Adiabatic Shear Deformation of TB2 Rivet with Different Electromagnetic Riveting Initial Temperature
In order to improve the quality of electromagnetic riveting joints,TB2 rivets at 20 ℃ and heated to 100,200 and 300 ℃ were tested for electromagnetic riveting technology under 210 V discharge voltage.The rivet heads were observed by optical microscope(OM),scanning electron microscope(SEM)and scanning transmission electron microscope(STEM),the effects of different initial temperatures on riveting effects were discussed,and the microstructure evolution in adiabatic shear band(ASB)and the formation mechanism of the shear band were analyzed.The results showed that with the increase of initial temperature,the material was softened by heating,the maximum deformation of TB2 gradually increased,and the plasticity becomes better,which was helpful for rivet form-ing.At 20 ℃,the maximum deformation was 38％,when the deformation reached 40％,the specimen shears along 45° direction.When the temperature reached 200 ℃,the maximum deformation of TB2 samples was 52％,which increased by 8％.When the tem-perature reached 300 ℃,the maximum deformation of TB2 sample was close to 58％,and the maximum deformation was increased by 20％.Through SEM observation on TB2 rivet upsets with different deformation,it was found that ASB would initially appear around the rivet upsets,and the width of ASB would decrease with the increase of deformation within a certain amount of deformation.During the whole adiabatic shear deformation process,ASB had experienced three stages of initiation and extension to fracture,which was a shear band from the formation of the main shear band to the formation of both the primary and secondary shear bands,and then to the final fracture.With the increase of the initial temperature,ASB would be delayed.ASB width ranged from 20 to 50 μm at different tempera-tures and deformations.With the increase of deformation,the width of ASB at 20,100 and 200 ℃ showed a downward trend.The adia-batic shear band would be roughly formed at 30％of the deformation,but when the temperature rose to 300 ℃,30％of the deforma-tion was not enough to occur,and it could be formed only when it reached about 40％.For the shear band width at 300 ℃,it showed a trend of decreasing first and then increasing,mainly because the deformation played a leading role in the formation of the shear band at the beginning of the deformation of the header,and the width of the shear band decreased with the increase of the deformation.When the deformation reached a certain stage,the temperature of the header rose rapidly due to the adiabatic effect.At this time,the temperature played a leading role in the influence on the width of the shear band.The rise of temperature led to more uniform deforma-tion of the header,instability of materials in more areas,and finally the adiabatic shear band.When the deformation continued to in-crease,the shear area would be more concentrated.The width of the shear band might decrease when it rose to a certain value.When the deformation reached a certain stage,the header would break.Focused ion beam(FIB)was used to cut the inner and outer edges of the shear band of TB2.TB2 samples with 40％deformation at 20 and 300 ℃ were taken for STEM,and it was found that the disloca-tion density in the direction formed the transition zone to the center of ASB decreased gradually,and dislocation cells existed in the elongated large grains.With the increase of deformation,the large grains were gradually broken into equiaxed nano grains,consistent with the characteristics of rotational recrystallized grain formation.
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