Low-chromium ferritic stainless steel that is subjected to a second laser-induced thermal cycle is susceptible to intergranular corrosion. Precipitation of carbides and nitrides depletes the adjacent regions of chromium. For this material, despite its low carbon content of 0.007%, laser transformation hardening has achieved a considerable increase in hardness. For certain thermal conditions during overlapping of tracks the above precipitation mechanism can take place. Numerical modelling was applied to analyse the laser parameters that could be critical for precipitation to take place. The results are compared to experiments that were screened with a standardised test method. Avoiding susceptibility to intergranular corrosion when laser transformation hardening was shown to be difficult during a second thermal cycle of an overlapping pass that passes a critical though narrow temperature range. Based on this, measures to mitigate the susceptibility was introduced, in the form of heating the material above melting temperature. When melting the material, even during the first pass, corrosion behaviour changes. It is shown that the risk of corrosion can be avoided during subsequent passes. Different thermal cycles are analysed to find limits for avoiding susceptibility to intergranular corrosion. By laser treating and mapping the critical thermal cycles, the material can be used in a wider range of applications.
Welding distortion is an important engineering topic for simulation and modeling, and there is a need for experimental verification of such models by experimental studies. High-speed pulsed digital holography is proposed as a measurement technique for out-of-plane welding distortion. To demonstrate the capability of this technique, measurements from a laser spot weld are presented. A complete two-dimensional deformation map with submicrometer accuracy was acquired at a rate of 1000 measurements per second. From this map, particular points of interest can be extracted for analysis of the temporal development of the final distortion geometry.
Recently the fiber and the disk laser have been developed as advanced high power lasers of continuous wave type. Their beams are fiber-guided. Their low beam parameter product determines the focused beam as a high power density tool for laser materials processing. The lateral and axial power density distribution is crucial for the process. The measurement of a focused Yb:fiber laser beam is compared with its theoretical profile. While a Gauss beam describes the real beam in the far field, in the vicinity of the focus the beam is similar to a top-hat profile. In particular, the peak power density is lower around the focus than for a Gaussian beam. This provides a flatter, less varying optical tool along a certain axial range. A suitable model of the focused laser beam is achieved by superposition of a Gauss beam with a second but negative Gauss beam, each with matched parameters for power, spot diameter, and Rayleigh length. Owing to its much better accuracy with less than a 20% error, the model beam is applicable, e.g., for simulation of the welding process.
KEYWORDS: Laser welding, Capillaries, High speed photography, Liquids, Photography, Zinc, Cameras, Digital photography, High speed cameras, Standards development
Recent developments in digital high-speed photography allow us to directly observe the surface topology and flow conditions of the melt surface inside a laser evaporated capillary. Such capillaries (known as keyholes) are a central feature of deep penetration laser welding. For the first time, it can be confirmed that the liquid capillary surface has a rippled, complex topology, indicative of subsurface turbulent flow. Manipulation of the raw data also provides quantitative measurements of the vertical fluid flow from the top to the bottom of the keyhole.
The paper reviews recent results on modeling a viscous liquid flow driven by ablation pressure. Based on the analysis of
the Navier-Stokes equation various strongly different manifestations of this phenomenon are explained. These are: (i) a
"clean" laser ablation, when laser spot has a clean sharp spot border, free from a re-solidified melt dross; (ii) a new form
of material removal in laser ablation - expulsion on a poly(methyl methacrylate) target of long (up to 1 mm) nanofibers
with a radius about 150-200 nm to the exterior of the spot under the action of a single pulse of KrF excimer laser; and
(iii) a new way of laser surface nanostructuring - the formation of a surface foam having a structure of micro-pores
interconnected with nanofilaments of diameters about 100 nm as a result of single pulse KrF laser irradiation of
biopolymer films.
Theoretical and experimental studies have been carried out in order to improve the understanding of the mechanism of pore formation in keyhole laser spot welding in a qualitative and also quantitative manner. A semi-analytical mathematical model of the keyhole collapse illustrates the different characteristic time scales of the contributing physical processes: post-vaporization (order of magnitude: 100 ns typically), excess keyhole vapor relaxation flow 10 μs), inertia driven collapse (100 μs), followed by bubble contraction, re-condensation and rising (10 ms), and re-solidification (10 ms). The conditions of the keyhole just before switching off the laser beam, observed by X-ray imaging, are essential for the subsequent collapse mechanism. In case of a bottleneck-shaped keyhole, which can easily form due to the paradox of vapor flow inversion, bubble formation is likely to occur due to necking. When the thermally contracting bubble is trapped by the re-solidification front, a pore is formed. The model is complementary to high speed X-ray observations of the keyhole shape, particularly in liquid Zn that enables investigation of keyhole and bubble formation not constrained by surrounding solid.
Laser-assisted generating by the blown powder process is investigated, both, experimentally and theoretically, with respect to layer cross section, grain growth, mechanical testing, track remelting and wall surface roughness compared to postremelting.
In order to improve the understanding of the mechanisms of laser surface treatment and to optimize industrial applications, a general model of laser surface treatment has been accomplished which enables calculation of the thermodynamic phenomena and analysis of the process. With particular emphasis on laser cladding and alloying, powder heating has been investigated in detail, which can be divided into heating during travelling through the laser beam and subsequent heating by the melt pool. While the time scale of the former heating mechanism is of the order of 1 ms, pool heating takes place within 10 microsecond(s) , hence almost immediately. While the pool temperature determines whether powder will be melted or not, in principle any powder, even WC, can be melted when passing the beam, as long as the calculated threshold intensity range is exceeded. Laser cladded tracks prove to be determined by the melt pool shape which is calculated analytically for non-gaussian beams, as well. While for low powder feeding rates cladding is limited by the mass balance, for substantial powder delivery the energy balance turns out to be the limiting criterion, while dilution diminishes. Increased degree of overlap between two tracks decreases the roughness of the cladded surface.
Applications of high power lasers in material processing can be divided into surface treatment, welding and cutting. An overview of the main physical mechanisms underlying these applications shall give a systematic basis for modeling the processes. The absorption behavior of a CO2-laser beam is discussed, mathematical attempts for describing the heat conduction are compared and a distinction between the several appearing melt flow phenomena is made. Further treated effects are evaporation, plasma formation and time dependence. Models of hardening, deep welding and cutting are presented to give examples for the mathematical treatment and the predictions and analysis that can be done.
A mathematical model for deep penetration laser welding has been developed that
considers only main phenomena. A moving linesource describes the net energy
input given by the difference between laser power entering the keyhole and
evaporation loss. Energy flow continuity yields the dependence of welding speed
on the wall temperature of the keyhole. Since it is generally assumed that the
keyhole is held open by the vapour pressure, a second condition for the keyhole
wall temperature is obtained.
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