Da-Bin Lee

In this work, the microstructure and corrosion behavior of a novel Al-6Mg alloy were investigated. The alloy was prepared by casting from pure Al and Mg+Al₂Ca master alloy. The ingots were homogenized at 420 °C for 8 h, hot-extruded and cold-rolled with 20% reduction (CR20 alloy) and 50% reduction (CR50 alloy). The CR50 alloy exhibited a higher value of intergranular misorientation due to a higher cold rolling reduction ratio. The average grain sizes were 19 ± 7 μm and 17 ± 9 μm for the CR20 and CR50 alloys, respectively. An intergranular corrosion (IGC) behavior was investigated after sensitization by a nitric acid mass-loss test (ASTM G67). The mass losses of both the CR20 and CR50 alloys were similar at early periods of sensitization, however, the CR20 alloy became more susceptible to IGC as the sensitization time increased. Grain size and β phase precipitation were two critical factors influencing the IGC behavior of this alloy system.

High strength Al-Mg alloy is an attractive material that has the characteristic of increasing both strength and elongation by adding more solute Mg. However, there is a limitation in the oxidation issue during the casting process when it contains high amounts of solute Mg. New Al-Mg alloy was developed using Mg+Al₂Ca master alloy by making a stable CaO/MgO mixed layer that no significant oxidation occurred. Here, the intergranular corrosion (IGC), electrochemical, and mechanical properties of new Al-Mg alloys fabricated through a combined process of extrusion and cold rolling were studied after the specimens went through artificial aging heat treatment at 200 °C. The results show that the grain size and the volume fraction of anodic β-precipitation (Mg₂Al₃) forming on the grain boundary influence the intergranular corrosion results. Corrosion potential and current density were achieved by potentiodynamic polarization electrochemical test. The results show that corrosion potential remains irrespective of the manufacturing process, while current density increases with artificial aging treatment. Both hardness and tensile mechanical properties decrease on cold rolled specimens after the heat treatment, while increase in extrusion and annealed specimens.

For the purpose of applying a modified 5083 alloy (New 5083M alloy) with high Mg content in various automotive sheet parts, the stretch forming behavior of the 5083M alloy was studied in tensile mode at a wide range of processing conditions. The tensile tests were conducted by using a tensile test machine under the temperature ranges of 100–400 °C and the strain rate ranges of 0.001–1 s⁻¹. The test results showed that the 5083M alloy has superior mechanical properties to that of the commercial 5083 alloy at elevated temperatures. The microstructure before and after the stretch forming was analyzed using optical microscope (OM) equipped with a polarizing filter and electron backscattered diffraction (EBSD) unit. Deformed microstructure was observed under low temperature conditions and dynamic recrystallized microstructure under high temperature conditions. However, regardless of microstructure evolution, developed deformation textures were distributed in orientation distribution functions (ODF) images. In addition, at high temperature and low strain rate condition, complex shaped cavities which were detrimental to mechanical properties appeared at the grain boundary and grain triple junction. Based on the test results data, a constitutive equation predicting the deformation behavior of the 5083M alloy was derived. The calculated curves by the constitutive equation were compared with the measured curves by experiment and agreed well.