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Fundamentals of Joining Processes

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Fundamentals of Joining Processes Outline ME3072 – MANUFACTURING ENGINEERING II BSc Eng (Hons) in Mechanical Engineering • Introduction to Welding Semester - 4 • Fusion-Welding Processes • Solid-State Welding Processes Fundamentals of Joining • Metallurgy of Welding Processes • Weld Quality • Brazing & Soldering Prepared By : R.K.P.S Ranaweera BSc (Hons) MSc Lecturer - Department of Mechanical Engineering University of Moratuwa 2 (for educational purpose only) Joining Processes Classification of Joining Processes 3 4 1 Introduction to Welding • Attention must be given to the cleanliness of the metal surfaces prior to welding and to possible • Is a process by which two materials, usually metals oxidation or contamination during welding process. are permanently joined together by coalescence, which is induced by a combination of temperature, • Production of high quality weld requires: pressure and metallurgical conditions. Source of satisfactory heat and/or pressure Means of protecting or cleaning the metal • Is extensively used in fabrication as an alternative Caution to avoid harmful metallurgical effects method for casting or forging and as a replacement for bolted and riveted joints. Also used as a repair • Advantages of welding over other joints: medium to reunite metals. Lighter in weight and has a great strength • Types of Welding: High corrosion resistance Fusion welding Fluid tight for tanks and vessels Solid-state (forge) welding Can be altered easily (flexibility) and economically 5 6 • Weldability has been defined as the capacity of • Steps in executing welding: metal to be welded under the fabrication conditions Identification of welds, calculation of weld area by stress imposed into a specific, suitably designed structure analysis, preparation of drawings & to perform satisfactorily in the intended service. Selection of appropriate welding process Welding procedure – welding sequence, testing, etc • The following metals have good weldability in the Execution of welding with supervision & inspection descending order: Iron, Carbon Steel, Cast Steel, Slag removal, weld dressing Cast Iron, Low Alloy Steels and Stainless Steels. Stress relieving by proper treatment • Welding is extensively used in the following fields: Testing, preferably by nondestructive methods automobile industry, aircraft machine frames, tanks, • Process of joining similar metals with the help of structural work, machine repair work, ship building, filler rod of the same metal is called autogeneous pipe line fabrication ,thermal power plants and welding, and joining of metals using filler rod of is refineries, fabrication of metal structures. called heterogeneous welding. 7 8 2 • Types of welded joints: • Welding positions: Flat position, Horizontal position Lap joint Vertical position and overhead position. Butt joint Corner joint Edge joint T-joint • Welders have to protect themselves against spark, hot metal, ultraviolet, infrared and visible light rays, welding fumes, and other hazards. 9 10 Fusion-Welding Processes • Oxyfuel Gas Welding (OFW) Refers to a group of welding processes that use, • Introduction as their heat source, the flame produced by the Is defined as the melting together & coalescing combustion of fuel gas and oxygen. of materials by means of heat, with or without Types of Gas used: the application of pressure and with or without Oxyacetylene – high temperature the use of filler metal. Hydrogen – low temperature Thermal energy required for these operations is Methylacetylene propadiene – low temperature usually supplied by chemical (oxy-fuel gas, Heat is generated in accordance with a pair of thermit) or electrical ( arc, resistance, electron chemical reactions: beam, laser beam) means. Welds undergo important metallurgical & physical changes that will effect its performance. 11 12 3 Three basic types of oxyacetylene flames used in Oxyfuel-gas (a) General view of and (b) cross-section of a torch used in welding and cutting operations: (a) neutral flame; (b) oxidizing oxyacetylene welding. The acetylene valve is opened first; the flame; (c) carburizing, or reducing, flame. The gas mixture in (a) gas is lit with a spark lighter or a pilot light; then the oxygen valve is basically equal volumes of oxygen and acetylene. is opened and the flame adjusted. 13 14 Filler Metals Used to supply additional material to the weld zone Available as rod or wire made of metals compatible with those to be welded Consumable filler rods may be bare, or they may be coated with flux. Purposes of the flux: - Retard oxidation of the surfaces of the part being welded, by generating gaseous shield around the weld zone - Helps to dissolve and remove oxides and other substances from the workpiece and form a stronger joint - Slag developed protects the molten puddle of metal against oxidation as it cools Basic equipment used in Oxyfuel-gas welding. To ensure correct connections, all threads on acetylene fittings are left- - Provides means of adding various alloying elements into the handed, whereas those for oxygen are right-handed. Oxygen weld metal to enhance the properties of the joint regulators are usually painted green, acetylene regulators red. - Stabilizes the arc by providing certain chemicals 15 16 4 Pressure Gas Welding • Arc Welding: Consumable Electrode Heat is obtained from electrical energy. Arc is produced between the tip of the electrode and the workpiece to be welded, by the use of an AC or a DC power supply. Arc produce temperatures about 30,000 0C Schematic illustration of the pressure-gas welding process. 17 18 • Shielded Metal Arc Welding (SMAW) Current used generally ranges between 50 A to 300 A & power requirements are generally 10kW About 50% of all industrial and maintenance welding is currently performed by this process. Type of current: DC – straight & reverse polarity Also known as stick welding. AC Electric arc is generated by touching the tip of a Schematic coated electrode against the workpiece and then illustration of withdrawing it quickly to a distance sufficient to the shielded metal-arc maintain the arc. welding operations Schematic illustration of the (also known as shielded metal-arc welding stick welding, process. because the electrode is in the shape of a stick). 19 20 5 • Submerged Arc Welding (SAW) Electrical current typically range between 300 A to 3000 A & weld speed is high as 5 m/min. Weld arc is shielded by granular flux , consisting of lime, manganese oxide, calcium fluoride, silica, and other compounds. It prevents spatter & sparks and suppresses the intense ultraviolet radiation and fumes. Flux also act as a thermal insulator promoting deeper penetration of heat into the workpiece. Consumable electrode is a coil of bare round wire 1.5 mm – 10 mm in diameter. Applications include thick plate welding for shipbuilding and for pressure vessels. Schematic illustration of the submerged-arc welding process and equipment. The unfused flux is recovered and reused. 21 22 • Gas Metal Arc Welding (GMAW) Schematic illustration of Formerly called metal inert gas (MIG) welding. the gas metal-arc Weld area is shielded by an effective inert welding process, formerly known as MIG atmosphere of argon, helium, carbon dioxide, or (for metal inert gas) various other gas mixtures. welding. In addition, deoxidizers are usually present in the electrode metal itself, prevent oxidation of the molten weld puddle. Suitable for welding a variety of ferrous and nonferrous metals. Basic equipment used in Metal can be transferred by three methods: gas metal-arc welding spray, globular and short circuiting. operations. 23 24 6 • Flux-Cored Arc Welding (FCAW) Similar to GMAW, with the exception that the electrode is tubular in shape & is filled with flux. Produce a more stable arc, improve weld contour , and produce better mechanical properties of the weld metal. Electrodes are usually 0.5 mm – 4 mm in diameter & the power required is about 20 kW. Used for welding of variety of joints, mainly on steels, stainless steels and nickel based alloys. Self-shielded cored electrodes are also available Schematic illustration of the flux-cored arc-welding process. This operation is similar to gas metal-arc welding. 25 26 • Electrodes Is classified according to the strength of the deposited weld metal, the current (AC or DC), & the type of coating. Identified by numbers or letters or by color code. Typical coated electrode numbers are 150 to 460 mm in length & 1.5 to 8 mm in diameter. (Wire diameter must not vary more than 0.05 mm & Coatings must be concentric with wire) Electrodes are coated with claylike material that include silicate binders & powder materials such as oxides, carbonates, fluorides, metal alloys, and cellulose. Designations for Mild Steel Coated Electrodes 27 28 7 • Arc Welding: Non-consumable Electrode • Gas Tungsten Arc Welding (GTAW) Unlike arc-welding processes, non-consumable Also know as tungsten inert gas (TIG) welding. electrode processes typically use a tungsten Filler metal is supplied from a wire & are similar electrode . to the metals to be welded. Shielding gas is supplied from external source. Shielding gas is usually argon or helium. Stable arc gap is maintained because the Is used for wide variety of metals & applications, electrode is not consumed. particularly aluminium, magnesium, titanium & refractory metals. Power supply is either DC at 200 A or AC at 500 A and power requirements range from 8 kW to 20 kW. 29 30 • Atomic Hydrogen Welding (AHW) The gas tungsten-arc welding process, formerly known as Uses an
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