Improvement of microstructure and mechanical properties of A356 alloy through ultrasonic treatment of Al-5Nb-1B refiner and Sr modification

It has become the most common grain refinement in the foundry industry through adding master alloys containing effective heterogeneous nucleation cores. In the present work, a new Al-5Nb-1B refiner was developed via ultrasonic vibration treatment (UVT) at 20 kHz and 1000 W for 3 min. The influence of ultrasound on the distribution and morphology of NbB 2 and Al 3 Nb precipitates in master alloy was examined. Concurrently, the synergistic influence of the refiner and 0.04 wt.% Sr addition on the morphology and mechanical properties of A356 alloy were evaluated. The results demonstrated that ultrasonic treatment efficiently breaks up the agglomeration of NbB 2 phase and the large block Al 3 Nb can refine into fine regular particles. By incorporating 3 wt.% Al-5Nb-1B alloy and 0.04 wt.% Sr into the A356 alloy, the α-Al dendrites were refined into a fine equiaxed structure with the average secondary dendrite arm spacing (SDAS) decreasing from 16.7 μm to 3.0 μm. Simultaneously, the long needle-like eutectic Si phase was transformed into fibrous morphology. Tensile tests revealed that the ultimate tensile strength (UTS) was raised by 15.8% from 171 MPa to 198 MPa, the yield strength (YS) was increased by 23.0% from 69 MPa to 85 MPa and the elongation (El) was raised by 62.3% from 7.7% to 12.5%. Following the heat treatment, the eutectic silicon phase were further refined into nearly spheroidized particles. At the same time, the UTS, YS and El of the alloy were further increased to 280 MPa, 135 MPa and 15.7%, respectively. Additionally, the strengthening and toughening mechanisms were evaluated.