Download
1 / 55
E N D
Plasma is commonly known as fourth state of matter after solid, liquid and gas. This is an extremely hot substance which consists of free electrons, positive ions, atoms and molecules. It conducts electricity.How it works:By positioning the electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope. Plasma is then forced through a fine-bore copper nozzle which constricts the arc. There are three operating modes which can be produced by varying bore diameter and plasma gas flow rate:•Microplasma: 0.1 to 15A.•Medium current: 15 to 200A.•Keyhole plasma: over 100A.The plasma arc is usually operated with a DC, drooping characteristic power source. Because its unique operating features are results of the special torch arrangement and separate plasma and shielding gas flows, a plasma control console can be added on to a normal TIG power source. Full plasma systems are also available. The plasma arc is not stabilised with sine wave AC. Arc reignition is difficult when there is a long electrode to workpiece distance and the plasma is constricted, extreme heating of the electrode during the positive half-cycle causes balling of the tip which can disturb arc stability. Special-purpose switched DC power sources are available. By misbalancing the waveform to reduce the duration of electrode positive polarity, the electrode is kept passably cool to maintain a pointed tip and achieve arc stability.
ElectrodeThe electrode used for the plasma process is tungsten-2%thoria and the plasma nozzle is copper. The electrode tip diameter is not as critical as for TIG and should be maintained at around 30-60 degrees. The plasma nozzle bore diameter is critical and too small a bore diameter for the current level and plasma gas flow rate will lead to excessive nozzle erosion or even melting. Large bore diameter should be carefully used for the operating current level.Because too large a bore diameter, may give problems with arc stability and maintaining a keyhole.Plasma and shielding gasesThe normal combination of gases is argon for the plasma gas, with argon plus 2 to 5% hydrogen for the shielding gas. Helium can be used for plasma gas but because it is hotter this reduces the current rating of the nozzle. Helium's lower mass can also make the keyhole mode more difficult.
Applications:Microplasma welding:Microplasma was traditionally used for welding thin sheets (down to 0.1 mm thickness), and wire and mesh sections. The needle-like stiff arc minimises arc wander and distortion. Although the alike TIG arc is widely used, the newer transistorised (TIG) power sources can produce a very stable arc at low current levels.Medium current welding:When used in the melt mode this is a substitute to normal TIG.The advantages are:1-Deeper penetration (from higher plasma gas flow).2-Greater tolerance to surface contamination including coatings (the electrode is within the body of the torch).The major disadvantage lies in the bulkiness of the torch, making manual welding more difficult. In mechanised welding, greater attention must be paid to maintenance of the torch to ensure consistent performance.
Keyhole welding:This has several advantages which can be exploited: deep penetration and high welding speeds. Compared with the TIG arc, it can penetrate plate thicknesses up to l0mm, but when welding using a single pass technique, it is more usual to limit the thickness to 6mm. The normal methods is to use the keyhole mode with filler to ensure smooth weld bead profile (with no undercut). For thicknesses up to 15mm, a vee joint preparation is used with a 6mm root face. A two-pass technique is employed and here, the first pass is autogenous with the second pass being made in melt mode with filler wire addition.
As the welding parameters, plasma gas flow rate and filler wire addition (into the keyhole) must be carefully balanced to maintain the keyhole and weld pool stability, this technique is only suitable for mechanised welding. Although it can be used for positional welding, usually with current pulsing, it is normally applied in high speed welding of thicker sheet material (over 3 mm) in the flat position. When pipe welding, the slope-out of current and plasma gas flow must be carefully controlled to close the keyhole without leaving a hole.
Arc SprayingArc spraying is the highest productivity thermal spraying process. A DC electric arc is struck between two continuous consumable wire electrodes which form the spray material. Compressed gas (usually air) atomises the molten spray material into fine droplets and propels them towards the substrate The process is simple to operate- Can be used manually or in an automated manner. Possible to spray a wide range of metals, alloys and metal matrix composites (MMCs) in wire form. A limited range of cermet coatings (with tungsten carbide) can also be sprayed in cored wire form, where the hard ceramic phase is packed into a metal sheath as a fine powder.The combination of high arc temperature (6000 K) and particle velocities in excess of 100 m.sec-1 gives arc sprayed coatings superior bond strengths and lower porosity levels when compared with flame sprayed coatings. However, the use of compressed air for droplet atomization and propulsion gives rise to high coating oxide content.
PLASMA SPRAYING PROCESS • Uses a DC electric arc to generate a stream of high temperature ionised plasma gas, which acts as the spraying heat source. • The arc is struck between two non-consumable electrodes, a tungsten cathode and a copper anode within the torch. • The torch is fed with a continuous flow of inert gas, which is ionised by the DC arc, and is compressed and accelerated by the torch nozzle so that it issues from the torch as a high velocity (in excess of 2000 m/sec), high temperature (12000–16000 K) plasma jet. • The coating material, in powder form, is carried in an inert gas stream into the plasma jet where it is heated and propelled towards the substrate. • Because of the high temperature and high thermal energy of the plasma jet, materials with high melting points can be sprayed. • Plasma spraying produces a high quality coating by a combination of a high temperature, high energy heat source, a relatively inert spraying medium and high particle velocities, typically 200–300 m.sec-1. • However, inevitably some air becomes entrained in the spray stream and some oxidation of the spray material may occur. The surrounding atmosphere also cools and slows the spray stream.
Applications • Plasma spraying is widely applied in the production of high quality sprayed coatings. • Spraying of seal ring grooves in the compressor area of aeroengine turbines with tungsten carbide/cobalt to resist fretting wear. • Spraying of zirconia-based thermal barrier coatings (TBCs) onto turbine combustion chambers. • Spraying of wear resistant alumina and chromium oxide ceramic onto printing rolls for subsequent laser and diamond engraving/etching. • Spraying of molybdenum alloys onto diesel engine piston rings.
HIGH VELOCITY OXYFUEL SPRAYING The most recent addition to the thermal spraying family, high velocity oxyfuel spraying (HVOF SPRAYING) has become established as an alternative to the proprietary, detonation (D-GUN) flame spraying and the lower velocity, air plasma spraying processes for depositing wear resistant tungsten carbide-cobalt coatings. This differs from conventional flame spraying in that the combustion process is internal, and the gas flow fates and delivery pressures are much higher than those in the atmospheric burning flame spraying processes. The combination of high fuel gas and oxygen flow rates and high pressure in the combustion chamber leads to the generation of a supersonic flame with characteristic shock diamonds. Flame speeds of 2000ms-1 and particle velocities of 600–800ms-1 are claimed by HVOF equipment suppliers. A range of gaseous fuels is currently used, including propylene, propane, hydrogen and acetylene.
Although similar in principle, potentially significant details, such as powder feed position, gas flow rates and oxygen to fuel ratio, are apparent between each system. • The HVOF process produces exceptionally high quality cermet coatings (e.g., WC-Co), but it is now also used to produce coatings of metals, alloys and ceramics. Not all HVOF systems are capable of producing coatings from higher melting point materials, e.g., refractory metals and ceramics. The capability of the gun is dependent upon the range of fuel gases used and the combustion chamber design. • A liquid fuel (kerosene) HVOF system, has just been launched, which is capable of much higher deposition rates than the conventional gas-fuelled units.
Comparison of Thermal Spraying Processes and Coating Characteristics
LASER BEAM WELDING(LBW) • LASER- Light Amplification by Stimulated Emission of Radiation • Focusing of narrow monochromatic light into extremely concentrated beams (0.001 mm even) • Used to weld difficult to weld materials, hard to access areas, extremely small components, In medical field to weld detached retinas back into place • Laser Beam- coherent Laser production- complex process.
The LASER, an acronym for "Light Amplification by Stimulated Emission of Radiation," is a device that produces a concentrated, coherent beam of light by stimulating molecular or electronic transitions to lower energy levels, causing the emission of photons.PFN- Pulse Forming Network
Al2O3 + 0.05% Chromium • solid state RubyLaser- Neon flash tube emits light into specially cut ruby crystals- absorbs light -electrons of chromium atoms get stimulated- • Increase in stimulation ---- electrons increase from normal(ground) orbit to an exited orbit. More energy input- energy absorbed exceeds thermal energy- no longer to heat energy. • Electrons drop back to intermediate orbit- emits PHOTONS (light) called spontaneous emission • With continued emission, released photons stimulate other exited electrons to release photons- calledstimulated emission • Causes exited electrons to emit photons of same wave length.
LASER WELDING Slide 17 of 18
LASER WELDING Slide 18 of 18
Power intensities > 10 kw/cm2 • No physical contact between work and welding equipment • 2 mirrors- coherent light reflected back and forth, becomes dense, penetrates partially reflective mirror, focused to the exact point • Very little loss of beam energy • Solid state, liquid, semiconductor and gas lasers used. • Solid state uses light energy to stimulate electrons Ruby, Neodymium, YAG • Gas lasers use electrical charge to stimulate electrons Gas lasers- higher wattage outputs. Used for thicker sections - CO2, N2, He • Liquid- nitrobenzene; Gas- based on gallium arsenide
Laser Welding Facts • Laser Welding Advantages• Processes high alloy metals without difficulty• Can be used in open air• Can be transmitted over long distances with a minimal loss of power• Narrow heat affected zone• Low total thermal input• Welds dissimilar metals• No filler metals necessary• No secondary finishing necessary• Extremely accurate• Welds high alloy metals without difficulty • CO2 Laser Welding Speeds
The solid-state laser utilizes a single crystal rod with parallel, flat ends. Both ends have reflective surfaces. A high-intensity light source, or flash tube surrounds the crystal. When power is supplied by the PFN (pulse-forming network), an intense pulse of light (photons) will be released through one end of the crystal rod. The light being released is of single wavelength, thus allowing for minimum divergence
One hundred percent of the laser light will be reflected off the rear mirror and thirty to fifty percent will pass through the front mirror, continuing on through the shutter assembly to the angled mirror and down through the focusing lens to the workpiece. • The laser light beam is coherent and has a high energy content. When focused on a surface, laser light creates the heat used for welding, cutting and drilling. • The workpiece and the laser beam are manipulated by means of robotics. The laser beam can be adjusted to varying sizes and heat intensity from .004 to .040 inches. The smaller size is used for cutting, drilling and welding and the larger, for heat treating
Laser Welding Limitations• Rapid cooling rate may cause cracking in certain metals• High capital cost• Optical surfaces easily damaged• High maintenance cost
Laser beam cutting • Along with beam, oxygen used to help cutting. Ar, He, N, CO2 also for steel, alloys etc. Two ways to weld • Work piece rotated or moved past beam • Many pulses of laser (10 times/sec)used. Narrow HAZ., speeds of 40 mm/sec to 1.5 m/sec Cooling system to remove the heat- gas and liquid cooling used
Klyston tubes (glass to metal sealing), capacitor bank, triggering device, flash tube, focusing lens, etc. in the setup. • Cathode of molybdenum, tantalum or titanium used.
ULTRASONIC WELDING Ultrasonic welding is an industrial technique whereby two pieces of plastic or metal are joined together seamlessly through high-frequency acoustic vibrations. One component to be welded is placed upon a fixed anvil, with the second component being placed on top. An extension ("horn") connected to a transducer is lowered down onto the top component, and a very rapid (~20,000 Hz), low-amplitude acoustic vibration is applied to a small welding zone. The acoustic energy is converted into heat energy by friction, and the parts are welded together in less than a second
Unique - no connective bolts, nails, soldering materials, or adhesives are necessary to bind the two parts together. • Thus, saves on manufacturing costs and creates unnoticeable seams in products where appearance is important. • A largely automated process • But, it is only applied to small components - watches, cassettes, plastic products, toys, medical tools, and packaging. • For example, the chassis of an automobile cannot be assembled with ultrasonic welding because the energies involved in welding larger components would be prohibitive.
In 1960 Sonobond Ultrasonics, originally known as Aeroprojects Incorporated, developed the first metal ultrasonic welding machine to be awarded a United States Patent • Since early 90s, rapid developments occured • The range of materials that can be joined together using this technique is increasing
Earlier, only non-flexible plastics could be welded because their material properties allowed the efficient transmission of acoustic energy from part to part. • Nowadays, less rigid plastics such as semicrystalline plastics can be welded because large amounts of acoustic energy can be applied to the welding zone. • As the technology matures and becomes more versatile, it is likely to obsolete large classes of historical techniques for joining materials together.
Ultrasonic welding. When bonding material through ultrasonic welding, the energy required comes in the form of mechanical vibrations. The welding tool (sonotrode) couples to the part to be welded and moves it in longitudinal direction. The part to be welded on remains static. Now the parts to be bonded are simultaneously pressed together. The simultaneous action of static and dynamic forces causes fusion of the parts without having to use additional material. This procedure is used on an industrial scale for linking both plastics and metals • Anvil • Parts to be welded • Sonotrode • Ultrasonic oscillation Differences in the process for welding plastics and metals with ultrasonics
Systems are composed of the same basic elements: • A press to put the 2 parts to be assembled under pressure • A nest or anvil where the parts are placed, allowing the high frequency vibration to be directed to the interfaces • An ultrasonic stack composed of a converter or piezoelectrictransducer, • An optional booster and a sonotrode (Horn). All three elements of the stack are specifically tuned to resonate at the same exact ultrasonic frequency (Typically 20, 30, 35 or 40 kHz) • Converter: Converts the electrical signal into a mechanical vibration • Booster: Modifies the amplitude of the vibration. It is also used in standard systems to clamp the stack in the press. • Sonotrode: Applies the mechanical vibration to the parts to be welded. An electronic ultrasonic generator (US: Power supply) delivering a high power AC signal with frequency matching the resonance frequency of the stack. A controller controlling the movement of the press and the delivery of the ultrasonic energy
The mechanisms during ultrasonic metal welding • Principle of ultrasonic metal welding • 1. Sonotrode • 2, 3 Parts to be joined • 4. Anvil • 5. Welding area
Applications • The applications are extensive and are in many industries including electrical and computer, automotive and aerospace, medical, and packaging. • Too thick pieces cannot be joined. This is the main obstacle in the welding of metals. • However, wires, microcircuit connections, sheet metal, foils, ribbons and meshes are often joined using ultrasonic welding. • Ultrasonic welding is a very popular technique for bonding thermoplastics. It is fast and easily automated with weld times often below one second and there is no ventilation system required to remove heat or exhaust. • This type of welding is often used to build assemblies that are too small, too complex, or too delicate for more common welding techniques
Computer & electrical industries • Used to join wired connections and to create connections in small, delicate circuits. Junctions of wire harnesses are often joined using ultrasonic welding • Wire harnesses are large groupings of wires used to distribute electrical signals and power. • Electric motors, field coils, transformers and capacitors may also be assembled with ultrasonic welding. • It is also often preferred in the assembly of storage media such as flash drives and computer disks because of the high volumes required. Ultrasonic welding of computer disks has been found to have cycle times of less than 300 ms. • Mostly used in microcircuits, since it creates reliable bonds without introducing impurities or thermal distortion into components. Semiconductor devices, transistors and diodes are often connected by thin aluminum and gold wires using ultrasonic welding.It is also used for bonding wiring and ribbons as well as entire chips to microcircuits. An example: in medical sensors used to monitor the human heart in bypass patients. • Has the ability to join dissimilar materials. Example: The assembly of battery components. When creating battery and fuel cell components, thin gauge copper, nickel and aluminum connections, foil layers and metal meshes are often ultrasonically welded together. Multiple layers of foil or mesh can often be applied in a single weld eliminating steps and cost.
Aerospace & automotive industries • Used in the assembly of large plastic components and electrical components such as instrument panels, door panels, lamps, air ducts, steering wheels, upholstery and engine components. As plastics are replacing other materials in the design and manufacture of automobiles, the assembly and joining of plastic components has increasingly become a critical issue. Some of the advantages for ultrasonic welding are low cycle times, automation, low capital costs, and flexibility. Also, ultrasonic welding does not damage surface finish, which is a crucial consideration for many car manufacturers, because the high-frequency vibrations prevent marks from being generated. • Used in the aerospace industry when joining thin sheet gauge metals and other lightweight materials. Aluminum which is a difficult metal to weld using traditional techniques because of its high thermal conductivity, is one of the easier materials to weld using ultrasonic welding because it is a softer alloy metal and thus a solid-state weld is simple to achieve. • Also, with the advent of new composite materials, ultrasonic welding is becoming even more prevalent. It has been used in the bonding of the popular composite material carbon fiber. Numerous studies have been done to find the optimum parameters that will produce quality welds for this material.
Medical industry • USW does not introduce contaminants or degradation into the weld and the machines can be specialized for use in clean rooms. • The process can also be highly automated, provides strict control over dimensional tolerances and does not interfere with the biocompatibility of parts. • Thus increases part quality and decreases production costs. • Items such as arterial filters, anesthesia filters, blood filters, IV catheters, dialysis tubes, pipettes, cardiometry reservoirs, blood/gas filters, face masks and IV spike/filters can all be made using ultrasonic welding. • Another important application is in textiles. Items like hospital gowns, sterile garments, masks, transdermal patches and textiles for clean rooms can be sealed and sewn using ultrasonic welding. This prevents contamination and dust production and reduces the risk of infection.
Packaging industry • Many everyday items are either created or packaged using ultrasonic welding techniques. • Eg: Sealing containers, tubes and blister packs .Also in the packaging of dangerous materials such as explosives, fireworks and other reactive chemicals. These items tend to require hermetic sealing but cannot be subjected to high temperatures. • One example of this application is the container for a butane lighter. This container weld must be able to withstand high pressure and stress and must be airtight to contain the butane.
Another example is the packaging of ammunition and propellants- which must be able to withstand high pressure and stresses in order to protect the consumer from the contents. When sealing hazardous materials safety is a primary concern. Thus, the reliability and automation of this process are strong benefits for companies. It is fast, sanitary and can produce hermetic seals. Milk and juice containers are examples of some products that are often sealed using ultrasonic welding. The paper parts to be sealed are coated with plastic, generally polypropylene or polyethylene, and then welded together to create an airtight seal. The main obstacle to overcome in this process is the setting of the parameters. If over-welding occurs then the concentration of plastic in the weld zone may be too low and cause the seal to break. If it is under-welded the seal is incomplete. Also, variations in the thicknesses of materials can cause variations in weld quality. Therefore, the preparation is extremely important. Other food items that are sealed include candy bar wrappers, frozen food packages and beverage containers.
In summary, It is increasing in popularity throughout many of the industries because of low cycle times, automation, low capital costs, flexibility, cleanliness, dimensional reliability and the bonding of dissimilar materials. • Some of the drawbacks of ultrasonic welding are that its use is limited by the thickness of the materials, it may require expensive specialized tooling and it may generate noise. As these drawbacks are overcome by continually developing technologies, it will be interesting to see how this unique welding technique continues to be utilized.
Safety There are risk of some hazards: exposure to high heat levels and voltages. This equipment to be operated using the safety guidelines provided by the manufacturer in order to avoid injury. Must never place hands or arms near the welding tip when the machine is activated. Also, operators should be provided with hearing protection and safety glasses. Operators should be informed of the OSHA regulations for the ultrasonic welding equipment and these regulations should be enforced. • Machines must receive routine maintenance and inspection. Panel doors, housing covers and protective guards need to be removed for maintenance with the power to the equipment off and only by the trained professional who is servicing the machine.
Sub-harmonic vibrations may create annoying audible noise, may be in larger parts near the machine due to the ultrasonic welding frequency. This noise can be dampened by clamping these large parts at one or more locations. Also, high-powered welders with frequencies of 15 kHz and 20 kHz typically emit a potentially damaging high-pitched squeal in the range of human hearing. Shielding this radiating sound can be done using an acoustic enclosure. In short, there are hearing and safety concerns with ultrasonic welding that are important to consider, but generally they are comparable to those of other welding techniques.
