AZ91 magnezyum alaşimlarinda soğuma hizlarinin mekani̇k ve korozyon özelli̇kleri̇ne etki̇si̇ni̇n i̇ncelenmesi̇

In this study, effects of cooling rates on microstructure, mechanical and corrosion properties of 0.5wt.% Ti or 0.5wt.% Pb added AZ91 Magnesium (Mg) alloys have been investigated. The cooling rates were enabled on the basis of various solidification velocities of the molten alloys by using four stage step mould. The microstructure of Mg alloys was investigated by using optical and scanning electron microscop. The mechanical properties of the alloys were determined by hardness measurements and tensile tests. The corrosion behaviour was avaluated by immersion tests and elektrochemical polarization experiments in 3,5wt.%NaCl solution. Addition of 0.5wt.% Ti or 0.5wt.% Pb to AZ91 alloy, reduced the amount of intermetallics (β (Mg17Al12)) and refined their form on grain boundaries. The results from the mechanical tests showed that tensile and yield strengths increased with anincrease of cooling rates of the alloys. When compared to 0.5wt.% Ti or 0.5wt.% Pb added Mg alloys, base AZ91 alloy showed relatively lower tensile and yield strengths. The results from the immersion tests showed that corrosion resistance of alloys increases progressively with increasing cooling rates for all the alloys used. Taking into account of the fastest cooling rates, the corrossion loss was decreased upto 70 and 90% by addition of 0.5wt.% Pb and 0.5wt.% Ti, respectively. SEM analysis showed that base AZ91 alloy was the most vulnarable to corrosion environment as compared to Pb or Ti added alloys which was attributed to finer grains and modified of β (Mg17Al12) phase as a result of cooling rates. The high corrosion resistance promoted by the addition Pb or Ti element could be related to the morphology and distributions of β phases along the matrix. Polarisation tests showed that corrosion current densities (Icorr) of AZ91 and 0.5wt.% Pb containing alloys were decreased with increasing cooling rates whereas addition of 0.5wt.% Ti reduced the corrosion current density slightly.