Impact toughness of an ingot hypereutectic Al–23 mass% Si alloy improved by rotary-die equal-channel angular pressing

The impact toughness of a hypereutectic Al–23 mass% Si alloy was improved by rotary-die equal-channel angular pressing (RD-ECAP). The alloy was successfully processed by RD-ECAP at high temperatures of 673 K and 623 K. The absorbed energy as measured by Charpy impact tests significantly increased with increasing the number of RD-ECAP passes, and finally reached 13.0 kJ/m2 after 32 passes. This value is 18 times higher than that of the as-cast Al–23 mass% Si alloy and 41% higher than that of the as-cast eutectic Al–11 mass% Si alloy. The grain or grain fragment size is about 150 nm. In addition, most of the large primary silicon particles (over 50 μm) were broken in the first 4 passes, and the primary silicon particle sizes decreased with increasing the pressing passes. These results indicate that the multipass RD-ECAP is effective in improving impact toughness through refining grains and changing the primary silicon particles. On the other hand, many voids were produced in the first 4 passes. These voids, however, were gradually eliminated with increasing the pressing passes, and finally only a few small voids were observed after 32 passes. The RD-ECAP processed Al–23 mass% Si alloy therefore showed ductile fracture during the impact testing.