Cold welding concepts are used in nano and micro fabrications in high-technology areas. For example, a gold nanowire can be cold welded instantly. The technique is also used in the nuclear field...
Welding of materials for construction is a very important component of any type of industrial infrastructure. There are different types of welding processes with different advantages. A weld can be defined as a coalescence of metals produced by heating to a suitable temperature with or without the application of pressure, and with or without the use of a filler material. In fusion welding a heat source generates sufficient heat to create and maintain a molten pool of metal of the required size.
The heat may be supplied by electricity or by a gas flame. Electric resistance welding can be considered fusion welding – because some molten metal is formed. Solid-phase processes produce welds without melting the base material and without the addition of a filler metal. Pressure is always employed, and generally some heat is provided. Frictional heat is developed in ultrasonic and friction joining, and furnace heating is usually employed in diffusion bonding. This article presents a brief summary on cold welding, its advantages, developments and characterisation of cold welds.
Welding is a technique used for joining metallic parts usually through the application of heat. This technique was discovered during efforts to manipulate iron into useful shapes. The welding technique—which involved inter-layering relatively soft and tough iron with high-carbon material, followed by hammer forging—produced a strong, tough blade. In modern times the improvement in iron-making techniques, especially the introduction of cast iron, restricted welding to the blacksmith and the jeweler. Other joining techniques, such as fastening by bolts or rivets, were widely applied to new products, from bridges and railway engines to kitchen utensils. Modern fusion welding processes are an outgrowth of the need to obtain a continuous joint on large steel plates. Riveting had been shown to have disadvantages, especially for an enclosed container such as a boiler. Gas welding, arc welding and resistance welding all appeared at the end of the 19th century. The first real attempt to adopt welding processes on a wide scale was made during World War I. By 1916, the oxyacetylene process was well developed, and the welding techniques employed then are still used. The main improvements since then have been in equipment and safety. Arc welding, using a consumable electrode, was also introduced in this period, but the bare wires initially used produced brittle welds. A solution was found by wrapping the bare wire with asbestos and an entwined aluminium wire. The modern electrode, introduced in 1907, consists of a bare wire with a complex coating of minerals and metals. Arc welding was not universally used until World War II, when the urgent need for rapid means of construction for shipping, power plants, transportation, and structures spurred the necessary development work. Resistance welding, invented in 1877 by Elihu Thomson, was accepted long before arc welding for spot and seam joining of sheet. Butt welding for chain making and joining bars and rods was developed during the 1920s. In the 1940s, the tungsten-inert gas process, using a non-consumable tungsten electrode to perform fusion welds, was introduced. In 1948 a new gas-shielded process utilized a wire electrode that was consumed in the weld. More recently, electron-beam welding, laser welding, and several solid-phase processes such as diffusion bonding, friction welding, and ultrasonic joining have been developed.
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing fusion, which is distinct from lower temperature metal-joining techniques such as brazing and soldering, which do not melt the base metal. In addition to melting the base metal, a filler material is often added to the joint to form a pool of molten material (the weld pool) that cools to form a joint that can be as strong as the base material. Pressure may also be used in conjunction with heat, or by itself, to produce a weld.
Arc welding: The electric arc used in welding is a high-current, low-voltage discharge generally in the range 10–2,000 amperes at 10–50 volts. An arc column is complex but, broadly speaking, consists of a cathode that emits electrons, a gas plasma for current conduction, and an anode region that becomes comparatively hotter than the cathode due to electron bombardment. Therefore, the electrode, if consumable, is made positive and, if non-consumable, is made negative. A direct current (DC) arc is usually used, but alternating current (AC) arcs also can be employed. Total energy input in all welding processes exceeds that which is required to produce a joint, because not all the heat generated can be effectively utilised. Efficiencies vary from 60 to 90 percent, depending on the process; some special processes deviate widely from this figure. Heat is lost by conduction through the base metal and by radiation to the surroundings. Most metals, when heated, react with the atmosphere or other nearby metals. These reactions can be extremely detrimental to the properties of a welded joint. Most metals, for example, rapidly oxidise when molten. A layer of oxide can prevent proper bonding of the metal. Molten-metal droplets coated with oxide become entrapped in the weld and make the joint brittle. Some valuable materials added for specific properties react so quickly on exposure to the air that the metal deposited does not have the same composition as it had initially. These problems have led to the use of fluxes and inert atmospheres. In fusion welding, the flux has a protective role in facilitating a controlled reaction of the metal and then preventing oxidation by forming a blanket over the molten material. Fluxes can be active and help in the process or inactive and simply protect the surfaces during joining. Inert atmospheres play a protective role similar to that of fluxes. In gas-shielded metal-arc and gas-shielded tungsten-arc welding an inert gas—usually argon—flows from an annulus surrounding the torch in a continuous stream, displacing the air from around the arc. The gas does not chemically react with the metal but simply protects it from contact with the oxygen in the air. Some of the best known arc welding methods include:
- Shielded Metal Arc Welding (SMAW) - also known as ‘stick welding or electric welding’, uses an electrode that has flux, the protectant for the puddle, around it. The electrode holder holds the electrode as it slowly melts away. Slag protects the weld puddle from atmospheric contamination.
- Gas Tungsten Arc Welding (GTAW) - also known as TIG (tungsten, inert gas), uses a non-consumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by an inert shielding gas such as Argon or Helium.
- Gas Metal Arc Welding (GMAW) - commonly termed MIG (Metal, Inert Gas), uses a wire feeding gun that feeds wire at an adjustable speed and flows an argon-based shielding gas or a mix of argon and carbon dioxide (CO2) over the weld puddle to protect it from atmospheric contamination.
- Flux-Cored Arc Welding (FCAW) - almost identical to MIG welding except it uses a special tubular wire filled with flux; it can be used with or without shielding gas, depending on the filler.
- Submerged Arc Welding (SAW) - uses an automatically fed consumable electrode and a blanket of granular fusible flux. The molten weld and the arc zone are protected from atmospheric contamination by being 'submerged' under the flux blanket.
Plasma welding: is an arc process in which hot plasma is the source of heat. It has some similarity to gas-shielded tungsten-arc welding, the main advantages being greater energy concentration, improved arc stability, and easier operator control. Better arc stability means less sensitivity to joint alignment and arc length variation. In most plasma welding equipment, a secondary arc must first be struck to create an ionised gas stream and permit the main arc to be started. This secondary arc may utilise either a high-frequency or a direct contact start. Water cooling is used because of the high energies forced through a small orifice. The process is amenable to mechanizstion, and rapid production rates are possible.
Thermochemical processes: One such process is gas welding. It once ranked as equal in importance to the metal-arc welding processes, but is now confined to a specialised area of sheet fabrication and is probably used as much by artists as in industry. Gas welding is a fusion process with heat supplied by burning acetylene in oxygen to provide an intense, closely controlled flame. Metal is added to the joint in the form of a cold filler wire. A neutral or reducing flame is generally desirable to prevent base-metal oxidation. By deft craftsmanship very good welds can be produced, but welding speeds are very low. Fluxes aid in preventing oxide contamination of the joint. Another thermochemical process is aluminothermic (thermite) joining. It has been successfully used for both ferrous and nonferrous metals but is more frequently used for the former. A mixture of finely divided aluminium and iron oxide is ignited to produce a superheated liquid metal at about 2,800° C (5,000° F). The reaction is completed in 30 seconds to 2 minutes regardless of the size of the charge. The process is suited to joining sections with large, compact cross sections, such as rectangles and rounds. A mold is used to contain the liquid metal.
Resistance welding: Spot, seam and projection welding are resistance welding processes in which the required heat for joining is generated at the interface by the electrical resistance of the joint. Welds are made in a relatively short time (typically 0.2 seconds) using a low-voltage, high-current power source with force applied to the joint through two electrodes, one on each side. Spot welds are made at regular intervals on sheet metal that has an overlap. Joint strength depends on the number and size of the welds. Seam welding is a continuous process wherein the electric current is successively pulsed into the joint to form a series of overlapping spots or a continuous seam. This process is used to weld containers or structures where spot welding is insufficient. A projection weld is formed when one of the parts to be welded in the resistance machine has been dimpled or pressed to form a protuberance that is melted down during the weld cycle. The process allows a number of predetermined spots to be welded at one time. All of these processes are capable of very high rates of production with continuous quality control. The most modern equipment in resistance welding includes complete feedback control systems to self-correct any weld that does not meet the desired specifications. Flash welding is a resistance welding process where parts to be joined are clamped, the ends brought together slowly and then drawn apart to cause an arc or flash. Flashing or arcing is continued until the entire area of the joint is heated; the parts are then forced together and pressure maintained until the joint is formed and cooled. Low- and high-frequency resistance welding is used for the manufacture of tubing. The longitudinal joint in a tube is formed from metal squeezed into shape with edges abutted. Welding heat is governed by the current passing through the work and the speed at which the tube goes through the rolls. Welding speeds of 60 m (200 feet) per minute are possible in this process.
Electron-beam welding: In electron-beam welding, the work piece is bombarded with a dense stream of high-velocity electrons. The energy of these electrons is converted to heat upon impact. A beam-focusing device is included, and the work piece is usually placed in an evacuated chamber to allow uninterrupted electron travel. Heating is so intense that the beam almost instantaneously vaporises a hole through the joint. Extremely narrow deep-penetration welds can be produced using very high voltages – up to 150 kilovolts. Work pieces are positioned accurately by an automatic traverse device; for example, a weld in material 13 mm (0.5 inch) thick would only be 1 mm (0.04 inch) wide. Typical welding speeds are 125 to 250 cm (50 to 100 inches) per minute.
Friction welding: In friction welding two work pieces are brought together under load with one part rapidly revolving. Frictional heat is developed at the interface until the material becomes plastic, at which time the rotation is stopped and the load is increased to consolidate the joint. A strong joint result with the plastic deformation and in this sense, the process may be considered a variation of pressure welding. The process is self-regulating, for, as the temperature at the joint rises, the friction coefficient is reduced and overheating cannot occur. The machines are almost like lathes in appearance. Speed, force and time are the main variables. The process has been automated for the production of axle casings in the automotive industry.
Laser welding: Laser welding is accomplished when the light energy emitted from a laser source is focused upon a work piece to fuse materials together. The limited availability of lasers of sufficient power for most welding purposes has so far restricted its use in this area. Another difficulty is that the speed and the thickness that can be welded are controlled not so much by power but by the thermal conductivity of the metals and by the avoidance of metal vaporisation at the surface. Particular applications of the process with very thin materials up to 0.5 mm (0.02 inch) have, however, been very successful. The process is useful in the joining of miniaturised electrical circuitry.
Diffusion bonding: This type of bonding relies on the effect of applied pressure at an elevated temperature for an appreciable period of time. Generally, the pressure applied must be less than that necessary to cause 5 percent deformation so that the process can be applied to finished machine parts. The process has been used most extensively in the aerospace industries for joining materials and shapes that otherwise could not be made – for example, multiple-finned channels and honeycomb construction. Steel can be diffusion bonded at above 1,000° C (1,800° F) in a few minutes.
Ultrasonic welding: Ultrasonic joining is achieved by clamping the two pieces to be welded between an anvil and a vibrating probe or sonotrode. The vibration raises the temperature at the interface and produces the weld. The main variables are the clamping force, power input, and welding time. A weld can be made in 0.005 second on thin wires and up to 1 second with material 1.3 mm (0.05 inch) thick. Spot welds and continuous seam welds are made with good reliability. Applications include extensive use on lead bonding to integrated circuitry, transistor canning, and aluminum can bodies.
Explosive welding: Explosive welding takes place when two plates are impacted together under an explosive force at high velocity. The lower plate is laid on a firm surface, such as a heavier steel plate. The upper plate is placed carefully at an angle of approximately 5° to the lower plate with a sheet of explosive material on top. The charge is detonated from the hinge of the two plates, and a weld takes place in microseconds by very rapid plastic deformation of the material at the interface. A completed weld has the appearance of waves at the joint caused by a jetting action of metal between the plates.
The philosophy that often guides the fabrication of welded assemblies and structures is 'to assure weld quality.' However, the term ‘weld quality’ is relative. The application determines what is good or bad. Generally, any weld is of good quality if it meets appearance requirements and will continue indefinitely to do the job for which it is intended. The appropriate methods of characterisation depend on the weld's function and the particular set of properties required for the application. In some instances, the ability of a weld to function successfully can be addressed by characterising the size or shape of the weld. An example of this is where factors related to the welding procedure, such as inadequate weld size, convexity of the bead, or lack of penetration, may cause a weld to fail. In other cases, it is important to characterise metallurgical factors such as weld metal composition and microstructure. Examples might include welds for which the goal is to avoid failures due to inadequate strength, ductility, toughness, or corrosion resistance. In general, the goals of weld characterisation are to assess the ability of a weld to successfully perform its function, to thoroughly document a weld and welding procedure that have been demonstrated to be adequate, or to determine why a weld failed. In the first part of this article, characterisation of welds will be treated as a sequence of procedures, each more involved than the last and concerned with a finer scale of detail. Initially, non-destructive characterisation procedures will be the focus. The first level of characterisation involves information that may be obtained by direct visual inspection and measurement of the weld. A discussion of nondestructive evaluation follows. This encompasses techniques used to characterise the locations and structure of internal and surface defects, including radiography, ultrasonic testing, and liquid penetrant inspection. The next group of characterisation procedures discussed are destructive, requiring the removal of specimens from the weld. The first of these is macrostructural characterisation of a sectioned weld, including features such as number of passes; weld bead size, shape, and homogeneity; and the orientation of beads in a multipass weld. Macroscopic characterisation is followed by microstructural analysis, including microsegregation, grain size and structure, the phase makeup of the weld, and compositional analysis. The third component of weld characterisation is the measurement of mechanical and corrosion properties. The goal of any weld is to create a structure that can meet all the demands of its service environment. In many cases, the best way of assessing the performance of a weld is to establish its mechanical properties.
In addition to a number of standard material tests, many mechanical tests are directed specifically at determining a weld's capabilities. Examples of mechanical properties typically characterised for welds include yield and tensile strength, ductility, hardness, and impact or fracture toughness.
Corrosion testing is often employed in situations where a welding operation is performed on a corrosion-resistant material, or in a structure exposed to a hostile environment. Although absolute corrosion performance is important, a major concern is to ensure that a weld and its Heat-Affected Zone (HAZ) are cathodic to the surrounding metal.
Following the discussion of the characterisation procedures, the second part of this article will give examples of how two particular welds were characterised according to these procedures.
Nondestructive weld examination: A standard should be established based on the service requirements. Standards designed to impart weld quality may differ from job to job, but the use of appropriate weld techniques can provide assurance that the applicable standards are being met. Whatever be the standard of quality, all welds should be inspected, even if the inspection involves nothing more than the welder looking after his own work after each weld pass.
A good-looking weld surface appearance is many times considered indicative of high weld quality. However, surface appearance alone does not assure good workmanship or internal quality.
Non-Destructive Examination (NDE) methods of inspection make it possible to verify compliance to the standards on an ongoing basis by examining the surface and subsurface of the weld and surrounding base material. Five basic methods are commonly used to examine finished welds: visual, liquid penetrant, magnetic particle, ultra-sonic and radiographic (X-ray). The growing use of computerisation with some methods provides added image enhancement, and allows real-time or near real-time viewing, comparative inspections and archival capabilities.
A review of each method will help in deciding which process or combination of processes to use for a specific job and in performing the examination most effectively.
Cold welding, the joining of materials without the use of heat, can be accomplished simply by pressing them together. When two surfaces without an interposing oxide layer are brought together, the similar atoms of either side collapse into each other. Unlike the conventional welding process, there is no application of severe heat or melting of the material at the interface. Both the surfaces continue to remain in solid phase throughout this forced adhesion process. The necessary force for the adhesion is applied through mechanical rolls and dies. Cold welding is also known as contact welding. Cold welding was initially discovered by modern societies in the early 1940s and thought of as a new phenomenon, but this process has actually been in existence for thousands of years. It was learned that two pieces of similar metals will bond together inside a vacuum as long as they possess clean, flattened surfaces and a strong initial force can be applied. During the process, deformities occur across 60 to 80% of the bonding surface, and this allows pure, clean metals to come in contact. Permanent bonding then takes place on the atomic level, with welds much stronger than what could be accomplished by other means. Another advantage is that there are absolutely no intermediary materials used as a type of solder, so as long as oxides are not allowed to reform across the metal’s surface, it should last for decades. Since the initial discovery period, researchers have shown that cold welding can also be accomplished without excessive force. By applying less pressure over a longer period of time, similar results can be achieved. Another method is to increase the surface temperature of the two materials being bonded for a short period of time to accelerate the molecules.
Cold welding process: Surfaces have to be well prepared, and pressure sufficient to produce 35 to 90 percent deformation at the joint is necessary, depending on the material. Lapped joints in sheets and cold-butt welding of wires constitute the major applications of this technique. Pressure can be applied by punch presses, rolling stands, or pneumatic tooling. Pressures of 1,400,000 to 2,800,000 kilopascals (200,000 to 400,000 pounds per square inch) are needed to produce a joint in aluminium; almost all other metals need higher pressures. A general flow of metal takes place between the die surfaces at room temperatures, stretching the mating surfaces of the metals. A true homogeneous weld is formed with no introduction of a bonding agent. While most ductile metals can be welded into similar or dissimilar metal joints, some of them are more readily joined together. Aluminium, copper, and ferrous metals clad with aluminium or copper flow together with relative ease. Copper or copper clad material must be electro less nickel plated to provide optimum weld ability. The weld zone is not only metallurgically homogeneous, but the metal is work hardened and stronger than the adjacent areas. Parts are joined without contamination from sparks or dusts and vapours. There can be no contamination from fluxes, solders or brazing alloys, which are traditionally used in certain industry sectors. The interior of an enclosure is as free from contamination after welding as it is before. Since no heat is required or generated during welding, the contents of the enclosure are not subjected to temperature cycling.
Cold weld enclosures are truly vacuum tight. While ordinary specifications call for seals with a Helium leak rate of 10-9 cc/sec/atm. Radiflow tests have been made on cold welded enclosures without detecting leaks at the instrument threshold of 10-11 cc/sec/atm. The weld can be made in an optimum environment for a given product, eliminating secondary pump-out operations. Seals may be made in high or low vacuum, dry nitrogen or other desired atmospheres at predetermined pressures.
The cold weld press consists of a 4 post arrangement allowing a stable and repeatable platform for cold welding. A 12 ton press has the capacity for welding most all standard device packages covering crystal and transistor outlines. Larger tonnage presses of either 20 ton or 40 ton are also available to cover multiple package welding, utilising multiple dies, and for the sealing of large diameter SCR devices (some as large as 5" in diameter).
The press system consists of a press base, mounted and sealed within the stainless steel glove box (optionally supplied mounted on a free standing bench assembly). The four posts extend through the top of the glove box and support the press crown assembly. Tonnage is developed by an air-hydraulic booster and operates on 100psi or less. This arrangement of hydraulic cylinders, oil reservoir, tools and boosters allows a low pressure ram advance for closing the welds tools and a short high pressure stroke for cold welding. Tonnage can be held indefinitely without oil heat up problems. The precision weld tools consist of upper and lower weld dies, which are permanently aligned and mounted on a precision two post die set. This method assures long life and optimum alignment. A floating adapter on the press ram seats in a tee slot atop the weld tool assembly eliminating any miss-guidance. Brass bushings on guide posts minimise lubrication contaminants within the clean atmosphere. The weld tools are designed for particular part configurations, considering material, material thickness, geometric shape, deformation characteristics, feed through requirements, finished dimension needs and reduction requirements in the weld area. Package design should consider requirements and limitations of the cold weld method in early stages of development. Each weld tool is machined from high-carbon-high-chrome tool steel and hardened, ground and polished with tolerances on some dimensions held to plus or minus .00025". The tools are tested for alignment, deformation and weld performance before shipment. Both upper and lower weld dies have a standard 2 1/4" diameter and 2.2" length. Dies have 'O' rings grooves in sides and bottoms for use with vacuum die set chambers. This die size is adequate for most all standard electronic device packages and will withstand the repeated application of 12 ton loads for many hundreds of thousands of welds.
When an evacuated device is required during the weld cycle, stainless steel vacuum chambers can be added to the weld tool. Dual interlocked safety pushbuttons are used to initiate a spring loaded upper and a stationary lower vacuum chamber surround the weld dies. The low pressure down stroke of the press ram is resisted by springs, allowing the vacuum chamber to be sealed for evacuation and backfill (nitrogen/helium) without applying pressure to the parts to be welded. This process cycle is PLC (Programmable Logic Controller) operated and allows adjustable vacuum and backfill gas time delays. The PLC is interlocked with pressure monitoring and automatic vacuum pump operation facilities. Complete evacuation and backfill are assured via ports in the tooling. The upper weld tool cavity retains a part by means of spring-ball plungers. Once the required vacuum level within the chamber is achieved the vacuum instrumentation will interface with the PLC to allow the weld follow through of the press at the correct time. The vacuum system provided is based on customers' requirements and Pyramid can provide pumping options covering rotary, turbo drag, turbo molecular and cryogenic vacuum pumps. Vacuum levels to 2 x 10-6 mbar are achievable based on pumping system and time of process. All vacuum systems are supplied with all necessary valving, vacuum instrumentation, interconnections and fittings.
The science and art of quality weld repairs: Proper selection and application of weld technologies for the repair of high-temperature components in heat-recovery steam generators and high-energy piping systems is critical to maintaining the high availability and reliability demanded of combined-cycle plants. For better decision-making, industry personnel responsible for weld repair of high-temperature components should come up to speed on (1) typical features of welds, (2) damage that may affect welds in service and the consequent need for repair, (3) the excavation design and geometry of full and partial weld repair, (4) welding processes, (5) welding and Post-Weld Heat Treatment (PWHT) requirements of the construction codes, (6) performance assessment of weld repairs, (7) principles of microstructural control within weldments – among many other things also covered in the report. Cold weld repair where preheat may be applied, but PWHT is excluded. The technique may help users mitigate the financial impact of lost production by allowing units to return to operation sooner than they would if PWHT were required. Cold-weld repair technology was pioneered in Russia in the 1960s and later advanced in the UK. The US National Board Inspection Code in 1977 allowed weld repairs without PWHT for several materials provided specific buttering techniques were used. In 1995, the NBIC accepted that certain welds were impractical to PWHT and allowed cold-weld repair provided the owner/operator could demonstrate properties equal to those for the original construction. Several controlled weld-metal deposition techniques were developed to enable cold-weld repairs. They use the thermal fields, generated by successive weld beads, to provide both the grain refinement necessary to assure success as well as some local tempering of the structure of the Heat-Affected Zone (HAZ). The so called half-bead, temper-bead, and heat-input control techniques all are based on controlled overlap of the adjacent weld bead segments and the use of several buttered layers.
Cold-weld repairs are not as simple as they might sound. Decisions on excavation configuration, filler-metal specifications, heat input at various stages of the repair, etc, are critical. Plus special training is required for welders performing the work. The ETD report is valuable for its perspective and recommendations on the process to avoid missteps. Keep in mind that toughness of the HAZ and weld strength depends significantly on use of a welding technique that minimises the coarse region from the parent metal. The first layer of weld should consist of small beads, deposited using low heat input to ensure minimum penetration into the parent metal. This can be achieved by using small electrodes, welding in the horizontal position, and adjusting the angle of the electrode to minimise penetration. Great care must be taken to avoid hydrogen cracking and lack-of-fusion defects. A 50/50 bead overlap will reduce the course-grained area, but not necessarily remove it altogether. Depositing a bigger weld bead on top of the smaller ones, such that its refined zone overlaps the coarse areas created by the original runs, is the preferred technique. Sometimes, the first beads are ground down slightly to enable the refined zones of the next beads to line up correctly. The final bead of any welding sequence should be deposited in the middle of the cap, away from the parent metal.
One of the takeaways is that cold-weld repairs of thermal fatigue cracks using Inconel-type weld metal normally are considered temporary, possibly providing up to five years of service before another crack is caused by thermal cycling. For a given repair situation, both the operating conditions and type of defect must be considered. Cold welding with nickel-based filler should be suitable for repair of manufacturing defects in castings, such as turbine casings, they said, or for repairs to previous weld repairs made at the manufacturing stage – because they generally experience low stress levels. Subject to periodic inspection, the repairs could be considered permanent. By contrast, in-service defects generally are caused by more severe loadings, so cold-weld repairs with nickel-based filler should be regarded temporary, as noted earlier.
Advantages of cold welding
As the process is performed at ambient temperature, there are no thermal effects on the parts being joined, and the process is fast, clean, very energy efficient and creates no heat affected zone to change the physical properties of either part.
Additionally, it creates a solid-state weld between either similar or dissimilar metals. Unlike some types of welding, no filler is needed. It is simple and inexpensive to operate once dies have been produced. However, it is highly specialised with respect to joint design and materials to be welded.
Limitations of cold welding
- The joint can fail in a reactive environment or a high oxygen environment.
- It is suitable for buried pipes and for components deployed in spaces, where there is no risk of oxygen contact.
- For the cold welding to be effective, the surfaces need to be brushed and cleaned effectively.
- If the outer layer of any one of the components has high oxygen content, adhesion is unlikely to occur.
- Another critical factor is the malleability of the metals used. At least one of the two metals to be joined must be malleable.
- Another setback is that since the weld takes place quickly, and is considered permanent, it is very difficult to verify the integrity of the weld, especially in thicker metals. Whether the result of poor parts fit-up, rapid cooling or a variety of possible contaminants – from the atmosphere, base material or filler metal – weld cracking carries with it significant consequences for any welding operation. Not only does this defect adversely affect the integrity of the finished weldment, but it also requires significant time and money to rectify.
- As the welds are made in the 'solid state' they are difficult to inspect; thus reliance must be placed on process control. With the exception of butt welds, or welds where the contact surfaces are sheared together, the thickness of the parts is reduced significantly at the weld.
Growing applications of cold welding
Modern uses for cold welding are numerous, but it is still definitely considered a situational process due to the aforementioned limitations. Perhaps the greatest use of cold pressure welding has been for joining of wire, foil to wire, wire to bi-metals, and sealing of heat sensitive containers such as those containing explosives (detonators for example). Rod coils are butt welded to permit continuity in post-weld drawing to smaller diameters. In the electronics industry, cold welding processes are used to seal tin plated steel crystal cans and copper packages for heat sensitive semiconductor devices. Glass packages are also sealed using an indium or tin alloy interlayer. An interesting application of the process is underground wire servicing where joins need to be made in hostile environments, such as in the presence of explosive gases.
- The technique makes it possible, however, to work in many hostile environments that were previously impossible, like welding underground pipelines that carry flammable gasses.
- Cold welding is a hermetic sealing process widely used in the crystal, transistor and high powered solid state electronic switching industries.
- The cold welding process uses a magnetic field to rapidly collapse one component onto another forming a metallurgical bond. The welding cycle is extremely short –typically less than 1 second. A typical magnetic pulse welding system includes a power supply, which contains a bank of capacitors, a high-speed switching system and a coil. The parts to be joined are inserted into the coil, the capacitor bank is charged and the high-speed switch is activated. As current is applied to the coil, a magnetic field is created, and the outer component is collapsed over the inner component. The technology is well-suited for joining dissimilar metals and cylindrical components, such as air conditioning tubing and tubular space frames. Magnetic pulse welding offers numerous benefits to assemblers, such as no heat input, fast cycle Arial, ability to join dissimilar materials, and base metal strength in most materials. The weld produced is a true solid-state bond. Magnetic pulse welding is a good alternative to brazing because it offers greater repeatability. Plum says magnetic pulse welding is being used for more and higher volume applications. For example, the technology appeals to auto part manufacturers because of its ability to join dissimilar materials, such as aluminium to steel, within very short cycle Arial.
- Cold welding concepts are used in nano and micro fabrications in high-technology areas. For example, a gold nanowire can be cold welded instantly. The technique is also used in the nuclear field. The welding of metals at the nanoscale is likely to have an important role in the bottom-up fabrication of electrical and mechanical nanodevices. Existing welding techniques use local heating, requiring precise control of the heating mechanism and introducing the possibility of damage. The welding of metals without heating (or cold welding) has been demonstrated, but only at macroscopic length scales and under large applied pressures. The high quality of the welds is attributed to the nanoscale sample dimensions, oriented-attachment mechanisms and mechanically assisted fast surface-atom diffusion. Welds are also demonstrated between gold and silver, and silver and silver, indicating that the technique may be generally applicable.