The influence from shape and size of tool surface defects on the occurrence of galling in cold forming of aluminium

Aluminium alloys are shaped in cold condition in a large number of industrial applications. Simple geometries can be produced to net shape in a single operation. Galling is an important factor limiting the tool life, the complexity of the shapes and the number of processing steps. A selection of ceramic coatings gives a strongly reduced tendency to galling, even without lubricant, as long as the coating surface is smooth. With rough coating surfaces there is no significant improvement compared to the conventionally used uncoated tool steels. The present investigation concentrates on the character of the surface roughness and its influence on galling. Here, surface defects including pyramidal indents and elongated scratches are introduced in a controlled manner and then characterised with optical microscopy and electron microscopy. After testing, the transfer of work material to the tool is studied using electron microscopy and surface analysis. The influence of the various types of local defects is studied with respect to galling tendencies and friction. The shape and extent of the surface roughness is important for galling resistance. Single indents, symbolizing occasional dents in the coating, initially pick up more aluminium than the surrounding coating but do not influence the galling and friction in total. This implies that the forming tools need fine polishing and gentle handling to avoid dents and initial aluminium pick up. The overall surface roughness level is of higher importance for galling and friction.