Towards an alloy design strategy by tuning liquid local ordering: What solidification of an Al-alloy designed for laser powder bed fusion teaches us

Abstract

High-strength aluminium alloys from the 2XXX, 6XXX, and 7XXX series suffer from severe hot cracking issues during 3D printing. Thus, there is a need to design new alloys with an improved hot cracking resistance while taking advantage of the non-equilibrium processing conditions of laser powder bed fusion to achieve optimized properties. Here, we report the successful fabrication of a new Al-Mn-Ni-Cu-Zr alloy designed for L-PBF. The as-built microstructure was characterized at different scales using synchrotron X-ray nano-tomography and microscopy with a special focus on automated crystallographic orientation mapping (ACOM) in transmission electron microscopy. Based on this multiscale microstructural investigation, we identify various mechanisms involving the presence of a locally ordered liquid during solidification under conditions typical of additive manufacturing. This local liquid ordering is often defined as Icosahedral Short Range Order (ISRO). The multiple consequences of ISRO in the liquid on the hierarchical microstructure inherited from additive manufacturing are discussed. ISRO enables refinement of the grain size thus improving the processability via an increase of the hot cracking resistance, producing equiaxed grains and twinned dendrites contributing to randomizing the crystallographic texture, and possibly extending supersaturated solid solutions through a cage effect. It is concluded that promoting ISRO should be considered a promising pathway to design alloys for additive manufacturing with good processability and optimized properties.