S.M.O. Tavares

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.

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.

Mechanical behaviour of AA 2024 friction stir overlap welds

Purpose – To investigate the effect of different welding configurations on the mechanical properties of friction stir welding (FSW) overlap joints. The application of FSW in an overlap configuration could be an attractive replacement to the riveting process for assembly of fuselage primary structures due to the similarity in tolerance management. However, the mechanical properties of welded overlap joints are often inferior to the respective riveted lap‐joint properties.Design/methodology/approach – In order to quantify the static and fatigue strength of FSW overlap joints, numerical and experimental investigation on overlap welds were performed in the current work. Several single shear overlap joints welding configurations were investigated, including single and multiple pass friction stir welds. The static and fatigue behaviour of these joints was assessed through tensile and fatigue tests.Findings – Static and fatigue behaviour were found to strongly depend on the welding process parameters and configu...

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.

Friction Stir Welding of T-Joints in Dissimilar Aluminium Alloys

The T-joint is a common joint type frequently used in transport industries because of the importance of increasing the inertia and strength of thin skins and shells without significant weight increase. This shape can be obtained by different processes as extruding, riveting, welding or others. However, the low weldability of some aluminum alloys, when using traditional welding processes, is an obstacle to the possible full benefit of such reinforced structures. The friction stir welding (FSW) process is suitable to join most aluminum alloys and should be considered as a feasible alternative to the other processes used to produce this type of geometry. This paper reports the results obtained concerning FSW T-joints with a new configuration. These joints simulate a typical reinforcement composed by two materials in order to optimize the damage tolerance. The skin is made of a 6xxx series alloy, and the reinforcement is made of a 7xxx series alloy. Mechanical properties were obtained and micro-structural analyses of the weld zone were performed, and the results were compared with those obtained in base materials and butt joints.Copyright

Impact of Integral Structures in the Design for Manufacture and Assembly of Airframes

From a lean philosophy viewpoint, the huge amount of fasteners used in airframes appears to be the inverse of what the design for manufacturing should seek. New design concepts with integral joints may be produced using welding processes, simplifying and optimizing the design for manufacturing and assembly process; however, crack arrest capability may be compromised. An introductory quality function deployment (QFD) analysis applying the Cross method evaluates the main design goals in the application of new joining processes in lightweight reinforced structures. The analysis shows that new welding process can improve airframe structures provided the structural integrity is ensured.

12.3 – Friction Stir Welding of T-joints Fabricated with Three Parts

Compared with conventional welding processes, friction stir welding (FSW) requires different structural designs to produce T-joints. The friction stir welded T-joint is a type of welding connection with important or even critical applications. Among those being considered by industry or already in industrial practice, the reinforcement of skins using stiffeners for aircraft fuselages, or the body of railroad passenger cars can be mentioned as examples. New geometries to produce this type of joint are presented. For one of these solutions a metallurgical and mechanical characterization was performed using stress/strain and bending tests comparing the behaviour of base material and FSW butt joints. Numerical simulations of the bending tests of T-joints using the non-linear finite element code ABAQUS were performed and their results compared successfully with experimental data.

Fatigue Crack Growth

Readers of this book will have previous knowledge of fracture mechanics, and as such no effort will be made to delve here into fundamental concepts.

Widespread Fatigue Damage and Limit of Validity

Multiple site damage (MSD) and multi element damage (MED) decrease the number of cycles up to failure, and concomitantly decrease the interval for inspection.