Guntram Wagner

Analysis of fatigue properties and failure mechanisms of Ti6Al4V in the very high cycle fatigue regime using ultrasonic technology and 3D laser scanning vibrometry.

Accelerated fatigue tests with Ti6Al4V were carried out using a 20kHz ultrasonic testing facility to investigate the cyclic deformation behavior in the Very High Cycle Fatigue (VHCF) regime in detail. Beside parameters like the ultrasonic generator power and the displacement of the specimen, a 3D laser scanning vibrometer was used to characterize the oscillation and fatigue behavior of the Ti-alloy. The course of the S-N(f) curve at the stress ratio R=-1 shows a significant decrease of the bearable stress amplitude and a change from surface to subsurface failures in the VHCF regime for more than 10⁷ cycles. Microscopic investigations of the distribution of the α- and β-phase of Ti6Al4V indicate that inhomogeneities in the phase distribution are reasons for the internal crack initiation. High resolution vibrometry was used to visualize the eigenmode of the designed VHCF-specimen at 20 kHz in the initial state and to indicate local changes in the eigenmodes as a result of progressing fatigue damage. Non-contact strain measurements were realized and used to determine the stress amplitude. The determined stress amplitudes were correlated with strain gauge measurements and finite element analysis.

Realization of Al/Mg-Hybrid-Joints by Ultrasound Supported Friction Stir Welding

To realize modern light weight constructions it is more and more necessary to combine the advantages of dissimilar materials. Fusion welding of dissimilar metals is in the most cases difficult or even impossible as a result of different melting points of the materials and the development of undesirable brittle intermetallic phases in the welding zone. This often leads to joint strengths considerable below the tensile strength of the base materials. By using Friction Stir Welding (FSW) as a pressure welding method, it is possible to reduce the development of the intermetallic phases of Al/Mg-joints significantly. But as calculated phase diagrams and high resolution microscopic SEM-investigations have shown it is not feasible to avoid them completely. The intermetallic phases form in the contact area very small continuous layers between the joining partners. On the other side it is known that ultrasonic energy can crack oxide layers. Hence a hybrid welding system at the Institute of Materials Science and Engineering (WKK) at the University of Kaiserslautern was developed called “Ultrasound Supported Friction Stir Welding (US-FSW)” with the aim to shatter the brittle interlayer lines and to scatter fragments in the welding area during the FSW process. Pre-investigations have shown that for US-FSW-joints between Al wrought alloys and Mg cast alloys the strength can be increased up to 30% in comparison to conventional friction stir welds. Currently, further investigations are carried out with joints between AC-48000 and AZ80.

Monitoring of the Friction Stir Welding Process to Describe Parameter Effects on Joint Quality

With the industrial use of FSW growing steadily, non-destructive testing methods that can detect the impending formation of flaws during welding must be developed. The present work accomplished two goals. First, the distribution of oxide fragments within aluminium welds could be correlated with certain welding process variables. Consequently, an approach was suggested to prevent the conglomeration of oxide fragments in the weld by reducing the surface roughness of the abutted edges. Second, welding forces can be used to predict the formation of elongated cavities inside the weld. This project showed that by monitoring the welding forces, the ability to change the welding variables in real time could prevent the formation of flaws in friction stir welds.

Microstructural and thermodynamic investigations on friction stir welded Mg/Al-joints

Abstract With friction stir welding it is possible to join light metals in the solid state. However joints between dissimilar metals are a special challenge. At the Institute of Materials Science and Engineering of the University of Kaiserslautern the friction stir weldability of magnesium–aluminium-joints was investigated. For the realization of high strength joints between these materials the knowledge of the developing microstructure in the welding zone is of especially high interest. Therefore, light microscopic and scanning electron microscopic investigations were carried out and the results compared with phase diagrams and phase fractions for the bonding zone which were calculated at Institute of Metallurgy of the University of Clausthal.

Helium‐tight Sealing of Glass with Metal by Ultrasonic Welding

Efficient joining methods between glass and metals, wich combine the individual advantages of both material classes, are of great technical importance. The present paper introduces the ultrasonic torsion welding technique for glass/metal welding. It could be shown that it is possible to join glass with metal by use of an aluminium interlayer

Low temperature joining of copper by Ag nanopaste: correlation of mechanical properties and process parameters

Nanoparticles exhibit a decrease in sintering and melting temperature with decreasing particle size in comparison to the corresponding bulk material. After melting or sintering of the nanoparticles, the material behaves like the bulk material. Therefore, high-strength and temperature-resistant joints can be produced at low temperatures, which is of big interest for various joining tasks. Joints (substrate: Cu) were prepared with an Ag nanoparticle-containing paste. The influence of the adjustable process parameters joining pressure, joining temperature, holding time, heating rate, thickness of paste application, surface treatment, pre-drying process, and subsequent heat treatment on the strength behavior of the joints was investigated. It is shown that in particular, the joining pressure exerts an essential influence on the achievable strengths. In addition, temperature, holding time, and thickness of paste application have a significant effect on strength behavior. In contrast, the pre-drying process, heating rate, surface pre-treatment, and subsequent heat treatment possess hardly any influence on joint strength.

Fügen artfremder Werkstoffe — Ultraschallschweißen von Kohlenstofffasertextilien mit Metallen

Kurzfassung Das Ultraschallschweißen wird in vielen Bereichen der kunststoff- und metallverarbeitenden Industrie erfolgreich eingesetzt. Insbesondere Bauteile aus artgleichen Werkstoffen werden mit diesem Pressschweißverfahren in großen Stückzahlen gefertigt. Das Kunststoff-Ultraschallschweißen wird zum Fügen von thermoplastischen Werkstoffen verwendet. Die Metallschweißvariante des Ultraschallschweißens wird beispielsweise zur Herstellung von Heizungsrohren oder Litzen-Steckerverbindungen eingesetzt. Seit einigen Jahren können mit dem Metallschweißverfahren auch artfremde Werkstoffpaarungen zwischen Glas bzw. Keramik und verschiedenen Metallen sowie deren Legierungen hergestellt werden. Aufbauend auf diesen Erfahrungen werden am Lehrstuhl für Werkstoffkunde der TU Kaiserslautern auch Ultraschallschweißungen zwischen Glasfaser- sowie Kohlenstofffasertextilien mit metallischen Werkstoffen erfolgreich durchgeführt.

New Sandwich Structures Consisting of Aluminium Foam and Thermoplastic Hybrid Laminate Top Layers

Sandwich structures consist of one light core layer and two top layers, which form the load-bearing structure. These layers have to be stiff and strong and have to protect the structure against indentations. The main task of the core layer is to keep the top layers in place and to generate a high shear stiffness. In order to obtain the required space between the top layers, the core layer has to have a high specific volume. Different sandwich materials with aluminium or steel top layers and cores of aluminium combs, corrugated aluminium sheets or aluminium foams are already known. In order to obtain better properties in terms of strength fibre-reinforced plastics (FRP) are utilised as top layers; this is the focus of numerous of the current research studies. The sole use of these materials leads to negative effects regarding the damage and impact behaviour. New top layers with high strength and high stiffness characteristics as well as good damage tolerances are to be expected by utilising metal layers in combination with endless fibre-reinforced plastics, so called hybrid laminates. These hybrid laminates combine the positive properties of metals (e.g. ductility) and fibre-reinforced plastics (e.g. tensile strength). The focus of this investigation lies on the production and characterisation of sandwich structures with aluminium foam core layers and hybrid laminate top layers. The foam cores consist of closed pore aluminium foams produced by utilising ingot and powder metallurgical techniques. The top layers consist of glass fibre-reinforced thermoplastics and aluminium layers. The production of the sandwich materials is realised by means of thermal pressing.

Joining of Aluminum Matrix Composites and Stainless Steel by Arc Brazing

To enlarge the field of application of aluminum matrix composites (AMC) suitable joining technologies are necessary. Especially dissimilar joints like between AMC and stainless steel are of interest. In this work the arc brazing of this material combination is investigated. A new filler based on the ternary system Al-Ag-Cu is used. The results of wetting tests of the base filler and adapted variants are described and discussed. Wetting angles and microstructure of the interfaces are regarded.

Development and Characterisation of Phenolic Resin Moulding Materials for the Production of New Short Fibre-Reinforced C/C-SiC Composites

This article focuses on the development of phenolic resin moulding materials for the production of new carbon fibre-reinforced ceramic composite materials based on C/C-SiC by utilising the LSI (liquid silicon infiltration) production method. The production of these moulding materials is being accomplished by combining phenolic resin and carbon fibres with the addition of a few selected parts of processing aids, during which the influence of the used lubricants on the processability of the moulding materials is examined. The starting materials, microstructures and mechanical properties of the materials were characterised at every step of the entire process (CFRP and C/C composites) as well as the end of the whole production (C/C-SiC composites). During this investigation a link between the portions of the lubricant used, the forming of the porosity and the impact on the mechanical properties was discovered. In regards to the optimisation of the process the involved parties were able to determine an optimal lubricant ratio.