PSI - Issue 69

C.A. Biffi et al. / Procedia Structural Integrity 69 (2025) 47– 52

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Fig. 1. Laser power distributions attained through beam shaping in the present work. The beam irradiance (I) were normalized with respect to the maximum irradiance (I max ) at the same power level. After welding the obtained welded beads were characterised by optical microscopy (Leitz Aristomet, operated with polarised light) to assess the shape, dimension and microstructural features of the melted and heat affected zones. Moreover, differential scanning calorimetry (DSC, TA mo. Q25) was performed under flowing nitrogen atmosphere in the [0 °C, 300 °C] temperature range with a constant heating rate of 10 °C/min. 3. Results and discussion Figure 2 shows the cross sections of all the welded beads, obtained by varying laser scanning speed and beam shape. The base material (BM), unaffected by the welding process, is characterised by large, irregular grains containing thin, martensitic needles oriented a preferential direction, which changes from grain to grain. Such relatively coarse microstructure results from the comparatively slow solidification process taking place at the end of plasma arc melting. The melted zone presents smaller grains, mainly oriented from along the highest thermal gradient direction, i.e. from the border to the centre of the welded bead (see Figure 3). No martensitic structures can be observed at the present scale, possibly because of the faster cooling of the solidified material taking place during the laser beam welding process. Finally, only very faint features suggesting the possible presence of a heat affected zone (HAZ) could be observed by optical microscopy: indeed, all samples presented a 50 μm thick layer surrounding the melted zone, which evidently did not undergo laser remelting but does not show the large martensitic needles typical of the base material. As far as processing defects are concerned, both porosities and cracks are present. Porosities, which could be observed only in samples welded with a gaussian power distribution (BS0) and at high scanning speeds, present a spherical shape, suggesting that they be the result of gas entrapment during the solidification of a bead generated in deep penetration mode. It may therefore be inferred that beam shaping allowed to avoid the formation of gas porosities, likely by granting a smoother distribution of power on the surface of the workpiece and avoiding excessive local vaporisation. Cracks are present at the highest scanning speed only (100 mm/s), independently from the used beam shaping factor, and always run vertically along the centreline of the melted zone, likely at the intersection between grains growing from either sides of the welded bead. This suggests that they may be due to the formation of elevated internal stresses after solidification: indeed, both the rapid cooling related to the use of the highest scanning speed and the possible formation of secondary phases weakening the grain boundaries may cooperate in determining this outcome. All the obtained welded beads present a high aspect ratio (depth/width), suggesting that, owing to the high beam intensity used during the experiments, a deep penetration was attained. At first sight, it is immediately evident that scanning speed has the strongest influence on the shape and dimension of the melted zone, as can be seen in the graphs reported in Figure 4.