Effects of heat treatment and build orientations on the fatigue life of selective laser melted 15-5 PH stainless steel

Effects of heat treatment and build orientations on the fatigue life of selective laser melted 15-5 PH stainless steel

Materials Science & Engineering A 755 (2019) 235–245 Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: w...

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Materials Science & Engineering A 755 (2019) 235–245

Contents lists available at ScienceDirect

Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea

Effects of heat treatment and build orientations on the fatigue life of selective laser melted 15-5 PH stainless steel

T

Sagar Sarkar, Cheruvu Siva Kumar, Ashish Kumar Nath∗ Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur, 721302, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Additive manufacturing Selective laser melting Build orientations Heat treatment Fatigue Stainless steel

Effects of heat treatment (aging and overaging) and build orientations on the fatigue life of Selective Laser Melted (SLM) 15-5 precipitation hardening (PH) stainless steel are reported. Low cycle fatigue (LCF) life of aged specimens is better than specimens in as-built condition for both vertical and horizontal build directions. On the contrary, high cycle fatigue (HCF) life of aged specimens is less than its as-built counterpart. Aging resulted in precipitation strengthening of the matrix through Cu-rich precipitation, but this makes specimen more defect sensitive in HCF regime. Overaging makes specimen ductile through coarsening of Cu-rich precipitates and increases the amount of retained austenite. This results overaged specimens to be less defect sensitive than aged specimens in HCF regime. Presence of irregular shaped defects e.g. voids, un-melted regions etc in SLM specimens reduces its fatigue life than its wrought counterparts. Relatively less number of defects is found for vertically built SLM specimens. However, orientations of defects being perpendicular to the loading axis, creates more stress concentration around a defect leading to lower fatigue life of the vertical SLM specimens. Present study could be used as a guideline to select proper build strategy and heat treatment for SLM 15-5 PH stainless steel for a desired fatigue life.

1. Introduction Over the last decade, manufacturing industries have shown increasing interests in Additive Manufacturing (AM) process which allows fabrication of a geometrically complex part layer by layer from its digital footprint. Two most common metal AM processes are Direct Metal Laser Deposition (DMLD) and Laser Powder Bed Fusion (LPBF) process. Selective Laser Melting, commonly known as SLM process is a LPBF process in which a fine layer of metal powder is spread over a build plate and a focused laser beam is scanned following a particular scan path and strategy along the powder bed under controlled/inert gas environment. As the laser beam is scanned, owing to its high heat flux it melts metal powder forming tiny molten pool in the scanned zone. Due to fast laser beam scanning speeds cooling of the molten pool is very fast, thereby it gets rapidly solidified. This causes formation of solid metal tracks along the laser scanning path, of pattern dictated by the laser scan strategy. Combination of the individual track within a plane forms a thin metal layer and the steps are repeated until the part is completely built. As compared to DMLD process, better powder catchment efficiency, precision in part features and smoother surface are obtained in SLM process. However, since the final part is submerged in the powder bed, powder particles stick to the surface along the outer ∗

periphery of the part and degrade the surface quality. This calls for post-processing such as machining/electro-polishing to improve the surface quality before its functional usage. Although SLM process has several advantages, widespread adoption of SLM parts in industries seems to be still challenging due to uncertainty in its mechanical properties, mainly in dynamic loading conditions [1]. Possible reasons for this uncertainty may be the formation of inhomogeneous microstructure, residual stresses and micronsize defects. In SLM process, molten pool thermal history is governed by numerous independent and dependent parameters such as laser power, scan speed, hatching strategy, gas flow rate and direction, build orientations etc. and they give rise to unique and complex melt pool thermo-fluidic behavior, spatial-temporal temperature field in the part during its fabrication [2]. A highly localized non-optimal molten pool thermal history may adversely affect the melt pool wetting, its thermal and flow behavior which in turn can trigger the formation of pores and non-uniformity in cooling rates [3]. This leads to inhomogeneous microstructure and anisotropy in mechanical properties of SLM parts. Placement of parts along different build axes may also affect their mechanical properties. How a part is built will determine its mechanical properties under loading condition depending upon the relative orientation of the built-axis and loading direction. Aspect ratio of parts

Corresponding author. E-mail address: [email protected] (A.K. Nath).

https://doi.org/10.1016/j.msea.2019.04.003 Received 3 February 2019; Received in revised form 30 March 2019; Accepted 1 April 2019 Available online 05 April 2019 0921-5093/ © 2019 Published by Elsevier B.V.

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Table 1 Chemical compositions (% weight) of EOS SS PH1 powder. Fe

C

Mn

Si

Cr

Ni

Cu

Nb

Mo

Balance

0.07 (max)

1.0 (max)

1.0 (max)

14–15.5

3.5–5.5

2.5–4.5

0.15–0.45

0.5 (max)

low cycle regime where effects of micro-structural impurities are minimum. On the contrary, it was found that aging causes SLM specimens more sensitive to impurities in HCF regime resulting in poor fatigue life. Insufficient melting of powder particles was reported to cause majority of the ‘split-shaped’ defects found in SLM specimens. These defects have a more pronounced effect for specimens built in vertical direction where loading direction is perpendicular to the crack opening forming the defects. More research needs to be carried out for a better understanding of the type and mechanism of failure of SLM parts under fatigue loading to estimate, predict and obtain desired fatigue life. Few literatures [8,9] are available with a focus on high cycle fatigue (HCF) behavior of SLM 17-4 PH stainless steel. Effect of different surface conditions e.g. as-built and machining on HCF life of SLM 17-4 PH stainless steel was reported by Stoffregen et al. [9]. However, to best of authors’ knowledge, the present study reports effects of heat treatment (both aging and overaging) and build orientations (i.e. vertical and horizontal) on fatigue life of SLM 15-5 PH stainless steel for the first time. Fatigue tests are conducted under rotating bending fatigue testing condition. Fatigue behavior is analyzed and correlated with the data obtained from various micro-structural characterizations, and fatigue life of SLM 15-5 PH stainless steel parts has been benchmarked against its wrought counterpart.

may also influence the thermal history of the molten pool which may affect underlying microstructure and mechanical properties further. Precipitation hardening (PH) stainless steels are getting popular day-by-day due to its excellent mechanical and corrosion properties at both room and high temperatures. Among various PH stainless steel, 15-5 PH stainless steel is very popular which is martensitic steel and has several applications in aerospace, medical, chemical and various engineering applications [4]. Static mechanical properties such as tensile strength, hardness etc. of SLM 15-5 PH stainless steel is available in open data sheet [5] and is comparable to its wrought counterpart [6]. However, a major hindrance in application of SLM parts in actual service conditions comes from the fact that very limited database is available on fatigue properties of SLM parts. Since the majority of failures of engineering components occur due to the cyclic loading, there is a need for more rigorous research for establishing database and guidelines for the desired fatigue life of SLM parts. Heat treatment is widely used in SLM AM process as a postprocessing technique to remove residual stresses of as-built SLM parts. However, depending upon the material, mechanical properties such as fatigue can also be tailored following a set of heat treatment cycles. Although steel in conventionally manufactured form is highly researched, limited studies are available on the fatigue properties of SLM stainless steel. Till date, reports on the effect of heat treatment on fatigue life of SLM PH grade stainless steel are very few. Akita et al. [7] studied the fatigue behavior of SLM 17-4 PH with and without quenching and reported very low fatigue life as compared to that of the wrought 17-4 PH stainless steel. It was concluded that the presence of relatively less amount of martensite phase in the matrix and micron size pores in SLM specimens are some of the major reasons for the decreased fatigue life. Amount of martensite could be enhanced by heating SLM specimens up to 1050 °C followed by water quenching. However for both the cases i.e. SLM 17-4 PH with and without quenching, fatigue life was lower compared to that of wrought 17-4 PH stainless steel. Mower and Long [8] reported lower fatigue life of asbuilt SLM AlSi10Mg and Ti6Al4V compared to its wrought counterpart. Lower fatigue strength was attributed to the multiple crack initiation sites originated from surface defects, internal voids and micro-cracks. Fatigue strengths of horizontally built SLM 316L and 17-4 PH stainless steel were found to be comparable to that of wrought materials; while specimens built in vertical direction showed relatively lower fatigue life. Post-processing such as HIP was recommended to improve the fatigue life of SLM specimens [8]. Yadollahi et al. [3] studied the effect of aging on both low and high cycles fatigue behavior of SLM 17-4 PH steel. Heat treatment improved the tensile strength and fatigue life in

2. Experimental details 2.1. Specimen preparation Specimens were deposited in nitrogen environment using EOSINT M 270 Direct Metal Laser Sintering machine equipped with a single-mode 200 W Yb-fiber laser. Metal powder used is EOS stainless steel PH1 (manufacturer: EOS GmbH- Electro Optical Systems, Germany) which conforms to the chemical composition of 15-5 PH stainless steel (DIN 1.4540 and UNS S15500 [5]) and its chemical composition (%weight) is given in Table 1. Since the powder is produced using gas atomization process, it is mostly spherical in shape and very few of them are irregular (< 5%). Few powder particles were found to be bonded with each other and have neck between them. This is commonly observed for powders prepared using gas atomization process. Powder particle size is in the range of 5–60 μm. Powder layer thickness to build specimens was kept 20 μm with process parameters being the standard ‘EOS Direct Part’ and ‘EOS External Support’ to get almost fully dense specimens. Hatching strategy was kept as 45° rotation between two successive layers.

Fig. 1. SLM 15-5 PH stainless steel specimens built (a) vertically and (b) horizontally. 236

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Buehler) using an electrolyte having 10% perchloric acid, 30% butanol and 60% methanol with 14 V applied between specimen and counter electrode for 10 s.

Table 2 Different heat treatment schedules. Heat treatment schedules

Temperature/Time/Cooling

Solution Annealing (SA) Aging (H900) Over-aging (H1150)

1038 °C/30 min/Air cooling SA/482 °C/1 h/Air cooling SA/621 °C/4 h/Air cooling

2.4. X-ray Micro-Computer Tomography (MCT) X-ray Micro-Computer Tomography (MCT) (Make: Pheonix), a nondestructive technique, was used to get a quantitative estimate of micron-size pores in both horizontal and vertical specimens. In this process, an x-ray beam is focused at a particular region called region of interest (ROI) of the specimen and the specimen is rotated in steps using a CNC stage. The x-ray source was operated at 160 kV and 100 μA. A 0.5 mm Sn sheet was kept in front of the x-ray source to take care effect of beam hardening. The detector which is mounted on the opposite side of the x-ray source collects 2D data for each step. Finally, from these 2D data, 3D images are created using commercially available software. ROI scan of gauge length portion of the fatigue specimen can provide a deep insight about the shape and distribution of pores which is otherwise difficult to detect using any other conventional techniques such as Archimedes principle. These pores may have detrimental effects on fatigue life. Voxel size for vertical and horizontal specimens was kept as 13.11 μm3 and 10.43 μm3 respectively. A complete 360° rotation about build axis of the specimen was carried out and total 1000 images were taken. Exposure time for each image was 500 ms. 3D Volumetric rendering was carried out using Visual studio software from 2D data of each image. Defects less than ∼17 μm3 could not be detected due to attenuation of X-ray beam in the material, but the effect of the same on tensile and fatigue life does not count when relatively larger defects are present.

Fig. 2. Dimensions of fatigue specimens.

Two different groups of cylindrical specimens of 12 mm diameter and 80 mm length were deposited, one of them being vertical (Fig. 1a) and another being horizontal (Fig. 1b). Specimens were cut from the base plate using wire-cut electro-discharge machining (WEDM) process (Make: Electra Mexicut, India). Since precipitation hardening does not occur directly during part fabrication of SLM specimens [10] few specimens from above two groups were heat treated in a muffle furnace (Make: UR Biococtin, India) under argon gas environment following the heat treatment schedules mentioned in Table 2. After heat treatment, both heat treated and non-heat treated cylindrical specimens were machined using computer numerically controlled lathe as per Fig. 2 and their gauge lengths were polished using different grades of SiC papers to reduce any effect of surface machining. Surface roughness was measured using a non-contact laser baser surface profilometer (Model: optoNCDT, Make: Micro-epsilon, USA) and it came out to be ∼0.9 μm.

3. Results and discussions 3.1. Tensile strength Uni-axial tensile test of the as-built and heat treated SLM specimens for both the build directions were carried out using Instron 1344 following ASTM E8 standard [11] and crosshead velocity of the grips was kept as 1.5 mm/min. Three replicates were taken for each heat treatment conditions. Average yield strength and tensile strain at break (i.e. % elongation) are plotted in Fig. 3a and Fig. 3b respectively. Tensile tests results are benchmarked against wrought 15-5 PH stainless steel specimens heat treated under the same test conditions. Results indicate that aging (H900) causes significant increase in yield strength as compared to as-built SLM specimens for both the build direction, while % elongation i.e. ductility reduces. This is because, in case of aging, Cu gets precipitated and precipitation strengthens the martensite matrix, but makes the specimen brittle. Whereas, overaging (H1150) increases ductility at the cost of yield strength through coarsening of Cu-rich precipitates and formation of an increased amount of retained austenite. A detail micro-structural characterization and discussion regarding the same have been already reported in other works by the author [10,12,13]. Similar variations have been found for the wrought 15-5 PH stainless steel.

2.2. Fatigue test Specimens were tested under rotating bending fatigue testing condition using a rotating bending fatigue testing machine (Model: HSM20, Hi-Tech limited, UK) operating at 25 Hz frequency. S-N curves (Fig. 6 and Fig. 7) were plotted keeping stress amplitude along Y-axis and number of cycles to failure along X-axis. Specimens were declared ‘run-out’ if it did not fail even after 107 cycles. In order to benchmark the performance of SLM specimens, fatigue tests of wrought 15-5 PH stainless steel specimens were also carried out under identical test conditions. 2.3. Micro-structural characterizations In order to investigate the fracture surface morphology, 10 mm from the tip of the fracture surface (after fatigue tests) was cut using a 5 axis CNC WEDM and analyzed using Scanning Electron Microscopy (Model: Zeiss, EVO 18 Research). Amount of martensite and retained austenite before and after fatigue test (further details are discussed in the later part of the study) was found out using Electron Backscatter Diffraction (EBSD) (Model: Zeiss Auriga with OXFORD EBSD detector) analysis. Specimens were mirror-polished using different grades (from P400 to P2500) of silicon carbide paper followed by cloth polishing using 3 μm diamond paste on an automatic polishing machine (Model: Buehler Metaserv 250, USA). Further colloidal silica solution of 1 μm was used and finally all the specimens were cleaned in isopropyl alcohol using an ultrasonic cleaner (Make: Toshkon, India) for 45 min. Colloidal silica polished specimens were further electro-polished (Model: ElectroMet 4,

3.2. Defect analysis As discussed earlier, build orientations can influence cooling rate, thus the resulting microstructure and the way build layers are stacked one after another have a direct influence on the mechanical strength of SLM parts. For example, vertically built SLM specimens have deposited layers aligned perpendicularly to the load axis and may affect its tensile strength. Indeed, yield strength and ductility found to be relatively less for vertically built SLM specimens than specimens built in the horizontal direction where build layers are parallel to the load axis. In order to find out this anisotropy in mechanical properties, cross-sections parallel to build layers were examined under a scanning electron 237

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Fig. 3. (a) Yield strength and (b) percentage elongation for as-built and heat treated SLM and wrought 15-5 PH stainless steel. *It may be noted that the term ‘as-built’ condition is applicable for only SLM specimen.

Fig. 4. SEM image of cross-section parallel to build layers and Micro-CT analysis showing defects in as-built specimens built along vertical (Fig. 4a and c respectively) and horizontal (Fig. 4b and d respectively) build axis.

reveal the defects. Electro-polishing was done to minimize the effect of smearing of metal surface due to the mechanical polishing. It could be seen that relatively a large number of pores is present in horizontally built specimens. The distribution of defects characterized by X-ray micro-CT is shown in Fig. 4c and d and results are summarized in Table 3. The voids/pores and un-melted regions can be attributed to the release of entrapped gas after solidification of metals and insufficient laser energy for full penetration depth respectively. Associated stress concentration for these irregular slit-shaped voids is more detrimental to mechanical properties than stress concentration arises from spherical voids. Effect of molten pool thermal history for different build orientations may be confirmed from the difference in defect statistics for two build orientations. Although the average and maximum volume of a detected defect is more, defect per unit volume is relatively less (∼0.36% lesser)

Table 3 Defect statistics using X-ray micro-CT analysis.

3

Volume scanned (mm ) No of defect detected Total detected defect volume (mm3) Defects per unit volume (%) Average defect area (μm2) Average defect volume (μm3) Minimum volume of a defect detected (μm3) Maximum volume of a defect detected (μm3) Average sphericity (ɸ)

Vertical

Horizontal

195 1403 1.05 0.54 14.4 × 104 10.3 × 105 10.98 × 103 8.2 × 106 0.51

191 2413 1.73 0.90 8.5 × 104 7.1 × 105 7.43 × 103 5.9 × 106 0.45

microscope (SEM) (Fig. 4a and Fig. 4b). It may be noted that after polishing the specimen with different grade of SiC paper, specimens were electro-polished to remove few layers from the top surface and 238

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Fig. 5. EBSD analysis of as-built (a) vertical and (b) horizontal SLM 15-5 PH stainless steel.

exposed surface during the fabrication of horizontal SLM specimens being relatively large leads to higher cooling rate which refines the austenite grains more as compared to that of vertical as-built SLM specimens. It is reported in the literature [17] that occurrence of retained austenite is high at grain boundaries where atomic arrangements are irregular and further growth of martensite is not possible. Therefore, relatively fine grain size in case of SLM process contributes to more amount of retained austenite. Also, since specimens were deposited in nitrogen environment and nitrogen is a strong austenite promoter, presence of retained austenite is found for SLM specimen in the as-built state [12]. Another aspect which must be taken into account is that often gas atomization process is done in nitrogen environment and since the environment of EOS SS PH1 metal powder preparation is not known from the material data sheet provided by the manufacturer, preparation of metal powder in nitrogen environment could also promote austenite formation [18]. Starr et al. [19] and Gu et al. [20] have reported that the EOS powder was atomized in nitrogen, the “standard” atmosphere for stainless steel powder manufacturing. Energy dispersive X-ray spectroscopic (EDS) analysis on powder surfaces as well as two built up material was carried out to find out nitrogen content,

for specimens built in vertical orientations. Sphericity (ɸ) is defined as surface area (A) of a sphere of an identical void volume (V) divided by the actual surface area of the void [14]: ɸ = (4π V2/3)/[(4π/3)2/3. A] ≤ 1, assuming ɸ for an ideal sphere is 1. It can be seen from the results that ɸ is less than 1 indicating defects are irregular in shape or flake like as also evident from Fig. 4 for both build directions.

3.3. Presence of retained austenite and its effect on mechanical properties Presence of the retained austenite could be seen from the EBSD analysis (Fig. 5) of both vertical and horizontal SLM specimens in asbuilt condition. Due to very high cooling rates in SLM process, martensite gets formed [15], but a relatively fine austenite grain structure reduces martensite start temperature and leads to an incomplete martensite transformation, and hence gives rise to retained austenite [16]. Since the molten pool thermal history is completely different for two different build orientations, the amount of retained austenite varies for as-built horizontal (∼13%) and vertical (∼5%) SLM specimens. The

Fig. 6. Stress amplitude vs. reversals (cycles) to failures for as-built and aged (H900) SLM specimens. 239

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Fig. 7. Stress amplitude vs. reversals (cycles) to failures for as-built and overaged (H1150) SLM specimens.

accommodated around a defect than for a brittle material (specimens undergone H900) through localized plastic deformation and hence the effect of defects in HCF regime is less prominent. Figs. 6 and 7 show that the fatigue life for horizontally build SLM specimens, as-built SLM as well as heat treated (aged and overaged) are more than vertical counterpart. Although defect statistics in Table- 3 shows, smaller size defects in large number are present in horizontal SLM specimens, size of individual defects rather than their number and orientation with respect to loading axis play an important role in determining the fatigue life which is a localized phenomenon. Fig. 8 schematically depicts that the stress concentration around a defect is much more severe and hence more prone to crack initiation when the loading axis is perpendicular to the defect orientation, which is applicable for vertical SLM specimens and it is less when loading axis is parallel to the defect orientation, applicable for horizontal SLM specimens. For all the cases, fatigue life of the wrought 15-5 PH stainless steel has higher than that of SLM counterparts. Reduced fatigue life for SLM specimens is due to the defects caused by pores, inclusions, un-melted powder particles.

recognizing that it is a powerful austenite former. This will help estimate the extent to which this element is contributing to the presence of austenite in the as-built condition. It was found that virgin EOS SS PH1 (15-5 PH stainless steel) powder has an average N2 content of 3.63 wt% whereas the same is 3.91 wt% and 6.52 wt% for vertical and horizontal as-built SLM specimens respectively. This also supports the observation that increased amount of austenite is present in horizontal SLM specimens in as-built condition. Significant increase in nitrogen content in reused powder with repeated use in SLM process in nitrogen atmosphere has been reported [21]. It may be noted that usually EDS is used for qualitative elemental analysis, not quantitative analysis, since it has an error range of 7% [21]. However, the data has been taken using multiple measurements show a consistent trend, although the exact content values may not be accurate. During deposition, the base plate is kept at 100 °C which is well above martensite finish temperature; this may also contribute to the formation of retained austenite in as-built specimens [12]. Material becomes ductile with increased amount of retained austenite resulting in higher toughness at the cost of lower tensile strength and hardness [10,12].

3.4. Fatigue properties Fig. 6 shows stress vs. number of reversals (cycles) to failures for asbuilt and aged (H900) SLM specimens. In case of aging of SLM 15-5 PH stainless steel, dislocation movement gets hindered through Cu precipitation (∼15 nm diameter) strengthening the matrix which increases both tensile strength and hardness [12] and therefore, expected to give better fatigue life as compared to as-built SLM specimens. However, aging makes specimen brittle [10]. Results show for both build orientations, low cycle fatigue (LCF) life is more for H900 as compared to as-built SLM specimens. However, reverse trend is observed in High Cycle Fatigue (HCF) life regime where crack initiation stage dominates the total fatigue life [22]. Now, since defects (in the form of pores, inclusions) are present in SLM specimens relatively less stress concentration can be accommodated around a defect for a brittle through localized plastic deformation than that of a ductile material. Therefore, defects play an important role in the reduction of HCF for SLM specimens as opposed to wrought 15-5 PH stainless steel specimens. Similar results were reported for fatigue life of heat treated 17-4 PH stainless steel by Yadollahi et al. [3]. Fig. 7 presents stress vs. number of reversals (cycles) to failures for as-built and overaged (H1150) SLM specimens. Results show for both build orientations, LCF life is more for H1150 as compared to as-built SLM specimens. In case of overaging, both size (∼40 nm length and 25 nm width) and density of Cu-rich precipitates increase as well as amount of retained austenite in the matrix increases [12]. These make the material more ductile [10]. Since overaged SLM specimens are relatively ductile, relatively more stress concentration can be

Fig. 8. Schematic showing stress concentration around a defect is much more severe for vertically built SLM specimens. 240

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Fig. 9. EBSD analysis of gauge length portion of horizontal SLM 15-5 PH stainless steel specimen (a) before and (b) after fatigue test at stress amplitude of 700 MPa.

surface [3]. For other SLM specimens, a similar type of fracture surface morphology was obtained. Crack propagation zones for as-built SLM specimens showed trans-granular cleavage fracture, while both cleavage and micro-void coalescence (Fig. 13) could be found in the final fracture region. For heat treated SLM specimens, cleavage mode of failure was dominant. Also, un-melted region, powder particles could be found along most of the fracture surfaces (Fig. 14). Since aging makes specimen brittle, fracture surface for aged vertical SLM specimen failed in HCF shows flat type of surface without any signature of ductile pullout (Fig. 15). However, fracture surface for an overaged vertical SLM failed in HCF shows few ductile pullout regions (Fig. 16). As already discussed earlier, irregular shaped defects influence fatigue life most for SLM specimens. In fracture surface of vertical as-built SLM specimen (Fig. 17a) it could be observed that a relatively bigger but shallow un-melted region acted as a crack initiation site whereas a relatively small but deep un-melted region (Fig. 17b) had acted as a crack initiation site for horizontal as-built SLM specimens. Also, since the loading axis was perpendicular to the defects (refer Fig. 8) for vertically built specimens, crack opening would be much easier resulting in less fatigue life.

3.5. Cyclic strain hardening In order to find out whether there is any cyclic strain hardening due to the transformation of austenite to martensite phase, EBSD analysis of gauge length section after fatigue test for varied stress amplitude was carried out. At higher stress amplitudes, the effect of strain hardening was found for the as-built horizontal SLM specimens. Yadollahi et al. [3] reported similar type of results for SLM 17-4 PH stainless steel in which phase transformation induced cyclic hardening took place at high strain levels due to development of cellular structure and increase in density of shear band. Fig. 9 shows decrease in austenite phase from 13% to 8% after the fatigue test. Similar types of results (from 9% to 3%) were found for H1150 specimens as shown in Fig. 10a and Fig. 10b. However, for H900 specimens where the amount of retained austenite is relative ly less, this effect (Fig. 10c and d) is less (from 4% to 2%) pronounced. 3.6. Fractography analysis In case of cyclic loading, crack generates from a point where there is a large stress concentration. It may be due to surface flaws or microstructural defects such as voids and inclusions. For SLM process, subsurface defects act as a crack initiation points which can be confirmed from Fig. 11 and Fig. 12. In general, fracture surface consists of three major regions i.e. crack initiation point, crack propagation regime and fracture (sudden failure) region. Depending upon the applied stress amplitude and mean stress, area of stable crack propagation will differ [1]. Fig. 11 shows the fracture surface of a vertically built SLM specimen in as-built condition undergone HCF. Sub-surface flaw acts as a crack initiation point which is distinct in nature. In case of low amplitude stress, area of stable crack propagation is relatively larger (Fig. 11) as compared to the case when high amplitude stress is applied (Fig. 12). Fracture surface of specimens failed under LCF shows multiple crack initiation sites (Fig. 12), since crack propagation in LCF regime accounts for a larger fraction of the time to failure which provides enough opportunity for other potential crack initiation points to grow [23]. The size of sub-surface defects visible at fracture surface for HCF and LCF failure are relatively large and small respectively, indicating defect size has a less dominant effect than its distance from the

4. Conclusions From the study on the effect of build orientations, aging and overaging on fatigue life of SLM 15-5 PH stainless steel under rotating bending fatigue testing condition the following conclusions are drawn: 1. For all types of SLM specimens, irregular shaped defects such as pores, un-melted powder are found, which have major influences in their reduced fatigue life as compared to their wrought counterparts. 2. Due to orientations of defects perpendicular to the load axis for vertical SLM specimens, it shows lower fatigue life both in as-built and heat treated conditions compared to for horizontal specimens. 3. Phase transformation (austenite to martensite) induced cyclic strain hardening is more prominent for as-built horizontal SLM specimens undergone high stress amplitude. Among heat treated SLM 241

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Fig. 10. EBSD analysis of gauge length portion of horizontal SLM 15-5 PH stainless steel; H1150 condition (a) before and (b) after fatigue test at stress amplitude of 730 MPa, H900 condition (c) before and (d) after fatigue test at stress amplitude of 780 MPa.

Fig. 11. (a) Fracture surface morphology showing distinct crack initiation site for an as-built vertically built SLM specimens under low stress amplitude (380 MPa, Nf = 235425); (b) a magnified view of crack initiation site showing crack initiation from an un-melted sub-surface region. 242

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Fig. 12. (a) Fracture surface morphology showing multiple crack initiation sites for an as-built vertically built SLM specimens under high stress amplitude (680 MPa, Nf = 1523); (b) a magnified view of crack initiation site showing multiple crack initiations from un-melted sub-surface regions.

reason being specimen becomes brittle and more sensitive to defect after aging. Whereas slightly better fatigue life for SLM H1150 specimens could be observed as compared to as-built counterpart for both build directions, since material becomes ductile and less sensitive to defect after overaging. 5. Fractography analysis shows, cracks initiate from un-melted regions for all the cases. Distinct crack initiation point is found for HCF, whereas multiple crack initiation points could be found for specimens tested under LCF regime. Crack propagation zones for as-built SLM specimens showed trans-granular cleavage fracture, while both cleavage and micro-void coalescence could be found in final fracture regions. For heat treated SLM specimens, cleavage mode of failure was dominant. Considering the above observations on fatigue behavior of SLM 155 PH stainless steel, it may be noted that the fatigue life of SLM parts is largely dependent on various in-process parameters such as build orientations, micro-structural defects etc. as well as post -processing such as heat treatment. Since most of the commercially available SLM systems have their own preparatory process parameters for part preparation, it is difficult to follow a standardized set of guidelines to obtain reliable and consistent fatigue life database for a given material. Neverthe-less the present study demonstrates the need for standardizing the SLM process at various stages starting from design, specimen deposition to post-processing and making it industry-ready.

Fig. 13. Fracture surface morphology showing cleavage and micro-void coalescence at final fracture regime for a horizontal as-built SLM specimen failed under stress amplitude of 420 MPa, Nf = 375684.

specimens, this effect is less prominent for aged specimens where the amount of retained austenite is less but more prominent for overaged specimens with increased amount of retained austenite in the matrix. 4. For both build orientations, fatigue life of SLM H900 specimens in high cycle regime is poor as compared to as-built counterpart;

Fig. 14. Fracture surface morphology showing presence of un-melted powder particles for a horizontal SLM specimen treated under (a) aging i.e. H900 (stress amplitude of 400 MPa, Nf = 325364) and (b) overaging i.e. H1150 (stress amplitude of 420 MPa, Nf = 1325687) condition. 243

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Fig. 15. (a) Overview of fracture surface morphology and (b) magnified view shows flat type of surface without any signature of ductile pullout for an aged vertical SLM specimen failed in HCF (stress amplitude of 300 MPa, Nf = 753654).

Fig. 16. (a) Overview of fracture surface morphology and (b) magnified view shows signature of ductile pullout for an overaged vertical SLM specimen failed in HCF (stress amplitude of 358 MPa, Nf = 723568).

Fig. 17. A comparison of un-melted regions which act as a crack initiation site for as-built (a) vertical and (b) horizontal SLM specimens.

Acknowledgements

ongoing study.

Authors gratefully acknowledge the financial support from Department of Heavy Industry (DHI) and Ministry of Human Resource Development (MHRD), Government of India under IMPRINT project 6917, sanction letter 3-18/2015-T.S.-I (Vol.-III) dated 20-01-2017 to support the present research work. Authors would like to thank Prof. Debalay Chakrabarti, Department of Metallurgical and Materials Engineering, IIT Kharagpur and Prof. Soumitra Paul, Department of Mechanical Engineering, IIT Kharagpur for allowing the authors to use various testing facilities at their respective laboratories. Technical supports from Mr. Santanu Ghosh, Mr. Tanmay Baram and Mr. Swarup Roy throughout the entire study are gratefully acknowledged. The raw/processed data required to reproduce the experimental findings cannot be shared at this time as the data also forms part of an

References [1] S. Sarkar, C.S. Kumar, A.K. Nath, Effect of mean stresses on mode of failures and fatigue life of selective laser melted stainless steel, Mater. Sci. Eng., A 700 (2017) 92–106. [2] N. Shamsaei, A. Yadollahi, L. Bian, S.M. Thompson, An overview of direct laser deposition for Add. Manuf.; Part II: mechanical behavior, process parameter optimization and control, Add. Manuf. 8 (2015) 12–35. [3] A. Yadollahi, N. Shamsaei, S.M. Thompson, A. Elwany, L. Bian, Effects of building orientation and heat treatment on fatigue behavior of selective laser melted 17‐4 PH stainless steel, Int. J. Fatigue 94 (2017) 218–235. [4] S. Sarkar, C.S. Kumar, A.K. Nath, Investigation on the mode of failures and fatigue life of laser-based powder bed fusion produced stainless steel parts under variable amplitude loading conditions, Add. Manuf. 25 (2019) 71–83. [5] EOS GmbH—Electro Optical Systems, Material Data Sheet EOS Stainless Steel PH1 for EOSINT M 290, EOS GmbH - Electro Optical Systems, Munich, Germany, 2017 https://cdn2.scrvt.com/eos/2c79c109ca82d0e7/07f8e87d836c/SS-PH1_M290_ Material_data_sheet_01-17_en.pdf , Accessed date: 13 January 2019.

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Materials Science & Engineering A 755 (2019) 235–245

S. Sarkar, et al.

[14] A.K. Swain, H. Patra, G.K. Roy, Mechanical Operations, Tata McGraw-Hill Education, 2011. [15] N.S. Bailey, W. Tan, Y.C. Shin, Predictive modeling and experimental results for residual stresses in laser hardening of AISI 4140 steel by a high power diode laser, Surf. Coating. Technol. 203 (2009) 2003–2012. [16] H.K. Rafi, D. Pal, N. Patil, T.L. Starr, B.E. Stucker, Microstructure and mechanical behavior of 17-4 precipitation hardenable steel processed by selective laser melting, J. Mater. Eng. Perform. 23 (2014) 4421–4428. [17] W.D. Callister, Material Science and Engineering an Introduction, John Wiley & Sons Inc, New York, 1994. [18] K. Coffy, Microstructure and Chemistry Evaluation of Direct Metal Laser Sintered 15-5 PH Stainless Steel, Master’s Thesis (2014) https://digital.library.ucf.edu/cdm/ singleitem/collection/GTD/id/81493/rec/1. [19] T.L. Starr, K. Rafi, B. Stucker, C.M. Scherzer, Controlling phase composition in selective laser melted stainless steels, Solid Freeform Fabrication Symposium, 2012, pp. 439–446. [20] H. Gu, H. Gong, D. Pal, K. Rafi, T. Starr, B. Stucker, Influences of energy density on porosity and microstructure of selective laser melted 17-4 PH stainless steel, Solid Freeform Fabrication Symposium, 2013, pp. 474–489. [21] J. Zhang, B. Hu, Y. Zhang, X. Guo, L. Wu, · H. Park, J. Lee, Y. Jung, Comparison of virgin and reused 15-5 PH stainless steel powders for laser powder bed fusion process, Progr. Add. Manuf. 3 (2018) 11–14 https://doi.org/10.1007/s40964-0180038-2. [22] S. Suresh, Fatigue of Materials, second ed., Cambridge University Press, Cambridge, UK, 2006. [23] N. Shamsaei, A. Fatemi, Small fatigue crack growth under multi-axial stresses, Int. J. Fatigue 58 (2014) 126–135.

[6] AK Steel Corporation, Product Data Bulletin 15-5 PH Stainless Steel, AK Steel Corporation, West Chester Township, OH, 2007http://www.aksteel.com/pdf/ markets_products/stainless/precipitation/15-5_ph_data_sheet.pdf , Accessed date: 13 January 2019. [7] M. Akita, Y. Uematsu, T. Kakiuchi, M. Nakajima, R. Kawaguchi, Defect-dominated fatigue behavior in type 630 stainless steel fabricated by selective laser melting, Mater. Sci. Eng., A 666 (2016) 19–26 https://doi.org/10.1016/j.msea.2016.04.042. [8] T.M. Mower, M.J. Long, Mechanical behavior of additive manufactured, powderbed laser-fused materials, Mater. Sci. Eng., A 651 (2016) 198–213. [9] H.A. Stoffregen, K. Butterweck, A. Eberhard, Fatigue analysis in selective laser melting: review and investigation of thin-walled actuator housings, Solid Free Fabrication Symposium, 2013, pp. 635–650. [10] S. Sarkar, C.S. Kumar, A.K. Nath, Effect of different heat treatments on mechanical properties of laser sintered additive manufactured parts, ASME. J. Manuf. Sci. Eng. 139 (2017), https://doi.org/10.1115/1.4037437 111010-111010-11. [11] ASTM E8/E8M-16a Standard Test Methods for Tension Testing of Metallic Materials, ASTM International, West Conshohocken, PA, 2016https://doi.org/10. 1520/E0008_E0008M-16A. [12] S. Sarkar, S. Dubey, A.K. Nath, Effect of heat treatment on impact toughness of selective laser melted stainless steel parts, ASME. International Manufacturing Science and Engineering Conference, Volume 1: Additive Manufacturing; Bio and Sustainable Manufacturing, 2018, https://doi.org/10.1115/MSEC2018-6418 V001T01A005. [13] S. Sarkar, S.R. Jha, A.K. Nath, Effect of heat treatment on corrosion properties of selective laser melted stainless steel parts, ASME. International Manufacturing Science and Engineering Conference, Volume 1: Additive Manufacturing; Bio and Sustainable Manufacturing, 2018, https://doi.org/10.1115/MSEC2018-6429 V001T01A007.

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