Other academic)
  • 2.
    Ericsson, Mats
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Fatigue properties of friction stir overlap welds2007In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 29, no 1, p. 57-68Article in journal (Refereed)
    Abstract [en]

    Friction stir welding (FSW) is currently used for many applications involving lap or T-joints, e.g. hermetically closed boxes such as cooling elements and heat exchangers. The frequent pressure changes in these make them susceptible to fatigue. The fatigue characterization of lap joints involves a combination of shear and bending. Forces applied to the ends of lap joints result in non-axial stresses in the connection area. FSW lap joints of Al-Mg-Si alloy 6082 in the artificially aged condition T6 were studied. A pin (probe) based on the Triflute (TM) concept was used with two modifications to the pin, the pin end being either convex or concave. Tool shoulders of 15 and 18 turn respectively were utilized, producing four different weld series. Fracture was initiated in the highly stressed area where the weld cuts through the interface between the two sheets. The cracks typically propagated through the weld in the upper sheet (tool side). The broadest tool shoulder with a concave end of pin design gave the best fatigue performance. This was due to an improved flow path provided by the hollowed out end of the pin; allowing material flow around the pin which resulted in minimal hooking of the sheet interface adjacent to the weld nugget. Additionally heat energy was supplied by the increased contact area. The stress intensity factor Delta K was determined. It was found that a simplified approach, developed to estimate Delta K for overlap spot welds, could be applied to friction stir overlap joints. The corresponding crack propagation rates were in fair accordance with the experimental results.

  • 3.
    Jin, Lai -Zhe
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Non-stationary creep simulation with a modified Armstrong-Frederick relation applied to copper2009In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 46, no 2, p. 339-346Article in journal (Refereed)
    Abstract [en]

    A previously formulated model using back stress to handle non-stationary creep during power-law breakdown is further developed. In particular, the way to integrate the back stress is modified. Usually the Armstrong-Frederick relation has been applied, but it can give unphysical results in the sense that the back stress exceeds the tensile strength of the material. Such a problem can be solved by replacing the back stress term in this relation with the back stress deviator. The creep model is applied to copper canister in waste packages intended for encapsulating spent nuclear fuel. These waste packages will be placed in the bedrock at a depth of about 500 m as a final stage of disposal. During storage, radioactivity-induced thermal evolution raises temperature in repositories and water-saturation generates pressure directly on the copper canister. The thermally activated creep in copper canister occurs readily. To estimate the amount of creep deformation, a finite element model is set up to compute the evolution of creep deformation in copper canister. The creep model takes both stationary and non-stationary creep into account The computed maximum creep strain is shown to be 7.8% over 10 years, which should not cause failure since measured creep elongations are in the range of 15-40%.

  • 4.
    Jin, Lai-Zhe
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Creep of copper canisters in power-law breakdown2008In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 43, no 3, p. 403-416Article in journal (Refereed)
    Abstract [en]

    According to the Swedish KBS-3 concept the spent nuclear fuel will be placed in copper canisters 500 m down in the bedrock. In thestorage, the canister will creep under conditions that are well inside the power-law breakdown regime. To prevent creep rupture fromoccurring that could cause leakage of nuclides, finite element models are set up to study the evolution of creep deformation in the coppercanisters. In this paper, two finite element models for the secondary creep are formulated. The first one is based on a fundamental climb–glide creep law valid over a wide range of temperatures. The second one is on the basis of a generalised Norton equation fitted to secondarycreep data of phosphorus doped pure copper. The creep deformation is shown to be much larger in the lid and the bottom of the canistersthan in the cylindrical wall. In the latter a stationary creep state is reached only after very long time (30000 years). Since the deformation inthe copper canister is restricted by a cast iron insert and stress concentrations are reduced with time, the total creep strain is limited.

  • 5.
    Jin, Lai-Zhe
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Numerical simulation of residual stresses for friction stir welds in copper canisters2012In: Journal of Manufacturing Processes, ISSN 1526-6125, Vol. 14, no 1, p. 71-81Article in journal (Refereed)
    Abstract [en]

    In an attempt to map the residual stress distributions after friction stir welding of copper canisters, a three-dimensional thermo-mechanical model has been formulated by coupling heat transfer and elastoplasticity analyses. The transient temperature field around the tool is simulated by a moving heat source. The simulation shows that the residual stress distribution in a thick-wall copper canister is sensitive to the circumferential angle and asymmetrical to the weld line. Both tensile and compressive stresses emerge along the weld line and its vicinity. The maximum tensile stress appears in the circumferential direction on the outer surface. The maximum tensile stress, whether it is predicted by the finite element method or measured by the hole-drilling technique and the X-ray diffraction method, does not exceed 50 MPa in general.

  • 6.
    Jin, Lai-Zhe
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Steady non-Newtonian flows in copper and iron aluminide at elevated temperatures2007In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 189, no 1-3, p. 428-434Article in journal (Refereed)
    Abstract [en]

    On the basis of dislocation climb and mobility, a steady-state non-Newtonian flow law is derived for two metallic and one intermetallic material,namely electrolytic tough pitch copper, phosphorus alloyed pure copper, and Fe24AlMo iron aluminide. The purpose is to develop a flow lawapplicable to the finite element simulation for hydrodynamic flow of incompressible metals. The model can accurately represent the experimentalflow stress values and the viscosity of the materials. The mathematical form of the model is similar to that of the free-volume approach, which isused for liquids and amorphous metals. The study indicates that in the temperature and strain-rate regimes that are appropriate to the hot-workingprocesses, the Cohen–Grest model, which is essentially related to the total thermal expansion of fluid, can phenomenologically be extended to thecrystalline solid-state materials for the depiction of viscosity data.

  • 7.
    Källgren, Therese
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Jin, Lai-Zhe
    KTH, Superseded Departments, Materials Science and Engineering.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Finite Element Modelling of Friction Stir Welding on Copper Canister2004In: 5th International Friction Stir Welding symposium, 2004Conference 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.

  • 8.
    Källgren, Therese
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Jin, Lai-Zhe
    KTH, Superseded Departments, Materials Science and Engineering.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Finite element modelling of temperature distribution in friction stir welding process and its influence on distortion of copper canisters2004In: SCIENTIFIC BASIS FOR NUCLEAR WASTE MANAGEMENT XXVIII, 2004, Vol. 824, p. 57-62Conference 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.

  • 9.
    Källgren, Therese
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Flow modelling of friction stir welding using brass markers in copperIn: Science and technology of welding and joining, ISSN 1362-1718, E-ISSN 1743-2936Article in journal (Other academic)
    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.

  • 10.
    Källgren, Therese
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Influence of tool geometry on the material flow during friction stir welding of copperIn: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774Article in journal (Other academic)
    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.

  • 11.
    Källgren, Therese
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Material Flow during Friction Stir Welding of Copper2008In: 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.

  • 12.
    Sandström, Rolf
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Modified Armstrong-Frederick Relation for Handling Back Stresses in FEM Computations2009In: CREEP & FRACTURE IN HIGH TEMPERATURE COMPONENTS: DESIGN & LIFE ASSESSMENT ISSUES, PROCEEDINGS / [ed] Shibli IA, Holdsworth SR, LANCASTER: DESTECH PUBLICATIONS, INC , 2009, p. 836-847Conference paper (Refereed)
    Abstract [en]

    If transient phenomena are important in creep design, a non-stationary creep model must be applied. One way of achieving this is to apply a back stress. The back stress reflects the history of the creep deformation. During primary creep the back stress normally increases from zero to a stationary value representing secondary creep. A common way to derive the back stress in FEM-modelling is to use the Armstrong-Frederick relation. It is demonstrated that this can give rise to unexpected results in the sense that back stress components can be much larger than the corresponding applied stress components. Modification of the Armstrong-Frederick relation are proposed that avoid this problem. An alternative approach is also given. In this case the back stress is assumed to be a scalar. Then its value can be derived in the same way as in the uniaxial case, for example using the Bergstrom-Kocks-Mecking model for FCC alloys. In the Odqvist equation for the creep rate, the effective stress is reduced with the back stress, but the stress deviator is left unmodified. The models are applied to copper canisters for storage of nuclear waste. It is demonstrated that the models give virtually the same result.

  • 13.
    Sandström, Rolf
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Östling, H.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Modelling of creep in friction stir welded copper2013In: Materials research innovations (Print), ISSN 1432-8917, E-ISSN 1433-075X, Vol. 17, no 5, p. 350-354Article in journal (Refereed)
    Abstract [en]

    Copper canisters for storage of nuclear waste will be exposed to creep. The canisters will be closed with friction stir welding (FSW). To describe the creep behaviour of the welds, uniaxial creep tests have been performed. A previously developed fundamental creep model for parent metal is applied to the different weld zones. The differences in microstructure and yield strength between the weld zones are taken into account. Creep strain versus time curves for the weld zones have successfully been predicted without the use of any adjustable parameters. It should be noted that the temperature range of interest of 50-100 degrees C is deep down in the power law break down regime with Norton exponents between 25 and 100. The constitutive equations are used in FEM computations of creep in the canister weldments.

  • 14. Wu, R.
    et al.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Influence of multiaxial stresses on creep properties of phosphorus alloyed oxygen free copper2010In: Proceedings Of The ASME Pressure Vessels And Piping Conference 2009, ASME Press, 2010, no PART B, p. 1525-1532Conference paper (Refereed)
    Abstract [en]

    Phosphorus alloyed oxygen free copper (Cu-OFP) canisters are planned to be used for spent nuclear fuel in Sweden. The copper canisters will be subjected to creep under multiaxial stress states in the repository. Creep tests have therefore been carried out using double notch specimens having a notch acuity of 0.5 in Cu-OFP at 75°C. The creep results from the notched specimens are compared with those from the smooth ones. It shows that the creep lifetime for notched specimens can be estimated to be two orders of magnitude or more longer than that for the smooth ones, indicating notch strengthening for the investigated Cu-OFP material. Metallographic examinations after failure show that pores and creep cavities to a limited extent are observed only adjacent to fracture. To interpret the tests for the notched creep specimens, finite element computations have been performed with a new basic model for primary creep without fitting parameters. The creep strain versus time could be simulated successfully. Initially the stresses at the notches are almost twice as high as in the centre of the specimens. The highest stresses are relaxed rapidly. At the studied temperature 75°C, the creep exponent of Cu-OFP is about 85, thus, deep down in the power-law breakdown regime. This contributes strongly to the homogenous stress distribution across the centre section. Since the redistribution of stresses has taken place before large creep deformation has occurred, the specimens are not notch sensitive in agreement with observations.

  • 15. Wu, Rui
    et al.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Creep crack growth in phosphorus alloyed oxygen free copper2013In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 583, p. 151-160Article in journal (Refereed)
    Abstract [en]

    Creep crack growth (CCG) has been studied in phosphorus-alloyed oxygen-free copper (Cu-OFP) at 22, 75, 175, and 215 degrees C with compact tension (CT) specimens. At 175 and 215 degrees C, the cracks grew about 10 mm before final instantaneous failure. In contrast, there was no visible crack growth at 22 and 75 degrees C. Strongly deformed grains were observed adjacent to the cracks at 175 and 215 degrees C. Intergranular creep cavities were found around the cracks. At 22 and 75 degrees C, deformed grains and some cavities as well as microcracks were observed close to the crack tip. A model for crack propagation based on creep damage formation has been formulated to interpret the test results. Rupture criteria based on both creep ductility exhaustion and grain boundary cavitation were taken into account. The contribution from the ductility exhaustion to the creep damage dominated at the lower two test temperatures whereas the contribution from grain boundary cavitation at the higher test temperatures. The model can describe the influence of temperature on the observed creep crack propagation. It can also account for the observed cavitation in a qualitative way.

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