Effect of growth velocity on microstructure and mechanical properties of directionally solidified 7075 alloy

7075 alloys were prepared using metals of 99.99% high purity in the vacuum atmosphere. These alloys were directionally solidified upwards under various growth velocities (8.3-166.0 mu m/s) using a Bridgman-type directional solidification furnace. Dendritic spacings (lambda(1), lambda(2)) and microhardness (HV) were measured from both longitudinal and transverse sections of the samples. Ultimate tensile strength (sigma(U)) and yield strength (sigma(y)) of the solidified samples at room temperature were examined. The HV increased from 75.2 to 112.8 MPa, the sigma(U) increased from 155.8 to 210.1 MPa, and the sigma(y) increased from 198.4 to 319.8 MPa with increasing growth velocity from 8.3 to 166.0 mu m/s. Exponent values of the V for the lambda(1), lambda(2), HV, sigma(U) and sigma(y) were calculated as 0.27, 0.41, 0.05, 0.11 and 0.15, respectively. The results show that the hot tearing in the 7075 alloy is caused by the fracture of dendrites structures.