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This thesis undertakes a study of laser welding in terms of basic laser material interaction parameters. This includes power density, interaction time and specific point energy. A detailed study of the correlation between the laser material interaction parameters and the observed weld bead profiles is carried out. The results show that the power density and the specific point energy control the depth of penetration, whilst the interaction time controls the weld width. These parameters uniquely characterise the response of the material to the imposed laser energy profile, which is independent of the laser system. It is demonstrated that by studying the laser welding with respect to the basic laser material interaction parameters also helps explain some phenomenological phenomena in laser welding, such as the effect of beam diameter on the weld profile. In addition a new approach for parameter selection in laser and hybrid laser welding is investigated. A phenomenological model allowing achievement of a particular laser weld on different laser systems is developed. In the proposed method the user specifies the required weld profile, according to the quality requirements and then the model provides combination of laser parameters, which lead to this particular weld on a given laser system. This approach can be potentially used to transfer laser data between different laser systems with different beam diameters. An extensive study of residual stains in laser and hybrid laser welding is carried out. Both processes are compared either at a constant total heat input or at conditions required to achieve the same depth of penetration. The results demonstrate that there is a trade-off between the fit-up tolerance and the residual stress-induced distortion. Hybrid laser welding provides better ability to bridge gaps than the laser welding, but for the price of increased residual stress and distortion. Additionally, industrial study of the sensitivity to fit-up of hybrid laser welding with high deposition rate MIG sources is carried out. This thesis is a part of NEGLAP (Next Generation Laser Processing) project sponsored by EPSRC (Engineering and Physical Sciences Research Council) and Tata Steel. The main objective is to understand the process fundamentals and exploit the usefulness of laser technology in pipe industry.

Friction stir welding (FSW) is a maturing welding technique using a rotating tool for simultaneous heating and stir deforming th~ material interface to form a solid bond. Significant tensile residual stresses (RS) and component distortion may be produced even with optimized FSW parameters. Recent stress engineering techniques such as global mechanical or roller tensioning may reduce tensile RS and distortion. This dissertation reports on the first systematic investigation into the efficiency of insitu global mechanical tensioning (IS GMT) as well as roller tensioning applied in-situ (ISRT) and post welding (PWRT) for mitigation of tensile RS and plate distortion in high strength aluminium alloy plates joined by FSW. The techniques were evaluated by measuring the distribution of RS across the weld-line by means of neutron and synchrotron X-ray diffraction as well as' the levels of plate distortion. In each case the weld microstructure and hardness distribution were characterised. The data were rationalised against the ISGMT load and roller tensioning down force respectively. The results have shown that ISGMT and PWRT significantly mitigate longitudinal tensile RS and component distortion. ISGMT was found to decrease the tensile RS by an amount approximately equal to that of the load applied. Consequently, a stress free weld is produced with an ISGMT load equal to the magnitude of the weld-line RS in the as-welded condition~ PWRT decreases the tensile RS as the rolling down force increases and significant compression may be introduced once a certain magnitude is exceeded. ISRT, at least for the range tested, was found to be less effective. The component distortion reduced along with RSÇ'ú mitigation. No effects on the microstructure or hardness distribution due to mechanical stress engineering were observed. Furthermore, it was demonstrated that in order to make accurate stress measurements by diffraction, the effect of precipitation on the stress free lattice spacing must be taken into account for age hardening alloys. In order to complete this study an automated robotic sample manipulation system was developed. Finally, the degree of stress relaxation occurring on cutting down large welds was evaluated by progressively shortening test welds and determining the RS for each length. The amount of stress relaxation for each weld follows the same behaviour and appears to depend on the width of the tensile weld zone only.

Friction Stir Welding (FSW) is a new welding technique for joining and processing metals. The object of this thesis is to develop the numerical models based on scientific knowledge and to provide a useful tool for predicting field variables and understanding the FSW process at a fundamental level. The thesis is focused on developing a continuum solid mechanics-based, fully coupled thermo-mechanical numerical model of FSW using the finite element method to simulate coupling between the welding tool movement, material deformation and movement, energy dissipation and heat transfer in the welding system. The model can simulate the whole welding process.

Linear Friction Welding (LFW) is a relatively new process adopted by aircraft engine manufacturers utilising new technologies to produce better value components. With increasing fuel prices and economical drives for reducing CO2 emissions, LFW has been a key technology in recent years for aircraft engine manufacture in both commercial and military market sectors. For joining Blades to Discs (‘Blisks’), LFW is the ideal process as it is a solid state process which gives reproducibility and high quality bonds therefore improving performance. The welding process is also more cost effective than machining Blisks from solid billets, and a reduction in weight can also be achieved with the use of hollow blades. The LFW process also allows dissimilar materials to be joined and a reduction in assembly time. The main aim of the research is to create a simulation model of a Linear Friction Welding machine and also apply systems thinking to fully understand the LFW process with a view to reduce total production costs. As this EngD focuses on systems thinking, a holistic approach will be used. The hard systems parts of this project will involve the mechanics of the system and understanding relationships between the key system interactions during the welding process in order to create an analytical model of the machine to use for fault diagnosis and prediction. The soft systems parts will focus on the machine users to gain an understanding of how to effectively implement the model with the process and its users. The benefits of the new model include the ability to execute it in a real- time environment with machine operation, allowing weld anomalies to be detected as (and in some cases before) they occur, as well as the monitoring of the machine’s condition. Therefore the business benefits would be realised through a reduction in machine downtime enabling the timely supply of goods providing customer value. Further benefits will be the greater understanding of the complex operation of the whole system and the welding process. Developing a robust research investigation framework, a research hypothesis is introduced and subsequent research questions are developed. Through a combination of hard system investigation using mathematical modelling and soft systems understanding through an action case study intervention, a holistic model is developed.