Solid State Welding Process
A solid-state welding process produces coalescence at temperatures below the melting point of the base material being joined and without the addition of a filler metal but with the application of pressure. For satisfactory welding, at least one of the metals being joined must be highly ductile and should not exhibit extreme work-hardening.
Following are the types of solid-state welding processes:
(i) Cold welding
(ii) Diffusion welding
(iii) Ultrasonic welding
(iv) Explosive welding
(v) Friction welding and Inertia welding
(vi) Forge welding
Ordinary bright metallic surfaces consist of hills and valleys not seen by naked eye. A layer of metal oxide (20 to 200 molecule thick) exists on the metal surface carrying on its top at moisture-absorbed oxide layer. In cold-working, the surfaces are cleaned for degreasing and a part of oxide removed by wire brush. When two such partly cleaned surfaces are pressed together, the remaining thin oxide film from high spot fragments and metal behind that suffers plastic deformation under pressure. Metal-to-metal contact occurs. Pressure is applied over a narrow strip. The pressure applied is so much that the original thickness is reduced to nearly one-fourth. The ductility of the metals produces a true fusion condition.
Two metal sheets are brought into overlapping contact and a punch is pressed into them. The interface between the sheets is thereby subjected to a transverse tensile strain while it is under a high compressive stress. The tensile strain causes fragmentation of the oxide film, permitting metallic contact to bond the two sheets.
Cold welding is used to weld aluminium, copper and its alloys, aluminium to copper, nickel to iron, etc. The process finds extensive application in cladding and joining many similar or dissimilar metals. Cold welding is also used for joining metals in explosive areas.
In this, coalescence of the meeting surfaces is produced by the application of pressure and elevated temperatures to carefully cleaned and mated metal surfaces so that they actually grow together by atomic diffusion. Similar or dissimilar metals can be joined without the use of filler metal.
Diffusion welding involves several stages. To achieve intimate metal-to-metal. contact between the pieces of metal, pressure (350 to 700 kg/cm²) is applied that deforms high peaks of the metal surface breaking the surface oxide layers. For achieving the diffusion and grain growth to complete the weld, temperatures up to 1100°C may be used, After the metal-to-metal contact is established, the atoms are within the attractive force field of each other and hence a high strength joint is established.
Different stages of diffusion welding process: (a) Surfaces I and II to be welded shown with peaks and valleys (asperities) in a magnified view, (b) Deformed peaks and valleys under pressure (c) Metal-to-metal contact at places where oxide film disrupted. (d) High strength joint developed as atoms are within attractive force field of each other after metal-to-metal contact. (e) Development of a stable metallic bonding between two surfaces being welded
Diffusion welding is done by gas-pressure bonding and vacuum fusion bonding. In gas-pressure bonding, parts in intimate contact are heated to about 815°C and an inert gas atmosphere is used around the weld. This method is used for welding non-ferrous metals. Vacuum fusion welding is used for ferrous metals. Heating may be done up to 1150°C and pressure applied may be up to 700 kg/cm² and process carried out in a vacuum chamber.
Diffusion welding finds application in the fabrication of reactor components in atomic energy industry, rocket engines, helicopter rotor hub, missiles, bombers and space shuttle. It is also used for the fabrication of composite material, i.e. dissimilar metals. Diffusion welding is used for joining titanium alloys, zirconium alloys and nickel-base alloys.
Ultrasonic welding is a solid state welding process wherein coalescence is produced by the local application of high frequency vibratory energy to the workpieces as they are held together under pressure. The workpieces are clamped together under modest static force normal to their interface and oscillating shear stresses of ultrasonic frequencies (10 kHz to 75 kHz) are applied approximately parallel to the plane of interface for about one second. The combined effect of pressure and vibration causes movement of the metal molecules, and brings about a sound union between the faces of materials in contact.
Pieces to be welded are clamped between the welding (sonotrode) tip and an anvil. Both sonotrode tip and anvil are faced with high speed steel, since considerable wear can occur at the contacting faces. A frequency converter converts 50 cycles line power into high frequency electrical power and a transducer changes the high frequency electrical power into ultrasonic vibratory energy which is transmitted to the welding joint through the welding sonotrode tip attached to the transducer. The tip oscillates in the plane of the joint interface. To start with, some triggering mechanism lowers the welding head, applies necessary clamping force and starts the flow of ultrasonic energy.
Ultrasonic vibrations, combined with the static clamping force, induce dynamic shear stresses in the workpieces, then local plastic deformation of joint materials occurs at the interface. Oxide coatings and other surface films are shattered and dispersed so that intimate contact and bonding of the workpiece surfaces take place.
Ultrasonic welding is especially useful for welding plastic parts, eliminating solvents, heat or adhesive. It is used for welding of aluminium sheets (up to 2 mm thickness), electrical and electronic components, hermetical sealing of volatile substances and preparing bimetallic junctions.
Explosive welding is the technique of using explosive charges to form a metallurgical bond between two pieces of metal and wherein coalescence or fusion is effected by high velocity impact between the two mating surfaces. The essentials of two modes of explosive welding operation, (a) parallel arrangement and (b) inclined arrangement, are shown in Fig. The flyer plate (metal 1) and the parent plate (metal 2) are to be welded. The flyer plate is propelled by an explosive charge to impact on and unite with parent plate supported on anvil. The buffer above the flyer plate may be of rubber or cardboard and is for protection of flyer plate from the detonation of explosive.
Above the buffer is a layer of explosive which is detonated from the lower edge such that under the effect of tremendous pressure generated due to detonation of explosive, the flyer plate is driven down to give a high velocity impact on the parent plate. As the explosive is ignited, detonation wave-front progresses across the surface of flyer plate in a straight-forward manner. As a result of explosive impulse, extremely high normal pressure accompanied by some shear or sliding pressure takes place between the flyer plate and the parent plate. At the point of impact, say P, a high instantaneous pressure is generated (which is much more than the shear strength of the mating plates) which causes. a portion of the two mating surfaces (called jet) to become fluid and be expelled.
The severe deformation of the colliding surfaces and the resulting jet breaks up any surface film, forcing the surfaces into intimate contact. The jetting phenomenon, required for bonding, causes the collision point to become plastic and flow into the space between the two plates. By this mechanism, the necessary condition for the formation of a direct metal-to-metal bond occurs. Melted zones are formed, but at discrete intervals along the bond rather than as a continuous layer. The surface jetting contributes greatly to the strength of weld, providing a mechanical lock in addition to the metallurgical bond.
Photomicrograph of tantalloy explosively welded to a columbium alloy. Note the “Rippled” effect at the interface, providing a mechanical lock in addition to the metalurgical bond.
Advantages of explosive welding include simplicity of process, very large surfaces can be welded, thin foils can be bonded with thick plates, even heat treated components can be welded, plates of wide range of thickness can be welded and weld joint has no porosity and hence better strength of the weld joint is ensured.
Application of explosive welding is in welding and cladding of metals. A number of dissimilar metal combinations are explosive welded, for example, aluminium to steel, tungsten to steel, titanium alloys to Cr-Ni steels, etc. Metals such as zirconium, titanium, stellite, copper and nickel alloys are cladded on carbon steels and low alloy steels. The process finds use in space and nuclear application. Major application is in explosive cladding of heat exchanger tubes, pressure vessels, die-casting industry, chemical industry, ship building and cryogenic application.
Friction Welding and Inertia Welding
In friction welding, coalescence is produced by the heat obtained from mechanically induced sliding motion between the two rubbing surfaces. The temperatures developed are below the melting point of the metals being welded but high enough to create plastic flow and inter molecular welding. The workpieces are held together under pressure during welding. The operation is carried out on lathe machines [Fig. 1(a) and Fig. 2(b)].
Inertia welding is similar to friction welding with the difference that the energy supply in inertia welding is from a rotating flywheel whereas in friction welding, energy supply is from a conventional system of electric power or hydraulic power.
Friction welding variables include rotational speed, contact pressure and time of welding. The relative rotational speed varies from 1500 rpm (for carbon steels) to 3800 rpm (for aluminium). The contact pressure in heating phase is between 280 kg/cm² (for aluminium) and 525 kg/cm² (for steel) and in forging phase, the contact pressure may go up to 1120 kg/cm³. Welding time is up to 15 seconds.
Materials welded include aluminium and alloys, brass, bronze, stainless steel, nickel alloys, carbon steels, tool steels, etc. Dissimilar metals welded are alloy steel to carbon steel, super alloys to carbon steels, copper to carbon steel, copper to aluminium, etc..
Friction welding finds application in automobile industry, replaces brazing and arc welding in many situations like production of bimetallic shafts, joining of super alloys turbine wheels to steel shafts, cutting tools of high speed steel welded to carbon steel shanks, etc.
It is the oldest known welding process, although because of some difficulties involved in carrying out this process, it has restricted use for welding wrought iron and low carbon steels with job thickness up to 30 mm. Workpieces are heated to plastic stage in a furnace fired by coal, coke or oil and gas. Ends of the workpieces to be joined are prepared by upsetting to different geometrical shapes to facilitate welding; a typical swedged joint edge preparation is shown in below Fig. for a lap scarf joint. The workpieces are heated to above 1000°C to plastic stage and later placed on the anvil end-to-end and hammered down to form the joint. Flux is also used, mostly borax with sal ammoniac.
Forge welding is used for welding solid parts only. Metals forge-welded are wrought iron and low carbon steels. It is a slow process and needs furnaces etc. and skilled welder. Forge welding is used in blacksmith shop for general repair works. Also it is used in rail-road shops and for making pipes from plates rolled to cylindrical form wherein long edges are butted together in the dies at high forging temperature.
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