Pedro Vilaça

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.

Effect of shoulder cavity and welding parameters on friction stir welding of thin copper sheets

Abstract The aim of this investigation was to study the influence of shoulder cavity and welding parameters on torque, defect formation, microstructure and mechanical properties of friction stir welds in very thin sheets of deoxidised copper. Three types of tools were used: a flat shoulder tool and two tools with conical shoulder cavities of 3 and 6° respectively. The welding parameters analysed were tool rotation and traverse speeds. It was observed that the torque, the microstructure and hardness and the formation of defects in the welds are influenced mainly by tool rotation speed and, to a lesser extent, by the traverse speed and shoulder cavity. The tensile properties of welds carried out at high rotation speeds are little affected by the shoulder cavity.

Friction Stir Welding Technology

The friction stir welding (FSW) process was invented in 1991 by Wayne Thomas et al., one of the authors of this chapter. This machine tool based process is currently considered an important development in welding technology, saving costs and weight for a steadily expanding range of applications of lightweight metallic structures. Evidences of the disruptive character of the FSW process are the prompt adoption by world-wide industry of the significant advantages of FSW and the numerous technic-scientific papers and patents published. The FSW technology has been subjected to the most demanding quality standard requirements and used in challenging industrial applications over a wide range of structural and non-structural components. In this chapter, some of the basic fundamentals underpinning the invention of FSW technology are presented with emphasis for the concept of the third-body region. The state-of-the-art concerning tooling in FSW for conventional and bobbin stir welding approaches are introduced. The non-destructive testing assessment of the most relevant imperfections in FSW is also discussed for butt and lap joints. In summary, the FSW is a key joining technology for lightweight metallic structures. The international organization for standardization standard for welding aluminium alloys by FSW is available and the most recent European standards for design of structures—Eurocodes, already include guidelines for the application FSW process.

A reconfigurable digital signal processing system for eddy currents non-destructive testing

This paper presents a digital signal processing system specially designed for eddy currents non-destructive testing. This new system has a field programmable gate array based processing core, communication interfaces, data conversion and analog devices to interface the probes. Communication with personal computers is ensured by Ethernet 10/100 and universal serial bus 2.0 high speed interfaces. The proposed architecture enables to set several combinations of peripherals cards to generate or acquire probe signals. Also, the new system allows the digital generation and analysis of the probe signals through multiple digital signal processing algorithms. Two different peripheral cards have been developed to meet the needs for the new IOnic concept of eddy currents probe. The IOnic acquisition card is composed by a programmable gain amplifier and a high speed analog to digital converter. The current stimulus generation is achieved with a digital to analog converter and a high output current transconductance amplifier. Together, the two peripherals are able to operate the probe from 10 kHz up to 10 MHz. An additional peripheral card to interface stepper motors was designed for sensor positioning.

Microstructural mapping of friction stir welded AA 7075-T6 and AlMgSc alloys using electrical conductivity

Abstract AlMgSc and AA 7075-T6 alloys find applications in the aeronautic industry due to their lightweight associated with high mechanical strength and fatigue resistance. Both alloys have poor weldability when joined by fusion welding processes, which is overcome with the use of friction stir welding (FSW). Recent research work shows that electrical conductivity field analysis can be used as a material characterisation technique for solid state material welding exhibiting a microstructure gradient. This study aims to apply electrical conductivity field analysis to bead on plate FSW to identify the potential application of this technique to map and characterise microstructural transformations. The FSW was conducted on AlMgSc and AA 7075-T6 plates with different parameters, and electrical conductivity measurements were performed at half thickness, complemented by other techniques as hardness, scanning electron microscopy and energy dispersive spectroscopy. A good correlation was observed among electrical conductivity field, microstructure and hardness of different zones in FSW beads.

Developments in NDT for Detecting Imperfections in Friction Stir Welds in Aluminium Alloys

Friction stir welding (FSW) has dramatically changed how aluminium alloys can be welded. The quality of FS welds is usually excellent, but some imperfections periodically occur. The geometry, location, and microstructural nature of these imperfections bear no resemblance to the imperfections typically found in aluminium fusion welds. Consequently, it has been difficult to identify FS weld imperfections with common non-destructive testing (NDT) techniques. Therefore, further development of NDT techniques must be done to enable the detection of FS weld imperfections. This paper presents an integrated, on-line, NDT inspection system for FS welds, which employs a data fusion algorithm with fuzzy logic and fuzzy inference functions. It works by analyzing complementary and redundant data acquired from several NDT techniques (ultrasonic, Time of Flight Diffraction (ToFD), and eddy currents) to generate a synergistic effect that is used by the software to improve the confidence of detecting imperfections. The system was tested on friction stir welded AA5083-H111 specimens. The results indicate that by combining the output from various NDT processes, an improvement in finding imperfections can be obtained compared to using each NDT process individually. The methodology implemented in the QNDT_FSW system has given good results and improved reliability in the NDT of friction stir welds.

Computational Tools for Modelling FSW and an Improved Tool for NDT

The good quality of Friction Stir Welding (FSW) joints enables the significant development of industrial applications of solid state welding technologies. This high quality standard is even more significant when FSW is compared to the fusion techniques. Nevertheless, in FSW joints some defects may arise which are very sensitive to small variations in some process parameters. Moreover, the results from computational modelling of the FSW are only valid for non-defective welds. Thus, in order for modelling of the process to contribute to the industrial consolidation of the FSW process, the experimental implementation results need to be supported by a reliable, Non-Destructive Testing (NDT) system. This work addresses an integrated scheme of two computational tools which enables the support of a faster establishment of process parameters, addressing the material flow analysis, with numerical coupling between fluid dynamics and solid mechanics; using the analytical iSTIR code; and a new, NDT, eddy currents probe, able to detect the typical FSW root imperfections.

Joining Aluminium Alloys Dissimilar in Thickness by Friction Stir Welding and Fusion Processes

The industrial application field of the Friction Stir Welding process is growing and the initial high potential of this technique is being confirmed. Research centres worldwide along with some major industrial production companies are making progresses in developing this technique up to a know-how level where the transfer to the industrial environment is technologically reliable and economically successful. The present work starts with a comparison of relevant welding features for industrial application when resulting from the FSW and fusion welding processes typically applied to aluminium alloys. FSW has the potential to improve the construction of tailor blanks. The results of welding plates of AA1050; AA2024-T3 and AA5083-H111 with different thickness is presented in terms of surface finishing, residual deformation, metallurgical analysis and static strength efficiency of the joints performed by FSW, GMAW and GTAW.

Mechanical Characterization of Friction Stir Channels under Internal Pressure and In-Plane Bending

Friction Stir Channelling (FSC) is a simple and innovative technique of manufacturing integral and continuous channels (also referred as conformal channels) in monolithic plates in a single step. This paper is focused on the mechanical behaviour of integral and continuous FS channels produced in a monolithic plate of the aluminium alloy AA7178‑T6 with 13mm of thickness, typically used in structural aircraft applications. Internal pressure tests were conducted on specimens with a longitudinal friction stir channel. In-plane bending tests were carried out in specimens with longitudinal and transversal channels; different conditions were analysed for each FSC parameters set. Results were analysed and compared concerning the FS channels microstructure and base material mechanical properties. Bending tests results show that tool rotation speed has more influence in the FSC specimens bending strength than tool travel speed and internal pressure tests show that increase tool rotation speed increases the minimum pressure that leaking points arise.

Mechanical Behaviour of FSW Aluminium Tailored Blanks

The mechanical behaviour of homogeneous and inhomogeneous FSW aluminium tailored blanks is analysed in this paper. The heterogeneity in mechanical properties across the different weld zones is discussed based on hardness testing results. Tensile and formability test results are also shown and the mechanical behaviour of the welds is discussed in relation to the base materials. Despite the hardness tests have indicated very small differences in hardness, between the welds and the base materials, and the tensile test results also showed similarities in mechanical behaviour, the formability tests revealed additional difficulties in forming the welded sheets.