Study On Technology Of The Integration Of Rheo-casting And Forging Process For Aluminum Alloy

Conventional casting is suitable for the production of complex parts. But has the disadvantage of low mechanical properties. Conventional forging can produce high performance parts, while complex one is difficult to be fabricated, and the deformation resistance is larger. Semi-solid forming lies between liquid casting and solid forging, and has advantages of both methods. Semi-solid forming especially rheoforming technology has become a research hotspot for advantages of short process, low-cost, and high-performance, etc. A process of integration of rheoforming and forging was proposed, based on casting, forging, and semi-solid forming processes. This technology which combines features of casting, forging, and semi-solid forming can realize rheoforming and forging of aluminum alloy in one mold. The technology has advantages of short processes, fast and efficient, dense parts, and high mechanical properties. In this paper, a slurry making equipment for applying rotating gas bubble stirring and mechanical vibration was developed. A device for integration of rheoforming and forging was designed. Slurry making process of aluminum alloy using rotating gas bubble stirring and mechanical vibration, and integration of rheoforming and forging process for aluminum alloy were explored. Mechanisms of slurry making, and rheoforming and forging were investigated.The semi-solid slurry making equipment for aluminum alloy was designed applying principles of rotating gas bubble stirring and mechanical vibration. Controllable parameters are stirring temperature, stirring time, rotation speed, and gas flow rate. The mould for integration of rheoforming and forging was designed. Forming molds and specimen are both separated vertically. The convex and concave dies are both insert type structure. Hydraulic system to drive the forming device was designed. A displacement and speed sensor was applied to display the injection velocity and displacement.The slurry making process using rotating gas bubble stirring was investigated. The results show that in order to obtain non-dendritic semi-solid slurry, the melt should be cooled from above liquidus to semi-solid zone, and then has a temperature between 5℃to 20℃below liquidus temperature, and gas bubble stirring should be imposed simultaneously. The optimum parameters of the rotating gas bubble stirring under the experimental conditions are as flow:gas flow rate 2 L/min, stirring time 3 min, rotation speed 200 r/min, and holding time 60 s. The microstructure uniformity in the crucible was examined. The results show that average grain size in the top of the crucible is smaller than in the bottom, and the non-dendritic grains in the edge of the crucible are rounder than in the middle.The slurry making processes using mechanical vibration and combination of gas bubbling and mechanical vibration were studied. The results show that in order to obtain semi-solid slurry with non-dendritic grains, the melt should be cooled from above liquidus to semi-solid zone, and mechanical vibration should be exerted simultaneously. The optimal vibration time is 5 min. In the chosen vibration parameter range, the bigger the amplitude and frequency, the better the mechanical vibration effect. As for the vibration velocity, the most refined non-dendritic grains can be obtained at the vibration velocity of 150 mms-1, and the most spherical non-dendritic grains can be got at the vibration velocity of 80 mms1. The optimal vibration acceleration is 8000 mms-2. The vibration in vertical direction can be simplified as harmonic forced vibration of one freedom system. The formula of mechanical vibration on melt particle was deduced. The results show that the vibration effect is proportional tof3×A2, that is, the product of the vibration velocity f×A and vibration acceleration f2×A. Integrated slurry making process is a combination of the optimal rotating gas bubble stirring process and mechanical vibration process. Compared with the single process, more refined and spherical non-dendritic grains are obtained, with the average grain size of 64.5μm, and grain roundness of 0.67.Microstructure and mechanical properties of liquid and semi-solid metal casting in permanent mold were investigated. The results show that mechanical properties of semi-solid specimens are higher than the liquid casting in permanent mold, except for the tensile strength of the semi-solid specimen corresponding to the integrated slurry making process. Microscopic cavities and holes are easy to be induced in the semi-solid slurry by the mechanical vibration. Stomata are caused in the specimen, especially when the rotating gas bubble stirring is exerted, which will deteriorate mechanical properties of the specimen. There are no impure particles assemble together or distribute in the tensile fractures in the semi-solid specimen, because of the stirring and convection effect in the slurry making process. And high levels of impurities can be tolerated in the semi-solid specimen without deterioration of the mechanical properties. Impurity particles exist in the tensile fractures of the liquid specimen, and the elongation is lower.Effect of rheological casting and forging parameters on microstructure and mechanical properties of the semi-solid specimen were studied. The results show that higher injection pressure and shot velocity are both beneficial to the filling ability of the specimen, but stomata are produced when the shot velocity is too fast. The optimal injection pressure is 40 MPa, and the optimal shot velocity is 92 mm/s. The optimal mechanical properties of semi-solid die-casting are tensile strength of 189 MPa, and elongation of 12.6%. Semi-solid billet has different mechanical properties when forged at different temperature and solid fraction. It is equivalent to liquid forging when the semi-solid billet is forged with the solid fraction lower than 0.5, with its mechanical properties about the same with semi-solid die-casting. When the semi-solid billet is forged at the solid fraction of 0.5, mechanical properties are deteriorated, with the tensile strength of 170.3 MPa, and elongation of 6.9%. When forged at the solid fraction higher than 0.5, the mechanical properties are also close to semi-solid die-casting. The optimal mechanical properties can be obtained, when the semi-solid billet is hot forged in the solid state, with the tensile strength of 194.7 MPa, and elongation of 14.3%. An optimal forging ratio exists in the semi-solid forging process. The best mechanical properties can be obtained at the forging reduction of 4 mm, with the tensile strength of 197.0 MPa, and elongation of 11.3%, corresponding to the forging ratio of 10%. Casting hole class defects in the semi-solid billet can be eliminated after forging, and specimen with denser microstructure is obtained. Solid grains are dense in the middle of the billet, and grains are elongated at the edge of the billet.The mechanism of semi-solid forging and liquid segregation were investigated. When forged at high liquid fraction, semi-solid slurry can be regarded as single-phase fluid with solid grains suspending in liquid, whose rheological model can be assumed to be Newtonian fluid. Through the formula derivation, the relationship between apparent viscosity of the semi-solid slurry and ratio of height to diameter of the billet, and pressure, and strain was obtained. The result shows that the apparent viscosity of the billet in the forging or compression processes is proportional to the square of the height to diameter ratio, and the pressure, and inversely proportional to the strain of the billet. When forged at high solid fraction, semi-solid billet should be considered as two-phase body, whose deformation model can be assumed to be porous materials. The total stress imposed on the billet is the sum of the stress imposed on the solid part and liquid part. In the case of a fixed total stress, the higher the solid fraction is, the bigger the stress on the solid grains is, and the solid grains are easy to be deformed. The liquid segregation in the billet is mainly formed in the rheological die-casting process, in which a pressure gradient exists in the filling stage of the semi-solid slurry. The relationship between liquid segregation and pressure gradient, liquid fraction, viscosity, number of seepage paths, and tortuosity factor of the seepage paths are obtained, through the formula derivation. The liquid segregation is proportional to the pressure gradient. The results show that, there is no liquid segregation in billet with dendritic microstructure, and Al and Si contents have no difference between the edge and central of the billet. Obvious liquid segregation exists in billet with spherical microstructure, and Al element decreases from the central to the edge of the billet, while Si element increases slightly, proving the existence of the liquid segregation.