10.11.3 Thermal (cold expansion)

10.11.2 Thermal (hot shrunk)

a mechanical compression joint consists of an oversize peg being forced into an undersize hole so that the peg is compressed and the hole is stretched. In a lightly compressed joint, friction will be generated between the two components as they try to spring back to their original dimensions.

Fig.10.30 Mechanical compression joint.(a)the bush becomes compressed as it is inserted in the bush plate and the bush plate expands;(b)the bush plate springs back on the bush and grips it-the bush plate is in tension and the bush is in compression

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You may remember that when metals are heated they expand. So, if the outer component is heated sufficiently it expands to a size where it can be slipped easily over the inner component. As the outer component cools it shrinks back to its original size and forms a compression joint on the inner component as shown in Fig.10.31 (a).Heating must be uniform to prevent distortion and the temperature has to be closely controlled. *Too low a temperature results in insufficient expansion for the components to be assembled.*Too high a temperature can result in changes in the properties and structure of the material used for the outer (heated) component.

a. D1>D2 when shaft and collar are at room temperature D2>D1 when the collar is heated, allowing the shaft to enter A compression (shrink) joint is made when the collar cools and shrinks onto the shaft. The shaft is in compression and the collar is in tension

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In a cold expansion joint, the inner component is cooled down until it will slip easily into the hole in the outer component, as it warms up to room temperature, it expands and forms a compression joint with the outer component, as shown in Fig.10.31 (b).This technique requires the use of such coolants as solid carbon dioxide(dry ice) or liquid nitrogen. These are best used under carefully controlled workshop conditions as such low temperatures are potentially dangerous and special equipment is required in their use. The appropriate codes of practice and safety regulations must be rigidly adhered to.          Cooling has the advantage that it does not affect the physical properties of the material, whereas heating may do so

b. D1>D2 when shaft and collar are at room temperature D2>D1 when the shaft is cooled in liquid nitrogen. The collar can now slip over the shaft A compression (expansion) joint is made when the shaft warms up to room temperature and expands into the collar. The shaft is in compression and the collar is in tension

• The process of soldering exploits the fusibility (low melting temperature) of tin-lead alloys

• The molten 'solder', as the alloy is called, bonds to an unmelted parent metal by the application of heat and a suitable flux.

• The parent metal is the metal from which the components being joined are manufactured

• the solder must have a lower melting temperature than the parent metal and it must also be capable of reacting together with the parent metal to form a bond.

The purpose of a flux is to:• Remove the oxide film from the surfaces to be soldered.• Prevent the oxide film from reforming during the soldering

process.• 'Wet' the surfaces being joined so that the molten solder will

run out into an even film• Allow itself to be easily displaced by the molten solder so that a

metal to metal contact is achieved. Back to slide7

10.12.1 Active fluxes

Active fluxes such as Baker's fluid (acidified zinc chloride solution) quickly dissolve the oxide film and prevent it reforming.

Unfortunately all active fluxes leave a corrosive residue which has to be washed off immediately after soldering and the joint has to be treated with a rust inhibitor.

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• Passive fluxes, such as resin, are used for those applications where it is not possible to remove any corrosive residue by washing

• Unfortunately passive fluxes do not remove oxide films to any appreciable extent; they only prevent them from reforming during the soldering process. Therefore the initial mechanical scouring of the joint faces has to be very thorough.

• Hard soldering is the general term used for silver soldering and brazing

• Can be defined as : A process of joining metals in which a molten filler metal is drawn by capillary attraction into the space between closely adjacent surfaces of the parts to be joined.

• hard solders also have a melting temperature range below that of the parent metal. However, this melting temperature range (generally 500°C) is well above that of soft solder and a soldering iron cannot be used to heat the joint and load the solder into it.

Fig. 10.33 Flame brazing: (a) typically hand torches used for hard soldering and brazing, (b) fire bricks or other suitable insulating material is packed around the component to form a brazing hearth which contains and reflects the torch's heat

The success of all hard-soldering processes depends upon the following conditions:•Selection of a suitable filler alloy which has a melting range appreciably lower than the parent metals being joined.•Thorough cleanliness of the surfaces to be joined by hard solder.•Complete removal of the oxide film from the joint surfaces before and during hard soldering by means of a suitable flux.•Complete 'wetting' of the joint surfaces by the molten filler alloy. When a surface is 'wetted' by a liquid, a continuous film of the liquid remains on that surface after draining. This condition is essential for hard soldering and the flux, having removed the oxide film, must completely wet the joint surfaces. This 'wetting' action by the flux assists the spreading and feeding of the molten filler alloy into the joint by capillary action. This ensures a completely filled joint.•Since the molten filler alloy is drawn into the joint by capillary attraction, the space between the joint surfaces must be kept to a minimum and it must also be kept constant. Any local increase in the gap can present a barrier to the filling of the filter alloy. This will prevent the joint from being uniformly filled, resulting in serious loss of strength.•Melting the filler alloy alone is not sufficient to produce a sound joint. The parent metal must itself be raised to the brazing temperature so that the filler alloy melts on coming in contact with the joint surfaces even after the flame has been withdrawn.

10.13.2 Brazing spelters

usually referred to as 'self-fluxing' alloysThese alloys contain silver, phosphorus and

coppercheaper and stronger than silver solders, but

they can only be used to braze copper and copper alloy components in the air

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• are brass alloys and are the oldest filler alloys used

• It is from the use of these brass alloys that the process called 'brazing' gets it name

• These spelters make the strongest joints but they also have the highest melting temperatures. They are mainly used to braze copper, steel and malleable cast iron components.

• additional material added to the joint has a similar composition and strength to the metals being joined

Fig. 10.34 Fusion welding: (a) before - a single 'V' butt requires extra metal; (b) after - the edges of the 'V' are melted and fused together with the molten filler metal

10.14.2 Metallic arc welding

• the heat source in a mixture of oxygen and acetylene burning to produce a flame whose temperature can reach 3250°C and this is above the melting point of most metals.

• The welding gases form a highly flammable and even explosive mixture, so this equipment must only be used by a suitable qualified person or a trainee under the direct instruction of such person

Fig. 10.35 Oxyacetylene welding equipment

Fig. 10.36 Oxyacetylene welding techniques: (a) the leftward method of welding - this is the easiest technique for a right-handed operator; it is used for sheet metal; (b) the rightward method of welding - this method is used for thicker plate as it gives better preparation

• the heat energy required to melt the edges of the components being joined and also the filler rod is supplied by an electric arc. The arc is the name given to the prolonged spark struck between two electrodes.

• The dangers with arc welding arise from the very high temperatures and very heavy electric currents involved. Also high voltages are present in the primary circuit (supply side) of the transformer, and these can lead to accidents involving electrocution

Fig. 10.37 Comparison of (a) oxyacetylene welding and (b) manual metallic arc weldingFigure 10.38 shows the general arrangement of

metallic arc-welding installation

The structures on a welded joint range from the wrought structures of the parent metal to the cast structures of the weld itself, all of which will have been subjected to heat treatment by the high temperatures involved in the process. The heat-affected zone of the parent metal will exhibit the effects of heat treatment. The unaffected regions, where the temperature has not been so high, will retain the original wrought structure of the parent metal. Therefore the effects of welding can be studied under the following headings.:

•The weld-metal deposit.•The heat-affected zone.

the weld metal can be considered as a miniature casting which has cooled rapidly from an extremely high temperature

Long columnar type crystals may be formed giving rise to a relatively weak structure, as shown in Fig. 10.39(a)

Long columnar type crystals may be formed giving rise to a relatively weak structure, as shown in Fig. 10.39(a). In a multi-run weld each deposit normalizes the preceding run and considerable grain refinement occurs with a consequent improvement in the mechanical properties of the joint. In this case, only the top run exhibits the coarse 'as-cast' structure as shown in Fig. 20.39(b).

Fig. 10.39 Weld metal deposit structure: (a) large single-run weld; (b) metallic arc weld

The heat-affected zone of the parent metal is difficult to define. It will depend upon such factors as:•The temperature f the weld pool.•The time taken to complete the weld.•The thermal conductivity of the parent metal.•The specific heat of the parent metal and the dimensions of the parent metal.•The method of welding used.

The heat-affected zone in mild steel plate can exhibit various structures. These range from an overheated structure for those parts adjacent to the weld pool and, therefore, heated to well above the upper critical temperature, to those parts whose temperature has hardly risen above room temperature

Fig. 10.40 Macrostructure of single-run welds in mild steel: (a) oxyacetylene weld; (b) metallic arc weld

The properties of the material will change these changes in structure. The coarser grains will show greater ductility and softness but reduced strength. The finer crystals will show less ductility but greater hardness and strength. These effects become more apparent as the carbon content of the steel increases

This is a resistance welding process widely used in the sheet metal industry

The joint is produced by making a series of spot welds side by side at regular intervals

Apart from ensuring that the joint faces are clean and free from corrosion, no special joint preparation is required

Fig. 10.42 Structure of spot welds: (a) correct welding temperature; (b) temperature too high