Papers by Author: Guntram Wagner

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Authors: Matthias Wolf, Guntram Wagner, Dietmar Eifler
Abstract: Aluminum matrix composites (AMCs) are characterized by improved mechanical properties in comparison to their unreinforced matrix alloys. But the knowledge about the fatigue behavior of AMCs in the HCF-and in the VHCF-regime is limited until now. Due to this AMC225xe and AMC xfine225 with an average SiC particle content of 25 vol.-% and particle sizes of 2.5 μm and 0.7 μm, respectively, as well as the base alloy AA2124 were fatigued up to 1010 cycles using the ultrasonic testing facility of the type "UltraFast-WKK-Kaiserslautern".To describe the fatigue behavior of the specimens several measuring devices were used to monitor and record the central process parameters. A very sensitive value to detect specimen failure at an early stage is the dissipated energy which can be determined as the integral of the generator power depending on the ultrasonic pulse time.In comparison to AA2124 the investigated AMCs have shown a considerably enhanced fatigue performance for stress amplitudes higher than 140 MPa. But below this stress amplitude for the matrix alloy run outs at 1010 cycles were realized whereas the AMCs failed at lower number of cycles still at lower stress amplitudes. Moreover, while crack initiation of the matrix alloy in all cases started at the surface for the AMCs the crack initiation point changes from surface to subsurface for more than 107 cycles. The subsurface failures of the composites were caused by microstructural inhomogeneities which could be identified with EDX and micro-CT as particle clusters and copper-iron-rich inclusions.
1597
Authors: Benjamin Strass, Guntram Wagner, Dietmar Eifler
Abstract: 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.
1814
Authors: Benjamin Strass, Guntram Wagner, Christian Conrad, Bernd Wolter, Sigrid Benfer, Wolfram Fürbeth
Abstract: Fusion welding of dissimilar metals is in the most cases difficult or even impossible as a result of different melting points and the development of undesirable brittle intermetallic phases. This often leads to joint strengths considerable below the tensile strength of the base materials. By using Friction Stir Welding (FSW) it is possible to reduce the development of the intermetallic phases of Al/Mg-joints significantly but not to avoid them completely. Hence a hybrid welding system at the WKK of 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 Al/Mg-US-FSW-joints the strength can be increased up to 30% in comparison to conventional FSW. Moreover for the reliable detection of nonconformities in the weld during a post-process inspection by suitable non-destructive testing (NDT) methods is necessary. Also there is a strong need for better process monitoring and control by in-process NDT methods. Furthermore the corrosion behavior of the basic materials and hybrid-joints was investigated by electrochemical methods indicating an increased corrosion of the Mg alloy in the area of the Al/Mg-butt weld.
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