Magnus Hanson

Influence from tool roughness on the risk of work material adhesion and transfer

Influence from surface roughness of a forming tool surface on its ability to pick up work material was investigated by laboratory testing. One powder metallurgical tool steel was given different surface roughnesses and tested in sliding contact against one austenitic stainless steel (SS) and one aluminium alloy (Al). Generally, the risk of work material adhesion and transfer increased with the roughness of the tool material. However, SS gave a low friction and very little metal transfer for a tool roughness of the order of the relatively soft SS oxide thickness. The Al alloy, on the other hand, which oxide is much harder than the parent metal, has no critical surface roughness below which adhesive transfer will not occur. The oxide fractures and oxide fragments indent the tool surface increasing the risk of adhesion.

On adhesion of work material in metal forming

The transfer of work material to the tool surface limits the tool life in many forming operations. Using a dedicated load-scanning test equipment with crossed-cylinder geometry, dry forming of austenitic stainless steel was simulated by provoking adhesion to the TiN-coated tool specimen. High-resolution electron microscopy combined with analytical techniques was used to examine the interface between tool and work material. The decisive mechanism for adhesion and transfer of steel to the TiN surface is suggested. The oxide layer on the steel surface, especially the Fe-oxide, initiates the metal transfer. The interfacial oxide acts as a glue between stainless steel and TiN and increases the adhesive forces. Obviously, the adhesion and internal strength of the oxide layer is far stronger than anticipated. It may even be stronger than the bonding to the austenitic steel itself. A consequence of these findings is that the development of galling resistance-forming tool materials and coatings for austenitic stainless steels should not only aim to improve the bulk tool material, but also to reduce the adhesion strength between the tool surface and the oxide layer on the work material.