Jesus Ruiz-Hervias

Influence of heating source position and dilution rate in achieving overmatched dissimilar welded joints

Abstract The investigation addresses the overall performance of black and white joints (BWJ) of low carbon steel (LCS) and stainless steel thin sheets achieved by laser hybrid welding. Assuming that the structural integrity is directly influenced by the processing temperature, a thermal simulation of BWJ of thin sheets was developed. Afterwards, the base metals apportionment at joint formation, namely their distinct dilution rate, was originally estimated from the top surface temperature variation. Defect-free laser hybrid dissimilar welds were experimentally obtained using the selected filler metal and the energetic input from the process simulation, even for a critical analysed case of heat source displacement from the weld gap centreline towards LCS. Detailed macro and microstructural examination of the BWJ and related microhardness analysis results are presented. The tensile tests results indicate that in the case of transversally loaded BWJ, the positive difference in yield between the weld metal and the base materials protects the weld metal from being plastically deformed; the flat transverse tensile specimens loading up to failure reveals large strains in LCS, far away from the weld.

Residual Stress Distributions in Bi-Metal (Ferritic to Austenitic Steel) Joints Made by Laser Welding

In this work, autogenous laser welding was used to join thin plates of low carbon ferritic and austenitic stainless steel. Due to the differences in the thermo-physical properties of base metals, this kind of welds exhibit a complex microstructure, which frequently leads to an overall loss of joint quality. Four welded samples were prepared by using different sets of processing parameters, with the aim of minimizing the induced residual stress field. Microstructural characterization and residual strain scanning (by neutron diffraction) were used to assess the joints’ features.

Specific properties of ferritic/austenitic Dissimilar Metals Welded Joints

This paper addresses several peculiar properties of ferritic/austenitic welded joints (Black and White Joints-BWJ) of thin sheets, which are emerging nowadays due to cost saving and satisfactory service performance. Starting from 3D thermal field modelling approached by considering Goldak’s double ellipsoidal heat source, together with a contribution of the authors, a 2D FEM-based model was used for arc welding of low carbon steel (CS) and stainless steel (SS) of thin sheets (3 mm). Conclusion helped to conduct experiments of BWJ arc welding (MMA and TIG), and laser-GMA hybrid welding, whilst infrared thermographic measurements were compared with the FEM results. Besides microstructural examination and mechanical characterization (hardness and tensile), fracture toughness and stress corrosion tests were performed on the BWJ laser hybrid welded. Whilst the model has offered important qualitative information about the asymmetric thermal field and the heat apportionment on the molten metal pool formation, the experiments have shown inadequate microstructures and constitutions of the welds for MMA and TIG welding, with respect to laser-GMAW hybrid joints. The description of the experimental program focused on B&W laser-GMA hybrid welding of thin plates aiming a characterization procedure using fracture mechanics methods is briefly presented.

Failure Criteria for Unirradiated PWR Cladding Subjected to Ring Compression Tests

The ring compression test applied to nuclear fuel cladding is relatively easy to perform but difficult to interpret. It can be representative of the loading state associated to a hypothetical spent fuel assembly drop accident. This is particularly important for spent fuel cladding subjected to drying operations previous to storage and transportation, because they may produce hydride reorientation along the radial direction of cladding. In this paper, experimental testing and numerical simulations are combined to obtain operative failure criteria from the results of the ring compression tests on unirradiated pre-hydrided samples with radial hydrides, simulating drying, storage and subsequent transport conditions.Copyright

Residual stress measurements of alumina-zirconia ceramics by time-of-flight neutron diffraction

Neutron strain scanning and Rietveld analysis were used to study the residual stresses in Al2O3/Y-TZP ceramic composites fabricated by different green processing techniques (a novel tape casting and conventional slip casting) and with different zirconia content. The results show that the residual stresses in zirconia particulates are tensile and the ones in alumina matrix are compressive, with almost flat through-thickness residual stress profiles in all bulk samples. The residual stresses for both phases were mainly dependent on the zirconia content, irrespective of the measurement direction and the fabrication process.

Laser shock processing influence on constitutive behaviour of graded structures produced by laser welding of ferritic to austenitic stainless steel

Abstract The main objective of the present investigation was to assess the constitutive behaviour of the graded structures produced by laser shock processing (LSP) on laser welded joints made between thin sheets of ferritic and austenitic stainless steel. First, the evolution of surface strain patterns during uniaxial loading of the as welded and LSP treated joints was experimentally obtained by using a video-image correlation technique. Overall and local constitutive behaviour across the weld, heat affected zone and parent metals resulted by coupling the applied load level with the corresponding strain average of each zone. The differences found in mechanical resistance of the LSP treated zones when comparing with the as welded ones were due to the processing features, namely the generation of compressive stresses and the development of graded surface structures. Special attention was paid on changes occurred in the ferritic steel side of the joint, which acts as a mechanical cut-out fuse in structures working in energy related applications. Then, by extrapolating the fatigue fracture behaviour of macroscopic notches to the surface cavities and irregularities from which surface topography results it was found that LSP improves the ferritic steel side fatigue resistance. Additionally, the growth of eventual surface flaws is delayed by the compressive stress state induced by LSP, as indicated by in-depth X-ray measurements.

Residual strain scanning of alumina-based ceramic composites by neutron diffraction

Residual strain profiles were measured by neutron diffraction in alumina-aluminum titanate ceramic composites sintered at two different temperatures, namely 1450 and 1550°C. The results show that irrespective of the direction and the sintering temperature, the obtained profiles are almost flat, with very similar results for both temperatures. In addition, the results demonstrate that the alumina is in compression whereas the aluminium titanate is subjected to tensile residual stresses.

Effect of Initial Residual Stress on Stress Relaxation in Cold-drawn Steel Rods

This work shows the effect of the initial residual stress state on the stress relaxation behavior of cold-drawn steel rods. The evolution of residual strains at several locations along the rod diameter was measured in-situ by neutron diffraction during a stress relaxation experiment. It was found that if residual stresses are significant, stress relaxation is not homogeneous in the cross-section of the rods. This also explains the higher stress losses found in the rods with high residual stresses.

Residual Strain Profiles in Alumina-Zirconia Ceramic Composites

Residual strain profiles were measured by synchrotron X-ray radiation in Al2O3/Y-stabilized ZrO2 (YSZ) ceramic laminates. Different stacking sequences were employed, including alternating layers containing 5 and 40 vol.% YSZ. Residual strains were found to be fairly constant within each layer; although they change at the interface between layers with different compositions. Different behaviour is observed for the strains along the in-plane and normal directions.

Effect of Varying Cold-Drawing Deformation on Residual Stresses Measured by Neutron Diffraction in Steel Rods

Cold-drawing is employed to fabricate wires and rods, which are mainly used as structural reinforcements in construction as well as in the tyre industry. As a consequence of processing, a residual stress profile is developed. In this paper, residual stress profiles are measured by neutron diffraction in cold-drawn pearlitic steel rods subjected to different deformations (true strain from 0.3 to 1.7). The results show that the residual stress profile produced by cold-drawing is similar in all the samples, irrespective of the degree of deformation.