| 1. |
- Decrozant-Triquenaux, Justine, 1994-
(författare)
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High Temperature Tribology of Aluminium : Effect of Lubrication and Surface Engineering on Friction and Material Transfer
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Lightweight design for automotive applications has been pursued for several decades and continues to increase. The main driving forces are new and increasingly stringent emission regulations as well as the increasing popularity of electric, or hybrid, vehicles where the increased weight of the batteries need to be compensated by light weight structures. It is also critical to maintain or improve passenger safety while creating components and structures with lower weight. Materials exhibiting a high strength-to-weight ratio, such as high strength aluminium alloys, are highly interesting to realise the next generation of lightweight vehicle structures.The high strength aluminium alloys include the 5XXX, 6XXX and 7XXX series. In order to increase their formability and minimise springback induced during forming at room temperature, these alloys are preferentially formed at elevated temperatures. Different forming processes such as warm forming and hot stamping (e.g. hot forming and quenching) have been developed to enable forming of components with high geometrical complexity and mechanical properties. However, hot forming of aluminium alloys leads to a challenging tribological interface. Aluminium alloys are ductile and reactive metals, prone to severe adhesion (also termed as seizure or galling) when sliding against a harder metallic counter surface. Aluminium transfer to the forming dies affect the tool lifetime and impacts the quality of the formed component which leads to significant maintenance costs and reduced productivity. These are the main limitations that hinder the implementation of hot aluminium forming for mass production.Lubrication as well as surface engineering strategies are potential methods to control friction and wear in the hot aluminium-tool steel interface. Solid lubricants such as graphite and hexagonal boron nitride (hBN) have been studied for aluminium forming. Polymer-based lubricants are also increasingly evaluated for high-temperature applications. Surface engineering techniques includes both the control of the tool topography and the use of protective coatings. Surface roughness has been observed as a crucial parameter in the initiation of aluminium transfer to the counter surface. PVD and CVD thin coatings are increasingly studied as ways to alleviate galling. Among others, CrN and DLC coatings are known to reduce adhesion when sliding against aluminium. Despite the research efforts in this field, there is still lack of systematic studies where synergistic effects of lubrication, surface topography and coatings are explored in the context of hot aluminium forming.The aim of this research is to enhance the understanding of the tribological behaviour of aluminium sliding against tool steel at elevated temperatures. The effect of tool steel composition, surface roughness (as-received and post-polished), and PVD surface coating composition (CrTiN, CrAlN, CrN and DLC ta-C) has been evaluated under dry and lubricated conditions (hBN-based and polymer-based).High temperature tribological tests were carried out in a reciprocating sliding flat-on-flat configuration. In dry conditions, the aluminium-tool steel tribosystem is characterised by severe adhesive wear and high friction. Effective control of friction and wear was found to be highly dependent on the ability of the lubricant to remain in the contact zone. The combined use of a polymer-based lubricant with post-polished surface topography on a PVD coated tool led to the best improvements in terms of frictional stability and reduced material transfer. This was mainly attributed to prevention of direct contact between the tool material and aluminium together with minimised mechanically initiated material transfer. Post-polished uncoated tool steels resulted in the development of a protective tribolayer in the contact and together with flattening of the aluminium surface, led to friction and wear reduction. In case of post-polished PVD coatings, the lubricant entrapment in the contact zone as well as the development of mechanically mixed layers on the aluminium lowered friction and wear.
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| 2. |
- Gebretsadik, Daniel, et al.
(författare)
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Friction and wear characteristics of PA 66 polymer composite/316L stainless steel tribopair in aqueous solution with different salt levels
- 2020
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Ingår i: Tribology International. - : Elsevier. - 0301-679X .- 1879-2464. ; 141
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Tidskriftsartikel (refereegranskat)abstract
- Friction and wear behaviour of a PA 66 composite with 25% glass fibre reinforcement sliding against a 316L stainless steel have been investigated using different aqueous solutions under mixed/boundary lubrication. The aqueous solutions used are deionized water, seawater, seawater without group II metal salts, solution of dissolved Group II metal salts and solution of group II metal salts and NaHCO3. Lower friction and wear was obtained when lubricated with seawater. However, when deionized water or salt solutions without group II metal salts is used, increased wear was observed. The lower friction and wear in seawater solution is due to group II metal ions and bicarbonate ions that facilitate formation of the sparingly soluble carbonates that act as a lubricating tribofilm.
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| 3. |
- Moghaddam, Pouria Valizadeh, et al.
(författare)
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Dry sliding wear of nanostructured carbide-free bainitic steels : Effect of oxidation-dominated wear
- 2020
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Ingår i: Wear. - : Elsevier. - 0043-1648 .- 1873-2577. ; 454-455
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Tidskriftsartikel (refereegranskat)abstract
- The microstructure has a profound impact on sliding wear behaviour. This paper aims to understand the effect of carbide-free bainitic microstructure on oxidation-dominated wear in a self-mated dry sliding contact and compare the results with quenched and tempered martensitic microstructure. The results show improved wear resistance of the carbide-free bainitic steel austempered at low temperature. Hence, the retained austenite content of the carbide free bainitic microstructure is not the only indicator of excellent wear resistance. Compared to tempered martensitic microstructure, the carbide-free bainite offers a higher resistance against the formation of brittle white etching layer during the sliding wear. In summary, the formation of a thin and mechanically stable compositional mixed layer on top of the surface together with a hard underlying substrate are the main reasons behind the improved wear performance of carbide-free bainitic steel austempered at low temperature.
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| 4. |
- Moghaddam, P. Valizadeh, et al.
(författare)
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Effect of retained austenite on adhesion-dominated wear of nanostructured carbide-free bainitic steel
- 2020
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Ingår i: Tribology International. - : Elsevier. - 0301-679X .- 1879-2464. ; 150
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Tidskriftsartikel (refereegranskat)abstract
- Sliding wear performance of carbide-free bainitic steel is significantly affected by microstructure. This investigation is an attempt to find a correlation between microstructure and wear resistance of carbide-free bainitic steel and compare the results with that of a conventional tempered martensitic microstructure. Under predominantly adhesive wear conditions, carbide-free bainitic microstructure with the highest amount of retained austenite offers the best wear resistance. This is mainly attributed to the more pronounced work hardening and TRIP-effect of carbide-free bainitic microstructure austempered at higher temperature. Therefore, higher initial bulk hardness is not the only indicator of wear resistance. Moreover, when both oxidation and adhesion are active, a microstructure with an optimum content of retained austenite and bainitic ferrite shows the highest wear resistance.
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| 5. |
- Moghaddam, P. Valizadeh, et al.
(författare)
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Influence of fracture toughness on two-body abrasive wear of nanostructured carbide-free bainitic steels
- 2020
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Ingår i: Wear. - : Elsevier. - 0043-1648 .- 1873-2577. ; 460-461
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Tidskriftsartikel (refereegranskat)abstract
- This study aims to understand the effects of toughness and hardness on two-body wear of nanostructured carbide-free bainitic steels. For this purpose, three different steel grades were austempered at 250 °C to obtain carbide-free bainitic microstructures with different mechanical properties. The mechanical properties were determined in terms of fracture toughness and hardness. The wear tests were carried-out at three different loads with two different types of abrasive papers. The results show that a carbide-free bainitic steel with an optimum combination of hardness and fracture toughness exhibits the highest wear resistance. It has been seen that hardness is not the only parameter in determining the abrasive wear resistance and retained austenite has a beneficial effect on two-body abrasive wear.
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| 6. |
- Moghaddam, Pouria Valizadeh
(författare)
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On the role of microstructure in wear of nanostructured carbide-free bainitic steels
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The significance of steel production for the development of sustainable society and economy is immense. Today’s fast growing global economy poses an increasing demand for improving the properties of steel. The development of nanostructured carbide-free bainitic steel with an excellent combination of strength and toughness is an attempt to satisfy this global demand. During austempering, the precipitation of cementite can be suppressed by addition of approximately 1.5 wt% silicon and a duplex microstructure comprising of extremely fine aggregates of retained austenite and bainitic ferrite can be obtained. Owing to their excellent mechanical properties, these novel steels exhibit considerable potential to replace quenched and tempered bearing steel or pearlitic rail steel. In these applications, wear play a crucial role in determining the performance of the components. However, the majority of studies have been limited to mechanical properties of these steels but not much attention has been paid to their tribological behaviour. Notably, the role of retained austenite and bainitic ferrite on wear performance has not yet fully understood. Furthermore, machining process is an inevitable step in the manufacturing of metal products. During the machining, contact temperature can rise to several hundred degrees. However, high temperature tribological behaviour of these steels in conjunction with coated cutting tool has not yet been addressed. Therefore, the aim of the present research work is to gain a deeper understanding of the correlation between microstructure and tribological performance of carbide-free bainitic steels in various conditions.To achieve this aim, tribological behaviour of nanostructured carbide-free bainitic steels has been investigated under dry rolling/sliding, sliding and two-body abrasive wear conditions. A number of steel grades were austempered under a wide range of temperatures and durations to obtain different carbide-free bainitic microstructures. The results have been compared with that of quenched and tempered bearing steel. Moreover, high temperature tribological behaviour of carbide-free bainitic and 316L stainless steels during interaction with TiAlN PVD coating has also been studied under dry reciprocating sliding condition.The results show that a relatively higher retained austenite content and its stability enhance wear resistance under rolling/sliding condition. Moreover, wear performance of carbide-free bainitic steel has been found to be superior to that of the quenched and tempered bearing steel. However, under sliding condition, the effect of retained austenite on wear heavily depends on sliding speed. At low sliding speed and under adhesive-dominated wear condition, higher retained austenite content results in improved wear performance due to the higher work hardenability. In contrast, at high sliding speed where oxidative wear is dominant, a microstructure with the lowest content of retained austenite exhibits the highest wear resistance. The higher amount of bainitic ferrite provides a hard underlying substrate for a thin and mechanically stable compositional mixed layer and thereby enhances the wear resistance. The formation and microcracking of a brittle white etching layer of quenched and tempered steel is responsible for its inferior wear resistance. Under two-body abrasive wear conditions, higher retained austenite content leads to improved wear resistance. A microstructure providing an optimum combination of hardness and toughness shows the best abrasive wear resistance. During reciprocating sliding, the tribological response of carbide-free bainitic steel is altered with increasing temperature. Higher temperature results in severe material transfer from carbide-free bainitic steel to TiAlN coated cemented carbide. Furthermore, at elevatedtemperature, a porous oxide layer grows on the transferred materials and reduce friction coefficient.In summary, there is no simple and general relationship between microstructure and wear resistance. Depending upon the dominant wear mechanisms and operating conditions, retained austenite and bainitic ferrite affect the wear behaviour in radically different manners.
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