Volume 59, Issue 11

This paper presents the characterization of the microstructure evolution during flow forming of austenitic stainless steel AISI 304L. Due to plastic deformation of metastable austenitic steel, phase transformation from γ-austenite into α’-martensite occurs. This is initiated by the formation of shear bands as product of the external stresses. By means of coupled microscopic and micromagnetic investigations, a characterization of the microstructure was carried out. In particular, this study shows the distribution of the strain-induced α’-martensite and its influence on material properties like hardness at different depths. The microstructural analyses by means of electron backscattered diffraction (EBSD) technique, evidence a higher amount of α’-martensite (ca. 23 %) close to the outer specimen surface, where the plastic deformation and the direct contact with the forming tool take place. In the middle area (ca. 1.5 mm depth from the outer surface), the portion of transformed α’-martensite drops to 7 % and in the inner surface to 2 %. These results are well correlated with microhardness and micromagnetic measurements at different depths. EBSD and atomic force microscopy (AFM) were used to make a detailed characterization of the topography and degree of deformation of the shear bands. Likewise, the mechanisms of nucleation of α’-martensite were discussed. This research contributes to the development of micromagnetic sensors to monitor the evolution of properties during flow forming. This makes them more suitable for closed-loop property control, which offers possibilities for an application-oriented and more efficient production.

High temperatures and a pronounced segmented chip are characteristic of titanium machining processes. The formation of segmented chips induces an alternating high frequency mechanical load on the tool and thus promotes tool failure. The presence of oxygen triggers a multitude of chemical interactions during the chip forming process. However, the oxygen content’s impact during titanium machining, especially on the chip formation, is unknown. In order to draw conclusions on this process, the machining tests were carried out while varying the oxygen content. The thus produced chips were metallographically examined. The varying degree of segmentation could then be visualized by boundary etching and subsequently be evaluated.

A metallographer is often rewarded with a beautiful image when looking through the microscope. These images deserve to be made accessible to the general public. Within the project “Lumen & Colours: Faces of a city – a participatory project combining media art and urban design” funded by the German Federal Cultural Foundation (German: Kulturstiftung des Bundes ), the opportunity arose to map images recorded via light optical as well as scanning electron microscopy onto different building facades of the city. A selection of images from research and failure analysis conducted at the Institute for Silicon Technology (ISIT), for example, were projected onto the facade of the cleanroom. The appeal lies in the fact that an image is associated with a location, that matter is merged with imagination and natural sciences with art. On the cleanroom building, which is a work of art in itself as its structural design is modelled after a silicon wafer, the image and the facade merge to become one. Art and science are surprisingly close to each other. A new dimension is revealed on the huge 36-meter wide facade of the Fraunhofer ISIT by a giant video projector with a weight of around 100 kg and an output of 30 000 ANSI lumens. [1] Moreover, the images were animated and projected onto different buildings in the city as part of a virtual presentation. This contribution will explain the technical background of the images obtained in battery research and from the analysis of silicon wafers and solder joints and will show the different views offered by microscopic observation and how these images become one with different facades.

After surface grinding, linear and dotlike fluorescent displays were found on the blade platform of a gas turbine guide vane. Through detailed inspections of macro- and micro morphologies and the crack fracture, the fluorescent displays are fine cracks in nature, and the difference in morphology is related to the distribution of the fine cracks. The fine cracks should be shrinkage cracks in castings. The grinding force caused internal cracks to appear on the grinding surface.