In this work, a 316L stainless steel to Inconel 625 functionally graded material (FGM) was built using different deposition strategies (named as direct and smooth-type interfaces) by Twin-Wire and Arc Additive Manufacturing (T-WAAM). This combination of materials is of interest in chemical plants, oil & gas, and nuclear applications, where high corrosion and wear resistance are essential requirements. Although these properties are superior in Inconel 625, replacing Inconel with stainless steel in strategic regions of structural components can reduce the overall costs and parts’ weight. Both direct and smooth transition interfaces were tested and characterized. Microscopic analysis revealed that each interface and the as-built samples had an austenitic matrix, and every sample was well bonded and free of defects. Different types of microstructures evolved at the interfaces due to distinct gradients in composition. Synchrotron X-ray diffraction measurements showed that the smooth-gradient produced secondary phases, such as δ-phase (Ni 3 Nb) and carbides, that were not present with the direct interface strategy. Overall, the properties were superior in the FGM with a direct interface, which experienced higher strengths and elongations upon failure. Moreover, neutron diffraction measurements revealed that lower residual stresses developed in the direct interface FGM than in the smooth gradient FGM. 

Wire and Arc Additive Manufacturing of Stainless Steel 316L/Inconel 625 Functionally Graded Material: Development and Characterization

In this research, additively manufacturing a functionally graded material (FGM) with a compositional range of Ni-based Inconel superalloy (Inconel 718) and Fe-based stainless steel (SS 316L) via directed energy deposition (DED) was examined. The microstructural transformation, defect behavior, and Vickers hardness of the material were each determined as a function of the discrete chemical composition of the FGM varied in steps of 10 wt% of the two materials across its length. In particular, for the specific compositions of 30 wt% Inconel 718/70 wt% SS 316L and 20 wt% Inconel 718/80 wt% SS 316L, critical pores and cracks (defects) initiated by ceramic oxides occurred due to the presence of intermetallic and carbide compounds. In addition, the results of electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) analyses of the FGM demonstrate that the thermal and residual stresses due to constitutional supercooling and columnar-to-equiaxial transition (CET) became concentrated at the grain boundaries, thereby further contributing to the formation of the defects. The measured Vickers hardness was inevitably found to be minimal near the defective compositional range regardless of laser parameter optimization due to the reduced generation of segregants in the inter-dendritic regions and the increased formation of precipitates at the grain boundaries. The results of microstructural and mechanical analyses indicate that deliberate and strategic removal of the defective compositional range helped obtain a robust FGM composed of Inconel 718 and SS 316L without noticeable defects.

Selective compositional range exclusion via directed energy deposition to produce a defect-free Inconel 718/SS 316L functionally graded material

The precise shape of a shift block support for the turbine blade of a hovercraft was additively manufactured using 17–4 precipitation-hardened stainless steel (STS 630) powder through laser powder ...

Additive manufacturing of a shift block via laser powder bed fusion: the simultaneous utilisation of optimised topology and a lattice structure

The customization of plastic injection molding dies is technologically and economically limited by conventional manufacturing processes. Recent advances in hybrid additive manufacturing (HAM) have provided more geometrical freedom for the manufacturing of parts with desired properties. In this paper, we report manufacturing of a hybrid 420/Corrax stainless steel with a reliable interface that can be applied in the manufacturing of next-generation geometrically complex plastic injection molding dies with enhanced strength and corrosion resistance. AISI 420 martensitic stainless steel is used as a cost-effective substrate, and a maraging stainless steel grade, known as Corrax, is printed on top of it using laser powder bed fusion (LPBF). A hybrid heat treatment cycle is applied to improve metallurgical properties and to enhance mechanical compatibility between the martensitic and the maraging stainless steels. Tensile tests coupled with scanning electron microscopy are carried out for analysis of failure, which show the development of shear bands in the microstructure of the 420 stainless steel substrate while a limited amount of deformation occurs in the interface region and Corrax microstructure. Void nucleation, growth, and coalescence are found at the 420/Corrax interface due to mechanical incompatibility and decohesion; however, microstructural instability mainly occurs along the shear bands on the 420 side and leads to fracture, which is quantified using high-resolution X-ray computed tomography. Nanoindentation tests show that the maximum level of hardness occurs at the interface due to the existence of sub-micron grains and the formation of AlN nanoparticles. Also, the formation of β-NiAl precipitates enhances the Corrax strength after heat treatment. In addition to a high strength, elevated corrosion resistance of the cooling channels is essential to extend the service life of plastic injection molding dies. Potentiodynamic corrosion testing at the interface shows that Corrax has remarkable corrosion resistance compared to 420. Therefore, additive manufacturing of the critical die areas such as the cooling channels using Corrax increases the service life of the mold.

A hybrid additively manufactured martensitic-maraging stainless steel with superior strength and corrosion resistance for plastic injection molding dies

For multi-materials having different compositions, delamination or crack may occur at the interface of two dissimilar materials due to differences in a lattice structure, residual thermal stress, and coefficient of thermal expansion. To solve such problems, functionally graded material (FGM) that gradually changes the properties of the interfacial boundary has been attracting attentions. Since the chemical compositions of FGM materials are gradually changed according to position, inter-layer residual stress between two heterogenic materials can be relieved. In this study, a functionally graded material (FGM) containing Inconel 718 and stainless steel (STS) 316L was fabricated by directed energy deposition (DED), with STS 316L volume fraction changing in the range of 0 wt% to 100 wt%. The fabricated FGM includes depositions of 25% graded (Graded (25)) and 10% graded (Graded (10)) materials. In non-graded samples, cracks were observed at the interface of Inconel 718 and STS 316L. In all three conditions, cracks in the vertical direction were observed in specific regions, and these cracks were caused by precipitation, inclusion, and columnar-to-equiaxial transition (CET). As far as hardness is concerned, a smooth decrease in hardness was observed in the graded-materials, but the hardness decreased rapidly at the interface of the non-graded sample. The highest tensile strength and elongation were obtained in the Graded (25) material. Microstructural and mechanical properties indicated that mechanical properties vary depending on the gradient composition, and it is essential to choose the appropriate gradient composition of FGM fabricated by DED. 

Defect of Functionally Graded Material of Inconel 718 and STS 316L Fabricated by Directed Energy Deposition and Its Effect on Mechanical Properties

Additive manufacturing (AM) has been applied in various industrial fields owing to its advantages with respect to manufacturing complex shapes, which are typically difficult to fabricate. However, AM also presents certain limitations, including low surface quality (of finished parts), residual stress, and heterogeneous microstructure, as per the investigations of various post-processing studies. In this regard, only a few post-heat treatment studies have employed a directed energy deposition (DED) process for fabricating SUS 630 parts. In this study, the deposition characteristics were observed according to the main DED process variables involved in the fabrication of a single track and multiple layers. Solution annealing (SA) and precipitation hardening (PH) were performed, as post-heat treatments, on the as-deposited sample, and their effects on the microstructure and strength were analyzed. A microstructure and phase analysis confirmed that the as-deposited sample was dominant, with the ferrite of the BCC structure constituting 96.7% and the retained austenite of the FCC structure constituting 3.3%. The fraction of martensite in the sample undergoing SA (SA sample, hereafter) was increased by 3.1%, when compared to that of the as-deposited sample, although the austenite fraction was decreased by 3.1%. The hardness values of both the SA sample and SA + PH sample with higher martensite phase fractions were higher than that of the as-deposited sample. Meanwhile, the tensile test showed a clear yield point and high elongation in the as-deposited sample owing to the retained austenite effect. Conversely, the SA sample showed increased strength but decreased elongation upon the removal of the retained austenite. However, the SA + PH sample exhibited a tensile strength similar to that of the SA sample, as well as an elongation similar to that of the as-deposited sample owing to the reverted austenite. These characteristics were also observed in the D-S type specimen containing both deposits and a substrate, and in the D-only type containing only the deposits. The impact test confirmed that the impact of the absorption energy was the highest in the as-deposited sample (138 J), whereas that of both the SA (65 J) and SA + PH (27 J) samples was lower. Accordingly, we observed a change in the microstructure primarily into martensite through the post-heat treatment of the deposited material, which subsequently improved the tensile properties and hardness.

Seung Weon Yang

Papers

Selective compositional range exclusion via directed energy deposition to produce a defect-free Inconel 718/SS 316L functionally graded material

Defect of Functionally Graded Material of Inconel 718 and STS 316L Fabricated by Directed Energy Deposition and Its Effect on Mechanical Properties

Solution annealing and precipitation hardening effect on the mechanical properties of 630 stainless steel fabricated via laser melting deposition