OPTIMIZED FATIGUE AND FRACTURE PERFORMANCE OF FRICTION STIR WELDED ALUMINIUM PLATE: A STUDY OF THE INTER-RELATIONSHIP BETWEEN PROCESS PARAMETERS, TMAZ, MICROSTRUCTURE, DEFECT POPULATION AND PERFORMANCE

Friction stir welding (FSW) is an exciting new solid-state welding process with the potential to advantageously impact many fabrication industries. Current take-up of the process by industry is hindered by lack of knowledge of suitable welding parameters for any particular alloy and sheet thickness. The FSW process parameters are usually chosen empirically and their success is evaluated via simple mechanical property testing. There are severe drawbacks with such methods of determining manufacturing conditions. These include indirect relationships between tensile and fatigue properties, particularly for welds, and a high probability of totally missing real optimized conditions. This research is therefore undertaken as a first step in providing information that will assist manufacturing industry to make sound decisions with respect to selecting FSW parameters for weldable structural alloys. Some of the key issues driving material selection for manufacturing are weld quality in terms of defects, fatigue strength and crack growth, and fracture toughness. Currently a very limited amount of data exists regarding these mechanical properties of FSW welds, and even less information exists regarding process parameter optimization. This is due to the mechanical microstructural complexity of the process and the relatively large number of process parameters (feed, speed, force and temperature) that could influence weld properties. In order to advance predictive understanding and modeling for FS welds, it is necessary to develop force and energy based models that reflect the underlying nature of the thermo-mechanical processes that the material experiences during welding. This project aims at determining the influence and effect of Friction Stir Welding process control parameters on the microstructure of the thermo-mechanically affected zone, the defect population in the weld nugget, hardness, residual stresses, tensile and fatigue performance of 6 mm plate of 5083-H321 aluminium alloy, which is known to be susceptible to planar defect formation. Welds were made with a variety of process parameters (that is feed rate and rotational speed) to create different rates of heat input. Forces on the FSW tool (horizontal and vertical), torque and tool temperature were measured continuously during welding from an instrumented FSW tool. Detailed information on fatigue performance, residual stress states, microstructure, defect occurrence, energy input and weld process conditions, were investigated using regression models and contour maps which offer a unique opportunity to gain fundamental insight into the process-structure-property relationships for FS welds. Weld residual strains have been extensively measured using synchrotron X-ray diffraction strain scanning to relate peak residual stresses and the widths of the peak profiles, taken from a single line scan from the mid depth of the FS welds, with the weld process conditions and energy input into the welds. Several residual stress maps were also investigated. The optical and scanning electron microscope were used to determine the type of intrinsic defects present in the FSW fatigue and tensile specimens. Vickers hardness measurements were taken from the mid depth of the welds and were compared with the weld input parameters. The main contribution of this thesis is as follow: (i) the relationship between input parameters and process parameters; (ii) the relationship between input weld parameters (that is feed rate and rotational speed) and process parameters (that is vertical downwards force Fz, tool temperature, tool torque and the force footprint data), energy input and tensile strength, fatigue life and residual stresses to obtain regions of optimum weld conditions; (iii) identification of the defects present in FSW, their relationship with process parameters and their effect on tensile strength and fatigue life; and (iv) the usefulness of the real time process parameter monitoring automated instrumented FSW tool to predict the mechanical properties of the welds.Nelson Mandela Metropolitan University, South Afric

A Comparative Study of Tool-Pin Profile on Process Response of Friction Stir Welding of AA6082-T6 Aluminium Alloy

This paper presents research work conducted to experimentally establish the process response of two diverse shaped tool-pin profiles for friction stir welding (FSW) AA6082-T6 aluminium. The dwell time was optimised by plunging each tool-pin into a plate sample until the spindle torque stabilised thus ensuring sufficient plasticised material in contact with tool shoulder and the tool-pins. The welds were conducted by employing the optimised dwell time, which in turn revealed a minimised process response time and distance to reach weld stability with respect to (1) the force exerted on the tool-pin in the welding direction, Fx , and (2) the spindle torque, T, during the welding process. Both Fx and T stabilised well within the set (pre-determined) ramp-up distance of 20 mm, indicating that the effective (useful) weld length is maximised. The macrographs also revealed good dynamic material flow within the nugget zone regions and more evident in the nugget zone of the flared tool

Development of friction stir welding techniques for multi-axis machines

Le soudage par friction et malaxage (SFM) est un procédé d'assemblage innovant à l'état solide qui a été inventé en 1993 et qui présente des avantages significatifs par rapport aux techniques de soudage par fusion. En raison des grandes forces appliquées sur l'outil et la nécessité de maintenir un angle constant sur tout le chemin de soudage (angle d'inclinaison), ce processus est normalement effectué sur des machines coûteuses conçues spécifiquement à cette fin. La présente thèse est une tentative de faciliter l'application de soudage par friction malaxage sur les centres d'usinage CNC très communément rencontré en industrie. Une variante peu connue et peu développée de ce processus, à savoir le soudage par friction et malaxage à angle droit dans lequel l'axe d'outil est toujours perpendiculaire à la surface de la pièce a été étudié de près dans cette thèse. Des outils spéciaux pour le soudage par friction et malaxage qui sont appropriées pour cette nouvelle orientation ont été développés et les paramètres de fonctionnement de ces outils ont été mis en place. En outre, des techniques ont été investiguées pour réduire la force axiale par optimisation de la conception de l'outil et des paramètres de soudage. De plus, l'une des principales difficultés qui pourraient survenir durant les applications industrielles du soudage par friction et malaxage est l’alignement horizontal et vertical des pièces pour le soudage de joints aboutés. Une méthodologie est aussi proposée pour effectuer le soudage par friction et malaxage sur des contours 3D. La Méthode Taguchi a été utilisée pour la conception d'expériences et des modèles de réseaux neuronaux artificiels ont été formés pour l'analyse des résultats des expériences et pour l’optimisation. Il a été démontré que le soudage par friction et malaxage à angle droit a la capacité de faire des soudures saines avec limites ultimes acceptables en utilisant des valeurs plus basses de force axiale d’environ 50% par rapport au soudage par friction et malaxage typique. En outre, les plages utilisables des paramètres de conception de l'outil et des paramètres de fonctionnement ont été trouvées. Elles conduisent à la réduction de la force axiale du soudage par friction et malaxage à angle droit. Les erreurs de placement de pièces dans des joints aboutés ont aussi été investiguées conduisant à une définition des plages acceptables d’erreur avec la méthode de soudage à angle droit. Les techniques développées ont aussi été validées dans la mise en œuvre du soudage par friction et malaxage pour les joints 2D et 3D. De plus, la méthodologie proposée pour le soudage sur contours 3D a été validée avec succès en soudage sur une pièce particulière en utilisant une machine CNC à 5 axes dans les deux configurations de joints aboutés et superposés.Friction stir welding (FSW) is an innovative solid state joining method invented at the end of twentieth century and having significant advantages over fusion welding techniques. Due to the high amount of forces applied on the FSW tool and the need to keep a constant angle all over the welding path (tilt angle), this process in normally performed on costly machines designed specifically for it. The present thesis is an attempt to facilitate the implementation of friction stir welding on common CNC machining centers. A less considered variant of this process, namely right angle FSW in which the tool axis is always perpendicular to the surface of workpiece has been closely studied and investigated. Special FSW tools which are appropriate for this new orientation have been developed and operating parameters for these tools have been established. In addition, techniques were developed to reduce the axial force through optimization of tool design and welding parameters. Moreover, one of the major difficulties which could be encountered during industrial applications of FSW, joint fit-up issues have been explored and attempts were made to manage these issues. A methodology has been proposed for FSW over 3D contours. Taguchi method has been used for design of experiments and artificial neural network models have been trained for analysis of results of experiments and optimization. It has been shown that the right angle FSW have the capacity of making sound welds with acceptable UTS employing lower values of axial force in comparison to typical FSW. Furthermore, workable ranges of tool design and welding parameters were found that leads to reduction of axial force within right angle FSW. To tolerate for joint fit-up issues, regions of operating parameters were established that could manage typical values of gap and mismatch. The developed techniques have also been validated and implemented for joining on 2D and 3D paths. In addition, the 3D methodology has been successfully validated in welding a complex part using a 5 axis CNC machine in both butt and lap configurations

Friction Stir Welding of Copper Canisters Using Power and Temperature Control

This thesis presents the development to reliably seal 50 mm thick copper canisters containing the Swedish nuclear waste using friction stir welding. To avoid defects and welding tool fractures, it is important to control the tool temperature within a process window of approximately 790 to 910°C. The welding procedure requires variable power input throughout the 45 minute long weld cycle to keep the tool temperature within its process window. This is due to variable thermal boundary conditions throughout the weld cycle. The tool rotation rate is the input parameter used to control the power input and tool temperature, since studies have shown that it is the most influential parameter, which makes sense since the product of tool rotation rate and spindle torque is power input. In addition to the derived control method, the reliability of the welding procedure was optimized by other improvements. The weld cycle starts in the lid above the joint line between the lid and the canister to be able to abort a weld during the initial phase without rejecting the canister. The tool shoulder geometry was modified to a convex scroll design that has shown a self-stabilizing effect on the power input. The use of argon shielding gas reduced power input fluctuations i.e. process disturbances, and the tool probe was strengthened against fracture by adding surface treatment and reducing stress concentrations through geometry adjustments. In the study, a clear relationship was shown between power input and tool temperature. This relationship can be used to more accurately control the process within the process window, not only for this application but for other applications where a slow responding tool temperature needs to be kept within a specified range. Similarly, the potential of the convex scroll shoulder geometry in force-controlled welding mode for use in applications with other metals and thicknesses is evident. The variable thermal boundary conditions throughout the weld cycle, together with the risk of fast disturbances in the spindle torque, requires control of both the power input and the tool temperature to achieve a stable, robust and repeatable process. A cascade controller is used to efficiently suppress fast power input disturbances reducing their impact on the tool temperature. The controller is tuned using a recently presented method for robust PID control. Results show that the controller keeps the temperature within ±10°C of the desired value during the 360º long joint line sequence. Apart from the cascaded control structure, good process knowledge and control strategies adapted to different weld sequences i.e. different thermal boundary conditions have contributed to the successful results

Mechanics of the solid-state bonding under severe thermomechanical processes

Friction stir welding (FSW) has found increased applications in automotive and aerospace industries due to its advantages of solid-state bonding, no fusion and melting, and versatility in various working conditions and material combinations. The extent and quality of the solid-state bonding between workpieces in FSW is the ultimate outcome of their industrial applications. However, the relationship among processing parameters, material properties, and bonding extent and fidelity remains largely empirical, primarily because of the lack of the mechanistic understanding of (1) tool-workpiece frictional behavior, and (2) bonding formation and evolution. In this dissertation, to study the underlying mechanism of tool-workpiece frictional behavior and bonding evolution at workpiece-workpiece interface during solid-state bonding process, firstly, a numerical model that take advantage of Coupled Eulerian Lagrangian (CEL) method is implemented to investigate the stick-slip behavior at tool-workpiece interface. An analytical model is also developed to correlate the stick-slip fraction to processing parameters such as the tool spin rate, and further to derive dimensionless functions for torque and heat generation rate predictions. These analyses provide the critical strain rate and temperature fields that are needed for the bonding analysis. Then, we note that the interfacial solid state bonding process under applied thermomechanical loading histories is a reverse process of the high temperature creep fracture of polycrystalline materials by grain boundary cavities, in this regard, a general modeling framework of bonding fraction evolution was derived, which directly depends on the stress, strain rate, and temperature fields near the interface. Finally, Based on the stick-slip contact analysis and the understanding of solid-state bonding mechanism, an approximate yet analytical solution has been developed to derive the bonding fraction field from the given processing, geometric, and material constitutive parameters, and the predicted ultimate bonding extent with respect to these parameters becomes a figure of merit for the study of processing window for industrial applications and design of the FSW process

Electrically-Assisted Friction Stir Welding of Aluminum Alloy to Advanced High Strength Steel.

Growing concerns on energy consumption increase the demand of lightweight vehicles. One of the most efficient solutions is to use multi-material structure. As a solid state process, friction stir welding (FSW) is promising for joining dissimilar materials. However, the processing window for achieving successful dissimilar joints is still narrow. Besides, a large axial welding force is required when steel is involved. In order to address these challenges, a material softening phenomenon, electro-plastic effect (EPE) is proposed to be incorporated into the process. In this research, first, EPE on various materials is systematically reviewed and a hypothesis is proposed for understanding the softening mechanisms. The effectiveness of EPE is then evaluated on one type of advanced high strength steel, TRIP 780/800 steel. Second, traditional FSW process is experimentally studied for joining dissimilar Al 6061 to TRIP steel. Effects of process parameters on joint microstructure evolution are analyzed based on the mechanical welding force and temperature measured during the process. Intermetallic compound (IMC) layer of FeAl or Fe3Al with thickness of less than 1 μm is formed at the Al–Fe interface in the advancing side. The maximum ultimate tensile strength can reach 85% of the base aluminum alloy. Third, analytical and numerical models are developed for friction stir welding of dissimilar materials. For plunge stage modeling, the field variable is introduced to identify regions of steel and aluminum and define the generalized material properties. Conservation equations are separately developed at the two materials interface for the discontinuities. The stable welding stage is modeled based on Eulerian formulation using multiple phase flow theories. The developed model can capture the material and temperature distribution measured from experiments. Finally, high density electrical currents are applied to the FSW process. Plunge stage of FSW is studied on aluminum alloy Al 6061 and TRIP 780 steel respectively. Effective reduction of the axial welding force can be obtained with good repeatability. During the FSW of Al 6061 to TRIP steel, the axial welding force can be consistently reduced under various welding conditions, which is a synergic result of both electro-plastic effect and Joule heating.PhDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133252/1/xunxliu_1.pd

Slip-Stick Contact Conditions for the Thermo-Mechanically Coupled Flow Drill Screw Process

Automakers have adopted a heavy focus towards lightweighting their fleets due to the stringent emission standards placed upon them. Lightweighting can be done using several methods but material substitution is proven to be most effective considering the traditional powertrains are on the border of theoretical limits. Designing multi-material body structures is a recognized strategy, replacing steels with lightweight metals such as aluminum, magnesium, and fiber-reinforced composites. The issue now arises on how to join these materials that possess such varied thermo-mechanical properties, with resistance spot welding (RSW) currently not an option. One of the newly-adopted joining technologies is Flow Drill Screwing (FDS) which is currently the only structurally viable joining technology that does not require access to the back side of the joint. FDS is a coupled thermo-mechanical process due to the frictional behavior between the rotating screw and stationary workpiece. An understanding of the process is limited to empirical methods mainly based on experimental findings with little known about the frictional behavior at the screw-workpiece interface. This lack of understanding not only inhibits the potential of the process, but more importantly, whether its application borders on the edge of reliability; and without an understanding to the transient contact conditions, accurate torque and temperature modeling is not feasible. Current models have limited accuracy as their methodology couples a friction coefficient and material strength term. A modeling approach that incorporates both a slipping and sticking condition is theorized to be more appropriate for frictional processes of this nature, but no coupled models currently exist. The following research aims at integrating these two conditions under a single model to enable more accurate modeling and prediction of the FDS process performance. A secondary objective presented in this research is to determine whether FDS processing time could benefit from the assistance of supplementary energy sources. Replacing RSW with these alternate joining technologies, such as FDS, comes at the expense of an increased process time. This research aims at augmenting FDS with heat to lower the impact of this decreased process efficiency while also testing the potential to open the design space to thicker/stronger materials

Surface composites and functionalisation : enhancement of aluminium alloy 7075-T651 via friction stir processing

Abstract: This research work is aimed at modifying and enhancing the properties of aluminium alloy 7075- T651 through the friction stir processing (FSP) technique, in order to improve the mechanical, electrochemical, structural, tribological as well as the metallurgical properties which include micro- and macro- structural analysis through XRD and Image processing of grain size and grain flow patterns determination, by reinforcing the parent metal. The surface modification of the parent metal has been made possible in the past via different techniques,such as laser surfacing, electronbeam welding and thermal spraying; but in recent years, the friction stir processing (FSP) technology has been adopted to cater for the complex methods of surface enhancement. FSP is well-renowned for its short route of fabrication, densification, grain refinement, homogenization of the precipitates of composite substances, nugget zone homogeneity. These have led to the efficient surface enhancement, significant and remarkable improvement in hardness, ductility, strength, increased fatigue life, as well as formability within which the bulk properties are still intact. The use of FSP in the fabrication of metal matrix composites (MMCs), especially aluminium matrix composites (AMCs) and aluminium hybrid composites (AHCs) were dealt with in this study...Ph.D. (Mechanical Engineering