![EMPT welding of space frames at PSTproducts](https://upload.wikimedia.org/wikipedia/commons/c/c5/EMPT_welding_of_space_frames_at_PSTproducts.png)
Magnetic pulse welding (MPW) is a solid state
welding
Welding is a fabrication (metal), fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing Fusion welding, fusion. Welding is distinct from lower ...
process that uses
magnetic
Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particle ...
forces to weld two workpieces together. The welding mechanism is most similar to that of
explosion welding
Explosion welding (EXW) is a solid state (solid-phase) process where welding is accomplished by accelerating one of the components at extremely high velocity through the use of chemical explosives. This process is often used to clad carbon ste ...
.
Magnetic pulse welding started in the early 1970s, when the
automotive industry
The automotive industry comprises a wide range of company, companies and organizations involved in the design, Business development, development, manufacturing, marketing, and selling of motor vehicles. It is one of the world's largest industry ...
began to use solid state welding. The biggest advantage using magnetic pulse welding is that the formation of brittle
intermetallic
An intermetallic (also called an intermetallic compound, intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic elemen ...
phases is avoided. Therefore, dissimilar metals can be welded, which cannot be effectively joined by
fusion welding
Fusion welding is a generic term for welding processes that rely on melting to join materials of similar compositions and melting points. Due to the high-temperature phase transitions inherent to these processes, a heat-affected zone is created i ...
. With magnetic pulse welding high quality welds in similar and dissimilar metals can be made in microseconds without the need for shielding gases or welding consumables.
Process
![EMPT welding of pressure vessels at PSTproducts](https://upload.wikimedia.org/wikipedia/commons/a/a7/EMPT_welding_of_pressure_vessels_at_PSTproducts.jpg)
Magnetic pulse welding is based on a very short
electromagnetic pulse
An electromagnetic pulse (EMP), also a transient electromagnetic disturbance (TED), is a brief burst of electromagnetic energy. Depending upon the source, the origin of an EMP can be natural or artificial, and can occur as an electromagnetic fie ...
(<100 µs), which is obtained by a fast discharge of
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
s through low inductance switches into a coil. The pulsed current with a very high
amplitude
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplit ...
and
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
(500 kA and 15 kHz) produces a high-density magnetic field, which creates an
eddy current
Eddy currents (also called Foucault's currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction or by the relative motion of a conductor in a mag ...
in one of the work pieces. Repulsive
Lorentz force
In physics (specifically in electromagnetism) the Lorentz force (or electromagnetic force) is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge moving with a velocity in an elect ...
s are created and a high magnetic pressure well beyond the material yield strength causing acceleration and one of the work pieces impacts onto the other part with a collision velocity up to .
During magnetic pulse welding a high
plastic deformation
In engineering, deformation refers to the change in size or shape of an object. ''Displacements'' are the ''absolute'' change in position of a point on the object. Deflection is the relative change in external displacements on an object. Strain ...
is developed along with high shear strain and oxide disruption thanks to the jet and high temperatures near the collision zone. This leads to solid state weld due to the microstructure refinement, dislocation cells, slip bends, micro twins and local recrystallization.
Principles
In order to get a strong weld, several conditions have to be reached:
*''Jetting condition:'' the collision has to be subsonic compared to the local material's speed of sound to generate a jet.
*''High pressure regime:'' the impact velocity has to be sufficient to obtain a hydrodynamic regime, otherwise the parts will only be crimped or formed.
*''No fusion during the collision:'' If the pressure is too high, the materials can locally melt and re-solidify. This can cause a weak weld.
The main difference between magnetic pulse welding and explosive welding is that the collision angle and the velocity are almost constant during the explosive welding process, while in magnetic pulse welding they continuously vary.
Advantages of MPW
* Allows welding of designs which with other processes are challenging or not possible.
* High-speed pulse lasts from 10 to 100 µs, the only time limitation is loading and unloading and capacitor charge time.
* Lower down-time due to lack of consumable parts (e.g. electrodes) and no need for cleaning.
* Suited to mass-production: typically 1-5 million welds per year.
* Dissimilar metals welding is possible.
* Weld with no
heat-affected zone
In fusion welding, the heat-affected zone (HAZ) is the area of base material, either a metal or a thermoplastic, which is not melted but has had its microstructure and properties altered by welding or heat intensive cutting operations. The heat ...
.
* No need for filler materials.
* Green process: no smoke, no radiation and no extraction equipment required.
* Bulk and surface purity is preserved.
* Can produce welds with no shielding gas, can be used to seal parts under vacuum.
* Mechanical strength of the joint is stronger than that of the parent material.
* High precision obtainable by adjustment of magnetic field, weld parameters can be changed electronically.
* Zero distortion can be achieved depending on part materials & geometry.
* Almost zero residual stresses.
* No corrosion development in the welding area.
Disadvantages
* Challenging to apply to welds that are not roughly circular.
* Geometry of parts may have to be changed to allow the magnetic pulse process.
* If the parts cannot be slid into and out of the pulse coil a more complex multi-part coil must be designed.
* Pulse coil may have to be re-designed if materials or dimensions are changed.
* Brittle components can be fractured by the shock (does not exclude the use of materials like glass, but must be considered).
* May produce an
EMP effect on any electronics present inside or near the part.
* Initial investment cost may outweigh the lower price-per-weld for low volume parts.
Numerical simulations of MPW
Various numerical investigations were carried out to predict the interface behavior of the MPW and the in-flight behavior of the flyer to determine the collision conditions. Generally, the flyer velocity prior to the impact governs the interfacial phenomena. This is the characteristic parameter that should be known based on the process and adjustable process parameters. Although, Experimental measurements using laser velocimetry methods provide an accurate assessment of the flyer velocity, (one example of such measurement is
Photon Doppler velocimetry (PDV)), numerical computation offers a better description of the flyer velocity in terms of spatial and temporal distribution. Moreover, a multi-physics computation of the MPW process take into account of the electrical current through the coil and compute the physical behavior for an electromagnetic-mechanical coupled problem. Sometime, these simulations also allow to include the thermal effect during the process. A 3D example model used for
LS-DYNA
LS-DYNA is an advanced general-purpose multiphysics simulation software package developed by the former Livermore Software Technology Corporation (LSTC), which was acquired by Ansys in 2019. While the package continues to contain more and more p ...
simulation is also described in , and it also provides some details of the physical interactions of the process, the governing equations, the resolution procedure, and both boundary and initial conditions. The model is used to show the capability of 3D computation to predict the process behavior and particularly, the flyer kinematics and macroscopic deformation.
[I. Çaldichoury and P. L’Eplattenier, EM Theory Manual, Livermore Software Technology Corporation, California, USA, 2012.]
References
External links
''The Electromagnetic Pulse Technology (EMPT): Forming, Welding, Crimping and Cutting''by R. Schäfer, P. A. Pasquale and S. W. Kallee
''3D Impacts Modeling of the Magnetic Pulse Welding Process and Comparison to Experimental Data''by J.-P. Cuq-Lelandais*, G. Avrillaud, S. Ferreira, G. Mazars, A. Nottebaert, G. Teilla, V. Shribman
''Automotive Applications of Electromagnetic Pulse Technology (EMPT)''by S. W. Kallee, R. Schäfer and P. A. Pasquale.
''Special Issue "Impulse-Based Manufacturing Technologies"''by Verena Psyk et al, J. Manuf. Mater. Process. 2021, 5(3), 96, ISSN 2504-4494.
{{DEFAULTSORT:Magnetic Pulse Welding
Welding