Development and application of advanced plasma welding techniques for high strength aluminium alloys
The main aims of this study are to generate a methodology for the optimisation of welding procedures for plasma welding of thin sheet aluminium alloys and to investigate weld quality modelling. Emphasis is focused on the recognition and evaluation of the consecutive stages of welding procedure development and on the formulation of a generalised procedural methodology that is potentially applicable to other processes and materials. The materials under investigation were 1.6 mm thick sheet 6013, 2024 and 7475 alloys, representing the major medium and high strength heat treatable aluminium groups used in the aerospace industry. Initial experimentation generated procedures relating to specimen and equipment pre-weld preparation. Bead on plate and square butt joint trials were performed in the downhand position. The plasma keyhole mode was operated autogenously while filler wire addition was employed for the melt-in mode. Arc monitoring techniques were used to log the arc voltage and welding current values. The effect of background and control parameters on process performance and joint quality was studied and used to generate operating envelopes and reveal optimum welding conditions. Geometrical data from the melt-in joints of all alloys were employed to build joint geometry prediction statistical models. Numerical algorithms, based on the information generated by the statistical models, were used to create joint geometry optimisation techniques. Calorimetric experiments and x-ray examination of joints revealed the relationship between major operating parameters and arc efficiency and the incidence of porosity, suggesting desired welding conditions that were incorporated in the optimisation process. Finally, two software tools for Joint Geometry Prediction and Welding Procedure Optimisation were developed, incorporating the knowledge and information created during the modelling and optimisation stages. These provide the end user with a means of process parameter selection and visualisation of the influence of parameter variation on weld bead geometry.