Papers by Author: John Banhart

Abstract: Nano-crystalline-amorphous Al based alloys with minor additions of rare earth elements and transition metals are of technical interest, because of their extraordinary high mechanical strength. This strengthening effect depends strongly on the alloy composition and the pathway of crystallisation. The crystallisation behaviour of Al87Ni6La7 and of Al87Ni5La7Zr metallic glass was studied with complementary methods such as XRD, TEM and 3D-AP. The amorphous matrix of the Zr-containing glass shows fluctuations of all minor elements on the nanometer scale. It is suggested, that these fluctuations act as nucleation zones for the crystallization of the glass during annealing.

Abstract: The decrease of the distance between particle centers due to the growth of the sinter necks can be explained by the well known two-particle model. Unfortunately this model fails to provide a comprehensive description of the processes for 3D specimens. Furthermore, there is a significant discrepancy between the calculated and the measured shrinkage because particle rearrangements are not considered. Only the recently developed analysis of the particle movements inside of 3D specimens using micro focus computed tomography (μCT), combined with photogrammetric image analysis, can deliver the necessary experimental data to improve existing sintering theories. In this work, μCT analysis was applied to spherical copper powders. Based on photogrammetric image analysis, it is possible to determine the positions of all particle centers for tracking the particles over the entire sintering process and to follow the formation and breaking of the particle bonds. In this paper, we present an in-depth analysis of the obtained data. In the future, high resolution synchrotron radiation tomography will be utilized to obtain in-situ data and images of higher resolution.

Abstract: Aluminium foam sandwich panels (AFS) made of a low-density aluminium alloy AlSi6Cu6 foam core and two dense 6082 alloy face sheets were fabricated, after which the panels were subjected to two different heat treatments. First, the AFS panels were aged to increase their strength without further solution heat treatment and fast quenching, a process which resembles a T5 treatment. Second, to define a reference point the face sheets of AFS samples were cut off the foam and subjected to a full T6 treatment. Hardness profiles were measured across the thickness of the face sheets after the two different treatments and the microstructure was investigated. The main conclusion is that mechanical performance of AFS panels can be considerably increased by heat treatment without full solution heat treatment (T5), but without reaching the level of a full T6 treatment. The potential use of an easy to apply T5 treatment is an important cost reducing factor.

Abstract: The aim of this work was to improve the joining between the fastening elements and the aluminium alloys foams. The research work was carried out on joining fastening elements into aluminium alloy foams during the foaming process, i.e., foaming around fastening elements. The foamable precursor material was produced by hot pressing the powder mixture of metal and a small fraction of the blowing agent. A steel mould containing a foamable precursor material and the fastening elements were heated to temperatures above the melting point of metallic matrix of foamable precursor material in order to obtain the final specimens. Each aluminium foam specimen (6061 and AlSi12) has 200x80x80mm and contains two fastening elements. The steel moulds design, the fastening elements geometry, the aluminium alloy composition, as well as the foaming parameters were studied in order to optimise the quality of the joints produced. The quality of the joints were determined by means of visual inspection and mechanical tests.

Abstract: Metal foams are quite a challenge to materials scientists due to their difficult manufacturing. In all processes the foam develops in the liquid or semiliquid state. Liquid-metal foams are complex fluids which contain liquid metals, solid particles and gas bubbles at the same time. An X-ray transparent furnace was developed to monitor liquid metal foam evolution. Aluminium foams - similar to the commercial Metcomb foams - were produced by feeding argon or air gas bubbles into an aluminium composite melt. The foam evolution was observed in-situ by X-ray radioscopy under normal gravity. Drainage and rupture were evaluated during the 5 min foam decay and 2 min solidification. Argon blown foams showed significant drainage and cell wall rupture during the first 20 s of foam decay. Air blown foams were stable and neither drainage nor rupture occurred. We demonstrated the feasibility of experiments during parabolic flight or drop tower campaigns. However, the development of a foam generator for low gravity is needed.