On the basis of lspsf semi-solid pulping process in laboratory, the lspsf control system was upgraded, a set of semi-solid vertical squeeze casting equipment with translational pouring and injection system was designed and developed, and a special squeeze casting mold was developed. High quality semi-solid slurry was produced by lspsf process on this equipment, and rheo squeeze casting of forged aluminum alloy was studied. The solidification process of 7075 alloy wheel casting was simulated by AnyCasting software. The rheo squeezeof cast aluminum alloy A356 was also studied. The main conclusions are as follows
(1) Upgraded lspsf based on PLC programmable control The semi-solid pulping system can accurately control the process parameters of semi-solid pulping (multi-point temperature, slurry conveying pipe speed); the semi-solid vertical squeeze casting equipment with translational pouring and injection system is developed, which mainly includes the clamping locking mechanism, injection part, hydraulic system and electrical control system, etc. the equipment can realize new extrusion The pressure solidification method of squeeze casting is “translational pouring pressure solidification method”; the special squeeze casting die is designed and developed, which adopts the insert structure. Through the innovative design of the punch structure and the connection of the clamping cylinder, the “secondary inner cavity extrusion (forging)” function in squeeze casting can be realized.
(2) By analyzing the microstructure of A356 alloy parts formed by lspsf pulping process, the results show that the microstructure of solidification and crystallization under pressure is uniform, and the primary α – al phase is fine and non dendritic; the A356 alloy parts formed by lspsf process have good appearance precision, high surface cleanliness, stable properties and average tensile strength of 0 5 MPa, the average elongation is 13.13%, and the maximum elongation is more than 19%. It is easy to form complex A356 parts by rheo squeeze casting. Through the setting of exhaust and overflow tank, the defects such as cold shut and inclusion are easy to control, and the quality and performance of the parts tend to be stable.
(3) The thermophysical property model and parameters of 7075 aluminum alloy wheel shaped squeezewere set. The process parameters and boundary conditions were set according to the experimental data. The parts were modeled based on AnyCasting calculation modules, and the forming process of 7075 aluminum alloy wheel shaped squeeze casting parts was numerically simulated. The results show that: pouring temperature and mold temperature have a significant impact on the quality of the parts, with the increase of pouring temperature, the probability defects of distribution gradually disappear, but the probability defects of the core increase, near 630 ℃, there is the best pouring temperature; mold temperature has a significant impact on the temperature field of the parts, and the influence of mold temperature on the temperature field is obvious There is a better mold temperature between 160 ℃ and 200 ℃; the mold filling speed has a significant effect on the quality of the parts. When the mold filling speed is 0.5m/s, the air entrainment is serious, and the probability of defects increases. When the mold filling speed is 0.2m/s, the smooth high-quality parts can be obtained; through the orthogonal experiment method, 7075 is obtained The results show that when the pouring temperature is 635 ℃, the mold filling speed is 0.2m/s, and the mold temperature is 200 ℃, the probability defect parameter distribution is the least, and the quality of squeeze casting parts is excellent.
(4) Continuous and high-quality 7075 semi-solid slurry can be obtained by lspsf process with automatic control, which can meet the mass production of parts with weight of about 2kg. When the pouring temperature is 630 ~ 636 ℃, mold temperature is about 200 ℃, holding time is 15s and specific pressure is 80MPa, 7075 can be formed by rheo squeeze casting The results show that the wheel shaped aluminum alloy parts have good appearance quality, no cracks, pores and shrinkage porosity, and the microstructure grains are fine and non dendritic; the as cast mechanical properties of 7075 alloy by rheo squeeze casting are greatly improved compared with those by conventional casting, and the average tensile strength is 299mpa, the average elongation is 15.32%, and the maximum elongation is more than 23%.
(5) With the increase of specific pressure, the grain size of metallographic structure in the same part of the workpiece decreases gradually. Therefore, increasing the pressure is conducive to grain refinement, but according to the number of nuclei in unit volume, zv is related to the number of nuclei N and the linear growth velocity g of the crystal.
(6) The heat treatment system of 7075 aluminum alloy wheel shaped parts formed by rheo squeeze casting was analyzed and tested. Through the DSC analysis of the parts, different solid solution temperature ranges were determined. Through a large number of samples, a variety of heat treatment schemes with different solution temperature, solution time, aging temperature and aging holding time were explored. The microstructure and hardness of the products after heat treatment were analyzed and compared, and the best T6 single-stage aging heat treatment system was obtained. The heat treatment verification experiments of small batch samples were carried out under the heat treatment system. Under the experimental conditions, the optimal T6 single-stage aging heat treatment process specification of 7075 alloy rheo squeeze casting parts is: 475 ℃ solution treatment, holding for 2 hours, room temperature water quenching, aging temperature of 140 ℃, holding for 16 hours, air cooling; according to the optimized T6 heat treatment process, the mechanical properties of 7075 alloy rheo squeeze casting parts are significantly improved, and the average tensile strength is 520mpa, average yield strength is 467mpa, average elongation is 9.05%, the maximum tensile strength is 552mpa, the maximum yield strength is 500MPa, the maximum elongation is 13.77%, the best performance index reaches the forging level. When the precipitate phase is very hard or the interphase distance is too large, the dislocation can continue to move around the precipitate, which is easier than cutting through the particles. The stress required for dislocation movement is expressed as: 2GB / L, where l is the r-spacing of particles, and the smaller the particle spacing, the greater the stress. In this paper, the grain size of 7075 rheo squeeze casting parts is very small, the particle spacing is very small, and the solid solubility of Cu and Mg is large, there are a lot of dislocations, vacancies and other defects in the matrix, so heat treatment can greatly improve the mechanical properties of 7075 rheo squeeze casting parts.
(7) The microstructure of 7075 alloy by rheo squeeze casting was observed. It was found that after solution treatment, there were holes of different sizes left by eutectic phase with low melting point near the grain boundary. The tensile fracture also found a variety of primary melting holes near the grain boundary. These holes will seriously affect the plasticity and comprehensive mechanical properties of the parts. Based on the analysis of the microstructure of tensile fracture, the tensile fracture mechanism of the formation of tear band is found. With the increase of tensile stress, the pores and pores aggregate and expand to form a tearing ridge until the final fracture. The equivalent stress field model of the hole under tensile stress is established by ANSYS software, and the simulation results are consistent with the above analysis. Therefore, it can be inferred that the significant decrease of elongation after T6 heat treatment is mainly caused by the existence of primary melting holes.