Several welding processes are available; however, their application is dictated by the mechanical properties, type of welded joints, the quality required in the service condition, cost and availability of the machine and operator’s skill.
The discussion below gives a comparative study of the different joining processes and their applicability to different types of materials and helps the welders select suitable welding processes.
The shielded metal arc welding process is widely used in many industries. All engineering materials can be welded. However, low melting and high reactive metals will be difficult to weld. This process is easy to operate and plates of thickness ranging from 1 mm to 25 mm can be readily welded. Preheating will be required in some alloy steels. Welding can be done in the flat, inclined, vertical and overhead positions. Edge preparations are essential in thick welding plates.
Manual arc welding is commonly used in the erection of structural works like storage tanks, bridges etc. In open breezy conditions, flux-cored self-shielded welding is better suited. Heavier plates are usually grooved weld.
TIG welding process is extensively used for welding cupronickel (70:30 alloy) for water pipe and condenser tubes. While welding carbon and alloy steel pipes by MMA (Manual Metal Arc) process for steam and power plants, backing rings are rarely used for piping in oil refineries and chemical plants.
With Submerged arc welding (SAW), carbon and alloy steels and copper alloys can be welded; generally applied for plate thickness above 10 mm. Best suited for automatic welding in boilers, pressure vessels and shipbuilding where high-quality welds for larger thickness plates are required. This process is generally used for flat and horizontal positions. Not suitable for cast iron.
3. Oxy-acetylene Gas Welding
Oxy-acetylene gas welding can be used for carbon steel, copper, aluminum, bronze welding, and Sheet metal welding. Small diameter pipe welding can be effectively carried out. Control of the flame is essential; plates of thickness up to 8 mm – 10 mm can be welded. Red brass and yellow brass are preferably welded by the oxy-acetylene process to minimize the vaporization of zinc.
All engineering materials except zinc can be welded using the gas metal arc welding (GMAW) process. The thickness of the plates ranges from 1 mm to 6 mm. TIG welding is applied to all non-ferrous and alloy steel welding and root pas in pipe welding. Welding equipment is more complex and costly. Challenging to weld small corners and, outdoor applications are limited. MIG welding process in semi-automatic or fully automatic form is used for non-ferrous and stainless steel pressure vessel parts. In manufacturing boiler units, many tube butt welds have to be made with the tubes positioned at any angle from horizontal to vertical, with restricted access. Automated orbital TIG welding with automatic cold wire feed is used in such cases. Titanium alloy tubes with a wall thickness of 1.6 mm and below are usually welded by the TIG process without filler wire. For heavier pipes, filler metals are used.
5. Spot, Projection and seam welding
These processes meant for sheet metals are widely applied in automobile parts, tube manufacturing parts and sheet metal industries. All engineering metals can be welded. Precautions are necessary for copper and aluminum alloys which are good thermal and electrical conductors.
Flash or induction welding is used for tubular joints in boiler construction. At the site, such welds are made by TIG for the root pass and manual metal arc welding for subsequent passes. Seam welding is normally limited to sheets up to 5 mm thick. Baffles and other interior parts are spots welded in place. A typical application of projection welding is in the manufacture of honeycomb panels. Propeller and drive shafts are commonly made of resistance welded tubing with the end forging welded by submerged arc or MIG/CO2 process.
Electro-slag welding (ESW) process is for thick section welding, 5 cm and above, of alloy steels; this is mainly used for pressure vessel parts, steel plant types of equipment, large shafts etc. Both Electro-slag welding and Submerged-arc welding (SAW) are best suited for thick plates; however, ESW is more specialized in its application and less flexible than SAW.
7. Electron and laser beam welding
Stainless steel, nickel-base alloys, titanium and zirconium and other reactive metals can be welded up to 10 to 25 mm. Special applications are in electronic industries, nuclear and aerospace industries. The process is rather costly. Laser welding has the ability to make tiny spot welds. So it is applied in microelectronic circuits. The Laser beam can weld metals on silicon and germanium.