Experimental Study on Electromagnetic Forming of TC4 Pipe Fitting Ends

Literature Overview

This paper, published in Machine Design and Manufacturing (Vol. 2019, No. 7, pp. 135-138) by Nie Peng et al. from Shenyang Aerospace University, addresses a long-standing challenge in aerospace manufacturing: the correction of roundness deviation at the ends of TC4 (Ti-6Al-4V) titanium alloy pipe fittings. The authors investigate electromagnetic forming (EMF) as a solution, studying the effects of discharge voltage, coil turns, discharge count, and driver thickness on the roundness of TC4 pipe fitting ends. The research was funded by the Liaoning Provincial Natural Science Foundation.

TC4 is a critical aerospace alloy used extensively in aircraft structures due to its excellent strength-to-weight ratio, fatigue resistance, and corrosion resistance. However, titanium alloys have poor room-temperature formability, making conventional cold forming methods difficult to apply. The electromagnetic forming process, which uses pulsed electromagnetic forces to deform the workpiece, offers a non-contact, high-speed forming alternative that is well-suited for difficult-to-form materials like titanium alloys.

Electromagnetic Forming Process Fundamentals

Electromagnetic forming operates on the principle of electromagnetic induction. A high-voltage capacitor bank discharges through a forming coil, generating a rapidly changing magnetic field. When a conductive workpiece is placed near the coil, eddy currents are induced in the workpiece, which interact with the magnetic field to produce a Lorentz force. This force acts as a radial pressure on the workpiece, causing it to deform plastically against a die.

The key process parameters studied in this research are:

The skin depth is a critical concept in electromagnetic forming. It is the depth at which the electromagnetic field (and hence the induced current and Lorentz force) decays to 1/e of its surface value. For TC4 titanium alloy, which has a relatively low electrical conductivity (~2.5 MS/m compared to ~60 MS/m for aluminum), the skin depth is larger than for more conductive materials. The driver thickness must be optimized to match the skin depth: if the driver is too thin, it cannot transmit sufficient force; if it is too thick, the outer layers contribute little to the forming force but add mass that must be accelerated.

Experimental Configuration and Results

The experimental setup used TC4 pipe fittings with an inner diameter of 23 mm and a wall thickness of 1 mm. These dimensions are representative of small-diameter aerospace tube fittings used in hydraulic and fuel systems. The roundness of the pipe end was used as the primary quality metric.

The experimental findings can be summarized as follows:

Discharge voltage effect: Increasing the discharge voltage significantly improves the roundness correction. This is because higher voltage produces stronger magnetic fields and greater Lorentz forces, resulting in more uniform radial pressure on the pipe wall. However, excessively high voltage can lead to over-deformation and surface damage.

Coil turns effect: At a fixed discharge voltage, increasing the number of coil turns improves the forming precision. More turns increase the energy density and create a more uniform magnetic field distribution around the pipe circumference, leading to more uniform deformation.

Discharge count effect: Multiple discharges can progressively correct roundness deviations. However, the improvement from the second discharge is significantly less than from the first, and additional discharges beyond two do not substantially improve roundness. This suggests that the initial discharge corrects the bulk of the deviation, while subsequent discharges address residual errors with diminishing effectiveness.

Driver thickness effect: The optimal driver thickness corresponds to the skin depth of the electromagnetic field in TC4. Drivers that are too thin cannot effectively transmit the electromagnetic force to the pipe wall, while drivers that are too thick have excessive mass that reduces the acceleration and force efficiency. The optimal thickness is at the skin depth, where the force transmission is maximized.

Material-Specific Considerations for Titanium Alloy Forming

The low electrical conductivity of TC4 (approximately 2.5 MS/m) presents unique challenges for electromagnetic forming. Compared to aluminum (60 MS/m) or copper (59.6 MS/m), TC4 requires:

The lower conductivity of TC4 means that the induced eddy currents are weaker, resulting in lower Lorentz forces for the same discharge energy. To compensate, the discharge energy must be increased (higher voltage or more coil turns), or the driver design must be optimized to maximize force transmission efficiency.

The higher yield strength of TC4 also means that the forming force must exceed 900 MPa to initiate plastic deformation. This is a significantly higher threshold than for aluminum or steel, requiring higher energy input. However, the electromagnetic forming process can achieve strain rates of 10^3 to 10^5 s^-1, at which TC4 exhibits strain rate hardening and potentially elevated flow stress. The dynamic flow stress of TC4 at high strain rates can be 20-30% higher than the quasi-static yield strength, further increasing the energy requirement.

Process Optimization Strategy

Based on the experimental findings, a systematic process optimization strategy can be developed:

1. Determine the skin depth for the specific TC4 alloy composition and the discharge frequency characteristics of the system.
2. Select the driver thickness to match the skin depth for optimal force transmission.
3. Set the initial discharge voltage to achieve the minimum energy required for plastic deformation (above the yield threshold).
4. Configure the coil turns to achieve the desired energy density and field uniformity.
5. Apply the first discharge and measure the roundness improvement.
6. Apply a second discharge if the roundness target is not met after the first discharge.
7. Do not apply additional discharges beyond two, as the marginal improvement is negligible.

This strategy can be implemented in a production environment with in-process roundness measurement between discharges, enabling adaptive control of the forming process.

Study Insights and Implications

This research addresses a practical manufacturing challenge that has limited solutions in current aerospace manufacturing. The roundness correction of TC4 pipe fitting ends is critical for applications requiring precise sealing (such as hydraulic connections, fuel system fittings, and bearing housings). Conventional methods—such as mandrel drawing, rotary swaging, or machining—each have limitations: mandrel drawing requires significant tooling investment, rotary swaging is limited by the machine's stroke and speed capabilities, and machining introduces work hardening and residual stresses that can affect fatigue life.

Electromagnetic forming offers a non-contact, high-speed alternative that avoids the tooling constraints of conventional methods and does not introduce residual stresses in the same way as machining. The process is also inherently repeatable, as the electromagnetic force is applied uniformly around the circumference of the pipe, leading to consistent roundness correction.

The finding that two discharges are sufficient for effective roundness correction is practically important. It means that the process cycle time is short, making the method suitable for high-volume production. The diminishing returns from additional discharges also suggest that the process reaches a natural equilibrium where the residual roundness deviation is determined by the material's elastic recovery and the precision of the die, rather than by the forming energy.

The skin depth optimization of the driver thickness is a fundamental insight that has broader applicability. For any electromagnetic forming application involving low-conductivity materials (titanium alloys, stainless steels, nickel-based superalloys), the driver thickness must be carefully selected based on the material's skin depth. This principle can be extended to other electromagnetic forming applications, such as electromagnetic spinning, electromagnetic deep drawing, and electromagnetic rolling.

The research also highlights the importance of process parameter interaction. No single parameter operates independently; the discharge voltage, coil turns, and driver thickness all interact to determine the effective forming force and deformation uniformity. This interaction complexity is why orthogonal experimental design or similar systematic approaches are necessary for process optimization, rather than relying on single-parameter studies.

For engineers in the aerospace manufacturing sector, this paper provides a validated process route for a challenging forming operation. The combination of electromagnetic forming with systematic parameter optimization represents a modern manufacturing approach that leverages high-strain-rate deformation physics to overcome the limitations of conventional forming methods for difficult-to-form materials.