Rolf Sandström

Influence of welding speed on the fatigue of friction stir welds, and comparison with MIG and TIG

Abstract The objective of this investigation was to determine whether the fatigue strength of friction stir (FS) welds is influenced by the welding speed, and also to compare the fatigue results with results for conventional arc-welding methods: MIG-pulse and TIG. The Al–Mg–Si alloy 6082 was FS welded in the T6 and T4 temper conditions, and MIG-pulse and TIG welded in T6. The T4-welded material was subjected to a post-weld ageing treatment. According to the results, welding speed in the tested range, representing low and high commercial welding speed, has no major influence on the mechanical and fatigue properties of the FS welds. At a significantly lower welding speed, however, the fatigue performance was improved possibly due to the increased amount of heat supplied to the weld per unit length. The MIG-pulse and TIG welds showed lower static and dynamic strength than the FS welds. This is in accordance with previous comparative examinations in the literature on the fatigue strength of fusion (MIG) and FS welds. The TIG welds had better fatigue performance than the MIG-pulse welds. The softening of the alloy around the weldline has been modelled. Using a model without adjustable parameters, a fair description of the hardness profiles across the weld as a function of welding speed was obtained. The softening in front of the Friction Stir Welding tool was also estimated. At the low and high welding speeds a full and partial softening is predicted, respectively.

A model for hot working occurring by recrystallization

Abstract A theoretical model is presented for hot working when dynamical recrystallization occurs. The workhardening and recovery of the dislocation structure as well as the recrystallization and the continuous grain growth are taken into account. The model is based on a description of the volume distributions of dislocations, which are produced as a result of the continuous recrystallization. Stress-strain curves are computed for iron and nickel and compared with experimental results of Luton and Sellars, and of Glover and Sellars. The model reproduces a number of features in the experimental curves as, for example, the flow stress oscillations at low strain rates, the distinct peak stress at high strain rates, the strain rate dependence of the peak stress and of the steady state stress, the strain rate dependence of the recrystalized grain size. The model suggests that the distinct peak stress at high strain rates is largely an effect of the variation in grain size during the dynamical recrystallization.

Shape memory materials and hybrid composites for smart systems: Part II Shape-memory hybrid composites

By hybridizing or incorporating shape-memory materials with other functional materials or structural materials, smart composites can be fabricated which may utilize the unique functions or properties of the individual bulk materials to achieve multiple responses and optimal properties, or, to tune their properties to adapt to environmental changes. A variety of shape-memory hybrid composites have been designed and manufactured, with shape-memory elements being either the matrix or the reinforcement. The hybrid composites provide tremendous potential for creating new paradigms for material–structural interactions and demonstrate varying success in many engineering applications. This review, from the standpoint of materials science, will give a state-of-the-art survey on the various shape-memory hybrid smart composites developed during the last decade. Emphasis is placed on the design, fabrication, characterization and performance of fibre-reinforced, particle-reinforced and multi-layered thin-film shape-memory composites.

New modelling of the B2 phase and its associated martensitic transformation in the Ti-Ni system

A symmetric two-sublattice model (Ni, Ti, Va){sub 0.5}(Ni, Ti, Va){sub 0.5} is applied to describe the intermediate B2 compound in order to cope with the order-disorder transition in the Ti-Ni system. Using this model, the ordered B2 and the disordered Ti-rich b.c.c. are described by a single Gibbs free energy function. The B2 phase is the parent phase of the martensitic transformation in the TiNi shape memory alloys (SMAs), and its thermodynamic properties are then reassessed with emphasis on its composition range that is critical for SMAs. The low temperature B19{prime} phase is also evaluated on the basis of the selected experimental data from the martensitic transformation. Properties related to the transformation are studied in comparison with experimental data. The magnetic contribution is examined for the martensitic transformation. All calculations are in satisfactory agreement with experimental phenomena.

On recovery of dislocations in subgrains and subgrain coalescence

Abstract A model is presented for the recovery of dislocations in the subgrain interiors and in the subboundaries. In addition to the pairwise annihilation the transfer of dislocations between the interior and the boundaries is considered. It is demonstrated that the recovery of the dislocations in the subgrain interiors in general occurs in two steps: first by pairwise annihilation and then by absorption at the boundaries, the latter process being essentially slower. The contribution from two mechanisms for subgrain coalescence is derived: extraction and emission of dislocations from the subboundaries. These contributions are compared to that from migration of boundary dislocations derived by Li. A comparison is also made to the subgrain growth rate due to subboundary migration. The two latter contributions which both involve the collective migration of dislocations, and are therefore conveniently referred to as such, are identical up to a numerical factor and occur consequently always together. Comparison to experiments will be made in a forthcoming paper.

Comparison of experimental, calculated and observed values for electrical and thermal conductivity of aluminium alloys

A model has been developed for calculating the electrical resistivity of commercial aluminium alloys from composition and heat treatments using the Matthiessen rule. The model is based on the approximation that the solubility of the alloying elements in heat treated alloys is equivalent to the equilibrium solubility at a higher temperature. These temperatures were determined from heat treatment data. The resistivity of a wide range of commercial alloys was calculated using the model, showing an agreement with most observed resistivity values of within 3 nΩm, except for alloys with special composition characteristics. According to the model, magnesium and manganese are important contributors to the resistivity for all main groups of alloys. In heat treated alloys the contribution of precipitates is 6–17% of the total resistivity. Thermal conductivity was calculated for alloys given in the literature using the Wiedeman–Franz law and the calculated resistivity. The calculated thermal conductivity agreed with the experimental values for the AlMg-alloys, but it was lower than the experimental values for pure aluminium and the AlCu-alloys in the annealed condition.

Measurement of the ductile-to-brittle transition temperature in a nickel aluminide coating by a miniaturised disc bending test technique

Nickel aluminide coatings are often employed to enhance the corrosion and oxidation resistance of nickel base gas turbine blades and vanes, as the high near-surface content of Al increases the ability to form an Al2O3 protective scale. The ductility of the coating depends on the type of aluminisation process and Ni-base material. In order to prevent coating degradation during service it is important to assess the ductile-to-brittle transition temperature (DBTT) in ductility of the coating. To determine the DBTT a miniaturised disc bending test (MDBT) technique is used, where a biaxial tensile stress is applied to a disc specimen. The DBTT of a NiAl coating, applied by a high-activity aluminium pack cementation process to a polycrystalline Ni-base superalloy (IN738 LC), was evaluated using the MDBT technique between room temperature (RT) and 860 degreesC. Test results gave a DBTT in biaxial ductility of the coating of approximately 760 degreesC. Above 760 degreesC, a significant increase in ductility was noted. Fractographic examination showed that the coating fractures in a mainly transgranular mode at RT but in a predominately intergranular mode at elevated temperatures, even at temperatures above DBTT.

Subgrain growth in Al and Al-1% Mn during annealing

The subgrain growth during annealing of cold-worked Al-1% Mn and Al(4N) has been measured in the temperature intervals 300 to 400° C and 100 to 200° C respectively. For Al-1%Mn the subgrain diameter showed a parabolic growth, while for Al the diameter gradually reached a constant value. The microstructural investigations which included in situ annealing in a high voltage electron microscope demonstrated that the operating mechanism for growth in the higher temperature range was collective migration of sub-boundary dislocations. In the lower temperature range extraction of dislocations was found to be the dominating mechanism, i.e. dislocations partly lying in the boundaries are pulled out by the stress field in the subgrain interior. The observed growth rates were consistent with models for these mechanisms presented in two previous papers.

Relationship between Charpy V transition temperature in mild steel and various material parameters

Abstract A simple theory for the Charpy V transition temperature T CV in mild steel is proposed. The yield stress σy and the internal cleavage fracture stress σf play an important role in the theory and, therefore, have been the subject of detailed experimental and theoretical studies. Experimentally recorded values of T CV in a C-Mn steel of varying grain size and a microalloyed steel are compared with the corresponding theoretical values. The results are critically discussed.

Low temperature creep ductility of OFHC copper

Abstract Creep tests have been carried out on oxygen-free high conductivity (OFHC) copper between 75 and 245°C. Two of the batches exhibited poor ductility at temperatures between 180 and 245°C and ruptured at creep strains of