Study On Microstructure And Properties Of Mg/ZrO₂ Composites Prepared By Friction Stir Processing

Magnesium（Mg）and its alloys have attached more and more attention because of their potential as a new type of biodegradable metal materials and orthopedic matrix materials.In complex physiological environments,due to the high corrosion rate of and mechanical integrity declines of Mg alloy implants,which makes it difficult to prepare the implants.Therefore,this work proposes a new composite material preparation technology to prepare Mg/ZrO₂ composite material,namely the friction stir processing（FSP）technology.In this study,AZ31 Mg alloy was used as the object,and the AZ31/ZrO₂composite materials with good uniformity were successfully prepared through multi-passes FSP,respectively.The scanning electron microscope（SEM）was to characterize the microstructure of composites.The microhardness,tensile properties at room temperature,friction and wear properties,electrochemical corrosion properties and biological properties were evaluated.The evolution of the microstructure and properties of the composite materials during various processes was explored.In addition,the effect of reinforced particles（ZrO₂）on the microstructure and properties of the composite was studied by compared with FSP AZ31 Mg alloy.The main conclusions of this work are as follows:The results show that,compared with the base metal（BM）,the AZ31/ZrO₂ composite material achieves homogenization,densification,and grain refinement after 5 and 6 passes FSP,respectively.During FSP,not only dynamic recrystallization leads to grain refinement of the Mg alloy,but the reinforcing particles（ZrO₂）also significantly refines the grains,so the average grain size is 1.6μm（5-passes）and 3.1μm（6-passes）in the processing zone.After FSP,a strong basal texture is generated,the c-axis of grain is deflected under the compression stress of shoulder and the shear stress of pin,and the maximum polar density is 76.03.The ultimate tensile strength（UTS）of BM was 283 MPa,and the elongation was 15.5%.After adding reinforced particles（ZrO₂）,the UTS of FSP AZ31/ZrO₂ composite materials for 5 and 6 passes were 397 MPa and 427 MPa,respectively,and the elongations were 9.5%and 13.8%,respectively.The area with ZrO₂particles is prone to secondary cracks during the deformation process,which increases brittleness,reduces the deformation ability and the plasticity.The reinforced particles（ZrO₂）are the main reasons for the improvement of the microhardness and tensile properties of AZ31 Mg alloy.The existence of ZrO₂ particles and the increase of passes have an effect on the wear resistance.The wear rate of 6-pass FSP AZ31/ZrO₂ composites is the lowest.Both FSP and the addition of reinforced particles reduced the average friction coefficient of AZ31 Mg alloy.The wear depth of FSP AZ31/ZrO₂ composites in air and SBF solution was less than that of FSP AZ31 Mg alloy.The wear volume decreased obviously,which was mainly attributed to the increased hardness of the composites due to the addition of ZrO₂ particles.The corrosion resistance of FSP AZ31/ZrO₂ composites varies with the change of processing passes.The I_corrof 5-passes and 6-passes FSP AZ31/ZrO₂ composites were 0.139 m A/cm² and 0.217 m A/cm²,respectively.The grain refinement and the change of grain orientation caused by FSP improve the corrosion resistance of Mg alloy.The corrosion current density(I_corr)of 5-passes and 6-passes FSP AZ31 Mg alloys were 0.111 m A/cm² and 0.090 m A/cm²,respectively.The static corrosion properties of FSP AZ31/ZrO₂ composites were lower than those of FSP AZ31 Mg alloys due to the addition of reinforced phase particles（ZrO₂）.Compared with BM,the cells cultured with FSP AZ31 magnesium alloy extract have higher proliferation activity,indicating that FSP treatment could improve the biocompatibility.Compared with FSP AZ31 magnesium alloy,the proliferation activity of FSP AZ31/ZrO₂ composites sample extract is further improved,indicating that the addition of ZrO₂ reinforced particles can further improve the biocompatibility of magnesium alloy.