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Laboratory test simulation of aluminium cold forming – influence from PVD tool coatings on the tendency to galling
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
2010 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 204, no 21-22, 3606-3613 p.Article in journal (Refereed) Published
Abstract [en]

Cold forming of aluminium is a group of very efficient methods used in a number of industrial applications. Two of the major factors limiting the tool life and restricting the complexity of the shapes produced, are galling, i.e. transfer of aluminium to the tool surface, which leads to problems in successive forming, and high stresses occurring when forming complex shapes. Both phenomena are closely related to friction and adhesion in the tool to workpiece interface. Earlier investigations have shown that aluminium is transferred to the tool steel surface regardless of the surface roughness of the tool. This has been proposed to be due to the hard native oxide on the soft aluminium leading to a mechanical gripping in the tool surface.The present paper investigates the influence of ceramic coatings on the tendency to galling. This is done by plastic forming of aluminium against coated tool steel rods in a lab test. The test scans over a wide load interval while monitoring the coefficient of friction. The transfer of work material to the tool is studied in the SEM. The test is focused on the friction level and on the number of contacts before a critical friction level is reached.The test set-up comprises two crossed cylinders in sliding contact, one made of coated tool material and the other of work material. Eight commercial coatings were included; DLC, TiAlN, TiN and TiCN as single layer or as multilayer combinations, with hardness values ranging from 800 HV to 3500. HV. All substrates were the same tool steel material (H13), prepared to two different surface finishes. The aluminium work piece cylinders of AA6082 were prepared by two different pre-treatments, soft-annealing followed by industrially used solid lubrication and pickling, respectively.The coating material, preparation of the tool surface finish and the aluminium surface pre-treatment all proved important with respect to galling tendencies and friction levels. Three of the DLC coatings showed very promising behaviour, also without lubricant, but surface roughness deteriorates the galling resistance. The other low-friction coatings tested neither improved the galling resistance nor reduced the friction significantly, compared to the uncoated tool steel.

Place, publisher, year, edition, pages
2010. Vol. 204, no 21-22, 3606-3613 p.
Keyword [en]
Aluminium alloy, Ceramic coating, Friction, Galling
National Category
Engineering and Technology
URN: urn:nbn:se:uu:diva-134845DOI: 10.1016/j.surfcoat.2010.04.025ISI: 000279696400036OAI: oai:DiVA.org:uu-134845DiVA: diva2:373935
Available from: 2010-12-02 Created: 2010-12-02 Last updated: 2016-04-18Bibliographically approved
In thesis
1. On Transfer of Work Material to Tools
Open this publication in new window or tab >>On Transfer of Work Material to Tools
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Bulk forming and cutting are widely used to shape metals in industrial production. Bulk forming is characterized by large strains, extensive plastic deformation and large surface expansions. Cutting is characterized by high speeds, high pressures and high temperatures. The prevailing conditions during these processes lead to transfer of work material to tools. In bulk forming this is a significant problem. The transferred work material is hardened and becomes harder than the work material, causing galling. This leads to high friction and high forming forces, bad surface finish of the formed products and significant difficulties to produce complicated geometries. In cutting, transfer of work material can be desired for protection of the tool surface. However, the transfer film has to be of the correct type to provide a stable and predictive behaviour during operation.

In this thesis the influence from tool material and surface treatment on work material transfer has been studied for both applications, with the use of simplified laboratory test methods followed by extensive surface studies. Both the tendency to, appearance of and chemical composition of work material transfer is evaluated. The results are compared with real industrial examples, to ensure that the correct mechanisms are mimicked.

In forming, the problems arise when poor lubrication prevails, due to high forming forces or large surface expansions. The transfer of work material can then be avoided with the use of a galling resistant coating, offering low adhesion. However, the coating has to be as smooth as possible, to avoid activation of the work material and subsequent transfer.

In cutting, the desired transfer film can be obtained by choosing the correct cutting parameters. The geometry and material of the fabricated component is often predetermined, setting the general cutting conditions, but the cutting speed influences the formation of the transfer film. Too low speed or too high speed leads to an unstable cutting process and poor surface finish of the piece. The speed intervals for each mechanism are partly determined by the tool material and thus by the tool coating.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Uppsaliensis, 2012. 68 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 894
Tribology, Friction, Forming, Cutting, Galling, Material transfer, Tribological coatings, Surface finish, Micro mechanisms, Aluminium
National Category
Engineering and Technology
Research subject
Materials Science
urn:nbn:se:uu:diva-165828 (URN)978-91-554-8261-9 (ISBN)
Public defence
2012-03-02, Häggsalen, Lägerhyddsvägen 1, Uppsala, 10:15 (Swedish)
Available from: 2012-02-10 Created: 2012-01-10 Last updated: 2013-04-08

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