Effect Of Continuous Casting And Extrusion Process On Microstructure And Room Temperature Mechanical Properties Of Zn-Al Alloys

As a typical superplastic alloy,Zn-Al alloy has been studied deeply for many years.At the same time,as a commonly used thermal spray material,Zn-Al alloy rods and wires are widely used in the field of anti-corrosion of steel structures.Zn-Al alloys can obtained room temperature superplasticity by rolling,equal channel extrusion(ECAP),hot extrusion,high pressure torsion(HTP)and other forming methods.However,these forming methods have the disadvantages of complicated processes and wasted energy.Continuous casting and extrusion(CASTEX),acting as a new short-flow forming method,has great development potential and research value.The current researches on CASTEX Zn-Al alloys mainly focused on the microstructure evolution of metal flow during deformation and tissue regulation of CASTEX.However,its mechanical properties still cannot meet the requirements of thermal spray rods and wires.Therefore,in this paper,two important parameters including extrusion ratio and the squeegee roller speeds are adjusted in the CASTEX process,and successfully prepared Zn-Al alloy rods and wires of varying specifications,which could meets the mechanical performance requirements of thermal sprayed rods and wires,and a certain room temperature superplasticity is obtained at the same time.In this experiment,scanning electron microscope(SEM),electron backscatter diffraction(EBSD),transmission electron microscope(TEM),and room temperature tensile at different strain rates were adopted,to study the microstructure and mechanical properties of CASTEX Zn-Al alloy.We have build a bridge between composition,process,structure and mechanical properties of CASTEX Zn-Al alloy.The main results are as follows:(1)Adjust the extrusion ratios and squeegee roller speeds of the CASTEX Zn5Al alloy to obtain a certain room temperature superplasticity.As λ=6 and n=8 r/min,the Zn5Al alloy possess the optimum plasticity,the elongation can reach 123%,and the corresponding tensile strength is 160 MPa.In the strain rate range of 1×10-5-1 ×10-3 s-1,the m value of Zn5Al alloy reaches 0.33.In this process,the average grain size of theη-phase in the alloy is 2.44 μm,and no obvious texture generated.(2)Adjust the process to λ=6 and n=14 r/min.At this time,Zn10Al alloy has obtained a certain room temperature superplasticity.It can reach an elongation of 173%and a tensile strength of 168 MPa.In the strain rate range of 1×10-5～1×10-3 s-1,the value of m reaches 0.25.In this process,the average grain size of the η-phase in the alloy is 2.33 μm,and no obvious texture generated.(3)When squeegee roller speed is fixed at 8 r/min,as the extrusion ratio increased from 4 to 12.5,the tensile strength of Zn5Al and Zn10Al alloys decreases first and then increases.The tensile strength of the Zn5Al alloy is between 160 and 182 MPa;the tensile strength of the Zn10Al alloy is between 170 and 245 MPa.The elongation is increased first and then decreased,with a range of 37%～123%and 41%～122%,respectively.Increasing the extrusion ratio cannot eliminate the lamellar structure of the Zn-Al alloy,but it can change its texture from non-basal texture to basal texture.Whenλ=6 or 11,the Zn5Al alloy and Zn10Al alloy can meet the mechanical performance requirements of thermal sprayed rods and wires.(4)Fixing the extrusion ratio of 6,as the squeegee roller speeds increases from 10 r/min to 16 r/min.The tensile strength of Zn5Al alloy decreases first and then increases,ranging from 155 to 190 MPa,and the elongation decreases from 113%to 46%;the tensile strength of Zn10Al alloy decreases first and then increases,ranging from 168 to 184 MPa.The growth rate first increase to 173%and then decrease to 38%.When λ=6,the Zn-Al alloys at different squeegee roller speeds all exhibit non-basal textures,and still a mixture of spherical and lamellar structures.In order to make the mechanical properties of the thermal sprayed rods and wires meet the supply requirements,the squeegee roller speed of the Zn5Al alloy should not exceed 10 r/min;the squeegee roller speed of the Zn10Al alloy should be less than 14 r/min.(5)The strain rate sensitivity of as-cast extruded Zn5Al alloy and Zn10Al alloy was studied after optimized process.At a strain rate of 1 × 10-4 s-1,the elongation of the Zn5Al alloy and Zn10Al alloy are the highest,reaching 172%and 177%,respectively;at the strain rate of 1×10-5 s-1,the elongation fell to 119%and 135%,respectively.The elongation of as-cast extruded Zn10Al alloy can still reach 102%at a strain rate of 1×10-2 s-1.After tensile deformation,the lamellar structure of the Zn-Al alloy basically disappears,and the equiaxed structure is maintained when the strain rate is below 1×10-2 s-1,After tensile at a strain rate of 1×10-3 s-1,the η phase grains of Zn5Al alloy and Zn10Al alloy are the finest,refining to 1.85 μm and 1.88 μm,respectively.When the strain rate drops below 1×10-4s-1,the η-plase grains begin to coarsen.(6)Trial production and process optimization for CASTEX Zn22Al alloy,but changing the extrusion ratio and the squeegee roller speeds could not achieve room temperature superplastic.As λ=11 and n=8 r/min,the elongation is the highest,reaching 87%,and the corresponding tensile strength is 170 MPa.There are a large number of lamellar clusters under different processes.Part of the lamellar structure is formed into a coarser spherical and lamellar morphology through continuous coarsening and discontinuous coarsening.