Microstructure And Properties Of Fine-Grained WE43Magnesium Alloy Prepared By Friction Stir Processing

As a novel severe plasticdeformation (SPD) tecnology, friction stir processing (FSP)hasmade a significant achievement in the recent20years since it has been developed. FSP hascurrently attracted extensive attention for producingthe fine-grained materials and surfacecomposites, and modifying the microstructure of heterogeneousmetallic materials. In thisstudy, we used FSP to treat as-cast WE43magnesium alloy. The processes of fabricatingfine-grained magnesium alloy and microstructure evolution mechanism were studied. Theeffect of processing parameter on grain size and mechanical properties were investigated. Thesuperplasticitybehavior of FSP WE43magnesium alloy was analyzed and the superplasticitydeformation mechanism and fracture mechanism was discussed. The conclusion are asfollows:(1) The results shows that defect-free material can be gained under the rotation speed of400-1500r/min and the processing speed of30-150mm/min in as-cast WE43magnesiumalloy during FSP, indicating the processing parameters for FSP WE43magnesium alloy canbe chosen in a wide range. With the synacticeffect of friction heat and mechanical stirring, themicrostructure of processing zone can be divided into four parts: stired zone (SZ),thermo-mechanical affected zone (TMAZ), heat affected zone (HAZ) and basematerial(BM).FSP results in the generation of fine-grained microstructures and fundamentalbreakup and dissolution of the coarse second phases.With the rotation speeds increasing, theaverage grain size first decreases and then increases, and the finest microstructure with anaverage size of2μm is produced at the rotation speed of800r/min. Similarity, the averagegrain size first decreases and then increases, and the finest microstructure (~2μm) is preparedat the processing speed of60mm/min with the processing speeds increasing. The results ofEBSD show that the fraction of the high angle grain boundariesis94%, which is a result ofDRX.(2) Single-pass FSP can improve the mechanical properties of WE43alloy significantly,in particular its ductility due to microstructure refinement. When the rotation speed increases,the best mechanical property is obtained at800r/min with a tensile strength of290MPa andan elongation of17.2%. When the rotation speed increases, the maximum tensile strength and elongation of the FSP WE43alloy is obtained at60mm/min.Microhardness of FSP WE43alloys is also higher than that of the as-cast alloy. Because of the fine-grained microstructure,there is no significant difference in microhardness among all the FSP samples. The as-castWE43alloy fails through cleavage fracture under tensile test, while ductile fracture is themain fracture mode for the FSP alloys.(3) High temperature tensile tests show that the FSP WE43alloys exhibit excellent highstrain rate superplasticity, with a large elongation of939%at723K with a strain rate of3×10-3s-1.Fine second phase particles precipitatedat the grain boundaries in the FSP WE43alloy plays an important role during superplastic deformation. It can prevent the grow-up ofthe fine grains gained from FSP, thus exhibitesexcellent high strain rate superplasticity at hightemperatures. Grain growth is considered to be the dominating mechanism for themicrostructure evolution during superplastic deformation at high temperature. The mainfailure mechanism during superplastic deformation is cavity coalescence. Cavities nucleatearound the second phase particles and connect to each other due to the grain boundary sliding(GBS).(4)The effect of heat treatment on the microstructure and properties of FSP WE43isstudied. The results indicates that solution treatment combined with FSP could make furtherimprovement on the grain refinement in WE43magnesium alloy. Therefore the mechanicalproperties can be enhanced. But FSP with subsequentsolution treatment maake the secondphase particlesdissolve in the matrix. Pinning effect of the second phase particlesdisappearesand lead to the deterioration of the the microstructure and properties of FSPWE43.Appropriate heat treatment process on the FSP WE43can improve the microstructureand properties.