A look at magnetic pulse weldingTony Pallone | June 24, 2019
The innovation of magnetic pulse welding (MPW), born in the nuclear energy industry just a half-century ago, expands the possibilities for solid-state welding through the application of electromagnetic force theory.
The technique uses short electromagnetic pulses, typically 10 to 100 microseconds in duration, to produce a high-density magnetic field in the target workpiece. This creates an opposing loop of electrical current, known as an eddy current, in the flyer piece.
The resulting repulsive forces and high magnetic pressure exceed material yield strength. The weld joint is fabricated by a high-speed (up to 500 m/s), short-distance, workpiece collision driven by the magnetic repulsion. This causes plastic deformation along the workpiece interfaces and drives the two pieces to share electrons at the atomic level. MPW creates a weld that is typically stronger than its parent materials, with near-zero residual stress.
One of the primary advantages of the approach is that, unlike fusion welding techniques, the pieces are not melted to the welding interface, which keeps material properties intact and allows similar and dissimilar metals to be joined. It can also weld metals to non-metallic materials such as ceramics, polymers, rubbers and composites. Moreover, the cold-welding process eliminates the hazardous emissions involved in fusion welding, making it an environmentally friendly manufacturing technique.
Several other manufacturing advantages are realized by MPW. The precise nature of the technique offers single-micron adjustability, while the reliability and speed of the process makes it suitable for high-volume production and automation. In addition, pre- and post-weld material cleaning is unnecessary, as metal jets form at the collision point to remove surface contamination and eliminate the development of corrosion. MPW is not an inexpensive process, but it does eliminate the need for additional resources such as shielding gases and filler materials, also known as welding consumables. And because no heat-affected zone is produced, it is possible to handle the welded parts immediately.
There are some necessary conditions for creating a strong weld through MPW. If impact force is insufficient, for instance, the pieces will only be crimped or formed. Or if pressure is too high, the materials can melt locally and create a weak weld upon re-solidification. In addition, the plastic deformation inherent in the process works best on flyer materials with good electrical conductivity. Materials with less conductivity can be used, but higher energy levels – corresponding to higher process costs – are required.
MPW is similar in nature to explosion welding, which creates acceleration force via chemical explosive. Both techniques rely upon plastic deformation, acceleration force and impact, and both can be used to join dissimilar metals. Yet explosion welding relies upon constant maintenance of collision angle and impact velocity, limiting the range of producible geometries; MPW employs a continuously variable relationship between those two factors. As a result, it supports a wide range of designs that might otherwise be impossible to create. It also does not require explosives knowledge for safe operation.
The automotive industry, in particular, has found myriad applications for the process, which is especially well suited for use with highly conductive alloys. Aluminum alloys, for instance, are a key material currently being explored to reduce vehicle body structure weight. MPW also allows metal parts containing plastic, such as those found in automotive HVAC components or fuel filters, to be welded without danger of heat damage.
Nevertheless, MPW deployment has historically been held back by the challenges of optimizing the necessary conditions. The short electromagnetic pulse used in the technique is produced by fast capacitor discharge through low inductance switches into a coil; such coils must be prepared to correspond to material shapes and types. Capacitor banks and power switches also must be sufficiently downsized for commercial viability. In recent years, however, developments in simulation software and electrical pulse power components have given way to wider MPW use.