Thermit Welding (TW) Heat source utilized for fusion in the thermit welding is the exothermic reaction ( in which heat is produced ) of the thermit mixture  Thermite = mixture of Al ( aluminium )and Fe 3 O 4 ( Iron oxide ) fine powders is kept in crucible in a proportion of 1 part of Al and 3 part of Fe 3 O 4. Some fluxes ( borax & sodium carbonate ) are also added into this mixture.  The igniting mixture which is normally barium peroxide powder is placed at the top of the thermit mixture. This igniting mixture is ignited by means of heated metal rod.

Thermit Welding (TW)  8Al(s) + 3Fe 3 O 4 (s) 4Al 2 O 3 (l) + 9Fe(l) + Heat ( 3.01 MJ/ mol )  the thermit reaction takes place in a total of 1 minute as follows :  where kJ stands for “kilojoules” a unit of energy, and “mol” stands for the quantity of matter known as a “mole”  Since density of liquid Aluminum oxide produced is less than molten steel, it floats over the top of molten steel and it is removed.  When the thermit mixture reaches ti its ignition temp of c, the reaction starts, and temp reaches to C and large amount of heat is liberated which melts both iron & aluminum oxide. Due to which Al is converted into Al2O3 and Fe ( Iron ) is melted. This molten metal acts as a filler metal in between the joint gap.

Thermit Welding (TW)  Now the bottom plug of crucible is opened & molten metal is allowed to drain into the prepared mould in between the joint to be weld  After the weld cools, excess material is removed by machining, Grinding or any other cutting methods.  Though this process used for joining but has more common with Casting than welding

Figure: Thermit welding: (1) Thermit ignited; (2) crucible tapped, superheated metal flows into mold; (3) metal solidifies to produce weld joint. Thermit Welding

Thermit Welding : Applications  Joining of railroad rails  Repair of cracks in large steel castings and forgings  Weld surface is often smooth enough that no finishing is required

TW Applications On site welding of Rails

TW Applications

Brazing, Braze welding, Soldering A suitable flux( borax, boric acid, chlorides, fluorides ) is used in the joint to prevent oxidation of the joint. Fluxes are available in the form of powder, paste,or liquid Brazing, Soldering, braze welding Normal Welding Filler metal is having the melting point less than base ( parent ) Metal Filler metal is having melting point nearly equal to base metal. Base metal does not meltBase metal is melted Joint takes place due to capillary action of melted filler metal rod or due to gravity action of the melted filler rod. Joint takes place due to fusion Less distortion in joint formed due to less temperature More distortion in joint due to more temperature Filler metal is non ferrous metal alloy Filler metal may non ferrous or ferrous metal alloy

Brazing, Braze welding, Soldering Soldering is a method of joining two pieces of a metal in which a fusible alloy or metal is introduced in a liquid state between the work pieces to be joined and allowed to solidify. The filler metal is distributed between the closely fitted surfaces of the joint by capillary action. The melting point of the filler metal is below c.The filler metal ( Lead –Tin alloy ) is called the solder. Types of Soldering : Hand Soldering ( Soldering iron method ), Dip Soldering Figure : Hand Soldering ( Soldering iron method ) Electronic PCB

Brazing, Braze welding, Soldering Brazing : It is a process of joining two work pieces of metals in which a non ferrous filler metal or alloy is introduced between the pieces to be joined. The melting point of the filler metal is above 450 o C but lower than the melting temperature of base metal. The filler metal is distributed between the closely fitted surfaces if the joint by capillary action It uses a filler metal like Al, Cu, Mg ( magnesium ) Figure : (a) Brazing and (b) braze welding operations.

Brazing, Braze welding, Soldering Braze ( Bronze )welding : It is similar to brazing in that joint is obtained by means of filler metal rod whose melting temp is above C, but below the melting temperature of the base metal. Difference is that in braze welding filler metal enters the joint by gravity, since joint gap is more.( No capillary action in case of brazing ) It uses filler metal like copper, zinc

Weldability  It is the ease with which two similar or dissimilar metals are joined with a good weld quality to possess the required metallurgical properties to perform satisfactorily in intended service.  weldability depends on following factors :  Melting point  Thermal Conductivity  Thermal expansion  Surface condition  Change in microstructure

Weldability  If these metallurgical, chemical, physical & thermal characteristics of a metal are considered undesirable with respect to weld ability, they may be corrected by proper :-  shielding atmosphere  Proper fluxing material  Proper filler metal  Proper welding methods  In some cases proper heat treatment of the metal before & after deposition.