This article throws light upon the top three innovative methods of welding. The methods are: 1. Gravity Welding 2. Fire Cracker Welding 3. Welding of Ceramics.
Gravity welding invented in 1938, is an automatic welding method employing SMAW process. It utilises a simple low cost mechanism that includes an electrode holder attached to a bracket which slides down an inclined bar held at a predetermined angle to the bar as shown in Fig. 22.34. This method is used almost invariably for making fillet welds.
Once the electrode tip is positioned in the root of the joint and the arc is initiated the electrode melts and the bracket slides downwards along the bar at a rate which depends upon the angle of inclination of the bar.
The electrode tip keeps contact with the work throughout its travel as shown in Fig. 22.35, till the electrode has been reduced to a length of about 50 mm at which point either the movement of the bracket ceases and the arc is extinguished or the bracket and the electrode holder are automatically kicked up to break the arc.
A fresh electrode is clamped in the electrode holder which is repositioned to start the weld where the previous electrode had stopped. The successful operation of gravity welder not only demands that the electrode coating must continuously touch the work throughout its travel but also requires but also requires that the melting rate of the electrode must match its sliding rate.
The power source employed with gravity welder is of the constant current type normally used for manual SMAW but it is adjusted to give a duty cycle of about 90% compared with 60% duty cycle required for manual SMAW. Currents of upto 400A may be used depending upon the size and type of electrode.
The electrodes used with gravity welder are heavy coated and of E6027 and E7024 types though E7028 type is also sometimes used. The most commonly used electrodes with gravity welder are those of 5 and 6 mm diameters and of 800 mm length though the normal 450 mm length electrodes may also be used but with much less economic advantage.
The deposition rate is only marginally increased by the use of gravity welder over manual SMAW but because an operator can simultaneously operate upto 5 gravity welders it leads to increased productivity of welders, reduces welder fatigue, operator training is minimised, and there is substantial savings in welding labour cost. Table 22.7 shows the amount of metal deposited, in Kg/hr, when using manual SMAW compared with two to five gravity welders.
Gravity welding is best suited for making fillet welds in horizontal position and gives excellent results when a sufficient number of horizontal fillets are to be made in a small area since the closeness of the welds makes it possible to attend to all the gravity welding units by quickly moving from one unit to another to reload them, initiate the arc and let them operate unattended. Such a situation exists in the fabrication of ships. That is why this method is most used for welding of stiffeners to plate in shipbuilding throughout the world.
Gravity welding is also used in railway coach building and barge yards. Though the process has been very advantageously used by Japanese shipbuilders but its economic advantages have not been exploited adequately by fabricators. However, it is hoped that gravity welding will achieve, in due course of time, an important place in production welding.
Fire cracker welding, developed in 1930’s is a method of automatically making butt and fillet welds using long heavily coated electrodes of the E6024 and E 7028 types. In this process the electrode held in an electrode holder is placed horizontally in the gap of a butt joint or in the angle of a fillet joint with a copper mould of appropriate shape placed to cover full length of the electrode as shown in Fig. 22.36.
The arc is struck by shorting the bare end of the electrode to the work by using a carbon rod. The arc length depends upon the thickness of the coating. Once the arc is started, the electrode melts and deposits the material underneath it and the process proceeds to completion automatically like a fire cracker.
The electrodes used for fire cracker welding are usually 1 m long and have a diameter of 5 to 8 mm. Both a.c. and d.c. power sources can be used but a.c. is preferred with a view to avoiding arc blow.
Fire cracker welding is a simple method which can be used to increase the productivity of a welder because one operator can simultaneously make several fire cracker welds. It has, however, some difficulties associated with it including the requirement for the careful preparation of the joint edges, the need for special copper mould for every type and size of joint, the difficulty of controlling weld penetration and the need to procure extra long electrodes with heavy coatings.
Fire cracker welding is not much popular though it can be used with advantage for building bridges, tanks and railway coaches. It can be used for welding square butt welds in material with thickness range of 1 to 3 mm and fillet welds in plates with thickness of 5 mm and more. The quality of the welds made by Fire cracker welding is similar to the quality of welds made by manual SMAW process.
A variant of fire cracker welding employs coated electrodes laid in flux thus eliminating the use of copper moulds. The flux consists of silica sand or complex mixture of silicates with 8 to 10% liquid potassium silicate to act as binder to form a flux paste of sufficient porosity to allow the escape of gases during welding.
The flux layer used to cover the coated electrode is 10 – 20 mm deep. Other details of the process are similar to those of the normal fire cracker welding. The current setting is 10 – 20 % higher than used for manual SMAW. The slag formed by the melting of coating and flux is easily detachable.
Fire cracker welding can be used for making all types of fillet and butt joints in downhand welding position. Certain welds which are difficult of access or impractical to make by manual SMAW can often be made by this process.
Ceramics are inorganic nonmetallic compounds produced by the action of heat and include clay products, cements, silicate glasses and other refractory glass-like materials. Ceramics used for engineering applications are referred to as ‘engineering ceramics’ and include alumina, silicon carbide, silicon nitride, zirconia, etc.
Engineering ceramics generally exhibit higher hardness, greater dimensional stability higher elastic modulus, high corrosion resistance, lower coefficient of thermal expansion, lower density as well as higher strength at higher temperatures as shown in Fig. 22.37. Table 22.8 gives some of the physical properties of selected engineering ceramics and metals.
Shaped components of ceramics are usually produced by the process of powder technology. However, these components are often required to be joined together to produce more complex shape and many applications require the joining of ceramics to metals. Welding and allied processes are generally used to achieve that aim.