Insights into the Cracking Mechanism Affecting a Structural Aluminum Alloy Processed by Laser Powder Bed Fusion

Abstract

Structural Aluminum alloy- 6061 (Al-0.8Si-1.2Mg %wt) is frequently reported to becrack sensitive during laser powder bed fusion (L-PBF). The lack of in-depthunderstanding of the root causes of hot cracking is an impediment for designing partsfor safety-critical applications. In the present work, the ambiguity on crackingmechanism is resolved and is identified as solidification cracking. Cracking is found tooccur only along columnar grain boundaries having a large misorientation, θ > 15°.This is attributed to the coalescence undercooling that lowers the dendrite coalescencetemperature. Our findings using the Rappaz-Drezet-Gremaud (RDG) criterion andRosenthal calculations show that the existence of stable liquid films is linked tomisorientation, causing a sudden increase in pressure drop leading to cracking. In thiswork, hot cracking sensitivity (HCS) maps are developed using the RDG criterion underconditions typical of L-PBF to investigate:the effect of the processing conditions, the melting parameters (laser power andscanning speed) on the thermal gradient (G) and solidification Velocity (v) inferred fromthermal simulations, on the hot tearing sensitivity. This led us to an understanding ofthe required G and v, and therefore the required laser power and speed to decreasethe cracking susceptibility and propose improvements to process the 6061 Al-alloyusing L-PBF relying on the developed HCS maps.the effect of addition of alloying elements such as Si and Mg, on the crackingsensitivity. This can be further used as guidelines to suggest chemical compositionmodifications of 6061 Al-alloy to improve its processability by L-PBF.