| As "Green Engineering Materials in 21st Century", magnesium alloy has great potentialities of application in industries like aviation, spaceflight, auto and electronics for its unique excellent properties (low density, high specific strength and specific stiffness, etc.). But, magnesium alloy is limited to apply in industry widely to some extent due to the poor performance in wear resistance, corrosion resistance, etc. In order to expand the magnesium alloy's potential, we have to adopt some technology to improve its wear resistance and corrosion resistance which is very important to the application of magnesium alloy.Microstructure of alloy would be different when they solidify out. Firstly, solidification tests that AZ31 magnesium alloy cooled in air, water, quench oil and liquid nitrogen are carried out. Effect of differert cooling velocity on microstructure and performance is studied, and the cooling velocity of magnesium alloy is estimated. Secondly, In order to improve the wear resistance and the corrosion resistance, surface of AZ31B magnesium alloy is melted using a pulsed 500W Nd:YAG laser in two different cooling way:one group is cooled in air, and the other is in liquid nitrogen. Magnesium alloy and the laser melted layer are studied by OM, Vickers hardness tester, XRD, TEM, wear tester and electromical tester from the following aspects:microstructure, microhardness, corrosion resistance and wear resistance.The results of magnesium alloy solidification tests show that magnesium alloy is all consist of a-Mg andβ-Mg17Al12 phase under different cooling conditions. The grains of magnesium alloy solidified in water are more fine than that solidified in air, and the quantity oβ-Mg17Al12 is no significant difference. Theβ-Mg17Al12 phase of magnesium alloy solidified in oil is in the form of a treelike mark. Grain boundaries could not be observed because of the large quantities ofβ-Mg17Al12 phase. Magnesium alloy solidified in liquid nitrogen has the smallest amount ofβ-Mg17Al12 phase. The micro-hardness of magnesium alloy solidified in air, water, quench oil and liquid nitrogen is 53.7HV0.05,55.3 HV0.05,59 HV0.05 and 72.1 HV0.05 respectively. The cooling velocity of magnesium alloy cooled in water and liquid nitrogen is estimated to 50.1K/s and 324.9K/s respectively.The results show that the grain size of the laser melted layer cooled in air is more smaller than as-received AZ31B. The laser melted layer cooled in air is also consist ofα-Mg andβ-Mg17Al12 phase as as-received AZ31B magnesium alloy. Due to the grain refining strengthenings,solid solution strengthening,dispersion strengthening and dislocation strengthening, the microhardness, wear resistance and corrosion resistance of laser melted layer have significant improvement. The highest microhardness reaches to 94-102HV0.05,raised by 70.9%～85.5%. Under the same friction conditions, the wear mass loss of the laser melted layer was 2.2×10-3g,1.7×10-3g and 2×10-3g respectively at the laser melting velocity of 5mm/s,7mm/s and 9mm/s and was 78.6%,60.7% and 71.4% of as-received AZ31B magnesium alloy(2.8×10-3g). The result of electrochemical corrosion show that the corrosion voltages of laser melted layer cooled in air moves 63 mV to positive direction than as-received AZ31B magnesium alloy and rearches to -1461 mV. At the same time, the corrosion current is decreased an order of magnitude. The average corrosion rate of as-received AZ31B magnesium alloy is 0.0684 g/(cm2·h). The average corrosion rate of laser melted layer cooled in air are 0.036 g/(cm2·h),0.0324 g/(cm2·h) and 0.0396 g/(cm2·h) at the laser melting velocity of 5mm/s,7mm/s and 9mm/s. The result indicate that the corrosion resistance of the laser surface melted layer is improved obviously.The results show that the grain size of the laser melted layer cooled in liquid nitrogen is more smaller than that cooled in air. The laser melted layer cooled in liquid nitrogen have higher micro-hardness (95～148HV0.05) than that cooled in air(94～102HV0.05), compared to the as-received magnesium alloy (about 55HV0.05).The wear mass loss of the laser melted layer cooled in liquid nitrogen (1×10-3 g) is less than that cooled in air(1.7×10-3 g), decreased by 41.2%. The corrosion potential of the laser melted layer cooled in liquid nitrogen and is moved 85 mV to positive direction than as-received AZ31B magnesium alloy and rearched to -1439 mV.The corrosion current of laser melted layer cooled in liquid nitrogen is decreased about an order of magnitude than that of as-received AZ31B magnesium alloy. The average corrosion rate of laser melted layer cooled in liquid nitrogen is 0.0108g/(cm2·h), compared with 0.0684 g/(cm2·h) for as-received AZ31B magnesium alloy and 0.0324g/(cm2·h) for laser melted layer cooled in air. The results indicate that AZ31B magnesium alloys laser melted layer have better corrosion resistance under the liquid nitrogen cooling condition.The main defect in laser surface melting was cracks, blow hole and burning of alloying elements. Cracks were mainly solidification cracking, and blow hole was mainly hydrogen hole. |
PDF Full Text Request