Research On Extrusion And Drawing Process Of Pure Magnesium Thin-walled Tube And Biological Corrosion

Magnesium and its alloys have broad prospects in electronic information products, aerospace, automotive weight loss and other applications because of its unique physical properties, such as low density, high specific strength and specific stiffness. In addition, magnesium can react chemically with body fluid, and the reaction products have no toxic and side effects on the human body. As a new generation of implanted materials, magnesium has been caught more attention due to its excellent biocompatibility and biodegradability. However, the application of magnesium has been limited because of its poor performance in plastic deformation which is determined by its characteristics of lattice.Both the process of extrusion-drawing pure magnesium thin-walled tubes and the corrosion behavior of pure magnesium thin-walled tubes in the simulated body fluid(SBF) were mainly studied in this paper. The bars were manufactured by accumulative high ratio extrusion process, with fine grain and homogeneous microstructrue. We can get secondary extrusion bars with good quality using follow extrusion process:the first extrusion temperature below 200℃, extrusion ratio 10.5 and the second extrusion temperature 100℃, extrusion ratio 10.2. In this extrusion process, The average grain size of the secondary extrusion bars can be decreased to below 9μm, and the minimum grain size is 4.2μm. The yield strengths of the bars were between 150~160MPa, the tensile strengths were between 250~270MPa, the elongation rates were between 13~18.5%.Turning the extruded bars to tube billets, and process thin-walled tubes by long core stem drawing and long core stem drawing after empty pull process. It has been discovered that the minimum thickness of the tubes is 0.40mm by the later process with the minimum average grain size of 6.3μm, the maximum yield strength of 195.9 MPa, the maximum tensile strength of 248.5MPa, and the maximum extension rate is still more than 6.0%.The corrosion mechanism of pure magnesium thin-walled tubes were studied by SEM and EDS analysis. The factors relating the corrosion of tubes such as the grain size, cold drawing deformation degree and heat treatment process were studied either. It was found that the elements deposited on the pure magnesium thin-walled tubes in different periods of corrosion were different. The matrix of thin-walled tubes were not protected by Ca/P layers which were deposited in early corrosion time. The corrosion of the tubes was from pitting corrosion and then extension from the point. Moreover, the smaller grain size, the larger cold drawing deformation degree and the lower heat treatment temperature, the better the corrosion resistance property.The corrosion resistance property of low-temperature relief annealed tubes is better than that of high-temperature complete recrystalization annealed tubes.