05 Welding processes 7 misc processes

Welding processes7 misc processe

1 Welding Under water

2 Cold Welding / Pressure Welding

3 Flash Welding

4 Upset Welding

5 Percussion Welding

6 ElectroSlag & ElectroGas

7 Thermite Welding

8 Explosion Welding

9 Stud Welding

1 Under Water Welding (A variant of SMAW)

Underwater welding began during World War I when the British naval force used it to make temporary repairs of leaking rivets on ship hulls The introduction of covered electrodes enabled succesful underwater welding and the production of welds having approximately 80% of the strength and 40% of the ductility of similar welds made in air

Because of the somewhat diminished weld properties, this SMAW application is generally restricted to salvage operations or underwater repair work

Two major categories:

Welding in Wet Environment

Another disadvantage is the restricted visibility, which is due to the equipment and the existing local contaminants in the water, as well as those generated by the welding arc Under the most ideal conditions, welds produced in wet environments using covered electrodes are marginal, at best They can be placed in service for short periods of time under reduced operating conditions, but should be replaced with quality welds as quickly as possible The covered electrodes used for wet welding must be waterproofed prior to underwater use This can be done by wrapping them with waterproof tape or by dipping them in special sodium silicate mixes and allowing them to dry

1 Under Water Welding (A variant of SMAW)

The dry environment enables the production of high-quality welds that meet all code quality requirements The SMAW process is not very popular for welding in the dry environment, because large amounts of smoke and fumes are produced An extensive air moving, filtering, and refrigeration system must be employed when the SMAW process is used, because a dry-environment area will quickly fill with the welding fumes, making it impossible for the welder to see that weld area and to function For this reason, the gas-tungsten arc welding and gas-metal arc welding processes have broader use in dry welding applications

2 Cold Welding (or Pressure Welding)

Cold welding, the joining of materials without the use of heat, can be accomplished simply by pressing them together Surfaces have to be well prepared, and pressure sufficient to produce 35 to 90 percent deformation at the joint is necessary, depending on the material Lapped joints in sheets and cold-butt welding of wires constitute the major applications of this technique

2 Cold Welding (or Pressure Welding)

1. Involves destruction of surface oxide layers (common deformation of material in weld zone)

2. Clean metal surfaces when brought into contact, generate inter-atomic forces which form the weld

3. Thermally induced processes such as re-crystallization or diffusion are not required for welding

4. This makes cold-formable materials with a brittle cover layer especially suitable for cold pressure welding

FLASH, UPSET AND PERCUSSION WELDING

FLASH, UPSET, AND percussion welding constitute a family of welding processes used to join parts of similar cross section by making a weld simultaneously across the entire joint area, without adding filler metal

Upset force is applied at some point before, during, or after the heating cycle to bring the parts into intimate contact The method of heating and time of force application distinguish these three welding processes

Percussion welding may also be used to join the tip or end of a small part to a flat surface

3 Flash Welding (Flash butt welding)

3 Flash Welding (Flash butt welding)

FLASH WELDING (FW) is a resistance welding process in which a butt joint weld is produced by a flashing action and by the application of pressure In basic terms, it is a melting and a forging process The process is capable of producing welded joints with strengths equal to those of the parent materials Figure 1 is a schematic representation of a typical flash welding operation

Flash welding is used to join metallic parts that have similar cross sections, in terms of size and shape The process lends itself to joining nearly all grades of steel, aluminum, brass, and copper parts, in addition to selected dissimilar materials

CHAIN LINKS

· TRANSMISSION BANDS

· AUTOMOTIVE FLYWHEEL RING GEARS

· STRIPS THAT ARE JOINED FOR CONTINUOUS PROCESSING LINES

· WIRE AND BAR DRAWING OPERATIONS FOR CONTINUOUS STAMPING PRESS FEED LINES

· ROLL FORM LINES

· AIRCRAFT LANDING GEAR

· TUBE AND ROD EYE CLEVISES

· BAND-SAW BLADES

· DRILL EXTENSIONS USING TWO DIFFERENT MATERIALS

4 Upset Welding

DEFINITION

UPSET WELDING a resistance welding process that produces coalescence over the entire area of faying surfaces, or progressively along a butt joint, by the heat

obtained from the resistance to the flow of welding current through the area where those surfaces are in contact Pressure is used to complete the weld.

PRINCIPLES OF OPERATION

WITH THIS PROCESS, welding is essentially done in the solid state The metal at the joint is resistance heated to a temperature where recrystallization can rapidly take place across the faying surfaces A force is applied to the joint to bring the faying surfaces into intimate contact and then upset the metal Upset hastens recrystallization

at the interface and, at the same time, some metal is forced outward from this location This tends to purge the joint of oxidized metal

5 Percussion Welding

PERCUSSION WELDING (PEW)

A welding process that produces coalescence at the faying surface using the heat from an arc produced by a rapid discharge of electrical energy Pressure is applied percussively during or immediately following the electrical discharge

Capacitor Discharge Percussion Welding

WITH THE CAPACITOR discharge method, power is furnished by a capacitor storage bank The arc is initiated by the voltage across the terminals of the capacitor

bank (charging voltage) or a superimposed high voltage pulse Motion may be imparted to the movable part by mechanical

or pneumatic means.

Magnetic Force Percussion Welding

FOR MAGNETIC FORCE welding, power is supplied by a welding transformer The arc is initiated by vaporizing a small projection on one part with high current from the transformer The vaporized metal provides an arc path.

The percussive force is applied to the joint by an electromagnet that is synchronized with the welding current Magnetic force percussion welds arc made in less than one half cycle of 60 Hz Consequently, the timing between the initiation of the arc and the application of magnetic force is critical.

6 Electroslag and Electrogas Welding

ELECTROSLAG WELDING AND ELECTROGAS WELDING are two related procedures that are presently used to weld thick-section materials in the vertical or near- vertical position between retaining shoes

6 Electroslag and Electrogas Welding

Primarily applied for joining steels of thicknesses over 50 mm (2 in.), electroslag welding (ESW) involves high energy input relative to other welding processes, resulting in generally inferior mechanical properties, specifically lower toughness of the heat-affected zone (HAZ) However, the high deposition rate and relatively low cost of the process make it attractive for heavy structural fabrication

The as-welded properties of electrogas welding (EGW), usually applied to steels under 50 mm (2 in.), are generally superior to those of electroslag welds, and the process is commonly applied to the field erection of storage vessels and other less critical structures

Electrogas welding is a method of gas-metal arc welding (if a solid wire is used) or flux-cored arc welding (if a tubular wire is used), wherein an

external gas is supplied to shield the arc and molding shoes are used to confine the molten weld metal for vertical position welding

6 Electroslag and Electrogas Welding

Electroslag welding is a vertical welding process in which a slag melts the filler metal and the surface of the work to be welded

Electric arc occurs only at the beginning of the process, and once a molten bath is achieved, the arc is extinguished

Confined by cooling shoes, the molten weld pool is shielded by the molten slag, which moves along the full cross section of the joint as welding progresses

The conductive slag is maintained in a molten condition by its resistance to electric current passing between the electrode and the work ESW can be considered a progressive melting and casting process in which the heat of a bath of molten flux is used to melt the filler metal and the edges of the plates to be welded

During the process, flux is added periodically or continuously to maintain an adequate slag covering over the pool of molten metal Two or more retaining shoes hold the molten metal in place until it has solidified

7 Thermite Welding

Thermite welding is an exothermic welding process that uses thermite to melt metal, which is poured between two workpieces to form a welded

joint It was developed by Hans Goldschmidt around 1895

Commonly the reacting composition is 5 parts iron oxide red (rust) powder and 3 parts aluminium powder by weight, ignited at high temperatures

A strongly exothermic (heat-generating) reaction occurs that produces through reduction and oxidation a white hot mass of molten iron and a slag

of refractory aluminium oxide The molten iron is the actual welding material; the aluminium oxide is much less dense than the liquid iron and so floats to the top of the reaction, so the set-up for welding must take into account that the actual welding material is on the bottom and covered by floating slag

7 Thermite Welding

The aluminothermic reaction that occurs in thermite welding follows the general formula:

Metal oxide + Aluminium (reducing agent) - Aluminium oxide + metal +heat

whereas the typical reaction that occurs in the thermite welding of rails is:

FE2O3 + 2AL -2FE + AL2O3 (HEAT ®850 KJ)

and the typical reaction that occurs in the welding of copper conductors to steel rails is:

3CU2O + 2AL - 6CU + AL2O3 (HEAT ®1060 KJ)

8 Explosion Welding

8 Explosion Welding

Pressure is applied by detonating a layer of explosive that has been placed over

one of the components being joined, called the flyer plate.

The contact pressures are extremely high; the kinetic energy of the plate striking the mating component causes a wavy interface

The impact mechanically interlocks the two surfaces, so that pressure welding by

plastic deformation also takes place

(b) The flyer plate is placed at an angle Any oxide films present at the interface are broken up and propelled out of the interface The resultant bond strength is very high The explosive may be a flexible plastic sheet or cord or in granulated or liquid form, which is cast or pressed onto the flyer plate Process is particularly suitable for cladding a plate or slab with a dis-similar metal

Note: Significant safety issues

9 Stud Welding

9 Stud Welding

Similar to Flash Welding

The stud (a small part; a threaded rod, hangar, handle) serves as one of the electrodes while being joined to another component

The other component is usually a flat plate

Polarity for aluminum is usually DCEP

Polarity for steel is DCEN

Applications in auto bodies, electric panels, shipbuilding