Effect of rotational speed on friction stir welding microstructure and properties of cast and rolled (ZrB2 + Al3Zr) particle-reinforced aluminum matrix composites

Friction stir welding of cast and rolled-state aluminum matrix composites is extensively used in many industries, and the speed at which the mixture is stirred profoundly influences the structure and characteristics of the welded connections. The present study investigates the microstructure and mechanical properties of the as-cast and as-rolled (ZrB₂ + Al₃Zr) particle-reinforced aluminum matrix composites. The welded joints were subjected to friction stir welding at a welding speed of 60 mm/min and stirring speeds of 1000, 1200, and 1400 rpm, separately. At a rotational speed of 1200 rpm, the welded joints exhibit optimal performance with an average hardness of 71.07 HV, a tensile strength of 183.9 Mpa, and an elongation of 16.7%. The weld core region generates precisely shaped equiaxed grains, while the weld matrix facilitates a significant amount of dislocation entanglement. At a rotational speed of 1200 rpm, the grain refinement effect is fully optimized, resulting in an average grain size of 2.94 μm. The recrystallized organization accounts for 70.6% of the grain size. Both types of reinforcing particles are adequately crushed, with the smallest particle size of ZrB₂ particles being less than 100 nm and the smallest particle size of Al₃Zr particles being less than 1 μm. The two methods of weld reinforcement are fine crystal strengthening and Orowan strengthening.