Study On Plastic Deformation And Strengthening Mechanism Of AZ Magnesium Alloy Reinforced By Al-Si Master Alloy

As the lightest metal structure materials for engineering application,magnesium alloys have broad application prospect in aerospace,automobile,electronic communications and other industrial fields.However,owing to its relatively low absolute strength as well as poor plastic deformation ability,most products are still applied to non-bearing load structural parts as cast magnesium alloy,which greatly limits its large-scale application in the automotive industry.Therefore,it urgently needs to develop a number of new low cost and high performance wrought magnesium alloys.Whereas most of currently developed high-strength magnesium alloys contain rare earth elements,which leads to the increase of production cost as well as the deterioration of subsequent plastic formability.Aiming at the problems mentioned above,the most widely used AZ magnesium alloys were chosen to investigate in our study.The alloying method was designed by replacing Al with Al-Si master alloy combined with plastic forming process optimization,thereby introducing the Mg₂Si particulate phase and leading to the improvement of mechanical properties for AZ magnesium alloys.Furthermore,the mechanisms of microstructure refinement,dynamic recrystallization and mechanical properties strengthening were revealed,and a specific strengthening and toughening approach for AZ magnesium alloy based on reinforced phases was proposed,which contributed to the development of low-cost and high-performance magnesium alloy.In this paper,AZ magnesium alloy sheets with Al-Si alloying were successfully prepared by pre-extrusion and multi-pass hot rolling process from ingots through magnetic suspension induction melting.The microstructures of alloy sheets were characterized by optical microscopy?OM?,scanning electron microscopy?SEM?and transmission electron microscopy?TEM?,the constituent phases were identified by X-ray diffraction?XRD?and energy dispersive spectroscopy?EDS?,the texture evolution was analyzed by electron backscatter diffraction?EBSD?,and the mechanical properties were tested by electronic universal testing machine.The microstructures,dynamic recrystallization,texture evolution and mechanical properties of AZ magnesium alloy sheets with Al-Si alloying were investigated.The dynamic recrystallization models of AZ magnesium alloys reinforced by particle phases were established,and the strengthening mechanisms of AZ magnesium alloys reinforced by Mg₂Si particle phases were revealed.The main results of this paper are as follows:?1?The Al-Si master alloy and different initial microstructural states of alloys have significant effects on the microstructure and mechanical properties of AZ31 hot rolled sheets.Compared with direct rolling from as-cast ingot,the Mg-3?Al-Si?-Zn alloy rolled sheet following pre-extrusion exhibits remarkably microstructure refinement,forming bimodal grain sized microstructure with average grain size of⁴.5?m,which involved a large number of ultrafine grains surround by normal size grains around refined Mg₂Si phases.Both the alloy sheets exhibit typical{0002}base texture,and the grain boundary misorientation is dominated by low angle grain boundaries.The ultimate tensile strength of the alloy sheets with Al-Si master alloy addition as well as rolled from pre-extruded is increased to 326 MPa.?2?The coarse Mg₂Si phase in as-cast Mg-6?Al-Si?-Zn alloy has a certain inhibition effect on the formation of Mg₁₇Al₁₂ phase.Mg₂Si phase is refined to below 5?m after the multi-pass hot rolling,and furthermore,the microstructure is also refined dramatically with the average grain size reduced from 13.6 to 6.4?m,forming fine grained regions around refined Mg₂Si phases,which involved a great number of ultrafine DRXed grains.The Mg-6?Al-Si?-Zn alloy sheets exhibit typical{0002}base texture,whereas the base texture intensity is weakened.Compared with Mg-6Al-Zn alloy,the tensile strength of Mg-6?Al-Si?-Zn alloy increases from 324 to 355 MPa after hot rolling process,while the elongation decreases slightly with the strength is improved.?3?In addition to many coarse network distribution Mg₁₇Al₁₂2 phases,there are a large number of coarse Chinese script type and irregular polygonal Mg₂Si phase.The slender needle-like,hard and brittle Mg₂Si phases in Mg-9?Al-Si?-Zn alloy are more prone to be crushed than coarse Mg₁₇Al₁₂ phase to form microscopic void during hot rolling process,therefore,resulting in the dramatical deterioration in plasticity.Compared with that of Mg-9Al-Zn alloy,the yield strength and tensile strength from the tensile stress-strain curve of Mg-9?Al-Si?-Zn alloy sheets decline slightly.?4?For as-cast AZ91 magnesium alloys,the dendritic segregation and distribution of Mg₁₇Al₁₂2 phase can be effectively modified through pre-homogenizing treatment.Multi-pass with small rolling reduction is one of effective approaches to achieve severe plastic deformation of AZ91 magnesium alloy.With the decrease of rolling reduction per pass and the increase of rolling passes,the matrix grains are refined obviously,whereas some Mg₁₇Al₁₂phases are fragmented to form tiny cracks.With the increase of pass reduction and rolling passes,the edge crack width and depth presents increasing tendency and the Mg₁₇Al₁₂precipitated phase changes from lamellar to spherical particles with tinier size during hot rolling deformation concurrently promoting the dynamic recrystallization of the matrix.The micro-crack initiation appears around the Mg/Mg₁₇Al₁₂ phase interface and coarser second phase during tensile test at room temperature,which leads to the macroscopic cracks.?5?The dynamic recrystallization types of AZ magnesium alloys with Al-Si master alloy addition and hot rolling process could be summarized as follows:continuous dynamic recrystallization based on continuous absorption dislocations at low angle grain boundaries and twin induced dynamic recrystallization based on twinning and nucleation.In addition to grain boundary nucleation,the Mg₂Si particles?>1?m?could also promote the dynamic recrystallization nucleation by particle nucleation mechanism effect during hot plastic deformation.The mechanical properties of the AZ magnesium alloy with the addition of Al-Si master alloy are improved obviously.The strengthening mechanism of Mg₂Si particle reinforced AZ magnesium alloy sheets could be interpreted as two aspects:on one hand,the refined Mg₂Si particles could promote dynamic recrystallization,which forms more refined grain regions,so as to obtain a strong grain refinement strengthening effect;on the other hand,the refined Mg₂Si particles could also bring about a strong second phase strengthening effect,which includes dislocation strengthening effect through inhibiting the dislocation motion,as well as Orowan strengthening mechanism with dislocation by pass second phase particles.In short,in view of the relatively low absolute strength for conventional magnesium alloy,the mechanical properties of AZ magnesium alloys were further improved through Al-Si addition replacing Al on the basis of low cost.The strengthening mechanism of AZ magnesium alloy sheets reinforced by Mg₂Si phase,and a specific method of strengthening AZ magnesium alloys based on the strengthened phase were proposed,which provide a new idea for the development of low cost and high performance magnesium alloy.