Improvement of fatigue and corrosion-fatigue resistance of AZ31B cast alloy by cold spray coating and top coating

Abstract

Recent advancements in manufacturing of light-weight structures have caused high interest in industries toward magnesium alloys because of its excellent properties such as specific strength, low density, and fatigue strength. Application of magnesium in the vehicle structure saves energy and reduces gas emission which consequently improves the performance of the vehicle. Magnesium alloys have low corrosion resistance and high tendency to corrode in humid and aqueous environments. The results of recent research show that cold spray coating of pure aluminum powder results in significant improvement in magnesium corrosion. However when tested under cyclic loads, the coated samples developed surface cracks and delamination. Which lead to highly localized corrosion damages underneath of the cracks or any discontinuity in the components. The early cracking was attributed to the low fatigue resistance of pure aluminum. The results of two new approaches in enhancing the corrosion fatigue life of coated magnesium are presented in this thesis. First, an aluminium alloy coating powder with the higher fatigue strength was selected for coating. Al-7075 powder, with average particle size of twenty microns have successfully been coated on AZ31B samples with highly densed, low porosity coated layer. The coating was performed using nitrogen gas at 400oC temperature, and gas pressure of 200 psi. Electrostatic painting was then applied. Result of tests in the corrosive atmosphere of 3.5 % of NaCl solution as well as rotating fatigue test in corrosive environment will be presented. Improvement of fatigue resistance of cold spray coated samples and corrosion-fatigue resistance of cold spray coated, e-painted samples are studied in this thesis.