Effect Of Friction Stir Processing On The Microstructure And Properties Of Mg-Zn-Y-Nd Alloy For Biomedical Materials

When applied Mg and its alloys on biomedical materials as a degradable metal, there always exist two problems due to their high in-vivo corrosion rate and bad mechanical property. While Friction Stir Processing (FSP) can improve the mechanical and corrosion properties of metallic alloys for its significant refinement of grain and second-phases, FSP has became one of the most effective methods to broaden the application of Mg alloys on biomedical materials. In this paper, we studied the effect of different FSP conditions on the microstructure, mechanical and corrosion properties of Mg-Zn-Y-Nd alloy by OM, SEM&EDS, XRD, tensile tests, electrochemical experiments, pH changes and mass loss analysis during immersion tests, and the influence mechanism was also studied at the same time.The results indicate that:because of the dynamic recrystallization occurred during FSP under the interaction of severe plastic deformation and thermal cycle, microstructure in the processing region exhibited equiaxed fine grains, due to the friction and stirring caused by the stirring head with a high rotating rate. With the increasing of rotation rate, grains grew up gradually, and the second phase particles became extremely small and mostly distributed along the grain boundaries or in the grain interiors. Under a FSP condition of150mm/min-400rpm, the grains were refined to a size of～1μm, and the nano-scaled second phase particles distributed homogeneously in the grain interiors.The mechanical tests show that:microhardness of Mg-Zn-Y-Nd alloy improved significantly after FSP, with the larger microhardness value in the processing region and decreased along the direction to base alloy which had a microhardness value of41HV while the stir zone had a microhardness value of89HV, and the tensile properties were improved greatly at the same time after FSP. With the increasing of rotation rate under the same forward speed, ultimate tensile strength (UTS) increased to the highest UTS of238MPa under the condition of300mm/min-1100rpm, while the elongation decreased at the same time. When the condition was 300mm/min-700rpm, the elongation was the largest of33%.Immersion tests indicate that:the pH values of as-cast alloy and FSP alloy increased in the early stage after immersed into simulated body flood (SBF). But there existed a steady region of FSP alloy after24h immersion, while the pH value of as-cast alloy still increased. We can conclude that when the samples after FSP were immersed into SBF for a long time a compact production layer produced which could prevent substrate from further contacting with SBF, so the corrosion resistance was improved. These were consistent with the results of corrosion behavior observation that many corrosion pits appeared in the as-cast alloy, while the corrosion surface of FSP alloy was smooth with a homogeneous corrosion behavior.Based on the results before, we can conclude the corrosion mechanism of Mg-Zn-Y-Nd alloy that:the corrosion pits were due to the large second phase mostly distributed along the grain boundaries which may accelerate the micro-galvanic corrosion in this region preferentially. While the nano-scaled second phase particles of Mg-Zn-Y-Nd alloy after FSP were mostly distributed homogeneously in the grain interiors, and the compact corrosion production layer produced during the immersion prevented the substrate from further corrosion effectively.