Consumable development to tailor residual stress in parts fabricated using directed energy deposition processes☆
Graphical Abstract
Introduction
Additive manufacturing (AM) is a near net shaping process employing the layer-wise fabrication of three dimensional parts directly from computer-aided design (CAD) models [1], [2]. With a growing number of applications that demand large-scale parts with complex geometries, it is vital to develop scalable additive manufacturing processes. Directed energy deposition (DED) processes have attracted attention in this regard due to the easy scalability of these processes [1], [3], [4]. DED processes, as the name implies, utilize an energy source to create a melt pool into which the material is fed [5]. The energy source can either be a laser, electron beam or an electric arc and the feedstock can be either powder or wire depending on the process [5]. In this study, we employ a wire arc-based process, the details of which can be found elsewhere [6], [7]. While previous reports show acceptable mechanical properties [1], [7], one of the challenges in the fabrication of large scale parts is related to the development of tensile residual stresses, which could potentially reduce high cycle fatigue life, deteriorate resistance to stress corrosion cracking and also promote premature failure of parts [8], [9], [10].
In general, the residual stresses in the additively manufactured parts are due to the large thermal gradients, the cooling rates and the cyclic heating that the processes require [8], [9], [11], [12], [13], [14]. Accumulated tensile residual stresses typically lead to premature failure in applications where fatigue loading occurs. In addition failure due to environmentally assisted cracking is also accelerated under tensile residual stresses. The total internal stress under load is the sum of contributions from the applied tensile load and the locked in residual stresses. Tensile residual stresses lead to a high total internal stress acting on the part during reversals, which in turn leads to premature failure. When the tensile residual stresses are reduced or eliminated, the total internal stresses are drastically reduced, leading to a longer time and a higher threshold stress required to achieve crack nucleation, therefore improving the overall fatigue life. Similar arguments can also be made for environmentally-assisted cracking.
Due to the synergy between welding and additive manufacturing, several contemporary approaches used to control residual stress development in welds could be effective in the additive manufacturing scenario as well. A review of these approaches is discussed elsewhere and not discussed here [8], [15]. In this study, we use the concept of alloying the filler material to promote a martensitic transformation at a specific temperature to induce a compressive stress in the build. While the effects of inducing a compressive residual stresses are largely beneficial, the presence of a compressive residual stress along the weld centerline could contribute to buckling [16], [17]. It has been demonstrated that a compressive residual stress along the weld center line could lead to buckling when it exceeds a “critical buckling load” [18], [19], [20], [21]. However in the case of wire arc additive manufacturing the minimum sample thickness which can be fabricated is several millimeters and there has been no reported evidence for buckling distortion occurring in wire arc additive manufacturing.
The concept of a filler material with a low transformation temperature was originally developed by Jones and Alberry in 1977 [22]. They conducted an elegant experiment where they correlated residual stress evolution with martensitic start temperature. To illustrate these differences they contrasted the stress evolution between a 2.25Cr-1Mo, 9Cr-1Mo and 316L steel during cooling [22]. The results showed that, for the case of austenitic stainless steel, the residual stresses continuously increased during cooling. In the case of the ferritic steels, which experience an allotropic transformation from FCC→BCC upon cooling, the residual stress evolution followed a similar pattern until the martensitic transformation occurred. Due to the volume expansion associated with the martensitic transformation, a compressive stress was induced. Upon cooling from the martensite start (Ms) temperature, they observed a reduction in the compressive stresses due to the continued thermal shrinkage. Based on these observations, it was hypothesized that reduction of the Ms temperature would lead to the formation of compressive residual stresses [22]. Ohta and coworkers developed a 10Cr-10Ni low temperature transformation welding consumable 10Cr-10Ni which improved the fatigue life of the weld [23], [24].
The residual stresses in steel welds are sensitive to the Ms temperature which in turn is sensitive to alloying. The influence of Ms temperature on compressive stress has been the subject of several investigations. One can understand these correlations by measuring the expansion strain in various alloys as a function of the Ms temperature. Higher expansion strains provide greater compensation for shrinkage strains and thereby increase the compressive residual stresses in the part. For instance Ohta et al. reported an expansion strain (a consequence of the martensitic transformation) of 0.55 for alloys with Ms temperature of 180 °C [23], [24], [25]. However when the Ms temperature is decreased to 80 °C (by alloying) the expansion strain drops to 0.28 [26]. The expansion strain does not change up to a temperature of 300 °C and remains constant at about 0.55–0.54 [26]. With a further increase in the Ms temperature to 360 °C the expansion strain drops sharply to 0.24 [27], [28]. Based on this the Ms temperature of the alloys should be between 150 and 300°C to maximize the next compressive residual stress.
While the martensitic transformation-induced plasticity plays a role in inducing a compressive residual stress, a 100% martensitic microstructure is not beneficial to the toughness. Controlling the phase fractions of retained austenite (RA) and martensite is crucial to ensure that the a desired balance between ensuring that the weld is completely in compression and retaining weld metal toughness is maintained [13], [29]. For example, it has been shown that retaining about 5–8% retained austenite should be targeted to ensure desired toughness [14]. A comprehensive review of the activities related to the development of new welding consumables for residual stress reduction are presented in [30], [31], [32], [33].
While it is possible to leverage this vast body of extant literature and use the generic principles from welding to fabricate parts via AM, there are challenges involved. Since AM is a layer-by-layer manufacturing process, the already-deposited layers may experience thermal excursions of decreasing intensity as subsequent layers are deposited. This may lead to the destruction of the compressive residual stresses, especially if the previous passes lead to the tempering of the newly deposited weld or to incomplete transformations. These challenges may also exist during multi-pass welding of thick sections. However, multi-pass welding of LTT filler materials have not been well studied and there are a limited number of studies. A detailed study by Ramjaun et al. investigated the residual stress evolution in multi pass welds fabricated using LTT filler materials [34]. They reported that, during multi-pass welding, it is important to maintain the interpass temperature above the Ms temperature so that the entire build process occurs completely in the austenitic state until the build is complete. It is therefore possible to regain the stress reductions associated with the low transformation temperature.
Moat et al. have used the idea of controlling residual stress by maintaining a high interpass temperature [35]. They showed that, by maintaining a high enough interpass temperature while fabricating multi-pass welds, the compressive residual stresses generated increased from − 200 MPa to − 600 MPa when the interpass temperature was increased from 50 °C to 200 °C [36]. They attributed the increased compressive residual stress to the lack of transformation during deposition in the case where the temperature is maintained above 200 °C, confirming the hypothesis that the reduction of the Ms temperature would lead to the formation of compressive residual stresses [36]. This difference in the residual stresses as a function of the interpass temperature is not pronounced in the surface due to the completion of the transformation.
However, while these studies provide direction to control only the residual stresses, the maximum number of passes deposited was limited to 3. In addition, the associated microstructures and mechanical properties were not characterized. This study is aimed at addressing these gaps. We investigated the effect of the interpass temperatures on the evolution of residual stresses and the mechanical properties of the builds, fabricated using large-scale wire arc additive manufacturing with LTT filler materials.
The key challenge in maximizing the effects of compressive residual stresses from LTT materials is to control the interpass temperature. The techniques used to deposit material and the strategies that were used to control the interpass temperature in parts during additive manufacturing is outlined in the experimental techniques section. Details pertaining to the measurement of residuals stresses is also discussed in the experimental techniques section. To provide the reader with a detailed understanding as to how interpass temperatures should be selected, the results section present the residual stress measurements from the neutron diffraction experiments and subsequently the results from metallographic characterization. The results section attempts to set the stage by attempting to showcase the relationship between the martensite start temperature, interpass temperature and the microstructure. These relationships are further elaborated in the discussions section. The discussions sections provides historical context pertaining to the processing structure properties of LTT materials used for welding and is intended to provide insights to the reader to select LTT materials for future use.
Section snippets
Build fabrication
The builds were fabricated using wire arc additive manufacturing. The filler wire used was a metal cored wire with a proprietary composition developed by the Lincoln Electric company was used. Typically these wires contain between 12% and 14%Cr, 4–5%Ni and < 0.05%C and rest iron. The nominal martensite start temperature is ~250 °C. As pointed out in the introduction section the Ms temperature of the alloy used in the present study falls right in between 150 and 300 °C and according to [26] this
Residual stress measurements
The results from residual stress measurements are presented in Fig. 3(a)-(b). Fig. 3(a) show the residual stress distribution in the LD (longitudinal direction), ND (normal direction) and BD (build direction), respectively, for the samples fabricated with an inter pass temperature of 100 °C. The directions are illustrated and defined in Fig. 1(b). Fig. 3(b) show the residual stress distribution in the LD, ND and BD for the samples fabricated with an inter pass temperature of 300 °C. The
Discussion
The neutron diffraction measurements proved that the residual stresses in the samples fabricated using LTT fillers qualitatively showed the opposite trend compared to conventional materials [8], [45], [46]. To understand how these changes occurred, it is important to consider the residual stress evolution during the deposition of a conventional steel. During deposition, the heat from the arc causes the material adjacent to the arc to rapidly expand. This results in a compressive stress in the
Conclusions
The present work clearly demonstrates the beneficial effects of LTT materials; a reversal of stress states was achieved when compared with conventional filler materials. The samples fabricated with an interpass temperature of 100 °C showed that the advantageous compressive residual stress obtained after the deposition of layer 1 can be altered or severely reduced in magnitude by the deposition of additional layers. This could be attributed to the insufficient re-austenitization of the previous
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was performed under the auspices of the US Department of Energy Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing office. A portion of this research used resources at the High Flux Isotope Reactor, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory.
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