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Top 5 Solid-State Welding Processes | Metallurgy Welding Processes Explosive

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Top 5 Solid-State Welding Processes | Metallurgy

This article throws light upon the top five solid-state welding processes. These are: 1. Forge Welding 2. Friction Welding 3. Explosive Welding 4. Thermo-Compression Bonding 5. Diffusion Bonding.

Solid-State Welding Process # 1. Forge Welding:

Forge welding or smith welding is the oldest known welding process and its use has been reported from 1400 B.C. By this process the pieces to be welded are heated to above 1000°C and then placed together and given impact blows by hammering. In the more recent form of large welding the pressure is applied by rolling, drawing and squeezing to achieve the forging action.

The oxides are excluded by virtue of design of the workpieces and or by the use of appropriate temperature as well as fluxes. Fluxes commonly used for forge welding low carbon steels are sand, fluorspar and borax. They help in melting the oxides, if formed.

Proper heating of the workpieces is the major welding variable that controls the joint quality. Insufficient heating may not affect a joint while overheating results in a brittle joint of low strength. Also, the overheated pieces tend to be oxidised which shows itself by spongy appearance.

The joints most commonly employed are scarf, butt, cleft and lap types, as shown in Fig. 2.32.

Top 5 Solid-State Welding Processes | Metallurgy welding processes explosive

An excellent living example of forge welded component of the olden days is the Iron Pillar of Delhi which measures 7-6 m in length with an average diameter of 350 mm and weighs 5.4 tonnes. These days the process is mainly used for welding low carbon steel parts usually for agriculture implements in rural areas of third world countries.

Solid-State Welding Process # 2. Friction Welding:

In friction welding one piece is held stationary and the other is rotated in the chuck of a friction welding machine. As they are brought to rub against each other under pressure, they get heated due to friction. When the desired forging temperature is attained throughout the rubbing cross-section of the workpieces, the rotation is stopped suddenly and the axial pressure is increased to cause a forging action and hence welding. This method has been in use for welding of thermoplastics since 1945 but metals were first welded successfully by it in 1956.

The machine used for friction welding resembles a lathe but is sturdier than that. The essential features of the machine are that it should be able to withstand high axial pressure of the order of 50,000 N/cm2 and be able to provide a high spindle speed of upto 12,000 rpm though the usual range may rarely exceed 5000 rpm.

A less popular variant of the process is called INERTIA WELDING in which welding is achieved by the rotation of a flywheel which is detached at the desired moment and comes to a stop within the stipulated time, thus elimi­nating the braking unit. Fig. 2.33 shows the principles of continuous drive and inertia type friction welding processes.

Friction welding is a high speed process suited to production welding. However, initial trials are required to standardise the process parameters for a given job. Friction welding of two pieces rarely takes more than 100 seconds though it may be just about 20 seconds for small components.

One of the parts to be friction welded needs to be round which puts a serious limitation on the use of this process. However, it is increasing in popularity and can weld most of the metals and their dissimilar combinations such as copper and steel, aluminium and steel, aluminium and titanium, etc. Typical applications of the process include welding of drill bits to shanks, i.e. engine valve heads to stems, automobile rear-axle hub-end to axle casing.

Solid-State Welding Process # 3. Explosive Welding:

In explosive or explosion welding process the weld is achieved by making one part strike against the other at a very high but subsonic velocity. This is achieved by the use of explosives usually of the ammonium nitrate base. The process is completed in micro-seconds.

The setup, in principle, used for explosive welding is shown in Fig. 2.34. It shows the two plates to be welded placed at an inclination to each other. The included angle varies between 1° and 10°. The thicker plate called the target plate is placed on an anvil and the thinner plate called the flyer plate has a buffer plate of PVC or rubber, between it and the explosive charge, for protection against surface damage.

Top 5 Solid-State Welding Processes | Metallurgy welding processes explosive

The charge is exploded by a detonator placed at one end of the flyer plate. When the charge explodes, the flyer plate moves towards the target plate at a velocity of 150 to 550 m/sec and the pressure produced at the interface of the impacting plates by such a high velocity is of the order of 70,000 to 700,000 N/cm2.

Under such a high veloc­ity and pressure the metal flows ahead of the joining front acting like a fluid jet resulting in a bond of the interlocking type as shown in Fig. 2.35. This inter­locking is an essential aspect of an explosion weld and is the cause of its strength. The weld strength equal to the strength of the weaker of the two components (metals) can be achieved.

Explosive welding is normally an outdoor process and needs a large area to ward-off the persons coming close to the explosion site particularly when an explosive charge of high strength may have to be exploded.

Explosive welding can be used for welding dissimilar metal combina­tions like copper and steel, aluminium and mild steel, aluminium and Inconel (76% Ni + 15% Cr + Fe), aluminium and stainless steel, etc. It can also be used for welding tantalum, titanium, and nickel components.

Typical applications of explosive welding include cladding of thick plates by thin sheets, even foils. Tube to tub-sheet joints in heat exchangers, valve to pipe joint, as well as blocking of leaking tubes in boilers can be successfully achieved by this process.

Solid-State Welding Process # 4. Thermo-Compression Bonding:

It is a pressure welding process which is employed at a temperature above 200°C. The process deals with mainly small components in the electri­cal and electronic industries for welding fine wires of about 0.025 mm diameter to metal films on glass or ceramic.

There are many versions of the process, three out of which are shown in Fig. 2.41 and are referred to as chisel or wedge bond, ball bond, and parallel gap bond. In the chisel or wedge bond a wire is deformed, under pressure, and welded to the film with the help of wedge shaped indentor. In the ball bond a wire is heated by a micro-hydrogen flame to form a ball at the wire tip as shown in Figure (b), which is subsequently welded to the heated film on substrate by the pressure exerted through the pierced indentor.

Top 5 Solid-State Welding Processes | Metallurgy welding processes explosive

In the parallel gap bond the wire or strip is pressed to the film with the help of twin electrode made of high resistance material like tungsten. The flow of current through the wire or strip heats it up locally thus keeping the heat confined to the small zone around it.

For all these variants of the process local inert atmosphere is created around the joint being bonded. Ultrasonic vibrations replace heating in some of the applications of all these modes of the process.

Commercial applications of the process include welding of noble met­als, aluminium, and copper to substrates of glass, or ceramic.

Solid-State Welding Process # 5. Diffusion Bonding:

In diffusion bonding or diffusion welding a weld is achieved by the application of pressure, of the order of 5 to 75 N/mm2, while the pieces are held at a high temperature, normally about 70% of the melting point in degrees absolute i.e. about 1000°C for steel. The process is based on solid-phase diffu­sion which, obviously, is accelerated with rise in temperature.

Diffusion in metals takes place due to vacant lattice sites or along grain boundaries, and is expressed by the following mathematical relationship:

D= D0 e-ERT

where,

D = rate of diffusion.

Top 5 Solid-State Welding Processes | Metallurgy welding processes explosive

D0 = Constant having the same dimension as D,

E = activation energy,

R = gas constant,

T = Absolute temperature at which the workpieces are held.

Depending upon the extent of diffusion required the process may be completed in 2 to 3 minutes or may take many minutes or even hours. The quality of surfaces to be welded plays an important role. A good quality surface turned, milled or ground to a standard of 0-4 to 0-2 ┬Ám* CLA (centre­line average) is usually adequate. The surface must be degreased before weld­ing by using acetone or petroleum ether swab.

Presence of oxide layers on the surfaces being joined hinder diffusion but get dissipated over a period of time. Thus, metals which dissolve their own oxides such as iron and titanium, bond easily. On the contrary metals that form tough refractory oxide layers, like aluminium, are difficult to diffusion weld.

Diffusion bonding can be achieved by three methods viz.:

1. Gas pressure bonding,

2. Vacuum fusion bonding, and

Top 5 Solid-State Welding Processes | Metallurgy welding processes explosive

3. Eutectic fusion bonding.

In gas pressure bonding, the parts are held together in an inert atmos­phere and heated to a temperature of 800°C by a system resembling an autoclave. During heating the high pressure provides uniform pressure over all the surfaces. This method is used for bonding non-ferrous metals only be­cause it necessitates high temperatures for steels.

In vacuum fusion bonding the parts are held in an intimate contact in a vacuum chamber. The pressure on the parts is applied by mechanical means or a hydraulic pump, and heating is done in the same way as in gas pressure welding. Fig. 2.42 shows a schematic diagram for vacuum fusion bonding.

A vacuum pumping system which can quickly reduce pressure to about 10-3 torr (mm of mercury) needs to be used. High pressure created by the use of me­chanical or hydraulic means makes it possible to diffusion bond steels by this method. Successful joining of steel can be achieved at a temperature of about 1150°C under an applied pressure of nearly 70 N/mm2.

In eutectic fusion bonding a thin piece of a particular material is placed between the surfaces to be welded. This results in the formation of a eutectic compound by diffusion at an elevated temperature and the piece may com­pletely disappear and form eutectic alloy(s) at the interface. The material used for being placed in between the two parts is usually a dissimilar metal in foil form with a thickness of 0-005 to 0-025 mm.

Diffusion bonding can be used to join dissimilar metals e.g., steel can be welded to aluminium, tungsten, titanium, molybdenum, cermet’s (compounds of ceramics and metals), copper to titanium, titanium to platinum, etc. It finds use in radio engineering, electronics, instrument making, missile, aircraft, nuclear, and aerospace industries.

Typical applications of diffusion bonding include tipping of heavy cut­ting tools with carbide tips or hard alloys, joining of vacuum tube components, fabrication of high temperature heaters from molybdenum disilicide for resis­tor furnace that can operate in an oxidising atmosphere upto 1650°C. In aero­space industry it is used for fabricating complex shaped components of tita­nium from simple structural shapes. It is also used for surfacing components to resist wear, heat or corrosion.

Top 5 Solid-State Welding Processes | Metallurgy

Top 5 Solid-State Welding Processes | Metallurgy welding processes explosive

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