Answer MEP Question 27

Q. Write short note on the followings – (A). Metal–locking; (B). TIG and MIG welding; (C). Brazing; (D). Soldering.

A. Metal Locking or metal stitching is a precise mechanical method of making permanent repairs to broken casting and mechanical parts.
There is no heat involved in the method, thus post-repair there is no need to correct any misalignment and warpage, also the repair can be carried out in the restricted area where it is hazardous to carry out hot work. This method of repair does not require any major disassembly.
a. A Jig to drill the holes across the fracture equidistantly,
b. Dumbbell-shaped Nickel alloy metal-lock keys. Which is a ductile alloy. They are available in different sizes.
c. Studs whose heads can be sheared off.
d. A Mechanized chisel to join the drill holes, size is such that to match the contour of the key.
e. Drill machine and drill bits of required size to match the keys and studs.
f. Hammer to tap the keys.
g. A chisel for the removal of remnant rough metal.
h. Grinder to ground the burrs.

B. Cast Iron components when cracked are not advised to repair by welding whereas metal locking is a suitable method to carry out such repair. Also, steel, bronze, or aluminum structures or components are also repaired by metal locking.

C. Procedure:- A skilled technician or engineer is required to carry out this repair. Shore-based workshops approved by classification societies are generally available for such repairs.

Following are the steps to carry out metal locking:-
a. With the help of a Jig & Drill machine, series of holes are drilled across the line of fracture, to the depth of the workpiece.

b. Drill holes are then joined in common by using a mechanized chisel.

c. Keys are then driven into the aperture and peened into a metal-to-metal condition, causing the key to virtually become integral with the parent metal.

d. Holes are then drilled along the line of the fracture, then screwed in with the studs.

e. Stud as screwed in should bite into the predecessor, forming a pressure-tight joint. This makes the casting a more rigid body. The heads of the studs are sheared off.

f. The remnant rough metal is removed by chisel. Protruding burrs are grounded by a grinder and a coat of paint is applied as required.
 

Welding, Brazing, and Soldering:

TIG welding:-  Tungsten Inert gas welding Also know as Gas tungsten arc welding is a manual weld process that uses a non-combustible electrode made up of tungsten, an inert or semi inert gas mixture, and separate filler material. especially useful for welding thin materials, this method is characterized by a stable arc and high-quality welds. however, it is required significant operator skills and can only be accomplished at relatively low speeds. it can be used on nearly all weldable metals, though it is most often applied to stainless steel and light metals. it is often used when quality welds are extremely important.

MIG welding:- Metal Inert Gas welding, Also known as Gas metal arc welding is a semi-automatic or automatic welding process that uses a continuous wire feed as an electrode and an inert or semi inert shielding gas to protect the weld from contamination using an inert gas as a shield. A constant voltage DC power source is most commonly used, but a constant, as well as an alternating current, can also be used. Welding speed is relatively high due to automatically fed continuous electrodes but is less versatile because it requires more equipment than the simple shield metal arc welding (SMAW) process.  

Brazing:- Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the adjoining metal.

Brazing differs from welding in that it does not involve melting the workpieces. The filler metal flows into the gap between close-fitting parts by capillary action. The filler metal is brought slightly above its melting temperature while protected by a suitable atmosphere, usually a flux. It then flows over the base metal and is then cooled to join the workpieces together. A major advantage of brazing is the ability to join the same or different metals with considerable strength.
High-quality brazed joints require that parts be closely fitted, and the base metals exceptionally clean and free of oxides. Joint clearances of 0.03 to 0.08 mm are recommended for the best capillary action and joint strength. Cleanliness of the brazing surfaces is also important, as any contamination can cause poor wetting (flow). Cleaning prior to brazing may be done by chemical cleaning and abrasive or by mechanical cleaning. In the case of mechanical cleaning, it is important to maintain the proper surface roughness as wetting on a rough surface occurs much more readily than on a smooth surface of the same geometry.
Unless brazing operations are contained within an inert or reducing atmosphere environment (i.e. a vacuum furnace), a flux such as borax is required to prevent oxides from forming while the metal is heated. The flux also serves the purpose of cleaning any contamination left on the brazing surfaces. Flux can be applied in any number of forms including flux paste, liquid, powder or pre-made brazing pastes that combine flux with filler metal powder. Flux can also be applied using brazing rods with a coating of flux, or a flux core.
A variety of alloys are used as filler metals for brazing depending on the intended use or application method. In general, braze alloys are made up of 3 or more metals to form an alloy with the desired properties. The filler metal for a particular application is chosen based on its ability to: wet the base metals, withstand the service conditions required, and melt at a lower temperature than the base metals or at a very specific temperature.
Some of the more common types of filler metals used are Aluminium-silicon, Copper, Copper-silver, Copper-zinc (brass), Copper-tin (bronze), Gold-silver, Nickel alloy,  Silver, etc.

Soldering:- soldering is a technique where different components are placed in a PCB design. Soldering pencil, solder, flux, and soldering rod stand are the devices used for the process. The joints are made at very low temperatures.
De-soldering is a process where it may be necessary to disconnect or remove components or wires from the soldered board or wired circuit.
Before soldering a component on the board, the component is first tinned. To solder, a copper wire on the board, remove the sleeve from the wire, and clean the copper wire. Dip the copper wire in the flux and heat it with a solder machine along with the solder. This process is called tinning. Heat the tinned wire and solder till the solder flows freely over the tinned wire.
Soldering pencil (iron), has a polished tip that is used for heating and is designed with an insulated handle. Solder is a soft metal alloy that melts when contacted with the soldering iron, melting point can be as low as 90 deg cel and up to 450 deg cel. The available types of solders are lead, lead-free, hard, glass, and flux-core. The type of flux can be acid, rosin, or water-soluble.

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