Effect Of Ultrasonic Micro-extrusion Forming On ZK60 Mg Alloy Rheological Behavior At Room Temperature

As was known to all, magnesium alloy had superior performance, and its density was minimum in metal structural materials at present. Compared to other biomedical materials,magnesium alloy possessed of better mechanical compatibility and biocompatibility. As a result, magnesium alloy was an ideal material for cardiovascular stents. In view of HCP lattice structure, magnesium alloy had less independent slip systems at room temperature. Relatively speaking, the plastic deformation got more difficult. There was inestimable significance for promoting commercial application in biomedical materials field that exploring new micro-forming process on magnesium alloy.Duo to the conventional micro-forming process faced with many technical problems, a new process was proposed that the ultrasonic vibration was applied to micro bulk forming in this paper. The theory is not mature about ultrasonic vibration assisted metal plastic deformation at present. In addition, ultrasonic vibration was rarely applied to micro-forming field in current research. To investigate the feasibility of the process and the mechanism of ultrasonic vibration assisted metal plastic deformation, a series of ultrasonic micro-extrusion experiments were preformed for different grain sizes ZK60 magnesium alloy at room temperature. Meanwhile, the corresponding tests and analysis were conducted for the microstructure and the mechanical properties of micro-forming parts. The experimental results are as follows:Compared to conventional micro-extrusion deformation, ZK60 magnesium alloy plastic deformability has been greatly improved at room temperature after applying ultrasonic vibration. Formability increased as the amplitude of ultrasonic waves increased. It was found that flow uniformity became worse, surface grains became more and surface layer became thicker than macro extruded-forming by observing the cross-sectional microstructure of micro-forming parts. The plastic deformation of metal surface layer was larger and grain size was smaller than the central region in the material. The grain size of micro-forming parts was refined after applying ultrasonic vibration, and the degree of grain refinement got larger than conventional micro-extrusion deformation. Grain refinement became more obvious with the decrease of extrusion diameter, but the thermal effect produced by ultrasonic vibration made grain size to increase slightly with the increase of ultrasonic vibration. Moreover, though both the grain size of deformation zone and no deformation zone had been refined by observing the longitudinal-sectional microstructure of micro-forming parts, the degree of grain refinement was larger in deformation zone. It was considered that grain refinement mechanism was mainly derived from ultrasonic vibration instead of extrusion ratio by analyzing the microstructure of different zone. It was seen from the microstructure of deformation zone that the maximum plastic deformation occured in the dead zone, and a minimum of grain size also appeared in the zone.It was observed that ultrasonic vibration could effectively reduce the extrusion stress, and significantly improved friction condition during micro-extrusion. Furthermore, extrusion stress and the friction factor between the die and the sample significantly decreased with increasing ultrasonic vibration. The relationship between extrusion stress and grain size was fully consistent with Hall-Petch equation in conventional micro-extrusion and ultrasonic micro-extrusion. More importantly, size effects on extrusion stress have been strengthened by ultrasonic vibration.Finally, it was found in mechanical testing that the strength and elongation of micro-forming parts improved significantly when applying ultrasonic vibration to micro-forming, and the compression fracture of micro-forming parts also proved its strength to be higher. The strength presented a declining trend with increasing ultrasonic vibration, yet. As for the change of elongation, it had no regularity. The elongation was averagely increased by 15%-25% compared to conventional micro-extrusion parts.In a word, there was great significance to improve the formability, mechanical properties of micro-forming parts that ultrasonic vibration was applied to micro bulk forming on ZK60 magnesium alloy.