| Summary: | 博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 95 === In this study, the effect of RE content on the microstructure, creep behavior, weld and corrosion properties of Mg-8Al-xRE (x=0, 1, 2 or 3 wt.%) alloys were investigated. The alloys were prepared by melting and casting in a vacuum induction melting furnace in an atmosphere of argon gas. Chemical analysis of the alloys was performed by inductively coupled plasma-atomic emission spectrometer (ICP-AES). To avoid the possible casting defects, such as micro-pore or micro-segregation, the cast ingots were indirect hot extrusion to remove the cast-defects. The microstructural analysis and phase characterizations of alloys were performed by optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), and X-ray diffraction (XRD). The fracture surfaces of crept specimens were examined by SEM. Corrosion tests were carried out by potentiodynamic polarization and immersion tests. The study of weld property was accomplished by tungsten-arc inert gas and CO2 laser beam welding. The microstructure analysis, mechanical properties, and corrosion properties of welded specimens were investigated.
The microstructure of Mg-8Al alloy with 1-3 wt.% RE additions were conducted: (i) The as-extruded Mg-8Al-xRE alloys consisted of α-Mg matrix, with β (Mg17Al12) and Al11RE3 compounds. (ii) Raising the extent of RE in the alloy also increases the amount and coarsening of the Al11RE3 compounds, but the amount of β phase diminishes and turns into the fine particles. The creep rupture life increment measured at 150℃ is around 40-100 MPa, and the creep rupture life over 150℃ is also prolonged. The marked improvement of the high-temperature tensile creep properties is attributed to the fine rod-like Al11RE3 compound having high thermal stability in the alloys. The stress exponent of the Mg-8Al-xRE alloys is approximately 2, which suggested the creep mechanism of the alloys is controlled by the grain boundary sliding. The creep activation energy of Mg-8Al and Mg-8Al-2RE alloys are 114 and 104 kJ mol-1, respectively. In addition, the aluminum content also has great effects on the creep property of the Mg-Al alloy; when comparing the AZ31-1RE with the Mg-8Al-1RE alloys. The creep resistance of the AZ31-1RE is inferior to that of the Mg-8Al-1RE, as the applied load is high; at either higher or lower test temperature. On the contrary, the creep resistance of the AZ31-1RE is superior to that of the Mg-8Al-1RE, as the applied load is low at any test temperature. Therefore, the effect of aluminum content on the creep properties of Mg-Al alloy is great. The close-grain boundary microstructure of pose-crept Mg-8Al shows that high volume fraction of lamellar β precipitates close-grain boundaries during the creeping. Since the lamellar precipitation occurring during the creeping may effectively multiply the grain boundary area available for easy deformation by grain boundary sliding through the elevated temperature creep. Therefore the creep resistance of Mg-8Al is poor at elevated temperature. Oppositely, no significant change in the microstructure morphology after creep exposure has been observed in the RE-containing alloys. The addition of RE to the Mg-8Al alloy forms the Al11RE3 intermetallic phase, which may suppress the precipitation of lamellar β phase during the creep test. Consequently, the sliding of grain boundaries and the slip of dislocations in the matrix are effectively prevented at elevated temperature, improving the creep resistance of Mg-8Al base alloy.
The optimization welding parameters of TIG welding are current 110 A and welding speed 7 m/min. Because the TIG welding has low energy density and high heat input characteristics, the macrostructure of welded Mg-8Al-xRE alloys possess three clearly distinguished regions, including weld metal, heat-affected zone, and parent metal. The aspect ratio of the weld pool is low, about 0.5. Microstructure of weld metal is refined with the increased RE content; and a lot of needle-like or rod-like RE-containing compounds precipitated. The join efficiency is approximate 85%, and the bonding strength is increased with the increasing RE content. However, when the RE content over 2 wt.%, the strength is decreased. The hardness of weld metal is lower than the parent metal, and close to the heat-affected zone. Corrosion rate of the alloy may slightly decrease with the increase of the added RE contents. The corrosion resistance of welded specimens is poor, comparing with the as-extruded alloys.
The optimized welding parameters of CO2 laser beam welding are power 2.0 kW and welding speed 2,500 mm/s. The CO2 laser beam welding has high energy density and low heat input characteristics, so the cooling rate of the weld metal is high. The microstructure of the welds is finer than that of TIG welding; and no obviously heat-affected zone is observed, which is due to the laser welded alloys owns fine structure, thus the hardness is higher than that of the TIG welded alloy. In this study, the hardness of the welded metal is slightly lower than that of the parent metal. The aspect ratio of the laser welds is higher than that of the TIG welding, which is about 1.5.
|
|---|
