Solidification crack propagation and morphology dependence on processing parameters in AA6061 from ultra-high-speed x-ray visualization

X-ray Imaging of Alloy Solidification: Crystal Formation, Growth, Instability and Defects

“…Additive manufacturing (AM) techniques such as laser powder bed fusion (LPBF) and directed energy deposition (DED) have been increasingly explored for the manufacture of products for the aerospace, automotive and biomedical sectors, because of their capability to produce complex, near-net-shape components directly from a 3D CAD file [3,141]. In situ Xray imaging with high temporal resolution (microseconds) provided by synchrotron X-ray sources is being used to investigate the process-structure relationships in these processes, such as the high speed melt pool behaviour, heat source-powder bed interactions and defect formation [48][49][50][51][52][53][54]. It is also being used to shed light on other rapid solidification processes such as laser spot welding [68,142,143].…”

“…Once formed, hot tears can run the entire length of columnar grains, and may even propagate across multiple layers of the print [148]. So far, in situ studies of hot tearing have mainly focused on AA6061, given its high susceptibility [52,143,149], with a strong link proposed between hot tears and pores. On the one hand, pores acted as preferential sites for hot tear initiation, for example, in the cases of both large keyhole-induced pores and smaller trapped-gas-induced pores [52,149].…”

“…So far, in situ studies of hot tearing have mainly focused on AA6061, given its high susceptibility [52,143,149], with a strong link proposed between hot tears and pores. On the one hand, pores acted as preferential sites for hot tear initiation, for example, in the cases of both large keyhole-induced pores and smaller trapped-gas-induced pores [52,149]. On the other hand, once cracks were present in the material, gas trapped in the melt pool could nucleate as pores on the crack surfaces [52].…”

“…On the one hand, pores acted as preferential sites for hot tear initiation, for example, in the cases of both large keyhole-induced pores and smaller trapped-gas-induced pores [52,149]. On the other hand, once cracks were present in the material, gas trapped in the melt pool could nucleate as pores on the crack surfaces [52]. The strong interaction between pores and hot tears suggested that strategies for their avoidance should be considered in a conjugated way during future material-process design.…”

“…The focus of solidification research on Al alloys derives from the relatively easy-to-achieve melting temperatures of around 660 °C, and excellent absorption contrast between Al and typical alloying elements such as Cu and Zn. Further, the high temporal resolution now afforded also allows new insights to be gained on transient phenomena during non-equilibrium, rapid solidification processes, such as the effect of processing parameters on melt pool morphology and melt pool defects in additive manufacturing (AM) [48][49][50][51][52][53][54], and has also made a contribution to the development and validation of new AM processes [55,56].…”