Machine Elements -1 Week11

Machine Elements-1Week11

Design of Brazed and Bonding JointsDesign of Riveted joints

Design of Brazed/Soldered and Bonded Joints

Brazing and Soldering:Both brazing and soldering are the metal joining processes in which parent

metal does not melt but only filler metal melts filling the joint with capillary action. If the filler metal is having melting temperature more than 450°C but lower than the melting temperature of components then it is termed as process of brazing or hard soldering. However, if the melting temperature of filler metal is lower than 450°C and also lower than the melting point of the material of components then it is know as soldering or soft soldering.

During brazing or soldering flux is also used which performs the following functions:

• Dissolve oxides from the surfaces to be joined.• Reduce surface tension of molten filler metal i.e. increasing its wetting action

or spreadability.• Protect the surface from oxidation during joining operation.

The strength of brazed joint is higher than soldered joint but lower than welded joint.

Brazed Joints• The most commonly used filler metal is copper base zinc alloy

consisting of normally 50-60% Cu, approximately 40% Zn, 1% Ni, 0.7 % Fe and traces of Si and Mn, which is brass and termed as 'spelter'. In some cases around 10% Ni may also be added to filler alloys. Copper base alloys may be available in the form of rod, strip and wire. Silver brazing filler metal may consists of 30-55% Ag, 15-35% Cu, 15-28% Zn, 18-24% Cd and sometimes 2-3% Ni or 5% Sn. Silver brazing alloys are available in form of wire, strip, rods and powders.

• Borax and boric acid are commonly used fluxes for brazing with copper base filler metals. Many other commercial fluxes may be available in the form of paste or liquid solution leading to ease of application and adherence to the surface in any position.

Various commonly used method of brazing are followings:

• Torch BrazingTorch brazing utilizes the heat of oxy-acetylene flame with neutral or reducing

flame. Filler metal may be either preplaced in form of washers, rings, formed strips, powders or may be fed manually in form of rod.

• Dip BrazingIn dip brazing components with filler metal in proper form is preplaced at the joint

and assembly is dipped in bath of molten salt which acts as heat source as well as flux for brazing. Preplaced preform melts and fills the joint. Another variant is to dip assembled parts in metallic bath and metal of bath fills the joint.

• Furnace BrazingSelf fixturing assembly with preplaced filler metal is placed inside electrically

heated furnace with temperature control for heating and cooling. These furnaces may also be using protective atmosphere with inert gases like argon and helium or vacuum for brazing of reactive metal components.

• Infra-red BrazingThe heat for brazing is obtained from infra-red lamps. Heat rays can be concentrated

at desired area or spot with concave reflectors. Such method of brazing requires automation and parts to be joined should be self fixturing. Filler metal is to be preplaced in the joint. The operation can be performed in air or in inert atmosphere or in vacuum.

• Induction BrazingThe heat is generated by induced current into the workpiece from a water cooled coil

which surrounds the workpieces to be brazed. High frequencies employed vary from 5 to 400 kHz. Higher the frequency of current, shallow is the heating effect while lower frequencies of current lead to deeper heating and so it can be employed for thicker sections. Fluxes may or may not be used during brazing.

• Resistance BrazingIn resistance brazing the heat is generated at the interfaces to be brazed by resistive

heating. The components are connected to high current and low voltage power supply through two electrodes under pressure. Only those fluxes are used which are electrically conductive and filler metal is preplaced.

Soldering:• The soldering filler metal is called solder. The most commonly used solder is lead and

tin alloy containing tin ranging from 5 to 70% and lead 95 to 30%. Higher the contents of tin, lower the melting point of alloy. Other filler metal are tin-antimony solder (95% tin and 5% antimony), tin-silver solder (tin 96% and silver 4%), lead-silver solder (97% lead, 1.5 tin and 1.5 silver), tin-zinc solder (91 to 30% tin and 9 to 70% zinc), cadmium-silver solder (95% cadmium and 5% silver). These are available in the form of bars, solid and flux cored wires, preforms, sheet, foil, ribbon and paste or cream.

• Fluxes used in soldering are ammonium chloride, zinc chloride, rosin and rosin dissolved in alcohol.

• Various soldering methods are soldering with soldering irons, dip soldering, torch soldering, oven soldering, resistance soldering, induction soldering, infra-red and ultrasonic soldering.

• Soldering iron being used for manual soldering, consists of insulated handle and end is fitted with copper tip which may be heated electrically or in coke or oil/gas fired furnace. Solder is brought to molten state by touching it to the tip of the soldering iron so that molten solder can spread to the joint surface.

• Ultrasonic soldering uses ultrasonics i.e. high frequency vibrations which break the oxides on the surface of workpieces and heat shall be generated due to rubbing between surfaces. This heat melts the solder and fills the joint by capillary action.

Design of Rivets1. General Information

Figure1. Rivet and its parts

A Rivet is a short cylindrical rod having a head and a tapered tail. The main body of the rivet is called shank

General information(2)• Riveting is an operation whereby two plates are joined with the help of a

rivet. Adequate mechanical force is applied to make the joint strong and leak proof. Smooth holes are drilled (or punched and reamed) in two plates to be joined and the rivet is inserted. Holding, then, the head by means of a backing up bar as shown in figure, necessary force is applied at the tail end with a die until the tail deforms plastically to the required shape. Depending upon whether the rivet is initially heated or not, the riveting operation can be of two types:

(a) cold riveting riveting is done at ambient temperature and (b) hot riveting rivets are initially heated before applying force. After riveting is

done, the joint is heat-treated by quenching and tempering. In order to ensure leak-proofness of the joints, when it is required, additional operation like caulking is done .

2- Types of riveted joints and joint efficiency

Riveted joints are mainly of two types:

1. Lap joints 2. Butt joints

2.1. Lap Joints:The plates that are to be joined are brought face to face such

that an overlap exists, as shown in figure. Rivets are inserted on the overlapping portion. Single or multiple rows of rivets are used to give strength to the joint. Depending upon the number of rows the riveted joints may be classified as single riveted lap joint, double or triple riveted lap joint etc. When multiple joints are used, the arrangement of rivets between two neighbouring rows may be of two kinds. In chain riveting the adjacent rows have rivets in the same transverse line. In zig-zag riveting, on the other hand, the adjascent rows of rivets are staggered. Different types of lap joints are sketched in

Lap joints

2.2)Butt Joints In this type of joint, the plates are brought to each other without

forming any overlap. Riveted joints are formed between each of the plates and one or two cover plates. Depending upon the number of cover plates the butt joints may be single strap or double strap butt joints. A single strap butt joint is shown in figure a. Like lap joints, the arrangement of the rivets may be of various kinds, namely, single row, double or triple chain or zigzag. A few types of joints are shown in figure a-c.

The strength of a rivet joint is measured by its efficiency. The efficiency of a joint is defined as the ratio between the strength of a riveted joint to the strength of an unrivetted joints or a solid plate. Obviously, the efficiency of the riveted joint not only depends upon the size and the strength of the individual rivets but also on the overall arrangement and the type of joints.

Butt Joints