WT NOTES PDF - Workshop Tech Notes

Workshop Tech. UNIT – 1 Table 1.2 RP systems and related base materials Prototyping Technologies Base Materials Selective laser sintering (SLS) Thermoplastics, Metals powders Fused Deposition Modeling (FDM)...

Workshop Tech. UNIT – 1 Table 1.2 RP systems and related base materials Prototyping Technologies Base Materials Selective laser sintering (SLS) Thermoplastics, Metals powders Fused Deposition Modeling (FDM) Thermoplastics, Eutectic metals. Stereo lithography (SLA) Photopolymer Laminated Object Manufacturing (LOM) Paper Electron Beam Melting (EBM) Titanium alloys 3D Printing (3DP) Various materials Additive manufacturing Techniques: 1.Laser Engineered Net Shaping (LENS) The LENSTM process builds components in an additive manner from powdered metals using a Nd: YAG laser to fuse powder to a solid as shown in Figure 5.15. It is a freeform metal fabrication process in which a fully dense metal component is formed. The LENSTM process comprises of the following steps. Steps A deposition head supplies metal powder to the focus of a high powered Nd:YAG laser beam to be melted. This laser is typically directed by fiber optics or precision angled mirrors. The laser is focused on a particular spot by a series of lenses, and a motion system underneath the platform moves horizontally and laterally as the laser beam traces the cross- section of the part being produced. The fabrication process takes place in a low-pressure argon chamber for oxygen-free operation in the melting zone, ensuring that good adhesion is accomplished. When a layer is completed, the deposition head moves up and continues with the next layer. The process is repeated layer by layer until the part is completed. The entire process is usually enclosed to isolate the process from the atmosphere. Generally, the prototypes need additional finishing, but are fully dense products with good grain formation. Principle The LENS process is based on the following two principles: A high powered Nd: YAG laser focused onto a metal substrate creates a molten puddle on the substrate surface. Powder is then injected into the molten puddle to increase material volume. A “printing” motion system moves a platform horizontally and laterally as the laser beam traces the cross-section of the part being produced. After formation of a layer of the part, the machine’s powder delivery nozzle moves upwards prior to building next layer. Advantages Superior material properties. The LENS process is capable of producing fully dense metal parts. Metal parts produced can also include embedded structures and superior material properties. The microstructure produced is also relatively good. Complex parts. Functional metal parts with complex features are the forte of the LENS system. Reduced post-processing requirements. Post-processing is minimized, thus reducing cycle time. Disadvantages Limited materials. The process is currently narrowly focused to produce only metal parts. Large physical unit size. The unit requires a relatively large area to house. High power consumption. The laser system requires very high wattage. 2.Direct Metal Deposition (DMD) A direct laser deposition (DLD) or direct metal deposition (DMD) process is a laser- assisted direct metal manufacturing process that uses computer controlled lasers that, in hours, weld air blown streams of metallic powders into custom parts and manufacturing molds. Some processes use wire instead of powder, but the concept is similar. A representative process is called the Laser Engineered Net Shaping (LENS) process. It uses CAD file cross-sections to control the forming process developed by Optomec Inc. The DLD process can be used throughout the entire product life-cycle for applications ranging from materials research to functional prototyping to volume manufacturing. An additional benefit is its unique ability to add material to existing components for service and repair applications. Powder-metal particles are delivered in a gas stream into the focus of a laser to form a molten pool of metal. It is a layer-by-layer additive rapid prototyping process. The DLD process allows the production of parts, molds, and dies that are made out of the actual end-material, such as aluminum or tool steel. In other words, this produces the high-temperature materials that are difficult to make using the traditional RP processes. The laser beam is moved back and forth across the part and creates a molten pool of metal where a precise stream of metal powder is injected into the pool to increase its size. This process is the hybrid of several technologies: lasers, CAD, CAM, sensors, and powder metallurgy. This process also improves on other methods of metalworking in that there is no waste material or subtractive processes necessary. It can also mix metals to specific standards and specifications in a manner that has never been possible before. Advantages: The strength of DLD lies in the process’ ability to fabricate fully dense metal parts with good metallurgical properties at reasonable speeds. DLD is an efficient approach that reduces production costs and speeds time to market for high-value components. The DLD systems enable the fabrication of novel shapes, hollow structures, and material gradients that are not otherwise feasible. Disadvantages: Since DLD is a freeform process, there is a limit to the overhang angle that can be built.  The traditional DLD or RP processes are using three-axis tables, and thus support structures are very often needed in building overhang parts. These structures are not desirable in laser-based processes involving metals. One could use a high melting- point material to build the support structures and use other processes, such as chemical etching, to remove the support material afterward. 3.Sheet Lamination (Laminated Object Manufacturing (LOM): There are two approaches of LOM process. I. Cut and then paste  Handling the cut pieces is difficult if not impossible since  More than one piece may have to be handled for every layer  Such pieces may be odd-shaped  Paper being flexible further complicates handling  A support mechanism will be required.  Suitable for laminated tooling. II. Paste and then cut Handling is easy – indexing of the reel is all that is required. The remaining stock acts as the support material. The only drawback is the time-consuming decubing operation. Suitable for paper-like flexible materials. Steps If multiple parts are to be made, one has to arrive at a cluster of optimal packing (an automatic program for this is still not available!). It is preferable to pack as many pieces as possible in processes such as LOM, SLS, SGC and 3DPrinitng. The object/ cluster is positioned and oriented in the desired place. Some users tilt it by 10 to 15 deg. to avoid any surface becoming horizontal (why?). Set the machine with the desired process parameters such as beam diameter, beam offset flag, grid sizes, number of dummy layers, bridging gap between two cuts etc. Load the paper roll of appropriate width. Identify the location for the build on the table and feed it to the machine. Paste a double-sided adhesive in that zone. Each slice or layer is realized using the following steps: The paper reel indexes by a fixed distance. It has adhesive at the bottom surface. The table rises to the required height. A hot roller (laminating tool) rolls over it causing it to stick to the previous layer. The height is measured and it is passed on to the slicing software. The loops of the slice are cut by the laser. It is possible to offset the laser beam by beam radius in such a direction as to compensate for it. This is followed by grid cutting around the bounding box of the stock. Note that the grids of all layers coincide. Finally, a parting off cut is made. The table lowers by a considerable distance so that the cut portion is stripped off from the reel. After all layers are made, the built volume is a rectangular block. This is parted off from the table using a thin wire rope. The unwanted material inside and surrounding are removed using hand tools. This is called ‘decubing’. This operation takes several hours. The part is finished and painted as required. It can be given a lacquer coat to prevent it from absorbing moisture. Advantages  Only boundaries are to be addressed and not their interiors.  It employs CO2 laser which is cheaper. No protective environment is required.  Paper is very cheap.  It gives strong wood-like parts. Ideal as patterns for casting Limitations  Grid cutting takes much more time than object cutting.  Decubing also is time-consuming.  Horizontal surfaces pose problems. Although it is solvable, it has not been done till date. 4. 3D Printing Very similar to SLS except that a binder liquid is spayed in selected regions instead of laser. Raw material is powder. Concept models can be prepared rapidly using a multi-jet multi-color spray over starch (ZCorp). Green parts will require sintering inside another furnace. When a binder is sprayed through thin nozzles on the selected region over a layer of powder, the particles in that region stick together. The remaining powder acts as support as in the case of LOM. Binder spray makes use of mechanical movement. However, use of multiple jets make it faster. Explicit support structures are not required. A wide variety of powders can be used. Steps Raw material is powder. The binder liquid is selectively deposited on the layer of powder. This is followed by a curing after which unbound powder is separated. 4. Fused Deposition Modeling (FDM) Molten material inside a hot chamber is extruded through a nozzle. Use of the raw material in wire form as a consumable piston is a great idea. he nozzle size alone does not decide the layer thickness and road width. They together depend on speed of head and wire feed speed. Their relation can be obtained from the principle of conservation of mass. (Analogy: applying tooth paste on the brush.) Explicit support structures are required. Therefore, twin heads are used, one for model and the other for support. Steps Starting material is melted and small droplets are shot by a nozzle onto previously formed layer Droplets cold weld to surface to form a new layer Deposition for each layer controlled by a moving x-y nozzle whose path is based on a cross section of a CAD geometric model that is sliced into layers Work materials include wax and thermoplastics Advantages  Any thermoplastic material can be used as long as the appropriate head is available.  It does not employ lasers and hence no safety related issues.  It does not use liquid.  powder raw materials and hence clean. It can be kept in an office environment as a3D printer.  Very easy to remove the support. This is probably the easiest of all RP processes.  This is the cheapest machine. However, this is also due to their business policy since the costs of all RP machines are comparable. Limitations  As every point of the volume is addressed by a „mechanical device“, it is very slow.  Not very accurate compared SLA, SGC etc. 6. Electron Beam melting (EBM) Electron beam melting (EBM) has become a successful approach to PBF (Powder Bed Fusion). In contrast to laser-based systems, EBM uses a high-energy electron beam to induce fusion between metal powder particles. This process was developed at Chalmers University of Technology, Sweden, and was commercialized by Arcam AB, Sweden, in2001. Laser beams heat the powder when photons are absorbed by powder particles. Electron beams, however, heat powder by transfer of kinetic energy from incoming electrons into powder particles. As powder particles absorb electrons they gain an increasingly negative charge. This has two potentially detrimental effects: (1) if the repulsive force of neighboring negatively charged particles overcomes the gravitational and frictional forces holding them in place, there will be a rapid expulsion of powder particles from the powder bed, creating a powder cloud (which is worse for fine powders than coarser powders) and (2) increasing negative charges in the powder particles will tend to repel the incoming negatively charged electrons, thus creating a more diffuse beam. There are no such complimentary phenomena with photons. As a result, the conductivity of the powder bed in EBM must be high enough that powder particles do not become highly negatively charged, and scan strategies must be used to avoid build-up of regions of negatively charged particles. In practice, electron beam energy is more diffuse, in part, so as not to build up too great a negative charge in any one location. As a result, the effective melt pool size increases, creating a larger heat-affected zone. ConsequentlyThe minimum feature size, median powder particle size, layer thickness, resolution, and surface finish of an EBM process are typically larger than for an mLS process. As mentioned above, in EBM the powder bed must be conductive. Thus, EBM can only be used to process conductive materials (e.g., metals) whereas, lasers can be used with any material that absorbs energy at the laser wavelength (e.g., metals, polymers, and ceramics). Electron beam generation is typically a much more efficient process than laser beam generation. 7. Selective laser Sintering (SLS) It is developed by University of Texas, Austin.It is marketed by DTM, USA and EOS, Germany. Raw material is powder. Principle is similar to Powder Metallurgy but for the absence of compaction. Green part is prepared on the RP machine after partial sintering and sintering is completed inside another furnace. Just as SLA, here also laser light is used. When it is scanned on the selected region over a layer of powder, the particles in that region fuse together. The remaining powder acts as support as in the case of LOM. Laser beam is positioned using a small mirror capable of deflecting in two directions. Therefore, this has very low inertia and hence high speed and accuracy. The power of the laser decides the layer thickness. Explicit support structures are not required. A wide variety of powders can be used. Steps When the slicing is done, The working volume is maintained with appropriate temperature so that laser supplies the energy required to cross the threshold sintering temperature. An inert environment is created using continuous supply of gas such as Nitrogen. This is to minimize fire hazards as the fine particles have high activation. Each slice or layer is realized using the following steps: The table dips by a layer thickness. A layer of powder is spread and leveled using a contra-rotating roller. The beam scans the layer of powder. Thus, the required region is “selectively sintered”. After all layers are made, the table rises completely revealing a block of cake with the part inside. The surrounding powder is soft and it is removed using suitable brushes. This powder is reusable. The part is kept in a suitable hot chamber to complete the sintering. The metallic prototypes require copper impregnation in another furnace to improve their polish ability. The part is finished and painted as required. Advantages A wide variety of powders can be used. Fast due to tiny moving mirror parts as in SLA. Metallic parts can be made. Suitable for making injection molding tools. Limitations Surface finish is less and dictated by the particle size. Z accuracy is poor due to the absence of milling. 9.Photopolymerization (Stereo lithography (SL) When a light of appropriate wave length falls on liquid photopolymer, the energy absorbed causes polymerization. The polymerized photopolymer will be in solid state. Laser light is used. When it is scanned on the selected region over a layer of liquid polymer, that region become solid. The remaining liquid can be drained. Laser beam is positioned using a small mirror capable of deflecting in two directions. Therefore, this has very low inertia and hence high speed and accuracy. The power of the laser decides the layer thickness. Explicit support structures are required. This is achieved by modifying the geometry of the prototype. Typically bristles and thin structures are added. 1.At the start of the process, in which the initial layer is added to the platform 2.After several layers have been added so that the part geometry gradually takes form. Steps Support structures are automatically added to the model wherever required. Slicing is done. Each slice or layer is realized using the following steps: The table (called vat) dips and comes up to the required Z level. A blade wipes off the excess liquid. The beam scans the liquid layer. For each loop, the border is made and then area filling is done. Area filling is not in zig-zag pattern but in grids. After all layers are made, the table rises completely revealing the part. After the liquid has drained, it is removed from the table and the support structure is carefully cut off. The part is kept in a post-cure apparatus where it is kept under UV radiation for an hour or so. This completes polymerization. The part is finished and painted as required UNIT – 2 UNIT – 3 UNIT – 4  UNIT 3: WELDING Definition: Welding is a process of joining similar or dissimilar materials by the application of heat and/or pressure. Principle of welding: If two surfaces are brought together in such a way that nothing but the grain boundaries separate them then the two bodies with adhere with a very large force resulting in what we called welding. Types of welding: 1 Fusion Welding Processes Fusion welding is a joining process that uses fusion of the base metal to make the weld. The three major types of fusion welding processes are as follows: 1. Gas welding: Oxyacetylene welding (OAW) 2. Arc welding: Shielded metal arc welding (SMAW) Gas–tungsten arc welding (GTAW) Gas–metal arc welding (GMAW) Submerged arc welding (SAW) 3. High-energy beam welding: Laser beam welding (LBW) Electron Beam Welding (EBW) OXYACETYLENE WELDING The Process Gas welding is a welding process that melts and joins metals by heating them with a flame caused by the reaction between a fuel gas and oxygen. Oxyacetylene welding (OAW), shown in Figure 1, is the most commonly used gas welding process because of its high flame temperature. A flux may be used to deoxidize and cleanse the weld metal. The flux melts, solidifies, and forms a slag skin on the resultant weld metal. Figure 2 shows three different types of flames in oxyacetylene welding: neutral, reducing, and oxidizing (4), which are described next. Three Types of Flames A. Neutral Flame This refers to the case where oxygen (O2) and acetylene (C2H2) are mixed in equal amounts and burned at the tip of the welding torch. A short inner cone and a longer outer envelope characterize a neutral flame (Figure 2a). The inner cone is the area where the primary combustion takes place through the chemical reaction between O2 and C2H2, as shown in Figure 3. The heat of this reaction accounts for about two- thirds of the total heat generated. The products of the primary combustion, CO and H2, react with O2 from the surrounding air and form CO2 and H2O. This is the secondary combustion, which accounts for about one-third of the total heat generated. The area where this secondary combustion takes place is called the outer envelope. It is also called the protection envelope since CO and H2 here consume the O2 entering from the surrounding air, thereby protecting the weld metal from oxidation. For most metals, a neutral flame is used. B. Reducing Flame When excess acetylene is used, the resulting flame is called a reducing flame. The combustion of acetylene is incomplete. As a result, a greenish acetylene feather between the inert cone and the outer envelope characterizes a reducing flame (Figure 2b). This flame is reducing in nature and is desirable for welding aluminum alloys because aluminum oxidizes easily. It is also good for welding high-carbon steels (also called carburizing flame in this case) because excess oxygen can oxidize carbon and form CO gas porosity in the weld metal. C. Oxidizing Flame When excess oxygen is used, the flame becomes oxidizing because of the presence of unconsumed oxygen. A short white inner cone characterizes an oxidizing flame 2 (Figure 2c). This flame is preferred when welding brass because copper oxide covers the weld pool and thus prevents zinc from evaporating from the weld pool. Figure 3.1 Oxyacetylene welding: (a) overall process; (b) welding area enlarged. Figure 3.2 Three types of flames in oxyacetylene welding. 3 Figure 3.3 Chemical reactions and temperature distribution in a neutral oxyacetylene flame. Advantages and Disadvantages of Gas welding The main advantage of the oxyacetylene welding process is that the equipment is simple, portable, and inexpensive. Therefore, it is convenient for maintenance and repair applications. However, due to its limited power density, the welding speed is very low and the total heat input per unit length of the weld is rather high, resulting in large heat-affected zones and severe distortion.The oxyacetylene welding process is not recommended for welding reactive metals such as titanium and zirconium because of its limited protection power. SHIELDED METAL ARC WELDING The Process Shielded metal arc welding (SMAW) is a process that melts and joins metals by heating them with an arc established between a sticklike covered electrode and the metals, as shown in Figure 4. It is often called stick welding. The electrode holder is connected through a welding cable to one terminal of the power source and the workpiece is connected through a second cable to the other terminal of the power source (Figure 4a). The core of the covered electrode, the core wire, conducts the electric current to the arc and provides filler metal for the joint. For electrical contact, the top 1.5 cm of the core wire is bare and held by the electrode holder. The electrode holder is essentially a metal clamp with an electrically insulated outside shell for the welder to hold safely. The heat of the arc causes both the core wire and the flux covering at the electrode tip to melt off as droplets (Figure 4b). The molten metal collects in the weld pool and solidifies into the weld metal.The lighter molten flux, on the other hand, floats on the pool surface and solidifies into a slag layer at the top of the weld metal. Functions of Electrode Covering The covering of the electrode contains various chemicals and even metal powder in order to perform one or more of the functions described below. A. Protection It provides a gaseous shield to protect the molten metal from air. For a cellulose- type electrode, the covering contains cellulose, (C6H10O5)x. A large volume of gas mixture of H2, CO, H2O, and CO2 is produced when cellulose in the electrode covering is heated and 4 decomposes. For a limestone-(CaCO3) type electrode, on the other hand, CO2 gas and CaO slag form when the limestone decomposes. The limestone-type electrode is a low-hydrogen type electrode because it produces a gaseous shield low in hydrogen. It is often used for welding metals that are susceptible to hydrogen cracking, such as high-strength steels. B. Deoxidation It provides deoxidizers and fluxing agents to deoxidize and cleanse the weld metal. The solid slag formed also protects the already solidified but still hot weld metal from oxidation. Figure 3.4 Shielded metal arc welding: (a) overall process; (b) welding area enlarged. C. Arc Stabilization It provides arc stabilizers to help maintain a stable arc. The arc is an ionic gas (a plasma) that conducts the electric current. Arc stabilizers are compounds that decompose readily into ions in the arc, such as potassium oxalate and lithium carbonate. They increase the electrical conductivity of the arc and help the arc conduct the electric current more smoothly. D. Metal Addition It provides alloying elements and/or metal powder to the weld pool. The former helps control the composition of the weld metal while the latter helps increase the deposition rate. Advantages and Disadvantages of SMAW The welding equipment is relatively simple, portable, and inexpensive as compared to other arc welding processes. For this reason, SMAW is often used for maintenance, repair, and field construction. However, the gas shield in SMAW is not clean enough for reactive metals such as aluminum and titanium. The deposition rate is limited by the fact that the electrode covering tends to overheat and fall off when excessively high welding currents are used. The limited length of the electrode (about 35 cm) requires electrode changing, and this further reduces the overall production rate. 5  GAS–TUNGSTEN ARC WELDING The Process Gas–tungsten arc welding (GTAW) is a process that melts and joins metals by heating them with an arc established between a nonconsumable tungsten electrode and the metals, as shown in Figure 5. The torch holding the tungsten electrode is connected to a shielding gas cylinder as well as one terminal of the power source, as shown in Figure 5a. The tungsten electrode is usually in contact with a water-cooled copper tube, called the contact tube, as shown in Figure 5b, which is connected to the welding cable (cable 1) from the terminal. This allows both the welding current from the power source to enter the electrode and the electrode to be cooled to prevent overheating. The workpiece is connected to the other terminal of the power source through a different cable (cable 2). The shielding gas goes through the torch body and is directed by a nozzle toward the weld pool to protect it from the air. Protection from the air is much better in GTAW than in SMAW because an inert gas such as argon or helium is usually used as the shielding gas and because the shielding gas is directed toward the weld pool. For this reason, GTAW is also called tungsten–inert gas (TIG) welding. However, in special occasions a noninert gas can be added in a small quantity to the shielding gas. Therefore, GTAW seems a more appropriate name for this welding process. When a filler rod is needed, for instance, for joining thicker materials, it can be fed either manually or automatically into the arc. Figure 3.5. Gas–tungsten arc welding: (a) overall process; (b) welding area enlarged. 6 Polarity Figure 6 shows three different polarities in GTAW, which are described next. A. Direct-Current Electrode Negative (DCEN) This, also called the straight polarity, is the most common polarity in GTAW. The electrode is connected to the negative terminal of the power supply. As shown in Figure 6a, electrons are emitted from the tungsten electrode and accelerated while traveling through the arc. A significant amount of energy, called the work function, is required for an electron to be emitted from the electrode. When the electron enters the workpiece, an amount of energy equivalent to the work function is released. This is why in GTAW with DCEN more power (about two-thirds) is located at the work end of the arc and less (about one-third) at the electrode end. Consequently, a relatively narrow and deep weld is produced. B. Direct-Current Electrode Positive (DCEP) This is also called the reverse polarity. The electrode is connected to the positive terminal of the power source. As shown in Figure 6b, the heating effect of electrons is now at the tungsten electrode rather than at the workpiece. Consequently, a shallow weld is produced. Furthermore, a large-diameter, water-cooled electrodes must be used in order to prevent the electrode tip from melting. The positive ions of the shielding gas bombard the workpiece, as shown in Figure 7, knocking off oxide films and producing a clean weld surface. Therefore, DCEP can be used for welding thin sheets of strong oxide-forming materials such as aluminium and magnesium, where deep penetration is not required. C. Alternating Current (AC) Reasonably good penetration and oxide cleaning action can both be obtained, as illustrated in Figure 6c. This is often used for welding aluminum alloys. Figure 3.6 Three different polarities in GTAW. Figure 3.7 Surface cleaning action in GTAW with DC electrode positive 7 Electrodes Tungsten electrodes with 2% cerium or thorium have better electron emissivity, current- carrying capacity, and resistance to contamination than pure tungsten electrodes (3). As a result, arc starting is easier and the arc is more stable. The electron emissivity refers to the ability of the electrode tip to emit electrons. A lower electron emissivity implies a higher electrode tip temperature required to emit electrons and hence a greater risk of melting the tip. Shielding Gases Both argon and helium can be used. Table 1 lists the properties of some shielding gases (6). As shown, the ionization potentials for argon and helium are 15.7 and 24.5 eV (electron volts), respectively. Since it is easier to ionize argon than helium, arc initiation is easier and the voltage drop across the arc is lower with argon. Also, since argon is heavier than helium, it offers more effective shielding and greater resistance to cross draft than helium. With DCEP or AC, argon also has a greater oxide cleaning action than helium. These advantages plus the lower cost of argon make it more attractive for GTAW than helium. Because of the greater voltage drop across a helium arc than an argon arc, however, higher power inputs and greater sensitivity to variations in the arc length can be obtained with helium. The former allows the welding of thicker sections and the use of higher welding speeds. The latter, on the other hand, allows a better control of the arc length during automatic GTAW. Advantages and Disadvantages Gas–tungsten arc welding is suitable for joining thin sections because of its limited heat inputs. The feeding rate of the filler metal is somewhat independent of the welding current, thus allowing a variation in the relative amount of the fusion of the base metal and the fusion of the filler metal. Therefore, the control of dilution and energy input to the weld can be achieved without changing the size of the weld. It can also be used to weld butt joints of thin sheets by fusion alone, that is, without the addition of filler metals or autogenous welding. Since the GTAW process is a very clean welding process, it can be used to weld reactive metals, such as titanium and zirconium, aluminum, and magnesium. However, the deposition rate in GTAW is low. Excessive welding currents can cause melting of the tungsten electrode and results in brittle tungsten inclusions in the weld metal. However, by using preheated filler metals, the deposition rate can be improved. TABLE 3.1 Properties of Shielding Gases Used for Welding 8  GAS–METAL ARC WELDING The Process Gas–metal arc welding (GMAW) is a process that melts and joins metals by heating them with an arc established between a continuously fed filler wire electrode and the metals, as shown in Figure 8. Shielding of the arc and the molten weld pool is often obtained by using inert gases such as argon and helium, and this is why GMAW is also called the metal–inert gas (MIG) welding process. Since noninert gases, particularly CO2, are also used, GMAW seems a more appropriate name. This is the most widely used arc welding process for aluminum alloys. Figure 3.8 Gas–metal arc welding: (a) overall process; (b) welding area enlarged. Modes of metal transfer: Modes of metal transfer significantly affect the depth of penetration, stability of weld pool and amount of spatter loss. Various forces cause the transfer of metal into the weld pool. The mode of transfer depends on the intersection of these forces and governs the ability of welding in different positions. The major forces which take part in this process are those due to (i) gravity, (ii) surface tension, (iii) electromagnetic interaction 1. Metal transfer under the influence of gravity: The force due to gravity may be retaining or detaching force, depending on whether the electrode is pointing upward or downward. 2. Metal droplet under the action of surface tension: Surface tension always tends to retain the liquid drop at the tip of the electrode. This force depends on the radius of the electrode and the density of the liquid metal. 3. Metal transfer under the action of electromagnetic force: The electromagnetic force, known as Lorenz force, is setup due to the interaction of the electric current with its own magnetic field. This force acts in the direction of the current when the cross section 9  of the conductor is increasing in the direction of the current. Similarly, the force acts in the direction opposite to that of the current if the cross section of the conductor is reducing in the direction of current. The hydrostatic pressure is created due to the magnetic force. As a result, the liquid drop is projected along the line of the electrode, independent of gravity. All these forces interact in a complicated manner and give rise to two broad classes of metal transfer. 1. Free flight transfer. (a) Globular, (b) spray transfer. 2. Short circuit transfer. (A). Globular transfer: Discrete metal drop close to or larger then electrode diameter travel across the arc gap under the influence of gravity. Globular transfer often is not smooth and produce spatter at relatively low welding current. G T occurs regardless of the type of shielding gases. (b). Spray Transfer: Above a critical current level small discrete metal drops travel across the arc gap under the action of electromagnetic force at much higher frequency and speed than in globular mode. Metal transfer is much more stable and spatter free. (c) Short circuit transfer: In Short circuit transfer the liquid drop at the tip of the electrode gets in contact with the weld pool before being detached from the electrode. Thus, the arc is momentarily short circuited. However, due to the surface tension and the electromagnetic force, the drop is pulled into the weld pool and the contact with the electrode is broken. Short- circuiting transfer encompasses the lowest range of welding currents and electrode diameters. It produces a small and fast freezing weld pool that is desirable for welding thin sections and overhead position welding. Figure 3.9: Modes of metal transfer (a) Globular (b) Spray Advantages and Disadvantages Like GTAW, GMAW can be very clean when using an inert shielding gas. The main advantage of GMAW over GTAW is the much higher deposition rate, which allows thicker workpieces to be welded at higher welding speeds. The dual-torch and twin-wire processes further increase the deposition rate of GMAW (12). The skill to maintain a very short and yet stable arc in GTAW is not required. However, GMAW guns can be bulky and difficult-to-reach small areas or corners. 10  SUBMERGED ARC WELDING The Process Submerged arc welding (SAW) is a process that melts and joins metals by heating them with an arc established between a consumable wire electrode and the metals, with the arc being shielded by a molten slag and granular flux, as shown in Figure 10. This process differs from the arc welding processes discussed so far in that the arc is submerged and thus invisible.The flux is supplied from a hopper (Figure 10a), which travels with the torch. No shielding gas is needed because the molten metal is separated from the air by the molten slag and granular flux (Figure 10b). Direct-current electrode positive is most often used. However, at very high welding currents (e.g., above 900A) AC is preferred in order to minimize arc blow. Arc blow is caused by the electromagnetic (Lorentz) force as a result of the interaction between the electric current itself and the magnetic field it induces. Figure 3.10 Submerged arc welding: (a) overall process; (b) welding area enlarged. Advantages and Disadvantages The protecting and refining action of the slag helps produce clean welds in SAW. Since the arc is submerged, spatter and heat losses to the surrounding air are eliminated even at high welding currents. Both alloying elements and metal powders can be added to the granular flux to control the weld metal composition and increase the deposition rate, respectively. Using two or more electrodes in tandem further increases the deposition rate. Because of its high deposition rate, workpieces much thicker than that in GTAW and GMAW can be welded by SAW. However, the relatively large volumes of molten slag and metal pool often limit SAW to flat-position welding and circumferential welding (of pipes). The relatively high heat input can reduce the weld quality and increase distortions. 11  LASER BEAM WELDING The Process Laser beam welding (LBW) is a process that melts and joins metals by heating them with a laser beam. The laser beam can be produced either by a solid- state laser or a gas laser. In either case, the laser beam can be focused and directed by optical means to achieve high power densities. In a solid-state laser, a single crystal is doped with small concentrations of transition elements or rare earth elements. For instance, in a YAG laser the crystal of yttrium– aluminum– garnet (YAG) is doped with neodymium. The electrons of the dopant element can be selectively excited to higher energy levels upon exposure to high-intensity flash lamps, as shown in Figure 11a. Lasing occurs when these excited electrons return to their normal energy state, as shown in Figure 11b.The power level of solid-state lasers has improved significantly, and continuous YAG lasers of 3 or even 5 kW have been developed. In a CO2 laser, a gas mixture of CO2, N2, and He is continuously excited by electrodes connected to the power supply and lases continuously. Higher power can be achieved by a CO2 laser than a solid-state laser, for instance, 15kW. Figure 12a shows LBW in the keyholing mode. Figure 12b shows a weld in a 13-mm-thick A633 steel made with a 15-kW CO2 laser at 20mm/s (18). Besides solid-state and gas lasers, semiconductor-based diode lasers have also been developed. Diode lasers of 2.5kW power and 1mm focus diameter have been demonstrated (19). While keyholing is not yet possible, conduction mode (surface melting) welding has produced full-penetration welds with a depth–width ratio of 3 : 1 or better in 3-mm-thick sheets. Reflectivity The very high reflectivity of a laser beam by the metal surface is a well-known problem in LBW. As much as about 95% of the CO2 beam power can be reflected by a polished metal surface. Reflectivity is slightly lower with a YAG laser beam. Surface modifications such as roughening, oxidizing, and coating can reduce reflectivity significantly (20). Once keyholing is established, absorption is high because the beam is trapped inside the hole by internal reflection. Advantages and Disadvantages Like EBW, LBW can produce deep and narrow welds at high welding speeds, with a narrow heat-affected zone and little distortion of the workpiece. It can be used for welding dissimilar metals or parts varying greatly in mass and size. Unlike EBW, however, vacuum and x-ray shielding are not required in LBW. However, the very high reflectivity of a laser beam by the metal surface is a major drawback, as already mentioned. Like EBW, the equipment cost is very high, and precise joint fit-up and alignment are required. 12 Figure 3.11 Laser beam welding with solid-state laser: (a) process; (b) energy absorption and emission during laser action. Figure 3.12 Laser beam welding with CO2 laser: (a) process; (b) weld in 13-mm-thick A633 steel. 13  ELECTRIC RESISTANCE WELDING The electric resistance welding is commonly used. It can be applied to any metals. Electric current passes through the materials being joined. The resistance offered to the flow of current results in raising the temperature of the two metal pieces to melting point at their junction. Mechanical pressure is applied at this moment to complete the weld. Two copper electrodes of low resistance are used in a circuit. The mechanical pressure or force required after the surfaces are heated to a plastic temperature is approximately 0.3 N/m2 at the welded surface. This method of welding is widely used in modern practice for making welded joints in sheet metal parts, bars and tubes etc. Parameter Affecting Resistance Welding Successful application of Resistance welding process depends upon correct application and proper control of the following factors. 1. Current: Enough current is needed to bring the metal to its plastic state of welding. 2. Pressure: Mechanical pressure is applied first to hold the metal pieces tightly between the electrodes, while the current flows through them called weld pressure, and secondly when the metal has been heated to its plastic state, to forge the metal pieces together to form the weld, called forge pressure. 3. Time of Application: It is the cyclic time and the sum total of the following time period allowed during different stages of welding a. Weld Time Time period during which the welding current flow through the metal pieces to raise their temp. b. Forge Time Time period during which the forge pressure is applied to the metal pieces. c. Hold Time Time period during which the weld to be solidify. d. Off Time The period of time from the release of the electrodes to the start of the next weld cycle. 4. Electrode contact area: The weld size depends on the contact area of the face of the Electrodes TYPES OF RESISTANCE WELDING 1. Spot welding 2. Seam welding, 3. Projection welding Spot Welding Spot welding is used to lap weld joints in thin metallic plates (up to 12.7 mm thick) for mechanical strength and not for tightness. The metallic plates are overlapped and held between two copper electrodes. A high current, depending upon plate thickness, at a very low volt-age (4-12 volts), is passed between the electrodes. The contact resistance of the plates causes to heat rapidly to a plastic state. Mechanical pressure is applied. Supply is cut-off for the metal to regain strength. The pressure is released. The process is repeated at another portion of the plates. Thus, spot joints at regular interval depending upon the strength required are obtained. The electrodes are water cooled to avoid overheating and softening of the tips. Spot welding is 14 mainly used in the manufacture of automobile parts refrigerators, metallic toys, racks, frames, boxes, radio chassis, etc. Figure 3.13 (a) Spot Welding Seam Welding The metallic plates are held by two copper roller electrodes with one roller driven by motor so that the plates are moved between the rollers at a suitable speed. The high current is passed between the electrodes holding metallic plates pressed together with suitable force and pushes together to travel between the revolving electrodes as showing in Fig. 7.29. The plates between the electrodes get heated to welding (fusion) heat and welded continuously under constant pressure of rotating electrodes. This is a quicker operation than spot welding and gives a stronger joint. The process is employed for pressure tight joints on oil drums, tanks and boiler water pipes, refrigeration parts, motorcar body, utensils, stoves, etc Figure 3.13 (b): Seam welding 15 Projection Welding There are raised projections in the workpiece at all points where a weld is desired as shown in Fig. 13 (c). As the current is switched on the projection are melted and the workpieces pressed together to complete the weld. The melted projections form the welds. This method enables production of several spot welds simultaneously. Figure 3.13 (c): Projection welding ULTRASONIC WELDING 3.14 3.14 16 17  Weld Defects 15 3.15 15 3.15 18 3.16 16 16 3.16 19  3.17 17 17 3.17 20