Influence of substrate thickness on the microstructure and mechanical properties of Al–Si–Cu/Al–Cu hybrid structures additively manufactured by electron beam directed energy deposition

Electron beam directed energy deposition (EB-DED) has been successfully employed to additively manufacture and repair Al–Cu alloys, however, the effects of thermal histories on the microstructure and properties have been little investigated. In this study, a specially developed Al–Si–Cu wire (380D) was deposited on the Al–Cu substrates (2A14-T6) via EB-DED to fabricate hybrid components for additive repair purposes. By using both experimental and numerical methods, the effects of substrate thickness on the thermal histories experienced and the microstructures resulted across the structural interfaces, as well as the mechanical properties of the hybrid parts, are comprehensively investigated. Results show that the columnar to equiaxed transition (CET) occurs in the deposits when the temperature gradient G and solidification velocity R satisfy the critical condition of G 2 /R < 5.075 × 10 12 . A thicker substrate leads to a higher G within the molten pool, which postpones the CET and increases the area of columnar grains. The columnar grains show lower strength than the equiaxed grains and are more susceptible to fractures under tensile loads. In the bottom layer of deposits, the θ-Al 2 Cu and Si phases are apt to precipitate under the extremely high solidification cooling rate, forming very fine networks, and resulting in much higher strengths than the upper deposited layers. Multiple thermal cycles act as in-situ heat treatments to promote the precipitation of θ' and θ phases in the bottom deposit regions and the 2A14-T6 substrates. During the EB-DED deposition, the 2A14-T6 substrates are over-aged due to the θ'' phase coarsening and transforming into θ' and θ phases, and the decrease in strength is less when the substrate is thicker and the temperature is lower. Thus, there is a trade-off between the strengths of deposit and substrate in the hybrid parts. It is suggested that the CET in the deposit should be facilitated while maintaining the substrate temperature below 200 °C during the EB-DED process, so as to achieve a hybrid component with higher overall strength. • The Al–Si–Cu/Al–Cu hybrid structures with different substrate thicknesses were fabricated by EB-DED process. • The critical condition of columnar to equiaxed transition in deposit was determined. • The effect of thermal histories on precipitation behaviors is quantitatively studied. • It was found that there existed a trade-off between the strengths of deposit and that of substrate in hybrid parts.