Research On Integral Forming Of High-speed Filling And High-pressure Densifying Of AM50A Magnesium Alloy

In recent years, light weight of vehicle is paid more and more attention due torequirements for saving energy, saving emission and increasing efficiency. More andmore magnesium alloys have been widely used in vehicle industry. It is difficult todeform magnesium alloys plastically due to the hexagonal close-packed crystal structureand limited slip system. At present, most magnesium alloys are produced by die casting.Although die casting can produce complex components, mechanical properties of theformed components cannot meet application requirements.On the other hand, forging can form components with enhanced mechanical properties,but the shape for the components cannot be too complex. In order to realize theintegration forming of the complex components with enhanced mechanical properties, anovel integral forming called high-speed filling and high-pressure density is proposed,in which combines the advantages of the die casting and forging.Firstly, three-dimensional model of typical complex motorcycle wheel componentwith enhanced mechanical properties was carried out. At the same time, gating systemand a three-factor and four-level orthogonal experiment were designed in this research.Numerical simulation of high-speed filling process was done by using FLOW-3Dsoftware and optimal process parameters including pouring temperature of705℃,injection speed of4m/s and die temperature of210℃were achieved.Die casting and the technology of high-speed filling and high-pressure densifyingwere employed to form the motorcycle wheel components under the same conditions inorder to compare them. The results show that a large number of surface defects such asincomplete filling, pores, cracks and cold shuts were found in the components formedby die casting. However, high surface quality was obtained in the components formedby high-speed filling and high-pressure densifying. The results of spoke tensile tests atroom temperature indicate that the mechanical properties of spokes have beensignificantly improved due to the application of forging force. The average tensilestrength increases from151Mpa to207Mpa, and the average elongation rises from6.3%to9.9%. The mechanical properties at high temperature are also increased. SEMobservation of fractograph shows that fracture mechanisms have been changed frombrittle fracture to ductile fracture. Furthermore, Vickers hardness of components formedby high-speed filling and high-pressure densifying was significantly improvedcompared with die castings.The microstructure of components formed die casting is not very uniform and theaverage grain size of microstructure was60um. Some oxide inclusions, cracks, holes,dendrites with an average grain size of160μm were found in the microstructure of die castings. On the contrary, fine, uniform and dense microstructure was obtained in thecomponents formed by high-speed filling and high-pressure densifying and the averagegrain size was20um. TEM results shows that some twins were created during theforming process. Furthermore, the space and thickness between two twins decreasebecause of the existence of forging force. From the EBSD results, we find that thehigh-speed filling and high-pressure densifying make the grain size and misorientationangle small, and texture reduced.Above all，an important conclusion can be drawn that the mechanical propertiesand microstructure of components formed by high-speed filling and high-pressuredensifying were significantly improved compared to die castings. It illustrates thathigh-speed filling and high-pressure densifying is suitable for forming complexcomponents with enhanced mechanical properties.