Welding Process free essay sample

Energy beam welding methods, namely laser beam welding and electron beam welding, are relatively new processes that have become quite popular in high production applications. The two processes are quite similar, differing most notably in their source of power. Laser beam welding employs a highly focused laser beam, while electron beam welding is done in a vacuum and uses an electron beam. Both have a very high energy density, making deep weld penetration possible and minimizing the size of the weld area. Both processes are extremely fast, and are easily automated, making them highly productive. The primary disadvantages are their very high equipment costs (though these are decreasing) and a susceptibility to thermal cracking. Developments in this area include laser-hybrid welding, which uses principles from both laser beam welding and arc welding for even better weld properties. Electron Beam Welding Introduction: Electron Beam Welding (EEW) is a unique way of delivering large amounts of concentrated thermal energy to materials being welded. We will write a custom essay sample on Welding Process or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page It became viable as a production process in the late 1950s. At that time, it was used mainly in the aerospace and nuclear industries. Since then, it has become the welding technique with the widest range of applications. This has resulted from the ability to use the very high energy density of the beam to weld parts ranging in sizes from very delicate small components using just a few watts of power to welding steel at a thickness of 10 to 12 inches with 100 Kilowatts or more. However, even today most of the applications are less than 1/2 in thickness, and cover a wide variety of metals and even dissimilar metal joints How it works: The most common Electron Beam systems used in manufacturing today are of the high vacuum design. The other machine types are: 1- Partial vacuum equipment. 2- Non-vacuum equipment. These two types are used in mass production where high output is important. The diagram shown shows the classic triode gun and column assembly. The triode gun design consists of the cathode (Filament), Bias cup (Grid) and Anode. Other sub-assembly components that contribute to the triode are: High voltage insulator Feed-through, high voltage cable and deflection coils. All these components are housed in a vacuum vessel called the upper column. The column assembly is held under a high vacuum by an isolation valve positioned below the anode assembly. The vacuum environment provides several benefits: †¢Removes the bulk gas molecules necessary for a stable triode. †¢Provides protection for the incandescent filament against oxidization. †¢Provides a controlled environment to protect the gun against welding by product. BeamFormation:UpperColumn The beam formatting begins with the emission of electrons from the incandescently heated tungsten filament. During this process the filament is saturated by a determined amount of the electrical current. Electrons boil off the filament tip as it reaches operating temperatures and gathers in the grid cup assembly. A negative high voltage potential (acceleration voltage) is applied to the filament cathode assembly with the cathode assembly charged at 150 kV the only force preventing the electron beam from propagating is a secondary negatively charged voltage that resides on the grid cup or bias assembly. This voltage respectively lower than the accelerating voltage acts as a valve that controls the volume of electron energy that can flow from the cathode emitter to its attracting target. The anode at a positive potential is one of the attracting targets in the triode but its role is more of a beam formation device rather than a collector of electrons. The secondary target is the work piece which is usually metallic and offers a conductive path to earth to complete the circuit. The electron gun assembly design is a result of some extensive engineering studies and experimentation. Some of the early triode designs were mathematically modeled and their designs still produced today. Beam Delivery: Lower Column Other important components of the beam delivery column are the focus and deflection coils and isolation valve. The magnetic focus coil located beneath the anode assembly provides the means for squeezing the beam into a tightly focused stream of energy or can be used to widely dispersed energy resource. The deflection coil is another important component that will contribute to the latter discussion of beam control parameters but for now we will simply say that it is a steering device. The focus coil is circular in design and is concentric with the column. An electrical current is passed through the coil which produces the resultant magnetic fluxes that act to converge the electron beam. The deflection coil is configured with four separately wound coils positioned at right angles to the column. The four coils are segmented as sets (x and y) each axis becomes a separate control allowing the energizing of each axis on command, thus steering the beam. Many industrial applications require the precise manipulation of the beam energy so as to provide a pattern for processing. This is usually accomplished by superimposing an AC signal onto the four coils simultaneously therefore creating a specific pattern. The isolation valve serves to isolate the vacuum environment in the upper column from the lower. After the electron beam has passed through the lower column, it enters the chamber cavity. Another important part of the lower column of the (EBW) machine is the viewing optics, the optics are arranged in the lower column in such a manner that when viewing the beam energy through a video camera or magnified optics it gives the view from a parallel plane, giving the viewer the perception of looking down the column. Beam Interaction in Chamber Cavity: As the beam enters the chamber cavity it is aimed onto a target material placed at a determined height representative of the actual work piece. This procedure is typical in most pre-weld set-up requirements. The welding technician would then follow a process of beam alignment and beam parameter calibration. Unlike laser, the preparation is quite different in the fact that the technician must view the actual beam through the optical system in order to verify the beam alignment and focus. With a laser beam, the technician could not view the beam quality and therefore must rely on instrumentation to profile the beam energy. Once the beam has been tuned and calibrated the equipment is now ready for part processing. The focused beam of electrons is impinged at a targeted location on the weld joint at which point the kinetic energy of the electrons is converted to thermal energy. The work piece can either be stationary and the beam energy deflected or the work piece can be traversed along a desired axis of motion. This motion can be computer controlled such as a CNC table or simply a rotating mechanism can be employed. As the beam energy is applied to the moving part several physical transformations take place. The material instantly begins to melt at the surface and a rapid vaporization occurs followed by the resultant coalescence. Two welding modes are used in the (EBW): 1) Conductance mode: Mainly applicable to thin materials, heating of the weld joint to melting temperature is quickly generated at below the materials surface followed by thermal conductance throughout the joint for complete or partial penetration. The resulting weld is very narrow for two reasons: a) It is produced by a focused beam spot with energy densities concentrated into a . 010 to. 030 area. b) The high energy density allows for quick travel speeds allowing the weld to occur so fast that the adjacent base metal does not absorb the excess heat therefore giving the E. B. process its distinct minimal heat affected zone. 2) Keyhole mode: It is employed when deep penetration is a requirement. This is possible since the concentrated energy and velocity of the electrons of the focused beam are capable of subsurface penetration. The subsurface penetration causes the rapid vaporization of the material thus causing a hole to be drilled through the material. In the whole cavity the rapid vaporization and sputtering causes a pressure to develop thereby suspending the liquidus material against the cavity walls. As the hole is advanced along the weld joint by motion of the work piece the molten layer flows around the beam energy to fill the hole and coalesce to produce a fusion weld. The hole and trailing solidifying metal resemble the shape of an old fashion keyhole. Both the conductance and keyhole welding modes share physical features such as narrow welds and minimal heat affected zone . The basic difference is that a keyhole weld is a full penetration weld and a conductance weld usually carries a molten puddle and penetrates by virtue of conduction of thermal energy. Advantages: 1. Deeper and narrower: Ability to achieve a high depth-to-width ratio eliminating multiple-pass welds. 2. Low heat input: Minimal shrinkage and distortion as well as ability to weld in close proximity to heat sensitive components. 3. Superior strength: Vacuum melt quality can yield 95% strength of base material. 4. Versatility: From . 001 to 3 deep penetration welds, each performed with exceptional control and repeatability. 5. High purity: Vacuum environment eliminates impurities such as oxides and nitrides. 6. Superior process: Permits welding of refractory metals and combinations of many dissimilar metals not easily weld with conventional welding processes. Disadvantages 1. All metals which can be welded by arc processes can be also welded by Electron-Beam-Welding, including exotic ones. Equipment tends to be expensive though, but there are joints which cannot be economically welded by any other process. Finally Electron-Beam-Welding is successfully applied for repair, overhaul and maintenance of expensive items, because it has almost no influence on nearby material. . On the sides of disadvantages of Electron-Beam-Welding one must include the elevated cost of equipment, the relatively high pump-down time, the need for properly designed joints, special fixtures and expert personnel. Laser Beam Welding Introduction: Laser Beam Welding (LBW) is a modern welding process; it is a high energy beam process that continues to expand into modern industries and new applications because of its many advantages like deep weld penetration and minimizing heat inputs. The turn by the manufacturers to automate the welding processes has also caused to the expansion in using high technology like the use of laser and computers to improve the product quality through more accurate control of welding processes. Major Difference: The main difference between traditional electric arc welding processes is in the mode of energy transfer. Unlike electric arc energy transfer, laser energy absorption by a material is affected by many factors like the type of the laser, incident power density and the base metal’s surface condition. Two important factors to help characterizing laser welding are: ) The energy transfer efficiency, -The ratio of the heat observed by the work piece to the incident laser energy. b) The melting efficiency -The ratio of the heat to just melt the fusion zone to the heat observed by the work piece. The laser output is not electrical because does not require electrical continuity. It is also not influenced by magnetism and not limite d to electrically conductive materials. It can contract with any material and its function doesn’t require a vacuum nor does it produce x-rays. How it works: The focal spot is targeted on the work piece surface which will be welded. At the surface the large concentration of light energy is converted into thermal energy. The surface of the work piece starts melting and progresses through it by surface conductance. For welding, the beam energy is maintained below the vaporization temperature of the work piece material because hole drilling or cutting vaporization is required because the penetration of the work piece depends on conducted heat. The thickness of the materials to be welded is generally less than 0. 80 inches if the ideal metallurgical and physical characteristics of laser welding must be realized. Concentrated energy produces melting and coalescence before a heat affected zone is developed. When the materials to be welded are thick and have high thermal conductivity like aluminum. The advantage of having a minimal heat affected zone can be seriously affected because the heat source in this type of welding process is the energy of light, the work piece will be welded purely which means the fatigue strength of the welded joint will be excellent. Energy distribution across the beam is generated by the design of the resonant cavity, including mirror curvatures or shape and their relative arrangement. This combination results in photon oscillation within the cavity specific output beam energy patterns, these patterns are called Transverse Energy Modes (TEMs). The function of all laser beam welding processes whether they be gas (carbon dioxide, helium, neo, etc. ) or other lasing sources is based on the principles of the excitation of atoms using intense light, electricity, electron beams, chemicals and etc. The role of focusing lenses in this process is really important because it concentrates the beam energy into a focal spot as small as 0. 005 in diameters or even less. Like mentioned above there are many types of Laser Beam Welding (LBW) but the most popular types in the industry are: 1. Nd:YAG (neodymium-yttrium aluminum garnet) Laser: The Nd: YAG laser uses a man-made crystal as its active medium and produces light with a 1. 06-micron wavelength. 2. Carbon Dioxide Lasers: The CO2 laser uses a mixture of gases including CO2 as the active medium and produces light with a 10. 6-micron wavelength. 3. The Diode Laser: The diode laser uses a semi-conductor diode material as its active medium can be manufactured to produce one of several wavelengths. Industries Served: 1- Aerospace. 2- Defense/military. 3- Electronics. 4- Research development. 5- Medical. 6- Sensors instrumentation. 7- Petrochemical refining. 8- Communications energy. Advantages: 1) Deep and narrow welds can be done. 2) Absence of distortion in welds created. 3) Minimal heat affected zones in welds created. 4) Excellent metallurgical quality will be established in welds. 5) Ability to weld smaller, thinner components. 6) Increased travel speeds. 7) Non-contact welding. Laser-hybrid Welding It is a new type of welding process that combines the principles of laser eam welding and arc welding. Introduction: The combination of laser light and arc into an amalgamated welding process is known since the 1970’s, but for a long time thereafter no further research and development was undertaken. Recently, researchers have turned their attention to this topic again and attempted to unite the advantages of the arc with those of the laser in a hybrid weld process. W hereas in the early days, laser sources still had to prove their suitability for industrial use, nowadays they are standard technological equipment in many manufacturing enterprises. The combination of laser welding with another weld process is called â€Å"hybrid welding process†. This means that a laser beam and an arc act simultaneously in one welding zone, they influence and support each other. Laser: Laser welding not only requires high laser power but also a high quality beam to obtain the desired â€Å"deep-weld effect†. The resulting higher quality of beam can be exploited either to obtain a smaller focus diameter or a larger focal distance. For the projects that are currently underway, a lamp-pumped solid state laser with a laser beam power of 4 kW is used. The laser light is transmitted via a 600? m glass fibre, in which the beginning and the end is water-cooled. The laser beam is projected onto the work piece by a focusing module with a focal distance of 200 mm. Laser Hybrid process: For welding metallic work pieces, the Nd:YAG laser beam is focused to obtain intensities of more than 106W/cm2. When the laser beam hits the surface of the material, this spot is heated up to vaporization temperature, and a vapor cavity is formed in the weld metal due to the escaping metal vapor. The extraordinary feature of the weld seam is its high depth-to-width ratio. The energy-flow density of the freely burning arc is slightly more than 104 W/cm2. Unlike a sequential configuration where two separate weld processes act in succession, hybrid welding may be viewed as a combination of both weld processes acting simultaneously in one and the same process zone. Depending on the kind of arc or laser process used, and depending on the process parameters, the processes will influence each other to a different extent and in different ways. The combination of the laser process and the arc process, there is also an increase in both weld penetration depth and welding speed (as compared to each single process). The metal vapor escaping from the vapor cavity acts upon the arc plasma. Absorption of the Nd:YAG laser radiation in the processing plasma remains negligible. Depending on the ratio of the two power inputs, the character of the overall process may be mainly determined either by the laser or by the arc. Absorption of the laser radiation is substantially influenced by the temperature of the work piece surface. Before the laser welding process can start, the initial reflectance must be overcome, especially on aluminum surfaces. This can be achieved by starting welding with a special start program. After the vaporization temperature has been reached, the vapor cavity is formed, and nearly all radiation energy can be put into the work piece. The energy required for this is thus determined by the temperature dependent absorption and by the amount of energy lost by conduction into the rest of the work piece. In Laser Hybrid welding, vaporization takes place not only from the surface of the work piece but also from the filler wire, so that more metal vapor is available, which in turn facilitates the input of the laser radiation. Advantages Laser hybrid process provides a fast welding speed and large gap capability. Other benefits include the penetration properties of laser welding and the addition of consumables and bridge-building properties of MIG. The use of consumables in the hybrid method means that the metallurgy of the joint can be influenced. This makes the method suitable for use with high-strength steels, with which consumables are needed to get the required properties out of the joint. Duplex stainless steel, employed widely by the offshore industry, can only be welded with a consumable, making it suitable with the hybrid method, but not with conventional laser welding. Conclusion The arc welding processes are cheap and reliable, but in light of the demands from modern society certain important limitations have become crucial. Many properties such as speed, heat input, and environmental aspects may not be improved further due to the limitations caused by the physics of the processes and the distortions observed in arc welding. Of equal importance are the large difficulties met in robotizing the processes where control of the individual weld-pass geometry is a key issue. Substituting with energy beam welding results in many advantages such as low distortion, high speed, and natural automation. Especially introduction of laser/MAG (GMAW) hybrid welding instead of pure laser welding increases the ability to bridge a gap and provides a significant increase in speed when welding a wide gap, and excellent weld properties are obtained. Thus, energy beam welding has bring a lot of advantages to mankind especially in manufacturing sector which then catalyst the progression of economic of one country.