Effect Of Forging And Heat Treatment Process On Microstructure And Thermal Fatigue Properties Of Al-7Si-1.6Cu-0.5Mg Alloy

In this paper,the self-made new Al-7Si-1.6Cu-0.5Mg alloy refinement treatment,hot forging treatment.T6 heat treatment.The effects of forging heat treatment on the microstructure and properties of Al-7Si-1.6Cu-0.5Mg alloy were studied by mechanical properties and OM,SEM and TEM.At the same time,the influence of temperature on the piston in the engine is simulated under the condition of without increasing the mechanical stress.The surface crack initiation and expansion process of the four Al-7Si-1.6Cu-0.5Mg alloys were studied by means of crystal microscope,scanning electron microscope and so on.The effects of thermal stress and tissue defects on the growth of cracks and the mechanism and mechanism of crack initiation and propagation were analyzed.After detailed observation,statistical data,the location of the crack initiation and the trajectory of the crack propagation under different processes and cycles.The effects of the defects such as loose,shrinkage,oxidative corrosion holes and the initiation and propagation of the second phase on the initiation and propagation of the crack and the growth of the crack at different temperature.Under the experimental conditions,the following results are obtained:1?After homogenization(500?,8h),forging(forging temperature450?~380?,Strain rate10% ~ 20%),heat treatment((Solid solution480?,2h,70?water cooled),(aging180?,6h)),compared with the as-cast sample,its comprehensive performance greatly improved,tensile strength of 380 MPa,hardness of 120 HV,elongation of 8%.2?In the process of hot forging,the coarse lamellar eutectic Si phase under the action of forging pressure,broken and spheroidization,in the dynamic recrystallization process,the formation of equiaxed eutectic silicon phase.After the T6 heat treatment,precipitate a large number of uniform,dispersed distribution of nano-second phase,the matrix play a role in dispersion enhancement.3?The reason why the strength of the material is improved after forging is explained from the relationship between work hardening and dislocation density.From the dislocation curve,it can be seen that when the dislocation density is higher than the dislocation minimum point ?m.In the forging process,the dislocation densityincreases,the dislocation in the process of movement will occur to form a separate order,play a role in hardening.4?The thermal fatigue resistance of the new Al-7Si-1.6Cu-0.5Mg for forging heat treatment is better than that of the as-cast,refined and deteriorated state in the three temperature environments at 20?300?,20?350? and 20?400?.When the circulating temperature is 20?330?,the forging heat treatment specimen has better thermal fatigue resistance than the other three samples.When the lower limit temperature(Tmin)is constant,the bearing capacity of Al-7Si-1.6Cu-0.5Mg alloy in four states decreases with the increase of upper limit temperature(Tmax).When the maximum temperature(Tmax)in the interval of 300~350?,the material temperature sensitivity is slightly higher than the temperature range of 350~400?.5?Four kinds of samples in the process of thermal cycling will appear corrosion corrosion hole phenomenon,but forging the heat treatment of the sample on the hole at least.When the hole ? larger can delay the initiation of cracks.Forged samples,the presence of microcracks in the crystal will be weak and the thermal fatigue resistance of the specimen.