Laser welding of steel to aluminium: thermal modelling and joint strength analysis

The integrity of steel-aluminium dissimilar alloy joints is dependent on the thermal cycle applied during the joining process. The thermal field has a direct influence on the growth of the intermetallic compounds (IMC), which result from the reaction between iron (Fe) and aluminium (Al), but it also determines the size of the bonding area of the joint. A finite element (FE) thermal model was developed to predict the transient thermal cycle at the Fe-Al interface for different levels of applied energy by changing the power density and interaction time. The time-temperature profiles were correlated to the weld geometry, IMC layer thickness and mechanical strength. The experimental results showed that having a small bonding area is equally detrimental to the mechanical strength of the joint as having a thick IMC layer. The FE model suggested that comparing to time, the temperature is more important in laser welding of steel to aluminium as this is the factor which most contributes to the growth of the IMC layer and the formation of the bonding area. However, it was not possible to identify a thermal field able to produce simultaneously a large bonding area and a thin IMC layer to optimize the joint strength

ULTRA-FAST LASER ENHANCED PRINTING OF NANOMATERIAL FOR HIGH QUALITY TRANSPARENT ELECTRODE

Direct printing of nanomaterials, which integrate nanomaterials into a film via low cost mean, is designed to fabricate transparent conductive electrode (TCE) film. Following laser processing is utilized as the post treatment to enhance the film performance. The laser processing is proposed in order to weld nanomaterials in nanoscale and enhance the electrical conductance of the nanomaterials film after direct printing. Rigid glass substrate was chosen as the substrate to load nanomaterials printing; however, this laser processing also can be utilized to nanomaterials printed on flexible substrate like polymer and bendable glass. Aluminum doped zinc oxide nanoparticles and silver nanowires were chosen as the printable nanomaterials. The laser – nanomaterial interaction and temperature evolution was studied by Comsol Multiphysics software. The nature intrinsic of laser induced localized nanowelding was simulated by Molecular Dynamic simulation. The SEM, TEM and XRD results show that microstructure of nanomaterials film was improved significantly after laser induced nanowelding. The performance evaluation confirms the improved optoelectronic property of nanomaterials printing film. The theoretical study of the electrical conductance enhancement is presented in the thesis. The direct printing techniques and ultra-fast laser processing have the potential to boost the efficiency when used in commercial mass – production

Thermal Modeling of Al-Al and Al-Steel Friction Stir Spot Welding

This paper presents a finite element thermal model for similar and dissimilar alloy friction stir spot welding (FSSW). The model is calibrated and validated using instrumented lap joints in Al-Al and Al-Fe automotive sheet alloys. The model successfully predicts the thermal histories for a range of process conditions. The resulting temperature histories are used to predict the growth of intermetallic phases at the interface in Al-Fe welds. Temperature predictions were used to study the evolution of hardness of a precipitation-hardened aluminum alloy during post-weld aging after FSSW.The work described herein has been sponsored by the UK Engineering and Physical Sciences Research Council (EPSRC) via the following grants: Friction Joining—Low Energy Manufacturing for Hybrid Structures in Fuel Efficient Transport Applications (EP/G022402/1 and EP/G022674/1), and LATEST 2: Light Alloys Towards Environmentally Sustainable Transport, 2nd Generation Solutions for Advanced Metallic Systems (EP/H020047/1).This is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1007/s11665-016-2225-

Modelling phase change in a 3D thermal transient analysis

A 3D thermal transient analysis of a gap profiling technique which utilises phase change material (plasticine) is conducted in ANSYS. Phase change is modelled by assigning enthalpy of fusion over a wide temperature range based on Differential Scanning Calorimetry (DSC) results. Temperature dependent convection is approximated using Nusselt number correlations. A parametric study is conducted on the thermal contact conductance value between the profiling device (polymer) and adjacent (metal) surfaces. Initial temperatures are established using a liner extrapolation based on experimental data. Results yield good correlation with experimental data

Thermal Mechanical Numerical Modeling of Friction Element Welding

With the objective of minimizing carbon footprint of vehicles, different organizations across the world are increasingly enforcing higher fuel efficiency targets for the automobile manufacturers. To improve the fuel economy while retaining or further improving the structural integrity, the automobile industry is vigorously shifting towards substituting conventional heavy materials like cast iron with new age materials such as aluminum alloys, steel alloys, etc. which are not only much lighter but also offer superior strength-to-weight ratio. Engineers use a mix of these new age materials with the aim of maximizing the benefits from each material. However, the utilization of such materials is currently limited in the industry as welding them using conventional methods such as resistance spot welding or fusion welding process, is plagued with inherent difficulties such as formation of brittle inter-metallic compounds, irreversible and adverse changes in the thermal and mechanical properties of the materials. Dissimilar material joining is of critical importance in aiding the manufacturers realize the crucial objective of a safer and more fuel efficient vehicle. Friction element welding (FEW), a friction based joining process, has been proposed for joining highly dissimilar materials in minimal time and with low input energy. FEW process can join a variety of materials which differ significantly in their mechanical, thermal, and metallurgical properties without inducing any of the defects associated with conventional welding methods. The fundamental governing mechanisms that characterize the FEW process needs to be investigated to help optimize the process for specific applications. Conducting experimental investigation is undesirable and infeasible due to the highly complex thermal-mechanical procedures occurring simultaneously in a very short period of time of about one second. As such, the utilization of a finite element model to simulate and analyze the FEW process is warranted which would help understand the underlying mechanisms of the process in detail and provide an efficient yet effective tool to observe the effect of different process parameters on the weld quality. A coupled thermal-mechanical finite element model (FEM) is developed in this work to simulate the FEW process and gain an understanding of the physical mechanisms involved in the process and help predict the influence of variation of process parameters on the evolution of temperature, material flow, and their effect on weld quality. The primary difficulty in simulating a highly transient process like FEW, wherein not only the workpiece is subjected to deformation but also the auxiliary joining element i.e. friction element undergoes extensive deformation, is that the mesh elements are prone to distortion failure while trying to capture such high amount of deformation. The presence and importance of temperature effect on material properties further complicate the FEM. To help eliminate the distortion issue while simultaneously achieving an accurate simulation of the FEW process, the coupled Eulerian-Lagrangian (CEL) approach is adopted. The novelty of the current approach employed lies in using a Eulerian definition for the tool as against the more traditional convention of adopting a purely Lagrangian definition. The Eulerian definition enables to simulate the extreme deformation of friction element and capture the material flow without any computational issues. To inspect for the accuracy of the FEM results, mechanical deformation for different parts observed in the FEM is compared against the experimental results. To further validate the FEM, experimental measurements of temperature at different locations at the interface of two layers of workpiece are compared against the FEM results at same locations in the model. With respect to, both, thermal and mechanical measurements comparisons good agreement is shown between the simulation results and the experimental data. The simulation results for sets with varying process parameters show that the rotational speed of the friction element has the highest influence on the amount of frictional heat generated followed by the time period for different steps. Higher amount of heat is generated and conducted into the top aluminum layer for longer Penetration time, whereas for more heat concentration into the friction element to achieve the required deformation, longer Welding step with higher rotational speed is desired

Numerical optimisation of laser assisted friction stir welding of structural steel

Significant progress has been made on the implementation of friction stir welding (FSW) in the industry for aluminium alloys. However, steel FSW and other high-temperature alloys is still the subject of considerable research, mainly because of the short life and high cost of the FSW tool. Different auxiliary energies have been considered as a means of optimising the FSW process and reducing the forces on the tool during the plunge and traverse stages, but numerical studies on steel are particularly limited. Building on the state-of-art, laser-assisted steel FSW has been numerically developed and analysed as a viable process amendment. Laser-assisted FSW increased the traverse speed up to 1500 mm min −1, significantly higher than conventional steel FSW. The application of laser assistance with a distance of 20 mm from the rotating tool reduced the reaction force on the tool probe tip up to 55% when compared to standard FSW

RF-MEMS switches for reconfigurable antennas

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Reconfigurable antennas are attractive for many military and commercial applications where it is required to have a single antenna that can be dynamically reconfigured to transmit or receive on multiple frequency bands and patterns. RF-MEMS is a promising technology that has the potential to revolutionize RF and microwave system implementation for next generation telecommunication applications. Despite the efforts of top industrial and academic labs, commercialization of RFMEMS switches has lagged expectations. These problems are connected with switch design (high actuation voltage, low restoring force, low power handling), packaging (contamination layers) and actuation control (high impact force, wear, fatique). This Thesis focuses on the design and control of a novel ohmic RF-MEMS switch specified for reconfigurable antennas applications. This new switch design focuses on the failure mechanisms restriction, the simplicity in fabrication, the power handling and consumption, as well as controllability. Finally, significant attention has been paid in the switch’s electromagnetic characteristics. Efficient switch control implies increased reliability. Towards this target three novel control modes are presented. 1) Optimization of a tailored pulse under Taguchi’s statistical method, which produces promising results but is also sensitive to fabrication tolerances. 2) Quantification of resistive damping control mode, which produces better results only during the pull-down phase of the switch while it is possible to be implemented successfully in very stiff devices. 3) The “Hybrid” control mode, which includes both aforementioned techniques, offering outstanding switching control, as well as immunity to fabrication tolerances, allowing an ensemble of switches rendering an antenna reconfigurable, to be used. Another issue that has been addressed throughout this work is the design and optimization of a reconfigurable, in pattern and frequency, three element Yagi-Uda antenna. The optimization of the antenna’s dimensions has been accomplished through the implementation of a novel technique based on Taguchi’s method, capable of systematically searching wider areas, named as “Grid-Taguchi” method

Ultrasonic Spot Welding of Thin Walled Fibre-Reinforced Thermoplastics

Das Ultraschall-Punktschweißen von faserverstärkten thermoplastischen Kunststoffen hat in der letzten Zeit bei Forschern in der Luftfahrt- und Automobilindustrie großes Interesse hervorgerufen. Es bietet eine effiziente Lösung zum Verbinden großer thermoplastischer Verbundbauteile durch Punktschweißen mit einem hohen Automatisierungsgrad. In der vorliegenden Arbeit wurde eine neue Technik zum Fokussieren der Ultraschallschwingungsenergie an der gewünschten Fügestelle zwischen zwei Fügepartnern aus thermoplastischen Verbundlaminaten untersucht. Bei diesem untersuchten Verfahren waren keine zusätzlichen Energierichtungsgeber zwischen den Fügepartnern erforderlich, um die Vibrationsenergie zu fokussieren. Es wurde festgestellt, dass es durch Schweißen der Laminate zwischen einer Sonotrode und einem Amboss möglich war, eine lokalisierte Wärme durch Reibung zu erzeugen in dem die Sonotrode eine größere Kontaktfläche mit dem Laminat als mit dem Amboss aufwies. In der Anfangsphase des Schweißens wurden die Grenzflächenschichten durch die reibungsverursachte Erwärmung abgeschwächt. Folglich zentrierte sich die zyklische Verformung in diesen abgeschwächten Grenzflächen. Die Annahme des Vorhandenseins der Reibung und ihres Einflusses auf die Wärmeerzeugung wurde mittels mechanischer FEM-Analyse untersucht. Die mikroskopische Analyse des Schweißpunktes lieferte schließlich den Beweis für die Schmelzauslösung an einem Ring um den Schweißpunkt und das anschließende Punktwachstum. Um die räumliche Verteilung der Temperatur und ihre zeitliche Entwicklung in der Schweißzone während des Ultraschallschweißprozesses besser zu verstehen, wurde das thermische Problem numerisch modelliert. Zur Verifizierung der mathematischen Modelle wurden die berechneten Zeitverläufe der Temperatur im Schweißpunktzentrum mit den experimentell ermittelten Werten unter vergleichbaren Bedingungen gegenübergestellt. Es wurde festgestellt, dass nach einer bestimmten Schweißzeit die Temperatur im Schweißzentrum plötzlich anstieg und das Polymer an der Schweißstelle überhitzt und die Zersetzung begann. Es wurde beobachtet, dass der Zeitverlauf der verbrauchten Leistungskurve durch das Schweißgerät einem ähnlichen Muster folgte, wie der Zeitverlauf der Temperatur in der Schweißpunktmitte. Basierend auf dieser Beobachtung wurde ein Steuerungssystem entwickelt. Die zeitliche Ableitung der Schweißleistung wurde in Echtzeit überwacht. Sobald ein kritischer Wert überschritten wurde, wurde die Ultraschallschwingungsamplitude aktiv durch einen Mikrocontroller eingestellt. Bei diesem Ansatz wurde die Temperatur im Schweißpunkt indirekt gesteuert, um während der gesamten Schweißdauer in einem optimalen Bereich zu bleiben. Die Ergebnisse des gesteuerten Schweißprozesses wurden mittels Temperaturmessungen und Computertomographie bewertet. Aus der Studie wurde der Schluss gezogen, dass das leistungsgesteuerte Ultraschall-Punktschweißverfahren eine effiziente und stabile Methode zum automatischen Verbinden von faserverstärkten thermoplastischen Teilen ist.:1 Introduction 1.1 Motivation 1.2 State of the Art 1.3 Statement of the Theses and Methods 2 Theoretical Background 2.1 Ultrasonic Welder 2.1.1 Ultrasonic Stack 2.1.2 Working Principle of the Ultrasonic Welder 2.2 Viscoelasticity 2.2.1 Viscoelasticity of Continuous Fibre-Reinforced Laminates 2.2.2 Viscoelastic Heating of CFRTP during the DUS Welding 2.3 Frictional heating at the Weld Interface during the DUS Welding 2.4 Fusion Mechanism during the USW 2.4.1 Contact of the Matrix at the Weld Interface 2.4.2 Healing of the Weld Interface through Autohesion 3 Experimental Analysis of the DUS Process 3.1 Experimental Setup 3.2 Experimental Procedure, Results and Discussions 3.2.1 Weld Progress and Formation Analysis 3.2.2 The Influence of the Amplitude and Static Force on the DUS 3.2.3 Computed Tomography Analysis of the DUS Welded Spots 3.2.4 Influence of the Weld Parameters on the Weld Force at Break 3.2.5 Influence of the Main Process Variables on the Weld Strength 4 Process Modelling and Simulation 4.1 Dynamic Mechanical 3D Finite Element Analysis 4.1.1 Woven Fabric Laminate Models 4.1.2 Laminate Properties and Meshing 4.1.3 FEM Analysis Procedure 4.1.4 Results of the Dynamic Analysis 4.2 Numerical Analysis of the Temperature Temporal and Spatial Development 4.2.1 The Numerical Method 4.2.2 Matrix Loss Modulus Calculation at the Welding Frequency 4.2.3 Model Validation 4.2.4 Analysis of the Spatial and Temporal development of the Temperature 4.2.5 Influence of Uncontrollable Factors on the DUS Process 5 Logical Control Method and Industrialisation 5.1 Process Controlling Hypothesis 5.2 Control System and Instruments 5.3 Experimental Procedure for Analysing the Control System 5.4 Analysis of the Controlled DUS Process 5.5 Control System Validation and Industrialisation 5.6 Automation of the Ultrasonic Spot Welding Process 6 Summary and Outlook 6.1 Conclusions 6.2 Outlook References AppendixThe ultrasonic spot welding of fibre-reinforced thermoplastic composites has recently received strong interest among researchers mainly in the fields of aerospace and automotive industries. It offers an efficient solution to join large thermoplastic composite parts through the spot welding approach with a high level of automation. In this study, a new technique for focusing the ultrasonic vibration energy at the desired spot between two mating thermoplastic composite laminates was investigated. In this method, no additional energy directing protrusions between the weldments were required to focus the vibration energy. It was found that by welding the laminates amid an ultrasonic sonotrode and an anvil in which the prior had a larger contact surface with the laminate as the latter, it was possible to generate a localised frictional heating. In the initial phase of the welding, the frictional heating softened the interfacial layers and thus caused the focusing of the strain energy in the weld spot centre. The assumption for the presence of the friction and its influence on the heat generation was investigated by means of finite element method analysis. Microscopic analysis of the weld spot delivered the proof for the melt initiation at a ring around the weld spot and subsequent inwards growth of the weld spot. In order to gain a better understanding of the temperature spatial distribution and its temporal development in the weld zone during the ultrasonic welding process, the thermal problem was analysed using the explicit finite difference method. The mathematical model was verified through a comparison between the calculated temperature curves and the experimentally obtained counterparts. It was found that after a certain weld duration the temperature in the weld centre underwent a sudden increase and caused the overheating and decomposition of the polymer in the weld spot. It was observed that the time trace of the consumed power curve by the welder followed a similar pattern as the time trace of the temperature in the weld spot centre. Based on this observation, a control system was developed accordingly. The time derivative of the weld power was monitored in real time and as soon as it exceeded a critical value, the ultrasonic vibration amplitude was actively adjusted through a microcontroller. In this approach, the temperature in the weld spot was indirectly controlled to remain within an adequate range throughout the welding duration. The results of the controlled welding process were evaluated by means of temperature measurements and computed tomography scans. It was concluded from the study that the power-controlled differential ultrasonic spot welding process could be an efficient method to fusion bond the fibre-reinforced thermoplastic parts in an automated manner.:1 Introduction 1.1 Motivation 1.2 State of the Art 1.3 Statement of the Theses and Methods 2 Theoretical Background 2.1 Ultrasonic Welder 2.1.1 Ultrasonic Stack 2.1.2 Working Principle of the Ultrasonic Welder 2.2 Viscoelasticity 2.2.1 Viscoelasticity of Continuous Fibre-Reinforced Laminates 2.2.2 Viscoelastic Heating of CFRTP during the DUS Welding 2.3 Frictional heating at the Weld Interface during the DUS Welding 2.4 Fusion Mechanism during the USW 2.4.1 Contact of the Matrix at the Weld Interface 2.4.2 Healing of the Weld Interface through Autohesion 3 Experimental Analysis of the DUS Process 3.1 Experimental Setup 3.2 Experimental Procedure, Results and Discussions 3.2.1 Weld Progress and Formation Analysis 3.2.2 The Influence of the Amplitude and Static Force on the DUS 3.2.3 Computed Tomography Analysis of the DUS Welded Spots 3.2.4 Influence of the Weld Parameters on the Weld Force at Break 3.2.5 Influence of the Main Process Variables on the Weld Strength 4 Process Modelling and Simulation 4.1 Dynamic Mechanical 3D Finite Element Analysis 4.1.1 Woven Fabric Laminate Models 4.1.2 Laminate Properties and Meshing 4.1.3 FEM Analysis Procedure 4.1.4 Results of the Dynamic Analysis 4.2 Numerical Analysis of the Temperature Temporal and Spatial Development 4.2.1 The Numerical Method 4.2.2 Matrix Loss Modulus Calculation at the Welding Frequency 4.2.3 Model Validation 4.2.4 Analysis of the Spatial and Temporal development of the Temperature 4.2.5 Influence of Uncontrollable Factors on the DUS Process 5 Logical Control Method and Industrialisation 5.1 Process Controlling Hypothesis 5.2 Control System and Instruments 5.3 Experimental Procedure for Analysing the Control System 5.4 Analysis of the Controlled DUS Process 5.5 Control System Validation and Industrialisation 5.6 Automation of the Ultrasonic Spot Welding Process 6 Summary and Outlook 6.1 Conclusions 6.2 Outlook References Appendi