Stationary shoulder friction stir welding of military aircraft sheet alloys

Abstract

Stationary Shoulder Friction Stir Welding (SSFSW) allows joining of materials without the addition of mass and could be a potential solution to expedient helicopter repair in operational theatre. This research considers the suitability for SSFSW of two aluminium alloys commonly used within the Merlin helicopter airframe, AA8090 and BS L165. Techniques including optical microscopy, hardness, tensile and fatigue testing were used to characterise the two alloys and to establish a baseline for comparison with the welded specimens. The welds were performed in four configurations, with each welded both parallel and perpendicularly to the material’s rolling direction. This was in an effort to establish: the alloy’s suitability for SSFSW in both similar and dissimilar welds; the effect of changing the alloy positioned on the advancing side; and the influence of the material rolling direction on the quality of the weld. Techniques similar to those used in the material characterisation were used to determine the quality of each configuration. Encouraging tensile and fatigue strength results were achieved, especially for the similar materials welds with BS L165 welded parallel to the rolling direction achieving over 100% UTS weld efficiency. Inconsistent quality including the presence of kissing bonds was, however, observed. When welding parallel to the rolling direction, stronger welds were produced with BS L165 positioned on the advancing side. The presence of kissing bonds made analysis of those welded perpendicular to the rolling direction inconclusive. The material rolling direction also had an effect on the weld, as demonstrated through differences in weld appearance, degree of mixing of materials and tensile and fatigue test results. Additionally, a relationship between hardness and tensile strength was determined which revealed close correlation between the governing equation and the test results, with only 7% scatter. This will allow for prediction of tensile strength, based on the measured hardness, when the materials are joined using SSFSW.