High power selective laser melting of crack sensitive nickel-base alloy CM247LC, including dimensional analysis and modelling

Compared to reference parameters in the low power and scan velocity range, which lead to dense and crack-free CM247LC LPBF samples due to in-situ crack healing, high power, high scan velocities and increased laser beam diameters are investigated, to decrease the production time further. By keeping the maximum laser intensity from the reference and the laser power to scan velocity ratio constant, the intensity approach provides an initial estimation for the laser spot size regarding the measured Archimedean density and crack density in the high power and high scan velocity range. The investigated cracks are identified as re-melting cracks. Solidification or hot cracks are not observed, since the crack healing effect for those kinds of cracks still occurs. Furthermore, a melt pool depth range is discovered, where not only solidification cracks can be avoided, but also re-melting cracks, which are resulting from higher laser power inputs. This theory can be proven by further laser spot size optimization, where the melt pool depth comes closer to the mentioned range. The Archimedean density and crack density results, in case of the 600 W power parameter with 2400 mm/s scan velocity and a beam diameter of 164 µm, are close to the one obtained from the reference with 200 W, a scan velocity of 800 mm/s and a laser spot of 90 µm. With the intensity approach and laser beam diameter optimization, the production time can be reduced by 300%. Based on dimensional analysis, a model, which combines the samples density with the crack density through the melt pool depth, is presented. Six main and two additional process and laser parameters are taken into relation. The result from the model and the measured values from experiments are in good agreement. Additionally, the influence of the doubled layer thickness and an increased hatch distance by 50% with varying scan velocities on the Archimedean density and crack density is analysed.

An Innovative Approach to Automated Photo-Activation of Crop Acreage Using UAVs to Stimulate Crop Growth

Photoactivation of plants by laser treatment is a promising direction in the development of modern agricultural production. Treatment of plants with radiation with specified characteristics stimulates the development of plants, the formation of generative traits and an increase in yield. An approach based on the use of a specialized laser installation mounted on an unmanned aerial vehicle (UAV) is proposed to automate the process of photoactivation of large cultivated areas. It is possible to perform laser activation of large areas with minimal expenditure of time and human resources due to autonomous processing of the field with the help of UAVs. An algorithm for calculating a covering trajectory for covering large rectangular areas with a laser spot with given characteristics is proposed in the paper. A methodology for calculating the required power of the laser installation depending on the altitude and flight time of the UAV is presented. The advantage of the developed approach is its versatility, since this approach takes into account the characteristics of a laser installation and can be used with devices of various types. Depending on the laser parameters, the algorithm builds such a trajectory for the UAV so that the irradiation of plant seedlings is uniform throughout the entire processing process. Field experiments were conducted when the UAV moved along the calculated trajectory at a speed of 0.3 m/s and the average processing time for a field 200 m long and 1 m wide was 9 minutes. The results of field experiments show that laser irradiation on most of the studied crops increased the yield and height of the stand (in cereals - in four out of six crops, in legumes - in four out of five studied crops). The proposed algorithm for constructing a path for uniform laser irradiation of a site takes into account the area of the laser spot to ensure the required radiation characteristics when using any laser installation.

On the Determination of Thermal Conductivity From Frequency Domain Thermoreflectance Experiments

The Fourier and the hyperbolic heat conduction equations were solved numerically to simulate a frequency-domain thermoreflectance (FDTR) experiment. Numerical solutions enable isolation of pump and probe laser spot size effects, and use of realistic boundary conditions. The equations were solved in time domain and the phase lag between the temperature of the transducer (averaged over the probe laser spot) and the modulated pump laser signal, were computed for a modulation frequency range of 200 kHz to 200 MHz. Numerical calculations showed that extracted values of the thermal conductivity are sensitive to both the pump and probe laser spot sizes, while analytical solutions (based on Hankel transform) cannot isolate the two effects, although for the same effective (combined) spot size, the two solutions are found to be in excellent agreement. If the substrate (computational domain) is sufficiently large, the far-field boundary conditions were found to have no effect on the computed phase lag. The interface conductance between the transducer and the substrate was found to have some effect on the extracted thermal conductivity. The hyperbolic heat conduction equation yielded almost the same results as the Fourier heat conduction equation for the particular case studied. The numerically extracted thermal conductivity value (best fit) for the silicon substrate considered in this study was found to be about 82-108 W/m/K, depending on the pump and probe laser spot sizes used.

Sizing the Depth and Width of Narrow Cracks in Real Parts by Laser-Spot Lock-In Thermography

We present a complete characterization of the width and depth of a very narrow fatigue crack developed in an Al-alloy dog bone plate using laser-spot lock-in thermography. Unlike visible micrographs, which show many surface scratches, the thermographic image clearly identifies the presence of a single crack about 1.5 mm long. Once detected, we focus a modulated laser beam close to the crack and we record the temperature amplitude. By fitting the numerical model to the temperature profile across the crack, we obtain both the width and depth simultaneously, at the location of the laser spot. Repeating the process for different positions of the laser spot along the crack length, we obtain the distribution of the crack width and depth. We show that the crack has an almost constant depth (0.7 mm) and width (1.5 µm) along 0.7 mm and features a fast reduction in both quantities until the crack vanishes. The results prove the ability of laser-spot lock-in thermography to fully characterize quantitatively narrow cracks, even below 1 µm.

In-Process Height Displacement Measurement Using Crossed Line Beams for Process Control of Laser Wire Deposition

We propose the use of the line section method with crossed line beams for the process control of laser wire deposition. This method could be used to measure the height displacement in front of a laser spot when the processing direction changes. In laser processing, especially laser deposition of metal additive manufacturing, the laser process control technique that controls the processing parameters based on the measured height displacement in front of a laser processing spot is indispensable for high-accuracy processing. However, it was impossible to measure the height displacement in front of a processing laser spot in a processing route in which the processing direction changes as the measurement direction of the conventional light-section method comprising the use of a straight-line beam is restricted although the configuration is simple. In this paper, we present an in-process height displacement measurement system of the light-section method using two crossed line beams. This method could be used to measure the height displacement in a ±90° direction by projecting two crossed line beams from the side of a laser processing head with a simple configuration comprising the addition of one line laser to the conventional light-section method. The height displacement can be calculated from the projected position shift of the line beams irrespective of the measurement direction by changing the longitudinal position on the crossed line beams according to the measurement direction. In addition, the configuration of our proposed system is compact because the imaging system is integrated into the processing head. We could measure the height displacement at 2.8–4 mm in front of a laser processing spot according to the measurement direction by reducing the influence of intense thermal radiation. Moreover, we experimentally evaluated the height displacement measurement accuracy for various measurement directions. Finally, we evaluated continuous deposition in an “L” shape wherein the deposition direction was changed while using a laser wire direct energy deposition machine for the laser process control based on the in-process height displacement measurement result. We achieved highly accurate continuous deposition at the position wherein the processing direction changes despite the acceleration and deceleration of the stage by laser process control.