Recent trends in surface metrology

https://orbit.dtu.dk/files/5556704/2010_Keynote_FR21.pdf

Environmentally benign tribo-systems for metal forming

Experimental and numerical studies of springback in air v-bending process for cold rolled TRIP steels

Evaluation of stamping lubricants in forming advanced high strength steels (AHSS) using deep drawing and ironing tests

Tribological evaluation of the tool steel, focused on determining coefficient of friction and critical load for galling initiation against austenitic stainless steel as a function of surface topography was carried out in a load-scanning test rig. Surfaces investigated included turned, ground, polished, shot penned and laser surface textured cold work tool steel. Additionally, effect of surface roughness and post-polishing on galling resistance of TiN and DLC coated surfaces was investigated. Results of this investigation show that by removing sharp peaks polishing of the bearing surface gives plateau-like topography and improves galling properties of forming tool steel. This becomes even more important when using hard ceramic coatings (i.e. TiN). When superbly polished contact surfaces can give similar galling resistance as otherwise obtained through contact lubrication. However, application of low friction coatings or introduction of micro-dimples with proper density and contact lubrication will result in superior galling resistance of the surface.

Surface topography effect on galling resistance of coated and uncoated tool steel

Tribological properties of PVD tool coatings in forming operations of steel sheet

Paint appearance is an important factor in the overall product quality of steel sheet, especially in the automotive industry. The present paper deals with the functional behaviour of the surface texture according to paint appearance. The main results on the improvement of paint appearance of outer car body panels, obtained with coated and uncoated cold rolled steel, textured with EBT-rolls, is reviewed. There are different methods to relate steel sheet surface texture to the appearance after painting. From the literature it is found that waviness has a detrimental effect on the surface appearance after painting. Different roughness and waviness parameters are compared in order to find a good relation with paint appearance. The following surface texture parameters are evaluated in this study: roughness parameters Ra, peak count Pc, waviness parameters Wca and amplitude between 0.5 – 5mm (Fourier analysis) and envelope parameters motif W and Ra-macro, measured with a spherical stylus of radius 1.5mm. Classical parameters such as Ra and Pc are only able to evaluate paint appearance for one single texturing process, and within very limited range. The ability to predict paint appearance by Ra-macro and Wca also seems to be limited, because of their dependency on the roll texturing technique. Based on extensive testing, it is suggested that Fourier analysis might be the best approach to predict the appearance after painting, and this independently of the texturing process. The results clearly show the improved appearance of Sibetex textured sheets compared to conventional textured sheets. This is due to the fact that, with Sibetex sheet, a reduction in waviness can be achieved.

Assessment of steel surface roughness and waviness in relation with paint appearance

Assessment of steel surface roughness and waviness in relation with paint appearance.

The influence of the stress state on the plastic deformation of CMnSi, CMnSi(Nb), and CMnAlSi transformation induced plasticity (TRIP)-aided steel has been analyzed. Imposing hydrostatic pressures up to 800 MPa during tensile deformation made it possible to change the stress state of the tensile testing specimens. It was found that the ratio of normal to shear stresses has a pronounced effect on the evolution of the microstructure, the austenite volume fraction change during straining, and the fracture surface morphology. The CMnAlSi TRIP steel, which has the largest uniform elongation and the smallest equivalent strain at fracture in the absence of the hydrostatic pressure, had a more pronounced improvement of all plastic characteristics at increasing hydrostatic pressure. An increased austenite stabilization, brought about by the high hydrostatic pressure, was clearly observed. The austenite stabilization results in a decrease of 20 °C to 25 °C of M s for an increase of 100 MPa of the hydrostatic pressure. The implications of the observations could be far-reaching for new sheet forming technologies, such as hydroforming, as the full transformation potential is available for crashsensitive structural parts by avoiding the formation of the martensite during forming operations.

The influence of the stress state on the plasticity of transformation induced plasticity-aided steel

The paper describes the tribological behaviour in the steel sheet and die contact occurring in deep drawing applications. In more detail, sheet surfaces produced by the EBT (Electron Beam Textured) process have been investigated, both theoretical and experimental. By a stepwise theoretical analysis, it is demonstrated that a substantial part of the load supported by sheet surface can be generated by micro-hydrodynamic effects on the curved hills of the sheet surface. Thereto, an original concept of dominant summits has been introduced in order to distinguish from the classical definition of a roughness “peak”. It is shown that the latter fails in explaining the experimental behaviour, both in friction and in galling. The new approach clearly demonstrates its potential for a better modelling of the tribo-behaviour.

Michel Vermeulen

Papers

Tribological properties of PVD tool coatings in forming operations of steel sheet

Assessment of steel surface roughness and waviness in relation with paint appearance

Assessment of steel surface roughness and waviness in relation with paint appearance.

The influence of the stress state on the plasticity of transformation induced plasticity-aided steel

Micro-hydrodynamic effects in EBT textured steel sheet