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
typically 200–300 m.sec-1.
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.
Schematic of High Velocity Oxyfuel (HVOF) Spraying System
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
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Laser Welding Limitations• Rapid cooling rate may cause cracking in certain metals• High capital cost• Optical surfaces easily damaged• High maintenance cost
Two ways to weld
Narrow HAZ., speeds of 40 mm/sec to 1.5 m/sec
Cooling system to remove the heat-
gas and liquid cooling used
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
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
Differences in the process for welding plastics and metals with ultrasonics
Systems are composed of the same basic elements:
All three elements of the stack are specifically tuned to resonate at the same exact ultrasonic frequency (Typically 20, 30, 35 or 40 kHz)
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
Computer & electrical industries
Aerospace & automotive industries
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.
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.
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.
Some references site Robert Hopkins for having invented the Electroslag welding process in the 1930's. Most of his patents relate to Electroslag melting for ingot manufacture, not welding. However one US patent, number 2,191481 filed in June, 1939 does describe the surfacing of one material on another. The illustration, however looks more like a melting furnace than welding. In fact the fellow who invented Submerged Arc Welding, Harry Kennedy, was granted a US patent in October of 1950, number 2,631,344, assigned to Union Carbide that more closely related to Electroslag welding. However it too falls short of defining what we know today as this simple welding process.
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