Filler Wire is for Wimps

Magnetic pulse welding (MPW) is often viewed as a developing technology, but a growing group of experts know it as an efficient and reliable welding method filled with real-world advantages like reduction in overall cost, lowered cycle time, increases in first-time quality and elimination of heat-induced base-metal changes.

For example, we are currently investigating doors made from steel, aluminum and magnesium sheet welded with MPW into one cohesive structure that provides ideal strength properties with optimum weight savings. This is a shift from today?s paradigm, where only similar metals can be welded together.

Similarly, envision magnetic pulse forming in the assembly line to eliminate stamping operations and add differentiation in real time. These aren?t small incremental improvements, but truly revolutionary changes in the manufacturing and design model. Early adopters investigating or using magnetic pulse typically tend to be cost-conscious, with a cutting-edge willingness to change or make these radical breakthroughs.

Magnetic pulse processes include welding, forming, crimping and powder compaction. In magnetic pulse, capacitors store electrical potential and special switches are used to quickly release the energy to a work coil at high current (Figure 1).

The principle that anchors the technique relates to current flow in a wire, creating a magnetic field around that wire. Also, nearby objects that can carry electricity will have a current induced in them in the opposite direction of the current in the primary coil. These opposing currents coincide with opposing magnetic fields and cause a pressure to develop between the primary coil and the work piece. This pressure is known throughout the world of physics as the Lorentz Force.

At sufficient amounts of current, the pressure developed can be very high. Designed appropriately, this pressure can do work, such as forming sheet metal into simple one-sided dies or welding the flyer material to another metal it impacts.

MPW is comparable to explosive welding in some ways. The speed and the impact angle as the two materials strike one another is a key design component in forming a magnetic pulse weld (Figure 2). Jetting occurs and scrubs the faying surfaces in the joint of any small oxide layer. After impact, the parts are held in intimate contact at high pressure for an instant as a metallic bond is formed between the two bare metal surfaces.

Crimping and forming can be done at lower relative energy levels and currents than welding, but the work is done in a similar fashion. In these processes, high-velocity metal-to-metal impact is not required, and in many cases significantly lower speeds can be used. Markets investigating wire crimping include makers of future electric vehicles. The tension strength and extremely low resistance values across the joint created by the ultra-high pressure crimped wire terminal is outstanding (Figure 3).

Other areas of crimping interest include metal to glass, ceramic, hoses and plastics. Crimping without contact is beneficial as it allows for coated metals to be crimped without disturbing the covering. Another feature is eliminating springback in the crimped joints, which improves overall joint quality.

To date, the most common design for welding is a contiguous cross section. Pipes or tubing joined to tubular cross sections or caps are frequently seen. Other cross-sections also enjoy good applicability, including elliptical and rectangular cross sections.

Sheet metal welding with magnetic pulse is also gaining popularity. For applications with sharp corners that cannot be softened, hermetic sealing is impossible. Whether a gas-tight seal is required or not, however, the mechanical performance of the joint is superior to conventional welding and brazing processes. Simple application examples exist in automotive HVAC and fuel-filter applications (Figure 4).

The other significant advantages brought by MPW include:

Like any welding process, the joint needs to be designed for a successful weld. Specifically, an overlap and gap is required (Figure 5). Typically the joint tolerance is less restrictive than tolerances applied in brazing or welding. For future products, it is important to design with MPW in mind.

In terms of cost, MPW equipment has a higher upfront capital outlay than many conventional arc welding or torch brazing processes, but typically lower than other "new" welding processes. Recent results confirm the payback on the capital can happen quickly through increases in quality, decreases in cycle time and reductions in variable joint costs.

Magnetic pulse is not perfect for welding all materials. Materials with higher yield strength and lower conductivity can be problematic. There are examples of materials to avoid, but in general, for dissimilar metal joints, one of the two materials is typically suitable to be the moving material, even if the other is not. Crimping is an exception and can be done nicely with a truly wide range of materials.

A discussion of MPW would not be complete without explaining weld quality and process evaluation practices. In general, magnetic pulse welded parts can be inspected the same way their conventional welded counterparts are.

In a production environment, inline helium leak checks or regular destructive tests are typically still employed. Additionally, like in explosive welding, ultrasonic non-destructive testing is proving to be a reliable way to check MPW. Results of magnetic pulse welded parts exposed to high pressure, helium leak and mechanical shear tension and torsion testing have been outstanding. Results in salt-spray environments, like those consistent with automotive testing, are superior to analogous braze results.

Mike Blakely is the operations manager at Hirotec America, 4567 Glenmeade Lane, Auburn Hills, MI 48326, 248-836-5100, Fax: 248-836-5101, [email protected], www.hirotecamerica.com.