| dc.description.abstract | Abstract
This study aimed at investigating some important issues when dealing with the material, magnesium and its alloys, which are the most popular light metal applying to many common purposes. The study could be divided into few main subjects, they are combustion behaviors, machining and improving corrosion resistance of magnesium and its alloys. Regarding to the knowing of combustion behavior of magnesium materials under different atmospheres, it could help preventing the fire hazard and developing the protective atmosphere when melting. The optimizing of machining conditions, not only help preventing fire hazard from ignition of chips but also improving the properties of magnesium work-pieces after machining. Non-chrome conversion treatment is a substitute of dichromic treatment. It is expecting to provide less environmental impacts and maintain good corrosion resistance as well. The individual briefs are as following:
Part I: AZ61A cakes were heated by the gas torch and combusted. The progressive development of heated cake was recorded and described. The product of Mg(OH)2 covers on the surface of heated AZ61A cake and protects it from oxidation. Reactions of Mg(OH)2 → MgO + H2O occurred during heating by flame and after AZ61A melted. Molten metal continuously oozed out and produced MgO accompanied with great amount of heat to react with carbon film (2Mg + CO2→ 2MgO + C). This led to reaction of MgO + C → Mg + CO and the product of Mg carried out oxidation and formed MgO. The residual showed that all AZ61A completely burned out and became a mess of white powder. The specimens of AZ61A were also set on the specimen holder in the designed containers filled with several atmosphere gases (Ar, air, CO2 and different ratios of CO2/Ar). The responded temperatures from specimen holders were recorded and analyzed. Mechanisms for illustrating the reaction of Mg with different gases were schematically illustrated. Possible reactions of Mg with O2, CO2 and CO were also discussed in this study.
Part II: For magnesium alloys, optimizing the machining conditions is necessary to prevent ignition of chips. In this study, effects of point angles of drill bits and drilling parameters on surface roughness and cutting resistance forces were measured and studied. Surface roughness and cutting resistance forces are increased following the increase of point angle and material removal rate. Point angle (2p) descends from 118° to 55° producing the smoothest machined surface and minimum variance in the measured roughness. In addition, effect of drilling operation on varying microstructure of AZ61A was also investigated in this study. The drilled sample showed a minimum extent of deformation twinning layer, when the drill bit adopted a point angle of 55°. The drilled sample developed a superior surface roughness and a short extent of twinning layer generated on the matrix of machined sample, if a 5% NaOH solution was used as lubricant and a 55° point angle was used.
Part III: A chrome-free conversion coating treatment, mainly permanganate phosphate solution, for magnesium and its alloys was developed in this study. The morphology of conversion-coated layer was observed by using SEM; the crystal structures and compositions was analyzed and determined by using XRD and XPS. The XRD results indicate an existence of amorphous structure on the coated specimen and only show a greater intensity of Mg17Al12 than as received sample. The XPS results show that the coated layer includes products of MgO, Mg(OH)2, MgAl2O4, Al2O3, Al(OH)3, MnO2 or Mn2O3 and amorphous oxy-hydroxides for Mg-Al-Zn alloys but MgO, Mg(OH)2, MnO2, amorphous oxy-hydroxides and MgMn2O8 for pure Mg. The electrochemical polarization test results demonstrate that the presented conversion treatment for series of AZ alloys develop an equivalent capability in corrosion potential to JIS H 8651 MX-1 (similar to Dow NO. 1), chrome-based method. | en_US |