Effect of Post‐heat Treatment on the Tensile and Cryogenic Impact Toughness Properties of Inconel 718 Manufactured by Selective Laser Melting

Abstract

This study investigates the effect of post‐heat treatments (solution treatment + aging [SA] and hot isostatic pressing + aging [HIPed]) on the tensile property as well as the room temperature and cryogenic impact toughness of Inconel 718 alloy manufactured by selective laser melting (SLM). The porosities of the three specimens measure SLM SA: 1.10% and SLM HIPed: 0.06%, and wrought SA: 0.01%. Initial microstructural observation shows γ″ + γ′ phase in all three alloys (SLM SA, SLM HIPed, and wrought SA). SLM SA and wrought SA show δ phase of greater fraction compared with SLM HIPed. SLM HIPed and wrought SA show a generally uniform microstructure, and equiaxed grains are found. In tensile tests, SLM HIPed shows greater strength compared with that of wrought SA, but SLM SA shows lowest yield strength among them. The elongations of SLM alloys are ≈1/3 of that of wrought SA. An impact test result shows that impact toughness value at cryogenic temperature of SLM HIPed alloy shows 80% of that of wrought SA. However, SLM SA shows relatively lower impact toughness. In addition, this study attempts to identify the deformation behavior and fracture mechanism of SLM Inconel 718 alloy based on the microstructural characteristics.

Comparison of the Structure, Mechanical Properties and Effect of Heat Treatment on Alloy Inconel 718 Produced by Conventional Technology and by Additive Layer Manufacturing

“…SLM samples revealed half of the ductility value compared with ACR samples, as was also shown in [35]. A similar problem was observed in the determination of notch toughness at room temperature (see Figure 16).…”

“…However, the production of parts from Inconel 718 by additive manufacturing is very promising from the point of view of the very low machinability of the material. SLM samples revealed half of the ductility value compared with ACR samples, as was also shown in [35]. A similar problem was observed in the determination of notch toughness at room temperature (see Figure 16).…”

A critical review on the effects of process-induced porosity on the mechanical properties of alloys fabricated by laser powder bed fusion

“…Vertical samples (ZX) had the poorest fracture toughness, horizontal XZ samples the best Results were attributed to tensile stress-strain response XZ samples were also noted to contain lower porosity [130] AlSi10Mg K IC calculated from fatigue fracture surfaces for vertical, diagonal and horizontal samples were 12.8 MPa m 0.5 vs 13.1 MPa m 0.5 and 14.0 MPa m 0.5 , respectively LoF pores along melt-pool boundaries aided crack propagation in vertical samples [151] AlSi10Mg Horizontal Charpy samples contain more gas pores but performed better than vertical samples due to microstructural reasons-cracks go across rather than along melt-pool boundaries Pores enlarged during T6 caused toughness to decrease and horizontal samples now performed worse than vertical samples Attributed to pores causing cracks to deviate from their usual path and the reduced load-bearing section area HIP ? T6 mitigated the pore growth issue, but Si coarsening limited toughness improvements (i) coalescence of porosity during impact [149], (ii) altering of the crack propagation path from one based on microstructure toward one based on porosity distribution [150,151], (iii) increased stress-concentration effects [75], and (iv) a substantial reduction in the actual cross-sectional area occupied by material that is available to absorb impact energy [148]. Thus, when Wang et al surveyed existing literature on the impact toughness of LPBF 316L and found a wide scatter in the impact toughness, as shown in Fig.…”