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Role of melt pool boundary condition in determining the mechanical properties of selective laser melting AlSi10Mg alloy
Author’s Accepted Manuscript Role of melt pool boundary condition in determining the mechanical properties of selective laser melting AlSi10Mg alloy Z.H. Xiong, S.L. Liu, S.F. Li, Y. Shi, Y.F. Yang, R.D.K. Misra www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(18)31462-X https://doi.org/10.1016/j.msea.2018.10.083 MSA37078
To appear in: Materials Science & Engineering A Received date: 26 April 2018 Revised date: 16 October 2018 Accepted date: 18 October 2018 Cite this article as: Z.H. Xiong, S.L. Liu, S.F. Li, Y. Shi, Y.F. Yang and R.D.K. Misra, Role of melt pool boundary condition in determining the mechanical properties of selective laser melting AlSi10Mg alloy, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.10.083 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Role of melt pool boundary condition in determining the mechanical properties of selective laser melting AlSi10Mg alloy
Z.H. Xiong1, S.L. Liu2, S.F. Li1, Y. Shi3, Y.F. Yang1*, R. D. K. Misra4
1
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, No.1 Zhong guan cun North 2ndSt, Beijing, 100190, China
2
Institute of Advanced Steels and Materials, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China
3
State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, No.287 Lan gong ping Road, Lanzhou 730050, China
4
Laboratory for Excellence in Advanced Steel Research, Materials Science and Engineering Program, Department of Metallurgical, Materials and Biomedical Engineering, University of Texas at El Paso, 500 W. University Avenue, El Paso, TX 79968, USA
*
Corresponding author. Tel.: +86-10-82544907. [email protected]
Abstract We describe here a comprehensive study on the effect of cellular structure and melt pool boundary (MPB) condition on the mechanical properties, deformation and failure 1
behavior of AlSi10Mg alloy processed by selective laser melting (SLM). The morphology of melt pool (MP) on the load bearing face of tensile samples was significantly different with build directions. It resulted in different mechanical properties of the samples with different build directions. Furthermore, the microstructure analysis revealed that the MP in the SLM AlSi10Mg alloy mainly consisted of columnar α-Al grains which were made of ultra-fine elongated cellular structure. Electron back-scatter diffraction (EBSD) analysis revealed that the long axis of cellular structure and columnar grains were parallel to <100>, which resulted in <100> fiber texture in SLM AlSi10Mg alloy. However, Schmid factor calculation demonstrated that the anisotropy of mechanical properties of the SLM AlSi10Mg alloy built with different direction was mainly dependent on the distribution of MPB on the load bearing face, and not texture. The defects including pores, residual stress and heat affected zone (HAZ) located at MPB made it the weakest part in the SLM AlSi10Mg. The sample built along horizontal direction exhibited good combination of strength and plasticity and is attributed to the lowest fraction of MPBs that withstand load during tensile. MPB had strong influence on the mechanical properties and failure behavior of SLM AlSi10Mg built with different directions.
Keywords: Selective laser melting; AlSi10Mg alloy; Melt pool boundary; Cellular structure; Anisotropy
1. Introduction 2
Hypoeutectic AlSi10Mg alloy has been widely used for aluminum castings for aerospace and automotive components because of excellent casting properties [1, 2]. However, the mechanical properties of as-cast AlSi10Mg alloy are inferior because of brittle fracture resulting from the presence of coarse and acicular eutectic silicon [3, 4]. Selective laser melting (SLM) [5-8] is capable of producing a refined microstructure by using high heating and cooling rates (103–108 K/s) [9], and further improving the mechanical properties. This has proven to be a potential approach for making high-performance AlSi10Mg components. The early studies on SLM AlSi10Mg mainly focused on the feasibility of making components and optimizing processing parameters because of the high reflectivity of aluminum for the laser beam. High power lasers of 200 W or greater have been used [6, 10]. Such a high-energy input essentially changes the thermal conditions and modifies the microstructure, which currently constitutes the primary focus. Thijs et al. [5] obtained an extremely fine cellular-dendritic microstructure consisting of cellular primary Al decorated with fibrous Si particles in SLM AlSi10Mg alloy. The refined microstructure was characterized by high hardness of 127 Hv0.5 for the as-built AlSi10Mg alloy [5]. Subsequently, similar “cell-like” microstructure characterized with long cells and Al-Si eutectic at cell boundaries was observed in the SLMAlSi10Mg alloy by Wu et al.[11]. The in-situ compression tests in transmission electron microscopy (TEM) further indicated that the Si particles in the “cell-like” microstructure played an important role during plastic deformation of SLM samples. Heat treatments were attempted to refine the Si particles in the SLM AlSi10Mg sample 3
by Li et al.[12]. Tensile ductility of up to 25% was obtained on optimizing the heat treatment. Though the microstructural refinement could improve the mechanical properties of AlSi10Mg alloy produced by SLM, the anisotropy of SLM AlSi10Mg sample is well-known [7, 13, 14]. Building strategies were applied in SLM and had limited influence in reducing anisotropy [15, 16]. MPB is considered as a key issue in anisotropy of SLM fabricated components [17]. However, the influence of MPB on mechanical properties of SLM AlSi10Mg has not been systematically studied. We have conducted a systematic study on the effect of variable MPB conditions on the mechanical properties with focus on morphology and orientation of Al and Al-Si eutectic around the MPB, in the attempt to clarify the response of mechanical properties to the different MPB conditions. 2. Experimental procedure Spherical AlSi10Mg alloy powders (-325 mesh) supplied by Zhejiang Asia General Soldering and Brazing Material Company were used. To obtain different MPB conditions, the tensile bars (please see Fig. 1a, d and g respectively) were built with the SLM facility (SLM Solutions GmbH, SLM 250 HL) in three directions: horizontal (horizontal sample, Fig.1a), 45° (45°sample, Fig.1d) and vertical (vertical sample, Fig. 1g). The SLM was carried out in a argon gas shielding atmosphere at an optimal laser beam power of 350 W, laser scanning velocity of 920 mm/s and energy density of 40 J/mm3 (using these parameters, the relative density of SLM AlSi10Mg samples was ~98.6%). 4
To reduce the effect of surface roughness, standard cylindrical tensile specimens of dimensions 6 mm diameter and 25 mm length conforming to ASTM E8 [18] were further machined from the tensile bars. Tensile tests were performed at a crosshead speed of 0.01/min using a computerized tensile test system (CMT-5105). Three tests were carried out for each condition. Samples were cut from the tensile bars and mechanically polished using 50 nm colloidal alumina and etched with Kroll’s etchant. The microstructure of the samples built with different directions was studied using optical microscope (OM, Olympus BX51M) and field emission scanning emission microscope (SEM, ZEISSULTRA-55) equipped with electron back-scatter diffraction (EBSD) detector. The EBSD samples were mechanically polished using 0.05 μm silica suspension without etching. The EBSD experiments were conducted at an acceleration voltage of 20 kV, working distance of 17.5 mm, tilt angle of 70° and step size of 0.25 μm. The data was processed by Channel 5 software. The texture in the samples were detected by X-ray, X-ray measurements were performed on a SIEMENSD500 using a Cu tube (wave length of the Cu X-ray is 1.5406A) at 40 kV and 40 mA. The diffraction patterns between a 2θ angle of 23°and 90°were measured using a step size of 0.02°and detection time of 3 s. Fracture surface of tensile specimens was examined by SEM. Thin wafers were cut from the as-built SLM samples, ground to ~40 μm in thickness and thinned for TEM (JEOL 2100) studies by precision ion polishing system (Gatan PIPSTM). TEM studies were conducted at 200 kV. 3. Results 5
3.1 Morphology of melt pools on the load bearing face The half-cylindrical shape of MP essentially exhibited two types of morphologies when observed along and perpendicular to the build direction. Fig.1 shows the morphology of MPs along and perpendicular to the build direction in the horizontal sample, 45o sample and vertical sample. To analyze the relationship between the microstructure and tensile properties, the microstructure of load-bearing face and the face parallel to the tensile loading direction (referred as parallel face henceforth) was examined. It was clearly observed that the load bearing face of horizontal sample exhibits “semicircular” MPs (Fig. 1b), while the parallel face was stripe-like MPs (Fig. 1c). In contrast, the MP morphology on load bearing face (Fig. 1h) and parallel face (Fig. 1i) of vertical sample was stripe-like and “semicircular”, respectively. The morphology of MPs of 45° sample on the load bearing face and parallel face was nearly identical (Fig. 1e and 1f), which exhibited a combination of “semicircular” and stripe-like MPs. This clearly indicated the shape of MPs and MPBs on the load bearing faces of SLM sample were different with different building directions. In addition, it may be noted from Fig. 1 that the pores were always located along the MPBs in each sample, irrespective of the build direction. 3.2 Microstructure in the MP Fig.2 shows the results of EBSD analysis and microstructure of load bearing face of horizontal sample, vertical sample and 45° sample, corresponding to “semicircular” MP, stripe-like MP and the combination of “semicircular” and stripe-like MPs, respectively. The following are the observations: 6
(i) The “semicircular” MPs on the load bearing face of the horizontal sample were mainly composed of columnar α-Al grains, which grew from the MPB to the inner of MP(Fig. 2a). The length and diameter of columnar α-Al grains were ~ 50-100 μm and ~ 5-20 μm, respectively. It is also noted that some small equiaxed grains were located at the MPB. From Fig. 2b, the columnar α-Al grain in the “semicircular” MP was made up of elongated cell-like/cellular structure, the length and width of the elongated cell-like structure was ~ 1-3μm and 0.5μm, respectively (Fig. 2b1 and 2b2), which implied that the elongated cell-like structure was significantly finer than columnar α-Al grain. Fig. 2b also shows that the MPB include HAZ and remelted zone, the cells disappeared in the HAZ replaced by granular microstructure, and the cells in the remelted zone was coarse, which indicated that the microstructure in MPB was inhomogeneous. (ii) The stripe-like MP on the load bearing face of the vertical sample was composed of equiaxed grains (Fig. 2e), which is actually the cross-section of columnar α-Al grains in view of three-dimensional nature of the half-cylindrical shape of MP. There were also some small equiaxed grains located at the MPB and were formed at MPB during SLM. The equiaxed α-Al grains in stripe-like MP were composed of equiaxed cell-like structure (Fig. 2f, 2f1 and2f2), where the equiaxed cell-like structure is the cross-section of elongated cell-like structure. (iii) In the case of 45°sample on the load bearing face, the columnar α-Al grains and the equiaxed α-Al grains were located at the two adjacent MPs (see Fig.2c). Fig. 2d, and 2d1 and 2d2 shows that the columnar α-Al grain and the equiaxed α-Al grain 7
consisted of ultra-fine elongated cells and ultra-fine equiaxed cells, respectively. TEM studies of cellular structure together with X-ray mapping of corresponding elements are presented in Fig.3. No significant segregation was observed in the SLM samples, although the cooling rate was high. The cells were α-Al phase decorated with eutectic Si at the boundaries. Mg was also detected at the cell boundaries and Traces of oxygen and nitrogen were observed. 3.3 Crystal orientation in the melt pool Fig.4 a and b show corresponding EBSD-band contrast map of the microstructure in the melt pool on the load-bearing face and parallel face of the horizontal sample, corresponding to the “semicircular” MP and stripe-like MP, respectively. Clearly, the load bearing face was mainly composed of columnar grains, while the parallel face consisted of equiaxed grains. It was confirmed that the equiaxed grains in stripe-like MP were cross-section of columnar grains in “semicircular” MP. Fig.4c gives the Kernel average misorientation (KAM) map in the MPs of the marked area in Fig. 4a, KAM values obtained from EBSD data can be used to analyze stress concentration [19-22], since KAM has a similar distribution with stress, as illustrated by Ramazani etal. [20, 22, 23]. Thus, stress distribution was characterized in terms of misorientation (KAM) values in Fig.4c, yellow color corresponds to high stress concentration, while blue color represents low stress concentration. The high stress concentration is present inside the local melt pools, but a change occurred at the MPBs, suggesting that the stress distribution in the vicinity of MPBs is not uniform. Fig.5a is inverse pole figure map of melt pools on the load bearing face of the 8
horizontal sample. <100> fiber texture was observed. Fig.5b shows the distribution of <100> fiber texture highlighted by green and red. Figs.5c and 5d shows the crystal orientation of columnar grains processed by EBSD software. It is clear that the long axis of both cell-like structure and columnar grains was parallel to <100>, which indicated that the columnar Al grains grew along the <100>. This is the reason for the formation of <100> fiber texture in SLM AlSi10Mg. The existence of <100> fiber texture in the SLM AlSi10Mg is also confirmed by X-ray measurement as shown in Fig.5e, the <100> texture // build direction of the SLM AlSi10Mg. 3.4 Mechanical properties and fractography Fig. 6 shows the tensile properties of SLM AlSi10Mg samples with different build directions. The ultimate tensile strength (UTS) of the three samples followed the sequence: horizontal sample (360 MPa) ≈ vertical sample (363 MPa) > 45° sample (340 MPa), while the sequence for yield strength (YS) was: horizontal sample (220 MPa) > vertical sample (190 MPa) ≈45° sample (186 MPa). The elongation followed the sequence: horizontal sample (7.1%) > 45° sample (6.0%) > vertical sample (4.3%). The horizontal sample built along the horizontal direction had highest elongation and exhibited good combination of strength and plasticity. The fracture surface of SLM AlSi10Mg samples with different build directions are shown in Fig. 7. From Fig. 7a, b and c, there are flat fracture (highlighted by red curve) on the fracture surface of the three kinds of samples. The flat fracture implied that the microstructure experienced less deformation before fracture, while the microstructure on the non-flat fracture should experience more deformation before fracture (the high 9
magnification of the microstructure on the non-flat fracture surface and flat fracture of horizontal sample are shown in Fig. 7a1 and 7a2 respectively). In addition, it can be seen that there are holes (of size ~10-80 μm) on the fracture surface of the three kinds of samples. The holes are the enlarged pores developed from the pores and invisible small pores (under optical microscope) in the matrix during deformation. Interestingly, the geometry of the flat fracture seems to be consistent with the MPBs, suggesting that crack initiated and progressed along the MPBs. This was further confirmed by the crack path observed in the HAZ of MPB (Fig. 7d and e). In addition, it was noted that the fraction of flat surface in the three samples was significantly different (Fig.7a, b and c). The percentage of flat region on the fracture surface of three samples was: 10% (horizontal sample) < 40% (45° sample) < 75% (vertical sample). The reason for the significant difference between the fracture surface of three samples is discussed below. 4. Discussion 4.1 Effect of texture in SLM AlSi10Mg From the aforementioned results, the long axis of both cell-like structure and columnar grains was parallel to <100>, which indicated that columnar Al grains grew along <100>, which resulted in the formation of <100> fiber texture in SLM AlSi10Mg. The fiber texture should have an effect on the yield strength by influencing the Schmid factor of the samples built with different directions. In order to calculate the Schmid factor of the columnar grain in each sample, a simple model is illustrated in Fig.8. The loading direction of the vertical sample is parallel to <100> of columnar grain and long 10
axis of cellular structure (Fig. 8a and b). Fig. 8b shows a crystal with a normal cross-sectional area A on which a tensile load F is applied, generating a uniaxial stress F/A. The angle between the slip direction and loading direction is ，The normal direction of the slip plane makes an angle ɸ with the loading direction. The slip system in face-centered cubic (FCC) Al grain is {110}<111> (Fig. 8b). In the uniaxial tensile [24]:
F cosɸcosλ= σcosɸcosλ (1) A m=cosɸcosλ(2)
is the shear stress acting on the slip plane and along the slip direction, m is Schmid factor. Without considering the effect of inhomogeneous structure (including HAZ in MPB, defect such as pores in the vicinity of MPB), slip occurs when the shear stress on the slip plane, and in the slip direction, reaches a valueτcrss, the yield stress of the polycrystalline material can be expressed as:
Y
crss m
(3)
Y isyield stress, m is the equivalent Schmid factor of three samples. From equation (3), sinceτcrss is same for identical material, Y is related to m , Y decreases with the increase of m . m changes with the build direction of the three kinds of samples. Since the loading direction of the vertical sample is parallel to <100> of columnar grain and long axis of cellular structure, i.e., the angle between the loading direction and <100> of the columnar grain of vertical sample is 0°. The equivalent Schmid factor of vertical 11
samples was calculated to be 0.408 and has a constant value. While the angle between the loading direction and <100> of columnar grain of 45° sample and horizontal sample were 45o and 90o, respectively. The Schmid factor for them do not have constant values. Accordingly, the Schmid factors of 45° sample and horizontal sample were calculated using MATLAB(R2015b) and the results are presented in Fig.9. From Fig.9 it can be seen that the equivalent Schmid factor for the three samples followed the sequence: horizontal sample > vertical sample ≈ 45° sample, according to equation (3), the yield strength of the three samples should be: horizontal sample < vertical sample ≈ 45° sample. This result is not consistent with the yield strength (Fig.6) measured by the tensile test, which implies that the texture in SLM AlSi10Mg is not the main factor that influence yield strength. The yield strength should be closely related to the inhomogeneous structure in MPB.4.2 Role of melt pool boundary (MPB) Based on the experiment results, crack initiated and progressed along the MPBs, which suggests that MPB is the weakest part of SLM samples. The weakness of MPB is likely to be related to the nonuniform distribution of stress concentration at MPB (Fig.4c), HAZ in which the cell-like structure disappeared (Fig.2b, d, f, Fig.7e), and defects such as pores (Fig.1) at the MPB side. Given that MPB was the weakest part, the elongation of SLM AlSi10Mg sample is mainly determined by the fraction of MPBs that withstand load. The fraction of MPBs that withstand load followed the sequence: horizontal sample (Fig. 1c) < 45°sample (Fig. 1f) < vertical sample (Fig. 1i). High fraction of MPB that withstand load generally lead to low elongation and results in flat fracture facet on the fracture surface of vertical 12
sample (Fig. 7c). This is the underlying reason for the vertical sample to exhibit maximum flat fracture facets (the microstructure on it experienced less deformation before fracture) and lowest elongation. In contrast, the horizontal sample had least MPBs that withstand load during tensile. Thus, horizontal sample had least flat fracture facets, which is the reason for highest elongation of the horizontal sample. Thus, the distribution of MPB on the load bearing face of as-built SLM AlSi10Mg tensile samples governed the fracture behavior and mechanical properties of SLM AlSi10Mg.
Conclusions (1) The melt pool in SLM AlSi10Mg alloy was half-cylindrical in three-dimensional shape, which resulted in significant difference in the distribution of MP and MPBs on the load bearing faces of the tensile samples with different building directions. (2) The MP in the SLM AlSi10Mg samples was made of columnar α-Al grains, which was comprised of elongated cell-like eutectic Si and Al matrix. The crystal orientation <100> of columnar α-Al grains was the preferred growth direction, leading to fiber textures. (3) The anisotropy of mechanical properties of sample with different build directions was not governed by fiber texture, but by the distribution of MPB on the load bearing surface. The localization of pores, residual stress and HAZ at MPB made it the weakest part in the SLM AlSi10Mg alloy. Crack initiated and progressed along the MPBs. (4) The horizontal sample exhibited good combination of strength and plasticity and is attributed to the horizontal sample having least MPBs that withstand load during the 13
tensile.
Acknowledgements The work was supported by the Key Research Program of Frontier Sciences (Grant No. QYZDB-SSW-JSC045), Chinese Academy of Sciences and National Youth Thousand Plan Program, and the National Key Research and Development Program of China (No. 2018YFB1105100).
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Abed, V. Ji, P. Aubry, P. Peyre, T. Malot, Direct fabrication of a Ti-47Al-2Cr-2Nb alloy by selective laser melting and direct metal deposition processes, in: T. Chandra, N. Wanderka, W. Reimers, M. Ionescu (Eds.) Thermec 2009 Supplement: 6th International Conference on Processing & Manufacturing of Advanced Materials, Trans Tech Publications Ltd, Stafa-Zurich, 2010, pp. 586. [10] Y. Li, D. Gu, Additive Manufacturing, 1 (2014) 99-109. [11] J. Wu, X.Q. Wang, W. Wang, M.M. Attallah, M.H. Loretto, Acta Materialia, 117 (2016) 311-320. [12] X.P. Li, X.J. Wang, M. Saunders, A. Suvorova, L.C. Zhang, Y.J. Liu, M.H. Fang, Z.H. Huang, T.B. Sercombe, Acta Materialia, 95 (2015) 74-82. [13] F. Wang, X.H. Wu, D. Clark, Materials Science & Technology, 27 (2011) 344-356. [14] K. Kempen, L. Thijs, J.V. Humbeeck, J.P. Kruth, Physics Procedia, 39 (2012) 439-446. [15] L. Thijs, K. Kempen, J.-P. Kruth, J. Van Humbeeck, Acta Materialia, 61 (2013) 1809-1819. [16] D.-K. Kim, W. Woo, J.-H. Hwang, K. An, S.-H. Choi, Journal of Alloys and Compounds, 686 (2016) 281-286. [17] W. Shifeng, L. Shuai, W. Qingsong, C. Yan, Z. Sheng, S. Yusheng, J. Mater. Process. Technol., 214 (2014) 2660-2667. [18] ASTM, in, West Conshohocken, PA, 2016. [19] C. Motz, T. Schöberl, R. Pippan, Acta materialia, 53 (2005) 4269-4279. [20] A. Ramazani, K. Mukherjee, A. Schwedt, P. Goravanchi, U. Prahl, W. Bleck, 15
International Journal of Plasticity, 43 (2013) 128-152. [21] A. Ramazani, S. Bruehl, T. Gerber, W. Bleck, U. Prahl, Materials & Design, 57 (2014) 479-486. [22] M. Calcagnotto, D. Ponge, E. Demir, D. Raabe, Materials Science and Engineering: A, 527 (2010) 2738-2746. [23] J. Kadkhodapour, S. Schmauder, D. Raabe, S. Ziaei-Rad, U. Weber, M. Calcagnotto, Acta materialia, 59 (2011) 4387-4394. [24] M. Meyers, K. Chawla, Mechanical behavior of materials, Cambridge University Press,(2008) 374-375.
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Fig.1. Build strategies and morphology of melt pool on the load bearing face and the parallel face of (a, b, c) horizontal sample, (d, e, f) 45° sample and (g, h, i) vertical sample.
17
Fig.2. ESBD inverse pole figures and SEM images: (a) and (b) horizontal sample, (c) and (d) 45° sample, (e) and (f) vertical sample. (b1) and (b2), (d1) and (d2), (f1) and (f2) are magnified views of (b), (d) and (f), respectively. 18
Fig. 3. (a) TEM image of cellular structure, (b-f): X-ray mapping results of N, O, Al, Si and Mg.
19
Fig.4. EBSD-band contrast figure of (a) load-bearing face and (b) parallel face of the melt pools in the horizontal sample; (c) Kernel average misorientation map of the selected area in (a).
20
Fig.5. (a) Inverse pole figure and (b) crystallographic orientation of melt pools on the loading bear side in the horizontal sample, (c) and (d) are the microstructural morphology of grain ‘I’ and ‘II’ pointed in (b), (e) inverse pole figure parallel to the build direction obtained by X-ray measurements. 21
Fig.6. Tensile properties of three SLMAlSi10Mgsamples.
22
Fig.7. Fracture surface of (a) horizontal sample, (b) 45° sample, and (c) vertical sample. (a1), (a2) are the enlarged view of the non-flat fracture surface and flat fracture in (a). (d) and (e) are the microstructure beneath the fracture surface.
23
Fig. 8. (a) The loading model of cellular-like structure for the three samples; (b) slip model of vertical sample.
Fig.9.Schmid factor distribution of three samples
24
PII: DOI: Reference:
S0921-5093(18)31462-X https://doi.org/10.1016/j.msea.2018.10.083 MSA37078
To appear in: Materials Science & Engineering A Received date: 26 April 2018 Revised date: 16 October 2018 Accepted date: 18 October 2018 Cite this article as: Z.H. Xiong, S.L. Liu, S.F. Li, Y. Shi, Y.F. Yang and R.D.K. Misra, Role of melt pool boundary condition in determining the mechanical properties of selective laser melting AlSi10Mg alloy, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.10.083 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Role of melt pool boundary condition in determining the mechanical properties of selective laser melting AlSi10Mg alloy
Z.H. Xiong1, S.L. Liu2, S.F. Li1, Y. Shi3, Y.F. Yang1*, R. D. K. Misra4
1
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, No.1 Zhong guan cun North 2ndSt, Beijing, 100190, China
2
Institute of Advanced Steels and Materials, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China
3
State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, No.287 Lan gong ping Road, Lanzhou 730050, China
4
Laboratory for Excellence in Advanced Steel Research, Materials Science and Engineering Program, Department of Metallurgical, Materials and Biomedical Engineering, University of Texas at El Paso, 500 W. University Avenue, El Paso, TX 79968, USA
*
Corresponding author. Tel.: +86-10-82544907. [email protected]
Abstract We describe here a comprehensive study on the effect of cellular structure and melt pool boundary (MPB) condition on the mechanical properties, deformation and failure 1
behavior of AlSi10Mg alloy processed by selective laser melting (SLM). The morphology of melt pool (MP) on the load bearing face of tensile samples was significantly different with build directions. It resulted in different mechanical properties of the samples with different build directions. Furthermore, the microstructure analysis revealed that the MP in the SLM AlSi10Mg alloy mainly consisted of columnar α-Al grains which were made of ultra-fine elongated cellular structure. Electron back-scatter diffraction (EBSD) analysis revealed that the long axis of cellular structure and columnar grains were parallel to <100>, which resulted in <100> fiber texture in SLM AlSi10Mg alloy. However, Schmid factor calculation demonstrated that the anisotropy of mechanical properties of the SLM AlSi10Mg alloy built with different direction was mainly dependent on the distribution of MPB on the load bearing face, and not texture. The defects including pores, residual stress and heat affected zone (HAZ) located at MPB made it the weakest part in the SLM AlSi10Mg. The sample built along horizontal direction exhibited good combination of strength and plasticity and is attributed to the lowest fraction of MPBs that withstand load during tensile. MPB had strong influence on the mechanical properties and failure behavior of SLM AlSi10Mg built with different directions.
Keywords: Selective laser melting; AlSi10Mg alloy; Melt pool boundary; Cellular structure; Anisotropy
1. Introduction 2
Hypoeutectic AlSi10Mg alloy has been widely used for aluminum castings for aerospace and automotive components because of excellent casting properties [1, 2]. However, the mechanical properties of as-cast AlSi10Mg alloy are inferior because of brittle fracture resulting from the presence of coarse and acicular eutectic silicon [3, 4]. Selective laser melting (SLM) [5-8] is capable of producing a refined microstructure by using high heating and cooling rates (103–108 K/s) [9], and further improving the mechanical properties. This has proven to be a potential approach for making high-performance AlSi10Mg components. The early studies on SLM AlSi10Mg mainly focused on the feasibility of making components and optimizing processing parameters because of the high reflectivity of aluminum for the laser beam. High power lasers of 200 W or greater have been used [6, 10]. Such a high-energy input essentially changes the thermal conditions and modifies the microstructure, which currently constitutes the primary focus. Thijs et al. [5] obtained an extremely fine cellular-dendritic microstructure consisting of cellular primary Al decorated with fibrous Si particles in SLM AlSi10Mg alloy. The refined microstructure was characterized by high hardness of 127 Hv0.5 for the as-built AlSi10Mg alloy [5]. Subsequently, similar “cell-like” microstructure characterized with long cells and Al-Si eutectic at cell boundaries was observed in the SLMAlSi10Mg alloy by Wu et al.[11]. The in-situ compression tests in transmission electron microscopy (TEM) further indicated that the Si particles in the “cell-like” microstructure played an important role during plastic deformation of SLM samples. Heat treatments were attempted to refine the Si particles in the SLM AlSi10Mg sample 3
by Li et al.[12]. Tensile ductility of up to 25% was obtained on optimizing the heat treatment. Though the microstructural refinement could improve the mechanical properties of AlSi10Mg alloy produced by SLM, the anisotropy of SLM AlSi10Mg sample is well-known [7, 13, 14]. Building strategies were applied in SLM and had limited influence in reducing anisotropy [15, 16]. MPB is considered as a key issue in anisotropy of SLM fabricated components [17]. However, the influence of MPB on mechanical properties of SLM AlSi10Mg has not been systematically studied. We have conducted a systematic study on the effect of variable MPB conditions on the mechanical properties with focus on morphology and orientation of Al and Al-Si eutectic around the MPB, in the attempt to clarify the response of mechanical properties to the different MPB conditions. 2. Experimental procedure Spherical AlSi10Mg alloy powders (-325 mesh) supplied by Zhejiang Asia General Soldering and Brazing Material Company were used. To obtain different MPB conditions, the tensile bars (please see Fig. 1a, d and g respectively) were built with the SLM facility (SLM Solutions GmbH, SLM 250 HL) in three directions: horizontal (horizontal sample, Fig.1a), 45° (45°sample, Fig.1d) and vertical (vertical sample, Fig. 1g). The SLM was carried out in a argon gas shielding atmosphere at an optimal laser beam power of 350 W, laser scanning velocity of 920 mm/s and energy density of 40 J/mm3 (using these parameters, the relative density of SLM AlSi10Mg samples was ~98.6%). 4
To reduce the effect of surface roughness, standard cylindrical tensile specimens of dimensions 6 mm diameter and 25 mm length conforming to ASTM E8 [18] were further machined from the tensile bars. Tensile tests were performed at a crosshead speed of 0.01/min using a computerized tensile test system (CMT-5105). Three tests were carried out for each condition. Samples were cut from the tensile bars and mechanically polished using 50 nm colloidal alumina and etched with Kroll’s etchant. The microstructure of the samples built with different directions was studied using optical microscope (OM, Olympus BX51M) and field emission scanning emission microscope (SEM, ZEISSULTRA-55) equipped with electron back-scatter diffraction (EBSD) detector. The EBSD samples were mechanically polished using 0.05 μm silica suspension without etching. The EBSD experiments were conducted at an acceleration voltage of 20 kV, working distance of 17.5 mm, tilt angle of 70° and step size of 0.25 μm. The data was processed by Channel 5 software. The texture in the samples were detected by X-ray, X-ray measurements were performed on a SIEMENSD500 using a Cu tube (wave length of the Cu X-ray is 1.5406A) at 40 kV and 40 mA. The diffraction patterns between a 2θ angle of 23°and 90°were measured using a step size of 0.02°and detection time of 3 s. Fracture surface of tensile specimens was examined by SEM. Thin wafers were cut from the as-built SLM samples, ground to ~40 μm in thickness and thinned for TEM (JEOL 2100) studies by precision ion polishing system (Gatan PIPSTM). TEM studies were conducted at 200 kV. 3. Results 5
3.1 Morphology of melt pools on the load bearing face The half-cylindrical shape of MP essentially exhibited two types of morphologies when observed along and perpendicular to the build direction. Fig.1 shows the morphology of MPs along and perpendicular to the build direction in the horizontal sample, 45o sample and vertical sample. To analyze the relationship between the microstructure and tensile properties, the microstructure of load-bearing face and the face parallel to the tensile loading direction (referred as parallel face henceforth) was examined. It was clearly observed that the load bearing face of horizontal sample exhibits “semicircular” MPs (Fig. 1b), while the parallel face was stripe-like MPs (Fig. 1c). In contrast, the MP morphology on load bearing face (Fig. 1h) and parallel face (Fig. 1i) of vertical sample was stripe-like and “semicircular”, respectively. The morphology of MPs of 45° sample on the load bearing face and parallel face was nearly identical (Fig. 1e and 1f), which exhibited a combination of “semicircular” and stripe-like MPs. This clearly indicated the shape of MPs and MPBs on the load bearing faces of SLM sample were different with different building directions. In addition, it may be noted from Fig. 1 that the pores were always located along the MPBs in each sample, irrespective of the build direction. 3.2 Microstructure in the MP Fig.2 shows the results of EBSD analysis and microstructure of load bearing face of horizontal sample, vertical sample and 45° sample, corresponding to “semicircular” MP, stripe-like MP and the combination of “semicircular” and stripe-like MPs, respectively. The following are the observations: 6
(i) The “semicircular” MPs on the load bearing face of the horizontal sample were mainly composed of columnar α-Al grains, which grew from the MPB to the inner of MP(Fig. 2a). The length and diameter of columnar α-Al grains were ~ 50-100 μm and ~ 5-20 μm, respectively. It is also noted that some small equiaxed grains were located at the MPB. From Fig. 2b, the columnar α-Al grain in the “semicircular” MP was made up of elongated cell-like/cellular structure, the length and width of the elongated cell-like structure was ~ 1-3μm and 0.5μm, respectively (Fig. 2b1 and 2b2), which implied that the elongated cell-like structure was significantly finer than columnar α-Al grain. Fig. 2b also shows that the MPB include HAZ and remelted zone, the cells disappeared in the HAZ replaced by granular microstructure, and the cells in the remelted zone was coarse, which indicated that the microstructure in MPB was inhomogeneous. (ii) The stripe-like MP on the load bearing face of the vertical sample was composed of equiaxed grains (Fig. 2e), which is actually the cross-section of columnar α-Al grains in view of three-dimensional nature of the half-cylindrical shape of MP. There were also some small equiaxed grains located at the MPB and were formed at MPB during SLM. The equiaxed α-Al grains in stripe-like MP were composed of equiaxed cell-like structure (Fig. 2f, 2f1 and2f2), where the equiaxed cell-like structure is the cross-section of elongated cell-like structure. (iii) In the case of 45°sample on the load bearing face, the columnar α-Al grains and the equiaxed α-Al grains were located at the two adjacent MPs (see Fig.2c). Fig. 2d, and 2d1 and 2d2 shows that the columnar α-Al grain and the equiaxed α-Al grain 7
consisted of ultra-fine elongated cells and ultra-fine equiaxed cells, respectively. TEM studies of cellular structure together with X-ray mapping of corresponding elements are presented in Fig.3. No significant segregation was observed in the SLM samples, although the cooling rate was high. The cells were α-Al phase decorated with eutectic Si at the boundaries. Mg was also detected at the cell boundaries and Traces of oxygen and nitrogen were observed. 3.3 Crystal orientation in the melt pool Fig.4 a and b show corresponding EBSD-band contrast map of the microstructure in the melt pool on the load-bearing face and parallel face of the horizontal sample, corresponding to the “semicircular” MP and stripe-like MP, respectively. Clearly, the load bearing face was mainly composed of columnar grains, while the parallel face consisted of equiaxed grains. It was confirmed that the equiaxed grains in stripe-like MP were cross-section of columnar grains in “semicircular” MP. Fig.4c gives the Kernel average misorientation (KAM) map in the MPs of the marked area in Fig. 4a, KAM values obtained from EBSD data can be used to analyze stress concentration [19-22], since KAM has a similar distribution with stress, as illustrated by Ramazani etal. [20, 22, 23]. Thus, stress distribution was characterized in terms of misorientation (KAM) values in Fig.4c, yellow color corresponds to high stress concentration, while blue color represents low stress concentration. The high stress concentration is present inside the local melt pools, but a change occurred at the MPBs, suggesting that the stress distribution in the vicinity of MPBs is not uniform. Fig.5a is inverse pole figure map of melt pools on the load bearing face of the 8
horizontal sample. <100> fiber texture was observed. Fig.5b shows the distribution of <100> fiber texture highlighted by green and red. Figs.5c and 5d shows the crystal orientation of columnar grains processed by EBSD software. It is clear that the long axis of both cell-like structure and columnar grains was parallel to <100>, which indicated that the columnar Al grains grew along the <100>. This is the reason for the formation of <100> fiber texture in SLM AlSi10Mg. The existence of <100> fiber texture in the SLM AlSi10Mg is also confirmed by X-ray measurement as shown in Fig.5e, the <100> texture // build direction of the SLM AlSi10Mg. 3.4 Mechanical properties and fractography Fig. 6 shows the tensile properties of SLM AlSi10Mg samples with different build directions. The ultimate tensile strength (UTS) of the three samples followed the sequence: horizontal sample (360 MPa) ≈ vertical sample (363 MPa) > 45° sample (340 MPa), while the sequence for yield strength (YS) was: horizontal sample (220 MPa) > vertical sample (190 MPa) ≈45° sample (186 MPa). The elongation followed the sequence: horizontal sample (7.1%) > 45° sample (6.0%) > vertical sample (4.3%). The horizontal sample built along the horizontal direction had highest elongation and exhibited good combination of strength and plasticity. The fracture surface of SLM AlSi10Mg samples with different build directions are shown in Fig. 7. From Fig. 7a, b and c, there are flat fracture (highlighted by red curve) on the fracture surface of the three kinds of samples. The flat fracture implied that the microstructure experienced less deformation before fracture, while the microstructure on the non-flat fracture should experience more deformation before fracture (the high 9
magnification of the microstructure on the non-flat fracture surface and flat fracture of horizontal sample are shown in Fig. 7a1 and 7a2 respectively). In addition, it can be seen that there are holes (of size ~10-80 μm) on the fracture surface of the three kinds of samples. The holes are the enlarged pores developed from the pores and invisible small pores (under optical microscope) in the matrix during deformation. Interestingly, the geometry of the flat fracture seems to be consistent with the MPBs, suggesting that crack initiated and progressed along the MPBs. This was further confirmed by the crack path observed in the HAZ of MPB (Fig. 7d and e). In addition, it was noted that the fraction of flat surface in the three samples was significantly different (Fig.7a, b and c). The percentage of flat region on the fracture surface of three samples was: 10% (horizontal sample) < 40% (45° sample) < 75% (vertical sample). The reason for the significant difference between the fracture surface of three samples is discussed below. 4. Discussion 4.1 Effect of texture in SLM AlSi10Mg From the aforementioned results, the long axis of both cell-like structure and columnar grains was parallel to <100>, which indicated that columnar Al grains grew along <100>, which resulted in the formation of <100> fiber texture in SLM AlSi10Mg. The fiber texture should have an effect on the yield strength by influencing the Schmid factor of the samples built with different directions. In order to calculate the Schmid factor of the columnar grain in each sample, a simple model is illustrated in Fig.8. The loading direction of the vertical sample is parallel to <100> of columnar grain and long 10
axis of cellular structure (Fig. 8a and b). Fig. 8b shows a crystal with a normal cross-sectional area A on which a tensile load F is applied, generating a uniaxial stress F/A. The angle between the slip direction and loading direction is ，The normal direction of the slip plane makes an angle ɸ with the loading direction. The slip system in face-centered cubic (FCC) Al grain is {110}<111> (Fig. 8b). In the uniaxial tensile [24]:
F cosɸcosλ= σcosɸcosλ (1) A m=cosɸcosλ(2)
is the shear stress acting on the slip plane and along the slip direction, m is Schmid factor. Without considering the effect of inhomogeneous structure (including HAZ in MPB, defect such as pores in the vicinity of MPB), slip occurs when the shear stress on the slip plane, and in the slip direction, reaches a valueτcrss, the yield stress of the polycrystalline material can be expressed as:
Y
crss m
(3)
Y isyield stress, m is the equivalent Schmid factor of three samples. From equation (3), sinceτcrss is same for identical material, Y is related to m , Y decreases with the increase of m . m changes with the build direction of the three kinds of samples. Since the loading direction of the vertical sample is parallel to <100> of columnar grain and long axis of cellular structure, i.e., the angle between the loading direction and <100> of the columnar grain of vertical sample is 0°. The equivalent Schmid factor of vertical 11
samples was calculated to be 0.408 and has a constant value. While the angle between the loading direction and <100> of columnar grain of 45° sample and horizontal sample were 45o and 90o, respectively. The Schmid factor for them do not have constant values. Accordingly, the Schmid factors of 45° sample and horizontal sample were calculated using MATLAB(R2015b) and the results are presented in Fig.9. From Fig.9 it can be seen that the equivalent Schmid factor for the three samples followed the sequence: horizontal sample > vertical sample ≈ 45° sample, according to equation (3), the yield strength of the three samples should be: horizontal sample < vertical sample ≈ 45° sample. This result is not consistent with the yield strength (Fig.6) measured by the tensile test, which implies that the texture in SLM AlSi10Mg is not the main factor that influence yield strength. The yield strength should be closely related to the inhomogeneous structure in MPB.4.2 Role of melt pool boundary (MPB) Based on the experiment results, crack initiated and progressed along the MPBs, which suggests that MPB is the weakest part of SLM samples. The weakness of MPB is likely to be related to the nonuniform distribution of stress concentration at MPB (Fig.4c), HAZ in which the cell-like structure disappeared (Fig.2b, d, f, Fig.7e), and defects such as pores (Fig.1) at the MPB side. Given that MPB was the weakest part, the elongation of SLM AlSi10Mg sample is mainly determined by the fraction of MPBs that withstand load. The fraction of MPBs that withstand load followed the sequence: horizontal sample (Fig. 1c) < 45°sample (Fig. 1f) < vertical sample (Fig. 1i). High fraction of MPB that withstand load generally lead to low elongation and results in flat fracture facet on the fracture surface of vertical 12
sample (Fig. 7c). This is the underlying reason for the vertical sample to exhibit maximum flat fracture facets (the microstructure on it experienced less deformation before fracture) and lowest elongation. In contrast, the horizontal sample had least MPBs that withstand load during tensile. Thus, horizontal sample had least flat fracture facets, which is the reason for highest elongation of the horizontal sample. Thus, the distribution of MPB on the load bearing face of as-built SLM AlSi10Mg tensile samples governed the fracture behavior and mechanical properties of SLM AlSi10Mg.
Conclusions (1) The melt pool in SLM AlSi10Mg alloy was half-cylindrical in three-dimensional shape, which resulted in significant difference in the distribution of MP and MPBs on the load bearing faces of the tensile samples with different building directions. (2) The MP in the SLM AlSi10Mg samples was made of columnar α-Al grains, which was comprised of elongated cell-like eutectic Si and Al matrix. The crystal orientation <100> of columnar α-Al grains was the preferred growth direction, leading to fiber textures. (3) The anisotropy of mechanical properties of sample with different build directions was not governed by fiber texture, but by the distribution of MPB on the load bearing surface. The localization of pores, residual stress and HAZ at MPB made it the weakest part in the SLM AlSi10Mg alloy. Crack initiated and progressed along the MPBs. (4) The horizontal sample exhibited good combination of strength and plasticity and is attributed to the horizontal sample having least MPBs that withstand load during the 13
tensile.
Acknowledgements The work was supported by the Key Research Program of Frontier Sciences (Grant No. QYZDB-SSW-JSC045), Chinese Academy of Sciences and National Youth Thousand Plan Program, and the National Key Research and Development Program of China (No. 2018YFB1105100).
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Fig.1. Build strategies and morphology of melt pool on the load bearing face and the parallel face of (a, b, c) horizontal sample, (d, e, f) 45° sample and (g, h, i) vertical sample.
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Fig.2. ESBD inverse pole figures and SEM images: (a) and (b) horizontal sample, (c) and (d) 45° sample, (e) and (f) vertical sample. (b1) and (b2), (d1) and (d2), (f1) and (f2) are magnified views of (b), (d) and (f), respectively. 18
Fig. 3. (a) TEM image of cellular structure, (b-f): X-ray mapping results of N, O, Al, Si and Mg.
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Fig.4. EBSD-band contrast figure of (a) load-bearing face and (b) parallel face of the melt pools in the horizontal sample; (c) Kernel average misorientation map of the selected area in (a).
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Fig.5. (a) Inverse pole figure and (b) crystallographic orientation of melt pools on the loading bear side in the horizontal sample, (c) and (d) are the microstructural morphology of grain ‘I’ and ‘II’ pointed in (b), (e) inverse pole figure parallel to the build direction obtained by X-ray measurements. 21
Fig.6. Tensile properties of three SLMAlSi10Mgsamples.
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Fig.7. Fracture surface of (a) horizontal sample, (b) 45° sample, and (c) vertical sample. (a1), (a2) are the enlarged view of the non-flat fracture surface and flat fracture in (a). (d) and (e) are the microstructure beneath the fracture surface.
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Fig. 8. (a) The loading model of cellular-like structure for the three samples; (b) slip model of vertical sample.
Fig.9.Schmid factor distribution of three samples
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