Research On Microstructure And Properties Of ZK60 Magnesium Alloy Prepared By Friction Stir Processing

Magnesium alloys have many advantages such as low density,high specific strength and stiffness,good castability,good mechanical properties and excellent biocompatibility,and have been widely used in aerospace,transportation,electronics and biomedical fields.However,most of magnesium alloys show poor ductility and formability at room temperature due to their intrinsic hexagonal close-packed structure,which limits the industrial applications to some extent.Friction stir processing(FSP)is a novel sever plastic deformation technique,which has great potential in grain refinement and modification of microstructure.Grain refinement has been demonstrated as an effective method to improve the mechanical properties of magnesium alloys.In this study,cast ZK60 magnesium alloy was selected as base metal and subjected to FSP.Microstructure evolution during FSP,effect of processing parameters on grain size and mechanical properties,corrosion properties in simulated body fluid(SBF)in vitro were investigated.In addition,this work also studies the effect of processing parameters and overlapping direction of multi-pass friction stir processing(MP-FSP)on grain size and mechanical properties of ZK60 magnesium alloy.Firstly,cast ZK60 magnesium alloy is subjected to different processing parameters by FSP.The aim is to investigate the effect of rotation and processing speeds on microstructure,microhardness and tensile properties.The results shows that defect-free material can be gained under rotation speed of 400~1200r/min and processing speed of 50~100mm/min.With the synactic effect of friction heat and mechanical stirring,the whole processing area can be divided into four parts: stired zone(SZ),thermo-mechanical affected zone(TMAZ),heat affected zone(HAZ)and base material(BM).The coarse dendrites of BM are turned into fine equiaxed grains,and the coarse second phases are broken up and dissolved into the matrix.With the increasing of rotation speeds,the grain size first decrease and then increase.Similarity,the grain size first decrease and then increase with the increasing of processing speeds,and the onion ring phenomenon become more clear in SZ.The mechanical properties test results show that the micohardness of SZ is higher than BM,and the tensile strength and elongation are improved after FSP.The finest and most uniform microstructure(~6.9μm)is prepared at the rotation speed 800r/min and processing speed 100mm/min(800/100),which is subjected to corrosion test.And its microhardness,tensile strength and elongation are 70.1HV,276 MPa and 31.6%,respectively.Secondly,the effect of FSP on corrosion properties of ZK60 casting in SBF is investigated.Both the electrochemical and immersion corrosion tests indicate that the corrosion resistance of FSP specimens is improved compared to BM specimens.Micro-galvanic corrosion is the dominated corrosion mechanism for both kinds of specimens.Seriously localized corrosion morphology is shown in as-cast specimen,mainly due to the coarse and imhomogeneous microstructure.On the other hand,the uniform corrosion morphology is observed on FSP specimen with fine and homogeneous microstructure.In addition,after 6 days immersion in SBF,the maximum tensile load for BM specimen is decreased to 40% of the original value,while the maximum tensile load for FSP specimen is still 60% of the speciemen before corrosion test.At last,the effect of processing parameters and overlapping direction on microstructure and mechanical properties of MP-FSP are studied.The results show that defect-free material can be gained by MP-FSP under different conditions.The two-pass FSP with the overlapping ratio of 100% can achieve more uniform microstructure and higher elongation compared to single-pass FSP,while the microhardness and tensile strength are slightly decreased.The finest grain size(~6.0μm)is achieved with the following processing parameters: first pass with 800/100 FSP,second pass with 600/100 FSP.It has the best comprehensive tensile properties compared to other processing methods,and its tensile strength,elongation and microhardness are 272 MPa,42.5% and 65.3HV.This study used overlapping passes FSP to create fine-grained magnesium alloy plates.Each pass consumes the retreating side(MP-AS)or advancing side(MP-RS)of the stir zone of the previous pass with overlapping ratio of 50%.The overlaps either by MP-AS or by MP-RS creates higher mechanical properties than the base material.And the tensile strength of SZ in two directions is similar.However,the microstructure distribution of both overlapping directions are imhomogeneous,the center region and edge region can be observed over the whole area.And the tensile test specimens prepared in the longitudinal direction(LD)are less elongated than the transverse direction(TD).MP-AS produces more uniform microstructure and higher mechanical properties than MP-RS,the tensile strengths of LD and TD are 256 MPa and 258 MPa,and elongations are 30.3% and 22.7%,respectively.