Research On The Organization And Properties Of ZL702A Alloys For Diesel Engine Cylinder Head

ZL702A alloys has been widely used in the manufacture of engine components because of their unique advantages.As a key component of the engine,the service performance of cylinder head has a pivotal influence on the engine power.it is necessary to quantitatively analyze the relationship between the microstructure and performance of the cylinder head,so as to provide data support and theoretical guidance for the production process design of new generation of cylinder head.This paper takes the cylinder head of an engine as the research object.The cylinder head has a large external size and complex internal shape,and the cooling rate of each part in the casting process has large differences,resulting in casting defects and increasing tissue inhomogeneity,which causes fatigue failure of the cylinder head in service process.In this work,the cooling rate of the typical parts of the cylinder head was simulated and calculated by Any Casting software for the microstructural characteristics of the cylinder head and the casting process.The quantitative effect of the cooling rate on the microstructure and mechanical properties was also studied.The mechanism of tensile crack initiation and expansion was elucidated.The specific research results are as follows.The simulation results of the cooling rate of the cylinder head show that the cooling rate increased from 5.8 ℃/min in the thickest part of the wall type to 10 ℃/min in the combustion chamber,which leads to a significant change in the microstructure,a 23 % refinement of theα-Al grains,a 48 % reduction in the secondary dendrite arm spacing（SDAS）,and a change in the eutectic silicon from biased aggregation along the grain boundaries to uniform distribution within the matrix,affecting the tensile properties of the material.The average room temperature tensile strength of the specimens increased by 19.7 % and the elongation after break was increased by 146 %.The quantitative analysis results showed that there was a significant linear relationship between the spacing of the secondary dendritic arms and the tensile strength at room temperature.The increase of the eutectic polarization area rapidly weakened the tensile strength of the material,and there was a significant functional relationship between them.The results of room-temperature spin-bending fatigue experiments showed that fatigue cracks sprouted from sparse-type defects at the edges of the specimens and then expanded along the eutectic structure.The quantitative analysis of the microstructure shows that under low loading stresses（≤120 MPa）,the silicon phase and the smaller the area,the higher the fatigue life,and there is a more obvious linear relationship.The distribution of eutectic silicon particles also has a quantitative relationship with the fatigue life.The same is true for the SDAS,while lower the SDAS,the more obvious the influence.The results of high-temperature（350 ℃）SEM in-situ tensile experiments show that due to the difference in elastic modulus between the α-Al matrix and the second phase,stress concentration occurs at the interface of the two phases under the action of external forces,resulting in a large number of microcracks.These microcracks can sprout a large number of microcracks,and these microcracks sprouting within the eutectic silicon aggregation zone merge with each other to make microscopic voids inside the specimen,which reduced mechanical properties of the specimen.The eutectic silicon particles at the tip of the crack affect its expansion.When the eutectic silicon particles have a potential phase at an angle of about 45° to the applied load,it can provide a path for the crack expansion.The cracks expand and merge in this direction,resulting in the fracture of the sample.