Dynamic interplay of tool geometry and process variables in FSW of aerospace alloys AA7075

Abstract

Zinc and magnesium precipitate-strengthened AA7075, with its promising strength-to-weight ratio, is widely utilized in aerospace applications. The distribution and morphology of strengthening precipitates within the weld zones critically influence the microstructural evolution and mechanical properties of friction stir weldments. In this study, a systematic evaluation of the effects of tool rotational speed, feed rate, and pin profiles on FSW AA7075 weldments was conducted using a Taguchi L9 orthogonal array. Three-pin profiles, including threaded cylindrical, conical, and triangular geometries, were analyzed to assess their influence on material flow dynamics and the stirring mechanism. The combined impact of heat generation and stirring on mechanical properties was investigated using analysis of variance (ANOVA). Additionally, an in-depth microstructural analysis was performed based on variations in heat input and their correlation with mechanical performance. At lower heat input (560 rpm, 63 mm/min, threaded cylindrical pin), finer grains and uniformly distributed precipitates enhanced mechanical performance, with a slight grain size reduction yielding the highest properties. ANOVA and signal-to-noise (S/N) analyses confirmed the dominant influence of tool rotational speed, contributing 44.85% and 63% to variations in microhardness and impact energy, respectively. Process optimization, employing the S/N ratio method, revealed a strong alignment between experimental findings and predictive models, with minimal deviations. X-ray diffraction (XRD) analysis confirmed the presence of Zn- and Mg-based precipitates under lower heat inputs and their dissolution at elevated heat levels. Scanning electron microscopy (SEM) fractography of Charpy impact tests provided insights into the critical failure mechanisms. This comprehensive study highlights the significance of controlled heat generation and material stirring in achieving superior joint integrity and mechanical performance. The findings establish a robust framework for optimizing FSW parameters, advancing the development of solid-state welding technologies for aerospace-grade AA7075 alloys and other high-performance applications.