Florian Fetzer

The influence of a spatial beam oscillation on the dynamics of the capillary and the mechanism leading to an increased or reduced generation of process pores was investigated for deep penetration laser beam welding of the aluminum alloy AlMgSi. Welding with a feed rate of 4 m/min and a welding depth of 4 mm was examined with sinusoidal (longitudinal and lateral) and circular beam oscillation patterns at frequencies of 100 Hz and 200 Hz. The welding processes were analyzed by means of online X-ray imaging with a frame rate of 2 kHz, which provides the temporal and spatial resolution required to resolve the dynamics of the capillary. With conventional rectilinear welding, the weld seams are prone to the formation of process pores. By applying a circular beam oscillation, the weld seams were found to be virtually free from porosity. The mechanism leading to a reduced occurrence of process pores differed for the two investigated beam oscillation frequencies. At an oscillation frequency of 100 Hz bubbles are regularly formed in the melt pool but immediately removed at the subsequent pass of the laser beam by degassing into the vapor capillary. At 200 Hz the formation of process pores is completely avoided from the first. For a sinusoidal beam oscillation in longitudinal direction, the bubbles formed in the weld pool were found to be further inflated at each pass of the laser beam.

Aluminum (AA6016) sheets were welded in overlap configuration to investigate the influence of different beam oscillation patterns on the resulting temperature gradient, the local solidification rate and the resulting grain structure and to compare the results with those obtained with the conventional rectilinear welding. Two pyrometers were used to experimentally determine the temperature gradient. The weld pool boundaries on the surface plane of the work piece were determined by image processing of high speed videos, in order to evaluate the local solidification rates. Metallographic analysis of the weld seams proved that laser beam oscillation during welding can be used to reliably form an equiaxed dendritic grain structure, which reduces the susceptibility to the formation of hot cracks.

Abstract Three multi‐shell metalloid gold clusters of the composition Au 32 (R 3 P) 12 Cl 8 (R=Et, n Pr, n Bu) were synthesized in a straightforward fashion by reducing R 3 PAuCl with NaBH 4 in ethanol. The Au 32 core comprises two shells, with the inner one constituting a tilted icosahedron and the outer one showing a distorted dodecahedral arrangement. The outer shell is completed by eight chloride atoms and twelve R 3 P groups. The inner icosahedron shows bond lengths typical for elemental gold while the distances of the gold atoms in the dodecahedral arrangement are in the region of aurophilic interactions. Quantum‐chemical calculations illustrate that the Jahn–Teller effect observed within the cluster core can be attributed to the electronic shell filling. The easily reproducible synthesis, good solubility, and high yields of these clusters render them perfect starting points for further research.

Abstract The collective properties of self-assembled nanoparticles with long-range order bear immense potential for customized electronic materials by design. However, to mitigate the shortcoming of the finite-size distribution of nanoparticles and thus, the inherent energetic disorder within assemblies, atomically precise nanoclusters are the most promising building blocks. We report an easy and broadly applicable method for the controlled self-assembly of atomically precise Au 32 ( n Bu 3 P) 12 Cl 8 nanoclusters into micro-crystals. This enables the determination of emergent optoelectronic properties which resulted from long-range order in such assemblies. Compared to the same nanoclusters in glassy, polycrystalline ensembles, we find a 100-fold increase in the electric conductivity and charge carrier mobility as well as additional optical transitions. We show that these effects are due to a vanishing energetic disorder and a drastically reduced activation energy to charge transport in the highly ordered assemblies. This first correlation of structure and electronic properties by comparing glassy and crystalline self-assembled superstructures of atomically precise gold nanoclusters paves the way towards functional materials with novel collective optoelectronic properties.