The susceptibility to hot cracking depends on the alloy composition, the welding parameters and the clamping conditions. We combined the simplicity of the fan shaped cracking test introduced by Kutsuna with the digital image correlation method to evaluate the local strain rate at the solidification zone. The analysis gives quantitative information, which strain rate is needed to stop the centerline crack propagation. By this approach, the hot cracking susceptibility was quantified by measuring the critical strain rate for four different AlMgSi aluminum alloys at three different weld speeds.
The most important factor influencing the maximum cutting speed is the absorbed laser power distribution. As the polarization of the cutting laser beam influences the local absorption, the temperature of the cutting front varies using differently polarized laser beams. To determine the local temperature the emission spectra of cuts with a radially polarized CO2-laser were measured and compared with emission spectra of cuts with a circularly polarized beam. The spectra were used to determine the space resolved temperature by fitting with the Planck black-body radiation in the case of predominantly continuum emission and with the spectral lines of Fe I in the case of spectra dominated by line emission.
Laser ablation with ultrashort pulses allows to create precise and flexible geometries on various materials. However, the generation of complex surface geometries with low surface roughness, high contour accuracy and defined depth still represents a challenge on porous and inhomogeneous materials such as additively manufactured parts or carbon fiberreinforced plastics (CFRP). In the present work, optical coherence tomography (OCT) was utilized for high-resolution optical distance measurement. The measurement beam of the OCT-based measurement system and the processing laser beam were superimposed with a dichroitic mirror. Combining both beams allowed online, time-resolved recording of the ablated depth during laser processing. The comparison of actual ablation depth with the target ablation depth was used to select areas that had to be processed in the subsequent pass. This closed-loop control of the ablation process was used to generate complex 3D geometries in stainless steel. Furthermore, the closed-loop controlled ablation was utilized for postprocessing of additively manufactured aluminum parts in order to remove support structures and to significantly reduce the surface roughness. Moreover, the OCT-based measurements allowed to determine the orientation of the fibers during controlled laser ablation of CFRP for layer-accurate laser ablation, which served as preparation for repairing damaged CFRP parts.
The use of Carbon Fiber Reinforced Plastics (CFRP) in industrial mass production has been rising dramatically in the last few years due to its light weight and high mechanical strength. However, like any other material, structures made of CFRP may get damaged at high service load or by unpredictable impacts. The aim of this study was to maximize the ablation rate in order to optimize the removing of the damaged material for industrial applications. A nanosecond laser with an average power of 21 W and a galvano scanner were used to treat CFRP surface. The strategy presented in this paper comprised two consecutive steps of grooving and removing. In the grooving step, the fibers cut into small fragments. In the removing step, the defocused beam scanned between two adjacent grooves with sufficient energy to sublimate the plastic matrix. By implementing the "grooving/removing" strategy, an ablation rate of 1.8 mm3/s was achieved which is about four times higher than ablation with pure sublimation.
Heat accumulation due to successive laser pulse (HAP) incident on the same spot and heat accumulation due to successive scans (HAS) of the laser beam over the same spot significantly influences the process result, especially when the resulting temperature of the workpiece exceeds the melting temperature. In particular, heat accumulation is one of the dominating effects during short-pulse laser functionalization of surfaces and strongly affects the resulting surface structures. Within this study, a novel heat accumulation model is introduced to calculate the temperature increase in the workpiece for the whole process including the effects of HAP and HAS in which the latter is differentiated between heat accumulation due to multiple passes (HAS-I) and heat accumulation due to multiple layers (HAS-II). With the new model, the surface structure was successfully predicted when using an ultra-short pulsed (USP) laser with an average power of 420 W for laser surface structuring of polished AISI 316L.
End pumping of solid-state lasers with high-power diode lasers has become a standard method to produce fundamental-mode laser radiation with high efficiency. The scaling of end-pumped, single-rod, fundamental-mode lasers to higher powers, however, very soon borders on principal limitations due to thermally induced beam distortions. A general relation between the minimum of the mode radius ω min in the crystal rod and the range of the dioptric power ΔD of the thermal lens for which the resonator is stable, has been derived in ref. [1], For fundamental-mode operation in the whole stable region the effective pump spot has always to be smaller than the fundamental mode The possibility to enlarge the stability range with symmetric multirod Fabry-Perot resonators is discussed in ref [2]. We have found that a further enlargement of the stability range can be achieved with a ring cavity, where the stability region is given by the dotted area in fig. 2.
The evolution of the depth of holes drilled with a single ms laser pulse combined with a series of ns laser pulses for different processing parameters was studied numerically. The results show that the achievable hole depth can be increased by increasing the intensity of the ms pulses and by saving the absorbed energy that would be lost in overheating of the melt. The melt includes the one generated between subsequent ns pulses and the un-expelled melt after the effect of shock waves induced by the previous ns pulse. The overheating of the un-expelled melt is avoided by increasing the intensity of the ns pulses. Additionally, increasing the repetition rate of the ns pulses during the processing with a single ms pulse reduces the energy wasted for overheating the generated melt and the energy transferred radially to the rim of the hole. On the basis of the numerically obtained relations between the intensity of the ns pulse and the changed depth of the hole per pulse, a semi-analytical method was developed to predict the depth of the hole drilled with combined laser pulses. The applicability of the method is discussed and verified by comparing the semi-analytical results with the numerical and experimental results.
The generation of coherent long-pulse (25μs) power at 9300 and 5400MHz for weather radar transmitters will be described. Power levels of 125 and 250W at X and C bands, respectively, have resulted from the combination of four multiplier chains using fundamental power generation at L band.
In laser cutting the cut quality strongly depends on the removal of the molten material from the cutting kerf. The physical properties of the liquid layer like viscosity or absorption are strongly temperature dependent, thus the temperature distribution on the cutting front plays an important role influencing the cut quality.Spatially and spectrally resolved measurements of the optical process emissions from the cutting front were done during cutting of aluminium with a radially polarized CO2-Laser. The observed spectra show continuum radiation or emission of spectral lines, depending on the processing parameters. This paper concentrates on the continuum emissions for lower cutting velocities. With higher cutting velocities there is a higher amount of evaporation which leads to line emissions out of the evaporated material. At the processing parameters showing continuum radiation it is observed, that the temperature on the cutting front and therefore the cutting process itself is not constant during the cut. During one cut there are measurements showing no emission from the cutting front as well as measurements with continuum radiation.With the assumption of a constant emissivity over the observed wavelength range the temperature distribution on the cutting front is determined from the continuum emission spectra with two methods, fitting Planck's radiation law to the spectra and using the logarithmic Wien-plot.
