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Summary of Fabrication Processes - Short notes for Mechanical Engineers

In this article, we will study the important points related to Fabrication processes which is an important topic in the manufacturing engineering in GATE Mechanical Engineering, UPSC ESE and other engineering exams. 

Autogeneous joining: In this type of joining process, no filler material is added, e.g. cold welding, friction welding, diffusion, hot forge and resistance welding.

Homogeneous joining: In this type the filler material is used to provide the joint and is the same as the parent material, e.g. arc, gas and thermite welding.

Heterogeneous joining: In this type of joining process, the filler material used is different from the parent material, e.g. soldering and brazing.

Welding: It is defined as a metallurgical fusion process where the interface of the two parts to be joined are brought to a temperature above the melting point and then allowed to solidify so that a permanent joint is obtained.

After the above definitions of different joining processes, we will now study about the Arc Welding Processes.

Arc welding: In the arc welding process, an arc between an electrode and a workpiece or between two electrodes is utilised to heat the joint to be welded.

Carbon arc welding: It is an arc welding process in which a pure graphite or baked carbon rod of 4 to 19 mm diameter and 300 to 450 mm long is used as a non-consumable electrode to create an arc between it and the workpiece by holding it in an elecrode holder, with an electrode extension of 75 to 125 mm. The weld in this type of welding can be made by the application of heat with or without the addition of filler material. It is mainly used for copper alloys.

Shielded metal arc: It uses coated electrodes of 2.5 to 6.35 mm diameter and 300 to 450 mm long held in an electrode holder. It uses AC or DC type power source. It is used for welding aluminium alloys, fabrication of ships, bridges, pressure vessels and structures. 

Submerged arc welding: This process uses a granular flux and a copper coated wire in spooled form, thus making it possible to deposit long weld runs without interruption. Both AC or DC power source can be used. It is used for welding low carbon, low alloy stainless steels, high alloy steels, copper, aluminium and titanium. It finds extensive use for welding thick plates of pressure vessels, ships, bridges and structural work.

Gas tungsten arc welding (TIG): It uses a non-consumable tungsten electrode with an envelope of inert shielding gas like argon or helium around it. Both AC and DC power source can be used. It is used for welding of aluminium, magnesium, stainless steels, copper, nimonic alloys, monel, inconel, brass, tungsten, silver, molybdenum and titanium.

Gas metal arc welding: It uses a consumable wire of 0.8 to 2 mm diameter wire on a spool, which is fed through a welding torch wherein it is provided the electrical connection and the shielding gas like argon, helium, nitrogen, carbon dioxide, hydrogen or their mixtures. When inert gas is used, it is called MIG (Metal Inert Gas) and when carbon dioxide is used, it is called MAG (Metal Active Gas) welding. it finds extensive use in welding of steels, aluminium, magnesium alloys, nickel alloys, copper alloys and titanium.

Plasma arc welding: The plasma arc is created between a tungsten electrode and the workpiece. Energy for plasma welding is obtained from a DC power source of the constant current type. It is used in the aeronautical, precision instrument industry and jet engine manufacture.

Plasma-mig welding: In this type the electrode wire is enveloped in a plasma sheath which controls heat and droplet transfer in such a way that higher speeds and deposition rates are reached as comparable with MIG and MAG. It is used for welding thin and thick materials of mild, low alloy creep resistant, stainless and heat resistant steels, as well as non-ferrous metals such as aluminum and copper.

Atomic hydrogen welding: It used two tungsten electrodes held in a special atomic hydrogen torch. The electrodes are connected to a constant current AC power source. It is used for the manufacture of alloy steel chains and repair of dies and tool steel components.

Stud welding: This process is used for welding stud or stud-like pieces to flat workpieces like plates. It combines arc and forge welding processes. It finds wide applications in ship building and automatic rail road machinery manufacturing.

Electroslag welding: It is the process of joining heavy steel sections in a single run. The weld joint is made in a vertical position. It is used for construction of pressure vessels, press frames, water turbines and heavy plate fabrication industries. 

Electrogas welding: It is an arc welding process using the vertical orientation of the weld joint. The wire used is of the flux cored type. It is used for welding metal thickness of 12 to 75 mm in ship building and site fabrication of storage tanks.

After arc welding processes, we will now study Resistance Welding processes.

In resistance welding processes, the heat is generated at the interface of contacting workpieces, due to resistance offered to the flow of electric current.

Resistance spot welding: In resistance spot welding process, overlapping sheets are welded by the flow of current between two cylindrical electrodes. It is mainly used for lap welding of thin sheets particularly in the welding of automobile and refrigerator bodies, and high quality work in aircaraft engines. 

Resistance seam welding: In seam welding, the electrodes are in the form of wheels or rollers and welds are made in an overlapping way to give stitch welds. It gives leak proof joint. It is capable of welding thin material only in the range of 2.5 to 4.75 mm thickness like hot rolled grades of low alloy steels. It is used for making flange welds and water tanks.

Projection welding: In this process, a large number of spot welds are made at one time. It is used for high carbon and low alloy steels, stainless and high alloy steels, zinc die castings and titanium alloys. It finds applications for manufacture of steel mesh for reinforced concrete pads. 

Resistance butt welding: In this process, the pieces to be welded are held in clamps supported on two platens, one of which is fixed and the other moveable. The ends to be welded touch each other before the current is switched ON. After passing heavy current, the two pieces are pressed together at a temperature of   to  to get the weld. It is used for end joining of rods, tubes, bars and similar other sections.

Flash butt welding: It consists of one fixed and one moveable workpiece through which current is passed. As the distance between the workpieces decrease, a flash occurs and their ends are heated up. At the same time, the workpieces are forced together to get a weld. It is used to weld mild steels, medium carbon steels and alloy steels. With this process, dissimilar metals can also be joined. It is used to weld rails, steel strips, window frames, automobile axles etc.

Percussion welding: In this process, two pieces are welded by a high-intensity short duration arc followed by very rapid impacting of the work pieces. The molten surfaces are then squeezed together by collision and some of the metal is forced out to the side of the joint. This process is used to weld dissimilar metals, e.g. copper to nichrome. 

High frequency resistance welding: In this process, the phenomenon of 'skin effect' is utilized to weld materials at high speed of the range of 300 m/min. Skin effect makes the current flow near the surface of the conductor. The AC supply used has a frequency of 100 to 500 kHz. This process is used to weld steels, copper and its alloys. Mostly it is used to make thin sheet metal pipes. 

High frequency induction welding: In this process, the edges to be welded are heated by induction of high frequency current. As there is no direct contact between the induction coil and the workpiece, so the chances of wear and tear are eliminated. 

After the resistance welding processes, let us look at Solid State Welding Processes. 

In these processes, the material to be welded is heated to a temperature below or just upto the solid state. The coalescence of the parts is achieved with the help of pressure. 

Forge or smith welding: In this process, the pieces to be welded are heated to a temperature above  and then placed together and given impact blows by hammering. Commonly used fluxes are sand, fluorspar and borax. The process finds its applications in rural areas for agricultural equipments. 

Friction welding: In this process, one piece is held stationary and the other is rotated in a chuck brought to rub against each other under pressure. When the desired temperature is obtained the rotation is stopped and axial pressure is increased to cause welding. This process is used to weld drill bits to shanks, engine valve heads to stem etc.

Ultrasonic welding: In this process, a metallic tip vibrating at ultrasonic frequency is made to join a thin piece to a thicker piece supported on an anvil. Its applications include aluminium and gold lead wire connections to transistors and diodes, helicopter access doors etc.

The next type of welding is Diffusion welding, which is achieved by the application of pressure which is of the order of 5 to 75 MPa, while the pieces are held at high temperature of 75% of the melting point. 

Electron beam welding: In this process, a beam of electron is used to melt the metal where it has to be welded. It is used for welding of super alloys, refractory metals, reactive metals and stainless steels. it is extensively used in electronics , nuclear, missile and aircraft industries.

Thermit welding: Thermit is a mixture of aluminium powder and metal oxide which when ignited results in a non-explosive exothermic reaction. The heat so generated melts and reduces the metal oxide to metallic form at a high temperature. This molten metal is used for joining metal parts by pouring in between them as for railroad tracks. 

The next type of welding is gas welding. In this process, a fuel gas like acetylene, hydrogen, natural gas etc. is mixed with oxygen in proper proportions to burn to get a flame which can be used for fusing and joining different parts. The most common type is the oxy-acetylene welding. In this process acetylene is mixed with oxygen in the welding torch and is then burnt at the torch tip to give a flame which can melt most of the ferrous and non-ferrous metals in common use. A filler material and flux is used commonly in this process. Acetylene is stored in cylinders containing acetone and oxygen is stored in cylinders under pressure. 

Important reactions:

Oxy-acetylene welding

Thermit welding:

Important equations:

Welding load voltage in arc welding:

Where I = load current in amperes.

Melting rate in arc welding (MR) = 

where L = electrode extension in mm, a = 5.4 x 10-3 for Al, 8.6 x 10-3 for MS and b = 4.4 x 10-6 for Al, 2.5 x 10-5 for MS.

Heat generated in resistance welding: 

Voltage arc length characteristics:

 

 

Where  = Open circuit voltage

 = short circuit current

V = (20+4I) volts

Make sure you go through these points in detail and make your own short notes. 

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