Selective electron beam melting of high strength Al2024 alloy; microstructural characterization and mechanical properties

Abstract

Abstract To date, the most commonly used aluminum alloys for additive manufacturing are eutectic/near-eutectic alloys. However, fabricating high strength aluminum parts has not yet been fully investigated due to solidification cracking and porosity formation, which make these alloys very hard to be processed. Electron beam selective melting (EBSM) of aluminum alloys has some advantages over selective laser melting. The process is not affected by reflectivity of aluminum powders, and parts exhibit less induced thermal stresses. In addition, oxidation is reduced due to vacuum condition. Therefore, the process provides high potential for the fabrication of aluminum alloys, specifically, Al2024 alloy which is widely used in different industries such as automotive and airline industries. Hence, these industries are highly benefitted from being able to build defect-free AM parts. This study is significant because Al2024 alloy is highly susceptible to solidification cracking and porosity formation, and crack-free samples with high relative densities have not been built by EBM process previously. The baseplate preheating temperature was set to 350 °C, and samples were fabricated by applying a proper powder bed preheating configuration for each layer. Different sets of processing parameters were used to investigate their effect on the performance of the parts. Archimedes technique was used to measure the relative density of samples, ranging from 95% to full-dense. The results showed that higher relative densities could be achieved by increasing the input energy to an optimum value; however, by further increasing the energy, the relative density decreases, which can be attributed to porosity formation. SEM and EBSD results showed equiaxed grains in a crack-free microstructure. Grain structure consisted of high angle grain boundaries having different crystallographic orientations with a few sub-grains. Additionally, AlCuMnFe, AlCuMnFeSi, and eutectic Al2Cu phases were uniformly distributed in the α phase matrix. Microhardness results showed an almost uniform change of hardness values in both horizontal and vertical sections, ranging from 100 HV to 110 HV for as-built samples. Moreover, tensile and yield strengths of as-built samples reached 314 MPa and 191 MPa, respectively, which can be further increased by a proper post-processing treatment.