Ernst Wolfgang Kreutz

Dynamics of ripple formation and melt flow in laser beam cutting

The dynamical behaviour of the laser beam fusion cutting process of metals is investigated. Integral methods such as the variational formulation are applied to the partial differential equations for the free boundary problem and a finite dimensional approximation of the dynamical system is obtained. The model describes the shape of the evolving cutting kerf and the melt flow. The analysis is aimed at revealing the characteristic features of the resultant cut, for example, ripple formation and adherent dross. The formation of the ripples in the upper part of the cut, where no resolidified material is detectable, is discussed in detail. A comparison with numerical simulations and experiments is made.

Fabrication of erbium-doped planar waveguides by pulsed-laser deposition and laser micromachining

Laser radiation is used both for the deposition of dielectric Er:BaTiO3 thin films and for material removal to generate wave guiding structures for photonic applications. Pulsed laser deposition with KrF excimer laser radiation (wavelength 248 nm, pulsed duration 20 ns) is used to grow dense, transparent amorphous or crystalline erbium doped BaTiO3 thin films. Visible emission due to up-conversion luminescence (wavelength 528 nm and 548 nm) under excitation with diode laser radiation at a wavelength of 975 nm is investigated as a function of the erbium concentration and structural film properties. The dielectric films are micro machined to form optical wave guiding structures using Nd:YAG laser radiation (wavelength 532 nm, pulsed duration 40 ps) and Ti:sapphire laser radiation (wavelength 810 nm, pulse duration 63 - 150 fs) by scanning the focused laser beam relatively to the sample.

Melt Expulsion by a Coaxial Gas Jet in Trepanning of CMSX-4 with Microsecond Nd:YAG Laser Radiation

Trepanning of 200 μm holes in 2-5mm thick CMSX-4 sheets is done by laser radiation provided by a lamp-pumped Nd:YAG slab laser with pulse durations of 100 - 500 μs. Pulse energies <1J determine the material removal mainly by melt expulsion assisted by a processing gas jet coaxial to the laser beam. Stagnation and deflection of the gas jet at the entrance of the kerf, friction in the molten material, and friction at the liquid/solid interface hinder an efficient melt expulsion. A simulation tool for supersonic gas flow solving Euler equations by the Finite Volume Method is developed in order to investigate the gas flow through the trepanning kerf. Gas pressures above and within the kerfs while trepanning at different inclination angles, geometries and arrangements of nozzles as well nozzle reservoir pressures are presented. The computed gas flow is compared to melt expulsion investigated metallographically by the determination of kerf widths and the thickness of the resolidified melt.

Formation of subwavelength-laser-induced periodic surface structures by tightly focused femtosecond laser radiation

Sub-wavelength (1/4*λ-3/4*λ) laser induced periodic surface structures are generated by irradiation of either bulk fused silica and silicon or Er:BaTiO3 thin films by scanning a tightly focused beam (Θ = 1 μm) of femtosecond laser radiation (λ = 800 nm, tp = 100 fs) on the surface. The ripple pattern extends coherently over many overlapping laser pulses parallel and perpendicular to the polarization of the laser radiation. The dependence of the ripple spacing on the spacing of successive pulses, the direction of polarization and the properties of the material is investigated. The evolution of the ripples is investigated by applying pulse bursts with 1 - 20 pulses. The development conditions of the stuctures are specified and possible mechanisms of ripple growth are discussed.

Melt ejection during single-pulse drilling and percussion drilling of micro holes in stainless steel- and nickel-based superalloy by pulsed Nd:YAG laser radiation

Single pulse drilling and percussion drilling of micro holes (diameter 100 μm, depth 2 mm) with Nd:YAG laser radiation (pulse duration 100 - 500 μs) in stainless steel (X5CrNi18-10) and nickel-based superalloy (CMSX-4) are reported. The development of the hole geometry, especially the hole diameter depending on the focal diameter including caustics and the thickness of the melt at the hole wall, is investigated. The experimental results are compared to the calculations of a physical model describing the motion of the melt front, the phase boundary between liquid and solid. Closures resulting from melt in the hole are detected by recording the expansion of plasma using high speed photography (50000 fps). The quality of drilled holes, e.g. conicity, thickness of recast and closures, is investigated by metallography and optical microscopy. Timescales of physical processes as the onset of melting and vaporization, the time to stationary melt flow and geometrical scales like the hole depth, the closure depth and the hole diameter are determined experimentally and numerically.

Failure recognition and online process control in laser beam welding

By means of ISO 9000 and other by law enforced rules the manufacturing industry is forced to document and control the quality of their products. With the aim to improve the quality of the manufactured products and the reliability of the corresponding process an improved understanding of the observability and controllability is desired. This can only be achieved by reliable online process control systems.Actual quality monitoring systems commonly use spatially integrating plasma sensors like photo diodes as detectors. Recent research work and publications show that in the near future the importance of process diagnosis systems using imaging devices like cameras as detectors will steadily grow. The advantage of such systems is that they provide the information about the spatial distribution of the monitored radiation source, thus allowing a deeper view into process characteristics than spatial integrating sensors can give.Based upon a high-speed camera an online process control system was developed. The system can be adapted for different applications like welding or cutting. An overview over the capabilities of camera based process monitoring systems in comparison to photo detector based approaches is given. The latest research work and an outlook to further developments will be presented.By means of ISO 9000 and other by law enforced rules the manufacturing industry is forced to document and control the quality of their products. With the aim to improve the quality of the manufactured products and the reliability of the corresponding process an improved understanding of the observability and controllability is desired. This can only be achieved by reliable online process control systems.Actual quality monitoring systems commonly use spatially integrating plasma sensors like photo diodes as detectors. Recent research work and publications show that in the near future the importance of process diagnosis systems using imaging devices like cameras as detectors will steadily grow. The advantage of such systems is that they provide the information about the spatial distribution of the monitored radiation source, thus allowing a deeper view into process characteristics than spatial integrating sensors can give.Based upon a high-speed camera an online process control system was developed. The sys...

Nd:YAG laser micromachining of SiC precision structures for MEMS

Micromachining of SiC with 1(omega) , 2(omega) , 3(omega) -Nd:YAG laser radiation with pulse durations in the ps to ns regime is performed in various processing gas atmospheres as a function of processing variables showing the influence of the heat and pressure load onto the precision of geometric structures generated. The physical and chemical processes involved in micromachining with laser radiation are characterized by a machine vision system and the produced structures are analyzed by profilometry, optical and electron microscopy as well as X- photoelectron spectroscopy. 3D microstructures are produced by scanning and turning the laser beam onto the material surface, width of structures < 100 micrometers and surface roughness < 2 micrometers , for example, require an overlap < 0.8 independent of the type of processing gas under investigation.

Direct writing of conducting traces by chemical modification of solid coatings with laser radiation

Abstract A laser radiation induced direct writing process is described to deposit highly conducting traces on different glass substrates. Copper oxide precoated substrates are exposed to laser radiation (λ = 514 nm; P L = 0.5-4 W) in a reducing H 2 ambient. The remaining copper traces have a porous structure and resistivities down to 20 μΩ cm. The typical thickness of the traces, mainly given by the thickness of the precoating, is 5 to 10 μm. The width of the traces depends on beam width, laser power, and scanning velocity (typical values some 100 μm) and is usually greater than the width of the beam.

Reconditioning of nickel base HPT blades and vanes used in aero engines and power plant gas turbines by combination of direct laser forming, laser metal deposition and laser drilling

Parts like HPT (High Pressure Turbine) blades and vanes with integrated complex shaped cooling channels cannot be repaired. The parts are replaced by new ones. The aim is to develop and implement a reconditioning chain to avoid scrapping of the part. The reconditioning chain covers the three different Laser Applications 1. Direct Laser Forming (DLF), 2. Laser Metal Deposition (LMD) and 3. Laser Drilling (LD) which are arranged in subsequent Processing Steps. The damaged volume of the blade or vane is replaced by a fitting patch with integrated cooling channels which is generatively manufactured by DLF based on CAD data acquired by digitizing and reverse engineering in a first step. The generated patch is joined into the remaining part of the blade or vane by LMD. The non open cooling channels are opened and in case of necessity reshaped by LD in the third step. Also the cooling channels damaged or destroyed in the joint zone patch-substrate by the LMD joining process within step 2 are reworked or inserted by LD in the third step. First results achieved within basic R & D of DLF, LMD and LD of relevant Nickel Base Alloys are presented.Parts like HPT (High Pressure Turbine) blades and vanes with integrated complex shaped cooling channels cannot be repaired. The parts are replaced by new ones. The aim is to develop and implement a reconditioning chain to avoid scrapping of the part. The reconditioning chain covers the three different Laser Applications 1. Direct Laser Forming (DLF), 2. Laser Metal Deposition (LMD) and 3. Laser Drilling (LD) which are arranged in subsequent Processing Steps. The damaged volume of the blade or vane is replaced by a fitting patch with integrated cooling channels which is generatively manufactured by DLF based on CAD data acquired by digitizing and reverse engineering in a first step. The generated patch is joined into the remaining part of the blade or vane by LMD. The non open cooling channels are opened and in case of necessity reshaped by LD in the third step. Also the cooling channels damaged or destroyed in the joint zone patch-substrate by the LMD joining process within step 2 are reworked or inserted...

Investigation of movement of the melting front to analyse the formation of hole geometry during single pulse drilling with Nd:YAG laser radiation

Single pulse drilling of micro holes (diameter 100 µm, depth<2 mm) in stainless steel (X5CrNi18-10) and nickel-base superalloy (CMSX-4) is performed by pulsed Nd:YAG laser radiation (pulse duration 100-500 µs). The movement of the melting front is investigated by metallography and optical microscopy. The formation of the hole geometry is compared to the spatial and temporal distribution of the pulse power. The processing limits regarding the aspect ratio (diameter: depth) are extended and the quality of drilled holes regarding recast layers in drilled holes, cylindricity, and defined conicity is extended.Single pulse drilling of micro holes (diameter 100 µm, depth<2 mm) in stainless steel (X5CrNi18-10) and nickel-base superalloy (CMSX-4) is performed by pulsed Nd:YAG laser radiation (pulse duration 100-500 µs). The movement of the melting front is investigated by metallography and optical microscopy. The formation of the hole geometry is compared to the spatial and temporal distribution of the pulse power. The processing limits regarding the aspect ratio (diameter: depth) are extended and the quality of drilled holes regarding recast layers in drilled holes, cylindricity, and defined conicity is extended.