Investigation of the Self-organising Behaviour of Laser Implanted Tool Surfaces

The alignment of defined surface structures on sheet metal surfaces for metal forming operations by skin-pass rolling is a common approach for improving the formability, the durability of the used forming tools, and the optical appearance of the final product. Especially automotive applications impose high requirements on the properties and the repeatability of surface structures. Different roll structuring techniques have therefore been developed during the last decades and found entrance into industrial application. However, the lifetime of the structured rolls is still not satisfying, which necessitates expensive and time consuming chrome plating and roll replacement. Furthermore, a study of the state of the art reveals that the tribological potential of defined surface structures for enabling micro-lubrication mechanisms in subsequent forming steps is apparently not realised completely by these techniques. In experimental studies a new structuring method is investigated for optimising the tribological behaviour in metal forming operations. This new approach utilises the system immanent wear forces for the creation and maintenance of a surface topology. It bases on the initial creation of a material properties gradient on the surface of tool steels, which is gradually transformed into a surface topology under tribological load due to locally different wear rates. For the creation of the necessary material properties gradient, a discontinuous laser dispersing (laser implantation) of ceramic particles is further developed. The wear behaviour and its influence parameters are identified in sliding and rolling wear experiments. It is shown in a pin-on-disc test that after a certain time a steady state exists where the created surface topology does not change. This state is not reached in the rolling test due to the outstanding wear performance of the laser created structures, the comparably low relative sliding velocity and the distribution of the wear effect on the roll circumference. Afterwards, a mathematic model basing on contact-mechanical considerations is introduced, which allows a prediction of the wear behaviour and the steady state height. It shows a good agreement with the experimental results. Finally, the potential of the laser implantation texturing for being used in industrial applications is discussed.