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In automotive a lot of electromagnetically, pyrotechnically or mechanically driven actuators are integrated to run comfort systems and to control safety systems in modern passenger cars. Using shape memory alloys (SMA) the existing systems could be simplified, performing the same function through new mechanisms with reduced size, weight, and costs. A drawback for the use of SMA in safety systems is the lack of materials knowledge concerning the durability of the switching function (long-time stability of the shape memory effect). Pedestrian safety systems play a significant role to reduce injuries and fatal casualties caused by accidents. One automotive safety system for pedestrian protection is the bonnet lifting system. Based on such an application, this article gives an introduction to existing bonnet lifting systems for pedestrian protection, describes the use of quick changing shape memory actuators and the results of the study concerning the long-time stability of the tested NiTi-wires. These wires were trained, exposed up to 4years at elevated temperatures (up to 140°C) and tested regarding their phase change temperatures, times, and strokes. For example, it was found that A P-temperature is shifted toward higher temperatures with longer exposing periods and higher temperatures. However, in the functional testing plant a delay in the switching time could not be detected. This article gives some answers concerning the long-time stability of NiTi-wires that were missing till now. With this knowledge, the number of future automotive applications using SMA can be increased. It can be concluded, that the use of quick changing shape memory actuators in safety systems could simplify the mechanism, reduce maintenance and manufacturing costs and should be insertable also for other automotive applications.
Investigation of magnetic effects on austenitic stainless steels after low temperature carburization
(2018)
This work aims at investigating the magnetic effects of austenitc stainless steels which can occur after a low temperature carburisation depending on the alloy. Samples were prepared of different alloys and subjected to a multiple low temperature carburisation to obtain different treatment conditions for each alloy. The layer characterisation was carried out by light microscope and also by hardening profiles and shows that the layer develops with each additional treatment cycle. A lattice expansion could be detected in all treated samples by X-ray diffraction. Magnetisability was measured using Feritscope and SQUID measurements. Not all alloys showed magnetisability after treatment. In addition to MFM measurements, experiments with Ferrofluid were also used to visualize the magnetic areas. These studies show that only about half of the formed layer becomes magnetisable and has a domain-like structure.
Ferromagnetism is of increasing importance in the growing field of electromobility and data storage. In stable austenitic steels, the occurrence of ferromagnetism is not expected and would also interfere with many applications. However, ferromagnetism in austenitic stainless steels after low-temperature nitriding has already been shown in the past. Herein, the presence of ferromagnetism in austenitic steels is discovered after low-temperature carburization (Kolsterizing), which represents a novel and unique finding. A zone of expanded austenite is established on various austenitic stainless steels by low-temperature carburization and the respective ferromagnetism is investigated in relation to the alloy composition. The ferromagnetism occurring is determined by means of a commercial magnetoinductive sensor (Feritscope). Ferromagnetic domains are visualized by magnetic force microscopy and a ferrofluid. X-ray diffraction measurements indicate a clear difference in the lattice expansion of the different alloys. Furthermore, a different appearance of the magnetizable microstructure regions (magnetic domain structure) is detected depending on the grain orientation determined by electron backscatter diffraction (EBSD). Strongly pronounced magnetic domains show no linear lattice defects, whereas in small magnetizable areas linear lattice defects are detected by electron channeling contrast imaging and EBSD.
Thermal shape memory alloys show extraordinary material properties and can be used as actuators, dampers and sensors. Since their discovery in the middle of the last century they have been investigated and further developed. The majority of the industrial applications with the highest material sales can still be found in the medical industry, where they are used due to their superelastic and thermal shape memory effect, e.g. as stents or as guidewires and tools in the minimal invasive surgery. Particularly in recent years, more and more applications have been developed for other industrial fields, e.g. for the household goods, civil engineering and automotive sector. In this context it is worth mentioning that for the latter sector, million seller series applications have found their way into some European automobile manufacturers. The German VDI guideline for shape memory alloys introduced in 2017 will give the material a further boost in application. Last but not least the new production technologies of additive manufacturing with metal laser sintering plants open up additional applications for these multifunctional materials. This paper gives an overview of the extraordinary material properties of shape memory components, shows examples of different applications and discusses European trends against the background of the most recent standard and new production technologies.
The corrosion resistance of stainless steels is massively influenced by the condition of their surface. The surface quality includes the topography of the surface, the structure and composition of the passive layer, and the surface near structure of the base material. These factors are influenced by final physical/chemical surface treatments. The presented work shows significantly lower corrosion resistance for mechanical machined specimens than for etched specimens. It also turns out that the rougher the surface, the lower the corrosion resistance gets. However, there is no general finding which shows if blasted or grinded surfaces are more appropriate, but a dependency on process parameters and the characteristics on corrosive exposure in terms of corrosion behavior. The results show that not only the surface roughness Ra has an influence on corrosion behavior but also the shape of peaks and valleys which are evolved by surface treatments. Imperfections in the base material, like sulfidic inclusions lead to a weaker passive layer, respectively, to a decrease of the corrosion resistance. By using special passivating techniques the corrosion resistance of stainless steels can be increased to a higher level in comparison to common passivation.
This paper compares the surface morphology of differently finished austenitic stainless steel AISI 316L, also in combination with low temperature carburization. Milled and tumbled surfaces were analyzed by means of corrosion resistance and surface morphology. The results of potentiodynamic measurements show that professional grinding operations with SiC and Al2O3 always lead to a better corrosion resistance of low temperature carburized surfaces compared to the untreated reference in the used acidified chloride solution. Big influence on the corrosion resistance of vibratory ground or tumbled surfaces has the amount of plastic deformation while machining, that has to be kept low for austenitic stainless steels. Due to the high ductility, plastic deformation can lead to the formation of meta stable pits that can be initiation points of corrosion. The formation of meta stable pits can be aggravated by low temperature diffusion processes.
Mechanical properties after stretching testings were calcu-lated and experimentally determined via Tempcore method for bar core, bar surface and whole bar cross section. It was displayed on the base of experiments and imitating simulation that deformation in core and surface areas of a bar are equal and therefore influence of structural parameters in the core area is principally decisive for initiating of neck forming in the surface area. The results showed that resistance to destruction of martensite surface layer has rather less effect on bar properties in general in comparison with previous investigations. It is concluded that improvement of core structure quality can help to lower brittleness of the whole bar. It was also proved that used techniques provide good concordance between the obtained results and experimental data. Therefore, the additivity rule for structural components can be used successfully for determination of whole bar parameters, taking into account thickness of surface layer that can be measured easily using hardness sensor. It will simplify practically quality control of products.
Improving the tribological properties of Stainless Steels by low-temperature surface hardening
(2022)
Hot isostatic pressing (HIP) allows the production of complex components geometry. Generally, a high quality of the components is achieved due to the well managed composition of the metal powder and the non-isotropic properties. If a duplex stainless steel is produced, a heat treatment after the HIP-process is necessary to remove precipitations like carbides, nitrides and intermetallic phases. In a new process, the sintering step should be combined with the heat treatment. In this case a high cooling rate is necessary to avoid precipitations in duplex stainless steels. In this work, the influence of the HIP-temperature and the wall thickness on corrosion resistance, microstructure and impact strength were investigated. The results should help to optimize the process parameters like temperature and cooling rate. For the investigation, two HIP-temperatures were tested in a classical HIP-process step with a defined cooling rate. An additional heat treatment was not conducted. The specimens were cut from different sectors of the HIP-block. For investigation of the corrosion resistance, the critical pitting temperature was determined with electrochemical method according to EN ISO 17864. An impact test was used to determine the impact transition temperature. Metallographic investigations show the microstructure in the different sectors of the HIP-block.
The project aims for the development of a new material system from high tensile stainless steel wires as net material with environmentally compatible antifouling properties for off-shore fish farm cages. Therefore, current net materials from textiles (polyamide) shall be partially replaced by high strength stainless steel in order to have a more environmentally compatible system which meets the more severe mechanical loads (waves, storms, predators (sharks)). With a new antifouling strategy current issues like reduced ecological damage (e.g. due to copper disposal), lower maintenance costs (e.g. cleaning) and reduced durability shall be resolved.
Formgedächtnislegierungen
(2018)
Formgedächtnislegierungen sind »Legierungen, die nach geeigneter Behandlung aufgrund einer martensitischen Umwandlung ihre Gestalt in Abhängigkeit von der Temperatur ändern«. Derartige Materialien werden in den nächsten Jahrzehnten eine wachsende Rolle in der Technik spielen. Um die Eigenschaften dieser Werkstoffe optimal nutzen zu können, ist es wichtig, den Einfluss des Herstellungs- und Verarbeitungsprozesses auf ihre Anwendung zu kennen.
Das Buch behandelt die metallkundlichen Hintergründe und die Verwendungsmöglichkeiten der Formgedächtnislegierungen in verständlicher, auf den Anwender zugeschnittener Weise.
Ziel dieses Projektes war die Entwicklung eines Linearantriebes mittels Formge-dächtnislegierungen (FGL) zur Knochenverlängerung und Defektüberbrückung ent-sprechend der Methode nach Betz und Baumgart. Der zu entwickelnde Linearantrieb sollte im Idealfall folgende Eigenschaften aufweisen: • mechanisch einfach aufgebaut • leicht zu miniaturisieren • großer Arbeitsweg • variable Positionierung • hohe Leistung • kostengünstig Zur Entwicklung eines derartigen FG-Marknagels sollten die folgenden Teilprobleme gelöst werden: • Auswahl einer geeigneten FGL zur Realisierung der benötigten Kräfte innerhalb der möglichen Temperaturdifferenz • Auswahl der Heizspirale und Messung der Aufheiztemperaturen an der Marknagel-oberfläche • Ausarbeiten der konstruktiven Lösung hinsichtlich der ermittelten Werte bezüglich des Verhältnisses Kraft/Weg • Werkstoffauswahl hinsichtlich der Implantierbarkeit, der Schweißbarkeit mit Laser und der Aufheizung • Mechanische Sicherung (Arretiermechanismus) bei Belastung der Extremität durch den Patienten und Entwicklung eines Rückstellmechanismus • Verbindungstechnik der Komponenten • Erprobung der entwickelten Prototypen in Labortests