Microstructures And Performances Of Mg Products Made By Various Forming Processes And Related Issues During Processing

Magnesium alloys have potential as structural materials for aerospace, 3C and other transport applications due to their high specific properties, low density and high damping capacity. Currently, magnesium alloy products are mainly made by die-casting process. Though the theories and facilities of die-casting process are comparatively perfect, there are always many problems in the process of manufacture, such as the die failure. In addition, due to special processing and performance of the products, the magnesium products manufactured by plastic formation and thixomolding are more and more popular in the market. In this work, combined with practice, a series of problems in magnesium alloy were studied by many testing means including OM, SEM, XRD, TEM, and so on. Microstructures and performances of extruded AZ31 Mg alloy were studyed; The failure problem of embedded lump in the die was tried to analyse; The difference of performance of AZ91D Mg alloy produced by high pressure die-casting and Thixomolding were discussed. The main conclusions are as follows:1. It can be seen that the side plane and cross-section along extrusion direction have many compound twinings from the optical micrograph. The tensile deformation of specimens indicated that they were of dynamic recrystallization. SEM results revealed that the tensile fractures belong to dimple fracture. The microstructures from TEM observation showed the twining and precipitate interior grains. Polarization tests indicated that the corrosion potential of surface was higher than those of cross-section and side plane; however the corrosion current was lower. The corrosion resistant of surface was appreciably better than that of cross-section and side plane.2. Solution treatment of the thixomolding AZ91D alloy at 415℃caused theβ-phase to dissolve intoα-Mg solid solution at a faster dissolution rate than that in the same alloy prepared by high pressure die-casting. During the solution treatment of the Thixomolding AZ91D alloy, the microstructural evolution was accompanied by redistribution of the alloying elements. Both discontinuous and continuous precipitates of the p-phase were observed during the aging treatment.3. The experiment results indicated that the corrosion rate of the thixomolding AZ91D alloy was lower than high pressure die-casting AZ91D alloy by analysing the relationship between weight loss and corrosion time. The main reason was that the surface area of anode of thixomolding AZ91D alloy was smaller than that of high pressure die-casting AZ91D alloy. In addition, the porosity of thixomolding AZ91D alloy was lower than high pressure die-casting AZ91D alloy. It was another reason why thixomolding AZ91D alloy had lower corrosion rate.4. The failure of embedded lump in the die was caused by the continuous impact of the high-temperature and high-velocity molten metal liquid on the die. To avoid the problem, it was very important not to make the metal liquid plumb impact on the embedded lump. It was also efficient to advance mechanical property of the embedded lump by improving heat treatment technology.