| Al-Mg-Mn and Al-Mg-Mn-Sc-Zr alloy ingots were produced by semi-continuous casting technique.The welding wire containing Sc and Zr were prepared for the Al-Mg-Mn-Sc-Zr alloy. It was investigated that the applied properties of Al-Mg-Mn and Al-Mg-Mn-Sc-Zr alloy sheets and their related micromechanism using comparative study method. Meanwhile optical microscope, transmission electron microscope and scanning electron microscopy were employed. This study provided experimental and theoretic bases for user to understand the Al-Mg-Mn and Al-Mg-Mn-Sc-Zr alloy sheets fully, and it included in-plane anisotropy of tensile mechanical properties, superplasticity, fatigue properties, corrosion resistance, electrochemistry and welding properties of these two alloy sheets, especially their related micromechanism. The following are important results:(1)The finished alloy sheets of Al-Mg-Mn produced by cold roll and annealing for stabilization has {110}<112> rolling texture and {100}<001> cubic texture. The indexes of in-plane anisotropy of Al-Mg-Mn alloy sheet reduced by {100}<001> texture. In the Al-Mg-Mn-Sc-Zr alloy sheet, the disperse precipitated phase Al3(Sc1-xZrx) particles inhibit the recrystallization strongly and prevent the formation of {100}<001> cubic texture. The indexes of in-plane anisotropy(IPA%)of the Al-Mg-Mn-Sc-Zr alloy sheet are bigger than those of the Al-Mg-Mn alloy sheet.(2)The elongation to failure of Al-Mg-Mn-Sc-Zr alloy sheet is higher than that of Al-Mg-Mn alloy sheet at the same temperature and initial strain rate. In addition, Al-Mg-Mn-Sc-Zr alloy sheets exhibit high -rate and low-temperature superplastic properties. The addition of minor Sc and Zr decrease the activation energy of superplastic deformation and improve superplasticity of Al-Mg-Mn alloy.(3)The main mechanism of superplastic deformation of the two alloys is grain boundary sliding and grain rotation accommodated by dynamic recrystallization, dislocation motion, grain boundary diffusion, liquid phase, filament and cavity. The addition of Sc and Zr leads to low the flow stress and the uniform distribution of liquid phase, filament and cavity. These are helpful for grain boundary sliding and grain rotation.(4)The addition of Sc and Zr to Al-Mg-Mn alloy increases the fatigue strength and fatigue lives. The fatigue strength of Al-Mg-Mn alloys with and without Sc and Zr at 107 cycles was 150 MPa and 123 MPa, respectively. For the two alloys, their fatigue crack growth rate raise as the increasing of stress ratio R.(5) The Al-Mg-Mn-Sc-Zr alloy has higher fatigue lives and fatigue strength than the Al-Mg-Mn alloy, but it has higher macrocrack propagation rate. In the Al-Mg-Mn-Sc-Zr alloy, the nucleation and growth of microcrack are impeded effectively by the fine pancake-like grain structure, dispersion Al3(Sc1-xZrx) particles, high-density dislocation and grain boundary, high resistance force of dislocation motion, these are the main causes that lead to higher fatigue lives and fatigue strength for the Al-Mg-Mn-Sc-Zr alloy. It is notable in Al-Mg-Mn alloy that the effect of crack close induced by roughness and plastically deforming, but it is weak that the resistance for macrocrack propagation from fine grain and Al3(Sc1-xZrx) particles in Al-Mg-Mn-Sc-Zr alloy, So Al-Mg-Mn alloy has higher resistance for macrocrack propagation.(6)The exfoliation corrosion sensitivity of Al-Mg-Mn-Sc-Zr alloy is higher than that of the Al-Mg-Mn alloy. The satisfied combination of strength, plasticity and exfoliation corrosion resisting property can be obtained by means of adopting reasonable annealing practice to reduce or eliminate the precipitation ofβphase at the grain boundary.(7)The corrosion resisting property of the two alloys can be reflected by polarization resistance, corrosion current density, corrosion rate. The analysis results of polarization curves agree with the experimental results of exfoliation corrosion susceptibility.(8)During pit incubation of Al-Mg-Mn and Al-Mg-Mn-Sc-Zr alloy, the Nyquist diagram is mainly composed of a depressed capacitive arc at high-mediate frequency and an inductive arc at low frequency. The appearance of two capacitive arcs in the Nyquist diagram indicates the beginning of pitting corrosion. In Nyquist diagram, the disappearance of inductive arc and the emergence of two time constants of Al-Mg-Mn-Sc-Zr alloy are earlier than those of Al-Mg-Mn alloy. During pits propagation, the Nyquist diagram is composed of two overlapping capacitive arcs. The equivalent circuit is designed according to the structure of pit and the mechanism of electrochemical corrosion. The electrochemical impedance spectroscopy(EIS) of pits propagation of the two alloys are fitted. The good correspondence between the experiment results and the fitted results obtained using the equivalent circuit. EIS technique can evaluate the severity of corrosion quickly, semi-quantitatively and nondestructively.(9)For the 12 mm thick hot rolled sheet of Al-Mg-Mn-Sc-Zr alloy, the tensile strength, elongation and welding coefficient of friction stir welding(FSW) is 382MPa, 20.3% and 96.5%, respectively; the tensile strength, elongation and welding coefficient of tungsten inert gas welding(TIG) is 328MPa, 10.7% and 82.8%, respectively. For the 2 mm thick cold rolled and annealed sheet of Al-Mg-Mn-Sc-Zr alloy, the tensile strength, elongation and welding coefficient of FSW is 391Mpa, 10.9% and 92.4%, respectively; the tensile strength, elongation and welding coefficient of TIG is 365MPa, 10.2% and 86.3%, respectively. FSW can improve mechanical properties of the welding joint remarkably.(10)There aren't hot crack, gas cavity and other welding defect in the welding joints of friction stir welding, but the hot crack and gas cavity emerge generally in the welding joints of tungsten inert gas welding, dynamic recrystallization has happened in the weld nugget zone of FSW, there are very fine equiaxed grains in the weld nugget zone, the diameter of these grains is about 1-3μm, and there are a great deal of dislocation substructure and the second phase Al3(Sc,Zr) particles in the thermo-mechanically affected zone of FSW joints. But the microstructure of TIG welding seam is solidification structure, its grains are coarse. Dislocation substructure disappears, and the density of Al3(Sc,Zr) particles decreases remarkably in the TIG heat-affected zone. The major cause which enhances the mechanical properties of FSW joints is the improvement of microstructure.
|
PDF Full Text Request