Evaluation of TiB₂ coatings in cold forming of aluminium

Abstract

Aluminium alloy parts are often formed using cold forming. The transfer of aluminium to the tool is a major problem in these operations. TiB₂ has low reactivity with aluminium and has shown promising results in other forming tests. Here, cold forming is simulated in equipment comprising a TiB₂ coated tool cylinder and an aluminium cylinder in sliding contact. The coated surfaces are prepared to two surface finishes, and the aluminium cylinders were prelubricated or unlubricated respectively. The test is focused on friction level and number of contacts to reach a threshold friction level. The aluminium surface pretreatment was found to be the most important factor; lubrication lowers the friction significantly. The tool surface finish is also important; polishing lowers and stabilises the friction. The TiB₂ coating offered 20–30% better galling performance than the uncoated steel. However, this improvement was far from that of the best available diamond-like carbon coatings.

Keywords:

• Galling
• Friction
• TiB₂
• Aluminium
• Steel

The authors wish to thank Steertec Raufoss and SINTEF Raufoss Manufacturing for financial support via the programme BIP AluPart, partly financed by the Research Council of Norway, and for sharing their knowledge about the aluminium forming process. Professor S. Hogmark is acknowledged for valuable comments on the manuscript.