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| 2. |
- Seidlitz, M., et al.
(författare)
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Coulomb excitation of Na-29,Na-30: Mapping the borders of the island of inversion
- 2014
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Ingår i: Physical Review C (Nuclear Physics). - 0556-2813. ; 89:2
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Tidskriftsartikel (refereegranskat)abstract
- Nuclear shell evolution in neutron-rich Na nuclei around N = 20 was studied by determining reduced transition probabilities, i.e., B(E2) and B(M1) values, in order to map the border of the island of inversion. To this end Coulomb-excitation experiments, employing radioactive Na-29,Na-30 beams with a final beam energy of 2.85 MeV/nucleon, were performed at REX-ISOLDE, CERN. De-excitation gamma rays were detected by the MINIBALL gamma-ray spectrometer in coincidence with scattered particles in a segmented Si detector. Transition probabilities to excited states were deduced. The measured B(E2) values agree well with shell-model predictions, supporting the idea that in the Na isotopic chain the ground-state wave function contains significant intruder admixture already at N = 18, with N = 19 having an almost pure two-particle-two-hole deformed ground-state configuration.
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| 3. |
- Altenkirch, J., et al.
(författare)
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Comparison of methods to determine variations in unstrained unit cell parameter across welds
- 2011
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Ingår i: Journal of Strain Analysis for Engineering Design. - : SAGE Publications. - 0309-3247 .- 2041-3130. ; 46:7, s. 651-662
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Tidskriftsartikel (refereegranskat)abstract
- Many alloys undergo complex changes in local chemistry in the vicinity of weldments due to the thermal excursion during welding. The resulting changes in solute concentrations can lead to significant local variations in the unstrained unit cell parameter which, if not accounted for, can lead to serious error when determining residual stress by diffraction methods. Age-hardening aluminium alloys are particularly susceptible to such effects. The present paper compares three methods (plane stress assumption, sin(2)psi method, and comb correction method) for evaluating the stress-free unit cell parameter variation for friction stir welds in AA7449-W51 plates of two different thicknesses. All three methods gave comparable results for thin (5 mm) sheet, but for the thicker (12.2 mm) plate the results calculated on the basis of the plane stress assumption diverged from the other two, largely because in this case the other methods indicate there to be a significant triaxiality of stress. In the example cases, hardness and unstrained unit cell parameter variations were found to be strongly correlated across the welds. The advantages and disadvantages of the three methods are compared.
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| 4. |
- Altenkirch, J., et al.
(författare)
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Mechanical Tensioning of High-Strength Aluminum Alloy Friction Stir Welds
- 2008
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Ingår i: Symposium on Neutron and X-Ray Studies for Probing Materials Behavior held at the 137th TMS Annual Meeting and Exhibition. - : Springer Science and Business Media LLC. - 1073-5623. ; 39A, s. 3246-3259
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Konferensbidrag (refereegranskat)abstract
- The extent to which in-situ global mechanical tensioning (GMT) can be used to modify the residual stress state in friction stir (FS) welds is investigated in this article. Residual stress distributions have been determined by synchrotron X-ray and neutron diffraction for four sets of FS welds in high-strength AA7449-W51 and lithium containing AA2199-T8 aerospace aluminum alloys subjected to a systematic range of GMT levels. For the cases studied, the results indicate that the level of residual stresses present in the as-welded state is a function of the alloy. The rate of residual stress reduction brought about by GMT, however, is basically alloy independent; indeed, it is essentially linear with respect to the GMT load, so that the tensioning required to reduce the weld stresses to zero can be calculated directly from the stresses present in the untensioned case. For thin plates, proximity to the yield stress in the hot-softened zone means that a guideline rule is that 1 MPa of tensioning during welding reduces the tensile stress by approximately 1 MPa. The GMT was found to be less effective at greater depths in thick plates. Furthermore, a reduction in bending distortion and an increase in angular distortion was observed with increased GMT, while no effects on the weld microstructure and hardness were observed.
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| 5. |
- Altenkirch, J., et al.
(författare)
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The extent of relaxation of weld residual stresses on cutting out cross-weld test-pieces
- 2009
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Ingår i: Powder Diffraction. - : Cambridge University Press (CUP). - 0885-7156 .- 1945-7413. ; 24:2, s. 31-36
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Tidskriftsartikel (refereegranskat)abstract
- Weld residual stress (RS) measurements are often undertaken on test-pieces which have been Cut Out from large components, yet it remains unclear to what extent the RSs in test-pieces are representative of those present in the original component. Similarly weld mechanical performance tests are frequently undertaken on cross-weld test-pieces without a proper understanding of the level or influence of retained RS. We present a systematic study of the relaxation of longitudinal RS in thin-plate butt welds produced using different materials and welding methods (FSW, laser-MIG, and pulsed-MIG). In each case the RSs were measured repeatedly in the same location as the welds were progressively and symmetrically cut down. Although cutting inevitably leads to stress redistribution, significant relaxation of the longitudinal RS was only observed when the weld length or width was reduced to below a certain value. This critical value appears to correlate with the lateral width of the tensile zone local to the weld-line and may be considered to be the characteristic length as defined in St. Venant's principle. Further, it was found that the level of stress relaxation as a function of weld length for all the welds studied could be collapsed onto a single empirical curve using a simple approach based on the characteristic length scales of the weld. Given the range of materials and welding methods used, this relation appears to be of general use for thin-plate welds although further work is required to test the limits of its applicability. (C) 2009 International Centre for Diffraction Data. [DOI: 10.1154/1.3152580]
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| 6. |
- Barnes, S. J., et al.
(författare)
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Friction Stir Welding in HSLA-65 Steel: Part I. Influence of Weld Speed and Tool Material on Microstructural Development
- 2012
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Ingår i: Metallurgical and Materials Transactions A - Physical Metallurgy and Materials Science. - : Springer Science and Business Media LLC. - 1073-5623. ; 43A:7, s. 2342-2355
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Tidskriftsartikel (refereegranskat)abstract
- A systematic set of single-pass full penetration friction stir bead-on-plate and butt-welds in HSLA-65 steel were produced using a range of different traverse speeds (50 to 500 mm/min) and two tool materials (W-Re and PCBN). Microstructural analysis of the welds was carried out using optical microscopy, and hardness variations were also mapped across the weld-plate cross sections. The maximum and minimum hardnesses were found to be dependent upon both welding traverse speed and tool material. A maximum hardness of 323 Hv(10) was observed in the mixed martensite/bainite/ferrite microstructure of the weld nugget for a welding traverse speed of 200 mm/min using a PCBN tool. A minimum hardness of 179 Hv(10) was found in the outer heat-affected zone (OHAZ) for welding traverse speed of 50 mm/min using a PCBN tool. The distance from the weld centerline to the OHAZ increased with decreasing weld speed due to the greater heat input into the weld. Likewise for similar energy inputs, the size of the transformed zone and the OHAZ increased on moving from a W-Re tool to a PCBN tool probably due to the poorer thermal conductivity of the PCBN tool. The associated residual stresses are reported in Part II of this series of articles.
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| 7. |
- Steuwer, Axel, et al.
(författare)
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Friction Stir Welding of HSLA-65 Steel: Part II. The Influence of Weld Speed and Tool Material on the Residual Stress Distribution and Tool Wear
- 2012
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Ingår i: Metallurgical and Materials Transactions A - Physical Metallurgy and Materials Science. - : Springer Science and Business Media LLC. - 1073-5623. ; 43A:7, s. 2356-2365
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Tidskriftsartikel (refereegranskat)abstract
- A set of single pass full penetration friction stir bead-on-plate and butt welds in HSLA-65 steel were produced using a range of traverse speeds (50 to 500 mm/min) and two tool materials (W-Re and PCBN). Part I described the influence of process and tool parameters on the microstructure in the weld region. This article focuses on the influence of these parameters on residual stress, but the presence of retained austenite evident in the diffraction pattern and X-ray tomographic investigations of tool material depositions are also discussed. The residual stress measurements were made using white beam synchrotron X-ray diffraction (SXRD). The residual stresses are affected by the traverse speed as well as the weld tool material. While the peak residual stress at the tool shoulders remained largely unchanged (approximately equal to the nominal yield stress (450 MPa)) irrespective of weld speed or tool type, for the W-Re welds, the width of the tensile section of the residual stress profile decreased with increasing traverse speed (thus decreasing line energy). The effect of increasing traverse speed on the width of the tensile zone was much less pronounced for the PCBN tool material.
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| 8. |
- Steuwer, Axel, et al.
(författare)
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A combined approach to microstructure mapping of an Al-Li AA2199 friction stir weld
- 2011
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Ingår i: Acta Materialia. - : Elsevier BV. - 1873-2453 .- 1359-6454. ; 59:8, s. 3002-3011
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Tidskriftsartikel (refereegranskat)abstract
- A wide range of complementary techniques are used to build up a detailed picture of the microstructural zones found in friction stir welds (FSW) in an advanced AA2199 Al-Li alloy. Neutron and synchrotron X-ray diffraction, transmission electron microscopy, differential scanning calorimetry, small angle X-ray scattering, scanning electron microscopy, electron backscatter diffraction (EBSD) and hardness mapping are brought together to build up a detailed two-dimensional picture of the grain morphology, precipitate type, size, volume fraction and matrix solute content across the weld cross section and to explain the general lack of a W-shaped hardness profile across FSW in third-generation Al-Li-Cu Mg alloys. Dissolution of the age-hardening phases occurred in different regions of the weld, depending on their respective solvus temperatures, with 5' dissolving within the heat-affected zone and T-1 coarsening in the thermo-mechanically affected zone before going into solution in the weld nugget. Changes to the precipitate distribution, and especially to the T1 phase, are linked to a significant reduction in hardness (strength) and unstrained lattice parameter across the weld zone. It was also possible to show that the low recovery of nugget zone hardness is primarily due to its poor natural ageing response. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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