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

Arc welding-based additive manufacturing techniques are becoming promising for metal additive manufacturing due to their capability of economically producing large-sized components with relatively high deposition rates. This article introduces wire arc additive manufacturing (WAAM) and reviews the microstructure and mechanical properties of various metals and alloys fabricated through this technique. Research indicates positive results as the mechanical properties of WAAM fabricated materials, such as titanium and nickel-based alloys, are found relatively comparable to cast or wrought materials. A feasible state-of-the-art WAAM requires controlling several processing parameters to achieve sound components, so an elaborated review is presented on selecting suitable process parameters. High-quality production in WAAM is limited due to elevated levels of heat inputs, causing several materials processing challenges and defects. Common challenges faced in the fabrication of various metals and alloys through WAAM, including high residual stresses, porosity, delamination, and cracking, are described in this article. The defect mitigation techniques and the recent advances performed in the field of WAAM are summarized in the last section of this paper. This paper concludes that WAAM can be an alternative process for high-quality manufacturing and presents a future vision to make WAAM more industry-oriented technology. 

A review on wire arc additive manufacturing: Processing parameters, defects, quality improvement and recent advances

Mechanical testing and microstructural analysis of wire arc additively manufactured steels

Functionally graded materials (FGMs) are desirable in a variety of applications. Due to a lack of understanding of design and manufacturing methods, it remains a challenge to engineer FGMs. Gradient patterns and performance shown in biological materials present motifs for designing FGMs. The bottom-up characteristic of additive manufacturing (AM) enables unparalleled control over material and structure, making AM a perfect candidate for FGMs. The integration of the two provides an unprecedented opportunity for developing next generation advanced materials. In this review, we firstly detail the biological gradient materials/structures in terms of their functions. Subsequently, the art-of-the-state AM methods for fabricating FGMs are reviewed, with a focus on the technology types of AM that can prepare biomimetic material and structural FGMs, especially emphasizing the multiform gradients induced by processing parameters. Then, the performance and application cases of FGMs created by AM are summarized. Finally, an outlook for further developments of AM technologies towards FGMs is provided. 

Graded biological materials and additive manufacturing technologies for producing bioinspired graded materials: An overview

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. 

N. Bairrão

Papers

Steel-copper functionally graded material produced by twin-wire and arc additive manufacturing