| Extruded Mg-Al-Zn alloys exists the problem of low mechanical properties and poor corrosion resistance, especially the corrosion trend of fusion zone. According to the concept of multi-alloy, Mn element and mixed rare earth elements (main cerium) was added into AZ61. The aim is to study the impact to microstructure, mechanical properties and corrosion resistance of AZ61magnesium alloy after composite addition of those elements.In this thesis, direct reading spectrometer was used to detect elements content real-time. AZ61magnesium alloy ingots were gotten by flat die casting and processed after air cooling, while an ultrasonic detector was used to detect casting defects.After homogenizing annealing and hot extrusion processing, test and analysis means including optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy spectrum analysis (EDS), RST5200electrochemical workstation, DCTC-600P, salt spray chamber, micro hardness tester and universal testing machine was used to study the impact of different Mn/RE ratio to microstructure, mechanical properties and corrosion resistance of extruded AZ61magnesium alloy systematically.The study shows that, by composite addition of manganese and rare earth elements, second phases’type and quantity changed significantly. Due to the addition of Mn and RE, the amount of β-phase (Mg17Al12) had a significant reduction and some spotty β-phase distributes in a-Mg, at the same time a large number of rod and needle-like Al11RE3phase generated, however most of them distribute in the crystal grains and only a small amount of it exist along the grain boundaries. Because of the addition of RE, the original Al8Mn5existing in AZ61changed into AlioRE2Mn7ternary phase. This ternary phase showed clusters like distribution and has good coherent relationship with the magnesium matrix. After homogenization heat treatment and hot extrusion, the needle-like rare earth crushed and dispersed in the matrix phase homogeneously and the clusters Al10RE2Mn7phase was dispersed after hot extrusion. The grains refined, but with the change of Mn/RE ratio,the grain size of the extruded bar had a tendency to grow up.In the mechanical properties, a large number of Al10RE2Mn7ternary phase formed after combined addition of manganese and rare earth elements.Broken and dispersed Al11RE3phase in AZ61+Mn+RE matrix has been further improved the mechanical properties on the basis of AZ61. From component2to component6with the change of Mn/RE, the tensile strength and elongation of the magnesium alloy with respect to AZ61were improved to some extent. Component2has the highest tensile strength and elongation which is273.88MPa and17.30%respectively. Besides, with change of the Mn/RE ratio, both tensile strength and elongation had a process of decreased first and then increased. As to tensile fracture surface, the component with better tensile strength and elongation had a large number of the second phase (Al10RE2Mn7) in its fracture surface. The spherical Al10RE2Mn7reinforced mechanical properties of AZ61+Mn+RE. There was relatively more dimples and tear ridge on fracture surface as well as less and smaller cleavage plane. It shows that the ability of the plastic deformation is better and the crack growth requires more energy. Component with a lower tensile strength and elongation, fracture morphology is mainly of a large amount of the cleavage plane, a small number of dimples and almost no second phase, indicating that in the stretching process, the crack along the cleavage plane expand rapidly leading to specimen fracture, and therefore its elongation is lower after fracture. In addition, the change of AZ61and of AZ61+Mn+RE in compressive strength is also great. The compressive strength of the AZ61magnesium alloy significantly improved after combined addition of Mn and RE, showing the trend of increased and then decreased. Component2and component3have a higher yield ratio. As to bending performance, changes in the flexural strength are obvious and there is a downward trend with the changes of Mn/RE ratio. When the Mn/RE ratio was1.87, alloy has the highest bending strength.In terms of corrosion resistance, for AZ61contains less second phase, so it possess the higher open circuit potential,a lower corrosion tendency. The current density of the polarization curve obtained after fitting. Self-corrosion current density of AZ61+Mn+RE compared with AZ61magnesium alloy was significantly decreased. This shows that AZ61+Mn+RE have good corrosion resistance in the initial stage of corrosion. For the impedance, the larger the surface film resistor R2value is, the stronger the ability of surface film protects magnesium matrix. The greater the charge transfer resistor R3value is, the easier the resistance of electrochemical corrosion is blocked. According to the data obtained from the fitting, the corrosion resistance from component1to component6is substantially gradually increased. Found through different time immersion test in salt water immersion test, AZ61accelerated corrosion rate with the immersion time and corrosion rate of other magnesium alloy is rapid in the initial stage of corrosion, while the latter a protective layer formed to reduce and stabilize the corrosion rate. Salt spray corrosion experiment is broadly in line with salt water immersion test results. Composite addition of Mn and RE improved the corrosion resistance of AZ61,especially the ratio reaches to0.67. However, when more than this value is added into magnesium alloy, the corrosion resistance continues to improve, but slightly.Fe is the main impurity elements in magnesium alloy. Composite addition of Mn and mixed RE in AZ61magnesium alloy makes Fe content reduced from0.0042wt%, when the manganese is added separately, to0.0015wt%, wherein the lowest Fe content is0.001wt%. From the perspective of purifying the magnesium alloy matrix, the composite addition further reduced the impurity content Fe in the magnesium alloy, so that AZ61+Mn+RE corrosion resistance improved further. As to salt spray test, serious corrosion at AZ61fusion seam could be seen after72hours. But with the change of composition, the degree of corrosion at the fusion zone minimized gradually. |
PDF Full Text Request