Effects of Sn and Pb additions on microstructures and mechanical properties of As-cast, rolled and annealed AZ61 magnesium alloys

Abstract

Magnesium (Mg) alloys are promising structural metallic materials owing to their low density and high specific strength. Mg alloys have been used in many industries including aerospace and vehicle parts, and portable electronic devices, where weight saving is of crucial importance. Researchers around the world are engaged in improve the mechanical properties of Mg alloys with various approaches, such as alloying, heat treatment, and deformation process. Among Mg alloys, AZ series (Mg-Al-Zn) Mg alloys possess suitable corrosion resistance, castability, and relatively satisfied mechanical properties. Compared to AZ31 and AZ91, AZ61 can be used in the forms of both as-cast state and the deformed state. In addition, AZ61 alloy possesses moderate strength and ductility. Further improvement of the mechanical properties by introducing the rare earth elements increases the cost of the production. In this study, in order to improve the mechanical properties of AZ61 Mg alloy, Sn (tin) and Pb (lead) were selected as alloying elements to modify the microstructures and mechanical properties of as-cast and rolled AZ61 Mg alloys. Certain amounts of Sn or Pb element were added as the alloying materials for the casting of AZ61 alloys. With the introducing of Sn or Pb elements into the AZ61 alloy system, the modification in phase and morphology of the intermetallic phases were examined. The mechanical properties, including wear resistance, hardness, and tensile properties, were tested and analysed. The experiments and results are summarized as follows: (1) Different amounts of Sn alloying addition were managed to add into the AZ61 Mg alloy by using the high induction furnace. The XRD (X-Ray Diffraction) result and the optical microstructures indicate that primary α-Mg phase, eutectic α-Mg phase, divorced eutectic β-Mg17Al12 phase, and lamellar β-Mg17Al12 particles were formed in as-cast AZ61-Sn alloys. Moreover, Mg2Sn phase was found to coexist with Mg17Al12 phase in AZ61-3Sn and AZ61-5Sn alloys. SEM (Scanning Electron Microscope) examination indicated that Sn increased the elemental segregation, refined the dendrites and decreased the average grain sizes. The enhancement of wear resistance and hardness was observed in Sn-containingAZ61alloys. The yield strength of the alloy was improved with the increasing addition of Sn owing to the formation of Mg2Sn precipitates and refined average grain sizes. However, an exceeding amount of Sn addition leads to the decrease of the UTS (Ultimate Tensile Strength) and elongation because of the continuous distribution of the intermetallic compounds. Furthermore, the Sn addition has changed the fracture mode of as-cast AZ61 alloy from transgranular fracture to intergranular fracture. (2) As-cast AZ61-Pb alloys were prepared by the high vacuum induction furnace to study the effect of Pb addition on the microstructure and mechanical properties. The XRD result suggested that Pb alloying did not change the kinds of phases in as-cast AZ61 alloy, which is attributed to the large solubility of Pb in Mg. The elemental distribution of Pb in as-cast AZ61 alloy was studied through EDS (Energy Dispersive Spectrometer). Results showed that Pb inclined to exist in the interdendrites rather than in the dendrites, and preferred to segregate in the α-Mg phase rather than to exist in eutectic β-Mg17Al12 phase. The wear resistance and the hardness were improved slightly due to the solid solution strengthening effect. Pb also leads to the increase of yield strength. However, the tensile properties decreased with excess Pb additions due to the variation of morphology. The fracture mode of the as-cast Pb added AZ61 alloys remained as the transgranular feature. (3) Rolling deformation was applied on the homogenized AZ61-Sn alloy. The inter-pass annealing temperature is 340ºC and 400ºC respectively. The alloy samples were rolled with the total thickness reduction of 50% and 80%, followed by cooling in air. The microstructures of the rolled alloy were replaced with a large number of deformation twins. With increasing addition of Sn, the fraction of the deformation twins tended to rise after the rolling process. Another significant finding in the rolled AZ61-Sn alloys is that the fine precipitates of Mg2Sn phase were occurred. Subsequent isothermal annealing treatment was performed to release the residual work stress. The annealed alloys showed equiaxed grains owing to the static recovery and recrystallization. The grain growth was suppressed in the Sn-containing alloys with Mg2Sn precipitates. The mechanical properties of the rolled and annealed alloy were tested. The wear resistance, hardness, yield strength and UTS were increased with Sn additions due to the precipitation hardening, work hardening, and grain refinement effects. However, the elongation decreased with Sn addition in the rolled and annealed AZ61 alloys. (4) AZ61-Pb alloys were homogenized and rolled to a final reduction rate of 80% with the inter-pass annealing temperature of 400ºC. The results showed that the Pb alloying element increased the fractions of twinning structure in the rolled AZ61 alloys. The isothermal annealing treatment was performed after rolling. The microstructures of the annealed alloys revealed equiaxed grains rather than the deformation twins owing to the static recovery and recrystallization. The average grain sizes of the annealed AZ61 alloys were decreased with 0.5 wt.% Pb addition. However, excessive additions of Pb result in the coarsening of grains. The wear resistance of Pb added AZ61 of rolled and annealed state were decreased. However, the hardness, yield strength and the UTS were generally enhanced by the Pb addition. The elongation of the rolled state was improved by Pb addition, while that of the annealed state was decreased. In summary, by introducing the alloying additions of Sn or Pb to AZ61 Mg alloy system, the microstructures and mechanical properties of the as-cast, rolled and annealed alloys have been evaluated systematically. Sn alloying formed Mg2Sn phase in the as-cast AZ61 alloy, while Pb alloying did not change the kinds of phases in the system. The yield strength was improved with Sn addition due to the hard Mg2Sn particles. However, the UTS and elongation were slightly decreased owing to the coexistence of Mg2Sn and Mg17Al12 particles. Pb addition generally increased the strength and decreased the elongation in AZ61 alloys. The rolling deformation and subsequent annealing treatment endowed even better mechanical properties to AZ61 alloy by Sn and Pb additions, such as very high yield strength and UTS. Notably, the significant enhancement in strength of the Sn added alloys was achieved due to the existence of the precipitation after deformation. The possible strengthening mechanisms include the solid solution strengthening, precipitation strengthening, work hardening, and grain refinement strengthening. Meanwhile, the Pb addition generally improved the yield strength and UTS due to the solid solution strengthening. Furthermore, the ductility of rolled state AZ61 alloys was enhanced by Pb alloying.