Strengthening characteristic and mechanism of AlCoCrFeNi high-entropy alloy particles for Al-Cu dissimilar friction stir lap welded joint

Regarding the light-weight welding application, the strategy of non-tool penetration into lower hard sheet is considered as a promising dissimilar friction stir lap welding (FSLW) process for restriction of tool wear and reduction of welding force. In order to further improve the mechanical properties of the lap joint for such a case, AlCoCrFeNi high-entropy alloy (HEA) particles were selected as reinforcement for the Al-Cu dissimilar FSLW. A novel approach to incorporating HEA particles into lap weld, where a HEA particle-reinforced Al matrix was pre-prepared by FSLW, was proposed to prevent the overflow of particles from weld effectively during Al-Cu FSLW. The strengthening characteristic and mechanism of the HEA particles were systematically explored in this paper. Microstructural analyses indicate that the spherical HEA particles with large size contribute to the grain refinement via particle-stimulated nucleation mechanism, furthermore, nanoscale particles (Al3Ni and Al13Fe4) are formed adjacent to the HEA particles in the Al matrix. The induced grain refinement strengthening and secondary phase strengthening by the HEA particles are responsible for the significant improvement in the mechanical properties of Al-Cu lap joint. This study provides a technological approach and a theoretical foundation for the high-quality dissimilar FSLW used in light-weight manufacturing field.