General Motor China Science Laboratory

Nomhon, China

General Motor China Science Laboratory

Nomhon, China
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Xie H.,Shanghai JiaoTong University | Dong X.,Shanghai JiaoTong University | Liu K.,Shanghai JiaoTong University | Ai Z.,Shanghai JiaoTong University | And 4 more authors.
Materials Science and Engineering A | Year: 2015

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. © 2015 Elsevier B.V.


Xie H.,Shanghai JiaoTong University | Wang Q.,Shanghai JiaoTong University | Liu K.,Shanghai JiaoTong University | Peng F.,Shanghai JiaoTong University | And 2 more authors.
Journal of Materials Processing Technology | Year: 2015

The influence of electrical parameters pulse frequency (f) and peak current density (J) on the springback of AZ31B magnesium alloy sheet was studied through electrical pulse-assisted (EPA) V-bending at 373 K. To do this, a current pulse generator and a bending setup were designed and manufactured. In the test, controlling variable method in which only f or J can be changed while other testing parameters were kept constant was adopted. The results show that at the same testing temperature 373 K, varying f or J, different springback angle can be deserved: the higher the f or J, the smaller the springback angles. In addition, at 373 K, increase of effective current also leads to evident reduction in springback angle. It demonstrates that athermal effect of the electrical pulses exists in EPA V-bending of this material. To understand the mechanism of the EPA V-bending, microstructure evolution in the bent part was studied through optical microscope. The results indicate that the electrical parameters f and J not only decrease the grain size at the bent part, the higher the f or J, the finer the crystals but also decrease the number of twinning crystals during EPA V-bending. Both the decrease of grain size and the number of twinning crystals are considered to contribute to springback reduction. © 2014 Elsevier B.V.


Xie H.,Shanghai JiaoTong University | Wang Q.,Shanghai JiaoTong University | Peng F.,Shanghai JiaoTong University | Wang J.,General Motor China Science Laboratory | Dong X.,Shanghai JiaoTong University
Journal of Materials Processing Technology | Year: 2015

The influence of electrical parameters pulse frequency (f) and peak current density (J) on the springback of AZ31B magnesium alloy sheet was studied through electrical pulse-assisted (EPA) V-bending at 373. K. To do this, a current pulse generator and a bending setup were designed and manufactured. In the test, controlling variable method in which only f or J can be changed while other testing parameters were kept constant was adopted. The results show that at the same testing temperature 373. K, varying f or J, different springback angle can be deserved: the higher the f or J, the smaller the springback angles. In addition, at 373. K, increase of effective current also leads to evident reduction in springback angle. It demonstrates that athermal effect of the electrical pulses exists in EPA V-bending of this material. To understand the mechanism of the EPA V-bending, microstructure evolution in the bent part was studied through optical microscope. The results indicate that the electrical parameters f and J not only decrease the grain size at the bent part, the higher the f or J, the finer the crystals but also decrease the number of twinning crystals during EPA V-bending. Both the decrease of grain size and the number of twinning crystals are considered to contribute to springback reduction. © 2014 Elsevier B.V.


Xie H.,Shanghai JiaoTong University | Dong X.,Shanghai JiaoTong University | Wang Q.,Shanghai JiaoTong University | Peng F.,Shanghai JiaoTong University | And 4 more authors.
Mechanics of Materials | Year: 2016

Advanced high strength steels have been increasingly used in auto body manufacturing industry to reduce weight and increase safety. However springback often occurs after stamping and seriously affects the forming accuracy, which is one of the bottle necks of its application. Targeting to control the springback, a novel method which employs the electric pulses to promote the relaxation of residual stress is investigated. In order to achieve this goal, electrically-assisted stress relaxation tests were carried out which shows that at a relative lower temperature, 373 K, the residual stress decreases with increasing electric parameters, and the athermal effect of the electric pulses is observed. A model of electrically-assisted stress relaxation is proposed. By combining the proposed stress relaxation model and the perfect plastic material model, a springback angle prediction model considering electric pulses is also proposed. The proposed models are verified with experimental results. © 2015 Elsevier Ltd.


Xie H.,Shanghai JiaoTong University | Dong X.,Shanghai JiaoTong University | Ai Z.,Shanghai JiaoTong University | Wang Q.,Shanghai JiaoTong University | And 4 more authors.
International Journal of Advanced Manufacturing Technology | Year: 2016

The effect of the electric pulses on the ductility of AZ31B magnesium alloy sheet was studied through cylindrical deep drawing test. In order to show the advantages of the electric pulses during this test, deep drawing tests both at room temperature and at elevated temperatures induced by the electric pulses were carried out using the self-designed device which only allows the electric pulses flow through the flange and die corner regions. The result shows that as temperature rises, the limit of the deep drawing depth increases. The limit of the deep drawing depth increases even faster when the temperature is more than 373 K. Moreover, to find out whether the athermal effect exists or not in this process, experiments with only varying the current frequency and peak current at the same temperature were conducted. The result confirms the existence of the athermal effect. Microstructure evolution analysis shows that the dynamic recrystallization induced by the electric pulses at a relatively lower temperature facilitates the ductility improvement. © 2016 Springer-Verlag London

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