Effects of solution ageing in compressive behaviour of SLM-In718 triply periodic minimal surfaces

Abstract This study aims to investigate the effect of heat treatment on the microstructure and mechanical properties of triply periodic minimal surface (TPMS) structures manufactured by selective laser melting (SLM) from Inconel ® alloy 718 (In718). Although many aspects of SLM-In718 have been studied, its as-built (AB) and heat treated (HT) performance under compression has not been thoroughly investigated. To this end, sheet-based diamond, gyroid, and primitive morphologies along with bulk specimens were manufactured and tested under AB and HT conditions. Standard solution ageing (solution treatment + two-step ageing) was carried out to improve some of the inherent deficiencies of SLM-In718 such as the development of brittle Laves and $$\delta$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>δ</mml:mi> </mml:math> phases. It was found that the overall mechanical performance of the HT TPMS specimens were improved in terms of stiffness, strength, and energy absorption among others. Furthermore, the geometry of the TPMS specimens dictated the extent of enhancement such that the diamond geometry outperformed the others in many aspects. In contrast, the primitive structure demonstrated the most stable plateau stage in both AB and HT conditions. Microstructural analysis revealed occasional precipitation of second-phases carbide particles at grain boundaries. Moreover, niobium was made available by dissolution of Laves phases during solution treatment which was later used in precipitation of strengthening $$\gamma ^{\prime}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>γ</mml:mi> <mml:mo>′</mml:mo> </mml:msup> </mml:math> and $$\gamma ^{\prime\prime}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>γ</mml:mi> <mml:mo>″</mml:mo> </mml:msup> </mml:math> phases during ageing. Finally, a new scheme for analysing the energy absorption was devised which could provide a new perspective towards studying energy efficiency of porous structures. Accordingly, it was found that heat treatment improves the mechanical properties contributing the first two stages of energy absorption (initial and plateau) while limiting the fraction of the final stage. In conclusion, it was demonstrated that although heat treatment could address issues like residual stresses and microstructural shortcomings of SLM-In718, the extent of this rectification is further controlled by cell topology of the TPMS structure.