Investigation of building orientation and aging on strength–stiffness performance of additively manufactured maraging steel

1 Investigation of building orientation and aging on strength-stiffness performance of additively manufactured maraging steel A. R. Oliveira¹, J. A. A. Diaz², A. D. C. Nizes³, A. L. Jardini4, E. G. Del Conte¹* ¹ Center for Engineering, Modeling and Applied Social Sciences, UFABC - Federal University of ABC, Santo André, Brazil. ² UNESP - São Paulo State University, São João da Boa Vista, Brazil. ³ Thyssenkrupp Brasil Ltda. Division Springs & Stabilizers, Brazil 4National Institute of Biofabrication (INCT-BIOFABRIS), Campinas, Brazil. *Corresponding author [email protected]u.br Telephone/Fax +55 11 4996-8286 ABSTRACT The interaction between building orientation and heat treatment may change the strengthstiffness behavior of maraging steel manufactured by Powder Bed Fusion (PBF). Further investigations about the correlation of the fracture characteristics and microstructures with the measured properties are needed to improve the process findings. Thus, this study investigated the way an interaction between building orientation (0º, 45º, and 90º) and specimens condition (as-built and aged) may change the defects interaction mechanism and the mechanical properties of the maraging 300 steel manufactured by PBF. Aging treatment, microstructure characterization, density measuring, tensile and microhardness tests, and fractography were performed on the specimens. The aging treatment strengthened the material, which also reduced the probability of defect coalescence. The interaction of building orientation and aging affected the stiffness, rising this in ≈ 70.5 GPa for the 0º aged specimens. Furthermore, this interaction changed the melt pool stretching direction, and the behavior of the defect coalescence rising the elongation at break in ≈ 16.5% for 0º as-built specimens. The investigated mechanisms show the importance of considering the interaction between the building orientation and condition (as-built and aged) for the mechanical performance of additively manufactured maraging steel 300. Keywords: Powder Bed Fusion; Building orientation; Mechanical properties; Maraging steel. 1 INTRODUCTION The Powder Bed Fusion (PBF) additive manufacturing process occurs by applying a laser beam on a powder bed, in which its energy enables material fusion in successive layers [1–3]. The anisotropy caused by layer manufacturing affects the mechanical properties of specimens obtained from the PBF process [4, 5]. Previous studies have investigated the effects of building orientation in maraging steel 300 specimens [6, 7]. However, the fracture behavior mechanism and its effects on mechanical properties require further investigations. Maraging steel 300 presents a martensite matrix with a small amount of austenite, approximately 5% when processed by additive manufacturing [8, 9]. This alloy is composed of a high-content of nickel (≈ 19%) and low carbon content [10]. Maraging steel 300 has applications 2 in several segments, such as tooling, aerospace, automotive, and nuclear reactor components due to its excellent mechanical performance, characterized by a high microhardness [10–12]. Cyr et al. [13] showed an expressive change in maraging steel anisotropy due to building orientation rising the ultimate tensile strength from the vertical to the horizontal specimen (≈ 100 MPa). The authors have stated that these findings are relevant to developing specimens for aerospace, naval, and defense applications [13]. Lewandowski and Seifi [5] have further discussed the influence of building orientation in the PBF process due to the definition of different microstructures and crystallographic configurations in metals. The cooling rate performance of the PBF process creates segregations at the melt pool boundaries and makes them visible after a chemical etching, allowing the experimental analysis of its different configurations [12, 14]. Aging heat treatment is widely applied in the maraging steel 300 for the enhancement of mechanical properties. Under adequate temperature and heating time conditions, this treatment induces cobalt rearrangement, intermetallic compounds precipitation such as Ni3(Ti, Al, Mo) and Fe2Mo [14, 15], and the formation of reversed austenite from the martensite-austenite transformation [16]. As a result, the literature highlights the material hardening and the elevation of the ultimate tensile strength, in contrast to the loss of ductility [7, 10, 11, 14]. Also, the literature presents divergent γ effects on maraging steel 300 properties, as usually high temperatures (480 to 650ºC) or prolonged periods during aging may increase the content of retained austenite. Nevertheless, they may trigger precipitate growth and cause overaging [8]. The literature also discussed average ultimate tensile strengths from 1000 to 1340 MPa for the maraging steel 300 fabricated by PBF in the as-built condition [2, 6–8, 12, 14, 17–20]. Additionally, the aging treatment performed with 480 ºC for 3h increased the ultimate tensile strength to ≈ 2100 MPa [8]. Kim et al. [21] have also investigated mechanical anisotropy in maraging steel 300 produced by SLM, before and after aging heat treatments. The authors observed changes in material resistance where different building orientations were considered as well as better properties after heat treatment [20]. The direct aging treatment is an approach to avoid unnecessarily solution treatment, saving time, and cost during the post-processing of steel parts. The solution treatment follows by aging in a given temperature around 480 - 490 °C does not increase mechanical strength nor the strain capacity of the maraging steel processed by PBF [8, 20]. Depending on the solution treatment temperature, the melt pool boundaries might partially disappear, and the microstructure can change [8]. Although the high mechanical properties reported, microstructural and chemical changes can reduce mechanical performance [12, 22]. The defects in the microstructure created with PBF are identified even in high-density specimens. Casalino et al. [10] highlighted the presence of pores with diameters lower than 30 µm for maraging steel specimens with 99% relative densities. The parameters optimization through analysis of variance (ANOVA) has shown that lower relative density values resulted in fragile structures, whereas relative densities over 99% had a ductile fracture [12]. Pores are the most common defects in maraging steel, which are created by the material melting and cooling processes intrinsic to PBF [7]. The lack of fusion is a frequent source of pores with irregular shapes and sizes between 40 and 90 µm [23]. Moreover, inclusions of 3 Ti/Al-oxides have also been found in maraging steel specimens [23]. Such defects compromise maraging steel 300 mechanical properties [12], in which it is possible to analyze how the interaction between building orientation and specimens condition (as-built and with aging) may change the defects propagation mechanism. In this scenario, this work investigated the effects of building orientations (0º, 45º, and 90º) and aging treatment on the mechanical properties of maraging steel 300 specimens produced by PBF. 2 MATERIALS AND METHODS The experimental study used the maraging steel 300 metallic powder manufactured by Carpenter Additive/LPW (Cheshire, United Kingdom). The powder particles had dimensions between 12 and 50 μm, and the medium diameter was 24.3 ± 10.2 μm (Figure 1). The spherical shape was dominant, even considering that some irregular particles were detected. Figure 1: Distribution of the maraging steel 300 powder particle size. Optical Emission Spectroscopy and Volumetry analyzed the chemical composition of the maraging steel 300 as-built specimen (Table 1) following the ASTM A751:14a [24] recommendations. Table 1: Chemical composition of the maraging steel 300 as-built specimen. Elements Ni Co Mo Ti Al Cr Si Mn C N P S Fe % weight 19.00 9.36 4.54 1.00 0.038 0.02 0.07 0.04 0.024 0.017 0.009 <0.001 bal. The Design of Experiments (DoE) proposed by Montgomery [25] supported the study. A full factorial experiment was proposed for the tensile tests with two factors: the specimen condition with the levels as-built (AB) and aging heat treatment (HT), and the building orientation with its respective levels, 0º (XZ), 45º (XZ-45), and 90º (ZX) [26], as shown in Figure 2. The DoE resulted in 18 experiments considering three repetitions planned with the Action Stat Pro software. All specimens were manufactured by PBF, using an EOSINT M280 machine from EOS GmbH (Munich, Germany) with a Yb-fiber laser of 170 W. Layer thickness was fixed in 0.02 mm with a stripe width of 5 mm, hatch distance of 0.1 mm, laser beam diameter of 0.1 mm, laser 10 20 30 40 50 Particle size distribution (μm) 4 speed of 1,250 mm/s, and the linear scan strategy with 67º rotation for each layer (Figure 2). The atmosphere gas was Nitrogen, and the temperature of 40 ºC preheated the building platform. Figure 2 illustrates the configuration of the layers for each building orientation adopted in the study. Figure 2: Maraging steel 300 dog-bone specimens in the 0º (XZ), 45º (XZ-45), and 90º (ZX) building orientations and its layers configurations. All the specimens were directly produced in the building platform according to the final desired shape. Cubic specimens of 10 x 10 x 10 mm were used for hardness and microstructural analysis. Rectangular specimens of 32 x 32 x 10 mm were used to perform density measurements. Tensile tests used tensile specimens, known as dog-bone (Figure 3), according to the ASTM E8/E8M standard [27]. The fractography analysis was conducted on the dog-bone specimens. For heat treatment, direct aging without a previous solution was chosen to avoid extra steps during the post-processing stage [8, 20]. The chosen aging condition was of 480°C for 3h, as reported in the previous work [8]. This treatment was carried out in a Mufla Q318M oven from Quimis (Sao Paulo, Brazil). The Instron 3369 universal machine (Massachusetts, United States) carried out the tensile tests [28], and an optical extensometer captured the data performed with the load speed of 1 mm/min [15]. Figure 3 shows the experimental assembly. x y z x y z Scan strategy Rotational (67º for each layer) Building orientation Melted pool boundaries orientation 0º 90º 45º Building orientation Load direction 5 Figure 3: Tensile test experimental assembly. The maraging steel 300 specimen’s microstructures was analyzed using a Zeiss Scope A1 AxioCam optical microscope (Jena, Germany) and a Jeol scanning electron microscope (Tokyo, Japan). The 3% Nital etching was used to reveal the microstructural features. Using a Shimadzu AW220 balance (Kyoto, Japan), the density of the rectangular specimens was obtained by means of the Archimedes' principle [29]. The Vickers microhardness was conducted in the cubic specimens' cross-sections parallel to building orientation with a 0.5 kgf for 15 seconds, using a Homis Tester HV 1000 (São Paulo, Brazil). The phases present in the material were also characterized in the cubic specimens using the X-ray diffraction technique (XRD) with the Stresstech G2R equipment (Rennerod, Germany) with a voltage of 25 kV, current of 7 mA, and Cr-kα radiation. The retained austenite was calculated according to the ASTM E975-13 standard [30], following equation 1, in which the volume fraction of austenite (𝑉 𝛾) can be calculated with the ratio of measured integrated intensities (𝐼), and the parameter of the theoretical integrated intensity (𝑅) of austenite (𝛾) and ferrite (𝛼). Concerning this research, due to the low content of carbon, the martensite diffraction pattern is equivalent to that of the ferrite [9]. 𝑉 𝛾=(𝐼𝛾/𝑅𝛾) [(𝐼𝛼/𝑅𝛼)+(𝐼𝛾/𝑅𝛾)] Equation 1 The dog bone specimens' fractography used a Zeiss EVO MA10 (secondary electron detector) scanning electron microscope (Jena, Germany). Furthermore, ANOVA evaluated the ultimate tensile strength (σu), elongation at break (e), and modulus of elasticity (E), in terms of building orientations and specimens condition. Mechanisms of cause and effect among microstructure, building orientations, and mechanical properties of the maraging 300 steel in the Specimens: Maraging steel 300 Building orientations: 0º, 45º and 90º Tensile Test: Instron 3369 universal machine Load speed: 1 mm/min 14 mm Optical extensometer Specimen 6 as-built condition and after aging heat treatment were investigated through fracture behavior analysis, especially using micrographs. 3 RESULTS AND DISCUSSION 3.1 Microstructure The average relative density obtained in the as-built condition was of 99.47 ± 0.04%, regarding the bulk density of 8.1 g/cm³ [18, 19]. Figure 4 presents the microstructure images. Figure 4a illustrates the melt pool boundaries of the specimen and its overlaps. In the same view, the melt pool depth expressed varying dimensions, especially between 20 and 35 μm. Furthermore, the melt pools presented the semi-elliptic configuration expected for PBF- manufactured maraging steel, as discussed by Yin et al. [14] and Mutua et al. [12]. The top view presented in Figure 4b depicts melt pools configuration following the laser's linear scan strategy, rotated in 67º at every layer. The concentration of segregations in these melt pool regions evidenced these boundaries [12, 14]. This phenomenon derives from the higher cooling rates in these regions due to a thermal gradient defined from the location relative to the laser focus during the PBF process [12, 14]. Typical grain morphologies in PBF manufactured maraging steel can be seen in Figure 4c [22]. The literature further highlights that the grains present an epitaxial growth parallel to the building orientation [11, 31]. Figure 4: Microstructure obtained with an optical microscope for (a) transverse; and (b) top views for the 0º AB specimen; and (c) SEM image of the grains with i. columnar, ii. coarse equiaxed and iii. fine equiaxed morphologies along the transverse section of the melt pools. Figure 5 revealed the preferred crystallographic orientations of grains for each building orientation. The polarized light using a lambda filter enabled this characterization for the specimens manufactured with different orientations. 200μm a) 200μm b) 200μm c) 7 Figure 5: Polarized micrography of the transverse section of specimens (a) 0ºAB; (b) 45ºAB; and (c) 90ºAB, presenting maraging steel grains crystallographic orientations. After 3h of aging treatment, there was no microstructure change as only fine precipitation phenomena took place. Therefore, the microstructure remained identical to the one shown in the as-built condition. This fact was also reported by Conde et al. [8] with similar maraging steel like the one used in this study, which highlighted only a notable change for a solubilization treatment for 1h at 980 °C followed by an aging treatment at 480 °C for 6h. The phases volume fraction calculated with Equation 1 and the data shown in Table 2 verified a retained portion of the austenite phase (γ) for the as-built specimens with the XRD, corresponding to 5.6% of the martensite matrix (α) volume. After the heat treatment, the austenite phase increased to 8.2%. Conde et al. [8] reported an increase in retained austenite after an aging treatment at 480 ºC for 3h from 4% at the as-built condition to 5% after the aging treatment. Table 2: X-ray diffraction data summary, showing the peaks identifications, hkl crystallography planes, the theoretically integrated intensity (𝑅), and area under the peaks. Peak Phase hkl 2θ R Area 1 Austenite 220 130.0 51.5 0.0 1 Austenite 200 80.0 26.4 1996.4 2 Ferrite 211 159.4 183.1 100433.7 2 Ferrite 200 106.1 18.9 8965.3 3.2 Mechanical properties Figure 6 shows the engineering stress-strain diagrams. Table 3 highlights the results obtained for the assessed mechanical properties. A considerable increase in mechanical resistance and a decrease in deformation capacity for the heat-treated condition can be observed. Tensile strength results are consistent with literature researches [2, 6–8, 12, 14, 17–20]. Song et al. [32] achieved ultimate tensile strength values close to 1016 MPa and 1820 MPa for maraging steel 300 at a 90º orientation in the PBF process, before and after the aging treatment (490 ºC for 2h), respectively. Yin et al. [14] observed ultimate tensile strength close to 1170 MPa, which had increased to approximately 2150 MPa after aging (490 ºC for 3h) of PBF-manufactured maraging steel 300. Furthermore, the powder supplier specifications [33] shown in Table 3 were used in this study for comparison purposes. 8 Figure 6: Stress-strain engineering diagrams for maraging steel 300 manufactured by PBF with orientations of 0º, 45º, and 90º in the as-built (AB) and after the aging heat treatment (HT) - 480 ºC for 3h. Table 3: Tensile test results and supplier specifications [33] for the maraging steel 300. Specimens E (GPa) E supplier (GPa) σy (MPa) σy supplier (MPa) σu (MPa) σu supplier (MPa) e (%) e supplier (%) 0ºAB 106.3±8.3 150 to 170 954.7±35.5 1000 to 1100 1047.8±56.9 1000 to 1200 18.2±1.2 6 to 14 0ºHT 176.8±1.2 160 to 200 1701.1±57.1 1850 to 2050 1821.8±24.2 1900 to 2100 3.3±0.2 2 to 4 45ºAB 117.5±3.6 - 949.5±42.1 - 1024.0±37.6 - 14.4±1.5 - 45ºHT 144.0±9.3 - 1714.4±20.9 - 1726.6±19.2 - 1.7±0.3 - 90ºAB 107.1±8.6 140 to 160 973.4±27.0 900 to 1100 1012.3±24.1 1000 to 1200 13.7±2.4 6 to14 90ºHT 145.1±13.8 160 to 200 1710.1±77.8 1850 to 2050 1740.6±58.0 1900 to 2100 4.3±1.5 2 to 4 The building orientation did not show expressive change of the specimens' mechanical properties for the as-built condition, which was also expected regarding the powder supplier specifications [33]. Thus, the ANOVA allowed further investigations related to the interaction of factors. Tables 4, 5, and 6 respectively show the ANOVA of the modulus of elasticity (E), ultimate tensile strength (σu), and elongation at break (e). The interaction of building orientation with specimen condition (as-built and aged) for the tensile strength and the building orientation for the elongation at break were the only factors that did not show effects in the respective response variables (p-value > 0.05). All other factors/interactions were relevant to mechanical performance (Tables 4, 5, and 6). 0 400 800 1200 1600 2000 0 2 4 6 8 10 12 14 16 18 20 S [MPa] e [%] 0ºAB 0ºHT 45ºAB 45ºHT 90ºAB 90ºHT Tensile Test: Instron 3369 universal machine Specimens: Maraging steel 300 9 Table 4: ANOVA for modulus of elasticity. Factor D.F. S.S. F p-value Condition (AB or HT) 1 9,107.8647 76.1870 4.8953E-07 Building orientation 1 714.7923 5.9792 0.0283 Condition * Building orientation 1 789.0111 6.6001 0.0223 Residual 14 1,673.6458 Table 5: ANOVA for tensile strength. Factor D.F. S.S. F p-value Condition (AB or HT) 1 243,0887.133 1,353.2346 2.4836E-15 Building orientation 1 10,212.4597 5.6851 0.0318 Condition * Building orientation 1 1,562.9919 0.8701 0.3667 Residual 14 25,148.9438 Table 6: ANOVA for elongation at break. Factor D.F. S.S. F p-value Condition (AB or HT) 1 687.3396 254.2111 2.2638E-10 Building orientation 1 9.4164 3.4826 0.0831 Condition * Building orientation 1 22.1680 8.1988 0.0125 Residual 14 37.8534 The condition (AB or HT) was the main factor of mechanical properties change, considering the higher F statistical value (Tables 4, 5, and 6). HT increased the modulus of elasticity and tensile strength and decreased elongation. One of the resistance increasing factor after the heat treatment was the presence of intermetallic precipitates that limit the movements of discordances in the metal structure [2]. Figure 7 shows the interaction between building orientations and specimens conditions for the modulus of elasticity and elongation at break. The building orientation changed the behavior of the modulus of elasticity. Figure 7a shows a relevant rigidity gain in the 0º oriented specimen with the aging heat treatment (≈ 70.5 GPa or ≈ 66%). On the other hand, the 45º and 90º specimens showed similar behaviors in both conditions, with a small increase in the modulus of elasticity to 45º AB. This mechanism was associated with fracture behavior in topic 3.3. Kempen et al. [11] also observed an increase in the modulus of elasticity of maraging after the aging conduction. Maximum elongation occurred in the 0ºAB specimen, whereas the minimum took place in the 45ºHT specimen. The changes in the as-built elongation at break was mostly influenced by the defects' coalescence mechanism discussed in topic 3.3. Sudden drops were verified in the diagrams (Figure 6) for the aged specimens due to the low elongation at break. 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