Element of mechanical engineering

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ELEMENTS OF MECHANICAL ENGINEERING (MES 14)

SESSION 1

CHAPTER 6

SOLDERING, BRAZING & WELDING

INTRODUCTION

Some products cannot be manufactured as a single piece. The desired shape and size of such products can be obtained by joining two parts of same or different materials. These parts are manufactured individually and are joined together to obtain the desired product. Forexample, air craft and ship bodies, welded machine frames, furniture, computers, bridges and the transmission or electric towers etc., are all fabricated by joining several different parts.

Based on the type of joint produced joining processes can be classified as
1. Temporary Joint.
2. Permanent Joint.

If a product is in use for a long time and there is wear and tear, theparts need to be dismantled for maintenance, repair or replacement. A temporary joint can be easily dismantled separating the original parts without any damage to them. In case it is a permanent joint, an attempt to separate the parts already joined will result in the damage of the parts. In a permanent joint, the joint is made such that it has properties similar to the base metal of the two parts.The joined parts become one piece. These parts cannot be separated into their original shape, size and surface finish. Based on the process used for making the joint, the joining processes can be further classified as
1. Soldering.
2. Brazing.
3. Welding.
4. Mechanical Fasteners like bolts, nuts, rivets, screws etc.
5. Adhesive bonding.

Mechanical fasteners are mostwidely used for temporary joints. Joints obtained by bolts and screws are temporary in nature and can be dismantled easily whenever necessary. Rivets are semi-permanent fastening devices and the joint can be separated only by destroying the rivet without affecting the parent elements. Adhesive bonding has generally less strength than the mechanical fasteners. But adhesive bonding is used to join oddshaped parts or thin sheets which may not lend themselves to mechanical fastening. Brazing and soldering are considered to form permanent joints, but for repair or replacement these joints can be dismantled by heating. Welding is one of the most extensively used fabrication method. The joint strength obtained in welding is being equal to or some times more than that of the parent metal. Weldingis not only used for making structures, but also for repair work such as the joining of broken castings. The choice of a particular joining process depends on several factors such as application, nature of loads or stresses, joint design, materials involved and size and shape of the components.

SOLDERING

Soldering is a method of joining similar or dissimilar metals by the applicationof heat and using a filler metal or alloy called solder, whose liquidus temperature is below 4500 C. The molten filler metal is made to flow between the two closely placed adjacent surfaces by the capillary action.

Though soldering obtains a good joint between the two plates, the strength of the joint is limited by the strength of the filler metal used. Soldering is used for obtaining aleak proof joint or a low resistance electrical joint. The soldered joints are not suitable for high temperature applications because of the low melting temperatures of the filler metals used.

The purpose of using the flux is to prevent the formation of oxides on the metal surface when the same is heated. The fluxes are available in the form of powder, paste, liquid or in the form of corein the solder metal. It is necessary that the flux should remain in the liquid form at the soldering temperature and be reactive to be of proper use. The filler metals used are essentially alloys of lead and tin. The composition of solder used for different purposes are as given below

Soft solder - lead 37% tin 63%
Medium solder - lead 50% tin 50%
Plumber’s solder - lead 70% tin 30%...
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