Martin Sommer

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.

MXenes, two-dimensional transition metal carbides or nitrides, have recently shown great promise for gas sensing applications. We demonstrate that the sensitivity of intrinsically metallic Ti3C2Tx MXene can be considerably improved via its partial oxidation in air at 350 °C. The annealed films of MXene sheets remain electrically conductive, while their decoration with semiconducting TiO2 considerably improves their chemiresistive response to organic analytes at low-ppm concentrations in dry air, which was used to emulate practical sensing environments. We demonstrate that partially oxidized MXene has a faster and a qualitatively different sensor response to volatile analytes compared to pristine Ti3C2Tx. We fabricated multisensor arrays of partially oxidized Ti3C2Tx MXene devices and demonstrate that in addition to their high sensitivity they enable a selective recognition of analytes of nearly the same chemical nature, such as low molecular weight alcohols. We investigated the oxidation behavior of Ti3C2Tx in air in a wide temperature range and discuss the mechanism of sensor response of partially oxidized MXene films, which is qualitatively different from that of pristine Ti3C2Tx.