X-rays capture hot cracking in laser additive manufacturing

Laser Additive Manufacturing (LAM) is a transformative technology that builds complex 3D components layer by layer from metallic alloys, directly from a digital design. It utilizes a rapidly moving laser to melt metal powders or wires, which then fuse into a solid mass.

Despite its advantages over traditional manufacturing processes to create complex structures for applications in aerospace, power generation and biomedical industries, LAM is susceptible to the formation of hot cracks during solidification. These cracks can compromise the structural integrity of the parts produced. Hot cracking is a common issue in high-strength aluminium alloys, particularly in the 6xxx series, due to their rapid solidification rates.

To tackle this problem, extensive research has been dedicated to finding solutions. One strategy is to reduce the thermal stress that contributes to cracking by adjusting the laser modulation such as power or pulse shape. However, it is essential to first gain a deeper understanding of how solidification affects crack formation. 

This study employed high-resolution synchrotron X-ray imaging to observe in situ the effects of different laser pulse shapes on the solidification and cracking behaviour of two aluminium alloys: AA6061 and AlSi10Mg (a less crack-susceptible alloy).

Two different laser pulse shapes – rectangular and ramped-down – were tested. The rectangular pulse shape involved a 10 ms laser on-time at 1.4 kW, while the ramp-down pulse linearly reduced the power from 1.4 kW to zero within 10 ms after turning on for 5 ms.

Using the intense and coherent X-rays at beamline ID19, it was possible to achieve a frame rate of 100 000 images per second and a 4-µm pixel size. This high resolution allowed for detailed observations of the melt pool dynamics and crack propagation, recording the solidification pathway for the first time. Figure 1 depicts the experiment setup and an example image. 

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Fig. 1: In-situ X-ray imaging results of the solidification behaviour of aluminium alloys during laser processing. a) Schematic of the experimental setup, including an X-ray image captured during the laser processing. b) Two pulse shapes were used in the experiment: rectangular and ramp-down, respectively.

For the AA6061 alloy, using the rectangular pulse shape resulted in a melt pool that reached its maximum size just before the laser was switched off. As the melt pool solidified, cracks started to form from the boundary at a rate of 22.78 μm/ms. In contrast, the ramp-down pulse caused the solid–liquid interface to move more slowly towards the centre, which delayed crack initiation. Although cracks still formed, they were smaller and less severe compared to those observed with the rectangular pulse.

For the AlSi10Mg alloy, neither pulse shape resulted in cracks. The melt pool was longer and deeper, suggesting that the material absorbed laser energy more efficiently and had a longer solidification time. This helped the material to release thermal stresses more effectively, thus reducing the likelihood of cracking. However, radiographs revealed excessive porosity, which points to a need for further investigation into pore behaviour, even though no cracks were present.

Click image to enlarge

Fig. 2: X-ray tomography results of the AA6061 alloys after laser processing, showing 3D crack network. a) (1): 3D crack network formed after being treated with a pulsed laser with a rectangular pulse shape, shown in 3D. (2): Cross-section view of the crack network perpendicular to the laser direction; (3): cross-section view of the crack network along the laser direction. b) (1): 3D crack network formed after being treated with a pulsed laser with a ramp-down pulse shape, shown in 3D. (2): Cross-section view of the crack network perpendicular to the laser direction; (3): cross-section view of the crack network along the laser direction.

High-resolution synchrotron X-ray microtomography at beamline ID19 provided a comprehensive characterisation of the crack networks (Figure 2). In AA6061 processed with the rectangular pulse, cracks extended in a tree-branch shape along the centre of the laser track, reaching depths of about 300 μm with a total volume of 0.0020 mm³ (Figure 2a). The ramp-down pulse significantly improved crack distribution, resulting in smaller and less extensive cracks (Figure 2b).

Although the longest cracks were also found initiated from the bottom of the melt pool with a length of about 400 μm, the majority of the cracks were initiated in the centre area of the melt pool, with a total volume of about 0.0010 mm3, only 50% of the volume with the rectangular pulse. 

In conclusion, this study shows that by reducing the solidification rate, thermal gradient, and associated stresses, laser pulse modulation can be an effective tool to reduce crack susceptibility in hard-to-process alloys during LAM. These observations led to the development of an analytical hot cracking model, which can predict crack susceptibility. More fundamentally, the results demonstrate that modifying thermal conditions provides a pathway to crack elimination in LAM, setting the stage for more complex laser manipulations to improve the printability and mechanical properties of these materials.

This innovative approach to laser pulse shaping opens new avenues for improving the quality and reliability of parts produced by laser additive manufacturing.

Principal publication and authors

In situ observation and reduction of hot-cracks in laser additive manufacturing, Y. Chen (a,b), D. Zhang (a), P. O’Toole (a), D. Qiu (a), M. Seibold (c), K. Schricker (c), J.-P. Bergmann (c), A. Rack (b), M. Easton (a), Commun. Mater. 5, 84 (2024); https://doi.org/10.1038/s43246-024-00522-3
(a) RMIT Centre for Additive Manufacturing, RMIT University, Melbourne (Australia)
(b) ESRF
(c) Technische Universität Ilmenau, Ilmenau (Germany)