PROCESSESNUMPON MAHAYOTSANUN DEPARTMENT OF MECHANICAL ENGINEERING KHON KAEN UNIVERSITY
Joining can be categorized in terms of their common principle of operation as:Fusion welding involves melting and coalescing materials by means of heat, usually supplied by electrical or high-energy means. The processes consist of oxyfuel gas welding, consumable- and nonconsumable-elctrode arc welding, and high-energy-beam welding.Solid-state welding involves joining without fusion; that is, there is no liquid (mol-ten) phase in the joint. The basic categories of solid state welding are cold, ultra-sonic, friction, resistance, explosion welding, and diffusion bonding.Brazing and soldering use filler metals and involve lower temperatures than used in welding. The heat required is supplied externally.
Adhesive bonding is an important technology because of its unique advantages for applications requiring strength, sealing, insulation, vibration damping, and resis-tance to corrosion between dissimilar or similar metals. Included in this category are electrically conducting adhesives for surface-mount technologies.Mechanical fastening processes use a wide variety of fasteners, bolts, nuts, screws, and rivets. Joining nonmetallic materials can be accomplished by such means as mechanical fastening, adhesive bonding, fusion by various external or internal heat sources, diffusion, and preplating with metal.
Oxyfuel gas welding (OFW) is a general term to describe any welding process that uses a fuel gas, combined with oxygen to produce a flame, as the source of the heat required to melt the metals at the joint. The most common gas welding processes uses acetylene. The proportion of acetylene and oxygen in the gas mixture is an important factor in oxyfuel gas welding. At a ratio of 1:1, that is, when there is no excess oxygen, the flame is considered to be neutral. With a greater oxygen supply, the flame can be harmful, especially for steels, because it oxidizes the metal; hence
it is known as an oxidizing flame. If oxygen is insufficient for full combustion, the flam is known as a reducing (having excess acetylene) or caburizing flame. Filler metals are used to supply additional metal to the weld zone during welding. They are available as filler rods or wire and may be bare or coated with flux. The purpose of the flux is to retard oxidation of the surfaces being welded, by generating a gas-eous shield around the weld zone.
Three basic types of oxyacetylene flames used in oxyfuel gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing, flame. (d) The principle of the oxyfuel gas welding operation.
WELDINGIn this method, the two components to be welded are heated at their interface by means of a torch using and oxyacetylene gas mixture. After the interface begins to melt, the torch is withdrawn. An axial force is then applied to press the two components together and is maintained until the interface solidifies. Note the formation of a flash due to the upsetting of the joined ends of the two components.
Schematic illustration of the pressure gas welding process; (a) before, and (b) after. Note the formation of a flash at the joint, which can later be trimmed off.
In arc welding, the heat required is obtained through electrical energy. Using either a consumable or a nonconsumable electrode (rod or wire), an arc is produced be-tween the tip of the electrode and the parts to be welded.
The heat input in arch welding can be calculated from the equation
Where H is the heat input l is the weld length V is the voltage applied I is the current v is the welding speed e is the efficiency of the process (from 75% for shielded arc welding to 90% for gas metal arc welding and submerged arc welding)v
The heat input given can also be expressed as
Where u is the specific energy required for melting A is the cross section of the weld
Some typical values of u are given in the right table. Thus, the equation of the weld-ing speed is
( )( )AlH =
SHIELDEDMETAL ARC WELDING
In this process, the electric arc is generated by touching the tip of a coated electrode to the workpiece and then withdrawing it quickly to a distance sufficient to maintain the arc. The heat generated melts a portion of the electrode tip, its coating, and the base metal in the immediate area of the arc. A weld forms after the molten metal solidifies in the weld area. The electrode coating deoxidizes and provides a shielding gas in the weld area to protect it from the oxygen in the environment.
(a) Schematic illustration of the shielded metal arc welding process. About one-half of all large-scale industrial welding operations use this process. (b) Schematic illustra-tion of the shielded metal arc welding operation.
SUBMERGEDMETAL ARC WELDING
In submerged arc welding, the weld arc is shielded by granular flux (consisting of lime, silica, maganese oxide, calcium fluoride, and other elements) which is fed into the weld zone by gravity flow through a nozzle. The thick layer of flux completely covers the molten metal and prevents weld spatter and sparks and suppresses the intense ultraviolet radiation and fumes. The flux also acts as a thermal insulator, allowing deep penetration of heat into the workpiece. The unfused flux is recovered (using a recovery tube), treated, and reused. The consumable electrode is a coil of
bare wire and is fed automatically through a tube (welding gun). Because the flux is fed by gravity, the SAW process is somewhat limited to welds in a horizontal or flat position, with a backup piece.
Schematic illustration of the submerged arc welding process and equipment. Unfused flux is recovered and reused.
GASMETAL ARC WELDING
In gas metal arc welding, the weld area is shielded by an external source of gas, such as argon, helium, carbon dioxide, or various other gas mixtures. In addition, deoxidiz-ers are usually present in the electrode metal itself, in order to prevent oxidation of the molten weld puddle. The consumable bare wire is fed automatically through a nozzle into the weld arc, and multiple weld layers can be deposited at the joint.
(a) Gas metal arc welding process, formerly known as MIG welding (for metal inert gas). (b) Basic equipment used in gas metal arc welding operations.
The flux-cored arc welding process is similar to gas metal arc welding, with the exception that the electrode is tubular in shape and is filled with flux. Cored elec-trodes produce a more stable arc, improve weld contour, and improve the mechani-cal properties of the weld metal. The flux-cored arc welding process combines the versatility of SMAW with the continuous and automatic electrode feeding feature of GMAW. It is economical and is used for welding a variety of joints with dif-ferent thicknesses, mainly with steels, stainless steels, and nickel alloys. A major
advantage of FCAW is the ease with witch specific weld metal chemistries can be developed. This process is easy to automate and is readily adaptable to flexible manufacturing systems and robotics.
Schematic illustration of the flux-cored arc-welding process. This operation is similar to gas metal-arc welding.
Electrogas welding is primarily used for welding the vertical edges of sections in one pass with the pieces placed edge to edge. The weld metal is deposited into a weld cavity between the two pieces to be joined. The space is enclosed by two water-cooled copper dams (shoes) to prevent the molten slag from running off. Mechanical drives move the shoes upward. Single or multiple electrodes are fed through a conduit, and a continuous arc is maintained. Shielding is provided by an inert gas, such as carbon dioxide, argon, or helium, depending on the type of
material being welded. The gas may be provided from an external source, or it may be produced from a flux-cored electrode, or both. Typical applications are in the construction of bridges, ships, pressure vessels, storage tanks, and thick-walled and large-diameter pipes.
Schematic illustration of the electrogas welding process.
WELDINGIn electroslag welding the arc is started between the electrode tip and the bottom of the part to be welded. Flux is added and melted by the heat of the arc. After the molten slag reaches the tip of the electrode, the arc is extinguished; energy is supplied continuously through the electrical resistance of the molten slag. Single or multiple solid as well as flux-cored electrodes may be used, and the guide may be nonconsumable or consumable. Welding is done in one pass. The weld quality is good, and the process is used for heavy structural steel sections, such as heavy machinery and nuclear reactor vessels.
Equipment used for electroslag welding operations.
ARC WELDINGIn gas tungsten arc welding, fomerly known as TIG welding (tungsten inert gas), a filler metal is typically supplied from a filler wire. However, welding also may be done without filler metals, such as in welding close-fit joints. The composition of filler metals must be similar to that of the me