Intrinsic Heat Treatment of an Additively Manufactured Medium Entropy AlCrFe2Ni2-Alloy

The alloy AlCrFe2Ni2, known as medium entropy alloy (MEA, Delta S/R similar to 1.33) was processed using Laser Direct Energy Deposition (L-DED). The alloy is designed to develop a Widmanstatten type duplex microstructure following a solid state phase transformation which is controlled by the cooling rate. During L-DED this transformation is hardly accomplished, commonly calling for a post-build heat treatment. For the first time, an intrinsic laser-based heat treatment was applied to promote this phase transformation in the Additively Manufactured HEA in a layer by layer approach. Process parameters for the intrinsic heat treatment were varied and investigated in terms of temperature-time cycles, cooling rates and penetration depth. The microstructure of as-built and differently heat-treated samples was investigated. In the as-built condition, the duplex structure consist mainly of ordered and disordered bcc phase and a small fraction of thin fcc-plates (40%). It was found that the fcc phase fraction can be significantly increased up to 58% by applying an intrinsic heat treatment. The heat treatment involves nucleation of new fcc plates as well as thickening of the existing plates. The process-related inhomogeneity of the microstructure resulting from heat affected zones at melt pool boundaries is not eliminated due to the short interaction times. In contrast to the conventional post-process heat treatment at 900 degrees C for 6 h, the microhardness is not significantly reduced during intrinsic heat treatment and remains in the range of 400 HV0.3. Intrinsic heat treatment is however beneficial, since it can be applied selectively. Thus, it offers novel possibilities for surface cladding applications.

Automated summary

The medium entropy alloy AlCrFe2Ni2 was processed using Laser Direct Energy Deposition (L-DED). An intrinsic laser-based heat treatment was applied for the first time to promote phase transformation and increase the fcc phase fraction in the alloy. The microhardness remained stable during the heat treatment, offering novel possibilities for surface cladding applications.