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Self-lubricating laser claddings for friction control during press hardening of Al-Si-coated boron steel
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. AC2T research GmbH, Wiener Neustadt, Austria.ORCID iD: 0000-0003-1542-5872
AC2T research GmbH, Wiener Neustadt, Austria.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.ORCID iD: 0000-0003-1454-1118
2019 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 269, p. 79-90Article in journal (Refereed) Published
Abstract [en]

In recent years, the use of Al/Si coatings has become widespread in hot stamping in order to protect the work piece from detrimental mechanisms such as scale formation or decarburisation affecting the quality of the finished product. However, the formation of Al-Fe intermetallics due to diffusion at high temperature can lead to unstable friction and damage both the tool and the work piece.

In the present study, self-lubricating coatings with the addition of silver and MoS2 have been prepared by means of laser cladding deposition, aiming at their use in hot stamping in order to decrease friction and wear. The coatings were evaluated at high temperatures against Al-Si-treated boron steel using two different testing configurations featuring open and closed tribosystems. A significant reduction in friction for the self-lubricating claddings were observed along with decreased material transfer. This could be beneficial for hot stamping applications as it can ensure the stability of the process while preventing surface damage to the work piece. Additionally, closed configuration tribotesting has been found to underestimate friction and wear of the tool/work piece system, thus making it less suited for the lab-scale simulation of hot metal forming compared to open configuration tribometers. This finding has been considered relevant as many references in the available literature still report the use of closed configuration tribometers.

Place, publisher, year, edition, pages
Elsevier, 2019. Vol. 269, p. 79-90
Keywords [en]
High Temperature, Laser Cladding, Self-Lubrication, Adhesion Wear, Hot Stamping
National Category
Manufacturing, Surface and Joining Technology Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
URN: urn:nbn:se:ltu:diva-71708DOI: 10.1016/j.jmatprotec.2019.02.002ISI: 000464299300009Scopus ID: 2-s2.0-85061155760OAI: oai:DiVA.org:ltu-71708DiVA, id: diva2:1265145
Note

Validerad;2019;Nivå 2;2019-02-18 (svasva)

Available from: 2018-11-22 Created: 2018-11-22 Last updated: 2019-04-29Bibliographically approved
In thesis
1. Self-Lubricating Laser Claddings in the Context of Hot Metal Forming
Open this publication in new window or tab >>Self-Lubricating Laser Claddings in the Context of Hot Metal Forming
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Laser cladding is a coating technique with significant advantages like the high quality of the resulting layers, their excellent metallurgical bonding to the substrate or the possibility to repair/rework high-value mechanical components. In recent years, the incorporation of solid lubricants to the base powder in order to produce self-lubricating claddings has been shown in the literature to be possible, with several of the described coatings being able to operate at high temperatures with low friction and wear. This has been considered to hold a great potential for industrial applications involving high temperature work pieces like hot metal forming.

In recent years, the hot stamping of ultra-high strength steel has become increasingly popular due to the enhanced ductility of the work piece and the possibility to achieve a fully martensitic microstructure, to the point that this forming technique has become widespread in the automotive industry. However, the use of Al-Si-based protective coatings on the work piece in order to prevent oxidation and decarburisation is the source of a poor tribological behaviour due to the formation of Al-Fe intermetallics by diffusion from the steel substrate. This can lead to significant material transfer to the tool in addition to a decreased quality of the finished product due to surface damage.

In an attempt to improve the tribological contact in high temperature metal forming applications while at the same time decreasing the need for lubrication, nickel- and iron-based self-lubricating coatings have been prepared by means of laser cladding, featuring the incorporation of different combinations of solid lubricants including soft metals like silver and copper in addition to transition metal dichalcogenides like MoS2 and WS2. The resulting laser claddings were thoroughly characterised, including their microstructure, oxidational properties and their tribological behaviour at high temperatures under different contact configurations and counter bodies.

During the present study, it has been observed that the addition of sulfur-containing precursors to the base powder used for coating preparation leads to the encapsulation of silver, preventing it from floating to the melt pool surface during the cladding process and thus allowing for a uniform distribution of the soft metal across the whole thickness of the coating.

Additionally, it has been observed that the chromium sulfides resulting from the thermal degradation of transition metal dichalcogenides during laser cladding are effective solid lubricants at high temperatures, while silver also contributes to decreased friction at room temperature. Thus, the addition of Ag and MoS2 to nickel-based self-lubricating claddings has been considered optimum in terms of the resulting tribological behaviour, as it leads to decreased friction up to temperatures of 600°C. Additionally, it has been found that the addition of solid lubricants like MoS2 to the nickel-based claddings leads to negligible counter body wear at high temperatures, coupled to the formation of a protective tribolayer on the counter body composed of oxidised nickel, chromium and sulfur. This behaviour has been consistently observed under different testing configurations, like reciprocating against both steel- and aluminium-based counter bodies, in addition to high temperature sliding tests against Al-Si-coated boron steel, and it is expected to protect the surface of the work piece during hot metal forming processes.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2019
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
Tribology, High Temperature, Laser Claddings, Self-Lubricating, Hot Stamping
National Category
Metallurgy and Metallic Materials Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-72367 (URN)978-91-7790-290-4 (ISBN)978-91-7790-291-1 (ISBN)
Public defence
2019-03-12, E632, Lulea, 13:00 (English)
Opponent
Supervisors
Note

This work was funded by the Austrian COMET Programme (Project K2 XTribology, Grant No. 849109) and has been carried out within the “Austrian Excellence Center for Tribology” (AC2T research GmbH)

Available from: 2018-12-27 Created: 2018-12-21 Last updated: 2019-04-03Bibliographically approved

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Torres, HectorPrakash, Braham

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