Development of a numerical modelling approach to predict residual stresses in Ti-6Al-4V linear friction welds

Linear friction welding (LFW) is a solid-state joining process which has been successfully implemented to manufacture bladed-disks, chains and near-net shape components. During welding, large residual stresses are created as a consequence of a non-uniform heating of the component which can severely affect the integrity of the structure. Experimental measurement of residual stresses and temperatures on linear friction welds is difficult, so researchers have used modelling to provide a better understanding of these important characteristics. Models developed in the literature, replicate the welding process by including the oscillation of the workpieces, resulting in long computational times. Therefore, numerical models are mostly confined to 2D geometry and complex geometry cases such as keystone or bladed-disk welds are rarely considered. This thesis focuses on the development and validation of computational models capable of predicting the residual stress field developed in Ti-6Al-4V LFW without modelling the complex mechanical mixing occurring at the weld interface. Using a sequentially coupled thermo-mechanical analysis on a 3D model defined in ABAQUS, the heat was applied at the weld interface using the average heat flux post-processed from the machine data obtained during welding trials, for all the phases. The material deformation was ignored and the material expulsion is accounted for by sequentially removing rows of elements. The models were validated against thermocouples, neutron diffraction and contour method measurements. The shearing occurring at the interface while welding was found to have little effect on the final residual stress field and therefore can be omitted. The residual stress field was found to be driven by the temperature profile obtained at the end of welding, prior to cooling and by the weld interface dimensions. A low weld interface temperature, shallow thermal gradient across the weld and small weld interface dimensions should be sought to minimise the residual stress magnitude. Therefore, a low burn-off rate obtained with reduced welding frequency, amplitude and applied force should be used; however the impact of using these parameters on the microstructure and material properties may need to be considered. The modelling approach was successfully implemented on a blisk LFW and its peculiar geometry was found to have little effect on the residual stress field as the peak magnitude is driven by the overall length of the part and the thermal profile prior to cooling. Several cycles of post-weld heat treatment were also investigated for the blisk weld. The results showed that all post-weld heat treatments reduced the residual stresses, however the differences between the heat treatments on the resulting stress field was minimal. In conclusion, the thesis presents an innovative computationally efficient modelling approach capable of predicting the residual stresses within standard and complex geometry LFW

Fundamental Understanding of Bond Formation During Solid State Welding of Dissimilar Metals

Dissimilar metal welds are used in a wide range of applications to effect light weighting and for corrosion resistance. While fusion welding techniques are limited in their ability to fabricate dissimilar metal welds, solid state welding techniques are limited in their ability to fabricate complex geometries with dissimilar metal combinations. Hence alternative techniques need to be explored to fabricate complex geometries with dissimilar metals welds in the solid state. Ultrasonic additive manufacturing in a solid state additive manufacturing process that combines ultrasonic welding with mechanized tape layering to fabricate dissimilar metal welds in the solid state. Though extensive feasibility studies have been performed to fabricate dissimilar metal welds using ultrasonic additive manufacturing, the fundamental mechanisms related to the bond formation mechanism are not fully understood. In this work multi scale characterization using scanning electron microscopy, electron backscatter diffraction, nano indentation and atom probe tomography was performed to rationalize the mechanism of bond formation in dissimilar metal welds. The fundamental questions that needed to be answered were Is possible for a solid state bond to form with extensive plastic deformation occurring only on one metal in a dissimilar metal combination The effect of plastic deformation on the oxide layer at the interface of dissimilar metal welds. To answer the above questions various dissimilar metal combinations (Steel-Ta) BCC-BCC, (Al-Ti) FCC-HCP, (Al-Steel) FCC-BCC were fabricated using ultrasonic additive manufacturing and characterized using the above-mentioned techniques. Bonded regions were characterized to study the role of plastic deformation by analyzing the micro texture developed at the interface. The general conclusion is the presence of a strong shear texture in the softer metal while the harder metal did not show any evidence of change in texture. To understand the effect of plastic deformation on the oxide dispersion atom probe tomography analysis was performed and the results indicate the possibility of oxide breakdown resulting in oxygen super saturation in the lattice. The bond formation is hypothesized to occur as a result of plastic deformation localized in the softer metal alone

Modelling of Ti-6Al-4V linear friction welds.

Linear friction welding (LFW) is a solid-state joining process that is finding increasing industrial interest for the fabrication of Ti-6Al-4V preforms. The fundamental science behind the process needs to be better understood to aid further process implementation. In practice, many aspects of the process are difficult to measure experimentally. Consequently, many researchers use computational models to provide an insight to the process behaviour, such as the thermal cycles and flash formation. Despite these recent research efforts, the effects of the workpiece geometry and process inputs on Ti-6Al-4V linear friction welds are still not fully understood. This thesis focuses on the development and validation of computational models to address this issue. Two and three-dimensional (2D/3D) computational models were developed using the finite element analysis software DEFORM. The models were validated with a systematically designed set of experimental welds. The validated models and experimental data were used to characterise the effects of the process inputs and workpiece geometry on the: thermal fields, material flow, flash morphology, interface contaminant removal, microstructure, energy usage, welding forces, coefficients of friction and welding times. The results showed that there is a benefit to using larger pressures and oscillating the workpieces along the shorter of the two interface-contact dimensions when producing Ti- 6Al-4V welds. This is because the burn-off required to remove the interface contaminants is reduced. Hence for the same burn-off, the factor of safety on contaminant removal is greater. Furthermore, these conditions can also reduce the interface temperature and refine the weld microstructure, which may offer additional benefits, such as reduced residual stresses and improved mechanical properties. In conclusion, the thesis aim was successfully addressed, therefore increasing understanding of the LFW process. The work showed that although the 3D models captured the full multi-directional flow behaviour, 2D models were better suited to parametric and geometric studies.Engineering and Physical Sciences (EPSRC)PhD in Manufacturin

Development of a numerical modelling approach to predict residual stresses in Ti-6Al-4V linear fraction welds.

Linear friction welding (LFW) is a solid-state joining process which has been successfully implemented to manufacture bladed-disks, chains and near-net shape components. During welding, large residual stresses are created as a consequence of a non-uniform heating of the component which can severely affect the integrity of the structure. Experimental measurement of residual stresses and temperatures on linear friction welds is difficult, so researchers have used modelling to provide a better understanding of these important characteristics. Models developed in the literature, replicate the welding process by including the oscillation of the workpieces, resulting in long computational times. Therefore, numerical models are mostly confined to 2D geometry and complex geometry cases such as keystone or bladed-disk welds are rarely considered. This thesis focuses on the development and validation of computational models capable of predicting the residual stress field developed in Ti-6Al-4V LFW without modelling the complex mechanical mixing occurring at the weld interface. Using a sequentially coupled thermo-mechanical analysis on a 3D model defined in ABAQUS, the heat was applied at the weld interface using the average heat flux post-processed from the machine data obtained during welding trials, for all the phases. The material deformation was ignored and the material expulsion is accounted for by sequentially removing rows of elements. The models were validated against thermocouples, neutron diffraction and contour method measurements. The shearing occurring at the interface while welding was found to have little effect on the final residual stress field and therefore can be omitted. The residual stress field was found to be driven by the temperature profile obtained at the end of welding, prior to cooling and by the weld interface dimensions. A low weld interface temperature, shallow thermal gradient across the weld and small weld interface dimensions should be sought to minimise the residual stress magnitude. Therefore, a low burn-off rate obtained with reduced welding frequency, amplitude and applied force should be used; however the impact of using these parameters on the microstructure and material properties may need to be considered. The modelling approach was successfully implemented on a blisk LFW and its peculiar geometry was found to have little effect on the residual stress field as the peak magnitude is driven by the overall length of the part and the thermal profile prior to cooling. Several cycles of post-weld heat treatment were also investigated for the blisk weld. The results showed that all post-weld heat treatments reduced the residual stresses, however the differences between the heat treatments on the resulting stress field was minimal. In conclusion, the thesis presents an innovative computationally efficient modelling approach capable of predicting the residual stresses within standard and complex geometry LFW.PhD in Manufacturin

Tribology in the 80's. Volume 1: Sessions 1 to 4

A wide range of subjects extending from fundamental research with tribological materials and their surface effects up to the final applications in mechanical components were covered

Investigation of Tribochemical Reactions Using the Model System of Methyl Thiolate on Copper Foil in Ultrahigh Vacuum and Ab-Initio Calculations

Advancement in the understanding of tribochemical systems suffers from several obstacles that hinder the progress in advancing an understanding of the fundamental processes involved in the evolution of friction and wear. Characterizing ephemeral chemical states within a buried interface is an experimental challenge and work in this dissertation uses a model system, methyl thiolate on copper foil, that undergoes tribo-activated decomposition to investigate the rate of change of the chemical components in the interface. The elementary steps in the tribochemical reaction were identified and consist of a shear-induced decomposition of methyl thiolate species to produce gas-phase hydrocarbons and form surface sulfur, which is mechanochemically transported into the sub-surface copper region resulting in changes in the friction coefficient. A method has been developed to analyze the changes in sliding-induced gas-phase product formation and friction coefficient as a function of the number of passes over the surface with a tribopin. Finally, the Vienna Ab-Initio Simulation Package (VASP) is used to calculate the methyl thiolate decomposition energies on Cu(100) as a function of load and the results are compared to the extended-Bell model

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

Manufacturing modification through process manipulation in inertia friction welding: enhanced functionality rotary friction welding

Rotary friction welding has been around since the end of the Second World War. Until recently machines and the process have only improved with the introduction of easier control systems and more accurate instrumentation. Recently, however, Manufacturing Technology Inc developed a range of enhanced functionalities based on combining the two fundamental processing options: Inertia and Continuous drive. The aim of this thesis is to explore the new enhanced functionalities and gain an understanding of their influence on the weld to examine potential process improvements. Initially all the various forms of enhanced functionality are reviewed in order to assess the potential for further use. Torque modulation, where the rotational speed can be modified mid cycle in reference to a profile, was found to have the greatest impact on the standard cycle, substantially reducing the variation in upset, with no noticeable impact on the weld microstructure. Final length control demonstrated an impact on the microstructure away from the nominal if large energy inputs were utilized. Simulated inertia profiles used large energy inputs to purposefully modify the process and resulting microstructure. Links were found between the process, corresponding weld zone widths and resultant residual stresses in Titanium 6Al 4V

An Investigation on the Structure/Property Relationships of Solid State Welding Processes in a Titanium Matrix Composite Alloy (Ti6Al4V [plus] 10 wt.% of TiC)

TiC particulate reinforced Ti6Al4V metal matrix composites (Ti6Al4V 10 wt.% TiC) have high strength-to-weight ratio and good high temperature properties. Although this class of composite clearly perform better than the matrix alloy itself, the successful application of such particulate-reinforced materials depends on the availability of proven joining techniques that can produce high quality joints. Due to the high chemical reactivity of titanium that may lead to a chemical interaction with the reinforcing material a poor fusion welding performance is commonly observed in these materials making solid-state diffusion bonding and rotary friction welding potential processes to produce complex structural components. Despite recent advances in processing and manufacturing technology of Ti6Al4V 10 wt.% TiC there is still a lack of understanding in the solid state joining possibilities and its microstructural changes and mechanical properties. The main objective of this work is to investigate and analyse the feasibility of joining the particulate-reinforced composite alloy by rotary friction welding and diffusion bonding processes. It is also aimed the determination and establishment of the microstructure/properties relationships of the resultant welds as well as to investigate the bonding mechanisms and understand the weldability aspects of friction welded and diffusion bonded Ti6Al4V 10 wt.% TiC. Metallurgical characterization of both base material and welded joints was performed using Optical and Scanning Electron Microscope. Mechanical assessment was accomplished using tensile, microflat tensile and fracture toughness tests. A microstructural examination of the friction-welded joints has revealed two distinct welding zones (transformed and recrystallized zone as well as heat affected zone); while no metallurgical transformation has occurred in the diffusion bonding process. In the case of rotary friction welding best results were associated with low rotational speed and low friction pressure; while in the diffusion bonding process the best results were associated with a bonding temperature and pressure of 1000C and 5MPa together with bonding times ranging from 35 and 60 minutes

Processing and Characterization of Plasma Sprayed Iron Aluminide Coatings

Surface modification is a generic term now applied to a large field of diverse technologies that can be gainfully harnessed to achieve increased reliability and enhanced performance of industrial components. Intermetallic compounds find extensive use in high temperature structural applications. The Fe3Al based intermetallic alloys offer unique benefits of excellent oxidation and sulfidation resistance at a potential cost lower than many stainless steels. These are mainly used in heating elements, regenerator disks, wrapping wire, hot gas filters, tooling, and shields. To obtain functional surface coating on machine components exhibiting selected in-service properties, proper combination of processing parameters has to be planned. These combinations differ by their influence on the coating properties and characteristics. Plasma spraying is gaining acceptance as a development of quality coatings of various materials on a wide range of substrates. Coatings made with plasma route exhibit excellent wear, corossion resistance and high thermal shock resistance etc. Iron premixed with 30% aluminium is deposited on mild steel and copper substrates by atmospheric plasma spraying at various operating power level ranging from 11to 21kW . After plasma spraying, the coated materials have been subjected to a series of tests. The particle sizes of the raw materials used for coating (iron with, 30 wt% aluminium powder) are characterized using Laser particle size analyzer of Malvern Instruments make. Thickness of the iron aluminide coatings are measured by using an optical microscope. X-ray diffraction technique is used to identify the different (crystalline) phases present in the coatings. The coating adhesion strength is evaluated by coating pull out method, as per ASTM C- 633 standard. Coated specimens are studied by JEOL JSM-6480 LV scanning electron microscope in order to know the surface and interface morphology. The porosity of the coatings is measured by putting polished cross sections of the coating sample under a microscope using image analyser. Microhardness measurement is done to know the hardness of the optically distinguishable phases by using Leitz Microhardness Tester Solid particle erosion is a wear process where particles strike against a surface and promote material loss. In this work, room viii temperature solid particle (sand) erosion test is carried out by using ASTM G76 standards. Deposition efficiency is evaluated as the important factor that determines the techno-economics of the process. Statistical analysis i.e. Artificial Neural Networks is gainfully employed to simulate property-parameter correlations in a space larger than the experimental domain. It is evident that with an appropriate choice of processing conditions a sound and adherent iron aluminide coating are achievable using iron aluminium powders