Tribological behaviour of high thermal conductivity tool steels for hot stamping

In the last years, the use of High Strength Steels (HSS) as structural parts in car manufacturing, has rapidly increased thanks mainly to their favourable strength to weight ratios and stiffness, which allow a reduction of the fuel consumption to accommodate the new restricted regulations for CO2 emissions control, but still preserving or even enhancing the passengers’ safety. However, the formability at room temperature of HSS is poor, and for this reason, complex-shaped HSS components are produced applying the plastic deformation of the sheet metal at high temperature. The use of hot stamping technology, which was developed during the 70’s in Sweden, has become increasingly used for the production of HSS for the car body-in-white. By using this technology, several improvements have been made, if compared with the forming at room temperature, such as the reduction of spring back and the forming forces, the production of more complex shapes, a more accurate microstructure control of the final piece and the achievement of components with high mechanical properties. The hot stamping process of HSS parts consists mainly in heating a metal sheet up to austenitization temperature and then a simultaneous forming and hardening phase in closed dies, water-cooled, to obtain a fully martensitic microstructure on the final components; in this way, ultimate tensile strength passes from 600 MPa up to 1500-1600 MPa. Anyway, several tribological issues arise when the die and metal sheet interact during the forming process at elevated temperatures; the absence of any types of lubricant due to elevate process temperature and in order to preserve the quality of the part for the later stages of the process chain, leads to high friction forces at interface; moreover, and the severe wear mechanisms together with surface damage of forming dies, can alter the quality of the component and can also have an high impact on the process economy due to frequent windows-maintenance or reground of tools. Furthermore, considering that the thermal conductivity of the die material influences the cooling performance, obtained during the quenching phase, and being the quenching time the predominant part of the cycle time, the productivity of the process is influenced too. On this base tool steels play a capital role in this process, as they strongly influence the properties of the obtained final product and have a strong impact to investment and maintenance costs. The survey of the technical and scientific literature shows a large interest in the development of different coatings for the blanks from the traditional Al-Si up to new Zn-based coating and on the analysis of hard PVD, CVD coatings and plasma nitriding, applied on dies. By contrast, fewer investigations have been focused on the development and test of new tools steels grades capable to improve the wear resistance and the thermal properties that are required for the in-die quenching during forming. The research works reported are focused on conventional testing configurations, which are able to achieve fundamental knowledge on friction behaviour, wear mechanisms and heat transfer evaluation, with both a high accuracy for the process parameters and less information about situations that replicate the thermal-mechanical conditions to which the forming dies are subject during the industrial process. Alternatively, the tribological performance have been studied through costly and time-consuming industrial trials but with a lower control on process parameters. Starting from this point of view, the main goal of this PhD thesis is to analyse the tribological performance in terms of wear, friction and heat transfer of two new steel grades for dies, developed for high-temperature applications, characterized by a High Thermal Conductivity with the purpose to decrease the quenching time during the hot stamping process chain and overcome the limits in terms of process speed. Their performances are compared with a common die steel grade for hot stamping applications. To this aim, a novel simulative testing apparatus, based on a pin on disk test, specifically designed to replicate the thermo-mechanical cycles of the hot stamping dies, was used to evaluate the influence of different process parameters on the friction coefficient, wear mechanisms and heat transfer at interface die-metal sheet. Unlike other research works reported in the literature, which individually analyse the friction, the wear mechanisms and thermal aspects, by means of the methodology used in this thesis, the tribological characterization as a whole is obtained by means of a single approach, in order to analyse the simultaneous global evolution of the tribological system

Temperature Effects on Organic Lubricants in Cold Forging of AA1050 Alloy

Abstract Lubricants have a key role in cold forging when pressures can reach extreme levels, since they contribute to reduce the high frictional forces occurring at the interface between the tools and billet. However, the adiabatic heating due to the high deformation rates may influence their performances with unpredictable consequences on the process stability. The objective of the research work is to investigate the friction behaviour of new environmental-friendly solid lubricants under process conditions with particular attention to the dies temperatures. The case study refers to the impact backward extrusion of AA1050 alloy cans. The newly developed testing set-up allows heating up the dies and the billet in order to reproduce controlled conditions of the tool temperature in the range 20-200° C. By matching the extrusion loads from the experiments carried out at different temperatures and the results of numerical simulations, the friction factors for each lubricant were determined

Mille general practice governance (MilleGPG): an interactive tool to address an effective quality of care through the Italian general practice network.

The General Practitioner (GP) is the "gate-keeper" in patients' treatment and management. Herein, the use of Electronic Medical Records (EMR) could represent an effective support for GPs. Software capable of managing EMRs are available and they can be functional in adopting treatment guidelines by means of computerized prompts and reminders systems. These tools can be also programmed to include clinical algorithms with which to measure the quality of care to make possible the identification of clinical issues, and to take actions for addressing them. Given that similar tools were not available in Italy, we developed MilleGPG, an interactive tool aimed to evaluate, and subsequently improve the quality of care among patients with comorbidities

New genetic loci link adipose and insulin biology to body fat distribution.

Body fat distribution is a heritable trait and a well-established predictor of adverse metabolic outcomes, independent of overall adiposity. To increase our understanding of the genetic basis of body fat distribution and its molecular links to cardiometabolic traits, here we conduct genome-wide association meta-analyses of traits related to waist and hip circumferences in up to 224,459 individuals. We identify 49 loci (33 new) associated with waist-to-hip ratio adjusted for body mass index (BMI), and an additional 19 loci newly associated with related waist and hip circumference measures (P < 5 × 10(-8)). In total, 20 of the 49 waist-to-hip ratio adjusted for BMI loci show significant sexual dimorphism, 19 of which display a stronger effect in women. The identified loci were enriched for genes expressed in adipose tissue and for putative regulatory elements in adipocytes. Pathway analyses implicated adipogenesis, angiogenesis, transcriptional regulation and insulin resistance as processes affecting fat distribution, providing insight into potential pathophysiological mechanisms

Genetic associations at 53 loci highlight cell types and biological pathways relevant for kidney function.

Reduced glomerular filtration rate defines chronic kidney disease and is associated with cardiovascular and all-cause mortality. We conducted a meta-analysis of genome-wide association studies for estimated glomerular filtration rate (eGFR), combining data across 133,413 individuals with replication in up to 42,166 individuals. We identify 24 new and confirm 29 previously identified loci. Of these 53 loci, 19 associate with eGFR among individuals with diabetes. Using bioinformatics, we show that identified genes at eGFR loci are enriched for expression in kidney tissues and in pathways relevant for kidney development and transmembrane transporter activity, kidney structure, and regulation of glucose metabolism. Chromatin state mapping and DNase I hypersensitivity analyses across adult tissues demonstrate preferential mapping of associated variants to regulatory regions in kidney but not extra-renal tissues. These findings suggest that genetic determinants of eGFR are mediated largely through direct effects within the kidney and highlight important cell types and biological pathways

Tribological behaviour of high thermal conductivity tool steels for hot stamping

In the last years, the use of High Strength Steels (HSS) as structural parts in car manufacturing, has rapidly increased thanks mainly to their favourable strength to weight ratios and stiffness, which allow a reduction of the fuel consumption to accommodate the new restricted regulations for CO2 emissions control, but still preserving or even enhancing the passengers’ safety. However, the formability at room temperature of HSS is poor, and for this reason, complex-shaped HSS components are produced applying the plastic deformation of the sheet metal at high temperature. The use of hot stamping technology, which was developed during the 70’s in Sweden, has become increasingly used for the production of HSS for the car body-in-white. By using this technology, several improvements have been made, if compared with the forming at room temperature, such as the reduction of spring back and the forming forces, the production of more complex shapes, a more accurate microstructure control of the final piece and the achievement of components with high mechanical properties. The hot stamping process of HSS parts consists mainly in heating a metal sheet up to austenitization temperature and then a simultaneous forming and hardening phase in closed dies, water-cooled, to obtain a fully martensitic microstructure on the final components; in this way, ultimate tensile strength passes from 600 MPa up to 1500-1600 MPa. Anyway, several tribological issues arise when the die and metal sheet interact during the forming process at elevated temperatures; the absence of any types of lubricant due to elevate process temperature and in order to preserve the quality of the part for the later stages of the process chain, leads to high friction forces at interface; moreover, and the severe wear mechanisms together with surface damage of forming dies, can alter the quality of the component and can also have an high impact on the process economy due to frequent windows-maintenance or reground of tools. Furthermore, considering that the thermal conductivity of the die material influences the cooling performance, obtained during the quenching phase, and being the quenching time the predominant part of the cycle time, the productivity of the process is influenced too. On this base tool steels play a capital role in this process, as they strongly influence the properties of the obtained final product and have a strong impact to investment and maintenance costs. The survey of the technical and scientific literature shows a large interest in the development of different coatings for the blanks from the traditional Al-Si up to new Zn-based coating and on the analysis of hard PVD, CVD coatings and plasma nitriding, applied on dies. By contrast, fewer investigations have been focused on the development and test of new tools steels grades capable to improve the wear resistance and the thermal properties that are required for the in-die quenching during forming. The research works reported are focused on conventional testing configurations, which are able to achieve fundamental knowledge on friction behaviour, wear mechanisms and heat transfer evaluation, with both a high accuracy for the process parameters and less information about situations that replicate the thermal-mechanical conditions to which the forming dies are subject during the industrial process. Alternatively, the tribological performance have been studied through costly and time-consuming industrial trials but with a lower control on process parameters. Starting from this point of view, the main goal of this PhD thesis is to analyse the tribological performance in terms of wear, friction and heat transfer of two new steel grades for dies, developed for high-temperature applications, characterized by a High Thermal Conductivity with the purpose to decrease the quenching time during the hot stamping process chain and overcome the limits in terms of process speed. Their performances are compared with a common die steel grade for hot stamping applications. To this aim, a novel simulative testing apparatus, based on a pin on disk test, specifically designed to replicate the thermo-mechanical cycles of the hot stamping dies, was used to evaluate the influence of different process parameters on the friction coefficient, wear mechanisms and heat transfer at interface die-metal sheet. Unlike other research works reported in the literature, which individually analyse the friction, the wear mechanisms and thermal aspects, by means of the methodology used in this thesis, the tribological characterization as a whole is obtained by means of a single approach, in order to analyse the simultaneous global evolution of the tribological system.Negli ultimi anni, l'utilizzo degli acciai alto resistenziali per sviluppare parti strutturali nell'industria automobilistica è aumentato notevolmente, grazie soprattutto al loro favorevole rapporti resistenza-peso e rigidezza, consentendo una riduzione del consumo del carburante per assecondare le nuove restrizioni in termini di emissioni di CO2 e conservando nel frattempo, la sicurezza dei passeggeri. Tuttavia, la formabilità a temperatura ambiente degli acciai alto resistenziali è scarsa e per questo motivo, i componenti con geometrie complesse sono prodotti applicando la deformazione plastica ad elevata temperatura. L'uso della tecnologia dello stampaggio a caldo, che è stata sviluppata durante gli anni '70 in Svezia, è diventata sempre più popolare per la produzione di parti che costituiscono il telaio delle automobili. Utilizzando tale tecnologia, si sono ottenuti notevoli miglioramenti - se confrontata con la formatura a freddo - come la riduzione del ritorno elastico e delle forze di stampaggio, la possibilità di ottenere geometrie più complesse, un accurato controllo della microstruttura del componente e l'ottenimento di pezzi con elevate proprietà meccaniche. Il processo di stampaggio a caldo di parti in acciaio alto resistenziale consiste principalmente nel riscaldamento di una lamiera fino alla temperatura di austenitizzazione e poi nell’applicazione simultanea della fase di formatura e tempra in stampi chiusi per ottenere una microstruttura martensitica sui componenti finali; in questo modo, il carico di rottura passa da 600 MPa a 1500-1.600 MPa. Tuttavia, diversi problemi tribologici sorgono quando lo stampo e lamiera interagiscono durante il processo di formatura a temperature elevate; l'assenza di qualsiasi tipo di lubrificante a causa delle elevate temperature di processo e per preservare la qualità del pezzo per le successive fasi di lavorazione porta ad elevate forze di attrito all'interfaccia stampo-lamiera e i severi meccanismi di usura insieme ai danni superficiali degli stampi di formatura possono alterare la qualità del prodotto finale e possono anche avere un impatto negativo sull’economia del processo a causa della frequente manutenzione o sostituzione degli stampi. Inoltre, considerando che la conducibilità termica del materiale dello stampo influenza le performance di raffreddamento che possono essere ottenute durante la fase di tempra in stampo e quindi, la produttività del processo, essendo il tempo di tempra la parte predominante del tempo ciclo, gli acciai per stampi ricoprono un ruolo importante in questo processo; influenzano fortemente le proprietà finali del pezzo ed hanno un forte contributo sugli investimenti e costi di manutenzione. Un'analisi della letteratura tecnico-scientifica mostra un grande interesse per lo sviluppo di diversi rivestimenti per le lamiere alto resistenziali, dal tradizionale Al-Si fino al nuovo rivestimento base Zn e sull'analisi di rivestimenti PVD , CVD e nitrurazione plasma da applicare sugli stampi, mentre molte meno indagini sono state focalizzate sullo sviluppo e test di nuovi gradi di acciai per stampi, capaci di migliorare la resistenza all'usura e le proprietà termiche che sono necessari per la tempra in stampo durante la formatura. I lavori di ricerca riportati sono concentrati su configurazioni di test convenzionali, che sono in grado di raggiungere la conoscenza fondamentale sul comportamento dell’attrito, dei meccanismi di usura e della valutazione del trasferimento di calore, con una elevata precisione per quanto riguarda i parametri di processo, ma non riescono a replicare le condizioni termo-meccaniche a cui gli stampi di formatura sono soggetti ciclicamente durante il processo industriale. In alternativa, le prestazioni tribologiche sono studiate attraverso costose prove industriali in termini di tempo e denaro, ma con un basso controllo sui parametri di processo. Partendo da questo punto di vista, l'obiettivo principale di questa tesi è quello di analizzare le prestazioni tribologiche in termini di usura, attrito e di trasferimento di calore di acciai per stampi, sviluppati per applicazioni ad alta temperatura, caratterizzati da una elevata conducibilità termica al fine di diminuire il tempo di tempra durante le fasi dello stampaggio a caldo e superare gli odierni limiti in termini di velocità di processo. Le loro prestazioni sono confrontate con un comune acciaio per stampi utilizzato nella formatura a caldo. A questo scopo, un nuovo apparecchio di prova, basato su un pin on disk test, specificamente progettato per replicare sugli stampi i cicli termo-meccanici del processo della stampa a caldo, è stato utilizzato per valutare l'influenza dei diversi parametri di processo sul coefficiente di attrito, meccanismi di usura e trasferimento di calore all'interfaccia stampo-lamiera. A differenza di altri lavori di ricerca riportati in letteratura, i quali analizzano singolarmente l'attrito, i meccanismi di usura e gli aspetti termici, mediante la metodologia utilizzata in questa tesi, la caratterizzazione tribologica nel suo complesso è ottenuta mediante un unico approccio, al fine analizzare l'evoluzione globale simultanea del sistema tribologico nel suo complesso

Analisi del comportamento tribologico di materiali per stampi in operazioni di stampaggio a caldo di lamiere in 22MnB5. Tribological behaviour of dies materials for hot stamping of 22mnb5

Il lavoro presenta uno studio su lamiere in acciaio al boro alto resistenziale 22MnB5 rivestite, in termini di comportamento tribologico e morfologia, in funzione dei parametri di processo tipici del processo industriale di stampaggio a cald

Analisi del comportamento tribologico di materiali per stampi in operazioni di stampaggio a caldo di lamiere in 22MnB5. Tribological behaviour of dies materials for hot stamping of 22mnb5

Il lavoro presenta uno studio su lamiere in acciaio al boro alto resistenziale 22MnB5 rivestite, in termini di comportamento tribologico e morfologia, in funzione dei parametri di processo tipici del processo industriale di stampaggio a caldoreservedEmbargo permanente per motivi di segretezza e/o di proprietà dei risultati e informazioni di enti esterni o aziende private che hanno partecipato alla realizzazione del lavoro di ricerca relativo alla tes

Tribological behaviour of lubricants in hot stamping of AA6016

The increasing demand for fuel-efficient vehicles has led automotive industry to introduce new alloys in car manufacturing, characterized by a high stiffness and strength-to-weight ratio. Due to their mechanical and chemical properties, aluminium alloys appear potential candidates to replace traditional steels for several parts of the car-body-in-white, even if the limited formability at room temperature, the marked springback and the severe tribological behaviour have often represented important drawbacks in traditional properties. Recently, the use of temperature- assisted processes, such as hot stamping, allows overcoming such limits thanks to substantial increase of the formability and, at the same time, a drastic reduction of springback. However, the choice of proper process parameters, in terms of lubrication at the interfaces between the dies and the blank, and thermal parameters of the dies materials still represent critical points for the feasibility of the process. Recent investigations have proved the limits of assuming constant friction for all the areas of the dies and the steps of the deformations, especially with variable pressures and non-constant temperatures at the interfaces. Such factors, together with the lubricants and the lubricant deposition on the blank, the blank and dies coatings, the surface roughness, the stamping speed are not always considered, despite their considerable influence on the process tribology. In this paper the friction behaviour of commercially available automotive aluminium (AA6016 alloy) is studied. The friction coefficient is measured by means of a new machine for strip draw test at different levels of pressure, temperature, sliding speed and type of lubricant. The results report the investigation of the surface topography of the metal sheets, investigated by Energy-Dispersive X-ray spectroscopy and optical profilometry

Feasibility analysis of lubrication through magneto-rheological fluids in sheet metal forming processes

Magneto-rheological fluids represent one of the most recent technologies introduced in many industrial applications to design fast and reliable electro-mechanical devices. Such fluids, which consist of a dispersion of metallic particles in a fluid carrier, may drastically change their rheological properties when subjected to external magnetic fields with short response times and low power consumption. Up to date, the most common applications of MR fluids regard the integration in mechatronics applications mainly in civil and mechanical fields, where they are often used as active lubricants in rotary components such as bearings. However, no investigations can be found in literature on their tribological performances when applied in metal forming processes. The paper aims at investigating the tribological behaviour of magneto-rheological fluids when applied as lubricants in sheet metal forming processes. To this aim, a new experimental apparatus has been developed to test the friction coefficient at the interface between the tools and the metal sheet when different magnetic fields are applied. The stamping of AA alloys was taken as reference case to select the process parameters in terms of contact pressure, magnetic field and sliding speed