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The aim of this thesis is to model the extrusion process conditions for some aluminium alloys using Finite Element Modelling (FEM) program. All the simulations were performed with the implicit finite element code FORGE20 (2-D) and FORGE3® (3-D). In this work only the alloys AA2024, AA2014, Al-1 %Cu and AA6063 where experimental work is available were considered. The FORGE2® program was used to investigate and select an appropriate flow stress constitutive equation to describe the material behaviour to model the process conditions. The extrusion pressure and the temperature rise were predicted and the pressure-displacement trace and the events which take place in the deformed material during the extrusion process were also simulated. The effect of the initial billet temperature on friction, and the extent of the surface zone affected by surface friction and the consequence changes in material flow were investigated. The changes in the subgrain size during quasi-static deformation were predicted. This allows a construction of velocity-displacement profiles which would ensure consistent properties over the length of the extrudate. The FORGE3® program was used to simulate the effect of changing the die geometry on material flow during extrusion for rod, shapes and tube extrusion and the effect of the initial temperature on the deformation zone. The load required, temperature evolution, surface formation of the extrudate and material flow during the process, were also predicted. These included solid sections and the production of tubes using bridge die. Two most commonly used constitutiveflow stress equations,the Zener-Hollomon and the Norton-Hoff were analysed and compared with experimental results. It was found that the Zener-Hollomon relationship provided a better representation of the experimental flow stress under high working conditions than the Norton-Hoff relationship. FEM has been successfully applied to model the deformation patterns in the load/displacement traces and temperature evolution during the extrusion cycle. The effect of the initial billet temperature on the deformation zone pattern and its consequent effect on friction using both numerical simulation and experimental work are presented. A specific function relationship to measure directly interfacial friction under conditions approaching those encountered in the quasi-static deformation process is described. The results revealed that the friction factor increases with increase in initial billet temperature and varies during the extrusion cycle. The dead metal zone (DMZ) is observed to vary in form and has a greater volume at high temperatures. FEM proved to be a very effective and efficient way to design the ram speed profile to control the extrudate properties. The control of the properties of the extrudate under a constant (Z) parameter resulted in a more uniform distribution of the subgrain size across and along the extrudate cross-section. Furthermore, the speed profile under constant Z conditions resulted in an improved extrusion speed and hence greater productivity coupled with better control of the subgrain size and the exit temperature. This new extrusion process is termed iso-Z Extrusion, and is considered an improvement on Iso-Thermal extrusion. The usefulness and the limitation of FEM when modelling complex shapes are discussed. Methods to assess the difficulty of hollow and section shapes are presented. The work also illustrates the essentials of numerical analysis in the comprehension of the thermo-mechanical events occurring during extrusion through bridge and shape dies. Results are presented for velocity distribution in the extrusion chamber, Iso-temperature contours and pressure/displacement traces. It is shown that for most of the shapes investigated, the material making up the extrudate cross-sections originated from virgin material within the billet. The outside surface of the extrudate originates from the material moving along the DMZ and the core of the extrudate from the central deformation zone. When simulating tube extrusion, it is shown that the FE program is able to predict the pressure requirements: the pressure/displacement trace showing a double peak for tube extrusion which is discussed in some detail. The FE program appears to predict all the major characteristics of the flow observed macroscopically.

This thesis reports the analysis by FEM of both continuum and structural models describing the extrusion process. They were compared with experimental work and the agreement is satisfactory. All the simulations were performed with the implicit finite element code Forge2009® with user input written in Visual Fortran®. Alloys AA2024 and AA6063 were utilised as the source materials in order to compare with published experimental work. The Forge2009® 2D module was used to investigate both direct and indirect axisymmetric rod extrusions. The extrusion load and the temperature rise were predicted and the load-displacement curves and the events that took place in both extrusion modes were also simulated, discussed and again verified. The effects of the difference between the two modes, especially friction and its consequences on the process were investigated. The indirect results point to a good method of improving efficiency. For complex solid section, the 3D module has been used to study the load required, temperature evolution, surface formation of the extrudate and material flow during the process. These all showed good correlation with experimental results. The microstructure evolution during the extrusion process and the following solution soaking process were simulated with physically-based mathematical microstructure models integrated into FEM through its Fortran® subroutine interface,. The agreement between the predicted microstructures using associated models and experimental measurements were acceptable. For hollow section, the emphasis was placed on the study of the complicated metal flow and the seam welding quality. Novel analyses were developed to analyse the metal flow.

This document is an Executive Summary of individual submissions of work that the author has submitted towards the degree of Engineering Doctorate. The work comprises three main themes, which can be demonstrated in a broader sense as contributing towards improved productivity in fusion welding: i) The use of active fluxes for Tungsten Inert Gas (TIG) welding. ii) An investigation into the reduction of porosity when Metal Active Gas (MAG) welding galvanneal coated steel sheet used in the automotive industry. iii) The use of high power Nd:YAG laser welding for the production of large diameter, long distance land pipelines. Active fluxes give improved productivity by increasing the penetration depth of the TIG welding process by the simple addition of a flux applied to the surface. Although the productivity benefits of the process had been proven through a joint TWIIindustry project, the mechanism by which the fluxes produced this improvement was not fully understood. The first theme investigated the mechanisms at work in providing increased penetration and concluded that the primary mechanism responsible for the action of the fluxes was not due to a change in the flow of the molten pool but, as others had suggested, due to arc constriction. This work contributed to a greater understanding of the welding process and, furthermore, a greater understanding of the potential opportunities and limitations of the process when designing new fluxes for other alloy systems. MAG welds in coated steel sheet used in the automotive industry are prone to porosity leading to high reject rates. The second phase of work reported here determined welding procedures capable of delivering low porosity welds developed through statistical experimental design. These procedures demonstrated how low porosity welds could be made using conventional MAG welding techniques on steels that had been galvanneal coated to provide corrosion resistance. The procedures developed could be easily implemented at high production rates in an industrial manufacturing environment to reduce defect levels, and thus costly repairs or high scrap rates. The third theme of the work demonstrated how Nd:YAG laser welding could potentially be used to replace conventional arc welding techniques for land lay of gas transmission pipelines. The application of a single laser fill pass, made at high production rates, could replace the use of multiple MAG welding stations greatly reducing the costs associated with pipeline fabrication. BP has claimed that half pipeline cost savings of up to $300 million dollars are achievable through the implementation of such a technique. The justification for the use of lasers in pipelines is discussed in terms of both technical and economic suitability. Preliminary experimental work showed that high power Nd:YAG laser welds could achieve productivity targets, although in order to reduce defects and achieve the necessary structural performance it would be necessary to combine laser welding with a MAG welding process.

The demands placed on back-up rolls in hot strip mills have been investigated by a combination of literature and industrial studies. The tribological operating conditions have been established and the maximum local loads and pressure distributions at the work roll/back-up roll interface have been obtained by processing mill and roll schedule data using a computer program (commercial software developed by V AI Industries (UK) Ltd) and applying the theories of contact mechanics. After a study of the responses of the rolls to these demands and possible failure mechanisms, research has centred on surface initiated damage whereby cracks can propagate into the roll substrate potentially reaching the internal residual stress fields and leading to catastrophic failure. A proposed qualitative contact and fracture mechanics model, for the rolling contact fatigue and spalling failure, has been quantified theoretically using published methods for determining the stress intensity factors at the tips of pressurised and water lubricated, inclined rolling contact fatigue cracks. The predictions of the quantitative model in terms of crack directions and lengths have been validated by microscopic observation of the morphologies cracks produced in test discs used in the "SUROS" Rolling-Sliding Testing Machine and also in a sample of material spalled from a back-up roll. The quantitative failure model includes criteria for crack branching either upwards leading to micro spalling or downwards (potentially catastrophic) and the link between these two cases has been related quantitatively to the value of the mode I threshold for the roll material. After linking mechanics to microstructure and quantifying the interactions between wear and rolling contact fatigue in this case, practical quantitative recommendations have been made for the design of bainitic back-up roll materials, back-up roll redressing procedures and the surface roughness of both the work rolls and back-up rolls presented to the mill.

This project evaluated the metal transfer behaviour of the variable polarity (VP) GMAW process. Analysis was performed using high speed video that was synchronised with high speed data acquisition. Melting rate measurements were found to be very dependent on current waveform, polarity, and droplet size, and metal transfer if it occurred, for each waveform period. The transient conditions of current waveform and metal transfer produced rapid changes in arc behaviour which influenced the melting at the electrode tip and growing droplet. The concentrated melting theory was developed to explain the significant increase in electrode extension burnoff and droplet growth rate that occurred at short EN time as a function of current, and during EP peak pulse when the pre-pulse droplet volume was small. The highest electrode extension burnoff and droplet growth rate occurred when the arc was permitted to climb over the solid electrode tip producing rapid concentrated melting. Likewise, large molten droplets were found to promote a negative electrode extension burnoff and a decreased droplet growth rate. The arc rooted on large droplets providing additional heating but limited electrode melting. The droplet burnoff rate (DBR) method was developed and found to yield good experimental measurements for the arc and resistive heating coefficients used in a 2nd order melting rate equation developed for a complex waveform process, like VP-GMAW. For the EN period, the EN time affected the melting rate as a function of EN current. The greater melting rate that occurred at low EN time was measured by the changes in the resistive heating coefficient. Concentrated arc melting of the electrode extension at low EN time caused the slope of the burnoff diagram to increase, which represented the resistive heating coefficient. The melting rate of the EP pulse was related to the pre-pulse droplet volume. Large pre-pulse droplets decreased the arc heating coefficient, which could be negative, which meant the electrode extension was increasing and the arc length was decreasing in that waveform period. VP-GMAW power supplies offered stable operation for welding sheet structures on both carbon steel and stainless steel. Higher travel speeds were required as the %EN of the waveform increased to produce acceptable constant deposit area fusion. Welding speeds were up to 300% higher with VP-GMAW compared to the GMAW-P process when welding lap joints on 1.8 mm thick material with a 1.8 mm gap. VP-GMAW heat input was up to 47% less than GMAW-P for the same melting rate.

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