Controlling Solidification Microstructure of Aluminum Alloys using Eutectic/Peritectic Reactions in Laser Powder Bed Fusion Process

In recent, laser powder bed fusion (L-PBF) has emerged as one of the most representative metal additive manufacturing techniques capable of producing metallic components by using a scanning laser beam to melt consecutive bedded-powder layers selectively. The L-PBF process enables the fabrication of aluminum (Al) alloys with superior and anomalous mechanical properties. Such unique properties were responsible for the formation of non-equilibrium microstructure and metastable phases in rapid solidification (at an extremely high cooling rate of 10⁵-10⁷K/s) during the L-PBF process. It has been reported that the L-PBF processed Al-Fe alloys with a near eutectic composition exhibited significantly refined solidification microstructures, contributing to high mechanical performance. In the eutectic reaction in rapid solidification, alloy elements could be partitioned into a liquid phase rather than a primary solidified phase, resulting in the enhanced formation of the second solid phase (Al₆Fe phase in an Al-Fe system). It is assumed that alloy elements might be partitioned into the primary solidified α-Al phase (rather than the liquid phase) through a peritectic reaction in rapid solidification. The partitioned solute elements in the α-Al supersaturated solid solutions would play a significant role in solid-solution strengthening (or strengthening by atomistic clusters in the α-Al matrix). These insights can open an opportunity for controlling refined microstructures of Al alloys by elemental partitioning via solidification paths of eutectic or peritectic reactions in the L-PBF process. In this concept, we have selected alloy elements exhibiting different solidification paths in Al-X binary phase diagrams. Cu and Ti elements were used as third alloy elements for the Al-Fe-X ternary system in the present study. Cu element exhibits a eutectic reaction in an Al-Cu binary system (partition coefficient, <i>k</i>_Cu^S/L &lt; 1) and forms (Al,Cu)₆Fe phase in an Al-Fe-Cu ternary system. In contrast, Ti element exhibits a peritectic reaction in an Al-Ti binary system (partition coefficient, <i>k</i>_Ti^S/L &gt; 1) and independently forms the Al₃Ti phase (no partitioning into the Al₆Fe phase). Based on the alloy element selections, we designed two ternary alloy compositions of Al–2.5Fe–2Cu and Al–2.5Fe–1.5Ti (mass%) available to the L-PBF process utilizing the thermodynamic calculations.<br/>In this study, L-PBF processing was performed using gas-atomized ternary alloy powders with an average particle size of about 20 mm. Rectangular alloy samples were fabricated using a ProX DMP 200 machine (3D Systems, USA) under a wide range of laser scan speeds (0.6 ~ 1.4 m/s) and laser power (102 ~ 204 W). The results of measuring the sample density provided the optimum laser parameter sets for the manufacturing of both alloy samples with high relative densities above 99 %, indicating high L-PBF processability for the Al–Fe–Cu and Al–Fe–Ti ternary alloy powders. The L-PBF processed both alloys exhibited microstructure consisting of a number of melt pools in which regions locally melted and rapidly solidified due to scanning laser irradiation. The added Cu element was partitioned into the refined Al₆Fe phase, resulting in the formation of (Al,Cu)₆Fe phase with an orthorhombic structure (<i>oC</i>24). A certain amount of solute Cu (approximately 0.6 %) was detected in the a-Al matrix. A higher solute Ti content above 1 % was detected in the α-Al matrix of the L-PBF processed Al–Fe–Ti ternary alloy. Such a trend appeared more significantly in relatively coarsened cellular microstructures along melt-pool boundaries. These results indicate that the elemental distribution could be controlled via different solidification paths in the L-PBF process. Mechanical properties of the L-PBF processed ternary alloys will be presented and discussed in terms of Cu or Ti elemental distribution in the refined solidification microstructures.