Improvement of Formability and Corrosion Resistance of Az31 Magnesium Alloy by Pulsed Current-assisted Laser Shock Forming

This study adopted a novel pulse current-assisted laser shock AZ31B sheet micro-forming method (EP-LSF). The mechanism of improving the formability of AZ31B Magnesium Alloy by pulse current assisted laser shock forming and the reason of improving the corrosion resistance were studied for the first time. Through laser shock free bulging experiment, tensile test, optical microscope (OM), and X-ray diffraction, the change in formability was studied. After pulse current assisted laser shock forming, the forming height of AZ31B magnesium alloy increases by 28.8%, the thinning rate decreases by 6.7%, and the strain rate sensitivity coefficient increases to 0.1452. The results show that the decrease of grain size and texture density is the reason why EP-LSF can further improve the formability of AZ31B magnesium alloy. The changes in corrosion resistance were studied by scanning electron microscopy and electrochemical tests. The results show that after EP-LSF, the corrosion current density of AZ31 magnesium alloy decreased, and the electrochemical impedance increased, indicating that this method further improved the corrosion resistance.

Acquisition and Evaluation of Theoretical Forming Limit Diagram of Al 6061-T6 in Electrohydraulic Forming Process

The current study examines the forming limit diagram (FLD) of Al 6061-T6 during the electrohydraulic forming process based on the Marciniak–Kuczynski theory (M-K theory). To describe the work-hardening properties of the material, Hollomon’s equation—that includes strain and strain rate hardening parameters—was used. A quasi-static tensile test was performed to obtain the strain-hardening factor and the split-Hopkinson pressure bar (SHPB) test was carried out to acquire the strain rate hardening parameter. To evaluate the reliability of the stress–strain curves obtained from the SHPB test, a numerical model was performed using the LS–DYNA program. Hosford’s yield function was also employed to predict the theoretical FLD. The obtained FLD showed that the material could have improved formability at a high strain rate index condition compared with the quasi-static condition, which means that the high-speed forming process can enhance the formability of sheet metals. Finally, the FLD was compared with the experimental results from electrohydraulic forming (EHF) free-bulging test, which showed that the theoretical FLD was in good agreement with the actual forming limit in the EHF process.

Characterization of Stress-Strain Behavior of Superplastic Titanium Alloy by Free Bulging Tests with Pressure Jumps

The work is dedicated to determination of stress-strain behavior of Ti6Al4V alloy deformed in conditions of biaxial tension provided by free bulging testing. The dome height during each test was continuously measured and recorded using a magnetostrictive position transducer. All the tests were performed using stepped pressure regime with jump pressure changing between two values at evenly spaced time moments. This experimental technique provides the possibility to study strain rate sensitivity index variation during the test and subsequently construct strain and strain rate dependent material model. The output data of each test include the evolution of dome height, subsequent pressure regime and final thickness of the specimen at the dome pole. In the framework of this study the processing of such data in order to evaluate the material behavior is discussed. Inverse analysis with different material models was implemented as well as special direct technique allowing one to construct stress-strain curves based on the results of free bulging tests with pressure jumps. The obtained material model was verified by finite element simulation.

Process Planning for the Lateral Extrusion of a Pipe with a Lost Core

In this paper, the authors discuss process planning for the lateral extrusion of a pipe with a lost core. In this process, maximum longitudinal length of the bulged part is restricted by the balance of the extrusion speed of the material and the lost core. In the free bulging condition, longitudinal length is limited to the pipe radius, because the extrusion speed of the core is slower than that of the pipe material when the longitudinal length of the bulged part is longer. The authors designed a two-stage forming process using the transit shape of a truncated cone to solve this problem. The dimensions of the truncated cone were estimated through trial-and-error using a commercial FEM simulator and considering the stretch effect for wrinkles of the pipe by deformation and traveling of the lost core. Finally, the authors conducted experiments to confirm the design’s validity. As a result, a longer longitudinal length of the bulged part than the pipe radius was successfully obtained.