Regulation Of Nanosized Iron-Containing Phase In Al-Fe Alloy And Its Effect On Microstructure And Mechanical Properties

Due to the outstanding advantages of high current carrying capacity,good heat resistance and high safety,thermal-resistant Al alloy conductor is widely used in electric transmission line.For increasing the comprehensive performance of thermal-resistant Al alloy conductor,rare earth such as Er and Sc was added into Al-Zr alloy conductor.However,the cost of the novel thermal-resistant conductor therefore greatly increased.Fe element is the most common impurity element in Al alloy,and it is easy to form needle-like Fe-containing phase,which could severely degrade the mechanical properties of Al alloy.It is difficult to reduce the content of Fe element to an ideal level by existing impurity removal methods,and the modification of Fe-containing phase is not satisfactory,which becomes a long-term unresolved problem in Al processing industry.A novel method of modification Fe-containing phase by continuous rheo-extrusion,heat treatment and plastic deformation is proposed in this study.Under the surppot of National Natural Science Foundation of China(No.51674077),the Fe-containing phase in Al alloy was refined to nano-size by the rapid cooling and shear deofrmation in continuous rheo-extrusion.Sc and Zr elements were partly substituted by Fe element and Al-Sc-Zr-Fe alloy conductor was developed,for the thermal stability and strengthening effect of Fe-containing phase.Based on this,the process of removing Fe element was eliminated,and Fe element was turned from impurity element into alloying element,so the cost of thermal-resistant Al alloy conductor was greatly reduced.The following crucial results were obtained.(1)The formation mechanism of nanosized Al3Fe phase during continuous rheo-extrusion and its effect on mechanical properties of the alloy were obtained.After pouring into the roll-shoe gap of continuous rheo-extrusion machine,Al3Fe phase in eutectic structure of Al-1Fe(wt.%)alloy was refined due to the rapid cooling of extrnsion wheel and extrusion shoe.The Al3Fe phase was then refined to nano-scale by the shear deformation in the roll-shoe gap,at the exit of extrusion wheel and in expansion extrusion mold.The size of Al3Fe phase in rheo-extruded Al-1Fe(wt.%)alloy matrix distributed in two range,and one of which is the blocky phase with an average length of 300 nm whilst the other is the near-spherical phase with an average diameter of 20 nm.The Al3Fe phase in rheo-extruded Al-1Fe(wt.%)alloy was much refiner than the needle-like Al3Fe phase with an average length of 40 ?m in as-cast Al-1Fe(wt.%)alloy.The tensile strength and elongation of Al-1Fe(wt.%)alloy prepared by continuous rheo-extrusion were 135 MPa and 30%,respectively,and they were 58.8%and 19%higher than the tensile strength and elongation of as-cast Al-1Fe(wt.%)alloy in report,respectively.The tensile strength and elongation of the rheo-extruded alloy was 51.7%and 5.3%higher than that of as-cast Al-1Fe(wt.%)alloy modified by rare earth,respectively.(2)The evolution mechanism of nanosized Al3Fe phase and its effect on mechanical properties were revealed.During heat treatment,the near-spherical Al3Fe phase with an average diameter of 20 nm was dissolved into the matrix,and the blocky Al3Fe phase with an average length of 300 nm was spheroidized.The driving force for spheroidization was the reduction of surface energy and lattice deformation energy.However,with increasing heat treatment time,the spheroidized Al3Fe phase became coarse and grew along a particular plane.Then the morphology of Al3Fe phase transformed into small plate-like.The large surface curvature of nanosized Al3Fe phase significantly accelerated the spheroidization and coarsening of Al3Fe phase.The effect of phase size on the spheroidization time of Al3Fe phase was far greater than the effect of heat treatment temperature.The coarse plate-like Al3Fe phase in the as-cast Al-1Fe(wt.%)alloy showed no obvious change in size and morphology during the heat treatment process.During heat treatment,the tensile strength of rheo-extruded Al-Fe alloy gradually decreased and tended to be stable.Under the same heat treatment condition,the tensile strength of rheo-extruded Al-Fe alloy was always higher than that of the as-cast Al-Fe alloy.During heat treatment,the elongation of rheo-extruded Al-Fe alloy also gradually decreased and tended to be stable.After heat treatment at 500? for 8 h,the elongation of rheo-extruded Al-1Fe(wt.%)alloy was lower than that of as-cast Al-1Fe(wt.%)alloy.(3)The effect of the size of hard particle on continuous dynamic recrystallization was analyzed,and the formula for calculating the critical size of particle was obtained.There were both nano-Al3Fe phase promoting dynamic recrystallization and nano-Al3Fe phase inhibiting dynamic recrystallization in rheo-extruded Al-Fe alloy.It was found that continuous dynamic recrystallization took place preferentially in the region containing Al3Fe phase with the diameter of 20 nm,and it indicated that the small Al3Fe phase reduced the deformation required for continuous dynamic recrystallization.However,the Al3Fe phase with the length of 300 nm hindered dynamic recrystallization.Therefore,the continuous dynamic recrystallization of rheo-extruded Al-1Fe(wt.%)alloy occurred earlier than that of pure Al,while the rate of continuous dynamic recrystallization was still lower than that of pure Al.The experimental results validated the mathematical model established by the author before.(4)The effect of cold rolling on the mechanical properties of rheo-extruded Al-1Fe(wt.%)alloy was obtained,and the strengthening mechanism was also revealed.During cold rolling,with increasing reduction ratio,the yield strength and tensile strength of Al-1Fe(wt.%)alloy gradually increased,while its elongation decreased firstly and then increased.When the cold rolling reduction ratio was 60%,the elongation of Al-1Fe(wt.%)alloy was the lowest.In the later stage of cold rolling,continuous dynamic recrystallization resulted in grain refinement,which increased the elongation.When the rolling reduction ratio was 90%,the yield strength,tensile strength and elongation of rheo-extruded Al-1Fe(wt.%)alloy were 241.6 MPa,275.0 MPa and 27.7%,respectively.Under the same condition,the yield strength,tensile strength and elongation of pure Al were 137.6 MPa,156.0 MPa and 32.6%,respectively.The strengthening mechanism of pure Al was grain refinement strengthening and dislocation strengthening,and the strengthening mechanism of Al-1Fe(wt.%)alloy was a synthesis of grain refinement strengthening,dislocation strengthening and second phase strengthening.(5)The microstructural evolution of Al-1Fe(wt.%)alloy during accumulative continuous extrusion forming was revealed.After one pass of accumulative continuous extrusion forming,the average grain size of Al-1Fe(wt.%)alloy decreased from 13.0 ?m to 1.2 ?m and the grain refining effect of one pass accumulative continuous extrusion forming was better than 5 passes of cold rolling with the total rolling reduction ratio of 90%.Due to the thermal effect and shear deformation during accumulative continuous extrusion forming,the blocky Al3Fe phase was spheroidized,and its length was also reduced to 200 nm.(6)Al-0.05Sc-0.1Zr-0.5Fe(wt.%)alloy thermal-resistant conductor with excellent comprehensive properties and low cost was prepared.The heat treatment method of Al-0.05Sc-0.1Zr-0.5Fe(wt.%)alloy conductor was T5 heat treatment(heat treated at 300? for 6 h),and the tensile strength,elongation and electrical conductivity of the conductor were 135.8 MPa,21.95%and 60.77%IACS,respectively.The tensile strength,elongation and electrical conductivity of cold-drawn Al-0.05Sc-0.1Zr-0.5Fe(wt.%)alloy conductor with the diameter of 4.5 mm were 188.2 MPa,7.92%and 60.28%IACS respectively,and its allowable continuous operating temperature was 230?.Al-0.05Sc-0.1Zr-0.5Fe(wt.%)alloy conductor with the diameter of 4.5 mm was AT4 type thermal-resistant Al alloy conductor,and its comprehensive performance was superior to the standard AT1,AT3 and AT4 thermal-resistant conductors in IEC 62004-2007 standard.The tensile strength,elongation and conductivity of cold-drawn Al-0.05Sc-0.1Zr-0.5Fe(wt.%)alloy conductor with the diameter of 3.0 mm were 190.6 MPa,7.16%and 60.22%IACS respectively,and its allowable continuous operating temperature was 150?.Al-0.05Sc-0.1Zr-0.5Fe(wt.%)alloy conductor with diameter of 3.0 mm was AT1 type thermal-resistant Al alloy conductor,and its comprehensive performance was superior to the standard AT1 thermal-resistant conductor in IEC 62004-2007 standard.