Effect Of Extrusion Process On Microstructure And Mechanical Properties Of Typical Aluminum Alloys And Magnesium Alloys

Lightweight of equipment is the requirement of today's society, in which people pay more attention to environment. Thus, the development of new alloy with lightweight and high strength is important for the weight reduction of equipment. In the present thesis, the magnesium alloys, AZ31 B and AZ91 D, and the aluminum alloys, 7A04 and 2024, were taken as the research objects, the shaping properties of these four typical light alloys under the different extrusion conditions were investigated by using 638 T horizontal extrusion machines. The microstructure and mechanical properties of the four alloys extruded under different conditions were characterized, and the effects of the extrusion temperature and extrusion rate on the microstructure evolution of the alloys were investigated. The main conclusions are as follows:(1) The microstructure and mechanical properties of the magnesium and aluminum alloys before and after extrusion were comparatively studied, respectively.It is found that, for all the alloys in the present work, the extrusion result in the reduction of the shrinkage porosity, and the mechanical properties is greatly improved,especially for AZ91 D, The tensile strength of AZ91 D is increased by about 80% and its elongation is increased by about 150%. The dynamic recrystallization occurred in the AZ31 B and AZ91 D alloys, which leads to the significant refinement of the grains and the formation of an uniform equiaxed grain microstructure during hot extrusion.In contrast, for the 2024 and 7A04 alloys, the dynamic recrystallization did not occur during the extrusion process, which is evidenced by the fact that most of the grains in the two alloys are elongated along the extrusion direction.(2) The influences of the extrusion temperature on microstructure and mechanical properties of the magnesium and aluminum alloys were studied. The results show that,with the extrusion temperature increasing, the grain size of the extruded AZ31 B and AZ91 D alloys tends to to increase. Moreover, the coarse second phases in the AZ31 B,AZ91D and 2024 alloys are crushed, which arrange along the extrusion direction to form a strip of the particles. While, for the 7A04 alloys containing small particles, the extrusion does not change the size and distribution of the second phases. Instead the number of the coarse second phases in the alloy is increased after extrusion. The response of the microstructure of AZ91 D to extrusion is obvious, and the microstructure of AZ31 B and 2024 alloys are more sensitive to the extrusion temperature, in comparison with the 7A04 alloy.(3) The influences of the extrusion speed on microstructure and mechanical properties of magnesium and aluminum alloys were studied. The results show that, as the extrusion speed increases from 0.5mm/s to 1mm/s, the surface cracks in the extruded AZ91 D alloy can be observed. Further more, with the increase of the extrusion speed, the mechanical properties of the AZ31 B alloy increase firstly and then decreases. On the other hand, it is found that the size of the second phases in the2024 and 7A04 alloys the small particles in island shape and the point-like shape exhibit a decreasing trend with the extrusion speed increasing. The microstructure of AZ91 D is extremely sensitive to the extrusion speed in comparison with the AZ31 B alloy, while the 7A04 alloy exhibits little sensitive to the extrusion speed.