Experimental study of scandium addition and precipitate formation on the mechanical and tribological properties of Ultrasonic-assisted stir-cast AA7075 hybrid metal matrix composites

ABSTRACT Aluminium Alloy Hybrid Metal Matrix Composites (AAHMMCs) have been utilized in automotive, marine, military and aerospace applications due to their high strength-to-weight ratio, microhardness, tensile, impact, and compressive strength, along with superior tribological properties. This study focused on optimizing ultrasonic-assisted stir casting of AA7075/WC/SiC Hybrid Metal Matrix Composites (HMMCs) by varying the wt.% of WC and SiC (6, 8, and 10), melting temperature (700, 750, and 800°C), stirring time (5, 10, and 15 min), and stirring speed (200, 225, and 250 rpm), while keeping ultrasonic parameters constant. The inclusion of 0.25 and 0.5 wt.% Scandium increased microhardness by 18% due to the Hall-Petch effect and Al3Sc precipitates, which strengthened the material. Ultrasonic assistance improved grain refinement and uniform particle distribution. The optimal process parameters were determined as a 750°C melting temperature, 250 rpm stirring speed, 5 min stirring time, and 8 wt.% WC+SiC. Aging at 300°C improved microhardness (by 15%), tensile strength (by 14.3%), compressive strength (by 12.4%), and wear resistance (by 51%) due to Mg2Si and Mg2Zn precipitates. Aging at 400°C increased impact strength by 59%, attributed to Al2Cu precipitate.