The production of advanced multi-material metal components through additive manufacturing (AM) provides an opportunity to overcome the limitations of traditional materials by combining different and tailored properties in a single structure while maintaining a high degree of design freedom [1, 2]. In particular, additively manufactured functionally graded materials (FGMs), which provide a smooth transition between the two dissimilar materials, are increasingly emerging [3]. In this way, issues related to a sharp interface between the two materials with different physical properties are minimized, including the formation of brittle intermetallic compounds, solidification cracking, and more severe residual stress formation due to the sudden change in thermal properties [1, 4]. FGMs are distinguished as either discontinuous or continuous. In the former, the spatial variation in composition and properties follows a discrete stepwise pattern, while in the latter this occurs continuously (cFGMs) [3, 5]. Directed Energy Deposition – Laser Beam (DED-LB) and Powder Bed Fusion-Laser Beam (PBF-LB) are the most widely employed powder-based AM technologies for the fabrication of multi-material structures [2, 6]. Furthermore, because additive manufacturing typically involves layer-by-layer construction, most research has focused on structures where the spatial variation of the composition is along the build direction, especially using DED-LB, while studies where this is within the layer and utilizing PBF-LB are less common and have only recently been realized [2, 6–9].
PBF-LB exploits a laser source that melts the powder in a highly localized manner into a melt pool, which then undergoes rapid solidification to build a three-dimensional part in a layer-by-layer strategy. However, due to the extremely steep thermal gradients generated and the repeated thermal cycles to which the material is subjected, a complex and large residual stress field is developed, which can also cause the manufacturing process to fail [10, 11]. Residual stresses, if not properly analyzed and controlled, cause cracking during the process, detachment from the build platform, and significant distortion in the final part [2, 10–12]. They also have a major influence on the fatigue properties of AM parts, so that post-process machining or heat treatments are often required to improve their mechanical behavior [13–15].
The onset of residual stresses in AM is due to two mechanisms: the temperature gradient mechanism (TGM) and the cool-down phase, as defined by Mercelis and Kruth in [16]. The TGM defines the formation of residual stresses as a result of the large thermal gradients that occur around the laser spot. After laser passage, the irradiated area is characterized by tensile stresses and surrounded by a compressive region. The cool-down phase, on the other hand, is associated with the shrinkage of the molten top layer, which is constrained by the underlying solid material during solidification, thereby producing tensile stresses in the upper layer and compression in the layers below. In addition, the process parameters used, as well as the material properties, size and geometry, are critical not only to the manufacture of the part, but also to the complexity and magnitude of the residual stresses [10, 11, 17–24]. Laser power, scan speed and scan strategy must be optimized to achieve a fully dense part while minimizing residual stresses and are also material dependent [10, 22, 25].
Notwithstanding the steady growth of studies on the fabrication of FGM structures, the analysis of the contextual residual stresses generated is scarce, even considering all metallic AM technologies. In particular, residual stress measurements on FGM specimens are poorly reported in the literature. Woo et al. measured the through-thickness residual stress field in five different DED-LB discontinuous between-layer FGM ferritic-austenitic steel specimens using various techniques [8]. They found that by using a bidirectional scanning strategy, the stress trend took on a sinusoidal shape as the number of interlayers increased. In addition, due to the large change in the thermal expansion coefficient observed between the fully austenitic composition and the 50% austenitic and 50% ferritic composition, a significant change from tension to compression is measured, up to 950 MPa. However, by modifying the scanning strategy to orthogonal or island scanning, the residual stress range drops significantly to about 430 MPa and the stress profile across the thickness (direction of composition change) returns to a smoother trend. In a subsequent study, Shan et al. developed a multiscale framework based on the inherent strain method to predict residual stresses and distortion in FGMs fabricated by DED-LB [26]. The authors extended the previous study [8] with a numerical model and found that increasing the number of interlayers to nine mitigated residual stress oscillations and reduced distortion in the final part. This suggests that moving towards continuous composition variation, i.e. cFGMs, makes the residual stress field more uniform. Furthermore, coupling this effect with the use of an island scanning strategy also leads to a reduction in the residual stress range through the thickness. Ghanavati et al. [27] found similar results, where mitigating sharp changes in residual stress was achieved by adopting a smoother compositional change in a between-layer multi-material SS316L-IN718 structure realized by DED-LB. Li et al. found that implementing buffer layers of AISI 316L and Inconel 718 in multi-material depositions of Cu on AISI 304L by using DED-LB resulted in lower maximum tensile residual stress and defect-free copper [28]. In their investigation, Shin et al. measured only surface residual stresses by X-ray diffraction in steel FGM specimens produced by DED-LB, in which the composition changed from fully ferritic to austenitic along the build direction, by exploiting 3 interlayers of 5 mm each [29]. Zhao et al. have developed a numerical model for residual stress analysis of between-layer TC4-Inconel 718 FGMs [30]. However, their work did not take into account the typical process parameters of PBF-LB, which generate different residual stress fields, and in particular did not consider within-layer FGMs. Rodrigues et al. used wire and arc additive manufacturing to build a AISI 316L stainless steel to Inconel 625 FGM, with a smooth transition along the build direction in steps of 5% composition changes [31]. Compared to a direct transition between the two materials, the FGM specimen exhibited higher residual stresses as measured by neutron diffraction. This result was caused by the greater formation of detrimental precipitates in FGM, which created volumetric mismatches that ultimately generated higher residual stresses. Bodner et al. fabricated a between-layer AISI 316L-Inconel 625 cFGM specimen using a novel deposition technique called liquid dispersed metal PBF [32]. The authors observed that the residual stresses exhibited a C-shaped profile through the thickness of the specimen with superimposed fluctuations at the interfaces between the materials. Subsequently, the same authors measured residual stresses in a multi-material structure fabricated using the same technology, with both between-layer and intra-layer variations of these materials [33]. Extremely high residual stresses, even exceeding 900 MPa, developed as a result of the process. In addition, stress concentrations were observed at transitions between materials, both between- and intra-layer. However, to the authors' knowledge, there is no work that has measured residual stresses in FGMs made by PBF-LB. Furthermore, only Rodrigues et al. analyzed a structure with smooth composition variation, but due to the high number of precipitates, they obtained a counterintuitive result with a direct interface that performed better than FGM, therefore smooth variations in composition certainly require further investigation [31]. In addition, only Bodner et al. analyzed residual stresses in an intra-layer multi-material structure [33]. In fact, the feasibility of manufacturing such layer-level FGMs by using PBF-LB has been only recently demonstrated, and the contextual residual stress characterization is still lacking in the literature. The mechanical properties of additively manufactured specimens are significantly influenced by the building direction [34, 35]. This also applies to the direction in which the variation in material composition is made in FGMs. As a result, the quantification of residual stresses cannot be simply inferred from analogous studies on between-layer multi-material PBF-LB manufactured samples. Finally, none of the above studies analyzed the influence of material-specific process parameters on residual stresses in FGM structures.
The main objective of this work is to measure residual stresses in cFGM structures of AISI 316L steel and 18Ni Maraging 300 fabricated via PBF-LB, where the continuous variation in composition is within the same layer. To achieve this goal, the contour method, a destructive measurement technique capable of providing a 2D map of residual stresses normal to a section plane, was used [36]. This measurement technique has emerged especially for measurements on welds because it is insensitive to microstructural gradients, which also makes it particularly suitable for AM components [8, 37–39]. Additionally, it is more readily available than other techniques providing in-depth measurements, like neutron diffraction or synchrotron X-ray diffraction [37, 40]. In addition, the effect on residual stresses of solution annealing and aging heat treatment, which are required to improve the mechanical properties of martensitic steel, was investigated. Finally, by varying the scanning speed, the impact of adopting material-differentiated process parameters on the residual stress field has been examined.