Experimental Investigation of Electromagnetic Forming for TC4 Titanium Alloy Pipe Fitting End Correction

Literature Overview

This paper by Nie Peng et al., published in Machinery Design and Manufacturing (2019, Issue 7, pp. 135-138), investigates electromagnetic forming (EMF) as a solution to the long-standing challenge of end-of-pipe correction for TC4 (Ti-6Al-4V) titanium alloy fittings. The research was conducted at the Shenyang Aerospace University's National Key Discipline Laboratory for Digital Aviation Manufacturing Technology, supported by the Liaoning Provincial Natural Science Foundation (201602564). The study focuses on a specific engineering problem: the poor room-temperature formability of titanium alloys in the aerospace sector, which makes conventional mechanical correction of pipe fitting ends impractical.

Problem Statement and Technical Background

TC4 is the most widely used titanium alloy in aerospace applications, valued for its excellent specific strength, fatigue resistance, and corrosion resistance. However, its low room-temperature ductility and high elastic modulus create significant challenges for post-forming correction operations. Conventional methods such as mechanical mandrel drawing or hydraulic expansion often introduce unacceptable plastic deformation, residual stresses, or surface damage. Electromagnetic forming offers a non-contact, high-strain-rate alternative that can achieve precise dimensional correction without mechanical contact, making it particularly attractive for sensitive aerospace components.

The experimental specimen was a TC4 pipe with an inner diameter of 23 mm and a wall thickness of 1 mm. The end roundness of the pipe fitting was adopted as the primary evaluation metric, as dimensional accuracy at connection interfaces is critical for bolted or welded joint integrity in aerospace structures.

Key Experimental Parameters and Results

The most significant finding concerns the driver plate thickness. For TC4, a low-electrical-conductivity material, the driver plate thickness must be carefully selected to match the electromagnetic skin depth. If the driver plate is too thin, insufficient electromagnetic force is transmitted to the workpiece. If too thick, excessive eddy current losses reduce the effective forming force. The optimal thickness corresponds to the skin depth, which for titanium alloys at typical EMF frequencies (50-200 kHz) ranges from approximately 0.3 to 0.6 mm.

Technical Analysis and Interpretation

The electromagnetic forming process relies on the Lorentz force generated by the interaction between the transient magnetic field and the induced eddy currents in the conductive workpiece. For low-conductivity materials like TC4 (electrical conductivity approximately 2.6 MS/m, compared to 58 MS/m for aluminum), the skin depth is larger, meaning the electromagnetic force penetrates deeper into the material. However, the lower conductivity also means weaker eddy currents and thus lower forming forces.

The finding that two discharge pulses are sufficient is practically significant. Multiple pulses increase process complexity, energy consumption, and potential for cumulative thermal effects that could alter the microstructure of the titanium alloy. The diminishing returns beyond two pulses suggest that the material reaches a quasi-steady deformation state after the second pulse, where the strain hardening rate exceeds the rate of new deformation initiation.

The discharge voltage serves as the primary control parameter because it directly determines the peak current and, consequently, the peak Lorentz force. The coil layer number acts as a secondary parameter, as it modifies the magnetic field distribution and concentration without changing the fundamental energy input. This hierarchy is consistent with electromagnetic forming theory, where the force is proportional to the square of the current density.

Engineering Practice Considerations

For aerospace applications, the electromagnetic forming of TC4 pipe fitting ends requires careful attention to several quality aspects. First, the dimensional accuracy must be verified using coordinate measuring machine (CMM) or optical profilometry, as the correction tolerances are typically within ±0.05 mm for aerospace fittings. Second, residual stress measurement through X-ray diffraction (XRD) should be performed to ensure that the electromagnetic forming does not introduce excessive compressive or tensile stresses that could affect fatigue performance. Third, surface integrity inspection is essential, as electromagnetic forming can cause micro-cracking or surface roughening in thin-walled titanium components.

The skin depth concept is particularly important for process design. Engineers should calculate the skin depth for the specific material and frequency combination before selecting the driver plate thickness. The formula δ = √(2ρ/ωμ) provides the theoretical skin depth, where ρ is the electrical resistivity, ω is the angular frequency, and μ is the magnetic permeability. For TC4 at 100 kHz, this yields a skin depth of approximately 0.45 mm, which aligns with the experimental finding that the optimal driver plate thickness corresponds to this value.

Study Insights and Implications

This research contributes valuable experimental data for the electromagnetic forming of low-conductivity materials, a topic that has received relatively limited attention compared to aluminum and copper alloys. The systematic investigation of four parameters provides a practical framework for process development. The finding that discharge voltage is the dominant parameter simplifies process optimization, as engineers can prioritize voltage adjustment before fine-tuning coil geometry and driver plate dimensions.

A key implication for aerospace manufacturing is the potential for integrating electromagnetic forming into automated production lines for titanium fittings. The non-contact nature of the process eliminates tool wear and reduces contamination risk, both critical concerns for aerospace-grade titanium components. Future research should explore the scalability of this approach to larger diameter fittings and investigate the interaction between electromagnetic forming and subsequent welding or fastening operations.