PSI - Issue 53

Daniel R. Galán-Rivera et al. / Procedia Structural Integrity 53 (2024) 407–415 Daniel R. Galán-Rivera et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Regarding to repairing sequence, progress direction has been selected longitudinal to tramway rail. All beams were performed maintaining the same direction for every powder employed. for some materials the optimal results were obtained varying the laser beam direction, because the powder nozzle for injection is positioned on the opposite side. On the other hand, to avoid cracks formation after laser coating operations, substrate (tramway rail) was preheated. Customer indicates that the maximum temperature for heating should not exceed about 160 degrees (because higher temperatures can affect the embedding that surrounds the rails on the track). For that reason, temperature range used for tramway rail preheating was 100-120ºC, to ensure that the limiting temperature was not exceeded. After all layers have been deposited, thermal blankets were employed to ensure a slow cooling and subsequently, to avoid cracking of substrate due to an abrupt cooling. Regarding the nozzle used, a lateral nozzle was selected instead of a coaxial one because it does not allow access to the desired area for the coating process. Diameter of nozzle was 1.6 mm. 3. Results and discussion This section describes the difficulties founded and the solutions in the repairing operations, for all the analyzed materials (powders). Study also tries to achieve the optimal laser repairing parameters to obtain a good quality deposited surface, free of defects. After finished the repair process on each specimen, macrographs were performed in the cross sections in order to verify the soundness of the repair and the presence of defects. Optical images obtained from macro examination and the reported hardness values from hardness tests are attached below. The hardness test was performed according to the UNE EN 15614-7 standard, paying special attention to the values obtained in the heat affected zone (HAZ), with is the area with the most critical microstructures regarding to cracking phenomena. For that reason, five hardness measurements (indentations) were located in the heat affected zone of specimens. 3.1. Material 1 This material is a corrosion-resistant metal powder, recommended for welding austenitic base metals having a low carbon content. Deposited material reaches hardness values about 250HV (like rail hardness) which can be double with work hardening (Table 2). The optimization of laser process involves several difficulties: • Cracking of substrate after cooling operations: first probes performed in real workpieces suffered cracks along the fusion line, at latest stages of repairing. In order to solve this issue, rails were preheated to guarantee a temperature between 100-200ºC in the coating area, at the beginning of each layer, and ensuring that in the rest of the rail did not exceed that temperature • Presence of non-metallic inclusions deposited along the repairing area was observed in some of the samples. Inclusions largely correspond to silicates, and they appear in all the analyzed materials (metal powder). The causes of these defects are mainly two: first, the powder flow is excessive for the applied power. To solve this issue, either the laser power can be increased, or the powder flow rate reduced. In this case, it was decided to lower the powder quantity deposited, since increasing the power of laser would exceed the established temperature limit. On the other hand, the presence of inclusions is attributed to orientation of injected powder relating to laser beam movement. At the beginning, injection was carried on from the back part, but it did not guarantee the absence of inclusions. However, injecting the powder from the front, has allow ensuring the absence of non-metallic inclusions in the repairing areas (Fig. 3).