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A force for the future

A force for the future

Connect, no.61, November 1994, p.5

A new welding process which combines the use of centrifugal force in combination with an exothermic chemical reaction is under further development at TWI

Welding by exothermic chemical reaction was originally devised in about 1897. Now, research has shown that there are wider opportunities when combining it with centrifugal force. The technique has already been applied successfully to clad cylindrical components and is continuing to show potential for the manufacture of symmetrical and flat components.

Sequence of events for centrifugal exothermic cladding

Sequence of events for centrifugal exothermic cladding.

(Top left) Metal oxide (in this case copper alloy) powder and ignition powder rotating in a drum, with oxy-acetylene heating torch ready to ignite the powder reactants.

(Top right) Ignition
(Bottom left)
A finished machined copper alloy deposited into a recessed 3mm deep groove

Potential fabrication application for digger wheels and turbine rings/blade

Potential fabrication application for digger wheels and turbine rings/blade

Possible applications include clad corrosion resistant line pipe, wear resistant chutes, heavy goods vehicles brake drums, digger and cutter fabrication, turbine blade rings, bearings, and reclamation of copper chills.

TWI is carrying out further development trials with this 'one-shot' fabrication technique which works as follows.

An exothermic chemical reaction is achieved with heavy metal oxides, using aluminium as a reducing agent, between aluminium powder and iron oxide reactants. When ignited these reactants produce the necessary welding heat within a few seconds. The superheated molten metal produced by the reactions is so hot (between 2500-2960◦C for steel) that the contacting surface layers of the substrate or joint melt. These layers, together with the liquid weld metal, coalesce to form a welded joint or clad layer.

Almost any desired composition of steel can be obtained by introducing the required alloying elements into the exothemiic charge. These could be in the form of ferro alloys like ferro-manganese or ferro-chrome. Non-ferrous materials such as copper and nickel alloys can be produced by reduction of the appropriate oxide and aluminium to produce superheated liquid metal.

Safety

The new process demands stringent safety precautions. The reaction can be started by igniting magnesium ribbon or a barium peroxide/fine aluminium powder ignition mixture. It requires no external energy because of the strong reaction of aluminium and oxygen.

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Illustration of a concept for cladding/welding nominally flat product applications

Investigations are focusing on the level of inclusions/porosity sometimes found in the deposited material. This to some extent has limited the use of the technique under normal casting conditions. However, the process does provide accelerations in excess of 250g to segregate the dense metallic constituents, liberated during the reaction, from the less dense slag and impurities. Ultimately this will produce improved quality deposit material and prevent casting voids.

TWI has filed a patent application for some aspects of the above process.

Note:
The use of a proprietary exothermic formulation is recommended with proper safety precautions. Care must be taken with powder reactants - the reaction of cupric oxide (CuO) with aluminium to yield copper (Cu) metal and alumina (Al 2O 3) slag is known to be explosively violent. The reaction can be controlled by replacing cupric oxide with cuprous oxide (Cu 2O) and adding a flux such as lime, fluorite or metallic copper. This will reduce the violence of the reaction.

Exothermic powder was supplied courtesy of Thermit Welding (GB) Ltd and Erico Europa (GB) Ltd.

For further information please contact us.

For more information please email:


contactus@twi.co.uk