Non-Arc Welding Processes Continued

Non-Arc Welding Processes Continued

Non-Arc Welding Processes (Cont.)
Learning Activities Look up Keywords View Slides; Read Notes, Listen to lecture Do on-line workbook Do homework Lesson Objectives When you finish this lesson you will understand: High Energy Density Welding, Advantages and Disadvantages Soldering and Brazing Systems Plastics Bonding Adhesive Bonding Keywords: Laser Beam Welding (LBW), Electron Beam Welding (EBW), Plasma Plume, YAG, Soldering, Brazing, Flux, Wave Soldering, Hot Plate Welding, Hot Gas Welding, Vibration Welding, Ultrasonic Welding, Thermoplastic, Thermoset, Adhesive Bonding, Curing

Non-Arc Welding Processes
Introduction Non-Arc Welding Processes Resistive heating, chemical reactions, focused light and electrons, sound waves, and friction can also be used to join materials Resistance welding Oxy-Fuel Welding Friction welding (&Solid State) Laser and electron beam welding Brazing and soldering Plastics joining Adhesive bonding If a high precision, low distortion, or a fast, deep single pass weld is needed, then a High Energy Density (HED) welding process can be used. Both electron beams and lasers can accomplish this. To create an area of high density, a beam is focused down to a spot on the surface of a material. When the energy density is great enough, melting occurs. The beam intensity at the point of welding falls in the range of 105 W/cm2 for laser beam welding to about 107 W/cm2 for electron beam welding.

High Energy Density Processes
Focus energy onto a small area Laser CO2 gas: fixed position Nd-YAG crystal: fiber-optic delivery Electron Beam High energy density welding is used in a wide variety of applications and is capable of producing welds with a high degree of precision. The low heat inputs of each process make it possible to control the weld width and depth and act to reduce distortion and residual stress. The word “LASER” is an acronym for “light amplification by stimulated emission of radiation.” A laser beam that becomes highly focused is an excellent source of concentrated energy. This energy is used for many welding applications and also cutting and heat treating. Two basic types of lasers are used in welding: solid-state and gas lasers. The choice of laser type depends on the type of material to be welded (how it absorbs certain wavelengths of light) as well as the required speed and penetration. Non-metallic materials, such as plastics, can be laser welded. Nd:YAG (a doped crystal of neodymium with yttrium, aluminum, and garnet) is the most common solid-state laser used for welding today. The end surfaces of its elongated crystal rod are ground flat and parallel. These ends usually have a reflective coating placed on them; one end is totally reflective, the other end is partially reflective, leaving a small area for photons to escape. The Nd ions excite their electrons to a higher energy level. By doing this, photons are emitted at a wavelength of 1.06 micrometers. After the photons are emitted, the electrons are allowed to return to their original state.

These processes focus the energy onto a small area Laser inch thick stainless steel sheet Electron Beam inch weld width on 0.5 inch thick steel plate High energy density welding is used in a wide variety of applications and is capable of producing welds with a high degree of precision. The low heat inputs of each process make it possible to control the weld width and depth. In laser welding, the beam can be focused for different applications. Usually, a small focus size is used for cutting and welding, while a larger focus is used for heat treatment. In electron beam welding, a concentrated beam, composed of primarily high-velocity electrons, is used. As these high speed electrons bombard the material, heat is produced. T

Laser Beam Welding (LBW)
The word “LASER” is an acronym for “light amplification by stimulated emission of radiation.” A laser beam that becomes highly focused is an excellent source of concentrated energy. This energy is used for many welding applications and also cutting and heat treating. Two basic types of lasers are used in welding: solid-state and gas lasers. Solid-state lasers are made of a single elongated crystal rod. Nd:YAG (a doped crystal of neodymium with yttrium, aluminum, and garnet) is the most common solid-state laser used for welding today. The end surfaces of the rod are ground flat and parallel. These ends usually have a reflective coating placed on them. While one end is totally reflective, the other end is partially reflective, leaving a small area for photons to escape. The Nd ions excite their electrons to a higher energy level. By doing this, photons are emitted at a wavelength of 1.06 microns. After the photons are emitted, the electrons are allowed to return to their original state. The most common gas laser is the carbon dioxide laser. It is used most widely for welding. An electrical charge excites the carbon dioxide molecules, which on their return to their normal energy state, emit some photons. Much like solid-state lasers, reflective surfaces are placed at the ends of the tube in which the gas is contained. The one end is totally reflective, while the other allows a small amount of light to pass. This light is emitted at a wavelength of 10.6 microns. Drawing from Welding Handbook, 8th Edition, Volume 2, American Welding Society. Laser T

Laser Beam Welding (LBW)
High Energy Density Processes Laser Beam Welding (LBW) Laser beam Plasma plume Molten material shielding gas nozzle (optional) workpiece motion Single pass weld penetration up to 3/4” in steel Materials need not be conductive No filler metal required Low heat input produces low distortion Does not require a vacuum Plasma keyhole The most common gas laser is the carbon dioxide laser, which emits light at a wavelength of 10.6 micrometers. Most metals absorb its wavelength of light from a Nd:YAG laser (1.06 micrometers) better than that of a CO2 laser. Also the Nd:YAG laser can be delivered by flexible fiber-optic cable, making it more versatile than the fixed delivery system for a gas laser. Factors affecting the choice between gas and solid state lasers are summarized below. CO2 lasers: higher power, better beam quality in terms of focusability, higher speeds and deeper penetration for materials that don’t reflect its light, lower start-up and operation costs. Nd:YAG lasers: versatile fiber-optic delivery, easy beam alignment, easier maintenance, smaller equipment, more expensive safety measures than CO2 because of its wavelength. A typical laser can be focused to a spot size of mm ( inch), with power densities greater than 107 W/cm2. At these power densities, a phenomenon referred to as keyholing occurs, which allows continuous, deep-penetration welding of metal. The metal melts and vaporizes upon interaction with the beam; the pressure of the metal vapor pushes molten metal out of the way and forms a keyhole or cavity. Keyhole welding

Focusing the Beam The presence of contaminants in the weld joint such as grease or rust can lead to porosity in the weld itself and to spatter. It is spatter which can greatly affect the choice of operating parameters for a laser. In general, a specific focal point size can be obtained over a range of focal lengths. Larger focal lengths are considered in instances where soot and weld spatter are likely. In laser welding, the beam can be focused for different applications. Usually, a small focus size is used for cutting and welding, while a larger focus is used for heat treatment or surface modification. The focal spot of the beam can also be varied based on the application. A defocused larger spot diameter can be used for surface heat treatment. Localized surface modification can be achieved by focusing on the surface of the spot. The cross-section of laser weld exhibits an hourglass shape. This shape arises because the focal spot of the laser is within the material being welded, with the beam converging to and diverging from this spot. For laser cutting, the focus of the beam is toward the bottom or back side of the plate. Heat Surface Welding Cutting treatment modification

Advantages Single pass weld penetration up to 3/4” in steel
High Travel speed Materials need not be conductive No filler metal required Low heat input produces low distortion Does not require a vacuum There are many advantages to laser welding. A laser can penetrate just about any material up to 3/4 of an inch. The materials do not have to be electrically conductive. The heat of the laser creates melting. Filler metal is not a requirement due to the excellent quality of the weld. The low heat input produces low distortion. Electron beam welding requires a vacuum which is not necessary in laser welding. T

Limitations High initial start-up costs Part fit-up and joint tracking are critical Not portable Metals such as copper and aluminum have high reflectivity and are difficult to laser weld High cooling rates may lead to materials problems There are some disadvantages to laser welding. Laser welders are very expensive. Typical capital costs include ~ $100K-150K for a C02 laser and another $150K for the fixturing required to move the part under the stationary beam. The overall laser set-up involves large machines that can not be transported to the job site. All fabrication must be completed at the shop. As light reflects off mirrors, lasers can also reflect off shiny materials. Thus, highly reflective metals, such as aluminum and copper, can be difficult to laser weld. The high cooling rates associated with laser welding can lead to materials problems. With steels, for example, there is the potential to form martensite adjacent to the weld zone. Tight tolerances are required on joint fit-up, as would be expected when the focused spot can be inches across. For a butt joint, the maximum gap shouldn’t exceed 0.1 mm (0.004 inch), with lass than a 0.2 mm (0.008 inch) vertical mismatch between the workpieces. The beam focus position needs to be accurate within 0.25 mm (0.01 inch). The beam must track the joint within 0.05 mm (0.002 inch). Rofin Sinar Laser is acknowledged as a source of much of the information on lasers in this section.

Electron Beam Welding (EBW)
An electron beam welding machine is made up of a power supply, electron beam gun, vacuum chamber, and gun/work motion equipment. The gun produces accelerated electrons. It contains a filament (usually tungsten) as the cathode, the cup, an anode, and beam focusing parts. Electrons are liberated from the surface of the tungsten. The beam collects and semi-focused by its natural attraction to the anode. As it passes through the anode, the beam receives a final focus by magnetic deflection. The beam exits the gun and travels through the work. Drawing from Welding Handbook, 8th Edition, Volume 2, American Welding Society. EB Applications T

Electron Beam Welding (EBW)
High Energy Density Processes Electron Beam Welding (EBW) Advantages Deepest single pass weld penetration of the fusion processes 14-inch-thick steel Fast travel speeds Low heat input welds produce low distortion An electron beam welding machine is made up of a power supply, electron beam gun, vacuum chamber, and gun/work motion equipment. The gun produces accelerated electrons. It contains a filament (usually tungsten) as the cathode, the cup, an anode, and beam focusing parts. Electrons are liberated from the surface of the tungsten. The beam collects and semi-focused by its natural attraction to the anode. As it passes through the anode, the beam receives a final focus by magnetic deflection. The beam exits the gun and travels through the work. The deep penetration, single pass capability and fast travel speeds of electron beam welding can prove very economical in industrial situations. As with laser welding, electron beam welding provides low distortion due to low heat input. Most conductive materials are weldable. Drawing from Welding Handbook, 8th Edition, Volume 2, American Welding Society.

Limitations High initial start-up cost Not portable Part size limited by size of vacuum chamber Produces x-rays Part fit-up is critical High cooling rates may lead to materials problems Electron beam and laser welding have some similar disadvantages. These include: price, inability of the unit to be portable, part fit-up tolerance, and materials problems associated with high cooling rates. The vacuum chamber has limited space; therefore a part to be welded is limited to its size. The most crucial factor of them all is that the unit produces x-rays. Heavy shielding is necessary to produce a safe working environment.

Questions? Turn to the person sitting next to you and discuss (1 min.): In laser welding, materials with high reflectivity reflect the beam right off the surface and no heat is absorbed and thus they are difficult to weld. What might we do to make these high reflectivity materials more weldable?

Introduction Non-Arc Welding Processes Resistive heating, chemical reactions, focused light and electrons, sound waves, and friction can also be used to join materials Resistance welding Oxy-Fuel Welding Friction welding (&Solid State) Laser and electron beam welding Brazing and soldering Plastics joining Adhesive bonding Brazing and soldering are two of the oldest means of joining metals together. They are used today in industries that require large quantities of products to be made such as electronics and different kinds of fittings. The distinction between the processes is that soldering occurs at temperatures below 840° F (450° C) while brazing takes place at temperatures above 840°F (250°C).

Brazing (B) and Soldering (S)
Brazing and Soldering Brazing (B) and Soldering (S) In these processes, the base metals are heated but do not melt; only the filler metal melts Brazing filler metals having a melting point above 840° F (450°C) Soldering filler metals have a melting point below 840°F (450°C) Brazing and soldering are processes where only the filler metal melts and flows into the joint; the base material remains unmelted. The parts are fitted together with tight tolerances, which produce capillary action to draw filler metal into the joint. Wetting, the ability of a liquid filler to spread over a free surface, becomes critical during this process. Wetting can be related to wax on a car. If a car is waxed, the water beads off the car. In brazing and soldering, this would be referred to as poor wetting. If a car has no wax on it, water spreads over the surface of the car. This is a necessary condition for brazing and soldering. Any oxides or other film must be cleaned off the parts to be joined, generally by a fluxing agent, in order to ensure good wetting.

Brazing and Soldering Batch processing involves the simultaneous production of large numbers of parts. Before the parts are placed placed in the furnace, or on the conveyor belt to the furnace, filler metal and flux is placed in the joint. In the furnace, the filler metal melts and flows over the free surfaces by capillary action. As the part leaves the furnace, the filler solidifies rapidly and the part, either brazed or soldered, becomes one rigid piece. Soldering of electronic components, such as on computer circuit boards, would not be an economical process if each circuit element was individually soldered. In this instance, the circuit boards are passed over the surface of open vats of molten solder. All the connections are made in this one operation. Drawing from Welding Handbook, 8th Edition, Volume 2, American Welding Society. T

Application of Low Thermal Expansion Alloys
Thermal expansion mismatch in metal-ceramic joints can lead to cracks in the ceramic Thermal expansion coefficients at 25°C (10-6 mm / mm·°C) Alumina, 8.8 Nickel, 13.3 Iron, 11.8 Kovar, 5.0 Kovar lid Silicon chip Alumina substrate Invar - 36% nickel, 64% iron Kovar - 29% nickel, 17% cobalt, 54% iron Metals have higher thermal expansion coefficients than ceramics. If a metal-to-ceramic joint is heated to high temperatures and cooled, large stresses build up in the brittle ceramic due to the comparatively greater expansion in the metal. Metals have sufficient ductility to withstand these stresses; cracking, however, often results in the ceramic. A joint between a ceramic and Kovar/Invar is not subject to cracking due to the low thermal expansion coefficients associated with nickel-iron and nickel-iron-cobalt alloys. Of all the low expansion alloys, Alloy 42 (42% nickel, balance iron) has the thermal expansion coefficient closest to that of alumina, and is often used in glass-metal seals. Metal-to-ceramic brazing is often accomplished through the addition of reactive metal to braze material such as tin. Reactive metals include such elements as titanium and zirconium. Brazed joints T

Brazing Specifications
Brazing and Soldering Brazing Specifications AWS A5.8 Specification for Brazing Filler Metal 8 well-defined groups (B) plus a vacuum grade (BV) BAg-1 (44-46 Ag, Cu, Zn, Cd) BAu-1 (37-38 Au, remainder Cu) BCuP-1 ( P, remainder Cu) Standard forms: strip, sheet, wire, rod, powder Joint design tolerances, generally ~ inches Uses for each braze material AWS C3.3 Standard Method for Evaluating the Strength of Brazed Joints The eight groups of filler metals are: aluminum-silicon, copper-phosphorus, silver, precious metals, copper and copper-zinc, magnesium, nickel, cobalt. There is also a vacuum grade for vacuum tube and apparatus applications; these metals have minimum amounts of high vapor pressure elements. There is a standard nomenclature for these materials. “B” indicates brazing filler metal; the major elements in the metal are listed after. For example BCuP indicates a copper-phosphorus braze metal. An initial “R” indicates rod form of brazing metal. There can be several compositions within a classification; they are indicated by an additional number after the element symbols. For example BCuP-1 ( % P, remainder Cu) and BCuP-3 ( & P, % Ag, remainder Cu). A “V” after the “B” indicates vacuum grade of the braze metal, as in BVAg-8. Vacuum braze metals also have a final Grade suffix: “Grade A” indicates spatter free; “Grade B” indicates non-spatter free. An example is BVAg-6b. Both solidus and liquidus temperatures are specified for braze metals in order to provide the range over which solid and liquid phases are present together. AWS A5.8 provides tables of these temperatures for the different braze materials. BAlSi: brazing of aluminum and its alloys BMg: brazing of magnesium alloys BCuP: brazing of Cu and its alloys; limited use for Ag, W, Mo; not for Fe or Ni alloys BAg: brazing of ferrous and nonferrous metals except Al, Mg BAu: Brazing of Fe, Ni and Co-based alloys where oxidation resistance is required

Balchin & Castner, “Health & Safety…”,
McGraw Hill, 1993

Advantages Joins unweldable materials
Brazing and Soldering Advantages Joins unweldable materials Base metals don’t melt Can be used on metals and ceramics Joined parts can be taken apart at a later time Batch furnace can easily process multiple parts Portable when joining small parts The base metals remain intact and do not melt during brazing and soldering. Therefore, bonds can be made between otherwise unweldable materials. If at a future time, one of the joined parts needs to be replaced, the entire assembly can be reheated, taken apart, and rejoined with a new part. Batch processing involves the simultaneous production of large numbers of parts. Before the parts are placed in the furnace, or on the conveyor belt to the furnace, filler metal and flux is placed in the joint. In the furnace, the filler metal melts and flows over the free surfaces by capillary action. As the part leaves the furnace, the filler solidifies rapidly and the part, either brazed or soldered, becomes one rigid piece. Soldering of electronic components, such as on computer circuit boards, would not be an economical process if each circuit element was individually soldered. In this instance, the circuit boards are passed over the surface of open vats of molten solder. All the connections are made in this one operation. Although ceramics are not welded by fusion metals, they can be joined by brazing. In metal to ceramic bonds, there is a great difference in the thermal expansion of the two materials. Ceramics expand on the order of ten times less than metals during similar heating. As such, an interlayer of intermediate thermal expansion is often used to transition between the metal and ceramic. Low-expansion, iron-nickel alloys such as Invar and Kovar are often used.

Limitations Joint tolerance is critical
Brazing and Soldering Limitations Filler metal ring surrounded by flux Joint tolerance is critical Lower strength than a welded joint Large parts require large furnaces Manual processes require skilled workers Flux The major limitation of brazing and soldering is joint tolerance. The parts to be joined must be fit to tight tolerances in order to promote capillary action. If the piece fit-up is too wide, capillary action will not take place. AWS A5.8 gives suggested joint design gap spacings for the different filler metals. These dimensions are generally ~ inches. Parts must be clean, so that the filler metal can wet the surface of the pieces being joined. This can involve degreasing and the removal of any surface oxide layer. Fluxing agents are often employed for this purpose.

Questions? Turn to the person sitting next to you and discuss (1 min.): Why is joint tolerance so critical? What happens if the joint space is too large? What happens if the joint space is too small? Turn to the person sitting next to you and discuss (1 min.): What happens if we do not have sufficient flux?

Introduction Non-Arc Welding Processes Resistive heating, chemical reactions, focused light and electrons, sound waves, and friction can also be used to join materials Resistance welding Oxy-Fuel Welding Friction welding (&Solid State) Laser and electron beam welding Brazing and soldering Plastics joining Adhesive bonding Ultrasonic welding is considered a solid state process, but in fact, some melting occurs. An ultrasonic weld is created when two pieces of material are held together and vibrated very rapidly by very high frequency sound wave. The pressure, along with the heat created by the motion of the parts relative to each other, slightly liquifies those surfaces. Upon cooling, the parts are joined. Ultrasonics is mainly good for joining large objects together. Ultrasonic welding can also be used for joining plastics together.

Joining Plastics H C=C H H -C-C- ···
Welding of Plastics Joining Plastics (Poly)ethylene Polymer - a single building block (mer) is repeated to form a long chain molecule Thermoplastic polymers soften when heated, harden when cooled 2-liter bottles Thermosetting polymers don’t soften when heated Car tires, caulking compound H C=C add H2O2 H H -C-C- ··· The need to produce larger, more complex, and reinforced parts from polymers (plastics) has increased the need for joining in this area. To date, most semi-structural polymer parts have been made from thermoset polymers which are joined by mechanical fasteners and adhesives. However, greater impact resistance, processing ease, and potential for recycling are increasing the interest in thermoplastic parts. While adhesives and mechanical fasteners are used for joining thermoplastic parts, thermosetting polymers offer an alternative joining possibility, fusion bonding. Fusion processes for joining thermoplastics and thermoplastic composites involve heating the polymer to a viscous state and physically causing polymer chains to interdiffuse, usually by pressure induced flow. The fusion welding processes can be divided into the following two groups: Processes involving external heating (hot plate, hot gas, infrared). Processes involving mechanical movement (vibration, ultrasonic).

Joining of Plastics Plastic (polymer) is a material in which single building blocks (mers) join to form a long chain or network molecule Thermoplastic polymers soften when heated and harden when cooled Foam cups (polystyrene), 2-liter bottles (polyethylene), Leisure suits (polyester) Thermosetting polymers become permanently hard when heat is applied and do not soften upon subsequent heating Car tires (isoprene, isobutene), Epoxy, Caulks (silicones) The need to produce larger, more complex, and reinforced parts from polymers (plastics) has increased the need for joining in this area. To date, most semistructural polymer parts have been made from thermoset polymers which are joined by mechanical fasteners and adhesives. However, greater impact resistance, processing ease, and potential for recycling are increasing the interest in thermoplastic parts. While adhesives and mechanical fasteners are used for joining thermoplastic parts, thermosetting polymers offer an alternative joining possibility, fusion bonding. Fusion processes for joining thermoplastics and thermoplastic composites involve heating the polymer to a viscous state and physically causing polymer chains to interdiffuse, usually by pressure induced flow. The fusion welding processes can be divided into the following two groups: Processes involving external heating (hot plate, hot gas, infrared). Processes involving mechanical movement (vibration, ultrasonic). T

Hot Plate, Hot Gas, Infrared
Advantages Provide strong joints Reliable Used on difficult to join plastics Limitations Slow Limited temperature range Hot plate, hot gas, and infrared are all welding processes that involve applying external heat to the the area of the polymer parts that are to be joined together. The joint area is heated to a viscous state without burning or vaporizing. The melted or softened polymer surfaces in the weld area are then forged together resulting in interdiffusion of the molecular chains, which produces a weld. In hot plate welding the two parts to be joined together are clamped onto a machine with a vertical heated plate called a platen. The ends of the parts are forced against the heated platen until melted or viscous. At this time the parts are automatically pulled back from the platen, the platen is removed, and the parts are forged together to make the weld. Hot gas or hot air welding uses a stream of heated gas or air directed at a filler rod and the joint area to fuse the surfaces. The filler rod is then pushed or fed into the joint area causing the fused polymers to contact. Best results are obtained when the filler rod is the same material as the base material. Infrared heating uses infrared radiation as a heat source. The radiation is focused on a weld face and causes melting of the polymer surface. Removing the infrared source and forging the surfaces together forms a weld. T

Hot Plate, Infrared Welding
Welding of Plastics Hot Plate, Infrared Welding Hot plate welding

Hot Gas Welding Thermoplastics (hotmelts)
Welding of Plastics Hot Gas Welding Thermoplastics (hotmelts) Adhesive is heated until it softens, then hardens on cooling Hot gas softens filler and base material Filler is pulled or fed into the joint Hot gas or hot air welding uses a stream of heated gas or air directed at a filler rod and the joint area to fuse the surfaces. The filler rod is then pushed or fed into the joint area causing the fused polymers to contact. Best results are obtained when the filler rod is the same material as the base material. For example polypropylene would be joined with polypropylene filler material.

Vibration Advantages Limitations Speed Used on many materials Size
Requires fixturing Equipment costly Vibration or linear friction welding involves the rubbing of two thermoplastics together under pressure at a suitable frequency and amplitude until enough energy is expended to melt the polymer. The vibration is stopped at that point, the parts are aligned, and the molten polymer allowed to solidify creating a weld. The vibration can be in a linear motion or a circular motion. The amplitude of the vibrations ranges from about .010 to .100 inch. The frequency of the vibrations ranges from 100 to 500 Hz. Vibration welding is attractive because a typical vibration weld takes only a matter of seconds to complete. However, vibration welders are expensive, as are the required fixturing needed to hold the parts in the vibration welder. The size of parts that can be vibration welded is limited to approximately an 8-inch by 8-inch square. The automotive and domestic appliance industry is using vibration welding extensively. Automotive applications include front and rear light assemblies, fuel filler doors, spoilers, instrument panels, and power steering and vacuum systems. T

Ultrasonic Advantages Limitations Fast Can spot or seam weld
Equipment complex, many variables Only use on small parts Cannot weld all plastics Ultrasonic welding processes occur when vertical oscillations at frequencies of 10 to 50 kHz are transmitted through polymers and dissipated in a bond line. The parts to be joined are held together under pressure and are subjected to ultrasonic vibrations at right angles to the contact area. The high-frequency stresses produce heat in the material and, if the components are properly designed, this heat can be selectively generated at the joint interface. Heat is generated through a combination of friction and hysteresis. The amplitude of the oscillations can be in the range of 20 to 60 microns, significantly less than the amplitude of movement in friction welding. The sound energy oscillations are generated by the ultrasonic welder and transferred to the parts being welded by what is called a horn. The design of the horn as well as the anvil or base of the ultrasonic welder is critical to the success or failure of the process as it must transmit a specific wavelength of sound into a specific joint geometry. Ultrasonic welding equipment is typically costly, which makes it impractical for short production runs. Ultrasonic welding is probably the most commonly used method to join thermoplastics. It is fast (a few seconds or less), clean, and usually produces welds that are relatively free of flash. In addition, ultrasonic welding is relatively easy to automate since fixtures can act as anvils and the horn can be applied outside the part to produce a weld on an inside surface. Items commonly made by ultrasonic welding include, dashboard assemblies for automobiles, and 3-inch computer disks. Note that most of these items are small; size is a limitation of the process. T

Questions? Turn to the person sitting next to you and discuss (1 min.): Make a list of some thermoplastic items you have recently seen that have been wlded.

Introduction Non-Arc Welding Processes Resistive heating, chemical reactions, focused light and electrons, sound waves, and friction can also be used to join materials Resistance welding Oxy-Fuel Welding Friction welding (&Solid State) Laser and electron beam welding Brazing and soldering Plastics joining Adhesive bonding Adhesive joining occurs when an adhesive is placed between the surfaces of two or more materials. The adhesive solidifies to produce a bond between the pieces.

Adhesives Thermosets form long polymer chains by chemical reaction (curing) Heat is the most common means of curing Ultraviolet light, oxygen - acrylics Moisture - cyanoacrylates Thermoplastics (hotmelts) Adhesive is heated until it softens, then hardens on cooling -Polyethylene, PVC Adhesive bonding is used as an alternative to such mechanical fastening methods as screws, rivets and spot welds. They provide greater stiffness and a more uniform stress distribution. Most adhesives are polymers and can be divided into two main types: 1) thermosets are structural adhesives that set by chemical reaction, 2) thermoplastics do not cure in the traditional sense, they simply change physical state from solid to liquid on heating and back to solid on cooling. Among the thermosets are: Anaerobics are one-part adhesives that harden on contact with metal and in the absence of air. Cyanoacrylates are one-part adhesives that cure by reacting with moisture. Epoxies are the most widely used adhesives and are available as one (cure by heating) or two-part (warm or cold cure) systems. They have good gap-filling properties. Urethanes can be used with a wide variety of substrates, but are limited to service temperatures below 200°F. Silicones are inorganic adhesives and have a broad temperature range of application, -40° to 500°F. T

Adhesive Bonding Curing of Adhesives Thermosets form long polymer chains by chemical reaction (curing) Heat (epoxy) Ultraviolet light, oxygen (acrylics) Moisture (superglue) Most adhesives are thermosetting polymers, which form and/or crosslink chain molecules by a chemical reaction. Thermoplastics were discussed in the previous section on polymer welding. They do not cure in the traditional sense, they simply change physical state from solid to liquid on heating and back to solid on cooling. The chemical reaction of curing can involve three types of materials. 1) A catalyst causes a reaction to begin but is not consumed in the reaction. 2) A hardener (curing agent) becomes part of the chain molecule or forms crosslinks between the chains. An initiator can be used with catalysts or hardeners to accelerate the initial reaction stages. Too much catalyst can result in excessive heat generation and a brittle, low-strength adhesive. Excess hardener might not be consumed in the reaction and act to degrade performance; too little hardener can result in shorter chains or unreacted crosslinking sites. Ambient temperature cures: Epoxies, urethanes, acrylics, RTV (room-temperature-vulcanized) silicones, and most cements can cure at ambient temperatures (45-105°F). Except for some RTV silicones, ambient cure systems are almost always two-part systems (2-part epoxy). Accelerated temperature cures: One-part and two-part epoxies and phenloics cure above 250°F, while urethanes, acrylics and silicones cure < 250°F. Free radical cures: An initiator is added to an adhesive that has unsatisfied bonds to begin the curing process. These initiators can be formulated to: work only in the absence of air (anaerobic), be sensitive to ultraviolet light, require moisture for initiation (cyanoacrylates or superglues).

Stress Modes - Best to Worst
1. Compression 2. Shear 3. Tension 2000 psi would be a typical strength of an epoxy. A weld in medium strength steel can have a strength of 60,000 psi. As such, adhesives are not structural materials. The successful use of adhesive depends on good joint design. Epoxies and urethanes are good for shear, tensile and compressive loading. Brittle adhesives, however, should be avoided in peel or cleavage. Flexibilized adhesives perform better in peel or cleavage, but may creep in shear or tension. High loads require high surface area to distribute the stress. Do not use higher strength adhesive simply to make a smaller joint. The relationship is not necessarily linear. A bond area of 2 square inches may be only 40-50% stronger than a bond area of 1 square inch. When increasing the surface area to increase the absolute joint strength, width is preferred over increased overlap length. For example, let’s look at two joints with a bond area of 1 square inch. Joint 1: 1 inch overlap, 1 inch wide. Joint 2: 0.5 inch overlap, 2 inches wide. Joint 2 is stronger. 4. Peel 5. Cleavage T 1 2

Why Adhesive Bonding? Dissimilar materials
Plastic to metal Materials that can be damaged by mechanical attachments Shock absorption or mechanical dampening Laminate structures Skin to honeycomb structure Adhesive bonding is used as an alternative to such mechanical fastening methods as screws, rivets and spot welds. It provides greater stiffness and a more uniform stress distribution. Adhesives are used to bond widely differing materials, including those with poor weldability, large differences in thermal expansion, or complete dissimilarity, e.g. plastics to metals. They are used for materials that cannot withstand the added stresses of holes for bolts or rivets. Also they can be used where, due to poor design, the joint cannot be reached for assembly in a complex part or where there is no room for mechanical fasteners. Adhesive bonding can act as a shock absorber or mechanical dampener by dissipating vibrations throughout a structure. They are also used to hold pieces together while mechanical attachments are made. Adhesives are used when substrates are too thin to be welded or too cumbersome to be mechanically fastened (skin to honeycomb). The entire area of the adhesive bond participates in stress dissipation, as opposed to bonding techniques such as spot welding. Whenever stress distribution is important, adhesive bonding is indicated.

Adhesive Selection Adhesive selection is based primarily on Epoxy
Adhesive Bonding Adhesive Selection Adhesive selection is based primarily on Type of substrate Strength requirements, type of loading, impact requirements Temperature resistance, if required Epoxy Cyanoacrylates Anaerobics - metals Urethanes Silicones Pressure sensitive adhesives (PSAs) The selection of an adhesive involves several steps. An initial selection is based upon the substrate. There may be only one or two general classes of adhesives that can be used with a given substrate. Strength, loading, and impact requirements come into play in determining the exact composition. Additives can be added to a brittle adhesive, such as epoxy, in order to improve its flexibility and improve its performance under peel or cleavage. The composition in turn determines the service temperature of the adhesive. The remaining properties of the adhesive, humidity resistance, chemical resistance, electrical resistance, are set by the composition that was selected. Having determined the adhesive composition, surface preparation is undertaken to comply with the application and production speed. Anaerobics are one-part adhesives that harden on contact with metal and in the absence of air. Cyanoacrylates are one-part adhesives that cure by reacting with moisture. Epoxies are the most widely used adhesives and are available as one (cure by heating) or two-part (warm or cold cure) systems. They have good gap-filling properties. Urethanes can be used with a wide variety of substrates, but are limited to service temperatures below 200°F. Silicones are inorganic adhesives and have a broad temperature range of application, -40° to 500°F. Pressure sensitive adhesives include tape and label adhesives and contact cements. PSAs contain two polymer resins, one for strength and the other for tack (stickiness).

Factors that Influence Process Selection
Material joining needs Capabilities of available processes Cost Environment Required welding speed Skill level Part Fit-up

Advantages Joining dissimilar materials - plastic to metal
Materials that can be damaged by mechanical attachments Blind joints Shock absorption or mechanical dampening Temporary alignment Laminated structures Thin substrates - skin-to-honeycomb construction Stress distribution Adhesives are used to bond widely differing materials, including those with poor weldability, large differences in thermal expansion, or complete dissimilarity, e.g. plastics to metals. They are used for materials that cannot withstand the added stresses of holes for bolts or rivets. Also they can be used where, due to poor design, the joint cannot be reached for assembly in a complex part or where there is no room for mechanical fasteners. Adhesive bonding can act as a shock absorber or mechanical dampener by dissipating vibrations throughout a structure. They are also used to hold pieces together while mechanical attachments are made. Adhesives are used when substrates are too thin to be welded or too cumbersome to be mechanically fastened (skin to honeycomb). The entire area of the adhesive bond participates in stress dissipation, as opposed to bonding techniques such as spot welding. Whenever stress distribution is important, adhesive bonding is indicated. T

Adhesive Bonding Limitations Adhesives don’t do work, they distribute work; they are not structural materials Environmental degradation Temperature Oxidation Difficult to repair Curing or setting time Surface preparation Adhesives shouldn’t be used as substrate materials; they are used to distribute stress. Good joint design and minimization of bondline thickness help to avoid this pitfall. Excessive service temperature and corrosive atmospheres can degrade some adhesives. The selection of an adhesive should be made with consideration of the service environment of the part. To make a repair in a failed component requires complete removal of the old, oxidized material and proper treatment of the surfaces prior to repair. As in the brazing or soldering process, the adhesive must wet the materials being joined. This can require surface preparation such as a solvent degrease followed by grit blasting. - ( C - C )x ( C - C )x - O OH C = O CH3 Polyvinylacetate Polyvinyl alcohol A low volume percentage of these polymers is used to produce white glue and wood glue. Water and fillers such as clay constitute the bulk of these adhesives.

Homework Do Homework Assignment 3 on “More Welding Processes” and Turn in by next class period.

Non-Arc Welding Processes Continued

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Non-Arc Welding Processes Continued

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