For given spacing, current and electrode material, the voltage drop across an arc is schematically shown in following figure
The region of arc column is represented by
The voltage length characteristic of a direct current arc is given by
V= (30 + 60l) volts,
where l is the length of the arc in cm. The power source characteristic is approximated by a straight line with an open circuit voltage = 80 V and a short circuit current = 1000 amp. What is the optimum arc length?
Consider the following statements pertaining to arc welding:
1. AC arc welding electrodes are coated with sodium silicate binders.
2. DC arc welding electrodes are coated with potassium silicate binders.
3. Potassium has a lower ionization potential as compared with sodium.
4. Reignition requires a voltage higher than the normal arc voltage.
Which of the above statements are valid?
1. AC arc welding electrodes are coated with potassium silicate binders.
2. DC arc welding electrodes are coated with sodium silicate binders.
3. An AC arc must reignite itself after every crossing of the zero current instant. Reignition requires a voltage higher than the normal arc voltage.
4. The process of reignition of an arc is facilitated by the presence of ions having a low ionization potential.
Adiabatic temperature in thermit welding is of the order of
Thermit welding chemical reaction:
8 Al + 3 Fe3O4 = 2Fe + 4 Al2O3 + ΔH The adiabatic temperature is calculated to be of the order of 3000°C.
The soldering process is carried out in the temperature range
The process of joining metal sheet by means of a fusible alloy or metal applied in the molten state is called
Soldering is the joining process in which the metal items are joined together by flowing' a molten filler metal to into the joint. The filler metal used in the process is called solder and its melting point is less than 425°C. There is no melting of base metal and the joint is essentially an alloying between the solder and the base metal.
The main constituents of soldering alloy are
The solder used for the soldering operation may be
1. a soft solder which is an alloy of lead and tin in varying proportions.
2. a hard solder which is copper-zinc alloy, copper-silver alloy or a nickel-silver alloy.
An alloy of copper, zinc and silver often used in fabrication work is called
Flux is used in soldering to
(b) The action of flux in soldering joints is
(i) to break and remove the oxide layer formed along the joined faces.
(ii) to aid the capillarity of the molten metal and facilitate its flow around and through the joint.
(iii) to act as cleansing agent for the removal of dirt etc.
The process of joining metallic pieces by introducing non-ferrous alloys in the liquid state between the metallic piece and allowing to solidify is known as