Study On Microstructure And Mechanical Properties Of Magnesium Alloy Via Lost Foam Casting

The lost foam casting (LFC) is well suitable for the magnesium alloy. LFC can produce castings with higher dimensional tolerances and provide cleaner working conditions and is possessed of some especial advantages as followings: (1) The gaseous and foggy pyrolysis products of EPS foam mainly consist of alky, benzene and styrene et al, which can protect the molten magnesium alloy from oxidizing maturely during filling. (2) The dry sand and vacuum degree avoid the occurrence of defects which is produced by the contact between magnesium liquid and water in molding sand. (3) The better deformability of dry sand obviously alleviates the hot tear tendency of casting under LFC. (4) The slow and steadily filling of molten metal also avoids the entrapment of gas into casting, which makes the casting possible for heat treatment to increase its mechanical properties. (5) The investment expanse is low compared to die casting. As a result, LFC of magnesium will rapidly gain popularity in the foundry industry. Nowadays the magnesium trademarks are designed for sand or die casting, which may be unsuitable for LFC. This thesis will begin with the microstructure and mechanical properties of AZ91 alloy via LFC that is most widely used presently. The existing problems of mechanical and processing properties will be found and physical refining, modifying composition and heat treatment will be utilizing in order to obtain a novel AZ91 magnesium alloy, which suits to LFC. The investigation is very important for practical application of magnesium alloy and academic sense. The flowability and solidification characteristic of AZ91 alloy via LFC through electrical probes connected to data acquisition controlled by computer were studied. The results showed that the molten metal replaced the pattern steadily in radial shape and the filling velocity varied alternately. The pouring temperature and vacuum degree had furthest impact on the filling velocity in all processing factors. The molten metal front cooled quickly during filling, which resulted in temperature drop heavily. It is necessary to alter solidification characteristic of AZ91 alloy because the mixture of liquid-solid state metal and wider freezing range (128℃) make the casting produce the casting defects. The microstructure and mechanical properties of AZ91 alloy via LFC were studied. The results showed that the microstructure of the AZ91 alloy via LFC consisted of dominant α-Mg and β-Mg17Al12 as well as a new phase Al32Mn25 and the eutectic structure took a completely divorced form. The smaller distribution coefficient of solutes (Al and Zn) between the matrix and the precipitates was the cause of generation of divorced eutectic structure. The mechanical properties tests demonstrated that the as-cast values were higher than that of sand gravity casting because of chilling and cushioning effect of foamed pattern during the mould filling. However, the mechanical properties were inferior to that of die casting or permanent mould casting. It is necessary to improve the mechanical properties of AZ91 alloy under LFC. The microstructures and properties of AZ91 magnesium alloy were investigated after various heat treatment conditions. The results showed that the solutes diffused very slowly during heat treatment. For aging at low temperature, the discontinuous precipitates emerged at grain boundaries and had a massive form. However, for aging high temperature, the precipitates appeared in a continuous pattern at grain boundaries and in the grains. The mechanical properties of the alloys were significantly improved after heat treating. Aging at high temperature was more effective for improving tensile strength and yield strength and the work-hardening rate of plastic deformation. The aging kinetic was also studied by measuring the hardness and electrical resistivity, which was well explained, based on the variation of the quantity, size and shape of precipitates. For the first time, the effects of mechanical vibration and special molten liquid heat treatment on the microstructure morphology and mechanical properties were investigated via LFC. The results showed that mechanical vibration could produce finer dendrite and improve the mechanical properties. The value of tensile strength of the AZ91 alloy wasultimate (increase by 27% compared to the vibration force of 0KN) when the casting was produced under the vibration force of 1.5KN. However, the strength of the AZ91 alloy dropped due to the presence of the microporosity in the casting when the vibration force was increased. Mechanical vibration produced finer dendrite by breaking dendrite into pieces resulted by shear stress, melting the secondary dendrite arm and increasing the amount of crystal nuclei and undercooling degree. The special molten liquid mixing treatment also could produce finer dendrite and improve the mechanical properties (tensile strength increase by 24% compared to unfining treatment), which was superior to the traditional high-temperature refining. The special molten liquid mixing treatment resulted in refining effect by providing more nucleate sites in the mixed melt. For the first time, the mechanical properties and range of solidification of AZ91 alloy by utilizing Cd and mischmetal (MM) as modified elements were investigated under LFC process. The results showed that the compound addition of Cd and MM could harden, strengthen, toughen the AZ91 alloy and optimal composition was AZ91 with the addition of 0.5%Cd and 0.6%MM. The values of yield strength, tensile strength, elongation, hardness, impact tough were improved by 21.2%, 13.1%, 30.5%, 27.5%, 61.5% compare'd to AZ91 alloy (poured at 710℃) and were 123MPa, 177MPa, 3.08%, 69.5 HB, 10.5J/mm2, respectively. Differential thermal analysis showed that the range of solidification of AZ91+0.5Cd+0.6MM alloy was only 48.5℃with liquidus temperature 655.05℃and solidus temperature 606.55℃, which was much lower the range of solidification of AZ91 alloy with 128℃. The smaller range of solidification can improve the processing properties. The higher liquidus temperature can alleviate oxidation and combustion of magnesium alloy during melting. The flowability and feeding ability of the magnesium alloy can be improved by increasing pouring temperature. The microstructure of the AZ91+0.5Cd+0.6MM alloy via LFC consisted of dominant α-Mg, β-Mg17Al12, Al3.1Ce(La) and Al8.8Mn5Ce(La) phase. The main morphology of Al3.1Ce(La) precipitate was lath and massive. The Al8.8Mn5Ce(La) precipitate emerged by thecoupled growth of Al3.1Ce(La) and Al8Mn5 phase. The Al3.1Ce(La) and Mg17Al12 precipitates prevented the movement of the dislocations and toughened the AZ91+0.5Cd+0.6MM alloy, which fractured in transcrystalline and cleavage pattern with small cleavage plane. The effect of solid solution and aging treatment on the microstructure and mechanical properties AZ91+0.5Cd+0.6MM alloy were investigated. The results showed that the Al3.1Ce(La) and Al8.8Mn5Ce(La) precipitates could exist steadily at the temperature of solid-solution while the unstable Mg17Al12 precipitates dissolved into matrix, which precipitated with the morphology similar to that of AZ91 alloy. The mechanical properties of AZ91+0.5Cd+0.6MM alloy were significantly improved after heat treating. The values of tensile strength, elongation and impact toughness were 255MPa, 8.80% and 26J/mm2, respectively. The transition phase Mg5Al2 precipitated out during aging treatment of AZ91+0.5Cd+0.6MM alloy at lower temperature because forming resistance of nucleus was too greatly.