Research on Microstructure and Properties of Thin-walled Tube of AZ31Magnesium Alloy by Plastic Processing Technology
|School||Harbin Institute of Technology|
|Course||Materials Processing Engineering|
|Keywords||AZ31magnesium alloy thin-walled tubes extrusion-drawing compositetechnology body fluid corrosion|
Magnesium alloy thin-walled tube is an ideal cardiovascular stents biomedicalmaterial owing to its good biomechanical compatibility and mechanical properties aswell as non-toxic and biodegradable characteristics. Currently, forming magnesiumalloy thin-walled tube with large slenderness ratio, high strength and high toughness isvery difficult, and it determines whether or not the magnesium alloy cardiovascularstents could be manufactured successfully. The project studied a new extrusionmodeling technology to produce magnesium alloy thin-walled tube with largeslenderness ratio and high strength and high toughness at low temperature, with the helpof a force-transmission lubrication medium. At the same time, it also reveals themechanism of low-temperature extrusion plastic deformation, develops magnesiumalloy capillaries in highly precision sizes and has large slenderness ratio and highstrength and high toughness, and achieved the integration of the magnesium alloyorganization performance and precision forming control.In this paper, The experiments use the as-cast AZ31magnesium alloy bar stock asraw materials and adopt the hollow spindle forward extrusion approach. Thetechnological process is as follows: homogenization heat treatment as-cast bar stock—pre-compressing the bars (analyzing mold temperature’s influence)—tube extrusion(analyzing the effect of extrusion ratio and temperature of billet)—the heat treatmentprocess (analyzing the effect of heat treatment temperature and time)—drawing(analyzing the drawing amount)—simulating body fluid corrosion and analyzingtubes organization and performance by using electronic universal testing machine,metallographic microscope, SEM scanning electron microscope and XRD.Studies show that pre-compressing the bars under different mold temperature, ifthe experiment is set billet temperature200℃, grain compared to as-cast grains arerefined significantly when the mold temperature is200℃.In the process of extruding the bars which are formed by pre-compression, whenthe mold temperature is200℃, the extrusion speed is22mm/s, using the oil-basegraphite as a lubricant, for Φ16mm—Φ3.6mm tubes, when the extrusion ratio is18.68and billet temperature is200℃, the maximum tensile strength is303.540MPa and themaximum bending strength is1087.153MPa, the elongation is6.2%. After the tubeextrusion process, it can refine the grains again, and the average grain size can reach3.910μm.For extruded Φ3.6mm tubes, heated to225℃and hold that temperature for45minutes, the elongation is14.52%, which is the best tube elongation, but the tensile strength and bending strength falls slightly, and hence providing a good drawing billet.In the drawing process, when the accumulated deformation reachs17.95%, that isto say, drawing from Φ3.6mm to Φ3.0mm, it gets the largest tensile strength andflexural strength of AZ31magnesium alloy tube, and the grain size is the smallest,reaching4.384μm. The maximum tensile strength is374.45MPa and the maximumbending strength is1114.421MPa. Elongation is increased as the amount of drawing hasbeen reduced.When simulating body fluid corrosion, the grain size is related to corrosion rate,which means the smaller the grain size is, the smaller the corrosion rate will be. Whenthe grain size is between1~10μm, the corrosion rate is1~2.1mg/(cm2·day). At Φ3.4mmand Φ3.2mm, the priority was given to grain interior corrosion, while at Φ3.0mm andΦ2.8mm, gave priority to grain boundary corrosion. The product of body fluid corrosionis Mg(OH)2.