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The powder welding process basically uses a simple oxy-acetylene torch where finely divided powder is fed into the flame from a hopper attached to the torch. The powder melts to give a dense coating and forms a strong bond with the base material by surface alloying and diffusion. Fluxing elements such as boron and silicon are necessary in the powders for good metallurgical bond. This process is typically used for deposits having thickness between 0.2-12 mm, in particular for smaller parts or repairs. The process is applicable only in ironbase, nickel-base or copper-base material.
Hot metal powder alloys are mostly nickel-based. Nickel when added with boron gives hardness due to formation of borides. The percentage of boron can be monitored to achieve a particular hardness range. Boron also brings down the melting point of nickel and helps the powder to fuse much below melting point of steel. For enhanced protection against wear and increased corrosion resistance, silicon along with other special alloys is added. Some cobalt-based powders are also available but are not as cost effective as nickel-based powders. Tungsten carbide is added to a nickel boride matrix to combat high pressure abrasion.
The flow of powder and spreading on base metal depends primarily on two characteristics : Powder particle shape and size. The mesh size determines the acceptability of the powder to be used by hot metal spray process. Usual range is between 15-160 microns. The shape of powder is very important for the welder as it solely determines the ease of flow of the powder through the nozzle of hot spray gun. The powder shape is dependent on the method of manufacture of the powders. Gas atomized powder have spherical particles resulting in a smooth flow through the nozzle and higher deposition rate. Ground powders have irregular shape and the flow characteristics are poor.
Advantages of Powder Spray
  • Thin layer can be given with hardness upto 65 HRC (upto 0.15 mm thickness)
  • Coating is uniform; hardly any post-machining required
  • No heat treatment is required
  • Self-fluxing: no slag cleaning required
  • For machine parts, life obtained is much higher
The limitations are
  • Not economic for thicker build-up on big jobs
  • Not applicable for aluminium, magnesium components
  • For big jobs furnace preheating and melting is required
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