How Pulsed Current Improves Ti-6Al-4V V-Bending

Ti-6Al-4V has poor room-temperature formability and a high yield-strength-to-elastic-modulus ratio. Because springback increases with this ratio, the springback angle is large and final dimensions are difficult to control. To improve V-bending performance, this study examines how pulsed-current parameters, specifically supply voltage and pulse frequency, affect part quality, forming load, and springback. It identifies useful process parameters and explains the microscopic mechanisms of electroplastic bending.

Effect of Current Density on Ti-6Al-4V V-Bending

Effect of Current Density on Forming Force

The electroplastic V-bending tests were performed on a SANS tensile testing machine. Bakelite insulated the upper and lower bending dies from the machine, and electrodes applied pulsed current to the specimen through the dies. Each rectangular sheet specimen measured 50 mm × 30 mm. The crosshead speed was 10.5 mm/min, the bend radius was 1 mm, the bend angle was 90°, and the same displacement was applied to every specimen. Different supply voltages and pulse frequencies were used to evaluate their effects on springback and forming force.

At a pulse frequency of 200 Hz, the force-displacement curve has three stages. First, forming force rises with deformation during elastic bending. Second, displacement continues to increase while the force changes little, producing a stable-deformation stage. Finally, when the workpiece contacts the lower die, the force rises sharply during holding and continues until bending ends. Compared with room-temperature bending, pulsed current shortens the elastic stage and substantially reduces the stable-stage force. Increasing effective current density shortens the elastic stage further and produces a larger force reduction. At 38.67 A/mm², the stable-stage force was 568 N, 49.01% below the 1,114 N required at room temperature. This reduction can lower energy consumption and production cost.

Effect of Current Density on Forming Quality

At room temperature, the Ti-6Al-4V specimen fractured severely in the V-bend region, with long, deep cracks and poor surface quality. With a pulsed effective current density of 14.00 A/mm², the bent surface was smooth and free of cracks. Pulsed current increases elongation and lowers yield strength, helping prevent cracking and improve forming quality.

Forming quality of electroplastic V-bent specimens at different effective current densities:

During bending, pulsed current reduces both the yield strength and work hardening of Ti-6Al-4V. Lower yield strength and less work hardening reduce springback. The current therefore improves deformation performance and decreases the springback angle. As effective current density rises, yield strength and work hardening continue to decrease, so springback becomes progressively smaller.

Effect of Pulse Frequency on Ti-6Al-4V V-Bending

Tests at different frequencies show that the stable-stage forming force decreases as pulse frequency rises, whether the supply voltage is 80 or 100 V. A higher pulse frequency therefore reduces the load required for bending. At a given supply voltage, peak current density remains the same, but effective current density increases with frequency, making the electroplastic effect more pronounced.

The improved elongation and bending performance of Ti-6Al-4V under pulsed current result mainly from Joule heating and the intrinsic electroplastic effect. The intrinsic effect is associated with the force exerted by drifting electrons on dislocations, often called the electron-wind force, which reduces the driving force required for dislocation motion. Because the force from drifting electrons is proportional to current density, a higher effective current density strengthens the intrinsic electroplastic effect. It also raises temperature and increases Joule heating. The material consequently softens, dislocation tangles and dislocation density decrease, elongation rises, and stress and yield strength fall. Pulsed current therefore improves specimen quality while reducing springback and forming force, with greater improvements at higher effective current density.