Study On The Microstructures And Thermal Fatigue Properties Of A356 Alloy Under Different Technological Conditions

At present, governments arise the following requests: the lightweight car, reducing automobile energy consumption and pollution, improving fuel economy. In order to realize energy conservation and emissions reduction targets, A356 aluminum alloy are widely used in the manufacture of automobile transmission gear, engine, wheel hub and body structure, etc.because of its light quality, moderate price, higher specific strength. But, recently it was found that many aluminum engine internal parts and components, such as cylinder block, cylinder head, piston, etc were damaged by thermal fatigue in the process of engine start and stop which serious influenced its service life and caused security hidden danger. How to improve the thermal fatigue properties of aluminum alloy, expand its application field in high temperature in addition to the automotive industry is becoming the important problems.In this paper, A356 aluminum alloy is used as base material, five different state alloy specimens were prepared based on composite refining modification treatment, micro alloying process and T6 heat treatment.They are as-cast, composite metamorphic state, composite refining metamorphic+T6 heat treatment state, composite refining modification+microalloyed state, composite refining metamorphic + microalloyed+T6 heat treatment state respectively. Study of different process on the mechanical properties,microstructure of different states A356 alloy.morphology and distribution of the characteristics phase were investigated by X-ray diffractometer(XRD), Scanning electron microscope(SEM) and Optical microscope(OM), mechanical property test methods. The change rule and mechanism of action were analysised.The thermal fatigue experiments were carried out by the self-constraint thermal fatigue tester, thermal fatigue behavior and mechanism were studied,the results show that:1. Compared with the as-cast A356 alloy, composite refining metamorphism, microalloyed and T6 heat treatment can improve the mechanical properties of A356 alloy. Among them,composite refining metamorphic +microalloyed+T6 heat treatment can significantly improved mechanical properties.The tensile strength is 345 Mpa, the elongation is12.3%, brinell hardness is 118 HB.2. Compared with the as-cast A356 alloy, composite refining metamorphism, micro alloyed and T6 heat treatment can improve the thermal fatigue properties of A356 alloy., the mechanical properties, composition uniformity and microstructure uniformity of A356 alloy treated by composite refining metamorphic + microalloyed + T6 heat treatment state is the best. In addition, alloy the thermal fatigue life of alloy would significantly reduced with the temperature increasing of upper limit of thermal fatigue experiment, so alloy applied temperature was suggest not exceed 400 ℃.3. The crack initiation process of five states of A356 alloy is similar. The tiny pits â†'micro pit produced in the micro oxidation layerâ†'crater grew up, the number of crater is increased â†' micro crack â†' crack source, the main reason is that the expansion coefficient of substrate and the second phase are different, the formation of crack source is promoted with the synergy of oxidation. In early extension period the crack is mostly grown in the intergranular way; In late extension, the crack is mostly grown in the intergranular and transgranular mixed way.4. Five kinds of preparation process of A356 alloy thermal fatigue crack growth rate of five states of A356 alloy showed a trend of increase firstly then decrease. Extension rate depends on the stress intensity factor of the stress field in front of crack. The second phase particles such as Al2 Cu phase which are not out of the matrix as well as the formation of secondary crack, crack bifurcation can have the effect of lower growth rate during the crack extension period.5. Affected by the morphology and distribution of Si phase, there are two main types of crack propagation: "Through " and "bypass" mechanism. At the beginning of the cold-heat circulation period, the oxide film generated due to the oxidation corrosion can protect matrix. With the increase of the cold-heat cycles, the oxide film is destroyed, the superimposition of the additional stress of the oxidation corrosion and cyclic alternating stress accelerated the growth of crack.