Welding is a process of joining similar metals by application of heat with or without application of pressure and addition of filler material. The term “weldability” has been defined as the capacity of being welded into inseparable joints having specified properties such as definite weld strength, proper structure, etc. There are several factors that affect the weldability i.e. Melting Point, Thermal conductivity, Thermal Expansion, Surface condition, and Change in Microstructure.
There are basically two types of welding, Plastic Welding and Fusion Welding.
Plastic Welding: In Plastic welding or pressure welding, the pieces of metal to be joined are heated to plastic state and then forced together by external pressure.
Fusion Welding: In fusion welding or non pressure welding , the material at the joint is heated to molten state and allowed to solidify.
The welding process have been classified into the following 7 processes and these 7 processes further classified into sub processes.
|1. Gas welding
2. Arc Welding
|3. Resistance welding
i. Butt welding
ii. Spot welding
iii. Seam welding
iv. Projection welding
v. Percussion welding
4. Thermite welding
|6. Newer Welding Processes
i. Electron Beam
7. Related processes
• Thermite welding is the process of igniting a mix of high energy materials, (which is also called thermite), that produce a molten metal that is poured between the working pieces of metal to form a welded joint
• Thermite is a pyrotechnic composition of a metal powder and a metal oxide, which produces an aluminothermic reaction known as a thermite reaction. It is not explosive, but can create short bursts of extremely high temperatures focused on a very small area for a short period of time.
Steps in Thermite Welding
1. Thermit material is a mechanical mixture of metallic aluminum and processed iron oxide.
2. Molten steel is produced by the reaction in a magnesite-lined crucible.
3. At the bottom of the crucible, a magnesite stone is burned, into which a magnesite stone thimble is fitted.
4. This thimble provides a passage through which the molten steel is discharged into the mold.
5. The hole through the thimble is plugged with a tapping pin, which is covered with a fire-resistant washer and refractory sand.
6. The crucible is charged by placing the correct quantity of thoroughly mixed material in it.
7. In preparing the joint for welding, the parts to be welded must be cleaned, alined, and held firmly in place.
8. If necessary, metal is removed from the joint to permit a free flow of the metal into the joint.
9. A wax pattern is then made around the joint in the size and shape of the intended weld.
10. A mold made of refractory sand is built around the wax pattern and joint to hold the molten metal after it is poured.
11. The sand mold is then heated to melt out the wax and dry the mold.
12. The mold should be properly vented to permit the escape of gases and to allow the proper distribution of the metal at the joint.
NEWER WELDING PROCESSES
Electron Beam Welding
• Electron beam welding (EBW) is a fusion welding process in which a beam of high-velocity electrons is applied to the materials being joined.
• The work pieces melt as the kinetic energy of the electrons is transformed into heat upon impact, and the filler metal, if used, also melts to form part of the weld.
• The welding is often done in conditions of a vacuum to prevent dispersion of the electron beam.
• The EB system is composed of an electron beam gun, a power supply, control system, motion equipment and vacuum welding chamber. Fusion of base metals eliminates the need for filler metals. The vacuum requirement for operation of the electron beam equipment eliminates the need for shielding gases and fluxes.
• The electron beam gun has a tungsten filament which is heated, freeing electrons. The electrons are accelerated from the source with high voltage potential between a cathode and anode. The stream of electrons then pass through a hole in the anode. The beam is directed by magnetic forces of focusing and deflecting coils. This beam is directed out of the gun column and strikes the work piece.
• The potential energy of the electrons is transferred to heat upon impact of the work piece and cuts a perfect hole at the weld joint. Molten metal fills in behind the beam, creating a deep finished weld.
• The electron beam stream and work piece are manipulated by means of precise, computer driven controls, within a vacuum welding chamber, therefore eliminating oxidation, contamination.
• Single pass welding of thick joints
• Hermetic seals of components retaining a vacuum
• Low distortion
• Low contamination in vacuum
• Weld zone is narrow
• Heat affected zone is narrow
• Dissimilar metal welds of some metals
• Uses no filler metal
• High equipment cost
• Work chamber size constraints
• Time delay when welding in vacuum
• High weld preparation costs
• X-rays produced during welding
• Rapid solidification rates can cause cracking in some materials
Applications for electron beam welding
• aerospace, automotive, semi-conductor, electronic components and jewelry.
• The process has proved very reliable and cost-effective in high volume production due to the advent of small vacuum chamber machines.
SOLID STATE WELDING
• Friction welding (FW) is a class of solid-state welding processes that generates heat through mechanical friction between a moving work piece and a stationary component, with the addition of a lateral force called “upset” to plastically displace and fuse the materials.
• Technically, because no melt occurs, friction welding is not actually a welding process in the traditional sense, but a forging technique.
• However, due to the similarities between these techniques and traditional welding, the term has become common.
• Friction welding is used with metals and thermoplastics in a wide variety of aviation and automotive applications.
Friction Welding Machine
Advantages of Friction Welding
• Dissimilar materials normally not compatible for welding can be friction welded
• Creates narrow, heat-affected zone
• Consistent and repetitive process
• Joint preparation is minimal – saw cut surface used most commonly
• Greatly increases design flexibility; choose appropriate materials for each area of a blank
• Suitable for quantities ranging from prototype to high production
• No fluxes, filler material, or gases required
• Environmentally friendly process – no fumes, gases, or smoke generated
• Solid state process – no possibility of porosity or slag inclusions
• Creates cast or forge-like blanks, without expensive costs of tooling or minimum quantity requirements
• Reduces machining labor, thereby reducing perishable tooling costs while increasing capacity
• Full surface weld gives superior strength in critical areas
• Reduces raw material costs in bi-metal applications; only use expensive materials where necessary in the blank
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• The machine is used for diffusion welding process.
• It is a solid state welding process that creates fusion of surfaces to be joined by applying pressure at high temperatures.
• The welding process may or may not use filler material. If used, they may be in the form of electroplated surfaces.
• These welders are ideal for joining refractory metals and many other dissimilar metals.
Diffusion Welding Process
• Diffusion welding process is does not comprise microscopic deformation melting or relative motion of the parts.
• Heat required for melting the parts is commonly obtained by resistance, induction or furnace.
• Atmosphere and vacuum furnaces are used for welding general metals. But for joining most refractory metals, a protective inert atmosphere is used.
Laser beam welding
• Laser beam welding (LBW) is a welding technique used to join multiple pieces of metal through the use of a laser.
• The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates.
• The process is frequently used in high volume applications, such as in the automotive industry.
• Laser welding is a high energy beam process and in this regard is similar to electron beam.
• With that exception they are unlike one another. The energy density of the laser is achieved by the concentration of light waves not electrons.
• The laser output is not electrical, does not require electrical continuity, is not influenced by magnetism, is not limited to electrically conductive materials and in fact can interact with any material whether it be metal, plastic, wood, ceramic, etc.
• Finally its function does not require a vacuum nor are x-rays produced.
Laser Welding Process
(1) Laser beam welding (LBW) is a welding process which produces coalescence of materials with the heat obtained from the application of a concentrate coherent light beam impinging upon the surfaces to be joined.
(2) The focused laser beam has the highest energy concentration of any known source of energy. The laser beam is a source of electromagnetic energy or light that can be pro jetted without diverging and can be concentrated to a precise spot. The beam is coherent and of a single frequency.
(3) Gases can emit coherent radiation when contained in an optical resonant cavity. Gas lasers can be operated continuously but originally only at low levels of power. Later developments allowed the gases in the laser to be cooled so that it could be operated
continuously at higher power outputs. The gas lasers are pumped by high radio frequency generators which raise the gas atoms to sufficiently high energy level to cause lasing. Currently, 2000-watt carbon dioxide laser systems are in use. Higher powered systems are also being used for experimental and developmental work. A 6-kw laser is being used for automotive welding applications and a 10-kw laser has been built for research purposes. There are other types of lasers; however, the continuous carbon dioxide laser now available with 100 watts to 10 kw of power seems the most promising for metalworking applications.
(4) The coherent light emitted by the laser can be focused and reflected in the same way as a light beam. The focused spot size is controlled by a choice of lenses and the distance from it to the base metal. The spot can be made as small as 0.003 in. (0.076 mm) to large areas 10 times as big. A sharply focused spot is used for welding and for cutting. The large spot is used for heat treating.
(5) The laser offers a source of concentrated energy for welding; however, there are only a few lasers in actual production use today. The high-powered laser is extremely expensive. Laser welding technology is still in its infancy so there will be improvements and the cost of equipment will be reduced. Recent use of fiber optic techniques to carry the laser beam to the point of welding may greatly expand the use of lasers in metal-working.
There are almost unlimited applications for marking, engraving, cutting, and welding in a substantial amount of companies and industries. Below are just a few of these applications. Please contact us and we would be most happy to process one of your samples that you think has merit at no charge to you.
Electro slag welding (ESW)
1. Electro slag welding (ESW) is a highly productive, single pass welding process for thick (greater than 25mm up to about 300mm) materials in a vertical or close to vertical position.
2. (ESW) is similar to electro gas welding, but the main difference is the arc starts in a different location.
3. An electric arc is initially struck by wire that is fed into the desired weld location and then flux is added.
4. Additional flux is added until the molten slag, reaching the tip of the electrode, extinguishes the arc.
5. The wire is then continually fed through a consumable guide tube (can oscillate if desired) into the surfaces of the metal work pieces and the filler metal are then melted using the electrical resistance of the molten slag to cause coalescence.
6. The wire and tube then move up along the work piece while a copper retaining shoe that was put into place before starting (can be water cooled if desired) is used to keep the weld between the plates that are being welded.
7. Electro slag welding is used mainly to join low carbon steel plates and/or sections that are very thick.
8. It can also be used on structural steel if certain precautions are observed.
9. This process uses a direct current (DC) voltage usually ranging from about 600A and 40-50V, higher currents are needed for thicker materials. Because the arc is extinguished, this is not an arc process.
Flux Core Welding
1. FCAW, Flux Core Flux-cored, tubular electrode welding has evolved from the MIG welding process to improve arc action, metal transfer, weld metal properties, and weld appearance.
2. It is an arc welding process in which the heat for welding is provided by an arc between a continuously fed tubular electrode wire and the work piece.
3. Shielding is obtained by a flux contained within the tubular electrode wire or by the flux and an externally supplied shielding gas. A diagram of the process is shown in figure
4. FCAW, Flux Core Flux-cored arc welding is similar to gas metal arc welding in many ways.
5. The flux-cored wire used for this process gives it different characteristics.
6. Flux-cored arc welding is widely used for welding ferrous metals and is particularly good for applications in which high deposition rates are needed.
7. At high welding currents, the arc is smooth and more manageable when compared in using large diameter gas metal arc welding electrodes with carbon dioxide.
8. The arc and weld pool are clearly visible to the welder. A slag coating is left on the surface of the weld bead, which must be removed.
9. Since the filler metal transfers across the arc, some spatter is created and some smoke produced.
Soldering is defined as “the joining of metals by a fusion of alloys which have relatively lw melting points”. In other words, you use a metal that has a low melting point to adhere the surfaces to be soldered together. Soldering is more like gluing with molten metal than anything else. Soldering is also a must have skill for all sorts of electrical and electronics work. It is also a skill that must be taught correctly and developed with practice.