Microstructure of Multi-Pass Friction-Stir-Processed Al-Zn-Mg-Cu Alloys Reinforced by Nano-Sized TiB2 Particles and the Effect of T6 Heat Treatment.

In this work, a fine-grained structure with a uniform distribution of TiB₂ particles and precipitates was achieved in TiB₂ particle-reinforced (PR) Al-Zn-Mg-Cu alloys by friction stir processing (FSP). The effects of multi-pass FSP on the microstructure, and TiB₂ particle distribution, as well as the microstructural evolution in the following T6 treatment, were investigated by X-ray diffraction, scanning electron microscopy and associated electron backscattered diffraction. The results showed that the distribution of TiB₂ particles and alloy precipitates was further improved with an increase in the FSP passes. Moreover, compared with alloy segregation in the as-cast PR alloys during T6 treatment, a complete solution of the precipitates was achieved in the FSP-treated PR alloys. The fine-grained structure of the FSP-treated PR alloys was thermally stable without any abnormal growth at the high temperature of T6 treatment due to the pinning effect of dispersed TiB₂ particles. The strength and ductility of the PR alloys were simultaneously improved by the combination of FSP and T6 treatment. [ABSTRACT FROM AUTHOR]