Induction Welding
What is induction welding? Induction welding is similar to resistance welding. In resistance welding, however, the current is delivered through contacts to the work piece, instead of induction. The welding form that uses an induction coil energized with a radio-frequency electric current and electromagnetic induction to heat the work piece is what is referred to as induction welding. In most instances both an electrically conductive and ferromagnetic work piece will be acted upon by an electromagnetic field (EMF). Eddy currents are used and resistive heating takes place. In ferromagnetic induction welding hysteresis heating occurs. The EMF distorts the magnetic domains of the ferromagnetic material. Susceptors, metallic or ferromagnetic compounds, can be introduced to nonmagnetic materials and electrical insulators so that plastic can be induction welded. Suscpetors absorb electromagnetic energy from the induction coil, heat up, and thermal conduction disperses heat to the surrounding material.
Induction welding is usually carried out in highly automated long production runs. It is common for welding pipe seams. Tubes can be welded by inducing a current in a tube along the open seam. The edges are heated to a temp high enough for welding. Then the seam edges are forced together. The induction welding process can be done quickly; a lot of power can be localized so the faying surfaces melt quickly and can form a continuous rolling weld when pressed together. Temperature and metal composition will affect penetration depth. The depth the heating penetrates the surface is inversely proportional to the square root of the frequency.
Induction Heating
Electromagnetic induction heats an electrically conductive object (metal). Eddy currents are generated within the metal. Resistance leads to Joule heating (resistive heating). An induction heater consists of an electromagnet. A high frequency alternating current (AC) is passed through the magnet. Heat may also be a result of a magnetic hysteresis. AC frequency is dependent on object size, material type, penetration depth, and coupling between work coil and work piece.
Induction heating may be used for surface hardening, melting, brazing, or soldering. Induction heating has been used to heat molten metals and gaseous conductors. It is used to heat graphite crucibles and in the semiconductor industry. It is favored for lower cost as inverters are not required. Induction heating is common on iron and iron alloys, due to their ferromagnetic properties. Eddy currents can be generated in any conductor. Magnetic hysteresis can occur in any magnetic material.
How does induction heating work? A furnace heats metal such as iron, steel, copper, aluminum, or precious metals to the melting point. Most furnaces consist of water-cooled copper rings. The furnaces may be small or have a hundred ton capacity. They are used in today’s foundries as an alternative to reverberatory furnaces. Induction furnaces may emit a whine or hum but they can be used in a vacuum or inert atmosphere. This is cleaner and prevents oxidation. A high frequency magnetic field can stir metal, ensuring consistent alloy distribution.
Watch for a future article on heat treatment.