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Induction welding
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Induction welding is a form of welding that uses electromagnetic induction to heat the workpiece. The welding apparatus contains an induction coil that is energised with a radio-frequency electric current. This generates a high-frequency electromagnetic field that acts on either an electrically conductive or a ferromagnetic workpiece. In an electrically conductive workpiece, the main heating effect is resistive heating, which is due to induced currents called eddy currents. In a ferromagnetic workpiece, the heating is caused mainly by hysteresis, as the electromagnetic field repeatedly distorts the magnetic domains of the ferromagnetic material. In practice, most materials undergo a combination of these two effects.
Nonmagnetic materials and electrical insulators such as plastics can be induction-welded by implanting them with metallic or ferromagnetic compounds, called susceptors, that absorb the electromagnetic energy from the induction coil, become hot, and lose their heat to the surrounding material by thermal conduction.[1] Plastic can also be induction welded by embedding the plastic with electrically conductive fibers like metals or carbon fiber. Induced eddy currents resistively heat the embedded fibers which lose their heat to the surrounding plastic by conduction. Induction welding of carbon fiber reinforced plastics is commonly used in the aerospace industry.
Induction welding is used for long production runs and is a highly automated process, usually used for welding the seams of pipes. It can be a very fast process, as a lot of power can be transferred to a localised area, so the faying surfaces melt very quickly and can be pressed together to form a continuous rolling weld.
The depth that the currents, and therefore heating, penetrates from the surface is inversely proportional to the square root of the frequency. The temperature of the metals being welded and their composition will also affect the penetration depth. This process is very similar to resistance welding, except that in the case of resistance welding the current is delivered using contacts to the workpiece instead of using induction.
Induction welding was first discovered by Michael Faraday. The basics of induction welding explain that the magnetic field's direction is dependent on the direction of current flow. and the field's direction will change at the same rate as the current's frequency. For example, a 120 Hz AC current will cause the field to change directions 120 times a second. This concept is known as Faraday's Law.
When induction welding takes place, the work pieces heat up to under the melting temperature and the edges of the pieces are placed together impurities get forced out to give a solid forge weld.[2]
Induction welding is used for joining a multitude of thermoplastics and thermosetting matrix composites. The apparatus used for induction welding processes includes a radio frequency power generator, a heating station, the work piece material, and a cooling system.
The power generator comes in either the form of solid state or vacuum tube and is used to provide an alternating current of 230-340 V or a frequency of 50–60 Hz to the system. This value is determined by what induction coil is used with the piece.
The heat station utilizes a capacitor and a coil to heat the work pieces. The capacitor matches the power generators output and the induction coil transfers energy to the piece. When welding the coil needs to be close to the work piece to maximize the energy transfer and the work piece used during induction welding is an important key component of optimal efficiency.[3]
Some equations to consider for induction welding include:
Thermal calculation:
Where: is thermal mass
is resistivity
is efficiency
is surface density
Newton Cooling Equation:
Where: is heat flux density
h is the heat transfer coefficient
is the temperature of the work piece surface
is the temperature of the surrounding air[4]
See also
[edit]References
[edit]- ^ Babini, A; Forzan (January 2002). "Eddy Current Distribution in a Thin Aluminum Layer" (PDF). Flux Magazine (38): 11–12. Archived from the original (PDF) on 2014-03-26. Retrieved 9 Mar 2015.
- ^ "Induction Welding". Thermatool Corp. Retrieved 2019-02-15.
- ^ Lionetto, Francesca; Pappadà, Silvio; Buccoliero, Giuseppe; Maffezzoli, Alfonso (2017-04-15). "Finite element modeling of continuous induction welding of thermoplastic matrix composites". Materials & Design. 120: 212–221. doi:10.1016/j.matdes.2017.02.024.
- ^ "Scientific.net". www.scientific.net. Retrieved 2019-02-15.
- AWS Welding Handbook, Volume 2, 8th Edition
- Davies, John; Simpson, Peter (1979), Induction Heating Handbook, McGraw-Hill, ISBN 0-07-084515-8.
Induction welding
View on Grokipediafrom Grokipedia
Fundamentals
Definition and overview
Induction welding is a non-contact welding process that employs electromagnetic induction to generate heat within the workpiece or an inserted implant, enabling the coalescence of materials without the use of flames, arcs, or direct physical contact between the heat source and the joint. This method relies on alternating current passed through an induction coil to create a rapidly changing magnetic field, which induces heating through mechanisms such as eddy currents in conductive materials. The process is particularly valued for its precision, cleanliness, and ability to produce strong, consistent welds in both automated and high-volume production environments.[9] The technique encompasses two primary categories: metallic induction welding and thermoplastic implant induction welding. In metallic induction welding, applicable to electrically conductive metals like steel, aluminum, and copper alloys, heat is generated directly within the base material near the joint interface, often using high-frequency currents to form continuous seams in applications such as pipe and tube manufacturing. Conversely, thermoplastic induction welding targets non-conductive polymers and composites, where a susceptor implant—typically metal mesh, fibers, or particles embedded at the joint—absorbs the induced energy to melt the surrounding matrix material, facilitating fusion under applied pressure. This categorization allows induction welding to address diverse material classes, from metals to engineering plastics like nylon and polyethylene.[9][10] Key characteristics of induction welding include its high-speed capability, with welding rates reaching up to several hundred meters per minute for thin-walled steel pipes, making it ideal for continuous production lines. It excels in creating uniform, hermetic seams without fillers, leveraging alternating current frequencies (typically 10-450 kHz) to induce eddy currents for resistive heating in conductors or hysteresis losses in ferromagnetic susceptors. The process demands materials that are electrically conductive for direct heating or augmented with susceptors for non-conductive substrates, ensuring efficient energy transfer and minimizing distortion. These attributes contribute to its automation potential and suitability for industries requiring rapid, repeatable joins, such as automotive and aerospace manufacturing.[9][11][12]Physical principles
Induction welding operates on the principle of electromagnetic induction, where a time-varying magnetic field induces an electromotive force (EMF) in a nearby conductor, as described by Faraday's law of induction. This law states that the magnitude of the induced EMF is equal to the negative rate of change of magnetic flux through the conductor: Here, represents the magnetic flux, with as the magnetic field and as the differential area element. In practice, an alternating current (AC) flowing through an induction coil generates an oscillating magnetic field that links with the workpiece, inducing voltages that drive currents within it. This fundamental mechanism enables non-contact heating essential for welding both metals and thermoplastics.[13][14] Heat generation in induction welding arises primarily from two mechanisms in conductive materials: eddy currents and, for ferromagnetics, hysteresis losses. Eddy currents are circulating loops of induced current within the conductor that encounter electrical resistance, leading to Joule heating proportional to , where is the current and is the resistance. These currents are confined near the surface due to the skin effect but effectively raise the temperature of metallic workpieces to forging temperatures required for solid-state bonding. In ferromagnetic materials, such as iron or steel, additional heating occurs through hysteresis losses, where energy is dissipated as the magnetic domains repeatedly magnetize and demagnetize in response to the alternating field, with loss magnitude depending on the material's coercivity and the field's amplitude. For thermoplastic welding, non-conductive polymers incorporate a susceptor—a thin, conductive implant like metal mesh or particles—where induced eddy currents generate localized Joule heating at the joint interface, melting the surrounding plastic without broadly affecting the bulk material.[15][16][17][8] The frequency of the AC supply plays a critical role in controlling the depth and efficiency of heating through the skin effect, which limits current penetration. Typical frequencies for induction welding range from 10 kHz to 450 kHz, allowing precise adjustment for material thickness and desired heat zone. The skin depth , defined as the distance over which current density falls to of its surface value, is given by where is the angular frequency, is the magnetic permeability, and is the electrical conductivity. Higher frequencies reduce , concentrating eddy currents and heating near the surface, which is advantageous for thin welds or to minimize distortion in deeper sections.[18][19][20] The power delivered to the workpiece depends on the coil's design, including current magnitude and number of turns, which determine the magnetic field strength . For a solenoid-like coil, , where is the number of turns, is the current, and is the coil length. The resulting induced power from eddy current heating can be approximated as where is the effective penetration depth and is the resistivity; this simplified expression highlights the quadratic dependence on frequency and field strength, underscoring the need for balanced parameters to achieve efficient, controlled energy input without excessive coil losses.[21][22][23]Historical development
Origins in electromagnetic induction
The foundational principles of induction welding trace back to the discovery of electromagnetic induction in the early 19th century. In 1831, Michael Faraday conducted pivotal experiments that demonstrated how a changing magnetic field could induce an electric current in a conductor. Using a device known as the ring coil apparatus, Faraday wrapped two insulated coils of wire around an opposite sides of an iron ring; when he connected one coil to a battery and interrupted the current, a momentary current was induced in the secondary coil, even without direct electrical connection. This observation extended to nearby conductors, where moving a magnet near a wire loop generated detectable currents, establishing the core mechanism of electromagnetic induction.[24][25] Building on Faraday's work, other scientists refined the understanding of induction throughout the 19th century, laying groundwork for practical applications. American physicist Joseph Henry independently discovered electromagnetic induction around 1832, while experimenting with electromagnets; he observed both mutual induction between coils and self-induction within a single coil, which delayed current changes and enhanced efficiency in electromagnetic devices. Concurrently, in 1834, Heinrich Lenz formulated Lenz's law, stating that an induced current creates a magnetic field opposing the change in magnetic flux that produced it, a principle essential for predicting the direction and behavior of induced currents. These insights enabled early applications, such as improvements in telegraphy—where Henry's high-efficiency relays facilitated long-distance signal transmission—and the construction of rudimentary electromagnetic motors, which converted electrical energy into mechanical motion without physical contact.[26][27] By the late 19th century, researchers began recognizing induction's potential for heat generation, marking the transition toward thermal applications. In 1887, British electrical engineer Sebastian Ziani de Ferranti patented the first prototype for an induction furnace, described in British Patent Specification No. 700, which utilized alternating current in a coil to induce eddy currents in a metal charge, thereby generating heat without direct contact. This innovation built on the growing availability of alternating current systems and highlighted induction's efficiency for non-contact heating. Extending this into the early 20th century, in 1916, Edwin F. Northrup at Princeton University developed the first practical high-frequency coreless induction furnace, capable of melting metals like brass and steel at temperatures up to 1,200°C using a spark-gap power supply; this milestone validated induction as a reliable method for controlled, uniform heating, directly informing subsequent welding technologies.[28][29]Industrial adoption and advancements
Induction welding emerged in the 1920s and 1930s with the development of high-frequency generators utilizing spark-gap oscillators, enabling initial experiments in metal joining.[30] In the 1930s, German engineers, including developments by firms like Oestringer Maschinenbau around 1930, pioneered the application of high-frequency induction heating for sheet metal welding and the first machines for tubes and pipes, marking a key step toward practical industrial use.[31] These early systems laid the groundwork for high-frequency induction welding of tubes and pipes, focusing on longitudinal seams in welded pipe production.[32] The process saw accelerated adoption during World War II in the 1940s, particularly for military applications requiring high-speed joining. Vacuum tube oscillators improved the efficiency of these systems, allowing for more reliable operation in demanding wartime production environments.[29] This period marked a shift from experimental setups to mass-scale implementation, driven by the need for robust, rapid welding in defense manufacturing.[30] Post-war innovations in the 1950s and 1960s included the transition to solid-state power supplies, which offered greater reliability and efficiency compared to earlier vacuum tube designs.[29] The 1960s also saw the introduction of implant induction welding methods for thermoplastics, pioneered by companies like Emabond Systems, enabling precise heating of embedded susceptors for plastic assembly.[33] From the 1980s onward, advancements such as IGBT-based inverters provided precise control over heating parameters, enhancing weld quality and process repeatability in industrial settings.[34] Integration with robotics in the 2000s facilitated automated induction welding in sectors like automotive and aerospace, supporting high-volume production of complex components.[35] Energy-efficient designs developed during this era, incorporating optimized power electronics, improved inverter efficiency and reduced losses.Metallic induction welding
Process description
Metallic induction welding is a high-frequency (HF) solid-state welding process primarily used for joining conductive metals such as low-carbon steel, stainless steel, aluminum, and alloys by generating localized heat through electromagnetic induction without melting the base material. The process relies on the skin effect, where induced eddy currents concentrate heating at the surface of the abutting edges to forging temperature, followed by mechanical pressure to achieve coalescence.[11] The typical process for tube and pipe production involves:- Feeding a flat metal strip through roll-forming stands to shape it into an open tube with edges in a vee configuration (angle of 3–7 degrees, length 1.5–2 times the tube diameter).
- Passing the open tube through an induction coil, which generates an alternating magnetic field (frequencies 200–450 kHz) to induce eddy currents, heating the edges to 1000–1200°C in 1–2 mm depth within seconds.
- An impeder (ferromagnetic core) inside the tube concentrates the field and prevents circumferential heating.
- Forge rolls apply pressure (up to several MPa) at the weld point to upset the heated edges, expelling oxides and forming a strong bond through dynamic recrystallization.
- The weld is often trimmed to remove excess material, resulting in a narrow heat-affected zone (HAZ) of 1–2 mm.[11][36]
Equipment and setup
The primary equipment is an HF induction welder consisting of a solid-state power supply (IGBT-based, 50–1000 kHz) that delivers adjustable output to an induction coil. The coil, made of water-cooled copper tubing or sheet, encircles the tube 1–2 diameters ahead of the weld point, with a gap of 2–5 mm for optimal coupling. Ferrite or ceramic cores may enhance field concentration.[11][37] An impeder assembly, typically ferrite rods or powder-filled tubes, is inserted inside the workpiece to direct the magnetic flux axially and reduce power losses; it is water- or air-cooled and positioned to extend 1.5–3 mm beyond the vee apex. Forge rolls, mounted on a rigid frame, provide precise pressure control via hydraulic or pneumatic actuators. Additional components include edge scarfers for preparation, cooling systems for the weld zone (using air or water sprays), and monitoring tools like pyrometers or infrared sensors to maintain edge temperature within ±50°C.[11] Setup involves aligning the forming mill, coil, impeder, and forge rolls in an inline configuration for continuous production. Frequency and power are tuned based on material resistivity and thickness (e.g., lower frequencies for thicker aluminum walls to increase penetration depth). For non-tubular applications, custom coils enable localized heating of plates or profiles.[36]Applications
Metallic induction welding is predominantly used for manufacturing longitudinal seam-welded tubes and pipes from steel and aluminum strips, supporting diameters from 10 mm to over 1 m and thicknesses 0.5–20 mm. In the oil and gas sector, it produces line pipes for pipelines, with high-speed production enabling lengths up to 18 m.[11][37] Automotive applications include structural components like bumpers, subframes, exhaust systems, and hydroformed tubes, where the process allows post-weld forming due to the strong, recrystallized weld. HVAC systems utilize it for radiator and heat exchanger tubing, while construction employs it for scaffolding poles and guardrails. Other uses encompass furniture tubing, sporting equipment (e.g., bicycle frames), and electrical conduit. The process also supports specialized products like helical-wound pipes and spiral-fin tubes for heat transfer.[11][37]Thermoplastic implant induction welding
Process description
In thermoplastic implant induction welding, the process begins with preparation by embedding a metal susceptor, such as a steel mesh or foil, at the joint interface during the initial molding of the thermoplastic parts.[38] This susceptor is strategically placed to ensure precise alignment when the parts are assembled, allowing for targeted heating without affecting the bulk material.[38] Compatible polymers are typically semi-crystalline thermoplastics like polypropylene (PP) or high-density polyethylene (HDPE), which melt at relatively low temperatures and enable interdiffusion for strong bonds.[39] The heating phase involves applying an alternating magnetic field, generated by an induction coil, at frequencies ranging from 50 to 400 kHz to induce eddy currents and hysteresis losses in the susceptor.[8] This rapidly heats the susceptor to 150-300°C, melting the surrounding polymer matrix locally at the interface while minimizing thermal degradation elsewhere.[39] The welding steps proceed as follows:- Ramp up the magnetic field for 5-30 seconds to achieve the target melt temperature, optimizing cycle time based on part geometry and polymer type.[8]
- Apply controlled pressure, usually 0.1-1 MPa, to promote interdiffusion of molten polymer chains across the interface and consolidate the joint.[40]
- Allow cooling under sustained pressure to solidify the material, forming a hermetic seal with enhanced molecular entanglement.[38]