In this article we will discuss about the meaning and applications of metal spraying in industries.
Meaning of Metal Spraying:
Metal spraying or metallizing literally means to treat with or coat with a metal or metallic compound. Metallizing as a process normally includes the preparation of base material, the spraying on the metal and finally finishing the surface by grinding.
Basically, there are two types of equipment used for metal spraying. One is the metallic-gun which consists of a gas torch with a hole in the centre of the tip for the wire, a small air turbine and gears to feed in the wire through the tip into the flame as fast as it melts and an air cap around the torch tip and nozzle which supplies a blast of air to atomise the molten metal and deposit it on the prepared surface.
In other method, powdered metal is fed from a container through a rubber hose to spray-gun and out through the centre of the flame, similar to the wire gun. In this case metal is already in the atomised form and hence air needed is just sufficient to deposit the molten metal on the surface being coated.
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The applications of both the methods are important in their respective fields. The powder-gun having no turbine or gears, costs less, is lighter to handle and without the blast of atomising air can be used more effectively to heat the base metal when such heat is needed. On the other hand, metal is less expensive in wire form than in powder form; metal in wire form is more readily available, and more easily handled.
The metallic-gun using metal in the wire form is most commonly used. The wire is fed into the spray gun at a definite rate melted by an oxyacetylene flame, and then blown, on the surface being coated, by compressed air. A sectional view of metal spraying gun is shown in Fig. 8.1.
The surface of parent metal is properly prepared as the bond between the sprayed metal and parent metal is purely mechanical. The surface is cleaned by blasting with sharp silica sand or angular steel grit. Cylindrical objects are prepared by machining small grooves on the surface followed by rolling over the tops of these grooves with a tool similar to the knurling tool.
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Both of the methods roughen the surface and provide the necessary interlocking surfaces, so that the plastic material can be easily adhered to the surface. The molten material is blown with considerable force, which causes the interlocking with surface irregularities. The sprayed metal itself provides a suitable surface for successive coatings and permits the building up a layer of considerable thickness.
By spraying metal on the parent metal, the original properties are changed. There is an increase in porosity, compressive strength and hardness, but the tensile strength decreases. Also the wearing quality of sprayed metal is good.
Now-a-days, this process has become very popular because of its economy and the rapidity with which the metal can be applied. Also there is no distortion and no internal stresses are developed in the parts.
Applications of Metal Spraying:
Some of the applications of metal spraying are:
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(1) Corrosion-Protection:
The most extensive use of metal spraying is the application of aluminium and zinc into iron and steel. Calorizing is frequently accomplished by spraying of 0.1 mm aluminium on steel and then heating the part to form iron aluminium compound on the surface.
(2) Hard and Noble Surface:
One big application of metallizing is the application of special metal surface on large masses of less costly metal e.g., big shaft required to be corrosion resistant need not be made completely of stainless steel. It might be made first of any metal and then metallised with the noble metal.
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Similarly the metallizing may be applied for abrasion or wear resistance, corrosion protection and electrical or magnetic properties etc. Most of the regular hard surfacing materials are available in powdered form and can be sprayed when thin coatings are prepared.
(3) Soldering Surfaces:
Sprayed copper is frequently used on non-metallic parts when it is desired to attach parts by soldering. The procedure may also be adopted on hard solder metals such as magnesium, when galvanic corrosion is not a consideration.
(4) Electrical Conductivity:
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Conductive coating of copper and silver can be obtained on poor conductor e.g., by spraying copper on most of carbon brushes for motors and generators for better electrical connection.
(5) Thermal Conductivity:
Sometimes it is desirable to have surface to carry away heat from hot spots on poor conductors. In this case copper can be sprayed and it has further advantage of perfectly fitting into the non-conducting part and gives best efficiency.
(6) Other Applications:
Its other applications are in having decorative films, reflecting surface and special metal forms.