Ageing characteristics and high-temperature tensile properties of Al-Si-Cu-Mg alloys with micro-additions of Mo and Mn

Artificial ageing characteristics and high temperature tensile properties of the A1-7Si-lCu-0.5Mg (wt%) alloy modified with micro-additions of Mo and Mn, were examined. The modified alloy formed microstructure containing Cu- and Mg-rich phases along with Mo-and Mn-rich dispersoids of complex morphology. During artificial ageing, the alloy reached a peak hardness of 97 HRF after a cycle at 200 degrees C for 1 h. At room temperature, the major impact of Mo and Mn additions was an increase in ductility by 3.5 times and in ultimate tensile strength by 4%. With increasing the testing temperature from 25 degrees C to 300 degrees C, the yield strength and tensile strength of the modified alloy in T6 condition decreased from 195 MPa to 54 MPa and from 324 MPa to 139 MPa, respectively. It was found that the alloy strength and ductility are affected by the secondary dendritic arm spacing. The coarse intermetallics influenced the crack propagation. When at room temperature, the cleavage-type brittle fracture was dominant at higher temperature the fully ductile cracking mode was present. The role of Mn and Mo-containing dispersoid phases in modification of the alloy properties is discussed.