M. F. Gittos, TWI, Cambridge, United Kingdom; A. J. Leonard, BP Exploration Company Ltd, Middlesex, United Kingdom
Process variables affect joint quality. Successful, reproducible welds may be produced by operating within process “windows”. However, problems arise when welding conditions deviate from the operating window. A need exists to understand the types of flaws that may be generated and their causes. Welds were made in 6mm thick 2014A, in which processing parameters were varied outside of the operating window. They were assessed using radiography, ultrasonic inspection and light microscopy. The types of flaw produced could be categorised either as voids or joint line remnants. The former were produced in two ways: forging pressure exerted on the workpiece was reduced and welding speed was increased, reducing the working of the metal and the heat input. A joint line remnant was introduced by using a tool that was too short for the plate thickness. This left a region of the original interface that had not been adequately disrupted by tool rotation. A root flaw could be very small and may not be detected by NDE. A joint line remnant could also consist of a line of oxide particles from the original interface extending through the weld. This was achieved by anodising the surfaces, prior to welding. It was found that voids may be formed when insufficient forging pressure is applied to the weld or the welding speed is too high. A joint gap of up to 2mm could be tolerated. Joint line remnants occurred when pre-weld cleaning was inadequate but machining, prior to welding, is effective in restricting their occurrence. Joint line remnants, as root flaws, are introduced when either insufficient pin depth or tool plunge depth is selected, or when there is poor tool-to-joint alignment. Appropriate selection of such parameters may eliminate these flaws. However, for critical applications, machining of the weld root may be advisable.
Summary: A number of flaw types were introduced into welds in aluminium alloy 2014A by operating outside of the established process window for the alloy and tool type used. The flaws were evaluated by non-destructive examination and were characterised by light microscopy. It was found that voids may be formed when insufficient forging pressure is applied to the weld or the welding speed is too high. A joint gap of up to 2mm could be tolerated when welding 6mm thick 2014A plate without the formation of weld flaws. Joint line remnants may be produced if inadequate pre-weld cleaning is performed and machining the plates, prior to welding, is effective in restricting their occurrence. Joint line remnants, in the form of root flaws, are introduced when either insufficient pin depth or tool plunge depth is selected for the joint, or when there is poor tool-to-joint alignment. Appropriate selection of such parameters may eliminate these flaws. However, for critical applications, machining of the weld root may be advisable.
