There are several types of welding that fall into the category of solid state processes and, although some of them have only been developed recently, others date among the oldest known welding methods.
Solid state welding processes include:
1. Cold Welding (CW):
Cold welding uses the application of pressure at room temperature to create a join. The pressure creates a join where the materials contact, although this creates substantial deformation at the weld. Thin materials can be joined using hand tools, but thicker materials require greater pressures and require the use of a press. The materials being joined should be clean at the interface and some metals will have an oxide film covering that breaks up during the welding process to reveal clean metal surfaces that allow for defect-free bonds. The parts may also have indentations on the weld surfaces to aid joining. Cold welding is easily automated and can be used for a range of ductile metals. It is often used for aluminium, copper and nickel alloys as well as low carbon steels.
2. Diffusion Welding (DFW):
Diffusion welding uses temperature and pressure to join the part instead of relative motion or microscopic deformation. While filler materials can be used with DFW they are not always necessary. The metal surfaces to be joined should be clean before they are heated to below melting temperature. A bind is created by the diffusion of the interface atoms on both weld pieces. DFW, also called simply Solid State Welding (SSW), doesn’t create any appreciable deformation in the work pieces and can be used to join refractory metals without impacting their metallurgical properties. The pieces are typically welded in atmosphere or vacuum surfaces to prevent oxides or other airborne contaminants coming into contact with the weld surfaces. The heat used for this process is generated through induction, resistance or a furnace. DFW is frequently used to join dissimilar metals, although it becomes diffusion brazing with the addition of a later of filler material between the faying surfaces of the weld pieces. Used in industries including aerospace, electronics, nuclear and manufacturing, DFW can weld dissimilar metals that are difficult to join via other methods, including stainless steel to titanium, steel to tungsten, steel to niobium, and gold to copper alloys. DFW provides high quality welds with no inclusions, chemical segregation, distortion or pores. It can be used to join thick workpieces but has a relatively low rate of productivity since it is a time consuming process. DFW also requires thorough surface preparation and a precise fit between the parts to be joined. In addition, initial equipment costs can be high.
3. Explosion Welding (EXW):
Also known as explosive welding, this technique originates from the First World War and the decades after the Second World War where it was observed that pieces of shrapnel were welding themselves to armour plating rather than being embedded. It is a solid state process whereby parts are brought together at a high velocity using a controlled detonation. Heat is produced as a result of the shock-wave associated with the impact and the energy expended by the collision, rather than the explosion itself, which makes the metals molten at the interface during welding. The plastic deformation associated with jetting and ripple formation at the interface between the workpieces also releases heat. Surface jetting makes the plastic interaction between the metal surfaces particularly pronounced, while the plastic flow of the metal helps create a higher quality weld. EXW can be used to join most metals, including dissimilar metals that cannot be welded using arc methods. Self-contained, portable and capable of quickly welding large areas, EXW doesn’t disturb mechanical or thermal treatments or the effects of cold work. This process creates welds with strengths equal to that of the weaker of the two metals being joined. The process typically works by resting the base plate on an anvil and placing the flyer plate above along with an explosive charge. The detonation begins at the edge of the plate and propagates along the plate at a high velocity. The maximum velocity for the detonation is 120% of the materials’ sonic velocity. Contaminants such as oxides and nitrides are expelled by the jet ahead of the bonding front. Explosion welding is used to manufacture clad pipes and tubes, aerospace and ship structures, bi-metal sliding bearings, pressure vessels, heat exchangers, weld transitions, and corrosion-resistant chemical process tanks. It is often used to clad base metals with thinner alloys. Explosion welding can join steel to copper, nickel, aluminium, tungsten, or titanium as well as joining aluminium to copper. Advantages include being able to weld large surfaces as well as creating high quality bonds with high strength, no distortion, porosity or change in the metal microstructure. It is a low cost and simple process that doesn’t require surface preparation. However, there are some disadvantages such as only being able to join simple shaped parts such as cylinders and plates and can only join flyer plates of less than 2.5” (63mm). Brittle materials with low ductility and impact toughness cannot be used, while there are evident safety and security issues around the storage and use of explosives.
4. Forge Welding (FOW):
Once called hammer welding, this is one of the oldest known welding processes. The technique works by heating metals in a forge before applying pressure (or hammer blows) to cause a permanent deformation at the weld interface. Blacksmiths traditionally used this method, heating the metal parts to be joined to a heat below the molten temperature and then applying a flux to the interface. At this point the blacksmith would use a hammer and anvil to create enough pressure at the faying surfaces to cause coalescence. Forge welding is used by blacksmiths and for the manufacture of metal art pieces and welded tubes. FOW can produce good quality welds, even in intricate shapes and doesn’t always require filler material. However, this process can only be used for low carbon steel and requires a high level of operator skill. It is a slow welding process and the weld can become contaminated by the coke used in the heating furnace.
5. Friction Welding (FRW):
Friction welding joins materials through the heat that is produced by rubbing material surfaces together under pressure. There are different techniques to achieve this, including rotating one workpiece against another to create frictional heat. Once a suitable temperature has been reached, the movement ceases and extra pressure is applied to allow the parts to join. Friction welding can produce high quality welds quickly without the use of filler metal or flux. It can be used to weld common metals, including dissimilar metals. Friction welding is a fast process that is capable of joining a range of metals including alloy steels, carbon steels, tool and die steels, stainless steels, aluminium and copper alloys, as well as magnesium, nickel and titanium alloys. Friction welding does, however, require relatively expensive equipment.
6. Hot Pressure Welding (HPW):
This technique uses a combination of heat and pressure to form a weld through the macro-deformation of the base metal. The surface deformation cracks the surface of the oxide film, increasing the area of clean metal. Hot pressure welding is usually undertaken in a vacuum or with a shielding medium. It is mainly used by the aerospace industry. Another variation of HPW is hot isostatic pressure welding, where pressure is applied through a hot inert gas in a pressure vessel.
7. Roll Welding (ROW):
This method uses heating and the application of pressure to cause deformation at the faying surfaces of metals. ROW is similar to forge welding, except where forge welding uses hammer blows to apply pressure ROW uses rolls to create the pressure for the join. This process is used for cladding mild or low alloy steels with high alloy materials like stainless steel. It is also used to make bimetallic materials for the instrument industry.
8. Ultrasonic Welding (USW):
Ultrasonic welding uses the local application of high-frequency vibratory energy to join parts that are held together under pressure. An ultrasonic tip or electrode is clamped against the workpieces, where it oscillates parallel to the weld interface. The combination of these oscillating forces and the pressure creates dynamic stresses in the base metal, producing minute deformations that create a rise in temperature at the weld zone of the metal. When combined with the pressure, this temperature rise produces the weld. The ultrasonic energy also cleans the weld area by breaking up oxide films on the surface. As with all solid state processes, the weld temperature does not reach the material melting point. USW creates welds with a strength equal to the base metal and is capable of joining most ductile metals as well as some combinations of dissimilar metals. However, USW is restricted to relatively thin metals. It is widely used in the aerospace, automotive, electronics and instrument industries as well as for producing and sealing packages and containers. It is a fast welding technique that creates low levels of deformation in the workpieces, producing high quality welds with only a moderate level of operator skill. The process can be easily integrated into automated production lines, although it can only be used for small or thin parts and components can suffer fatigue due to the ultrasonic vibrations.
Solid state welding processes have a range of applications, often depending on which process is being used. The low temperatures associated with these welding techniques mean that they lend themselves to both similar and dissimilar material combinations. They tend to deliver high joint quality when compared to many fusion processes and have found applications in industries ranging from aerospace to power.
Through a combination of heat and pressure, solid state welding processes can deliver strong bonds, even with dissimilar metals. These processes do not cause the metals being joined to reach their melting point, which means that the properties of the metals are kept intact. While some of these processes are slow to complete others can create a join quickly. The speed of the process is typically determined by the heat that is used, with cooler techniques taking longer than those that use higher heats.