Aluminum alloy is widely used in automobile manufacturing as an important lightweight material because of its low density, high corrosion resistance and high specific strength. Cracking, wrinkling and springback are the three main quality defects in sheet metal stamping. The total elongation of aluminum alloy is smaller than that of steel plate. At room temperature, the formability of aluminum alloy is worse than that of steel plate, and it is more prone to fracture. Accurately simulating the fracture phenomenon in the sheet metal forming process is of great significance to the forming process and die design. The fracture of metal materials can be divided into brittle fracture and ductile fracture, and most aluminum alloys belong to ductile fracture. From the perspective of meso damage mechanics, the ductile fracture criterion considers that the defects of holes in the material continuously nucleate, grow and aggregate under the action of external force, and finally cause the damage of the material, which can better predict the formability of sheet metal with poor plasticity.
1、 There are many factors affecting ductile fracture, which can be divided into two categories.
1. One is related to materials, such as strain hardening level, hole volume fraction and phase particle composition;
2. Class II is process related, such as strain rate, friction and forming temperature.
2、 Two types of dimples on the fracture surface of aluminum alloy tensile specimens
1. The shape of a dimple is large and deep, indicating that 5052 aluminum alloy belongs to ductile fracture;
2. The shape of the two dimples is small, and a certain amount of spherical particles are distributed at the bottom and around the dimple, indicating that 5052 aluminum alloy has partial brittle fracture. Due to the small proportion of the two dimples, the main fracture mechanism of 5052 aluminum alloy belongs to ductile fracture.
3、 Simulation and experimental results of aluminum alloy ball joint bulging
During the process from forming to fracture, the sheet metal has obvious necking. The macro fracture is broken along the 45 ° direction with the thickness direction. The fracture of the cracked part is relatively flat without irregular fluctuation. The sheet metal thickness has obvious thinning near the edge. The finite element simulation results are in good agreement with the experimental results, and the location, shape and size of the crack at the plate crack are close to the experimental results.
4、 Aluminum sheet forming cracking conclusion
1) The uniaxial tensile test of aluminum alloy plate was carried out, and the test stress-strain curve was fitted by voce model. The Cockcroft Latham ductile fracture criterion is introduced into the unidirectional tensile finite element model. Combined with the test results, the failure parameters of 5052 aluminum alloy are inversely calculated.
2) The bulging cracking of 5052 aluminum alloy sheet metal ball joint is simulated and predicted by using the fitted voce model parameters and ductile fracture criterion parameters, and the experimental verification is carried out. The results show that the simulation prediction of sheet metal cracking position and crack shape are consistent with the experimental results.
3) Using this method to predict the forming cracking limit of aluminum alloy sheet has the advantages of convenient solution of material parameters and high calculation accuracy, which can provide reference for relevant research.