| Electromagnetic riveting(EMR) is a joining technology under the high speed impact. Compared with other riveting technologies, it has some advantages of a high loading rate, a huge impact and an stable deformation. EMR can effectively solve some joining technical bottlenecks including that composite sheets are prone to damge under the extruding effect of rivets, and titanium alloy rivets have the great deformation resistance at room temperature and is difficult to be formed with the pneumatic riveting. Consequently, this technology has been partly applied to assembling processes in the aerospace field, and will inevitably be an key developing direction of joining technologies. Based on the advantages of EMR and requirements from the aerospace industry, Φ10mm-2A10 aluminum alloy rivets and Φ6mm-TA1 titanium alloy rivets were used to mainly study dynamic plastic deformation behaviours and microstructure evolution of rivets, mechanical properties and formation qualities of riveted structures.The electromagnetic-mechanical-thermal coupling model was established using the finite element software ANSYS/LS-DYNA. The Johnson-Cook constitutive equation was employed to characterize the material properties of rivet materials, the factors of the plastic strain, the strain rate and temperature were taken into account in the model. The EMR process was systematically analyzed by considering the interplay of electromagnetic field, mechanical field and thermal field in real time. The numerical analysis results accorded with that of experimental investigations. Through numerical simulation, some parameters(such as magnetic pressure distribution, strain rate variation and thermal field distribution) which are difficult to measure quantitatively in the actual experiments can be obtained. Temporal-spatial magnetic pressure distribution indicated that the magnetic pressures were varying with the form of a decaying sinewave, and peaked at the opposite position between driver plate and the center of coil thickness direction.The interference distributions from numerical simulations presented uneven trend along the rivet shaft. For the phenomenon, this paper investigated the plastic deformation behaviours by combining elastic-plastic theory and stress wave transimtting law, and established the ditribution model of relative interference. This model was verified through experimental measuration. Results showed that the relative interference was larger at the position close to rivet tail, and decreased toward the cup head position in the form of the product of power function and exponential function. Moreover, the interference directly determined the size of plastic deformation areas around the hole wall of riveted sheets. The size law of plastic deformation areas along the sheet thickness direction was similar to the distribution law of relative interference.For 2A10 aluminum alloy rivets, the adiabatic shear band was an important deformation characteristic during the EMR process. The thermo-softening effect and the remarkble difference between the two sides of metal plastic flow resulted in the formation of it. In addition, experimental and simulative results demonstrated that the adiabatic shear band preferentially initiated at the diagonal position of rivet tail, and gradually developed to intersect in the center of rivet tail. The width of the final adiabatic shear band was 80 μm. There are many mutually tangled dislocations within the adiabatic shear band, and the dislocation slip contributed to the formation of some lamellar substructure with the width of 0.8 μm. The existence of the adiabatic shear band significantly affected the property distributions of rivet tails. And many strengthening phases Al2 Cu and high density dislocations caused that the microhardness within the adiabatic shear band was much higher than that of other positions. Moreover, adiabatic shear bands in the rivet tail also led to the nonuniform distribution of compressive strengthes along the radial direction of rivet tail and tensile strengthes along the thickness direction. However, the average compressive yield strength among different measured positions increased by 81% relative to the original strength of rivets, which had positive influence on the load-bearing capacity of riveted structures.For TA1 titanium alloy rivets, adiabatic shear bands occurred under the greater axial deformation of rivet tail. The width of adiabatic shear bands was 10 μm. Dislocation slip was a main mechanism of microstructure evolution in the adiabatic shear band for TA1 titanium alloys with the dense-hexagonal crystal structure. Recrystallized grains with the size of 100~200 nm existed in the adiabatic shear band, which resulted from the dynamic rotation of sub-grains.Mechanical properties of riveted structures were important standards to evaluate the joining reliability of them. The maximum shear load-bearing capability and pull-out load-bearing capability were 23.2 k N and 35 k N for riveted structures with the single Φ10mm-2A10 aluminum alloy rivet. And mechanical properties of these riveted structures with the 5~6 mm height of rivet tails were the optimal. In addition, the maximum shear load-bearing capability and pull-out load-bearing capability were 9.9 k N and 12.3 k N for riveted structures with the single Φ6mm-TA1 titanium alloy rivet.In order to explore engineering applications of the EMR technology, the comparative analysis was employed for riveting structures with the Φ10mm-2A10 aluminum alloy rivet and bolting structures with the Φ6mm-30 Cr Mn Si steel rivet. Relative to bolting structures, The maximum shear load-bearing capability and pull-out load-bearing capability were improved by 3.1% and 40%, respectively. Furthermore, each rivet could lose 15.8% weight than a set of bolt(including a screw nut, a spring washer and a flat washer). The load-to-weight ratio values for shear tests and pull-out tests were improved by 22.6% and 66.1%, respectively. Consequently, the strategy of substituting Φ10-2A10 riveted structures for Φ6-30 Cr Mn Si bolted structures can not only improve mechanical properties of joining structures, but also realize weight reduction, which is of engineering significance for assembling technologies in the aerospace field. |
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