Wheat gluten biopolymers generally become excessively rigid when processed without plasticisers, while the use of plasticisers, on the other hand, can deteriorate their mechanical properties. As such, this study investigated the effect of carbon black (CB) as a filler into glycerol-plasticised gluten to prepare gluten/CB biocomposites in order to eliminate the aforementioned drawback. Thus, biocomposites were manufactured using compression moulding followed by the determination of their mechanical, morphological, and chemical properties. The filler content of 4 wt% was found to be optimal for achieving increased tensile strength by 24%, and tensile modulus by 268% along with the toughness retention based on energy at break when compared with those of glycerol-plasticised gluten. When reaching the filler content up to 6 wt%, the tensile properties were found to be worsened, which can be ascribed to excessive agglomeration of carbon black at the high content levels within gluten matrices. Based on infrared spectroscopy, the results demonstrate an increased amount of β-sheets, suggesting the formation of more aggregated protein networks induced by increasing the filler contents. However, the addition of fillers did not improve fire and water resistance in such bionanocomposites owing to the high blend ratio of plasticiser to gluten.

In recent past, warm forming processes have been used successfully to increase the strength of age-hardenable alloys by dynamic precipitation. However, the influence of dynamic age hardening on the wear and friction behaviour of age-hardenable aluminium alloy is not clear. Therefore, in the present investigation the effect of static and dynamic ageing on the friction and wear behaviour of aluminium 6082 alloy (AA 6082) sliding against tool steel (TS) surface has been studied. The aluminium samples used in the present study were in as-cast, solitionised and peak aged conditions. Optical microscope revealed the presence of dendritic structure in both as-cast and solitionised samples. Scanning electron microscope analysis of the debris and worn surfaces revealed the wear mechanism and role of precipitates on the friction and wear results. At low temperature (40 C), the frictional behaviour of as-cast, solitionised and peak aged samples were similar. The wear rates at 40 C increased with increase in the amount of strain inside the specimens due to fine precipitations. At 180 C, a significant variation in the frictional behaviour of different specimens was observed. The wear rate of solitionised specimens at 180 C is higher compared to as-cast and aged specimens. The absence of hard phases at initial stage of the test and subsequent dynamic precipitates restricted to a thin layer were responsible for the increase in wear rate.

In the present investigation the effect of static and dynamic ageing on the wear and friction behavior of aluminum alloy (AA 6082) sliding against tool steel (TS) surface has been studied. The AA 6082 alloy samples used in the present study were in as-cast, solutionized and peak aged conditions. Scanning electron microscope analysis of the debris and worn surfaces revealed the role of precipitates on the dry sliding wear behavior. Frictional behavior varies significantly for all the conditions at elevated temperature (180 °C) compared to room temperature (40 °C). Such response was attributed to the dynamic precipitations during elevated temperature test.

Aluminium alloys are commonly used as lightweight materials in the automotive industry. This non-ferrous family of metallic alloys offers a high versatility of properties and designs. To reduce weight and improve safety, high strength-to-weight ratio alloys (e.g. 6XXX and 7XXX), are increasingly implemented in vehicles. However, these alloys exhibit low formability and experience considerable springback during cold forming, and are therefore hot formed. During forming, severe adhesion (i.e. galling) of aluminium onto the die surface takes place. This phenomenon has a detrimental effect on the surface properties, geometrical tolerances of the formed parts and maintenance of the dies. The effect of surface engineering as well as lubricant chemistry on galling has not been sufficiently investigated. Diamond-like carbon (DLC) and CrN physical vapour deposition (PVD) coated steel have been studied to reduce aluminium transfer. However, the interaction between lubricants and PVD coatings during hot forming of aluminium alloys is not yet fully understood. The present study thus aims to characterise the high temperature tribological behaviour of selected PVD coatings and lubricants during sliding against aluminium alloy. The objectives are to first select promising lubricant-coating combinations and then to study their tribological response in a high-temperature reciprocating friction and wear tester. Dry and lubricated tests were carried out at 300 °C using a commercial polymer lubricant. Tests using DLC, CrN, CrTiN, and CrAlN coated tool steel were compared to uncoated tool steel reference tests. The initial and worn test specimen surfaces were analysed with a 3-dimensional (3D) optical profiler, scanning electron microscope (SEM) and energy dispersive X-ray spectroscope (EDS) as to understand the wear mechanisms. The results showed formation of tribolayers in the contact zone, reducing both friction and wear. The stability of these layers highly depends on both the coatings’ roughness and chemical affinity towards aluminium. The DLC and CrN coatings combined with the polymer lubricant were the most effective in reducing aluminium transfer.

Aluminium alloys are commonly used as light-weight materials in the automotive industry. This non-ferrous family of metal alloys offers high versatility of properties and designs. In order to reduce weight and improve safety, grades with high strength-to-weight ratio, such as the 6XXX and 7XXX alloy series, are increasingly implemented in vehicles. These alloys, however, exhibit low formability and experience considerable springback when formed at low temperatures, and thus have to be formed at elevated temperatures. Severe adhesion and galling are known to be critical tribological challenges in hot forming of aluminium. During the forming operation, adhesion and transfer of aluminium onto the die surface take place. This phenomenon has a detrimental effect on the surface properties, geometrical tolerances of the formed parts and maintenance of the dies. The influence of surface engineering as well as lubricant composition on adhesion and galling has not been sufficiently investigated. Diamond-like-Carbon and Chromium Nitride PVD coatings applied on the tool steel have shown promising results for reducing aluminium transfer at high-temperatures, especially in the presence of a lubricant. However, the interaction between lubricants and PVD coatings during hot forming of aluminium alloys is not yet fully understood. The present study thus aims at characterising the high temperature tribological behaviour of selected PVD coatings and a lubricant during sliding against an aluminium alloy. The objectives are to select promising lubricant-coating combinations for the given application and to study their tribological response in a high-temperature reciprocating friction and wear tester. The tests were carried out at 300°C, under dry and lubricated conditions, in order to study the friction and wear performance. Uncoated tool steel reference tests were performed under dry and lubricated conditions and lubricated tests using DLC, CrN, CrTiN and CrAlN coated tool steel were performed and compared to the reference results. The initial and worn surfaces were analysed with white light 3D optical interferometry, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) with a view to understand the wear mechanisms.

Hot stamping is characterised by severe contact conditions, especially when forming aluminium components. In order to improve the tool lifetime, process economy, and component quality, understanding the initiation mechanisms behind aluminium transfer onto the tool surface at high temperatures is critical. To date, the tribological interaction between tools and aluminium sheets at high temperature has received limited attention. Lubricants, combined with surface engineering techniques (e.g. coatings, nitriding and surface topography control), show great potential for reducing the severity of material transfer at high temperatures. However, there is still, limited knowledge about their interaction and performance in this tribological context. In this study, high temperature tribological tests were carried out to characterise the synergetic effects of surface coatings/treatments with and without lubrication on friction and wear. A commercially available lubricant was evaluated when used in combination with uncoated, nitrided and CrWN- or DLC ta-C-based PVD coated tool steel. The tests were carried out on a hot strip drawing tribometer, employing an open contact configuration representative of the hot stamping contact conditions at two different temperatures. The counter-material was a 6082 aluminium alloy, heated up following a thermal cycle relevant for the hot stamping process. The results showed that the tribological response was highly dependent on the retention of the lubricant in the contact and the type of surface modification technique. The results show that bonding of the lubricant to the tool surface is critical. In the case of lubricant failure, severe adhesive wear and aluminium transfer onto the tool surface occurred, correlated with an increase in friction. The use of different surface engineering methods led to different results: lower friction levels could be reached when combining use of lubricant and PVD coatings compared to using uncoated or plasma nitrided tool steel. In this study, the best combination to minimise aluminium transfer and friction is the association of the lubricant with CrWN PVD coating in this study.

Usage of high-strength aluminium alloys is increasing for automotive applications due to high strength-to-weight ratios. Limited formability at room temperature requires hot forming for production of complex geometries. The occurrence of severe adhesion and material transfer negatively affects process economy and there is a need for effective solutions to overcome these issues. This work is focussed on the high-temperature tribological behaviour of Al6016 alloy and uncoated/PVD coated tool steels. Studies were conducted under dry and lubricated conditions using a high-temperature reciprocating tribometer. High friction in dry sliding for uncoated and coated tool steels was observed. Hexagonal boron nitride lubricant was not effective in reducing friction. DLC coating with a polymer lubricant resulted in the lowest friction and minimised material transfer.

The use of high strength aluminium alloys, such as 6XXX and 7XXX series, is continuously increasing for automotive applications in view of their good strength-to-weight ratio. Their formability at room temperature is limited and they are thus often formed at high temperatures to enable production of complex geometries. Critical challenges during hot forming of aluminium are the occurrence of severe adhesion and material transfer onto the forming tools. This negatively affects the tool life and the quality of the produced parts. In general, the main mechanisms involved in the occurrence of material transfer of aluminium alloys at high temperature are still not clearly understood. Therefore, this study is focussed on understanding of the friction and wear behaviour during interaction of Al6016 alloy and three different tool steels in as-received and polished state. The tribotests were carried out under dry and lubricated conditions, with two distinct lubricants, using a reciprocating friction and wear tester. The worn surfaces were analysed using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The results showed a high dependence of friction and wear behaviour on the tool steel roughness as well as on the stability of the lubricant films. Tribolayers were found to develop in the contact zone and their capacity to improve the tribological behaviour is seen to be drastically impacted by the surface roughness of the tool steel. When the tribolayers failed, severe adhesion took place and led to high and unstable friction as well as material transfer to the tool steel.

Severe adhesion, also referred to as galling, is a critical problem in press hardening, especially in stamping tools used for hot forming of Al–Si-coated ultra-high strength steel. Galling is known to develop rapidly on the tool surface and it negatively affects the quality of the formed products. Earlier research on this topic has focused on the galling initiation. However, studies on the galling development during extended sliding and the corresponding quantitative measurement still lack depth. In the present study, a tribological test is established to study the galling development under press hardening conditions. The tribological test set-up aims to simulate extended sliding between the Al–Si-coated boron steels and the tool die material. The contact conditions in the interface are studied by a numerical model of the tribological test. The friction coefficients and material transfer are discussed taking into account the variation of the different test conditions. Using the results from the tribological tests, the galling simulation is performed in the numerical model. A geometry-updated sample based on the galling (transferred material build-up) height is simulated and the consequent pressure fluctuation is obtained in the numerical model. This contributes to the explanation of the severe transferred material accumulation during the test.

Press hardened steels are commonly used as a lightweight choice for manufacturing car components because of the high ratio of strength to weight. The use of ultra-high-strength steels for the design of lightweight vehicles contributes to the reduction of emissions of carbon dioxide while maintaining passenger safety. Stamping tools used in press hardening processes suffer harsh contact conditionsin terms of dramatic temperature changes, cyclic loadings, and complex interactions between coatings and oxidation. In mass production, tool wear is an inevitable problem that increases maintenance costs. Severe adhesive wear, also called galling, substantially occurs in the stamping tool used against Al—Si-coated workpieces. The galling that takes place during press hardening not only degrades the production quality but also shortens the service life of the tool. In order to properly arrange tool maintenance and minimize galling through adjusting process parameters, engineers need to know when and where galling occurs, based on modelling of the galling in press hardening simulations. In order to implement a galling simulation for press hardening, a modified Archard wear model is employed in the present study, which is a contact-mechanics-based model. The specific wear rate in the model is calibrated by the quantitative galling measurements of a high-temperature tribometer test. The tribological test is designed to mimic the press hardening conditions, where the correlations between galling and process parameters such as temperature, pressure, and sliding distance are outlined. The galling simulation is implemented in a full-scale press hardening experiment, and the predicted galling is validated in terms of severe galling positions and galling profiles. The galling profile evolution is correlated to variations in the contact conditions. Uncertainties in the numerical model, such as the choice of penalty scaling factor and friction coefficient, are analysed with a parameter study and discussed. This study demonstrates finite element (FE) simulations involving galling prediction in press hardening so as to improve product development and production efficiency.

This work investigates the coefficient of friction (COF) at room temperature between tool steel 1.2343 and aluminum alloys EN AW-5182, EN AW-6016 as-delivered (T4) and EN AW-6016 naturally aged (T4*) using a strip drawing tribometer. In order to simulate the contact conditions of industrial sheet metal forming processes, the surfaces of the steel pins and of the aluminum strips were maintained as-delivered, i.e., the pins were wire-cut from a hardened and ground plate and the strips were cut from electrical discharge textured (EDT) and dry-lubricated sheets. Two sliding velocities, 50 mm/s and 250 mm/s, and two nominal contact pressures, 10 N/mm² and 20 N/mm², were considered. The sliding distance on each strip was 0.5 m. Each pair of pins was utilized for testing 10 or 20 strips to study the influence of increasing the sliding distance on the COF. Before and after the tribological experiments, surface topographies of selected pins and strips were analyzed using 3D optical surface profilometry, optical microscopy and scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS). Strain hardening due to plastic surface deformation of the strips was investigated using an automated hardness tester. In general, an increasing trend of the COF was observed with increasing sliding distance. The mean COF obtained for each of the tests was in the range of 0.09−0.17; however, it was considerably higher if aluminum was transferred from the strip to the pins. Moreover, moist pin surfaces were identified to increase the COF, as the originally dry lubricant became pasty and sticky which promoted entrapment of abraded aluminum particles. Slightly higher strain hardening of alloy EN AW-5182 compared to alloy EN AW-6016 caused less flattening of the strip asperities and more severe wear of the pin surface.

An accurate and reliable wear analysis requires detailed knowledge of the tribological conditions of the studied system. In this work, a numerical model which can quantify wear and is applicable to hydraulic motors is developed. Detailed tribological knowledge can be acquired through strategic experimental testing and numerical simulations. The model is constructed to include the effect on wear from varying lubricant film thickness. The development of the wear model includes consideration of wear observed in the Scanning Electron Microscopy (SEM) analysis of tested motors. The model is of the Archard type, in which the k-value is estimated from experiments, after considering the effect of lubrication. The contact pressure is the solution to a lubrication model that governs both the hydrodynamics of the lubricant film and the direct contact between the rough surfaces. To validate the model, a hydraulic motor is run at different operating conditions and the apparent wear depth is analysed after the tests. Numerical simulations mimicking the same configuration are performed and the predicted wear depths are compared to the experimental results. Similarities and differences are discussed and it is evident that a clear correlation exists between the wear predicted with the model and the measurement data of the apparent wear in the hydraulic motor. There are also discrepancies because of the model simplicity and the uncertainty in the specifications of the tested system. The results imply that wear analysis using numerical simulations aid the understanding of wear in machinery. The combined knowledge of physical conditions on different important scales enables in-depth analysis with numerical tools which cannot be achieved through experimental investigations alone. Furthermore, the numerical model can be refined leading to better wear predictions.

Recent years have seen a continuously growing interest in high temperature tribological research. A significant part of this is driven by the need for improved understanding and knowledge pertaining to friction and wear and their control in the context of hot forming of high strength steels. Friction and wear characteristics of a sliding system are highly dependent on the properties of the two interacting surfaces. At high temperatures, the surface and material properties become extremely important since these systems often operate under unlubricated conditions. High temperature tribological processes are highly complex as these involve changes in mechanical properties due to microstructural changes; thermal softening; surface chemical and morphological changes due to oxidation and diffusion; deterioration of the surface and bulk material as a result of adhesive/abrasive wear and thermal fatigue. Many of these changes occur on the surfaces and/or in the near surface region. The formation of surface oxide layers and near surface layers with a highly refined microstructure (nano-structured) has been reported to have a significant influence on the tribological behaviour. An improved understanding of these effects is a prerequisite in an attempt towards controlling friction and wear at high temperatures. The main aim of this work is to investigate the formation of oxide layers and near surface transformed layers during tool steel and boron steel interaction at elevated temperatures and their relation to the friction and wear response. The results from sliding wear tests showed that under favourable conditions of temperature and load, a reduction of wear by three orders of magnitude and reduced friction by 50% was obtained. This was attributed to the formation of a composite layer structure involving a refined workhardened layer and a protective oxide layer on top. In the case of three body abrasive wear of boron steel, a reduction in wear rate when temperature increased (100–200 °C) has also been found. This reduction in three-body wear is due to the formation of a workhardened layer with a mechanically mixed layer of wear debris and fragmented silica particles on top. At higher temperatures (>500 °C), the softer matrix due to recrystallisation and phase transformations was unable to maintain a lower wear rate despite the presence of embedded fragmented silica particles.

The usage of ultra-high-strength boron steel (UHSS) in automotive industry has increased rapidly in the recent past. Forming of UHSS components is performed at elevated temperatures, which also offers the possibility of hardening through quenching directly after forming. However, the influence of hardening on friction and wear during relative sliding between the tool and the workpiece is unclear. Therefore, the friction and wear characteristics at elevated temperatures of hardened and unhardened UHSS and tool steel pairs are investigated in this study. The results show that both friction and wear at all the investigated temperatures are affected by hardening of the UHSS. For uncoated UHSS, the hardening resulted in lower friction and the tool wear increased at low temperatures, but was not affected at elevated temperatures. This was attributed to the higher hardness after hardening combined with the presence of an oxide scale on the UHSS after heating and quenching. For Al-Si-coated UHSS, the hardening reduced friction and tool steel wear at elevated temperatures, and also reduced the wear of the Al-Si-coated high-strength steel at low temperature mainly owing to the formation of an intermetallic layer on the Al-Si-coated UHSS surface after exposure to elevated temperatures.

Abrasive wear in industrial applications such as mining, materials handling and agricultural machinery constitutes a large part of the total wear. Hardened high strength boron steels are known for their good wear resistance and mechanical properties, but available results in the open literature are scarce. This work aims at investigating how different quenching techniques affect the two-body abrasive wear resistance of hardened high strength boron steels. Furthermore, the wear as a function of depth in thicker hardened high strength boron steel plates has also been studied. The material characterisation has been carried out using microhardness, SEM/energy dispersive spectroscopy and three-dimensional optical surface profilometry. The results have shown that water quenched and tool quenched high strength boron steel had similar wear resistance. The main wear mechanisms appear to be microcutting combined with microfatigue. Workhardening during the abrasion process has been found to affect the abrasive wear.

The increasing interest in liquid metal cooled nuclear reactors provides technical and scientific challenges such as the understanding, prevention, and prediction of the degradation of materials in liquid lead. Critical components include the fuel rods, heat exchanger tubes, and pump impellers. These functional elements are exposed to mechanical loading (up to 40 MPa), high temperatures (450–550 °C), and fluid-induced vibrations (up to 25 Hz). Under such conditions, fretting wear occurs between e.g., the spacer wire and the outer surface of the fuel or heat exchanger tubes. This work is aimed to establish a laboratory-scale fretting wear test setup and develop test methodology to enable systematic material characterisation in liquid metal environments. The results obtained by using the described methodology indicate that adhesive wear is the dominant degradation mechanism, and 316L stainless steel shows a higher coefficient of friction but a lower wear volume/tribolayer volume compared to 100Cr6 bearing steel. These results are in agreement with those reported in open literature and demonstrates the suitability of the presented method for conducting fretting tests and analysis for various materials and contact configurations in liquid lead environment.

In recent years, additive manufacturing (AM) of metallic materials has achieved the production of virtually fully dense parts, extending the range of potential applications. There is a growing interest in the use of AM to produce forming tools for hot stamping. The possibilities of locally tailoring the die material to tackle wear challenges and producing more complex geometries to improve die cooling are key-features driving that interest. However, there is a lack of knowledge concerning the tribological behavior of AM materials, particularly at high temperature, as well as the influence surface finishing processes after additive manufacturing. The aim of this study is to investigate the high temperature friction and wear behavior of a tool steel, produced by selective laser melting, within the context of hot forming of AlSi-coated boron steel. A high temperature strip drawing tribometer was used to perform sliding tests at 600 °C and 700 °C. Three different surface finishes were used for the AM samples: ground, milled and shot-blasted. A conventionally produced steel with the same chemical composition and a ground surface finish was used as a reference. At 600 °C, a similar stable coefficient of friction of 0.4 was observed for both materials and all surface topographies. At 700 °C, all tests resulted in a sudden increase in friction up to 0.9 due to local rupture of the AlSi-coating, severe material transfer and ploughing. The wear mechanisms observed for the ground surfaces, both AM and reference tool steel, were a combination of adhesive material transfer and abrasive material removal that promotes material pile-up, resulting in wedge formation on the tool steel surface. The characteristic wedge formation was not common in the milled surface. This is attributed to strain-hardening and topographical features from the finishing process. For the shot-blasted AM surface, deformation and flattening of the large asperities was observed, as well as material transfer. Subsurface deformation associated with high adhesion during sliding was observed, mainly for the ground surfaces. The milled surface resulted in the least amount of tool steel transfer onto the counter body, while the shot-blasted one resulted in the largest amount. AM and reference ground tool steel showed very similar friction and wear behavior in this tribosystem.

Additive manufacturing (AM) of ferrous alloys has achieved a technological maturity level that allows for the production of high-performance steel components. Among these alloys, tool steels and maraging steels can be used in die manufacturing for different hot forming processes as both materials have good mechanical properties and thermal stability. In recent years, several works have successfully produced these alloys through selective laser melting, focusing on the optimization and characterization of their microstructure and mechanical properties. However, few studies look into the tribological aspects of these newly produced materials and even fewer consider high temperature friction and wear performance. Therefore, there is a need to understand the high temperature tribological response of tool materials produced by additive manufacturing. In this context, the aim of this work is to investigate the tribological behavior of a tool steel and a maraging steel, both produced by selective laser melting, at different elevated temperatures. A conventionally produced tool steel was used as reference. A reciprocating sliding tribometer was used to perform tests at 40 °C, 200 °C and 400 °C. The counter-body was a WC-Co cemented carbide. Friction and wear of the AM tool steel and reference tool steel were very similar, thus no signs of the AM route having an impact on the tribological behavior were observed. Both tool steels showed reduced wear volume with increasing temperature, while the opposite was observed for their respective WC-Co counter-bodies. Higher temperature also resulted in increased amount of W transferred onto the tool steel wear scars. AM maraging steel showed higher and more unstable friction level, as well as a much larger wear volume at all temperatures; material transfer onto WC-Co was observed at certain temperatures. Overall, tribolayer formation (or lack thereof) was the dominant aspect in the tribological responses.

The Ni-TiO2 and Ni-CeO2 composite coatings with varying hydrophilic/hydrophobic characteristics were fabricated by the electrodeposition method from a tartrate electrolyte at ambient temperature. To meet the requirements of tight regulation by the European Chemicals Agency classifying H3BO3 as a substance of very high concern, Rochelle salt was utilized as a buffer solution instead. The novelty of this study was to implement a simple one-step galvanostatic electrodeposition from the low-temperature electrolyte based on a greener buffer compared to traditionally used, aiming to obtain new types of soft-matrix Ni, Ni-CeO2, and Ni-TiO2 coatings onto steel or copper substrates. The surface characteristics of electrodeposited nickel composites were evaluated by SEM, EDS, surface contact angle measurements, and XPS. Physiochemical properties of pure Ni, Ni-CeO2, and Ni-TiO2 composites, namely, wear resistance, microhardness, microroughness, and photocatalytic activity, were studied. Potentiodynamic polarization, EIS, and ICP-MS analyses were employed to study the long-term corrosion behavior of coatings in a 0.5 M NaCl solution. Superior photocatalytic degradation of methylene blue, 96.2% after 6 h of illumination, was achieved in the case of Ni-TiO2 composite, while no substantial change in the photocatalytic behavior of the Ni-CeO2 compared to pure Ni was observed. Both composites demonstrated higher hardness and wear resistance than pure Ni. This study investigates the feasibility of utilizing TiO2 as a photocatalytic hydrophilicity promoter in the fabrication of composite coatings for various applications.

Hot stamping is a forming process widely used in the manufacturing of structural components in automobiles. It is a versatile process that enables the fabrication of complex-shaped components with high strength. It also facilitates the manufacturing of components that incorporate high-strength sections and high-ductility sections, by controlling the cooling rate. The process is versatile in terms of the microstructures and mechanical properties that can be obtained. This versatility, however, puts high demands on the materials pertaining their stability, wear resistance, costs, etc. This study has focused on understanding the effect of temperature on the tribological response of different tool materials when these are exposed to high temperatures. The results show that friction significantly stabilises with increased temperature for most tool steels. One tool steel behaves more unstably at high temperature, and this is attributed to the presence of Cr7C3, MoO3, and VO and severe wear on the workpiece material. The most severe wear on the workpiece is caused by a partially melted interdiffusion layer, which facilitates the detachment of the Al-Si coating and subsequent transfer onto the tool; this effect is maximised at the highest temperatures of the workpiece. An important finding is that friction and material transfer severity decrease as the workpiece temperature decreases, and friction is stabilised as tool temperature increases without minimising wear or the average friction coefficient.

Damage of forming tools at elevated temperatures is a major problem in metal forming processes such as hot stamping. Of special importance is the occurrence of adhesion and gross material transfer from the work-piece to the tool surface, a phenomenon known as galling. This type of damage adversely affects the quality of the produced parts and process economy due to frequent maintenance for refurbishing and/or replacing the tools. In view of its importance, several studies on galling have been conducted. However most of these studies pertain to conventional or cold forming processes.In this work, a systematic analysis of damage mechanisms of actual hot stamping tools and extensive high temperature tribological tests to investigate the tool-workpiece interaction have been carried out. The analysis of worn hot forming tools revealed that the main damage mechanisms encountered during hot forming of Al-Si coated ultra-high strength steel (UHSS) are fatigue, corrosion and material transfer. Amongst these mechanisms, material transfer (galling) is the most important from quality and productivity points of view. It was found that galling onto the tool steel is caused by accumulation and compaction of wear debris. Lumps of transferred material are formed through agglomeration of wear debris. The preferential sites for the accumulation of debris are defects on the tool surface or grinding marks generated during refurbishing. Hence, the surface topography of the tool is of great importance for controlling the material transfer. The most relevant parameters that influence galling during the interaction of Al-Si coated UHSS and tool steel were studied through laboratory tribological tests. It was found that controlling parameters such as Rvk, Rpk, Rsk and Sm can minimise galling on untreated tool steels. Furthermore, it was observed that sliding parallel to the surface lay of the tool results in reduced galling as the debris can escape the contact through the valleys of the tool surface.The usage of different PVD hard coatings, such as AlCrN, TiAlN and DLC, were considered as a possibility to alleviate galling. Additionally, the tribological response of plasma nitrided tool steel with and without a post oxidation treatment was also studied. The use of hard PVD coatings on the tool steel resulted in severe adhesion and material transfer during sliding against Al-Si coated UHSS at elevated temperature. This was due to the affinity between the constituents of the coating of the tool and the Al-Si coating. Plasma nitrided tool steel with post oxidation treatment showed negligible galling mainly due to the formation of protective oxide layers on the surface of the Al-Si coated steel. The formation of these layers reduced wear of the Al-Si coating and consequently the generation of Al-Si wear debris. The protective oxide layers are formed primarily by wear debris from the outermost layer of the post oxidised plasma nitrided tool steel. Heat treatments of the Al-Si coating revealed that the phases present in the coating are directly linked to the galling behaviour. Without sufficient temperature or time, stable and harder phases are not formed. This greatly increases the occurrence of galling by severe adhesion as the outermost layer of the coating (unalloyed Al) and the tool steel have high affinity. If the harder phases are formed, the mechanism for galling changes to accumulation and compaction of wear debris from the Al-Si coating and direct adhesion between the Al-Si coated UHSS and the tool steel is reduced.

In recent years, the use of ultra high-strength steels (UHSS) as structural reinforcements and in energy-absorbing systems in automobiles has increased rapidly; mainly in view of their favourable strength to weight ratios. However, due to their high strength, the formability of UHSS is poor, thus complex-shaped UHSS components are invariably produced through hot-metal forming processes. The use of hot stamping or press hardening, which was developed during the 1970’s in northern Sweden, has become increasingly popular for the production of ultra high strength steels. In hot stamping, different tribological problems arise when the tool and work-piece interact during the forming process at elevated temperatures. Wear and surface damage of forming tools can be detrimental to the quality of the final product and these can also have an adverse impact on the process economy due to frequent maintenance or replacement of tools. In this work, a literature review pertaining to tribology of hot sheet metal forming has been carried out. This review has revealed that the awareness of tribology and its application in metal forming processes at high temperature has increased in the recent years. A considerable amount of work has been done to enhance the understanding of the response of different materials and parameters involved and also to improve the process itself. However, despite these developments, there exist major gaps in knowledge pertaining to the occurrence of friction and wear in hot sheet metal forming. Extensive experimental studies have thus been undertaken to bridge some of the knowledge gaps related to tool wear and failure mechanisms in the hot stamping process. These studies have involved both the systematic analysis of actual worn tools as well as parametric tribological investigations in the laboratory. The analysis of worn tools showed that friction is a crucial parameter in their operating life. It was observed that severe mechanical stresses are generated due to high friction during the work-piece/tool interaction. As a result of the cyclic thermal and mechanical loads imposed during the hot forming process, the stresses generated eventually lead to the occurrence of fatigue damage at the tool surface. Another important mechanism observed was material transfer from the work-piece to the tool surface. This is particularly common and detrimental in hot forming of coated work-piece material. The most common coating applied to the ultra high strength steel is a hot dip aluminium based coating, commonly referred to as Al-Si coating. The parametric studies carried out were aimed at understanding of the initiation mechanisms of material transfer from the Al-Si coated steel to the tool material. The results showed that severe galling occurs by accumulation and compaction of wear debris and becomes enhanced in tools having rough surfaces. The roughness defects on the surface promote accumulation of wear particles. Furthermore, high contact pressure also enhances the compaction of wear debris and consequently the severity of material transfer. It was observed that the severity of galling can be reduced by the use of smooth and hard surfaces. Additionally, the use of different PVD coatings on the tool steels showed an increased tendency on adhesion, causing a severe material transfer onto the tool surface.

Galling is a severe form of adhesive wear encountered in metal forming operations. In hot stamping, an Al-Si coating is normally applied onto the ultra-high-strength steels to prevent decarburisation and to improve the corrosion resistance of the steel. Material transfer occurring from the coated ultra-high-strength steel to the tool surface has been identified as major issue in hot stamping. This transferred material impairs the quality of the produced parts and at the same time, it increases the costs of maintenance of the tools. This work focuses on the understanding of surface topography parameters and their effect on galling. Surface roughness level and orientation of the surface lay on tool surface have been studied. The results showed that a single parameter of the surface topography is not enough to describe the resistance to galling. Parameters such as Rv, Rp and Rsk are also important to consider in order to rank the galling resistance of the surface. The sliding direction with respect to the surface lay also had a significant influence on galling; sliding in the direction parallel to it resulted in substantially reduced material transfer.

In metal forming operations such as form fixture hardening, the interaction between the tools and the work-piece is strongly influenced by the tribological properties at the interface. Damage or excessive wear of the tools can be detrimental to the quality of the final component and it also has an impact on the process economy due to increased maintenance or more frequent replacement of tools. The objective of this study was to investigate the damage mechanisms encountered in real form fixture hardening tools in order to understand the causes of tool failure and ultimately to come up with possible solutions for this problem.Advanced techniques such as Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS) were used for obtaining an in-depth understanding of the different phenomena involved in the failure of form fixture hardening tools. Two different tools having different hardness values and microstructures that had been used in production were analysed.The damage mechanisms found included abrasive and adhesive wear, material transfer, corrosion and mechanical and thermal fatigue. The main damage mechanism was found to be cracking caused by mechanical stresses on the surface. Although both tools presented similar types of damage, the severity was different and it was strongly influenced by the microstructure.

Occurrence of galling in hot forming is detrimental to the quality of produced parts and process economy. Material transfer from Al-Si coated work-piece to the tool material has been studied in this work. PVD coatings (AlCrN, TiAlN and DLC) on tool steel substrate have been considered as well as plasma nitriding and their tribological behaviour was compared to the case of an untreated tool steel. Galling initiates through accumulation and compaction of wear debris when untreated tools are used whereas the PVD coatings resulted in increased galling due to adhesion. Plasma nitrided tool steel showed negligible galling due to formation of glaze layers and the formation of such layers depends on the occurrence of wear of the nitrided tool steel.

In the automotive industry, a significant amount of components are formed using hot stamping. This process allows formation of complex shapes whilst controlling microstructure and mechanical properties of the end product. There are different tribological challenges encountered within the process due to the elevated temperature. The current studies focused on the tribological behaviour of different tool steels sliding against Al-Si coated steel (typically used in hot stamping) at different temperatures. It was observed that the tool steel temperature and the work-piece material temperature had a direct effect on friction level and stability and on the wear mechanisms; particularly in the occurrence of adhesive wear and material transfer onto the tool steels. In general, larger amount of material transfer and higher friction was observed with lower temperatures on the tool steel (~200˚C) and higher temperature of the work-piece material (~900˚C). The presence of oxides on the tool steel reduced the severity of material transfer and stabilised friction. However, the tool steel composition also affected the effectiveness in the stabilisation of the friction coefficient

In the automotive industry, a significant amount of components are formed using hot stamping. This process allows formation of complex shapes whilst controlling microstructure and mechanical properties of the end product. There are different tribological challenges encountered within the process due to the elevated temperature. The current studies focused on the tribological behaviour of different tool steels sliding against Al-Si coated steel (typically used in hot stamping) at different temperatures. It was observed that the tool steel temperature and the work-piece material temperature had a direct effect on friction level and stability and on the wear mechanisms; particularly in the occurrence of adhesive wear and material transfer onto the tool steels. In general, larger amount of material transfer and higher friction was observed with lower temperatures on the tool steel (~200˚C) and higher temperature of the work-piece material (~900˚C). The presence of oxides on the tool steel reduced the severity of material transfer and stabilised friction. However, the tool steel composition also affected the effectiveness in the stabilisation of the friction coefficient.

Galling is a severe form of adhesive wear encountered in metal forming operations. In hot stamping, an Al-Si coating is normally applied onto the ultra high strength steels (UHSS) to prevent decarburisation and improve the corrosion resistance of the steel. Material transfer occurring from the coated UHSS to the tool surface has been identified as major issue in hot stamping. It is a known problem as this transferred material impairs the quality of the produced parts as well as increases the costs of maintenance of the tools. The present work focuses on the understanding of surface topography parameter and their effect on galling. Surface roughness level and orientation of the roughness marks (lay) on tool surface have been studied. The results showed that a single parameter of the surface topography is not enough to describe the resistance to galling. Parameters such as Rp, Rv and Rsk are also important to consider in order to rank the galling resistance of the surface. The sliding direction with respect to the surface lay also had a significant influence on galling; sliding in the direction parallel to it resulted in substantially reduced material transfer.

Galling is a severe form of adhesive wear associated with both cold and hot metal forming operations. In hot sheet metal forming of Al-Si-coated ultrahigh-strength steel (UHSS), transfer occurs from the coated UHSS to the tool surface. This leads to poor quality of produced parts, damage of expensive tooling, and increased downtime for maintenance of the tools. This study thus aims at identifying the salient mechanism(s), which give rise to initiation/occurrence of galling at elevated temperatures. This has been accomplished by analysing actual hot forming tools and through systematic parametric tribological investigations in the laboratory. The analysis of the actual tools has shown that the transferred layer consists of Al, Si, and Fe. The structure of the transferred materials is composed of sintered/compacted wear particles. The parametric study has shown that galling is dependent on the operating conditions. A strong relationship between the contact pressure and material transfer has been observed. The severity of galling is lower for smoother surfaces at low contact pressure. However, at high contact pressure, the influence of roughness under these conditions is insignificant. It has also been observed that hard-tool steel substrates reduce the severity of galling, particularly, at high contact pressure.

The increasing demands for light-weight components in vehicles contribute to the global expansion of hot sheet metal forming technologies. Structural components are typically produced using hot stamping of ultra-high strength steel (UHSS). This process allows forming of complex shapes whilst enabling control of the mechanical properties of the end product. Interest in zinc coated UHSS has increased in recent years in view of the corrosion protection it provides to the final components. There is a need for increased understanding of its tribological behaviour during the interaction with tool steel at elevated temperatures. In this work, tribological studies have been carried out in a novel hot strip tribometer. The aim was to study the effect of different operating conditions on the tribological behaviour of zinc coated UHSS sliding against a hot-work tool steel under un-lubricated conditions. The parameters studied in this work were; temperature, ranging from 400 °C to 700 °C; and contact pressure, from 5 to 30 MPa. The UHSS was initially heated up to austenitising temperature (840 °C) and then cooled down to the testing temperature. Upon stabilisation of temperature, the load was applied and sliding was carried out for a total of 1500 mm at 100 mm/s. The results showed a trend towards decreasing average coefficient of friction as temperature and contact pressure increased. Unstable friction behaviour was observed at low temperature (400 °C) and high contact pressure (30 MPa) whilst higher temperatures (600 °C) facilitated the development of a low and stable friction behaviour. It is proposed that the friction behaviour is controlled by the properties of the zinc phases in the coating developed during heating of the UHSS. The combination of high temperature and sliding conditions result in the removal of the uppermost oxide layer and the phases beneath control the friction behaviour.

The usage of the hot stamping process is of great importance due tothe high demands for production of ultra-high strength steels (UHSS).An Al-Si coating is normally applied to the steel to preventdecarburisation and scaling during heating and to improve thecorrosion resistance of the final component. During heating, the Al andthe Si from the coating combine with the Fe from the steel substrate toform hard intermetallic phases. Little is known about the influence ofthe heating conditions on the tribological behaviour of the Al-Sicoating during interaction with tool steels. The present workinvestigated different heat treatment parameters and the influencethey had on the microstructure of the coating and the gallingbehaviour. With low alloying temperatures (700C), severe gallingoccurred and increasing the alloying temperature to 900C resulted inalmost negligible material transfer. The reduction in galling wasassociated to the development of Fe2Al5 and FeAl2 at the surface.

In recent years, the usage of the hot stamping process has increased due to the high demands for production of ultra-high strength steels (UHSS). In particular, more studies are being carried out pertaining to the Al-Si coated UHSS to understand its behaviour during the forming process as well as the performance of the produced parts. The Al-Si coating is applied to the UHSS with the aim of reducing decarburisation and to avoid the formation of thick oxide scales on the work-piece during the heating stage. Usage of the Al-Si coating has the added benefit of greatly improving the corrosion resistance and paintability of the produced components [1,2].

The usage of the hot stamping process is of great importance due to the high demands for production of ultra-high strength steels (UHSS). An Al-Si coating is normally applied to the steel to prevent decarburisation and scaling during heating and to improve the corrosion resistance of the final component. During heating, the Al and the Si from the coating combine with the Fe from the steel substrate to form hard intermetallic phases. Little is known about the influence of the heating conditions on the tribological behaviour of the Al-Si coating during interaction with tool steels. The present work investigated different heat treatment parameters and the influence they had on the microstructure of the coating and the galling behaviour. With low alloying temperatures (700˚C), severe galling occurred and increasing the alloying temperature to 900˚C resulted in almost negligible material transfer. The reduction in galling was associated to the development of Fe2Al5 and FeAl2 at the surface.

Press hardening tools are expected to operate satisfactorily for thousands of forming cycles before being replaced. An approach to enhance the durability of tools in operation, and improve the process economy, is to refurbish them by depositing new material on the tool through welding, or hardfacing, when its surface is adversely damaged. The welding process changes the microstructure of the original tool steel and the deposited material also has a different microstructure, as well as different chemical composition to facilitate the welding process. Dissimilar friction behaviour between the original tool steel and the welded material can lead to unstable friction forces during forming. This work therefore focuses on understanding the tribological behaviour of different hardfacing materials, deposited through TIG welding, and their interaction with Al-Si coated boron steel at high temperatures. The tribological behaviour is evaluated using a hot-strip drawing tribometer capable of simulating the sliding conditions in the press hardening process. The effect of temperature on the microstructure and on the friction and wear behaviour of the weld material is also studied by conducting tests at room temperature and at 500°C. In general, friction and the governing wear mechanisms are similar for the evaluated hardfacing materials. Friction stability was observed to be closely related to the temperature of the workpiece material. Temperature of the tool significantly affects the formation and structure of material transfer layers onto tool steels.