Effect of Composite Micro-alloying and Rapid Solidification on Microstructure and Properties of Hypoeutectic Al-Si Alloy and Its Mechanism Research

Author:Li Yu Kun

Supervisor:du xiao dong wu yu cheng


Degree Year:2019





Material lightweight is a new development direction of transportation,aerospace and other industries.Aluminum alloy has become the preferred material for lightweight development because of its advantages of light weight,high specific strength and good corrosion resistance.With the increasing demand of the society,the performance requirement of aluminum alloy is getting more and more strictly.Nowadays,the development of new high-strength lightweight aluminum alloy has become a hot topic in the research of lightweight.This paper was based on the hypoeutectic Al-3Si alloy,the rare earth elements Sc composited other alloying elements Mg,Cu and Zr were added to modify it.The evolution mechanism of microstructure and properties were systematically studied on microalloying,T6 treatment and rapid solidification process by virtue of the optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM),electron back-scattered diffraction(EBSD)and atomic force microscope(AFM).And the influence of different technological synergies and related mechanisms were also studied.The main conclusions were summarised as following:(1)Sc has a dual modification effect on the hypoeutectic Al-3Si alloy,including grain refinement and modification of eutectic Si phase.With the increasing content of Sc,the microstructure changes from coarse dendrites to fine equiaxed grains and the morphology of eutectic Si phase gradually changes from continuous sheet to discontinuous fiber.The grain size of the alloy becomes a minimum when the Sc content reaches 0.6 wt.%.The regular shape particles can be clearly observed from the inside of the alloy,which acts as a heterogeneous nucleus during the solidification.The microstructure evolution of the alloy results in the change trend of mechanical properties.When the Sc content reaches0.6 wt.%,the hardness,yield strength,tensile strength and elongation of the alloy also reach the maximum value which are 82.2 HBS,158.9 MPa,286.5 MPa and 10.1%,respectively.(2)The primary Al3(Sc,Zr)phase in the as-cast alloy has two kinds of three-dimensional morphologies,including cubic and sharp cubic shape.The different two-dimensional morphologies are caused by interception of different sections(close to(100),(110)and(111)plane).The multilayer morphology of primary Al3(Sc,Zr)phase mainly consists of two structures:(a-Al+Al3(Sc,Zr)+a-Al+Al3(Sc,Zr)+×××)even number layer structure and(Al3(Sc,Zr)+a-Al+Al3(Sc,Zr)+×××)odd layer structure by the eutectic growth mode.The primary Al3(Sc,Zr)particles anda-Al matrix have the same growth orientation,which are good active particles and act as the heterogeneous nucleus ofa-Al matrix.Thea-Al matrix is coated with the primary Al3(Sc,Zr)particle for nucleation and epitaxial growth during solidification.(3)After solution treated at 540℃for 10 h,the spheroidizing effect of the eutectic Si phase in as-cast alloy is obvious,and the degree of spheroidization corresponds to the modification effect of Sc element on Si phase in the as-cast state.The mechanical properties of the alloy after T6 treatment are significantly improved compared with the as-cast alloy.When the Sc content reaches 0.6 wt.%,the hardness,yield strength,tensile strength and elongation of the alloy also reach the maximum value,which are 100.5 HBS,180.7 MPa,324.5 MPa and 12.5%,respectively.This is attributed to the aging strengthening effect of secondary Al3(Sc,Zr),CuAl2,Mg2Si and other precipitated phases after T6 treatment.The secondary Al3(Sc,Zr)phase precipitated by aging exhibits the facet morphology,including{110},{111}and{200}planes.There is a certain oritention relationship between the secondary Al3(Sc,Zr)phase anda-Al matrix:[010]Al3(Sc,Zr)//[010]a-Al,{100}Al3(Sc,Zr)//{100}a-Al,{110}Al3(Sc,Zr)//{110}a-Al,{111}Al3(Sc,Zr)//{111}a-Al,and there is also a certain oritention relationship between the Mg2Si phase anda-Al matrix:{410}Mg2Si//{111}a-Al,{210}Mg2Si//{100}a-Al.(4)The alloys with different Sc contents are transformed into fine equiaxed grains by means of rapid solidification treatment.In this process,the large supercooling degree are generated by rapid solidification,which is the dominant factor.The large supercooling degree inhibits the formation of the primary Al3(Sc,Zr)phase,therefore,the grain size of the alloys does not change significantly with the increase of Sc element.The synergistic effect of rapid solidification and Sc element improves the morphology and distribution of eutectic Si phases.When Sc content reaches 0.6 wt.%,the bone-like eutectic Si phases in the alloy become a minimum and dispersed most widely,which also partially evolve into granular morphology.The mechanical properties of the alloy after rapid solidification treatment are improved compared with the as-cast condition and the mechanical properties does not change regularly with the Sc content.As rapid solidification increases the solubility of alloying elements,the mechanical properties of the rapidly solidified alloys with different Sc content are significantly improved after aging treatment at 160℃for 3 h.When the Sc content reaches 0.6 wt.%,the hardness,yield strength,tensile strength and elongation of the alloy are 119.5 HBS,182.4 MPa,337.1 MPa and 10.4%,respectively.