| Lightweight materials is the main trend of the development of transportation,aerospace and other industries.due to its low density,moderate intensity,high plasticity,excellent specific strength as well as good corrosion resistance,aluminum alloy has become the preferred material for lightweighting.With the increasing demand of society,the requirements for the performance of aluminum alloy is becoming ever more demanding.The research for the high strength and light aluminum alloys has become a hot issue of lightweight research nowadays.From the point of view of material lightening,the research object of this paper is commonly used hypoeutectic Al-Si alloy:A356 aluminum alloy.Based on the traditional A356 material,further optimization of its composition design and heat treatment process are carried out in order to achieve material lightening.Based on the hypoeutectic Al-7Si-0.35 Mg alloy modified by Ti and Zr,this paper is going to apply to the field emission scanning electron microscopy(FESEM),differential scanning calorimeter(DSC),X-ray diffraction(XRD),transmission electron microscopy(TEM),tensile testing machine and Brinell hardness tester to discuss the effect of microstructure and properties of different content of alloying elements and design of heat treatment process on the T6 state microstructure of Al-7Si-0.35Mg-0.2Ti-x Zr Aluminum Alloy.Starting from the intermetallic compounds formed in the solidification process of the alloy and the precipitated phases in the aging process,this paper conducts a comparative research on the the evolution rules of the structure,composition and morphology of the compounds as well as the precipitation strengthening behavior and mechanism of the precipitated phases.The main conclusions are the following:(1)Ti and Zr elements can compound and refine the alloy structure,and alter the eutectic silicon morphology in as-cast structure synergistically.The addition of Zr element in the alloy can refine ?-Al and reduce the secondary dendrite spacing.The addition of Zr makes eutectic silicon change from continuous flake needle to discontinuous fibrous short rod.The main forms of Zr elements in the alloys are Al-SiZr and Al-Si-Ti intermetallic compounds with sizes ranging from 2 to 10 ?m.With the increase of Zr content,Brinell hardness,yield strength,tensile strength and elongation of as-cast alloys increase simultaneously,and reach the maximum when Zr content is0.2wt%.Zr can effectively improve the mechanical properties of the alloy,and excessive Zr will have adverse impact on the mechanical properties of the alloy.(2)The T6 heat treatment process of the alloy is determined by DSC analysis and peak aging test as follows: Solid solution 540?×5h + Aging 160?×6h.The eutectic silicon phase at grain boundary of as-cast Al-7Si-0.35Mg-0.2Ti-x Zr alloy has spheroidized transformation after T6 heat treatment.The modification effect is the best when 0.2wt.% Zr element is added,and the spheroidizing effect is the best.After T6 heat treatment,Al-7Si-0.35Mg-0.2Ti-0.2Zr alloy produced secondary precipitates with particle size of about 20-50 nm,which dispersed in the matrix,and could effectively improve the strength and plastic toughness of the Al-7Si-0.35Mg-0.2Ti-0.2Zr alloy.(3)With the increase of aging time,Brinell hardness,yield strength and tensile strength of the alloy will reach the maximum: 93.4HBS,245 MPa and 315 MPa at 160×6h,respectively.The tensile fracture surface of the alloy shows that the dimples in the fracture surface aged for 4 h and 6 h are large,deep and regular with better plasticity and toughness. |
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