The effect of melt shearing on the microstructure and corrosion behavior of Al-Zn-Mn-Fe

In this work, a vertical screw was used for applying shear stress to melted Al-Zn-Mn-Fe alloy under various times; and, the effete of this process (melt shearing) on microstructure and corrosion properties were investigated. Optical microscope and Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS) were used for evaluating the microstructure changes. The effect of melt shearing process on the phase changes was investigated by XRD method. The experimental investigations showed that, melt shearing with a vertical screw has a significant effect on the microstructure. When the shearing time increases, the dendritic grains change to equiaxed grains and also the large grains alter to fine grains. EDS results depicted that, the samples which are solidified without shearing process, include some grain boundary concentrated by iron, while the iron concentration would be uniform in the sample with melt shearing process. With increasing the shearing time, the coarse and continuous intermetallics in the grain boundaries change in to discontinuous and fine intermetallics. In order to investigate the corrosion resistance of samples, the Electrochemical Impedance Spectroscopy (EIS) and cyclic polarization tests were used. The results depicted that, when the shearing time increases, due to uniform distribution of cathodic intermetallics and reduction of their size, the corrosion resistance in sea water increases considerably.