V. Richter-Trummer

Friction stir welding of T-joints with dissimilar aluminium alloys: mechanical joint characterisation

Abstract Friction stir welding (FSW) is a high reliability joining process creating excellent opportunities for new design concepts. This paper discusses T-joints composed by dissimilar aluminium alloys, a configuration suitable for reinforced panels where the skin is made of an aluminium alloy with higher toughness, and the web (reinforcement or stiffener) is made of a higher strength aluminium alloy, creating a good damage tolerant arrangement. A T-joint configuration was proposed non-including overlap interfaces between the workpieces. This T-joint also promotes a good flow among the materials of the different workpieces during the FSW process resulting in sound welds. Mechanical properties were measured achieving high efficiency values of joint static and dynamic strenght but with the drawback of the loss of elongation. Microstructural analyses of the weld zone were performed, and the results were compared with those of base materials and FSW butt joints evidencing the possibility of joining two dissimilar aluminium alloys in a T configuration. Additionally, the residual stress field, which is an important parameter for a more reliable design of integral structures, was evaluated with a semidestructive and a destructive method. The feasibility to weld T-joints with dissimilar aluminium alloys was demonstrated achieving good quality results, which can be used for structure reinforcement and optimisation.

The through-the-thickness measurement of residual stress in a thick welded steel compact tension specimen by the contour method

Compact tension specimens are widely used for fatigue crack growth characterization. It is well known that when using this type of specimen for welded samples, residual stresses affect the crack growth behaviour, in particular, reducing the crack growth rate when the initial notch is parallel to the weldment. Several approaches have been used to characterize the residual stress field in welded compact tension specimens, including traditional destructive relaxation-based methods, as well as non-destructive diffraction-based techniques. The problem with the approaches presented so far is that the former do not provide through-the-thickness results, whereas the latter, although capable of providing through-the-thickness measurements, are only available in very few facilities worldwide. The interest in thick welded samples, where substantial through-the-thickness variations of residual stress fields are to be expected, leads to the need for a full-field characterization. In the present work, the residual stress perpendicular to the crack growth path in two welded steel compact tension specimens was measured by the contour method, an emerging destructive method. A full surface map of the residual stresses was obtained. This full field information on the residual stress distribution reveals substantial through-the-thickness variations. High compressive stresses were found in the centre of both specimens. The compact tension specimen with the crack plane in the centre of the weld bead presented slightly higher compressive residual stresses. The strong variation of the stress field, both in the longitudinal as well as in the through-the-thickness directions, emphasizes the need for full-field residual stress measurements for the correct interpretation of fatigue crack growth tests when using this type of specimen.

The Contour Method for Residual Stress Determination Applied to an AA6082-T6 Friction Stir Butt Weld

Residual stresses parallel to the welding direction on a cross-section of a 3 mm thick friction stir butt-welded aluminum alloy AA6082-T6 plate were determined using the contour method. A full contour map of longitudinal residual stresses on a weld cross section was determined in this way, revealing detailed information on the residual stress distribution in the inside of a friction stir weld, especially in the nugget zone. The typical M-shape, usually described for the residual stress distribution in friction stir welds, was found. The maximum residual stresses are below the yield strength of the material in the shoulder region and, outside of the welding region, low tensile and compressive residual stresses are responsible for the necessary stress equilibrium on the plane of interest. A comparison was made with the established incremental hole drilling technique on an equivalent plate for validation and good agreement of both techniques was obtained. The distribution, as well as the magnitude of the residual stresses measured by both techniques, is very similar, thus validating both the experimental and numerical procedures used for the contour method application, presented and discussed in the present paper.

Characterization of Fatigue Crack Growth Rate of AA6056 T651 and T6: Application to Predict Fatigue Behaviour of Stiffened Panels

Tensile and fatigue crack growth tests of the 6056 T651 and T6 aluminium alloys were carried out. The fatigue crack propagation tests were performed on compact tension 4mm thick (CT) specimens, under cyclic loading with R ratios 0.1 and 0.5. The resulting data was used to predict the fatigue behaviour of stiffened panels subjected to fatigue loading under similar R ratios. The AA6056-T651 panels were fabricated using High Speed Machining (HSM) starting with 30mm thick plates. AA6056-T651 CT specimens were cut from the panels mentioned above, whereas AA6056-T6 CT specimens were machined from 5mm thick material. It was found that the AA6056-T651 (HSM material) specimens, machined from a 30mm thick plate presented higher rupture and yield stress than the AA6056-T6 material extracted from a 5mm thick plate. When tested at the same R value the AA6056-T6 specimens present higher crack growth rate than the AA6056-T651 specimens.

Fatigue crack propagation behaviour in thick steel weldments

Purpose – The purpose of this paper is to study the fatigue crack growth (FCG) behaviour of the steel and weldments of a railway bridge.Design/methodology/approach – Tests were carried out on compact tension (CT) specimens using the thickness (B=32 mm) of a structural detail. The test matrix included three R values and three material conditions: base material (BM), heat affected zone (HAZ) and weld metal (WM). An evaluation of opening load behavior was carried out. The full field measurement of the residual stress perpendicular to the crack plane was performed using the contour technique. A simplified finite element analysis supported the interpretation of the results. Scanning electron microscopy (SEM) observation of the fracture surface of BM and HAZ specimens was carried out.Findings – Extensive crack closure effects were found in the welded specimens. Important through‐the‐thickness variation of residual stress was found using the contour technique. The residual stress fields of HAZ and WM specimens l...

Fibre Bragg grating sensors for monitoring the metal inert gas and friction stir welding processes

Fibre Bragg grating (FBG) sensors are finding increased usage in experimental mechanics for monitoring service conditions in structures and other equipment and are currently being tested for process monitoring. In FBG sensors, strain and temperature cause a shift in the Bragg wavelength reflected by the grating contained in these fibres. In situ monitoring of strain and temperature during welding processes increases knowledge of the welded material and the welding process itself. In the present work, two welding processes are monitored using FBG sensors and the complete measurement approach including sensor selection, calibration, instrumentation, welding monitoring and result interpretation is presented. Calibration for strain measurements at constant temperature was performed using a four-point bending test, and temperature calibration was carried out using an oven. Results for a sensor length of 5 mm are presented. Both transient and residual strains were recorded during experiments on metal inert gas and friction stir welding and the possible impact of this monitoring technology is discussed in the light of process optimization and subsequent structural health monitoring.

Fatigue Behaviour of FS, LB and MIG Welds of AA6061-T6 and AA6082-T6

The increasing use of aluminium alloys in transportation industry, not only in aeronautics but also in automotive industry, creates the need for research on more efficient and reliable welding processes to be used. In order to allow the industry to use novel manufacturing techniques as Laser Beam Welding and Friction Stir Welding, which promise high efficiency, research work on S-N and crack growth fatigue data from the weld zone is required to provide tools to assess the damage tolerance. This chapter is a contribution to this effort, contrasting the fatigue behaviour of joints made using a traditional process, Metal Inert Gas welding, with those made with Friction Stir and Laser Beam Welding.

Mechanical Characterization of Friction Stir Welds of Two Dissimilar Aluminium Alloys of the 6xxx Series

A study on the mechanical characterization of friction stir welds between aluminium alloys 6061-T6 and 6082-T6 was carried out. For comparison, single alloy joints made from each one of the two alloys were also performed. The work included microstructure examination, microhardness tests, tensile tests and bending tests of all joint types. An approximate finite element model of the joint, taking into account the spatial dependence of the tensile strength properties, was made, modelling a bending test of the weldments.

Fatigue Crack Growth Modeling in Stiffened Panels Considering Residual Stress Effects

A model to determine Stress Intensity Factors (SIFs) and simulate the fatigue crack growth in stiffened structures taking into consideration residual stresses is presented in this paper. The stress field required to estimate the SIF was calculated using the Finite Element Method (FEM) considering the residual stress as an initial condition. The residual stress field redistribution as a function of crack growth is taken into account using the Abaqus software. Specimens without and with residual stresses, resulting from different welding techniques, were considered for the present study. The residual stress fields can significantly deteriorate or improve the fatigue life of the structure, depending upon the location of the initial crack; consequently these effects should be analyzed and modelled in order to better understand the consequences of the application of the considered manufacturing processes.

Lightweight Stiffened Panels Fabricated Using Emerging Fabrication Technologies: Fatigue Behaviour

The need for lower cost and the emergence of new welding technologies has brought interest in large integral metallic structures for aircraft applications; however, new problems must be addressed, e.g. in integral structures, a crack approaching a stiffener propagates simultaneously in the skin and into the stiffener and breaks it. The use of manufacturing techniques such as high speed machining (HSM), laser beam welding (LBW) and friction stir welding (FSW) requires further experimental and numerical work concerning the fatigue behaviour of panels manufactured using those processes. This chapter is focused on an experimental test programme including fatigue crack growth rate characterization in panels fabricated using HSM, LBW and FSW. The work was developed in the frame of the European Union DaToN project. Data was obtained for panels tested in mode I crack propagation under load ratios (R) of 0.1 and 0.5. It was found that welded panels presented longer lives up to rupture. This result is associated to the residual stress fields existing in the welded panels, and also to the location of the initial artificial defect, placed in the skin midway the specimen’s two stiffeners.