Fundamentals of Joining Processes

Outline ME3072 – MANUFACTURING ENGINEERING II BSc Eng (Hons) in Mechanical Engineering Semester - 4 • Introduction to Welding • Fusion-Welding Processes • Solid-State Welding Processes • Metallurgy of Welding • Weld Quality Fundamentals of Joining Processes • 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) Classification of Joining Processes Joining Processes <ul style="display: flex;"><li style="flex:1">3</li><li style="flex:1">4</li></ul>1• Attention must be given to the cleanliness of the metal surfaces prior to welding and to possible oxidation or contamination during welding process. Introduction to Welding • Is a process by which two materials, usually metals are permanently joined together by coalescence, which is induced by a combination of temperature, pressure and metallurgical conditions. • Production of high quality weld requires: ꢀSource of satisfactory heat and/or pressure ꢀMeans of protecting or cleaning the metal ꢀCaution to avoid harmful metallurgical effects • Is extensively used in fabrication as an alternative method for casting or forging and as a replacement for bolted and riveted joints. Also used as a repair medium to reunite metals. • Advantages of welding over other joints: ꢀLighter in weight and has a great strength ꢀHigh corrosion resistance • Types of Welding: ꢀFusion welding ꢀSolid-state (forge) welding 5ꢀFluid tight for tanks and vessels ꢀCan be altered easily (flexibility) and economically 6• Weldability has been defined as the capacity of metal to be welded under the fabrication conditions imposed into a specific, suitably designed structure & to perform satisfactorily in the intended service. • Steps in executing welding: ꢀIdentification of welds, calculation of weld area by stress analysis, preparation of drawings ꢀSelection of appropriate welding process ꢀWelding procedure – welding sequence, testing, etc ꢀExecution of welding with supervision & inspection ꢀSlag removal, weld dressing • The following metals have good weldability in the descending order: Iron, Carbon Steel, Cast Steel, Cast Iron, Low Alloy Steels and Stainless Steels. ꢀStress relieving by proper treatment ꢀTesting, preferably by nondestructive methods • Welding is extensively used in the following fields: automobile industry, aircraft machine frames, tanks, structural work, machine repair work, ship building, pipe line fabrication ,thermal power plants and refineries, fabrication of metal structures. 7• Process of joining similar metals with the help of filler rod of the same metal is called autogeneous welding, and joining of metals using filler rod of is called heterogeneous welding. 82• Types of welded joints: ꢀLap joint • Welding positions: Flat position, Horizontal position 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. <ul style="display: flex;"><li style="flex:1">9</li><li style="flex:1">10 </li></ul>• Oxyfuel Gas Welding (OFW) Fusion-Welding Processes ꢀRefers to a group of welding processes that use, as their heat source, the flame produced by the combustion of fuel gas and oxygen. • Introduction ꢀIs defined as the melting together & coalescing of materials by means of heat, with or without the application of pressure and with or without the use of filler metal. ꢀ Types of Gas used: ꢁ Oxyacetylene – high temperature ꢁ Hydrogen – low temperature ꢀThermal energy required for these operations is usually supplied by chemical (oxy-fuel gas, thermit) or electrical ( arc, resistance, electron beam, laser beam) means. ꢁ Methylacetylene propadiene – low temperature ꢀHeat is generated in accordance with a pair of chemical reactions: ꢀWelds undergo important metallurgical & physical changes that will effect its performance. <ul style="display: flex;"><li style="flex:1">11 </li><li style="flex:1">12 </li></ul>3(a) General view of and (b) cross-section of a torch used in oxyacetylene welding. The acetylene valve is opened first; the gas is lit with a spark lighter or a pilot light; then the oxygen valve is opened and the flame adjusted. Three basic types of oxyacetylene flames used in Oxyfuel-gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing, flame. The gas mixture in (a) is basically equal volumes of oxygen and acetylene. <ul style="display: flex;"><li style="flex:1">13 </li><li style="flex:1">14 </li></ul>ꢀ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 lefthanded, whereas those for oxygen are right-handed. Oxygen regulators are usually painted green, acetylene regulators red. - Provides means of adding various alloying elements into the weld metal to enhance the properties of the joint - Stabilizes the arc by providing certain chemicals <ul style="display: flex;"><li style="flex:1">15 </li><li style="flex:1">16 </li></ul>4ꢀPressure 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. <ul style="display: flex;"><li style="flex:1">17 </li><li style="flex:1">18 </li></ul>ꢀCurrent used generally ranges between 50 A to 300 A & power requirements are generally 10kW • Shielded Metal Arc Welding (SMAW) ꢀAbout 50% of all industrial and maintenance welding is currently performed by this process. ꢀType of current: ꢁ DC – straight & reverse polarity ꢁ AC ꢀAlso known as stick welding. ꢀElectric arc is generated by touching the tip of a coated electrode against the workpiece and then withdrawing it quickly to a distance sufficient to maintain the arc. Schematic illustration of the shielded metal-arc welding operations (also known as stick welding, because the electrode is in the shape of a stick). Schematic illustration of the shielded metal-arc welding process. <ul style="display: flex;"><li style="flex:1">19 </li><li style="flex:1">20 </li></ul>5ꢀElectrical current typically range between 300 A to 3000 A & weld speed is high as 5 m/min. • Submerged Arc Welding (SAW) ꢀ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. <ul style="display: flex;"><li style="flex:1">21 </li><li style="flex:1">22 </li></ul>• Gas Metal Arc Welding (GMAW) Schematic illustration of the gas metal-arc welding process, formerly known as MIG (for metal inert gas) welding. ꢀFormerly called metal inert gas (MIG) welding. ꢀWeld area is shielded by an effective inert atmosphere of argon, helium, carbon dioxide, or various other gas mixtures. ꢀ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 gas metal-arc welding operations. ꢀMetal can be transferred by three methods: spray, globular and short circuiting. <ul style="display: flex;"><li style="flex:1">23 </li><li style="flex:1">24 </li></ul>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. <ul style="display: flex;"><li style="flex:1">25 </li><li style="flex:1">26 </li></ul>• 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 <ul style="display: flex;"><li style="flex:1">27 </li><li style="flex:1">28 </li></ul>7<ul style="display: flex;"><li style="flex:1">• Arc Welding: Non-consumable Electrode </li><li style="flex:1">• Gas Tungsten Arc Welding (GTAW) </li></ul>ꢀUnlike arc-welding processes, non-consumable electrode processes typically use a tungsten electrode. ꢀAlso know as tungsten inert gas (TIG) welding. ꢀFiller metal is supplied from a wire & are similar 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 electrode is not consumed. ꢀIs used for wide variety of metals & applications, 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. <ul style="display: flex;"><li style="flex:1">29 </li><li style="flex:1">30 </li></ul>• Atomic Hydrogen Welding (AHW) ꢀUses an arc in a shielding atmosphere of H2. ꢀArc is between 2 tungsten or carbon electrodes. ꢀHydrogen also cools the workpiece. The gas tungsten-arc welding process, formerly known as TIG (for tungsten inert gas) welding. • Plasma Arc Welding (PAW) ꢀA concentrated plasma arc is produced and is aimed at the weld area. ꢀArc is stable and reaches temperatures as high as 33,000 0C. Equipment for gas tungsten-arc welding operations. ꢀPlasma is ionized hot gas, composed of nearly equal numbers of electron and ions <ul style="display: flex;"><li style="flex:1">31 </li><li style="flex:1">32 </li></ul>8ꢀPlasma is initiated between tungsten electrode and the orifice by a low current pilot arc. • Thermit Welding (TW) ꢀInvolves exothermic reactions between metal oxides & metallic reducing agents and the heat produced in this reaction is used for welding. ꢀShielding is supplied by means of an outer shielding rings and the uses of gases, such as argon, helium or mixtures. ꢀCommon mixtures of materials used in welding steel & cast iron are iron oxide, aluminium oxide, iron and aluminium. ꢀTwo methods of plasma arc welding: transferred arc method (a) or nontransferred arc method (b). ꢀMixtures may also contain other materials to impart special properties to the weld. ꢀIs suitable for welding & repairing large forgings and castings. <ul style="display: flex;"><li style="flex:1">33 </li><li style="flex:1">34 </li></ul>ꢀProcedure - align the part to be joined ꢂ built a • Electron Beam Welding (EBW) mold ꢂallow to flow superheated products ꢀHeat is generated by high velocity narrow beam electrons & the kinetic energy of the electrons is converted in to heat as they strike the workpiece ꢀRequires special equipment to focus the beam on the workpiece in a vacuum. • Laser Beam Welding (LBW) ꢀUtilizes a high power laser beam as the source of heat to produce a fusion weld. ꢀThe beam has high energy density, therefore deep penetrating capability. <ul style="display: flex;"><li style="flex:1">35 </li><li style="flex:1">36 </li></ul>9ꢀComparison of Conventional and Electron- or Laser-Beam Welding • Cutting ꢀA piece of metal can be separated in to two or more pieces or into various contours by the use of heat source that melts and removes a narrow zone in the workpiece. ꢀOxyfuel Gas Cutting (OFC) ꢁ Cutting occurs mainly by the oxidation of the steel ꢁ Basic reaction with the steel are, The relative sizes of the weld beads obtained by conventional (tungsten arc) and by electron-beam or laser-beam welding <ul style="display: flex;"><li style="flex:1">37 </li><li style="flex:1">38 </li></ul>Solid-State Welding Processes • Introduction ꢀSolid-phase welds are produced by bringing the clean faces of components into intimate contact to produce a metallic bond with or without application of heat, but application of pressure is essential to induce plastic flow. (a)Flame cutting of steel plate with an oxyacetylene torch, and a cross-section of the torch nozzle. (b) Cross-section of a flamecut plate showing drag lines. ꢀArc Cutting ꢁ Air carbon arc cutting ꢁ Plasma arc cutting ꢁ Lasers and electron beams <ul style="display: flex;"><li style="flex:1">39 </li><li style="flex:1">40 </li></ul>10 <ul style="display: flex;"><li style="flex:1">• Cold Welding (CW) </li><li style="flex:1">• Ultrasonic Welding (USW) </li></ul>ꢀPressure is applied to the workpieces, through either dies or rolls. ꢀFaying surfaces of the two components are subjected to a static normal force and oscillating <ul style="display: flex;"><li style="flex:1">shearing (tangential) stress. </li><li style="flex:1">ꢀAlso known as roll bonding. </li></ul>ꢀShearing stresses are applied by the tip of a transducer and frequency of oscillation is generally in the range of 10 kHz to 75 kHz. ꢀPrior to welding, the interface is degreased, wire brushed, and wiped to remove oxide smudge. ꢀCan be used to join small workpieces made of soft, ductile metals. ꢀTemperatures generated usually in the range of one-third to one-half of the melting point. ꢀCan be used with wide variety of metallic and nonmetallic materials, including dissimilar metals and plastics. Schematic illustration of the roll bonding, or cladding, process <ul style="display: flex;"><li style="flex:1">41 </li><li style="flex:1">42 </li></ul><ul style="display: flex;"><li style="flex:1">(a) </li><li style="flex:1">(b) </li></ul>• Friction Welding (FRW) ꢀSteps of operation ꢁ On of the components remains stationary while the other is placed in a chuck or collet and rotated at a high constant speed. ꢁ Two members to be joined are then brought into contact under an axial force. ꢁ Rotating member is then brought to a quick stop, while the axial force is increased. ꢁ Pressure at the interface and the resulting friction produce sufficient heat for a strong joint to form. (a)Components of an ultrasonic welding machine for lap welds. The lateral vibrations of the tool tip cause plastic deformation and bonding at the interface of the workpieces. ꢀTypes of FRW processes: Inertia friction welding, Linear friction welding and Friction stir welding (b)Ultrasonic seam welding using a roller. <ul style="display: flex;"><li style="flex:1">43 </li><li style="flex:1">44 </li></ul>11 ꢀFriction Stir Welding (RSW) (a) ꢁ Use of a third body to rub against the faying surfaces ꢁ Probe at the tip heat and mix or stir the material (b) (a)Sequence of operations in the friction welding process (b)Shape of fusion zone in friction welding, as a function of the force applied and the rotational speed. The principle of the friction stir welding process. Aluminum-alloy plates up to 75 mm (3 in.) thick have been welded by this process <ul style="display: flex;"><li style="flex:1">45 </li><li style="flex:1">46 </li></ul>ꢀResistance Spot Welding (RSW) • Resistance Welding (RW) ꢁ Tips of two opposing solid cylindrical electrodes touch a lap joint of two sheet metals, and resistance heating produces a spot weld ꢀHeat required for welding is produced by means of electrical resistance across two components to be joined. ꢁ Currents range from 3000 A to 40,000 A ꢀActual temperature rise at the joint depends on the specific heat and on the thermal conductivity of the metals to be joined. (a) Sequence in resistance spot welding. (b) Crosssection of a spot weld, showing the weld nugget and the indentation of the electrode on the sheet surfaces. This is one of the most commonly used process in sheetmetal fabrication and in automotive-body assembly. ꢀMagnitude of the current in resistance welding operations may be as high as 100,000 A, although the voltage is typically only 0.5V – 10V. ꢀSimilar or dissimilar metals can be joined. <ul style="display: flex;"><li style="flex:1">47 </li><li style="flex:1">48 </li></ul>12 ꢀResistance Seam Welding (RSEW) ꢁ Is a modification of spot welding wherein the electrodes are replaced by rotating wheels or rollers. ꢁ Using a continuous AC power supply, the electrically conducting rollers produce a spot weld when ever the current reaches a sufficient high level in the AC cycle. ꢁ Can produce a joint that is liquid tight or gas tight. ꢁ Roll spot welding is an extension of RSEW. <ul style="display: flex;"><li style="flex:1">(a) </li><li style="flex:1">(b) </li></ul>Examples of Seam Welding (a) and (b) Seam-welded cookware and muffler. (a) Seam-welding process in which rotating rolls act as electrodes. (b) Overlapping spots in a seam weld. (c) Roll spot welds. (d) Resistance-welded gasoline tank. <ul style="display: flex;"><li style="flex:1">49 </li><li style="flex:1">50 </li></ul><ul style="display: flex;"><li style="flex:1">ꢀHigh-Frequency Butt Welding (HFRW) </li><li style="flex:1">ꢀResistance Projection Welding (RPW) </li></ul><ul style="display: flex;"><li style="flex:1">ꢁ Similar to seam welding </li><li style="flex:1">ꢁ Embossing one or more projections on one of the </li></ul>surfaces to be welded to increase the electrical resistance ꢁ High frequency current (up to 450 kHz) is employed ꢁ Types: ꢃHigh frequency resistance welding (fig. a) ꢃHigh frequency induction welding (fig. b) Schematic illustration of resistance projection welding Two methods of high-frequency butt welding of tubes Projection welding of nuts or threaded bosses and studs <ul style="display: flex;"><li style="flex:1">51 </li><li style="flex:1">52 </li></ul>13 <ul style="display: flex;"><li style="flex:1">ꢀFlash Welding (FW) </li><li style="flex:1">ꢀStud Welding (SW) </li></ul>ꢁ Heat generated from the arc as the ends of the two members begin to make contact and develop an electrical resistance at the joint ꢁ Similar to flash welding ꢁ Part to be joined serves as one of the electrodes while being joined to another component ꢁ Form a flash at the joint ꢁ Disposable ceramic ring (ferrule) is placed around the joint to concentrate the heat generated, prevent oxidation and retain the metal in the weld zone ꢁ Used to repair broken band-saw blades The sequence of operations in stud welding, which is used for welding bars, threaded rods, and various fasteners onto metal plates Flash-welding process for end-to-end welding of solid rods or tubular parts <ul style="display: flex;"><li style="flex:1">53 </li><li style="flex:1">54 </li></ul><ul style="display: flex;"><li style="flex:1">(c) </li><li style="flex:1">(d) </li></ul>• Explosion Welding (EXW) ꢀPressure applied by detonating a layer of explosives that has been placed over one of the components to be joined ꢀCold pressure welding by plastic deformation <ul style="display: flex;"><li style="flex:1">(a) </li><li style="flex:1">(b) </li></ul>(c) and (d) Cross-sections of explosion-welded joints Schematic illustration of the explosion welding process: (a) constant interface clearance gap and (b) angular interface clearance gap <ul style="display: flex;"><li style="flex:1">55 </li><li style="flex:1">56 </li></ul>14 <ul style="display: flex;"><li style="flex:1">• Diffusion Bonding/Welding (DFW) </li><li style="flex:1">• Standard Identification for Welds </li></ul>ꢀProcess in which the strength of the joint results primarily from diffusion and secondarily from plastic deformation of the faying surfaces ꢀAbility to fabricate sheet-metal structures by combining diffusion bonding with superplastic forming

Fundamentals of Joining Processes

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