WELDING AND WELDMENTS
An Introduction To Welding Processes
This summarizes the conventional, better known joining processes. The distinguishing features of the various processes, their attributes and limitations, and comparisons where applicable are identified. However, the information presented is a generalization of the subject and should not be used for selecting processes for specific applications. Where specific information and data are needed, the reader should consult additional sources.
The arc welding group of joining processes is widely used in industry. Other well known and often used joining processes are oxyfuel gas welding, resistance welding, flash welding, brazing and soldering. Diffusion, friction, electron beam, and laser beam welding along with adhesive bonding are being applied to an increasing number of applications in industry. Ultrasonic and explosive have only narrow fields of application, as does thermal spraying.
Relatively few individuals develop an expertise in the use of all processes and probably no one has had the occasion to join all of the currently available engineering materials. The purpose of this article, therefore, is to familiarize readers with all the major contemporary processes so that consideration may be given to those processes that might otherwise be overlooked.
Frequently, several processes can be used for any particular job. The major problem is to select the one that is the most suitable in terms of fitness for service and cost. These two factors, however, may not be totally compatible, thus forcing a compromise. Selection of a process can depend on a number of considerations, including the number of components being fabricated, capital equipment costs, joint location, structural mass, and desired performance of the product. The adaptability of the process to the location of the operation or the type of shop, and experience and abilities of the employees may also have an impact on the final selection.
The term arc welding applies to a large and diversified group of welding processes that use an electric arc as the source of heat to melt and join metals. the formation of a weld between metals when arc welded may or may not require the use of pressure or filler metal.
The welding arc is struck between the workpiece and the tip of the electrode. The electrode will be either a consumable wire or rod or a nonconsumable carbon or tungsten rod which carries the welding current. The electrode is manually or mechanically moved along the joint, or it remains stationary while the workpiece is moved. When a nonconsumable electrode is used, filler metal can be supplied by a separate rod or wire if needed. A consumable electrode, however, will be designed not only to conduct the current that sustains the arc but also to melt and supply filler metal to the joint. It may also produce a slag covering to protect the hot weld metal from oxidation.
Resistance welding incorporates a group of processes in which the heat for welding is generated by the resistance to the flow of electrical current through the parts being joined. It is most commonly used to weld two overlapping sheets or plates which may have different thicknesses. A pair of electrodes conduct electrical current to the joint. Resistance to the flow of current heats the faying surfaces, forming a weld. These electrodes clamp the sheets under pressure to provide good electrical contact and to contain the molten metal in the joint. The joint surfaces must be clean to obtain consistent electrical contact resistance to obtain uniform weld size and soundness.
There are three major resistance welding processes: spot welding, projection welding, and seam welding. In spot welding, the welding current is concentrated at the point of joining using cylindrical electrodes. Spot welds are usually made one at a time. With this process, a projection or dimple is formed in one part prior to welding. The projection concentrates the current at the faying surfaces. Large, flat electrodes are used on both sides of the joint. Several projections may be formed in one of the components to produce several welds simultaneously. As an example, a stamped bracket may have three or four projections formed in it so that it can be welded to a sheet with one welding cycle.
In seam welding, leak-tight welds can be made by a series of overlapping spot welds. These are produced by introducing timed, coordinated pulses of current from rotating wheel electrodes.
Flash welding is classified as a resistance welding process. Heat for welding is created at the faying surfaces of the joint by resistance to the flow of electric current, and by arcs across the interface. When the faying surfaces are heated to welding temperature, force is applied immediately to consummate a weld. Molten metal is expelled, the hot metal is upset, and a flash is formed. Filler metal is not added during welding.
The usual flash weld joins rods or bars end to end or edge to edge. Both components are clamped in electrodes which are connected to the secondary of a resistance welding transformer. One component is moved slowly towards the other, and when contact occurs at surface irregularities, the current flows and initiates the flashing action. This flashing action is continued until a molten layer forms on both surfaces. Then the components are forced together rapidly to squeeze out the molten metal and dross, and upset the adjacent hot base metal. This produces a hot worked joint free of weld metal. The mechanical properties of flash welds are often superior to other types of welds.
Flash welding is usually an automatic process. Parts are clamped in place by a welding operator who simply presses a button to start the welding sequence.
Oxyfuel gas welding includes a group of welding processes that use the heat produced by a gas flame or flames for melting the base metal and, if used, the filler metal. Pressure may also be used. Oxyfuel gas welding is an inclusive term used to describe any welding process that uses a fuel gas combined with oxygen to produce a flame having sufficient energy to melt the base metal. The fuel gas and oxygen are mixed in the proper proportions in a chamber which is generally a part of the welding torch assembly. The torch is designed to give the welder complete control of the welding flame to melt the base metal and the filler metal in the joint.
Oxyfuel gas welding is normally done with acetylene fuel gas. Other fuel gases, such as methylacetylene propadiene and hydrogen, are sometimes used for oxyfuel gas welding of low melting metals. The welding flame must provide high localized energy to produce and sustain a molten weld pool. With proper adjustment, the flames also can supply a protective reducing atmosphere over the molten weld pool. Hydrocarbon fuel gases such as propane, butane, natural gas, and various mixtures employing these gases are not suitable for welding ferrous metals because the heat output of the flame is too low or the flame atmosphere is oxidizing.
Diffusion welding is a specialized process generally used only when the unique metallurgical characteristics of the process are required. Components to be diffusion welded must be specifically designed and carefully processed to produce successful joints. It is useful for applications concerned with:
- the avoidance of metallurgical problems associated with fusion welding processes
- the fabrication of shapes to net dimensions
- the maintenance of joint corrosion resistance with titanium and zirconium
- the production of thick parts with uniform through thickness properties, as with titanium laminates
Diffusion welding occurs in the solid state when properly prepared surfaces are in contact under predetermined conditions of time, pressure, and elevated temperature. The applied pressure is set above the level needed to ensure essentially uniform surface contact but below the level that would cause microscopic deformation. The temperature is generally well below the melting point. A filler metal, usually preplaced as an insert of plating, may be used. The function of the filler metal generally is to lower the required temperature, pressure, or time required for welding or to permit welding in a less expensive atmosphere.
Friction welding machines are designed to convert mechanical energy into heat at the joint to be welded. The usual method of accomplishing this is to rotate one of the parts to be joined and force it against the other which is held in a stationary position. Normally, one of the two workpieces is circular or nearly circular in cross section, such as a hexagon. Frictional heat at the joint interface raises the metal to forging temperature. Axial pressure forces the hot metal out of the joint. Oxides and other surface impurities are removed with the soft, hot metal.
Two major techniques for friction welding are available. With the conventional technique, the moving part is held in a motor-driven collet and rotated at a constant speed while axial force is applied to both parts. The fixed part must be held rigidly to resist the axial force and prevent it from rotating. Rotation is continued until the entire joint is heated sufficiently. Then simultaneously, the rotation is stopped and an upsetting force is applied to complete the weld. The process variables are rotational speed, axial force, welding time, and upset force. During the welding period, the drive motor must provide energy at the rate necessary to make the weld. Therefore, a relatively high powered motor is required.
A second technique is called inertia welding. With it, energy is stored in a flywheel which has been accelerated to the required speed by a drive motor. The flywheel is coupled directly to the drive motor by a clutch and to the collet which grips the rotating member. A weld is made by applying an axial force through the rotating part while the flywheel decelerates, transforming its kinetic energy to heat at the joint. When properly programmed, the weld is completed when the flywheel stops. The welding variables are flywheel moment of inertia, flywheel rotational speed, axial force, and upset force if used.
There are numerous other solid state welding processes, including ultrasonic welding, explosive welding and cold welding.
Ultrasonic welding employs a combination of a static normal force and a high-frequency oscillating shear force to cause coalescence at joint interfaces. Frequencies normally range from 10,000 to 60,000 Hz. The process has been very successfully applied to fine wire bonding in the electronic industry.
Explosive welding is accomplished by accelerating one of the components to be welded to an extremely high velocity with high energy explosives. The kinetic energy of motion is converted to heat at the joint interface. The high pressures generated at the interface cause coalescence and welding. The process is most commonly applied to the cladding of plates with a dissimilar metal. Slabs are also clad using explosive welding, and subsequently hot rolled into plates.
In cold welding, plastic deformation causes the generation of a new, clean surface at the joint interface, which in turn promotes solid-state welding. The process has been used to join soft, ductile metals in numerous applications including the welding of aluminum wire stock.
Electron beam welding is accomplished with a stream of high-velocity electrons which is formed into a concentrated beam to provide a heating source. The process produces intense local heating through the combined action of the stream of electrons. Each electron penetrates its own short distance and gives up its kinetic energy in the form of heat. With high beam energy, a hole can be melted through the material (a keyhole).
The hole is moved along the joint by moving either the electron gun or the workpiece. It is maintained as the metal at the front melts and flows around to the rear where it solidifies. Welds can be made without a keyhole, where melting takes place by conduction of heat from the surface, but welding speeds are lower.
Electron beam welding has major advantages. It can produce deep, narrow, and almost parallel-sided welds with a low total heat input and comparatively narrow heat affected zones.
Similar to the electron beam, a focused high-power coherent monochromatic light beam used in laser beam welding causes the metal at the point of focus to vaporize, producing a deep penetration column of vapor extending into the base metal. The vapor column is surrounded by a liquid pool, which is moved along the joint producing welds with depth to width ratios greater than eight to one. Yttrium aluminum garnet (YAG) lasers may be used for spot welding thin materials, joining microelectronic components, and other tasks requiring precise control of energy input to the workpiece. Initial applications were limited by their low power, but later devices could produce 10 kW pulses having one millisecond duration. in seam welding, speeds are relatively low because the welds are formed by a series of overlapping spot welds.
Laser beam welding has been used successfully to join a variety of metals and alloys including low alloy and stainless steels which do not exhibit high hardenability, aluminum alloys, lead, titanium, refractory metals, and high temperature alloys. Porosity-free, ductile welds can be attained with average tensile strengths equivalent to those of the base metal.
Brazing is a group of welding processes in which the joint is heated to a suitable temperature in the presence of a filler metal having a liquidus above 840F and below the solidus of the base metal. The filler metal is distributed between the close fitted faying surfaces of the joint by capillary action. Braze welding is differentiated from brazing because the filler metal is deposited in a groove or fillet exactly at the point where it is to be used and capillary action is not a factor. Brazing is arbitrarily distinguished from soldering by the filler metal melting temperature. In soldering, filler metals melt below 840F.
To produce acceptable brazed joints, considerations must be given to four basic elements: joint design, filler metal, uniform heating of the joint, and protective or reactive shielding. The various brazing processes are primarily designated according to the sources or methods of heating. Those which are currently of industrial significance include torch brazing, furnace brazing, induction brazing, resistance brazing, dip brazing, infrared brazing and diffusion brazing.
Soldering involves heating a joint to a suitable temperature and using a filler metal (solder) which melts below 840F. The solder is distributed between the closely fitted surfaces of the joint by capillary action. Heat is required to raise the joint to a suitable temperature, melt the solder, and to promote the action of a flux on the metal surface so that the molten solder will wet and flow into the joint.
Successful soldering involves shaping the parts to fit closely together, cleaning the surfaces to be joined, applying the flux, assembling the parts, and applying the heat and solder. Flux residues may be removed when the joint is cooled.
Adhesive bonding is a joining process which is gaining acceptance as an assembly method for joining metals. The method has several advantages and limitations. On one hand, it is capable of joining dissimilar materials, for example, metals to plastics; bonding very thin sections without distortion and very thin sections to thick sections; joining heat sensitive alloys; and producing bonds with unbroken surface contours. Furthermore, bonding can be accomplished at a low cost.
On the other hand, joints produced by this method may not support shear or impact loads. Also, such joints must have an adhesive layer less than 0.005" thick and must be designed to develop a uniform load distribution in pure shear or tension.
Thermal spraying is a process in which a metallic or nonmetallic material is heated and then propelled in atomized form onto a substrate. The material may be initially in the form of wire, rod, or powder. It is heated to the plastic or molten state by an oxyfuel gas flame, an electric or plasma arc, or an explosive gas mixture. The hot material is propelled from the spray gun to the substrate by a gasjet. Most metals, cermets, oxides, and hard metallic compounds can be deposited by one or more of the process variations. The process can also be used to produce free-standing objects using a disposable substrate. It is sometimes called metallizing or metal spraying.
When molten particles strike a substrate, they flatten and form thin platelets that conform to the surface. These platelets rapidly solidify and cool. Successive layers are built up to the desired thickness. The bond between the spray deposit and substrate may be mechanical, metallurgical, chemical, or a combination of these. In some cases, a thermal treatment of the composite structure is used to increase the bond strength by diffusion or chemical reaction between the spray deposit and the substrate.
Thermal spraying is widely used for surfacing applications to attain or restore desired dimensions; to improve resistance to abrasion, wear, corrosion, oxidation, or a combination of these; and to provide specific electrical or thermal properties. Frequently, thermal sprayed deposits are applied to new machine elements to provide surfaces with desired characteristics for the application.
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