Microstructure, segregation and fracture behavior of 6061 aluminum alloy samples formed by semi-solid or traditional high pressure die casting

Abstract

A serpentine channel pouring (SCP) process for efficiently preparing semi-solid 6061 aluminum alloy slurry and its rheological high pressure die casting (Rheo-HPDC) technology were introduced. The microstructure, segregation and fracture behavior of 6061 aluminum alloy samples obtained via the traditional HPDC or Rheo-HPDC process were investigated and compared. Furthermore, the effects of pouring temperature and Si content on the distribution, grain size and volume fraction of the primary phase were analyzed. The results indicated that the Rheo-HPDC process can effectively refine and improve the morphology of the primary phase compared with the traditional HPDC samples, which evolved from coarse dendrites into fine spherical grains. The pouring temperature had a significant effect on the primary α1-Al grains during slurry preparation, but had little effect on the secondary solidified α2-Al grains in the Rheo-HPDC samples. Moreover, with increasing Si content, the casting capability of the alloy was significantly enhanced, while the grain size of the primary phase was no obviously changed. Additionally, the liquid segregation led to varying microstructures, elemental distributions and fracture mechanisms of the samples. The volume fraction of the primary phase increased with increasing the distance from the sample surface, while the Si, Mg, Fe and Cu contents were enriched at edge of the testing bars. Besides, the liquid segregation tendency gradually increased along the filling direction in the Rheo-HPDC samples. Under the same die casting process parameters, the Rheo-HPDC samples had excellent mechanical properties than that of the traditional HPDC samples, and its ultimate tensile strength (UTS) increased from 90.8 MPa to 106.9 MPa. The fracture mechanism of the traditional HPDC samples was mainly brittle quasi-cleavage fracture, while the Rheo-HPDC samples was mainly mixed-fracture mechanism of dimple and quasi-cleavage fracture.