Load-Adapted Surface Modifications to Increase Lifetime of Forging Dies

Diffusion treatments offer possibilities to enhance the performance and the service lifetime of hot forging tools. In combination with coating after nitriding, the surface layer hardness can be further increased. Within the scope of this study, a surface layer hardness above 2,000 HV0.005 was determined for borided or DLC (diamond-like carbon) coated surface layers. An increased surface layer hardness improves the abrasive wear resistance of forging dies. Furthermore, the plastic deformation of thermally softened forging die areas can be reduced. Beside these desirable effects, the ductility of diffusion treated or coated near surface layers is reduced and thermomechanical cracks are promoted. Therefore, additional approaches were developed to improve the thermomechanical crack behavior of forging dies. Patterned plasmanitriding by the use of coverages to prevent areas from nitrogen diffusion, new combination processes of plasmanitrocarburizing (PNC) followed by plasmanitriding (PN) and the innovative boriding were investigated on different abstraction levels. A system of several testing rigs was set up to enable the abstraction of the thermal shock conditions in different stages. The patterned nitriding, boriding and combination plasma process (PN + PNC) were evaluated in a series of industrial field tests to derive recommendations for suitable tool treatments

Wear mechanisms in press hardening of boron steel : identification and laboratory study

Press hardening, also known as hot stamping, is a hot sheet metal forming technique which allows producing lightweight components with complex geometry and outstanding mechanical properties. For this reason, use of press hardened components is steadily increasing in the automotive industry. Several variants of the press hardening process exist. The most common is direct press hardening of AlSi coated boron steel. In this process, a coated sheet metal blank is austenitised, transferred to a press and formed and quenched in a single step. One of the factors affecting the competitiveness of press hardening is tool wear. Press hardening dies require intensive maintenance, which increases production costs and limits the efficiency of the process. In the case of press hardening of AlSi coated boron steel, wear due to material transfer from the coating is particularly severe. The present thesis deals with wear on press hardening tools, following a methodology of integral analysis beginning at the industrial system and ending with laboratory tribological studies. As a first step, a non-destructive methodology for the analysis of industrial forming tools was developed, based in surface replication. Using this technique, the surface of the tool is transferred into a stable and highly accurate polymeric replica. Replicas obtained this way were studied in the laboratory using optical profilometry techniques. This methodology was applied to the characterization of wear in industrial press hardening tools. Replicas were obtained in normal production stops, allowing complete study of the process without affecting production. Analysis of these replicas allowed identifying the main wear mechanisms appearing in press hardening tools. Laboratory tests were then developed to study and obtain full comprehension of the tool-component interaction. These studies were not based in the design of new press hardening tribosimulators; instead, simplified test setups were devised which allowed the isolated study of the relevant wear micromechanisms and the main factors affecting them. In the first test, interaction of aluminium and tool steel was studied at different temperatures. System behaviour was observed to be highly dependent on the generation of hard oxide particles and on mechanical interaction between the surfaces, with adhesive processes taking place at high temperature. In a second series of tests, the role of adhesive forces was studied independently from mechanical interaction, and it was observed that surface engineering techniques modifying the chemical composition of the tool surface (nitriding, coating and application of lubricant) resulted in modified affinity. To complete this study, the effect of mechanical interaction was studied; wear micromechanisms were observed to change based on the surface finish of the tool specimen. Finally, the applicability of PVD coatings on press hardening tools was studied. On the one hand, a test methodology was developed able to characterize the mechanical response of coated steel surfaces at temperatures similar to those in hot metal forming. Samples with different surface engineering solutions were studied to determine how temperature affected system performance, and to evaluate the contribution of substrate nitriding under a PVD coating. On the other hand, an industrial test was carried out using coated production tools on a press hardening shop. Commercial PVD coatings appeared to reduce the buildup rate of transfer layers, but did not eliminate the issue of material transfer. Results of this work show that tool wear appears from a complex combination of chemical and mechanical interaction. While a solution to wear in press hardening still does not exist, the relative importance of these wear micromechanisms can be influenced by modifying the chemical, topographical and mechanical characteristics of the tool surface.L'estampació en calent és una tècnica de conformació en calent de xapa metàl·lica que permet obtenir components de baix pes, gran complexitat geomètrica i excel·lents propietats mecàniques. Per aquesta raó, és un procés particularment atractiu per a la indústria de l'automoció. Existeixen diverses variants d'aquest procés industrial, però la més comú és l'estampació directa d'acer amb recobriment d'Alumini-Silici. En aquest procés, la xapa recoberta es porta a temperatura d'austenització i es trasllada a una premsa, on en un sol pas es du a terme el conformat i el tractament tèrmic. Un dels factors que afecten la competitivitat de l'estampació en calent és el desgast de les eines. Les matrius utilitzades requereixen un manteniment intensiu, cosa que afecta negativament l'eficiència del procés. En el cas de l'estampació d'acer amb recobriment AlSi, el desgast és molt sever i està relacionat amb la transferència de material del recobriment a l'eina. El focus d'aquesta tesi és l'estudi del desgast en estampació en calent seguint una metodologia de treball que abarca des de l'anàlisi d'eines industrials fins al desenvolupament d'assaigs tribològics al laboratori. En un primer pas, es va desenvolupar una metodologia d'assaig no destructiu basada en les rèpliques superficials. Amb aquesta tècnica es genera una còpia de la superfície de l'eina utilitzant un material polimèric, que es pot estudiar al laboratori utilitzant tècniques d'anàlisi topogràfica. Aquesta metodologia va ser aplicada a la caracterització de desgast en eines industrials d'estampació en calent a partir de rèpliques extretes durant parades de la línia de producció. En aquest treball, es van identificar els principals mecanismes de desgast que apareixen en aquest sistema. A continuació, es van desenvolupar assaigs de laboratori que permetessin comprendre totalment la interacció entre eina i component. Aquests estudis no es van centrar en el desenvolupament d'un tribosimulador complex, sinó en utilitzar configuracions experimentals simplificades per a estudiar de forma aïllada i controlada els diferents mecanismes. El primer assaig es va utilitzar per a explorar la interacció entre alumini i acer d'eines a diverses temperatures. El comportament del sistema va resultar ser fortament afectat per la generació de partícules dures d'òxid i la interacció mecànica entre les superfícies, amb una component d'afinitat química a alta temperatura. En un segon dispositiu experimental, la contribució de les forces adhesives sorgides d'aquesta afinitat es va estudiar de forma aïllada. Els resultats obtinguts mostren que els tractaments que modifiquen la composició química de la superfície (nitruració, recobriments i lubricants) afecten de manera important aquestes forces adhesives. L'estudi al laboratori es va completar amb una avaluació de l'efecte de la interacció mecànica: es va poder observar que els micromecanismes de desgast varien en funció de l'acabat superficial de l'eina. Per últim es va estudiar l'aplicabilitat dels recobriments durs PVD a les eines d'estampació en calent. Per una banda, es va desenvolupar un assaig de laboratori per a caracteritzar sistemes d'eines recoberts a temperatures equivalents a les del conformat en calent de metall. Utilitzant aquest assaig, es va avaluar l'efecte de la temperatura sobre el rendiment mecànic d'acer d'eines recobert, així com l'efecte de la nitruració del substrat. Per altra banda, es va dur a terme una prova industrial utilitzant eines d'estampació en calent amb recobriments PVD comercials. Tot i que no es va eliminar el desgast, es va observar que el material transferit s'acumulava a menys velocitat en aquestes eines. Els resultats d'aquesta tesi mostren que el desgast a l'estampació en calent apareix per una combinació de factors d'interacció química i mecànica. La importància relativa d'aquests factors pot modular-se modificant les característiques químiques, mecàniques i topogràfiques de la superfície de l'eina

Wear mechanisms in press hardening of boron steel : identification and laboratory study

Tesi per compendi. La consulta íntegra de la tesi, inclosos els articles no comunicats públicament per drets d'autor, es pot realitzar prèvia petició a l'Arxiu de la UPCPress hardening, also known as hot stamping, is a hot sheet metal forming technique which allows producing lightweight components with complex geometry and outstanding mechanical properties. For this reason, use of press hardened components is steadily increasing in the automotive industry. Several variants of the press hardening process exist. The most common is direct press hardening of AlSi coated boron steel. In this process, a coated sheet metal blank is austenitised, transferred to a press and formed and quenched in a single step. One of the factors affecting the competitiveness of press hardening is tool wear. Press hardening dies require intensive maintenance, which increases production costs and limits the efficiency of the process. In the case of press hardening of AlSi coated boron steel, wear due to material transfer from the coating is particularly severe. The present thesis deals with wear on press hardening tools, following a methodology of integral analysis beginning at the industrial system and ending with laboratory tribological studies. As a first step, a non-destructive methodology for the analysis of industrial forming tools was developed, based in surface replication. Using this technique, the surface of the tool is transferred into a stable and highly accurate polymeric replica. Replicas obtained this way were studied in the laboratory using optical profilometry techniques. This methodology was applied to the characterization of wear in industrial press hardening tools. Replicas were obtained in normal production stops, allowing complete study of the process without affecting production. Analysis of these replicas allowed identifying the main wear mechanisms appearing in press hardening tools. Laboratory tests were then developed to study and obtain full comprehension of the tool-component interaction. These studies were not based in the design of new press hardening tribosimulators; instead, simplified test setups were devised which allowed the isolated study of the relevant wear micromechanisms and the main factors affecting them. In the first test, interaction of aluminium and tool steel was studied at different temperatures. System behaviour was observed to be highly dependent on the generation of hard oxide particles and on mechanical interaction between the surfaces, with adhesive processes taking place at high temperature. In a second series of tests, the role of adhesive forces was studied independently from mechanical interaction, and it was observed that surface engineering techniques modifying the chemical composition of the tool surface (nitriding, coating and application of lubricant) resulted in modified affinity. To complete this study, the effect of mechanical interaction was studied; wear micromechanisms were observed to change based on the surface finish of the tool specimen. Finally, the applicability of PVD coatings on press hardening tools was studied. On the one hand, a test methodology was developed able to characterize the mechanical response of coated steel surfaces at temperatures similar to those in hot metal forming. Samples with different surface engineering solutions were studied to determine how temperature affected system performance, and to evaluate the contribution of substrate nitriding under a PVD coating. On the other hand, an industrial test was carried out using coated production tools on a press hardening shop. Commercial PVD coatings appeared to reduce the buildup rate of transfer layers, but did not eliminate the issue of material transfer. Results of this work show that tool wear appears from a complex combination of chemical and mechanical interaction. While a solution to wear in press hardening still does not exist, the relative importance of these wear micromechanisms can be influenced by modifying the chemical, topographical and mechanical characteristics of the tool surface.L'estampació en calent és una tècnica de conformació en calent de xapa metàl·lica que permet obtenir components de baix pes, gran complexitat geomètrica i excel·lents propietats mecàniques. Per aquesta raó, és un procés particularment atractiu per a la indústria de l'automoció. Existeixen diverses variants d'aquest procés industrial, però la més comú és l'estampació directa d'acer amb recobriment d'Alumini-Silici. En aquest procés, la xapa recoberta es porta a temperatura d'austenització i es trasllada a una premsa, on en un sol pas es du a terme el conformat i el tractament tèrmic. Un dels factors que afecten la competitivitat de l'estampació en calent és el desgast de les eines. Les matrius utilitzades requereixen un manteniment intensiu, cosa que afecta negativament l'eficiència del procés. En el cas de l'estampació d'acer amb recobriment AlSi, el desgast és molt sever i està relacionat amb la transferència de material del recobriment a l'eina. El focus d'aquesta tesi és l'estudi del desgast en estampació en calent seguint una metodologia de treball que abarca des de l'anàlisi d'eines industrials fins al desenvolupament d'assaigs tribològics al laboratori. En un primer pas, es va desenvolupar una metodologia d'assaig no destructiu basada en les rèpliques superficials. Amb aquesta tècnica es genera una còpia de la superfície de l'eina utilitzant un material polimèric, que es pot estudiar al laboratori utilitzant tècniques d'anàlisi topogràfica. Aquesta metodologia va ser aplicada a la caracterització de desgast en eines industrials d'estampació en calent a partir de rèpliques extretes durant parades de la línia de producció. En aquest treball, es van identificar els principals mecanismes de desgast que apareixen en aquest sistema. A continuació, es van desenvolupar assaigs de laboratori que permetessin comprendre totalment la interacció entre eina i component. Aquests estudis no es van centrar en el desenvolupament d'un tribosimulador complex, sinó en utilitzar configuracions experimentals simplificades per a estudiar de forma aïllada i controlada els diferents mecanismes. El primer assaig es va utilitzar per a explorar la interacció entre alumini i acer d'eines a diverses temperatures. El comportament del sistema va resultar ser fortament afectat per la generació de partícules dures d'òxid i la interacció mecànica entre les superfícies, amb una component d'afinitat química a alta temperatura. En un segon dispositiu experimental, la contribució de les forces adhesives sorgides d'aquesta afinitat es va estudiar de forma aïllada. Els resultats obtinguts mostren que els tractaments que modifiquen la composició química de la superfície (nitruració, recobriments i lubricants) afecten de manera important aquestes forces adhesives. L'estudi al laboratori es va completar amb una avaluació de l'efecte de la interacció mecànica: es va poder observar que els micromecanismes de desgast varien en funció de l'acabat superficial de l'eina. Per últim es va estudiar l'aplicabilitat dels recobriments durs PVD a les eines d'estampació en calent. Per una banda, es va desenvolupar un assaig de laboratori per a caracteritzar sistemes d'eines recoberts a temperatures equivalents a les del conformat en calent de metall. Utilitzant aquest assaig, es va avaluar l'efecte de la temperatura sobre el rendiment mecànic d'acer d'eines recobert, així com l'efecte de la nitruració del substrat. Per altra banda, es va dur a terme una prova industrial utilitzant eines d'estampació en calent amb recobriments PVD comercials. Tot i que no es va eliminar el desgast, es va observar que el material transferit s'acumulava a menys velocitat en aquestes eines. Els resultats d'aquesta tesi mostren que el desgast a l'estampació en calent apareix per una combinació de factors d'interacció química i mecànica. La importància relativa d'aquests factors pot modular-se modificant les característiques químiques, mecàniques i topogràfiques de la superfície de l'eina.Postprint (published version

The making of nickel and nickel-alloy shapes by casting, powder metallurgy, electroforming, chemical vapor deposition, and metal spraying

Casting, powder metallurgy, electroforming, metal spraying, and chemical vapor deposition techniques for producing nickel and nickel-alloy shape

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

Powder metallurgy in aerospace research: A survey

The various techniques by which powders can be produced, as pure metals or as alloys, are discussed; the methods by which these powders can be formed into the final parts are explained as well as further processing that may be necessary to meet specific requirements. The NASA developments are detailed, and references are provided for those who wish to obtain further information characteristic of any methodology