LS Metallkunde und Werkstofftechnik
Refine
Year of publication
Document Type
- Conference Proceeding (40)
- Scientific journal article peer-reviewed (35)
- Scientific journal article not peer-reviewed (19)
- Article (14)
- Doctoral thesis (11)
- Part of a book (chapter) (7)
- Book (1)
- Habilitation (1)
- Patent (1)
Language
- English (102)
- German (26)
- Chinesisch (1)
Keywords
- Fatigue (2)
- Gamma titanium aluminides (2)
- Mechanical properties (2)
- Aging heat treatment (1)
- Cr/CrN multilayer, Erosion, PVD, Coatings (1)
- Creep (1)
- Erosion, MAX Phase (1)
- High cycle fatigue (1)
- Lotus effect (1)
- MAX Phase (1)
Institute
This paper serves as an overview of the ongoing research in the field of multiscale and multidisciplinary analysis of deformation and damage in the case of oligocrystalline structures. The research focuses on experimental measurement and numerical calculation of ductile failure in the X2CrNiMo18-15-3 (AISI 316L) stainless steel. An embedding numerical technique is employed where crystal plasticity theory is used to represent plastic deformation in the material and element removal technique based on Rice&Tracey damage model for ductile void growth to simulate damage initiation inside the material, which is observed in the experiments. Additionally, the crystal plasticity model is supported by a hierarchical multiscale approach connecting nano-, micro- and meso-scales.
The topic of fatigue strength of joints of flat and honeycomb-structured thin sheets has not yet been adequately studied. This concerns, for example,the development of an appropriate test method. In many cases, there is still a lack of information, how effective joints of flat and structured sheet metals are in omparison with reference joints of flat sheets. Combinations of resistance spot-welded specimens of flat and honeycomb-structured thin sheets of deep-drawing steel DC04 with a thickness of 0.5 mm were examined. Adopted clamping plates were applied for the testing. Joined smooth specimens were used as a reference material. The analysis of the frequency [1] was applied for comparison. The fatigue limit of a joint of a flat and a structured sheet metal is not significantly affected by spot welding. The fatigue limit of these spot-welded joints is comparable to that of an non joined honeycomb-structured sheet. By means of frequency or stiffness analysis, stability and damage mechanisms of differently shaped joints were explained. The joining of bridges of the structure shows a better stability than the combination of honeycombs. The stiffness analysis is a favored method for the clarification of the damage mechanisms in a spot-welded joint. The reasons for the low fatigue strength values of the joined flat-structured specimens compared to flat-flat joints are strong notch effects in the bridge area and momentum effects by the structure height of the studied honeycomb structure. The spot weld with a pronounced indentation also affected the fatigue strength of both flat and structured joined sheet metals.
The literature review reveals that the honeycomb-structured thin sheet metals are not adequately investigated with regard to their fatigue strength. A question that remains almost completely ignored in the process is the direct comparability of structured thin sheet metals with flat reference specimens. A thin sheet is not a classic specimen to determine the fatigue strength because of the known problem of the fracture mechanics. Structured thin sheet metals of deep-drawing steel DC04 with a thickness of 0.5 mm were examined. Flat specimens were used as reference material. The frequency analysis was used for comparison.
Fundamental differences in the mechanical behavior between flat and structured sheets under cyclic load were observed. Whereas an extremely flat S-N curve in the low-cycle region with a high slope coefficient was observed for the smooth thin sheets, the structured specimens show a curve typical for notched specimens. Moreover, the smooth sheets have a specific cyclic hardening and/or softening behavior. The state of plane stress of a fine sheet in contrast to a multi-axial one for the structured sheet complicates the direct comparison. For this reason, such a comparison with respect to their fatigue strength in the Wohler diagram should only be considered with caution.
Investigation of the wear resistance properties of Cr/CrN multilayer coatings against sand erosion
(2015)
The wear of metallic components used in gas and steam turbines due to erosive sand particles lead to a tremendous decrease in their lifetime. This wear can be reduced by the use of suitable erosion resistant coatings resulting in lower maintenance costs. In this context, multilayer Cr/CrN PVD coatings using an industrial coater was designed and applied on Inconel 718, a material which finds its application in gas turbines. A variation in the bimodal period has been induced in order to achieve an optimal coating architecture providing optimum properties needed for the erosion resistant coatings. The coating was deposited using a single Cr- target with an induction of N2 during the nitriding phase at a temperature of 480-500 °C and the coating thickness of 24-26 µm was kept constant throughout. The erosion tests were conducted at angles of 30°, 60° and 90°. The sand used for the test is an irregular shaped SiO2. The erosion tests were followed by a detailed microscopic examination of the eroded coating structure in combination with nanoindentation and scratch tests.
This research work provides information about the influence of Ti2AlC MAX phase coatings on the
erosion resistance of Ti624 2alloy.Flat specimens were coated witha20 µm Ti2AlC film using an industrial siz emagnetron sputtering coater and annealed at 800 °C for 1h in vacuum for the formation of the MAX phase.The erosion resistance of the specimens was tested using high velocity particles at room
temperature.The results indicate that the erosion resistance of Ti6242 has been significantly enhanced
by the employment of MAX-phase coatings. This demonstrates the capability of using MAX phase
coatings as erosion protective medium for engine components exposed to similar damage conditions.
Additionally,the results show that uncoated Ti6242 samples with duplex microstructure have a better
durability against erosion attack compared to uncoated samples with globular microstructure.
Coronary heart disease is still one of the most common sources for death in western industrial countries. Since 1986, a metal vessel scaffold (stent) has been inserted to prevent the vessel wall from collapsing. Most of these coronary stents are made from CrNiMosteel (316L). Due to its austenitic structure, the material shows a good combination of strength, ductility, corrosion resistance, and biocompatibility. However, this material has some disadvantages like its non-MRI compatibility and its poor fluoroscopic visibility. Other typically used materials are the CoBase alloys L-605 and F-562 which are MRI compatible as well as radiopaque. Another interesting fact is their excellent radial strength and therefore the ability to produce extra thin struts with increased strength. However, because of a strut diameter much less than 100 μm, the cross section consists of about 5 to 10 crystal grains (oligocrystalline). Thus, very few or even just one grain can be responsible for the success or failure of the whole stent. To investigate the relation between microstructure, mechanical factors and stent design, commercially available Cobalt-Chromium stents were investigated with focus on distinct inhomogeneous plastic deformation due to crimping and dilation. A characteristic, material related deformation behavior with predominantly primary slip was identified to be responsible for the special properties of CoCr stents.
In the cast condition g titanium aluminide alloys that solidify completely through the b phase are
characterized by fine and homogeneous microstructures, weak textures and low segregation. For these reasons such alloys have a relatively good workability and can be closed-die forged without preceding ingot breakdown even if the alloys contain no large fractions of the b phase at the working temperature. The present work was devoted to a combined study of the constitution and microstructural morphologies that develop in various two-step heat treatments of a single-step forged b solidifying alloy. The
study included high-energy X-ray diffraction for in situ investigations of the constitution at the heat treatment temperature. It was observed that the phase ransformations are quite sluggish in the material which results in fine microstructures and some conditions that significantly deviate from thermodynamic equilibrium. Further, tensile and creep testing was carried out on the different material conditions in order to identify the range in which the properties can be varied. It is found that this easily forgeable material exhibits comparable strength, ductility and creep strength as more conventional peritectically solidifying alloys.
Multi-layer constructions become more and more relevant in lightweight applications due to their high strength to
weight ratio. They offer excellent crash, damping and recycling properties. Also, the morphology of thermoplastic
carbon fibre reinforced plastics (CFRP) render them interesting for large scale manufacturing processes.
Nevertheless, a major disadvantage results in a poor resistance against wear and tear, e.g. erosion, which is
attributed to weak hardness properties. Hence, this work deals with tribological investigations on orthotropic
carbon fibre reinforced polymers (PA 6) either with protective ceramic coating or without. The chosen coating
system is a well-known protective covering of metal components, e.g. metal cutting tools, produced by physical
vapor deposition (PVD). To characterize the coating system on thermoplastic CFRP, standard analyzing methods are utilized, like optical and scanning electron microscopy (SEM). The tribological investigations are conducted by the tribological ball on disk method to generate wear tracks on the sample surfaces and hence to calculate the
wear rates. These results are compared to literature findings with respect to a certain protective coating system (TiN) and a second nano-structured gel coating system, where both systems are deposited on a thermosetting
material, i.e. carbon fibre reinforced epoxy resin, respectively. For this purpose the feasibility of depositing a protective ceramic layer on thermoplastic CFRP is demonstrated. First results on suitable surface pre-treatments have shown a significant influence on the coating quality. The improved performance regarding the wear behavior with respect to tribology compared to the poor substrate and existing technologies is shown additionally.