Microstructures and Mechanical Properties of ...

This is a preprint version of the paper published in Materials Science Forum, 783-786 (2014), pp. 898-903

Microstructures and Mechanical Properties of Stainless Steel AISI 316L Processed by Selective Laser Melting A. Mertens1a, S. Reginster1b, Q. Contrepois1c, T. Dormal2d, O. Lemaire3e and J. Lecomte-Beckers1f 1

University of Liege (ULg), Faculty of Applied Science, Department of Aerospace and Mechanics, Metallic Materials Science Unit, Chemin des Chevreuils, 1 B52/3, B 4000 Liege, Belgium 2

Sirris Research Center, Rue Bois St-Jean, 12, B 4102 Seraing, Belgium 3

CRM Group, Avenue Bois St-Jean, 21, B 4000 Liege, Belgium

a

[email protected], [email protected], [email protected], [email protected], [email protected], [email protected]

d

Keywords: Selective laser melting, Microstructures, Stainless steel AISI 316L, Mechanical properties

Abstract. In this study, samples of stainless steel AISI 316L have been processed by selective laser melting, a layer-by-layer near-net-shape process allowing for an economic production of complex parts. The resulting microstructures have been characterised in details in order to reach a better understanding of the solidification and consolidation processes. The influence of the processing parameters on the mechanical properties was investigated by means of uniaxial tensile testing performed on samples produced with different main orientations with respect to the building direction. A strong anisotropy of the mechanical behaviour was thus interpreted in relation with the microstructures and the processing conditions. Introduction Selective laser melting (SLM) was developed in the late 1990s as an economic layer-by-layer nearnet-shape process allowing for the production of complex parts from a wide variety of materials [1]. Practically, a layer of powder is deposited in a bed and molten locally by a laser, according to the desired shape. The powder bed is then lowered, a fresh layer of loose powder is deposited and these steps are repeated until the part is completed. An important characteristic of SLM is that the metallic melt pools cool down and solidify very rapidly, thus producing strongly out-of-equilibrium microstructures that might exhibit high internal stresses [1, 2]. Epitaxial growth of the grains from a given layer on the grains formed during the solidification of the previous layer may also occur [3-7]. The resulting microstructure hence exhibit elongated grains that are roughly parallel to the building direction. In turn, this can give rise to a strong mechanical anisotropy in parts produced by SLM, as previously reported e.g. for Ti alloy Ti-6Al-4V [4, 5]. Such a mechanical anisotropy with respect to the building direction has also been observed by Tolosa et al. [8] for stainless steel AISI 316L that is very suitable for SLM and that is widely used e.g. in the chemical, energy and biomedical industries [6]. However, Tolosa et al. [8] did not report on the microstructures of their specimens hence making it difficult to fully understand the influence of the processing conditions on the mechanical properties. As a consequence, the present paper aims to shed more light on the correlation between the processing conditions, the microstructures and the mechanical properties of samples from stainless steel AISI 316L processed by SLM with various main orientations with respect to the building direction. Microstructures were characterised in details in order to better understand the


consolidation and solidification processes. A strong anisotropy of the mechanical behaviour was thus interpreted in relation with the microstructures and the processing conditions. Experimental procedures Samples for tensile testing were produced using a MTT SLM 250 laser melting deposition manufacturing system, in an argon purged production chamber, and following three different orientations ox, oy and oz with respect to the building direction (oz), as illustrated in Fig. 1. All samples were processed simultaneously, as parts of one single job, and the processing parameters were kept constant i.e. a laser power of 175 W, a travel speed of 700 mm/s, a focus offset of 1 mm and a powder layer thickness of 60 µm. The nominal chemical composition of the AISI 316L stainless steel powder was as follows (weight %): Cr – 17.3, Ni – 10.9, Mo – 2.3, Mn – 1.39, Si – 0.46, P – 0.028, N – 0.031, Fe-bal. The substrate was not preheated.

Fig. 1. Schematic representation of the various orientations of the tensile samples with respect to the building direction (oz). Samples for metallographical examinations were embedded in resin and polished following standard practices. The overall quality and soundness of the produced parts have been assessed directly from observations of “mirror polished” samples, and the average volume fraction of porosities was determined by image analysis using the imageJ software. A more detailed microstructural characterisation was then carried out after etching with aqua regia i.e. 55% HCl, 20% HNO and 25% of methanol. Microstructural observations have been carried out by means of an Olympus BX60M optical microscope and a SEM-FEG FEI XL30 scanning electron microscope. Moreover, EBSD observations have been performed using an EDAX MSC-2200 system, so as to shed more light on the solidification process. Uniaxial tensile tests have been performed according to the ISO 6892-1 B25: 2099 standard on samples 2.6 mm thick, 5 mm wide and with an initial gauge length of 35 mm. To ensure for sufficient statistics, five samples have been tested for each orientation. Average values of the yield stress, of the ultimate tensile strength and of the maximum uniform elongation have been obtained from the tensile stress-strain curves. The fractured surfaces have also been observed using SEM, in order to better understand the mechanisms leading to failure.


Results and discussion Microstructures. Fig. 2(a) shows a representative optical micrograph from a mirror polished sample, where the porosities present in the specimen can be clearly distinguished. For specimens processed in the ox and oy orientations, most of these porosities exhibit a spherical shape typical of gas bubbles that were trapped in the build during solidification [1, 4]. The volume fractions of porosities were moreover measured by image analysis, as a function of the manufacturing orientations. As can be seen in Fig. 2(b), samples processed in the ox and oy directions exhibit reasonably low volume fractions of porosities, that are comparable to values commonly reported in literature for AISI 316L parts produced by SLM [9, 10]. Samples produced in the oz direction, on the other hand, exhibit a volume fraction of porosities that is significantly higher (Fig. 2(b)).

(a) (b) Fig. 2 (a) optical micrograph from a mirror polished sample (processed in the ox direction) and (b) volume fraction of porosities, as measured by image analysis on mirror polished samples.

(a) (b) Fig. 3 (a) Optical micrograph of a sample processed in the oy direction, the black circle marks a wetting defect localised between two successive layers; (b) SEM micrograph of a sample processed in the oz direction showing a wetting defect associated with a lack of fusion, as indicated by the unmolten powder particles (after etching with aqua regia). This difference in quality between samples processed in the ox/oy and the oz directions can be better assessed thanks to more detailed microscopical observations carried out on etched samples (Fig. 3). Indeed, etching with aquia regia reveals semi-circular shapes (Fig. 3(a)), that are oriented perpendicularly to the building direction (oz) and that correspond to the melting scan tracks. Fig. 3(a) clearly shows that the spherical gas bubbles are distributed randomly inside the individual melt pools, while other defects, bigger in size and more elongated in shape, are localised


preferentially between melt pools corresponding to two successive layers. Such elongated defects remain scarce in the samples produced with the ox and oy orientations. They are present in a much greater number in the samples processed with the oz orientation, where they are generally associated with unmolten powder particles (Fig. 3(b)), thus accounting for the higher volume fraction of porosities in the oz specimens. Elongated defects, localised in between two successive layers, can be ascribed either to balling (i.e. an instability of the melt pool due to thermocapillary phenomena [6]) or to the lack of proper wetting of the melt pool on the solid that had been previously deposited [1, 11]. Wetting can be significantly impaired by the formation of oxide films at the surface of the previously solidified layers. As a consequence, upon depositing a new layer, sufficient remelting of the previous layers must take place to ensure for a clean surface [1, 11]. As demonstrated by the presence of a great number of interlayer defects associated with unmolten powder particles (Fig. 3(b)), such a sufficient remelting has not taken place during the fabrication of the oz specimens. This observation strongly suggests that, although all samples were produced in a single job with constant processing parameters, the oz specimens were produced under globally colder processing conditions in comparison with the ox/oy samples (which exhibit a smaller number of interlayer defects). This can be further rationalised as follows: (1) The first deposition steps involved the production of both the ox, oy and oz samples. As a consequence, the overall surface scanned by the laser was quite big, leading to a high energy input and in turn to a fairly efficient (pre-)heating of the system formed by the substrate and the powder bed as a whole. (2) After completion of the ox and oy specimens, the later deposition steps only involved the production of the oz samples and the overall surface scanned by the laser was smaller, leading to a smaller energy input and in turn to a less efficient (pre-)heating of the substrate and powder bed. In future work, this could be corrected by adjusting the processing parameters during the later production steps, so as to keep a high overall energy input [6].

(a) (b) Fig. 4 (a) Optical micrograph showing the successive melt scan tracks and deposition layers (after etching with aquia regia) and (b) EBSD map corresponding to the marked square area. Fig. 3(b) also shows a cellular morphology that is typical of the solidification of similar stainless steel grades under ultra-fast cooling conditions [12]. In order to shed more light into the solidification phenomena, Fig. 4 shows an optical micrograph where successive melt scan tracks/deposition layers can be clearly distinguished, along with the corresponding EBSD map. Fully austenitic grains can be identified from the EBSD map (Fig. 4(b)). These grains are elongated following the building direction. Comparison with the optical micrograph of Fig. 4(a) shows that these elongated grains cross over several adjacent melt pools, thus confirming that solidification occurred to some extent through a process of epitaxial growth.


Mechanical properties are summarised in Fig. 5 that shows the average values of the yield stress, the ultimate tensile strength and the maximum uniform elongation obtained for the three different processing orientations (Fig. 1). These mechanical properties exhibit a strong anisotropy with respect to the building direction (oz). Samples processed in the ox and oy directions, on the one hand, exhibit very similar properties, along with a typical ductile fracture behaviour. Samples processed with the oz orientation, on the other hand, exhibit significantly lower strengths and elongations. The deterioration of the mechanical properties in the oz samples can be ascribed to their greater volume fraction of defects (Fig. 2(b)), that exhibit moreover a very detrimental orientation (i.e. perpendicular) with respect to the loading direction [4]. This is further supported by the representative fractograph of Fig. 5(d) which clearly shows the “lack of melting” defects at the origin of failure in the oz specimens. Besides, the peculiar morphology of the austenite grains, that are elongated along the building direction (oz) as a result of the occurrence of epitaxial growth (Fig. 4), may also contribute to the strong mechanical anisotropy observed between the ox/oy and the oz specimens. Indeed, the seemingly bigger grain size in the loading direction for the oz samples might also contribute to explain their lower yield stress when compared with the ox/oy samples.

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(c) (d) Fig. 5. Mechanical properties for samples processed with the three different orientations (as illustrated in Fig. 1) (a) Yield stress (MPa); (b) Ultimate tensile strength (UTS) (MPa); (c) Maximum uniform elongation (%) and (d) representative fractograph of a sample fabricated in the oz direction. “Lack of melting” defects are highlighted by white circles. Conclusions Samples from stainless steel AISI 316L have been processed by selective laser melting, a layer-bylayer near-net-shape technique, according to three different main orientations (ox, oy and oz) with respect to the building direction (oz). The resulting mechanical properties exhibit a strong anisotropy between samples fabricated in the ox/oy and the oz directions, that was interpreted in terms of microstructural differences. On the one hand, the occurrence of epitaxial growth of the grains from a given layer on top of the previously solidified layer led to a microstructure characterised by elongated austenitic grains roughly parallel to the building direction oz. On the other hand, samples fabricated along the oz direction were shown to exhibit a higher volume


fraction of defects due to lack of melting, which played a detrimental role during failure. The choice of better optimized path-dependent processing parameters should be recommended in future work in order to limit the presence of such defects. Acknowledgements The authors wish to acknowledge the financial support of the European Fund for Regional Development and the Walloon Region under convention FEDER 1784 TipTopLam, and of the Interuniversity Attraction Poles Programme initiated by the Belgian Science Policy Office, contract IAP7/21 “INTEMATE”. The authors also wish to thank Mrs S. Salieri (ULg) and the Additive Manufacturing Team from the Sirris Research Centre for their help with samples preparation. References [1] J.P. Kruth, G. Levy, F. Klocke, T.H.C. Childs, Consolidation phenomena in laser and powderbed based layered manufacturing, Annals of the CIRP 56 (2007) 730-759 [2] M. Shiomi, K. Osakada, N. Nakamura, T. Yamashita, F. Abe, Residual stresses within metallic model made by selective laser melting process, Annals of the CIRP 53 (2004) 195-198 [3] L. Thijs, F. Verhaeghe, T. Craeghs, J. Van Humbeeck, J.P. Kruth, A study of the microstructural evolution during selective laser melting of Ti-6Al-4V, Acta Mater. 58 (2010) 3303-3312 [4] T. Vilaro, C. Colin, J.D. Bartout, As-fabricated and heat-treated microstructures of the Ti-6Al4V alloy processed by selective laser melting, Metall. Mater. Trans. A 42 (2011) 3190-3199 [5] A. Mertens, Q. Contrepois, T. Dormal, O. Lemaire, J. Lecomte-Beckers, Ti alloys processed by selective laser melting and by laser cladding: microstructures and mechanical properties, in: Proc. 12th European Conference on Space Structures, Materials and Environmental Testing, Noordwijk, The Netherlands (ESA SP-691, July 2012) [6] I. Yadroitsev, P. Krakhmalev, I. Yadroitsava, S. Johansson, I. Smurov, Energy input effect on morphology and microstructure of selective laser melting single track from metallic powder, J. Mater. Process. Technol. 213 (2013) 606-613 [7] X. Su, Y. Yang, Research on track overlapping during selective laser melting of powders, J. Mater. Process. Technol. 212 (2012) 2074-2079 [8] I. Tolosa, F. Garciandia, F. Zubiri, F. Zapirain, A. Esnaola, Study of mechanical properties of AISI 316 stainless steel processed by « selective laser melting », following different manufacturing strategies, Int. J. Adv. Manuf. Technol. 51 (2010) 639-647 [9] R.H. Morgan, A.J. Papworth, C. Sutcliffe, P. Fox, W. O’Neill, High density net shape components by direct laser remelting of single-phase powders, J. Mater. Sci. 37 (2002) 3093-3100 [10] E. Yasa, J.P. Kruth, Microstructural investigation of selective laser melting 316L stainless steel parts exposed to laser re-melting, Procedia Eng. 19 (2011) 389-395 [11] J.P. Kruth, L. Froyen, J. Van Vaerenbergh, P. Mercelis, M. Rombouts, B. Lauwers, Selective laser melting of iron-based powder, J. Mater. Process. Technol. 149 (2004) 616-622 [12] J.W. Elmer, S.M. Allen, T.W. Eagar, Microstructural development during solidification of stainless steel alloys, Metall. Trans. A 20 (1989) 2117-2131