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Mechanical properties of 304 austenite stainless steel manufactured by laser metal deposition
Materials Science & Engineering A 758 (2019) 60–70
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Mechanical properties of 304 austenite stainless steel manufactured by laser metal deposition
T
Weibo Huanga, Yimin Zhangb,∗, Weibing Daia, Risheng Longb a b
School of Mechanical Engineering and Automation, Northeastern University, Shenyang, 110006, China Equipment Reliability Institute, Shenyang University of Chemical Technology, Shenyang, 110142, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser metal deposition Microstructure Hardness Static tensile properties Fatigue strength Fracture morphology
Laser metal deposition (LMD) is a common manufacturing technique of laser additive manufacturing (LAM) which belongs to coaxial powder feeding method. The LMDed SS304 specimens are fabricated by the same combination of the process parameters so that they have the similar properties. The microstructure is similar to that of the selective laser melted (SLMed) specimen. The grain sizes of the LMDed specimen are compared with those of the specimens manufactured by the traditional method and SLM. Through the calculation of Ni and Cr equivalents in the LMDed SS304 specimen and the analytical results of electron backscattered diffraction (EBSD) and energy dispersive spectroscope (EDS), the phase compositions of the LMDed SS304 specimen are evaluated. The hardness of the LMDed specimen is compared with that of the conventionally manufactured wrought SS304 and SLMed specimens. And due to the differences of the grain size and phase composition on the different directions, the hardness of the LMDed specimen is anisotropic. The static tensile properties (ultimate tensile strength (UTS), yield strength (σ0.2) and elongation (EL)) and fatigue strength (FS) of the LMDed specimen are compared with those of the specimens made by the traditional method and SLM. The SeN curve is established by the multiple experiments. Scanning electron microscopy (SEM) is applied to observe the fracture morphology of the static tension and tension-compression fatigue tests. And EDS is applied to analyze the chemical compositions of the particles on the fracture surface. As expected, LMD has the higher static tensile properties and fatigue strength than those of the specimen manufactured by the traditional method.
1. Introduction Additive manufacturing (AM) is also known as three-dimension printing (3DP) which adopts laser or electron beam to melt the adhesive materials such as powder or wire so that the specimen is constructed in layer-by-layer fashion [1–4]. According to the type of energy source, AM can be divided into two categories: laser AM (LAM) and electron beam AM (EBAM). And LAM can be divided into powder feeder and powder bed on the basis of the different feeding methods [5]. Laser metal deposition (LMD) is a common manufacturing technique of LAM which belongs to powder feeding method. The principle of LMD is similar to laser cladding [6]. The metal powders are delivered into the laser spot by the shielding gas stream and then melted to form the molten pool [7]. The advantages of LMD are high efficiency and low consumption [8–11]. To date, LMD has been applied in the different industrial fields [12]. So far, the researches on LMD could be roughly divided into two categories, laser repair and direct metal forming. Some literature has
∗
been published with respect to laser repair. Sun et al. [13] applied LMD process to repair the HSLA-100 steel plates with the large volume trapezoidal grooves. The appearance, microstructure, hardness, tensile strength, bending strength and low temperature toughness of the repaired specimens with two different filling powders were researched. Song et al. [14] applied LMD process to repair the 304 austenite stainless steel (SS304) substrates with the large volume trapezoidal grooves. The results showed that the repaired part had no micro-defects, and the hardness, wear resistance and fatigue property were better than those of the substrate. In addition to being used for repair, LMD has been increasingly applied in the overall manufacture. Many documents on the manufacture of various alloys by LMD have been published. A variety of alloys have been successfully manufactured by LMD and their mechanical properties were also studied. Liu et al. [15] successfully applied LMD to produce the Ti6.5Al3.5Mol.5Zr0.3Si alloy specimens and studied the fatigue properties. The research revealed that the columnar crystal boundaries were detrimental for the fatigue properties. Lu et al. [16] studied the anisotropic mechanical properties
Corresponding author. E-mail addresses: [email protected] (W. Huang), [email protected] (Y. Zhang).
https://doi.org/10.1016/j.msea.2019.04.108 Received 28 February 2019; Received in revised form 26 April 2019; Accepted 28 April 2019 Available online 03 May 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
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good Hall fluidity. LMD apparatus applied in this experiment is shown in Fig. 1. As shown in Fig. 1, the device consists of laser device, sealed box, computer control system, system power source, shielding gas cylinder, powder feeder, and cooler. The laser device applies an YLS – 4000 Ytterbium laser system. The main parameters of the apparatus are listed in Table 3. The apparatus adopts the coaxial powder feeding technology. The process principle diagram of the apparatus is shown in Fig. 2. As shown in Fig. 2, the laser generated by the laser device is transmitted to the machining head through the optical fiber; the powder feeder carries the powders to the machining head through the argon gas, and the powders coincide with the laser focus; and the CNC (Computerized Numerical Control) machine moves the machining head to manufacture the current layer and superimposes the layers, until the entire part is finished. The optimized process parameters were applied to fabricate the fully density SS304 specimens in this experiment. The SS304 powders were coaxially delivered by the argon gas with a flow rate of 7 L/min, and the rotating speed of powder plate was 1.0 r/min. The laser power, scan speed and overlap rate in this experiment were 3 KW, 840 mm/s and 50%, respectively. And the scan strategy is shown as Fig. 3. The concentrations of oxygen and water were kept underlying 50 ppm in the sealed box. All specimens were fabricated under the same combination of the process parameters so that the specimens had the similar density and mechanical properties. The specimens were designed according to Chinese GB/T228-2002 standard [27] which was nearly similar to ISO 6892–1998 [28]. The dimensions of the specimen are shown as Fig. 4. According to the literature [23,29], the mechanical properties of the specimens which were parallel to the deposition direction were lower than those of the specimens which were perpendicular to the deposition direction. As shown in Fig. 5(a), the rod is perpendicular to the deposition direction. The rods were subjected to subsequent machining and polishing to form the experimental specimens, which are shown in Fig. 5(b). The microstructure of the LMDed specimen was different from that of the specimen manufactured by the traditional manufacturing process [30,31]. In order to observe the microstructure of the specimen, nitrohydrochloric acid was applied to corrode the surface. The optical microscopy (OM) was employed to observe the microstructure of the LMDed SS304 specimen. ImageJ software was applied to measure the grain size. Electron backscattered diffraction (EBSD) was applied to ensure the phase compositions of the LMDed specimen. Energy dispersive spectroscope (EDS) was applied to analyze the contents and distributions of the elements on the points and surfaces. The hardness of the LMDed specimen was evaluated by the hardness tester. The mechanical properties of the LMDed specimen were evaluated by a PC controlled EHFEV200K2-040-1A machine in the room temperature. The mechanical properties contain static tensile properties (UTS, σ0.2 and EL) and fatigue strength (FS). Uniaxial tension was employed for static tension and fatigue tests in this paper. The tensile rates of the static tension test were 200 N/s for stress control and 0.07 mm/s for displacement control. The tension-compression fatigue tests were carried out under the sinusoidal cycle loads with the 10 Hz frequency and a stress ratio (ratio of maximum stress to minimum stress) of −1. Scanning electron microscopy (SEM) was applied to observe the fracture morphology of static tension and fatigue tests.
of the laser metal deposited (LMDed) Ti6Al4V alloy. The research indicated that the anisotropic microstructural distribution of the LMDed specimen caused the anisotropic mechanical properties. And the LMDed specimen had the higher strength, lower elongation (EL) and smaller reduction of area in the transverse direction, which were compared with those in the perpendicular direction. Blackwell [17] studied the mechanical and microstructural characteristics of the LMDed IN718 alloy. The results showed that ultimate tensile strength (UTS) and yield strength (σ0.2) of the LMDed IN718 alloy were higher than those of the typical wrought IN718, but the reduction of area was lower. Xu et al. [18] researched the morphologies, microstructures and mechanical properties of the LMDed 316 L stainless steel. The experiments indicated that LMD could successfully fabricate the 316 L specimens with fine cellular and good metallurgical bonding between layers. And the fracture mode was ductile fracture after tensile test. SS304 is a common austenitic stainless steel which has been widely applied in varieties of fields, such as petroleum, nuclear and chemical industries due to its excellent corrosion resistance, mechanical properties and high cost-effect [19,20]. SS304 had high Cr content so as to produce the excellent corrosion property, and the austenitic phase resulted in the satisfactory mechanical properties [21]. The AM technology such as selective laser melting (SLM) has been applied in the manufacture of SS304 specimen. Guan et al. [22] studied the effects of the process parameters of SLM on the static tensile properties of SS304 specimen. The results indicated that SLM could produce the full density specimens with the higher σ0.2 and UTS than those of the wrought specimens under the appropriate process parameters. The effects of the layer thickness, building direction, overlap rate and hatch angle on the static tensile properties were studied in this paper. Yu et al. [23] studied the mechanical anisotropies of the SLMed Tie6Ale4V alloy and SS304 alloy. The research explained that the SLMed Tie6Ale4V alloy had more pronounced mechanical anisotropies than those of the conventionally manufactured wrought SS304 alloy. And heat treatment could effectively improve the mechanical anisotropies of the SLMed Tie6Ale4V alloy. Elghany and Bourell [24] researched the property evaluation of SS304L during SLM. The effects of powder layer thickness and laser scan speed on the density, microstructure, surface hardness and static tensile properties of the specimens were explored. The largest difference between LMD and SLM is the different powder feeding mode. LMD adopts the coaxial powder feeding method, while SLM adopts powder bed method. To date, few papers were published on the direct manufacture of the SS304 specimens by LMD. Arrizubieta et al. [25] adopted the numerical simulation and experiment to predict the hardness, grain size and porosity of the LMDed SS304. The predicted results of the numerical simulation were very similar to the experimental results. From the above, the most studies on the mechanical properties of the LMDed specimens focused on titanium-based alloys, nickel-based alloys and SS316L alloy. Up to now, fatigue fracture accounted for 90% engineering failures [26]. As a consequence, the in-depth understanding of the fatigue property of the LMDed SS304 specimens is very important and necessary. Up till now, the mechanical properties of the SS304 specimens manufactured by LMD technology, including static tensile properties (UTS, σ0.2 and EL) and fatigue strength (FS) were still unclear. In this paper, the mechanical properties of the LMDed SS304 specimens were evaluated to fill in the gaps in this field. The microstructure and hardness of the LMDed SS304 specimen were also studied in this paper.
3. Results and discussion 3.1. Microstructure
2. Experimental details Due to the unique forming mode of LMD, the formed specimen has the different microstructure from the conventionally manufactured wrought material. The microstructures on the XOY and XOZ planes are shown in Fig. 6(a) and (b). It is clearly seen that the microstructures on the both planes are different. The grain shape on the XOY plane is similar to honeycomb, and the grain shape on the XOZ plane is columnar.
The SS304 powders applied in this experiment were made by gas atomization method. According to the data provided by the supplier, the chemical compositions of SS304 powder and the volume fractions of the particle size are shown in Table 1 and Table 2, respectively. The oxygen concentration is approximate 280 ppm. And the powder has 61
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Table 1 Chemical compositions of SS304 powder. Element
C
Cr
Ni
Mo
Mn
Si
Cu
Co
P
S
N
Fe
wt.%
0.043
19.56
9.66
0.89
0.054
0.05
0.002
0.003
0.027
0.01
0.014
Balance
Table 2 Volume fractions of particle size.
Table 3 Main parameters of LMD apparatus.
Volume fraction (%)
Particle size (μm)
Parameters
Values
Dv Dv Dv Dv
95.2 145 178 367
Forming scope Maximum scan speed Laser type Laser wavelength Laser power Laser spot radius Shielding gas
800× 600× 900 mm 5 m/min Optical fiber laser 1070 nm 2–10 KW 1–2.5 mm Ar
(10) (35) (50) (90)
The results are same as those of the SLMed specimen in the literature [22]. The grain size on the XOY plane approximates 6 μm and the length of the columnar grain on the XOZ plane is tens of microns. The aspect ratio of columnar grain is the main factor to affect property anisotropy [23]. The grain size of the conventionally manufactured wrought SS304 material was 60 μm [32]. The grain size of the LMDed specimen is significantly smaller than that of the conventionally manufactured wrought SS304 material, but larger than that of the SLMed
specimen which had the nanoscale grain size [22]. The cooling rate during LMD was usual between 102 K/s to 104 K/s [33] which was higher than that during the conventionally manufactured wrought process and lower than that during SLM. The differences in cooling rate lead to the different grain sizes. The grain size and shape have the
Fig. 1. LMD apparatus (a) sealed box and computer control system (b) power source (c) shielding gas cylinder, powder feeder and cooler (d) laser device. 62
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(eq) and Cr (eq) are calculated by the equations [36].
Ni (eq) = Ni% + 30 × (C % + N %) + 0.5 × Mn% = 9.66% + 30 × (0.043% + 0.014%) + 0.5 × 0.054% = 11.397%
(1)
Cr (eq) = Cr % + Mo% + 1.5 × Si% + 0.5 × Nb% = 19.56% + 0.89% + 1.5 × 0.05% = 20.525%
(2)
In the literature [37], the solidification mode has four types: A, AF, FA and F modes. The Cr (eq)/Ni(eq) is 1.8 which indicates that the solidification mode belongs to FA mode. But considering the growth mode of ferrite and the Cr (eq)/Ni(eq), the solidification mode during LMD belongs to AF mode. Due to the larger cooling rate during SLM, the microstructure of the SLMed specimen was full austenite [22]. Considering the effects of the Cr (eq)/Ni(eq) and cooling rate, the phase compositions of the LMDed SS304 specimen are austenite and a small amount of ferrite. This conclusion is consistent with the previous one. In the SS304 specimen, the most important elements are Cr and Ni which have the significant effects on the phase compositions. Fig. 9 (a) and (b) show the distributions of Cr and Ni elements on the XOY plane. It is clear that Cr and Ni elements are evenly distributed on the XOY plane without significant component segregation. Fig. 10(a) and (b) show the distributions of Cr and Ni elements on the XOZ plane. It is clear that Cr and Ni elements are unevenly distributed on the XOZ plane with significant composition segregation. The place where Cr element is enriched on the plane is also the region where Ni element is poor. In Fig. 10, the ferrite is formed in the place where Cr element is enriched and Ni element is poor. It proves that two phases of austenite and ferrite occur in the LMDed specimen.
Fig. 2. Process principle diagram of LMD.
Fig. 3. Scan strategy.
significant effects on the properties of the specimen. Some micro-voids can be seen from Fig. 6, which were attributed to the flow behavior of the molten pool during the laser melting process [34,35]. Macroscopic segregation is also called regional segregation. Macroscopic segregation is mostly due to the physical movement of the liquid or solid phase during solidification. In Fig. 7(a) and (b), the macroscopic appearances on the XOY plane and XOZ plane are presented, respectively. Through careful observation of the macroscopic appearances on the two planes, there is no macroscopic segregation on the planes except for the presence of pores. Since the LMDed specimen is formed in the layer-by-layer mode and has the high cooling rate, macroscopic segregation is difficult to occur. EBSD is an effective method for analyzing the phase compositions of the alloy. The phase compositions on the two planes are analyzed by EBSD. And the EBSD maps are shown in Fig. 8 (a), (b) and (c). In Fig. 8, the red part represents ferrite which is body-centered cubic (BCC) structure, and the background is austenite which is face-centered cubic (FCC) structure. As shown in Fig. 8(a), no obvious ferrite aggregates on the XOY plane. Fig. 8 (b) and (c) are EBSD maps of any two positions on the XOZ plane. As shown in Fig. 8(b) and (c), the ferrite exists on the XOZ plane. And most ferrite forms at the grain boundary of austenite. After measurement of ferrite content in the multiple locations, the ferrite content is about 10% on the XOZ plane. For austenitic stainless steel, the Ni equivalent (Ni(eq)) is critical for the formation and stability of austenite. If the Ni(eq) is less than 12%, the γ austenite phase is considered to be metastable [32]. The effect of Cr element is to promote the formation and stability of ferrite. The Ni
3.2. Hardness Nine points on the XOY and XOZ planes are selected as shown in Fig. 11, and the readings of the points are recorded in Table 4. The average hardness on the XOY and XOZ planes is 203.6 HV and 184.9 HV, respectively. It is clear that the hardness on the XOY plane is larger than that on the XOZ plane. The hardness of the conventionally manufactured wrought SS304 material was 168 HV [32]. The hardness of the LMDed specimen is obviously larger than that of the conventionally manufactured wrought SS304 specimen. The main reason for this result is grain size. The phenomenon can be explained by Hall-Petch equation [38]. It can be clearly seen from Table 4 that the hardness on both directions is different. Grain size and phase composition are the two main reasons to cause the result. As shown in Fig. 7 that the grain size on the XOY plane is smaller than that on the XOZ plane, which causes the larger hardness on the XOY plane. As shown in Fig. 8, the ferrite exists on the XOZ plane. The larger grain size and the ferrite phase result in the lower hardness on the XOZ plane. 3.3. Static tensile properties 3.3.1. Tensile properties Fig. 12(a) indicates the stress-strain (σ-ε) curve of the LMDed specimen. It is clear that the static tensile process of the LMDed specimen can be divided into three stages, namely elastic deformation stage (oa), uniform plastic deformation stage (ab) and non-uniform plastic deformation stage (bc). Fig. 12(b) presents the comparison of the Fig. 4. Dimensions of specimen.
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Fig. 5. (a) 3D schematic and (b) specimen.
Fig. 6. Microstructure on (a) XOY plane and (b) XOZ plane.
Fig. 7. Macroscopic appearances on (a) XOY plane, and (b) XOZ plane.
plastic deformation, and the reliability of the structural part is higher. The experimental results of this paper and the data of the other papers are listed in Table 5. It is clear that the all static tensile properties of the LMD specimen meet the ANSI standard, and are higher than those of the conventionally manufactured wrought SS304 specimen. In other words, without the complicated post-processing, LMD can obtain the specimens with the better static tensile properties. LMD has the wonderful ability to reduce the energy consumption and simplify the fabricating processes compared with the traditional manufacturing processes. Nevertheless, the UTS and σ0.2 of the LMDed specimens are lower than those of the SLMed specimens, but the EL of the LMDed specimen is higher. The yield strength ratio of the LMDed specimen is between the SLMed specimen and the conventionally manufactured wrought SS304 specimen. It shows that the LMDed specimen has the better plasticity than that of the SLMed specimen. The grain size is the main reason to affect the static tensile properties. The average grain size of the LMDed specimen was much smaller than that of the conventionally manufactured wrought SS304 specimen which was introduced in Section 3.1. The smaller the grain size, the
specimens before and after fracture. It is clear in Fig. 12(b) that the gauge section becomes finer and longer after fracture. The fracture occurs near the center of the specimen where the necking happens. The static tensile properties of the LMDed specimens are compared with those of the SLMed specimen [22], conventionally manufactured wrought SS304 specimen [32] and ANSI standard [22]. ANSI is the abbreviation of the American National Standards Institute, which is established in 1918. It is mainly composed of the American Society for Testing and Materials (ASTM), the American Society of Mechanical Engineers (ASME), the American Association of Mining and Metallurgical Engineers (ASMME), the American Society of Civil Engineers (ASCE), and the American Institute of Electrical Engineers (AIEE). The main process parameters of SLM were laser power, scan speed, overlap rate, hatch angle and building direction, which were 200 W, 15 m/min, 40%, 90° and horizontally built, respectively [22]. The SS304 specimen was solution treated at 1080 °C for 1 h [32]. The ratio of σ0.2 to UTS (σ0.2/UTS) is called the yield strength ratio. The lower yield strength ratio indicates that the material has the better plasticity; the higher yield strength ratio indicates that the material has strong resistance to 64
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Fig. 8. EBSD maps on (a) XOY plane, (b) and (c) XOZ plane.
Fig. 9. EDS maps of (a) Cr and (b) Ni on the XOY plane.
Fig. 10. EDS maps of (a) Cr and (b) Ni on the XOZ plane.
3.3.2. Fracture morphology analysis Fig. 13 shows the fractured specimen produced by the static tensile test. It can be seen from Fig. 13(a) that the fracture caused by the static tension occurs at the position close to the center of the specimen, and the significant necking phenomenon occurs at the fracture. Fig. 13(b) shows that the angle between the necked slope and the axial direction is approximate 45°. Necking is the basic feature of plastic fracture. Fig. 14(a) shows the overall fracture morphology. As shown in
more the grain boundaries are in the same area. Reduction of grain size leads to grain boundary strengthening. This phenomenon can still be explained by Hall-Petch equation [38]. When the grain was refined to a few micrometers or less, the plastic deformation ability of the material exhibited the remarkable decrease, and the work hardening ability was lost [39]. This is the reason that the SLMed specimen has the larger strength but lower plasticity.
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Table 5 Static tensile properties of LMDed, SLMed, conventionally manufactured wrought SS304 and ANSI specimens. Type
UTS (MPa)
σ0.2 (MPa)
EL (%)
σ0.2/UTS
LMD SLM [22] Conventionally manufactured wrought SS304 [32] ANSI standard [22]
609 713 586
273 551 242
67.3 43.6 65
0.45 0.77 0.41
≥520
≥205
≥40
0.39
3.4. Fatigue strength 3.4.1. SeN curve The SeN curve is drawn according to the fatigue life of the LMDed specimen under the different stress levels, which is shown in Fig. 16. The stress corresponding to the cycle number of 106 is taken as the fatigue strength of the specimen. The fatigue strength of the SS304 specimen manufactured by LMD is 255 MPa, which is larger than those in the literature [40,41]. The fatigue strengths of the conventionally manufactured wrought SS304 specimens both approached 230 MPa at ambient temperature in the both literature. It can be seen from Section 3.1 that the grain size of the LMDed specimen is smaller than that of the conventionally manufactured wrought SS304 specimen, which causes more grain boundaries in the LMDed specimen. Fine grain hinders the formation and expansion of the fatigue cracks in the specimen and prolongs the fatigue life of the specimen [42]. Fatigue fracture usually belongs to transgranular fracture. That is to say, crack propagation needs to pass through the grain interior. The grain boundary has the effect to slow the crack growth rate because the crack tip must have higher energy to pass through the grain boundary. Grain boundary strengthening is the main reason for the higher fatigue strength of the LMDed specimens.
Fig. 11. Nine points selected on the plane. Table 4 Hardness (HV) on XOY and XOZ planes. No.
1
2
3
4
5
6
7
8
9
Mean
XOY XOZ
205 181
209 179
201 194
206 183
204 193
200 188
207 178
198 184
202 175
203.6 184.9
Fig. 13(a), the color of the cone-shaped fracture is dark. This is because the fracture surface has the weak reflection ability to light. Fig. 14(b) and (c) show the local fracture morphologies at the magnifications of 2000× and 5000×, respectively. It is clear that the dimples are evenly distributed. The dimple is the typical feature of plastic fracture. The formation mechanism of the dimple feature is void aggregation, that is to say, the process of micro-void nucleation, growth, accumulation, and fracture. The micro-voids are formed by LMD and internal separation of material. The micro-voids grow continuously under the action of external force, and the cross-sections of the matrix between several adjacent micro-voids are continuously reduced until they are connected to each other to cause fracture, and forming the fracture morphology of the dimples. As can be seen from Fig. 14(b) and (c), all dimples have the same shape and are approximately equiaxed. This is because under the action of normal stress, the growth rates of the micro-voids in the three directions of space are same. Fig. 14(d) is the enlarged image of the red box in Fig. 14(c). Points 1 and 2 are the particles on the tensile fracture surface. The EDS module in SEM is applied to analyze the chemical compositions of the particles. The chemical compositions of the particles are shown in Fig. 15. It indicates that they are steel particles, which are possible to be formed by the collapse of matrix.
3.4.2. Fatigue fracture morphology The fatigue fracture morphologies of the LMDed specimens under the different loads are shown in Figs. 17–19. As shown in Figs. 17(a), 18(a) and 19(a), the fatigue fracture surfaces are partitioned into the three disparate zones which are crack initiation zone (CIZ), crack propagation zone (CPZ), and instantaneous fracture zone (IFZ), respectively. CIZ is the crack nucleation and initiation region. As shown in Figs. 17(b), 18(b) and 19(b), the cracks all nucleate and initiate on the surfaces of the specimens under the different loads. And the characteristic of CIZ is smooth. As shown in Fig. 6, a certain number of holes exist in the LMDed specimens, but the cracks still nucleate on the surfaces of the specimens. Due to the presence of defects such as pits on the surface, stress concentration occurs at the defects during loading, which becomes the CIZ [43]. In addition, under the action of alternating
Fig. 12. (a) σ-ε curve, and (b) length comparison of LMDed specimens before and after fracture. 66
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Fig. 13. Macroscopic fracture feature: (a) fractured specimen and (b) fractured section. Fig. 14. Fracture morphology of static tensile specimen: (a) overall appearance, (b) local topography magnified 2000×, (c) local topography magnified 5000× and (d) fracture morphology in the red box of (c). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 16. S–N curve. Fig. 15. Weight percent (wt %) of major elements in the particles.
in the plane stress state, and plastic slip is likely to occur. Repeated cyclic slip strain will produce the metallic slip band to make the microcrack nucleate. The micro-crack nucleation is a slow process in the CIZ. Fig. 20(a) shows the specimen which is subjected to the cyclic tensile-
stress, although the nominal stress does not exceed the σ0.2 of the material, the unevenness of the material structure causes slip in the local area of the specimen. This is because the surface of the specimen is 67
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Fig. 17. Fatigue fracture morphologies (275 MPa, 3701 cycles): (a) overall appearance, (b) CIZ, (c) CPZ and (d) IFZ.
Fig. 18. Fatigue fracture morphologies (265 MPa, 24883 cycles): (a) overall appearance, (b) CIZ, (c) CPZ and (d) IFZ.
fatigue life. The stress intensity factor is a physical quantity that reflects the strength of the elastic stress field at the crack tip. It is related to crack length, geometric characteristic of specimen, and load. The relationship can be described by the following equation:
compressive loads. It is clearly shown in Fig. 20(b) that the slip bands occur on the surface of the specimen under the cyclic tensile-compressive loads. The slip bands are the places where the cracks may initiate. When the micro-crack slowly spreads to the critical length, it enters the CPZ. In the CPZ, river stripes are the most typical feature, which are shown as Figs. 17(c), 18(c) and 19(c). The width of each stripe represents the crack propagation distance at one stress cycle. When the maximum stress is 275 MPa, the width of the stripe approximates 12 μm. The width is 2 μm at 265 MPa, and it is 0.8 μm at 260 MPa. It is clearly seen that the higher stress causes the wider strip and lower
KI = βσ πa
(3)
where KI is the stress intensity factor; β is the shape factor; σ is the nominal stress; and a is the crack length. Fracture toughness is considered to be the inherent property of the material. It indicates the sensitivity of the material to the crack under the static load, and is expressed by KIC. When the stress intensity factor KI exceeds KIC, the 68
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Fig. 19. Fatigue fracture morphologies (260 MPa, 747112 cycles): (a) overall appearance, (b) CIZ, (c) CPZ and (d) IFZ.
Fig. 20. (a) Specimen under cycle loads, and (b) slip bands on the surface of specimen.
conclusions are enumerated as follows:
specimen generates fracture. It is clearly seen from Figs. 17(b), 18(b) and 19(b) that the fracture caused by the higher stress has the larger CPZ. Because the LMDed specimens are manufactured by the same material and have the same shape, the specimens have the same β and KIC. According to Eq. (3), the higher stress causes the stress intensity factor KI of the specimen to reach the fracture toughness KIC under a small crack length. And due to the wider strip, the fracture caused by the higher stress has the larger crack propagation rate. In summary, the fractures caused by the different stresses have the CPZs with the different areas. At different cyclic stress levels, when the crack spreads to a certain size, the specimen breaks. At this point, the area where the direct fracture occurs is the IFZ, which is shown as Figs. 17(d), 18(d) and 19(d). The instantaneous fracture is similar to the static tension fracture, which has the fracture morphology of static tension. And IFZ is the fracture area formed by the rapid instability of the crack. Because the LMDed specimen has good plasticity, the dimple is the typical feature of IFZ.
(1) The grain shapes of the LMDed specimen are similar to those of the SLMed specimen which are honeycomb on the XOY plane and columnar on the XOZ plane, respectively. The grain sizes of the LMDed specimen are 6 μm on the XOY plane and tens of micrometers on the XOZ plane which are smaller than those of the conventionally manufactured wrought SS304 specimen and larger than those of the SLMed specimen. No macroscopic segregation occurs by observing the macroscopic appearance of the two planes. Due to the Cr and Ni equivalents and cooling rate during LMD, the phase compositions of the LMDed specimen are austenite and ferrite, which is different from the full austenitic SLMed specimen. The ferrite content is about 10% on the XOZ plane and the ferrite forms at the grain boundary of austenite. And no obvious ferrite aggregates on the XOY plane. (2) Due to the smaller grain size, the hardness of the LMDed specimen is larger than that of the conventionally manufactured wrought SS304 specimen. And the hardness on the XOY plane is larger than that on the XOZ plane, which is attributed to the smaller grain size and less ferrite on the XOY. (3) The static tensile properties (UTS, σ0.2 and EL) of the LMDed specimen are all higher than those of the conventionally manufactured
4. Conclusions The microstructure, hardness, static tensile properties and fatigue strength of the LMDed specimen are explored in the paper. The main 69
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wrought SS304 specimen. Although the UTS and σ0.2 are lower than those of the SLMed specimen, the EL is higher. The obvious characteristics of necking and dimple show that the fracture type is plastic fracture. The particles on the fracture surface are steel particles, which are possible to be formed by the collapse of matrix. (4) The SeN curve is drawn by the multiple experiments. The fatigue strength of the LMDed specimen is higher than that of the conventionally manufactured wrought SS304 specimen at 106 cycles. This is attributed to the effect of grain boundary strengthening, which hinders the crack propagation. The CIZs are smooth, which nucleate and initiate at the surface under the different loads. In the case of the larger cyclic load, the slip band appears clearly on the surface of the specimen where the crack may originate. The main feature of the CPZ is river stripe. And the width of the stripe represents the crack growth rate. Since the KI of the specimen loaded with the larger cyclic load first reaches the KIC of the material, it has the smaller CPZ and larger crack growth rate. The main feature of the IFZ is dimple which is similar to that of the static tensile specimen.
[15]
[16]
[17] [18]
[19] [20]
[21]
[22]
[23]
In summary, the LMDed specimen has the better mechanical properties than those of the conventionally manufactured wrought SS304 specimen.
[24] [25]
Data availability [26]
The data are all presented in this paper. [27]
Acknowledgements
[28]
We would like to express our appreciation to Chinese National Natural Science Foundation of China (U1708254) for supporting this research.
[29]
[30]
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Mechanical properties of 304 austenite stainless steel manufactured by laser metal deposition
T
Weibo Huanga, Yimin Zhangb,∗, Weibing Daia, Risheng Longb a b
School of Mechanical Engineering and Automation, Northeastern University, Shenyang, 110006, China Equipment Reliability Institute, Shenyang University of Chemical Technology, Shenyang, 110142, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser metal deposition Microstructure Hardness Static tensile properties Fatigue strength Fracture morphology
Laser metal deposition (LMD) is a common manufacturing technique of laser additive manufacturing (LAM) which belongs to coaxial powder feeding method. The LMDed SS304 specimens are fabricated by the same combination of the process parameters so that they have the similar properties. The microstructure is similar to that of the selective laser melted (SLMed) specimen. The grain sizes of the LMDed specimen are compared with those of the specimens manufactured by the traditional method and SLM. Through the calculation of Ni and Cr equivalents in the LMDed SS304 specimen and the analytical results of electron backscattered diffraction (EBSD) and energy dispersive spectroscope (EDS), the phase compositions of the LMDed SS304 specimen are evaluated. The hardness of the LMDed specimen is compared with that of the conventionally manufactured wrought SS304 and SLMed specimens. And due to the differences of the grain size and phase composition on the different directions, the hardness of the LMDed specimen is anisotropic. The static tensile properties (ultimate tensile strength (UTS), yield strength (σ0.2) and elongation (EL)) and fatigue strength (FS) of the LMDed specimen are compared with those of the specimens made by the traditional method and SLM. The SeN curve is established by the multiple experiments. Scanning electron microscopy (SEM) is applied to observe the fracture morphology of the static tension and tension-compression fatigue tests. And EDS is applied to analyze the chemical compositions of the particles on the fracture surface. As expected, LMD has the higher static tensile properties and fatigue strength than those of the specimen manufactured by the traditional method.
1. Introduction Additive manufacturing (AM) is also known as three-dimension printing (3DP) which adopts laser or electron beam to melt the adhesive materials such as powder or wire so that the specimen is constructed in layer-by-layer fashion [1–4]. According to the type of energy source, AM can be divided into two categories: laser AM (LAM) and electron beam AM (EBAM). And LAM can be divided into powder feeder and powder bed on the basis of the different feeding methods [5]. Laser metal deposition (LMD) is a common manufacturing technique of LAM which belongs to powder feeding method. The principle of LMD is similar to laser cladding [6]. The metal powders are delivered into the laser spot by the shielding gas stream and then melted to form the molten pool [7]. The advantages of LMD are high efficiency and low consumption [8–11]. To date, LMD has been applied in the different industrial fields [12]. So far, the researches on LMD could be roughly divided into two categories, laser repair and direct metal forming. Some literature has
∗
been published with respect to laser repair. Sun et al. [13] applied LMD process to repair the HSLA-100 steel plates with the large volume trapezoidal grooves. The appearance, microstructure, hardness, tensile strength, bending strength and low temperature toughness of the repaired specimens with two different filling powders were researched. Song et al. [14] applied LMD process to repair the 304 austenite stainless steel (SS304) substrates with the large volume trapezoidal grooves. The results showed that the repaired part had no micro-defects, and the hardness, wear resistance and fatigue property were better than those of the substrate. In addition to being used for repair, LMD has been increasingly applied in the overall manufacture. Many documents on the manufacture of various alloys by LMD have been published. A variety of alloys have been successfully manufactured by LMD and their mechanical properties were also studied. Liu et al. [15] successfully applied LMD to produce the Ti6.5Al3.5Mol.5Zr0.3Si alloy specimens and studied the fatigue properties. The research revealed that the columnar crystal boundaries were detrimental for the fatigue properties. Lu et al. [16] studied the anisotropic mechanical properties
Corresponding author. E-mail addresses: [email protected] (W. Huang), [email protected] (Y. Zhang).
https://doi.org/10.1016/j.msea.2019.04.108 Received 28 February 2019; Received in revised form 26 April 2019; Accepted 28 April 2019 Available online 03 May 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
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good Hall fluidity. LMD apparatus applied in this experiment is shown in Fig. 1. As shown in Fig. 1, the device consists of laser device, sealed box, computer control system, system power source, shielding gas cylinder, powder feeder, and cooler. The laser device applies an YLS – 4000 Ytterbium laser system. The main parameters of the apparatus are listed in Table 3. The apparatus adopts the coaxial powder feeding technology. The process principle diagram of the apparatus is shown in Fig. 2. As shown in Fig. 2, the laser generated by the laser device is transmitted to the machining head through the optical fiber; the powder feeder carries the powders to the machining head through the argon gas, and the powders coincide with the laser focus; and the CNC (Computerized Numerical Control) machine moves the machining head to manufacture the current layer and superimposes the layers, until the entire part is finished. The optimized process parameters were applied to fabricate the fully density SS304 specimens in this experiment. The SS304 powders were coaxially delivered by the argon gas with a flow rate of 7 L/min, and the rotating speed of powder plate was 1.0 r/min. The laser power, scan speed and overlap rate in this experiment were 3 KW, 840 mm/s and 50%, respectively. And the scan strategy is shown as Fig. 3. The concentrations of oxygen and water were kept underlying 50 ppm in the sealed box. All specimens were fabricated under the same combination of the process parameters so that the specimens had the similar density and mechanical properties. The specimens were designed according to Chinese GB/T228-2002 standard [27] which was nearly similar to ISO 6892–1998 [28]. The dimensions of the specimen are shown as Fig. 4. According to the literature [23,29], the mechanical properties of the specimens which were parallel to the deposition direction were lower than those of the specimens which were perpendicular to the deposition direction. As shown in Fig. 5(a), the rod is perpendicular to the deposition direction. The rods were subjected to subsequent machining and polishing to form the experimental specimens, which are shown in Fig. 5(b). The microstructure of the LMDed specimen was different from that of the specimen manufactured by the traditional manufacturing process [30,31]. In order to observe the microstructure of the specimen, nitrohydrochloric acid was applied to corrode the surface. The optical microscopy (OM) was employed to observe the microstructure of the LMDed SS304 specimen. ImageJ software was applied to measure the grain size. Electron backscattered diffraction (EBSD) was applied to ensure the phase compositions of the LMDed specimen. Energy dispersive spectroscope (EDS) was applied to analyze the contents and distributions of the elements on the points and surfaces. The hardness of the LMDed specimen was evaluated by the hardness tester. The mechanical properties of the LMDed specimen were evaluated by a PC controlled EHFEV200K2-040-1A machine in the room temperature. The mechanical properties contain static tensile properties (UTS, σ0.2 and EL) and fatigue strength (FS). Uniaxial tension was employed for static tension and fatigue tests in this paper. The tensile rates of the static tension test were 200 N/s for stress control and 0.07 mm/s for displacement control. The tension-compression fatigue tests were carried out under the sinusoidal cycle loads with the 10 Hz frequency and a stress ratio (ratio of maximum stress to minimum stress) of −1. Scanning electron microscopy (SEM) was applied to observe the fracture morphology of static tension and fatigue tests.
of the laser metal deposited (LMDed) Ti6Al4V alloy. The research indicated that the anisotropic microstructural distribution of the LMDed specimen caused the anisotropic mechanical properties. And the LMDed specimen had the higher strength, lower elongation (EL) and smaller reduction of area in the transverse direction, which were compared with those in the perpendicular direction. Blackwell [17] studied the mechanical and microstructural characteristics of the LMDed IN718 alloy. The results showed that ultimate tensile strength (UTS) and yield strength (σ0.2) of the LMDed IN718 alloy were higher than those of the typical wrought IN718, but the reduction of area was lower. Xu et al. [18] researched the morphologies, microstructures and mechanical properties of the LMDed 316 L stainless steel. The experiments indicated that LMD could successfully fabricate the 316 L specimens with fine cellular and good metallurgical bonding between layers. And the fracture mode was ductile fracture after tensile test. SS304 is a common austenitic stainless steel which has been widely applied in varieties of fields, such as petroleum, nuclear and chemical industries due to its excellent corrosion resistance, mechanical properties and high cost-effect [19,20]. SS304 had high Cr content so as to produce the excellent corrosion property, and the austenitic phase resulted in the satisfactory mechanical properties [21]. The AM technology such as selective laser melting (SLM) has been applied in the manufacture of SS304 specimen. Guan et al. [22] studied the effects of the process parameters of SLM on the static tensile properties of SS304 specimen. The results indicated that SLM could produce the full density specimens with the higher σ0.2 and UTS than those of the wrought specimens under the appropriate process parameters. The effects of the layer thickness, building direction, overlap rate and hatch angle on the static tensile properties were studied in this paper. Yu et al. [23] studied the mechanical anisotropies of the SLMed Tie6Ale4V alloy and SS304 alloy. The research explained that the SLMed Tie6Ale4V alloy had more pronounced mechanical anisotropies than those of the conventionally manufactured wrought SS304 alloy. And heat treatment could effectively improve the mechanical anisotropies of the SLMed Tie6Ale4V alloy. Elghany and Bourell [24] researched the property evaluation of SS304L during SLM. The effects of powder layer thickness and laser scan speed on the density, microstructure, surface hardness and static tensile properties of the specimens were explored. The largest difference between LMD and SLM is the different powder feeding mode. LMD adopts the coaxial powder feeding method, while SLM adopts powder bed method. To date, few papers were published on the direct manufacture of the SS304 specimens by LMD. Arrizubieta et al. [25] adopted the numerical simulation and experiment to predict the hardness, grain size and porosity of the LMDed SS304. The predicted results of the numerical simulation were very similar to the experimental results. From the above, the most studies on the mechanical properties of the LMDed specimens focused on titanium-based alloys, nickel-based alloys and SS316L alloy. Up to now, fatigue fracture accounted for 90% engineering failures [26]. As a consequence, the in-depth understanding of the fatigue property of the LMDed SS304 specimens is very important and necessary. Up till now, the mechanical properties of the SS304 specimens manufactured by LMD technology, including static tensile properties (UTS, σ0.2 and EL) and fatigue strength (FS) were still unclear. In this paper, the mechanical properties of the LMDed SS304 specimens were evaluated to fill in the gaps in this field. The microstructure and hardness of the LMDed SS304 specimen were also studied in this paper.
3. Results and discussion 3.1. Microstructure
2. Experimental details Due to the unique forming mode of LMD, the formed specimen has the different microstructure from the conventionally manufactured wrought material. The microstructures on the XOY and XOZ planes are shown in Fig. 6(a) and (b). It is clearly seen that the microstructures on the both planes are different. The grain shape on the XOY plane is similar to honeycomb, and the grain shape on the XOZ plane is columnar.
The SS304 powders applied in this experiment were made by gas atomization method. According to the data provided by the supplier, the chemical compositions of SS304 powder and the volume fractions of the particle size are shown in Table 1 and Table 2, respectively. The oxygen concentration is approximate 280 ppm. And the powder has 61
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Table 1 Chemical compositions of SS304 powder. Element
C
Cr
Ni
Mo
Mn
Si
Cu
Co
P
S
N
Fe
wt.%
0.043
19.56
9.66
0.89
0.054
0.05
0.002
0.003
0.027
0.01
0.014
Balance
Table 2 Volume fractions of particle size.
Table 3 Main parameters of LMD apparatus.
Volume fraction (%)
Particle size (μm)
Parameters
Values
Dv Dv Dv Dv
95.2 145 178 367
Forming scope Maximum scan speed Laser type Laser wavelength Laser power Laser spot radius Shielding gas
800× 600× 900 mm 5 m/min Optical fiber laser 1070 nm 2–10 KW 1–2.5 mm Ar
(10) (35) (50) (90)
The results are same as those of the SLMed specimen in the literature [22]. The grain size on the XOY plane approximates 6 μm and the length of the columnar grain on the XOZ plane is tens of microns. The aspect ratio of columnar grain is the main factor to affect property anisotropy [23]. The grain size of the conventionally manufactured wrought SS304 material was 60 μm [32]. The grain size of the LMDed specimen is significantly smaller than that of the conventionally manufactured wrought SS304 material, but larger than that of the SLMed
specimen which had the nanoscale grain size [22]. The cooling rate during LMD was usual between 102 K/s to 104 K/s [33] which was higher than that during the conventionally manufactured wrought process and lower than that during SLM. The differences in cooling rate lead to the different grain sizes. The grain size and shape have the
Fig. 1. LMD apparatus (a) sealed box and computer control system (b) power source (c) shielding gas cylinder, powder feeder and cooler (d) laser device. 62
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(eq) and Cr (eq) are calculated by the equations [36].
Ni (eq) = Ni% + 30 × (C % + N %) + 0.5 × Mn% = 9.66% + 30 × (0.043% + 0.014%) + 0.5 × 0.054% = 11.397%
(1)
Cr (eq) = Cr % + Mo% + 1.5 × Si% + 0.5 × Nb% = 19.56% + 0.89% + 1.5 × 0.05% = 20.525%
(2)
In the literature [37], the solidification mode has four types: A, AF, FA and F modes. The Cr (eq)/Ni(eq) is 1.8 which indicates that the solidification mode belongs to FA mode. But considering the growth mode of ferrite and the Cr (eq)/Ni(eq), the solidification mode during LMD belongs to AF mode. Due to the larger cooling rate during SLM, the microstructure of the SLMed specimen was full austenite [22]. Considering the effects of the Cr (eq)/Ni(eq) and cooling rate, the phase compositions of the LMDed SS304 specimen are austenite and a small amount of ferrite. This conclusion is consistent with the previous one. In the SS304 specimen, the most important elements are Cr and Ni which have the significant effects on the phase compositions. Fig. 9 (a) and (b) show the distributions of Cr and Ni elements on the XOY plane. It is clear that Cr and Ni elements are evenly distributed on the XOY plane without significant component segregation. Fig. 10(a) and (b) show the distributions of Cr and Ni elements on the XOZ plane. It is clear that Cr and Ni elements are unevenly distributed on the XOZ plane with significant composition segregation. The place where Cr element is enriched on the plane is also the region where Ni element is poor. In Fig. 10, the ferrite is formed in the place where Cr element is enriched and Ni element is poor. It proves that two phases of austenite and ferrite occur in the LMDed specimen.
Fig. 2. Process principle diagram of LMD.
Fig. 3. Scan strategy.
significant effects on the properties of the specimen. Some micro-voids can be seen from Fig. 6, which were attributed to the flow behavior of the molten pool during the laser melting process [34,35]. Macroscopic segregation is also called regional segregation. Macroscopic segregation is mostly due to the physical movement of the liquid or solid phase during solidification. In Fig. 7(a) and (b), the macroscopic appearances on the XOY plane and XOZ plane are presented, respectively. Through careful observation of the macroscopic appearances on the two planes, there is no macroscopic segregation on the planes except for the presence of pores. Since the LMDed specimen is formed in the layer-by-layer mode and has the high cooling rate, macroscopic segregation is difficult to occur. EBSD is an effective method for analyzing the phase compositions of the alloy. The phase compositions on the two planes are analyzed by EBSD. And the EBSD maps are shown in Fig. 8 (a), (b) and (c). In Fig. 8, the red part represents ferrite which is body-centered cubic (BCC) structure, and the background is austenite which is face-centered cubic (FCC) structure. As shown in Fig. 8(a), no obvious ferrite aggregates on the XOY plane. Fig. 8 (b) and (c) are EBSD maps of any two positions on the XOZ plane. As shown in Fig. 8(b) and (c), the ferrite exists on the XOZ plane. And most ferrite forms at the grain boundary of austenite. After measurement of ferrite content in the multiple locations, the ferrite content is about 10% on the XOZ plane. For austenitic stainless steel, the Ni equivalent (Ni(eq)) is critical for the formation and stability of austenite. If the Ni(eq) is less than 12%, the γ austenite phase is considered to be metastable [32]. The effect of Cr element is to promote the formation and stability of ferrite. The Ni
3.2. Hardness Nine points on the XOY and XOZ planes are selected as shown in Fig. 11, and the readings of the points are recorded in Table 4. The average hardness on the XOY and XOZ planes is 203.6 HV and 184.9 HV, respectively. It is clear that the hardness on the XOY plane is larger than that on the XOZ plane. The hardness of the conventionally manufactured wrought SS304 material was 168 HV [32]. The hardness of the LMDed specimen is obviously larger than that of the conventionally manufactured wrought SS304 specimen. The main reason for this result is grain size. The phenomenon can be explained by Hall-Petch equation [38]. It can be clearly seen from Table 4 that the hardness on both directions is different. Grain size and phase composition are the two main reasons to cause the result. As shown in Fig. 7 that the grain size on the XOY plane is smaller than that on the XOZ plane, which causes the larger hardness on the XOY plane. As shown in Fig. 8, the ferrite exists on the XOZ plane. The larger grain size and the ferrite phase result in the lower hardness on the XOZ plane. 3.3. Static tensile properties 3.3.1. Tensile properties Fig. 12(a) indicates the stress-strain (σ-ε) curve of the LMDed specimen. It is clear that the static tensile process of the LMDed specimen can be divided into three stages, namely elastic deformation stage (oa), uniform plastic deformation stage (ab) and non-uniform plastic deformation stage (bc). Fig. 12(b) presents the comparison of the Fig. 4. Dimensions of specimen.
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Fig. 5. (a) 3D schematic and (b) specimen.
Fig. 6. Microstructure on (a) XOY plane and (b) XOZ plane.
Fig. 7. Macroscopic appearances on (a) XOY plane, and (b) XOZ plane.
plastic deformation, and the reliability of the structural part is higher. The experimental results of this paper and the data of the other papers are listed in Table 5. It is clear that the all static tensile properties of the LMD specimen meet the ANSI standard, and are higher than those of the conventionally manufactured wrought SS304 specimen. In other words, without the complicated post-processing, LMD can obtain the specimens with the better static tensile properties. LMD has the wonderful ability to reduce the energy consumption and simplify the fabricating processes compared with the traditional manufacturing processes. Nevertheless, the UTS and σ0.2 of the LMDed specimens are lower than those of the SLMed specimens, but the EL of the LMDed specimen is higher. The yield strength ratio of the LMDed specimen is between the SLMed specimen and the conventionally manufactured wrought SS304 specimen. It shows that the LMDed specimen has the better plasticity than that of the SLMed specimen. The grain size is the main reason to affect the static tensile properties. The average grain size of the LMDed specimen was much smaller than that of the conventionally manufactured wrought SS304 specimen which was introduced in Section 3.1. The smaller the grain size, the
specimens before and after fracture. It is clear in Fig. 12(b) that the gauge section becomes finer and longer after fracture. The fracture occurs near the center of the specimen where the necking happens. The static tensile properties of the LMDed specimens are compared with those of the SLMed specimen [22], conventionally manufactured wrought SS304 specimen [32] and ANSI standard [22]. ANSI is the abbreviation of the American National Standards Institute, which is established in 1918. It is mainly composed of the American Society for Testing and Materials (ASTM), the American Society of Mechanical Engineers (ASME), the American Association of Mining and Metallurgical Engineers (ASMME), the American Society of Civil Engineers (ASCE), and the American Institute of Electrical Engineers (AIEE). The main process parameters of SLM were laser power, scan speed, overlap rate, hatch angle and building direction, which were 200 W, 15 m/min, 40%, 90° and horizontally built, respectively [22]. The SS304 specimen was solution treated at 1080 °C for 1 h [32]. The ratio of σ0.2 to UTS (σ0.2/UTS) is called the yield strength ratio. The lower yield strength ratio indicates that the material has the better plasticity; the higher yield strength ratio indicates that the material has strong resistance to 64
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Fig. 8. EBSD maps on (a) XOY plane, (b) and (c) XOZ plane.
Fig. 9. EDS maps of (a) Cr and (b) Ni on the XOY plane.
Fig. 10. EDS maps of (a) Cr and (b) Ni on the XOZ plane.
3.3.2. Fracture morphology analysis Fig. 13 shows the fractured specimen produced by the static tensile test. It can be seen from Fig. 13(a) that the fracture caused by the static tension occurs at the position close to the center of the specimen, and the significant necking phenomenon occurs at the fracture. Fig. 13(b) shows that the angle between the necked slope and the axial direction is approximate 45°. Necking is the basic feature of plastic fracture. Fig. 14(a) shows the overall fracture morphology. As shown in
more the grain boundaries are in the same area. Reduction of grain size leads to grain boundary strengthening. This phenomenon can still be explained by Hall-Petch equation [38]. When the grain was refined to a few micrometers or less, the plastic deformation ability of the material exhibited the remarkable decrease, and the work hardening ability was lost [39]. This is the reason that the SLMed specimen has the larger strength but lower plasticity.
65
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Table 5 Static tensile properties of LMDed, SLMed, conventionally manufactured wrought SS304 and ANSI specimens. Type
UTS (MPa)
σ0.2 (MPa)
EL (%)
σ0.2/UTS
LMD SLM [22] Conventionally manufactured wrought SS304 [32] ANSI standard [22]
609 713 586
273 551 242
67.3 43.6 65
0.45 0.77 0.41
≥520
≥205
≥40
0.39
3.4. Fatigue strength 3.4.1. SeN curve The SeN curve is drawn according to the fatigue life of the LMDed specimen under the different stress levels, which is shown in Fig. 16. The stress corresponding to the cycle number of 106 is taken as the fatigue strength of the specimen. The fatigue strength of the SS304 specimen manufactured by LMD is 255 MPa, which is larger than those in the literature [40,41]. The fatigue strengths of the conventionally manufactured wrought SS304 specimens both approached 230 MPa at ambient temperature in the both literature. It can be seen from Section 3.1 that the grain size of the LMDed specimen is smaller than that of the conventionally manufactured wrought SS304 specimen, which causes more grain boundaries in the LMDed specimen. Fine grain hinders the formation and expansion of the fatigue cracks in the specimen and prolongs the fatigue life of the specimen [42]. Fatigue fracture usually belongs to transgranular fracture. That is to say, crack propagation needs to pass through the grain interior. The grain boundary has the effect to slow the crack growth rate because the crack tip must have higher energy to pass through the grain boundary. Grain boundary strengthening is the main reason for the higher fatigue strength of the LMDed specimens.
Fig. 11. Nine points selected on the plane. Table 4 Hardness (HV) on XOY and XOZ planes. No.
1
2
3
4
5
6
7
8
9
Mean
XOY XOZ
205 181
209 179
201 194
206 183
204 193
200 188
207 178
198 184
202 175
203.6 184.9
Fig. 13(a), the color of the cone-shaped fracture is dark. This is because the fracture surface has the weak reflection ability to light. Fig. 14(b) and (c) show the local fracture morphologies at the magnifications of 2000× and 5000×, respectively. It is clear that the dimples are evenly distributed. The dimple is the typical feature of plastic fracture. The formation mechanism of the dimple feature is void aggregation, that is to say, the process of micro-void nucleation, growth, accumulation, and fracture. The micro-voids are formed by LMD and internal separation of material. The micro-voids grow continuously under the action of external force, and the cross-sections of the matrix between several adjacent micro-voids are continuously reduced until they are connected to each other to cause fracture, and forming the fracture morphology of the dimples. As can be seen from Fig. 14(b) and (c), all dimples have the same shape and are approximately equiaxed. This is because under the action of normal stress, the growth rates of the micro-voids in the three directions of space are same. Fig. 14(d) is the enlarged image of the red box in Fig. 14(c). Points 1 and 2 are the particles on the tensile fracture surface. The EDS module in SEM is applied to analyze the chemical compositions of the particles. The chemical compositions of the particles are shown in Fig. 15. It indicates that they are steel particles, which are possible to be formed by the collapse of matrix.
3.4.2. Fatigue fracture morphology The fatigue fracture morphologies of the LMDed specimens under the different loads are shown in Figs. 17–19. As shown in Figs. 17(a), 18(a) and 19(a), the fatigue fracture surfaces are partitioned into the three disparate zones which are crack initiation zone (CIZ), crack propagation zone (CPZ), and instantaneous fracture zone (IFZ), respectively. CIZ is the crack nucleation and initiation region. As shown in Figs. 17(b), 18(b) and 19(b), the cracks all nucleate and initiate on the surfaces of the specimens under the different loads. And the characteristic of CIZ is smooth. As shown in Fig. 6, a certain number of holes exist in the LMDed specimens, but the cracks still nucleate on the surfaces of the specimens. Due to the presence of defects such as pits on the surface, stress concentration occurs at the defects during loading, which becomes the CIZ [43]. In addition, under the action of alternating
Fig. 12. (a) σ-ε curve, and (b) length comparison of LMDed specimens before and after fracture. 66
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Fig. 13. Macroscopic fracture feature: (a) fractured specimen and (b) fractured section. Fig. 14. Fracture morphology of static tensile specimen: (a) overall appearance, (b) local topography magnified 2000×, (c) local topography magnified 5000× and (d) fracture morphology in the red box of (c). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 16. S–N curve. Fig. 15. Weight percent (wt %) of major elements in the particles.
in the plane stress state, and plastic slip is likely to occur. Repeated cyclic slip strain will produce the metallic slip band to make the microcrack nucleate. The micro-crack nucleation is a slow process in the CIZ. Fig. 20(a) shows the specimen which is subjected to the cyclic tensile-
stress, although the nominal stress does not exceed the σ0.2 of the material, the unevenness of the material structure causes slip in the local area of the specimen. This is because the surface of the specimen is 67
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Fig. 17. Fatigue fracture morphologies (275 MPa, 3701 cycles): (a) overall appearance, (b) CIZ, (c) CPZ and (d) IFZ.
Fig. 18. Fatigue fracture morphologies (265 MPa, 24883 cycles): (a) overall appearance, (b) CIZ, (c) CPZ and (d) IFZ.
fatigue life. The stress intensity factor is a physical quantity that reflects the strength of the elastic stress field at the crack tip. It is related to crack length, geometric characteristic of specimen, and load. The relationship can be described by the following equation:
compressive loads. It is clearly shown in Fig. 20(b) that the slip bands occur on the surface of the specimen under the cyclic tensile-compressive loads. The slip bands are the places where the cracks may initiate. When the micro-crack slowly spreads to the critical length, it enters the CPZ. In the CPZ, river stripes are the most typical feature, which are shown as Figs. 17(c), 18(c) and 19(c). The width of each stripe represents the crack propagation distance at one stress cycle. When the maximum stress is 275 MPa, the width of the stripe approximates 12 μm. The width is 2 μm at 265 MPa, and it is 0.8 μm at 260 MPa. It is clearly seen that the higher stress causes the wider strip and lower
KI = βσ πa
(3)
where KI is the stress intensity factor; β is the shape factor; σ is the nominal stress; and a is the crack length. Fracture toughness is considered to be the inherent property of the material. It indicates the sensitivity of the material to the crack under the static load, and is expressed by KIC. When the stress intensity factor KI exceeds KIC, the 68
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Fig. 19. Fatigue fracture morphologies (260 MPa, 747112 cycles): (a) overall appearance, (b) CIZ, (c) CPZ and (d) IFZ.
Fig. 20. (a) Specimen under cycle loads, and (b) slip bands on the surface of specimen.
conclusions are enumerated as follows:
specimen generates fracture. It is clearly seen from Figs. 17(b), 18(b) and 19(b) that the fracture caused by the higher stress has the larger CPZ. Because the LMDed specimens are manufactured by the same material and have the same shape, the specimens have the same β and KIC. According to Eq. (3), the higher stress causes the stress intensity factor KI of the specimen to reach the fracture toughness KIC under a small crack length. And due to the wider strip, the fracture caused by the higher stress has the larger crack propagation rate. In summary, the fractures caused by the different stresses have the CPZs with the different areas. At different cyclic stress levels, when the crack spreads to a certain size, the specimen breaks. At this point, the area where the direct fracture occurs is the IFZ, which is shown as Figs. 17(d), 18(d) and 19(d). The instantaneous fracture is similar to the static tension fracture, which has the fracture morphology of static tension. And IFZ is the fracture area formed by the rapid instability of the crack. Because the LMDed specimen has good plasticity, the dimple is the typical feature of IFZ.
(1) The grain shapes of the LMDed specimen are similar to those of the SLMed specimen which are honeycomb on the XOY plane and columnar on the XOZ plane, respectively. The grain sizes of the LMDed specimen are 6 μm on the XOY plane and tens of micrometers on the XOZ plane which are smaller than those of the conventionally manufactured wrought SS304 specimen and larger than those of the SLMed specimen. No macroscopic segregation occurs by observing the macroscopic appearance of the two planes. Due to the Cr and Ni equivalents and cooling rate during LMD, the phase compositions of the LMDed specimen are austenite and ferrite, which is different from the full austenitic SLMed specimen. The ferrite content is about 10% on the XOZ plane and the ferrite forms at the grain boundary of austenite. And no obvious ferrite aggregates on the XOY plane. (2) Due to the smaller grain size, the hardness of the LMDed specimen is larger than that of the conventionally manufactured wrought SS304 specimen. And the hardness on the XOY plane is larger than that on the XOZ plane, which is attributed to the smaller grain size and less ferrite on the XOY. (3) The static tensile properties (UTS, σ0.2 and EL) of the LMDed specimen are all higher than those of the conventionally manufactured
4. Conclusions The microstructure, hardness, static tensile properties and fatigue strength of the LMDed specimen are explored in the paper. The main 69
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wrought SS304 specimen. Although the UTS and σ0.2 are lower than those of the SLMed specimen, the EL is higher. The obvious characteristics of necking and dimple show that the fracture type is plastic fracture. The particles on the fracture surface are steel particles, which are possible to be formed by the collapse of matrix. (4) The SeN curve is drawn by the multiple experiments. The fatigue strength of the LMDed specimen is higher than that of the conventionally manufactured wrought SS304 specimen at 106 cycles. This is attributed to the effect of grain boundary strengthening, which hinders the crack propagation. The CIZs are smooth, which nucleate and initiate at the surface under the different loads. In the case of the larger cyclic load, the slip band appears clearly on the surface of the specimen where the crack may originate. The main feature of the CPZ is river stripe. And the width of the stripe represents the crack growth rate. Since the KI of the specimen loaded with the larger cyclic load first reaches the KIC of the material, it has the smaller CPZ and larger crack growth rate. The main feature of the IFZ is dimple which is similar to that of the static tensile specimen.
[15]
[16]
[17] [18]
[19] [20]
[21]
[22]
[23]
In summary, the LMDed specimen has the better mechanical properties than those of the conventionally manufactured wrought SS304 specimen.
[24] [25]
Data availability [26]
The data are all presented in this paper. [27]
Acknowledgements
[28]
We would like to express our appreciation to Chinese National Natural Science Foundation of China (U1708254) for supporting this research.
[29]
[30]
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