Determining Elastic Behavior of Additively Manufactured IN718 Alloys using XRD Residual Stress Measurements

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1 Determining Elastic Behavior of Additively Manufactured IN718 Alloys using XRD Residual Stress Measurements K. Lau, D.T. Ardi 1, S.Y. Chan 2 and P. Chaturvedi 3 Data-driven Surface Enhancement, Advanced Remanufacturing and Technology Centre, 3 Cleantech Loop #01/01 More info about this article: Abstract Cleantech Two, Singapore keegan-lau@artc.a-star.edu.sg 1 Data-driven Surface Enhancement, Advanced Remanufacturing and Technology Centre. 2 Intelligent Product Verification, Advanced Remanufacturing and Technology Centre. 3 Intelligent Product Verification, Advanced Remanufacturing and Technology Centre. Additive manufacturing is a layer-by-layer building methodology that provides unique opportunities for producing intricate geometries through bottom-up processing. In this work, using a state-of-the-art stress diffractometer, we have determined the X-ray elastic constant of an additively manufactured IN718 alloy to understand its elastic behaviour. Flat test specimens were fabricated by direct metal laser sintering and bent under a varying external load. The strains induced during each step were derived by measuring changes in the interplanar distance of specific crystallographic planes. This data was used to calculate the plane specific elastic constant and the process was repeated on a set of different locations to assess its directional dependence. Since additive manufacturing is a highly non-equilibrium process, these results will enhance our understanding of anisotropic mechanical properties frequently exhibited by additively manufactured components. Keywords: additive manufacturing, residual stress, x-ray diffraction, elastic constant, Ni-base alloys [ID153] 1

2 1 Introduction Additive manufacturing (AM) has become an effective tool for custom small-batch production or for prototying in the recent years. The conversion of CAD designs into reality has been accelerated by this dependable process. Furthermore, additive manufacturing reduces design lead time and provides new geometric possibilities [1]. With additive manufacturing, limits of producibility of conventional manufacturing are no longer valid, especially for the generation of geometries with undercuts and internal freeform cavities. Additive manufacturing is divided into two systems; powder-bed systems and powder-fed systems. Powder-bed systems such as Selective Laser Melting (SLM), Laser Cusing and Direct Metal Laser Sintering (DMLS) use a powder deposition method consisting of a coating mechanism to spread a powder layer onto a substrate plate and a powder reservoir. Powder-fed systems such as Laser Cladding, Directed Energy Deposition and Laser Metal Deposition melt powder that flows through a nozzle right on the surface of the treated part which makes the way material is added layer-by-layer different from powder bed systems. While the properties of AM alloys have been reported to be uniform in the deposition (XY) plane, there is evidence of some anisotropy in the building direction (Z-axis). The anisotropy is generally due to the nature of the process, which creates parts by joining layers and involves multiple cycles of repeated heating and cooling of material[2]. In this study, one IN718 sample was fabricated via DMLS process. The X-ray elastic constant was examined by measuring changes in the interplanar distance of a 311 plane using an XRD diffractometer and a bending device. These results were compared with the corresponding results a wrought IN718 sample obtained from the same measurement technique. 2 Methodology 2.1 Material A beam measuring 170x20x5mm was fabricated via DMLS process using EOS recommended parameters. The sample was then micro-shotpeened. The fabricator was not able disclose exact information for the printing and shotpeening process as they were proprietary information. The build direction is shown in Figure 1 below. [ID153] 2

3 Figure 1 Build direction of IN718 beam sample Another wrought IN718 beam sample was cut and ground to the same size - 170x20x5mm, for comparison purposes. 2.2 Residual Stress Measurement Residual stress measurement techniques are generally classified into two groups: surface/near-surface measurement techniques and bulk measurement techniques. Surface/near-surface residual stress measurement techniques consisted of XRD and CHD techniques while bulk residual stress measurement techniques consisted of Contour Method (CM). In this work, we employed the X-ray diffraction method to measure residual stress and to obtain the elastic constants X-ray Diffraction The determination of residual stress through the XRD methods is well documented in the literature [3]. XRD residual stress measurements were carried out using Stresstech XStress 3000 G3R Stress Analyzer. They were performed in accordance to the requirements stated in BS EN 15305:2008[4]. The key measurement parameters were: X-ray souce: Manganese Filter type: Chromium (K-β) Measuring mode: Modified-χ [ID153] 3

4 2.3 Bending The IN718 samples were bent using Stresstech Elastic Constant Device HD (Figure 2). Figure 2 Stresstech Elastic constant device HD 5 steps of bending were carried out with each step utilizing different loads. 2.4 Obtaining Elastic Constant The sample was bent with load intervals of 1402, 764, 127, 764 and 1402 N. At each load interval, XRD data was measured according to the parameters listed in Table 1 is made. The recorded load cell reading was changed to load stress value by (1) : σ = 3 h 2 = ε (1) in which σ = stress in direction i a = moment arm, see Figure 2 b = width of the specimen h = thickness of the specimen E = Young s modulus (MPa) F = load cell force (N) ε = strain in direction i, and the Δd is measured by the Xstress Location of measurements The samples were measured in 3 different locations in the center, 5mm apart from each other, as shown in Figure 3 and 4 below. [ID153] 4

5 Figure 3 Distance between measurement locations (red dots) on beam sample are kept at 5mm Figure 4 Measurement location index on beam sample 3 Results The elastic constant values obtained are shown in Table 1 below. These values were obtained by measuring the deviation in XRD peak under pre-determined loading conditions. Table 1 Elastic constant values obtained Position Elastic Constant (MPa) AM Wrought [ID153] 5

6 4 Discussion The results of the study present a quantitative analysis of anisotropy inmechanical of additively manufactured parts. The difference in the elastic constant values suggests that, in contrast to conventional methods, DMLS can introduce anisotropy in the fundamental mechanical properties of metals and alloys. The elastic constant values from bending the DMLS-printed beam sample differ from each other and this shows that additive manufacturing changes fundamental mechanical properties. On the other hand, by observing the values obtained from bending the wrought beam sample, it was found that the elastic constant values were relatively closer suggestingisotropic mechanical behaviour. This is because conventional manufacturing methods do not tend to deviate so significantly from equilibrium conditions. In contrast, additive manufacturing, being a process far from equilibrium, it employs rapid heating and cooling during the printing process. These rapid heating and cooling cycles introduces anisotropy [5]. Future work can be considered on analyzing the crystallographic anisotropy of additive manufactured parts. using a full profile XRD analysis. 5 Reference [1] E. Umaras and Marcos S. G. Tsuzuki, Additive Manufacturing - Considerations on Geometric Accuracy and Factors of Influence, IFAC (International Federation of Automatic Control), [2] M. Krishnan, E. Atzeni, R. Canali, D. Manfredi, F. Calignano, E. Ambrosio and L. Iuliano, Influence of post-processing operations on mechanical properties of AlSi10Mg parts by DMLS, High Value Manufacturing, 2014 [3] M. Fitzpatrick, A. Fry, P. Holdway, F. Kandil, J. Shackleton and L. Suominen, Determination of residual stresses by X-ray diffraction - Issue 2, National Physical Laboratory, Middlesex, [4] B. E , Non-destructive Testing - Test Method for Residual Stress Analysis by X-Ray Diffraction, 2008 [5] Y. Huang, M. C. Leu, J. Mazumder and A. Donmez, Additive Manufacturing: Current State, Future Potential, Gaps and Needs, and Recommendations, Journal of Manufacturing Science and Engineering, vol. 137, [ID153] 6