Phase transformation and microstructural evolution in Al-Mn-Fe-Si 3104 aluminum alloy made by laser directed energy deposition

Phase transformations in 3104 fabricated by direct energy deposition (DED) additive manufacturing (AM) under different thermal conditions and microstructure evolution after different heat treatments were investigated. The mechanical properties were also investigated and compared to direct-chill (DC) -cast counterparts. The high cooling rate in DED contributed to the ultrafine cellular structure with the dominant α-Al (Fe,Mn)-Si particles and Al₆(Fe,Mn) particles on the cellular boundary. The inhomogeneous distribution of particles can be observed with ultrafine particles inside the melt pool and coarsened particles at the heat-affected zone (HAZ). Second, due to the high cooling rate after DED fabrication, the conventional Al₆(Fe,Mn) to α-Al (Fe,Mn)-Si phase transformation is barely observed during the post-DED heat treatments. Last, there was no sign of the formation of dispersoids during the heat treatments. The high density of particles (∼100 times higher than DC-Cast alloy) and characteristic cellular structure in DED 3104 resulted in the slightly higher UTS(σ_UTS=212.2 ± 5.3 MPa), yield strength (σ_ys=103 ± 3 MPa) and ductility (17 ± 1.8%) than DC-Cast counterparts. With the use of the 450 °C preheated substrate during deposition, the microstructure and the phase transformations in DED 3104 were closer to those observed in DC-Cast 3104.