Post 391 – by Gautam Shah
Fusion welding is a common term to describe a process of joining by melting or softening with heat, energy or chemical action, two similar or dissimilar materials.
Fusion joining systems are used in fabrication of metals and thermoplastics. There are basically THREE categories of fusion joining systems. For soldering and brazing, the work pieces are not melted, yet joined by using a melt-able filler material. For welding the work pieces, and in some instances, the filler material, both are melted. The joint is created with or without application of pressure. Some plastics are joined by solvents that dissolve (soften) the surface areas of the work pieces; this is often termed as a solvent welding, but this is truly an adhesion fixing.
Fusion joining requires a heat source, such as a gas flame, an electric arc, a laser, an electron beam, friction, or ultrasound. Fusion joining sometimes requires a slight to very heavy pressure. Fusion joining is done under many different environmental conditions like open air, rain, frost, underwater in vacuum or space and sometimes under the shield of inert gases like Nitrogen, Argon, Helium, Carbon dioxide, etc. Fusion joining in spite of all care is essentially a hazardous procedure. It involves risks of high electric currents, high temperatures, sparks, fumes, and radiation.
● Forge welding was the first fusion joining process. Blacksmiths used to beat the heated and overlapped metals for joining. Wood, charcoal and later mineral coals were used to heat the work pieces.
● Soldering and Brazing use external metal softening at low temperature. For the purpose gold, silver, tin, lead, were common in craft work.
During 1800s, DC power sparking, Oxygen-fuel, and arc processes were developed for fusion welding processes, The results were unreliable as joints cracked and came of the surfaces. It was in the 20th C that AC power was consistently produced and distributed. After world war II, Plasma, laser and electron welding systems, along with X-ray technology for checking the integrity of welded joints were developed. Deep and narrow welding became possible with concentrated heat of Electron beam welding (1958). Later in the 1960s the laser beam welding helped high speed clean profile cutting and automated precision welding. Though both processes are expensive are used for special applications. Today industrial plants have the robotic welding systems, using these technologies.
● Friction welding uses heat generated through friction and then pressure to achieve joining. Friction heating is generated by sliding the parts together, and as the surfaces soften, heavier pressure is applied.
● Oxy-fuel welding is one of the best-known forms of fusion welding. This uses fuel gases, like acetylene, liquid petroleum, hydrogen, propane, natural gas or propylene with oxygen. Typically Acetylene gas and pure oxygen can generate flame temperatures of 3500 C. This flame is hot enough to melt most industrial metals.
● Arc-welding uses electricity to generate an electric arc between an electrode and the pieces of metal to be joined. It is homogeneous welding. It is most widely used method of welding.
● Electric resistance welding is similar to the arc-welding. In this process two electrodes are placed on either side but close to the piece to be welded. As the electrodes are brought closer under pressure to create resistance and thereby heat. One of the known such process is butt or spot welding.
● Thermit welding is uses intense heat generated by igniting iron oxide and aluminum powder. This process is used for joining rails at remote sites where heavy equipment cannot be carried.
● Laser beam welding uses very precise heat source of a laser. The lasers penetrate deep and narrow areas without affecting the surrounding parts. It requires heavy industrial set-up.
Heat fusion processes are good for heat fusible substances like metals and some plastics, but induce stresses in the mass. Electric resistance processes of welding require electrically conductive mass. In fusion joining processes the deposited substances are heat-hardened and these are very difficult to grind out. Joint testing procedures such as X-ray and sonography are elaborate and not always perfect.
Fusion joining systems require power or energy input, a filler material, a flux and often a shield gas. Welding in certain situations are done without the use of a filler material. Filler materials where used, are invariably of compatible materials in terms of melting point temperature, alloying capacity, fluxing capacity. These are in the form of powders of pure metals, alloys, oxides and such other compounds, ceramics, granules, foils, wires, and rods. Flux or fluxing agents are required to dissolve the existing, and to be formed oxide. Welding rods are coated with borax and aluminum chloride. Stainless steel welding requires zinc chloride and hydrochloric acid in equal parts. Soldering of the Galvanized surfaces need hydrochloric acid. Brass soldering requires tallow or rosin. Soldering of tinned surfaces require hydrochloric acid or rosin. Copper, Brass and gun metal need aluminium chloride, hydrochloric acid or ammonium phosphate. Shield gases are inert or noble gases (constituting O group of a periodic table), such as Helium, Neon, Argon, Crypton, Xenon, Radon. These gases do not react with other materials so are used for forming an environment so that outside contaminants and other substances do not react with the weld. Some other non-inert gases are used for welding, are Carbon dioxide and Nitrogen.
Fusion Joining has few inherent issues. Distortion occurs due to substantial thermal expansion and contraction. Oxidation occurs at high temperatures, which prevents either joining or results in poor bonding. Oxidation can be prevented by welding in protected environments (metal inert gas -MIG, and tungsten inert gas TIG). In oxyacetylene welding, the gases produced by combustion prevent oxidation. In certain cases the electrodes are coated with a fluxing agent. Cracking occurs for many reasons such as inadequate heat and moisture in air. Stresses in the welded zone and surrounding areas (heat-affected zone -HAZ) occur due to high heat. Prolonged cooling anneals the steel.
This fusion joining can be also done for metal, ceramic and other polymer surfaces with use of alternating magnetic field to heat and melt a nano-crystalline iron-aluminium powder, which then cools to bind the surfaces of two parts.