Experimental investigation on electroplastic effect of DP980 advanced high strength steel

This paper investigated the influence of the electric pulses on the flow behavior and plasticity of the advanced high strength steel (AHSS) Dual Phase, DP980. In order to isolate the thermal effect of the electric pulses, two kinds of uniaxial tensile tests at the same testing temperature were carried out: (1) isothermal tensile test carried out in an environment cabinet and (2) electrically-assisted isothermal tensile test. The stress–strain curves were recorded and compared. The results indicate that at the same testing temperature, the stress–strain curves obtained by test (1) are generally lower than curves deserved by test (2). It demonstrates that electric pulses can not reduce the flow stress when compared with the case that without electric pulse which is contrary to the traditional electroplastic effect. Another result is that when the testing temperature is not more than 573 K, the stress–strain curves obtained by both tests are higher than the tension curve in room temperature and the cases are opposite when testing temperature is more than 573 K. In addition, the elongation improvement is not observed as well in the stress–strain curves. The difference between the two tests when compared the cross section shrinkage rate and the fracture elongation rate is not obvious. From another aspect to study the athermal effect of the electric pulse, tests with only varying the peak current density or pulse frequency at the same testing temperature were conducted. The results once again show that no evident difference between the stress–strain curves. It confirms that no athermal effect exists in DP980. However, with temperature elevated to 473 K, the material is strengthened and then weakened as temperature exceeds 473 K. Scanning electron microscope (SEM) analysis was adopted to better understand the observed phenomena. The results show that the decomposion of the martensite and the thermal effect are the main reason that attributes to the strength variation. Fracture surface morphology analysis indicates that the appearance of the shallower and smaller dimples as well as fracture platform represents the bad plasticity when increasing temperature. Meanwhile, fracture pattern is transformed from brittle fracture in test (1) to ductile fracture in test (2) when tensioned at 673 K.