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Thermomechanical material processing by friction


Connect, no.78, July-August 1996


A novel solid phase material processing technique is under development in TWI's Electron Beam and Friction Processes Department which has potentially far-reaching effects for certain primary metal industries. The process is practical and cost effective, and presents possibilities for reprocessing and refining both monolithic and particulate reinforced composite materials.

Friction Pillar Processing (FPP), like Friction Hydro Pillar Processing (FHPP), involves a thermomechanical treatment leading to intense plastic deformation so that a significant reduction in grain size is achieved.

The FPP technique involves rotating the consumable feedstock material co-axially within a cylindrical mould under an applied load thus generating a softened layer across the rotational interface. The softened material develops faster than the feed rate of the feedstock, causing the frictional interface to rise along the feedstock. This softened material is extensively worked during the processing operation, resulting in homogenisation of the microstructure(i.e. all solidification structures are removed), a refinement of the grain structure due to dynamic recrystallisation and removal of casting defects such as casting porosity or hot cracks. In ideal conditions, the rotational interface will move upwards in a virtually continuous fashion.

Any material processing technique which can improve material properties offers tremendous advantages to the processing and user industries. However, all conventional thermomechanical treatments, (e.g. forging, extrusion, rolling,etc) result in a large change in ingot shape. FPP is unique in that material can benefit from recrystallisation and homogenisation of the microstructure, but with little change in shape.

Mechanical properties can be produced so that impact properties of processed material can exceed by two and three times the impact properties of the original cast material. Preliminary experiments with straightforward low carbon steel (0.2%C, 0.4%Mn) have provided increased impact strengths of up to 115 Joules at +20 deg.C compared with the original parent material value of 31 Joules.

Investigations are underway to establish data on fatigue, yield, ultimate tensile strength, ductility and corrosion properties for a number of steel, aluminium and copper alloys which have been Friction Pillar Processed.

Radical microstructural changes typically occur during FPP of cupro-nickel chrome (CNC) and nickel-aluminium-bronze (NAB) alloys. Coarse grain size, and dendritic microstructure of NAB are eliminated, leading to a fine equiaxed microstructure. Similarly, the coarse transformation microstructure of as-cast CNC is removed.

TWI has filed for patents on the above technique.

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