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Effect of friction stir welding parameters on the microstructure and microtexture evolution of SAF 2205 stainless steel
Journal Pre-proof Effect of friction stir welding parameters on the microstructure and microtexture evolution of SAF 2205 stainless steel S. Emami, T. Saeid, A. Abdollah-zadeh PII:
S0925-8388(19)33030-0
DOI:
https://doi.org/10.1016/j.jallcom.2019.151797
Reference:
JALCOM 151797
To appear in:
Journal of Alloys and Compounds
Received Date: 4 June 2019 Revised Date:
5 August 2019
Accepted Date: 9 August 2019
Please cite this article as: S. Emami, T. Saeid, A. Abdollah-zadeh, Effect of friction stir welding parameters on the microstructure and microtexture evolution of SAF 2205 stainless steel, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.151797. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
Effect of friction stir welding parameters on the microstructure and microtexture evolution of SAF 2205 stainless steel S. Emamia, T. Saeida, A. Abdollah-zadeh*b1∗ a) Faculty of Materials Engineering, Sahand University of Technology, Tabriz, Iran. b) Department of Materials Engineering, Tarbiat Modares University, Tehran, Iran.
Abstract The present study investigates the influence of different welding parameters of welding and rotational speeds on the microstructure and the texture in the stir zone (SZ) during friction stir welding (FSW) of a SAF 2205 duplex stainless steel. The FSW process was conducted using a WC– based tool with welding speeds ranging 50 – 350 mm/min and rotational speeds ranging 400 – 800 rpm. Microstructure characterization and textural studies showed that fine and dynamically recrystallized grains developed through the microstructure of the SZs and the grain size of the developed microstructures in the SZs is decreased with increasing the welding speed and/or with decreasing the rotational speeds. The intensity of the resultant simple shear texture developed in the SZs increased with increasing the rotational speed and/or with decreasing the welding speeds. Increasing the welding speed tends to weaken the intensity of shear texture and finally randomize it. Therefore, the contribution of rotational speed in formation of simple shear texture in the SZs of the processed materials is greater than that of welding speed. Keywords: FSW, duplex stainless steel, microstructure, grain size, shear texture.
∗
Corresponding Author Tel.: +98 2182883522 E-mail address: [email protected] (A. Abdollah-zadeh)
1
1. Introduction Friction stir welding, as a solid-state joining technique has become increasingly prominent in joining and welding of duplex stainless steels (DSS). This is more probably contributed to the solid state nature of the FSW which efficiently retain the desired ratio of the constituent phases and produces welds without any significant changes in phase balance especially at welding parameters associated with low heat input [1-3]. In this regard, there have been conducted several studies on FSW of duplex stainless steels. Okamoto et al. [4] investigated the feasibility of the FSW on 6 mm thick plates of 329 DSS. Saeid et al. [5] examined the microstructure and the mechanical properties FS welded joints of a SAF 2205 DSS produced with different welding parameters. Santos et al. [6] also investigated the feasibility of FSW process on four types of superduplex stainless steels with different welding parameters. Santos et al. [7] recorded the thermal history of an UNS S32205 DSS. Recorded FSW thermal cycles exposed the welded material at high temperatures for a short time which disapproved the formation of any detrimental second phase precipitation. It is clear from the open literature that these early cited studies focused on the feasibility of FSW on the DSSs. Since an essential perception of microstructural evolution during FSW can provide useful information for both academic research and industrial applications, recent studies are mainly limited to microstructure development during the FSW of DSSs [8]. For example, Sato et al. [9] studied the evolved microstructure and the resultant mechanical properties of a SAF 2507 super DSS in the FSW process. They showed that the microstructures of both constituent phases of ferrite and austenite were dynamically recrystallized and very fine microstructures were developed. Saeid et al. [3] investigated the softening behavior of a SAF 2205 DSS subjected to 2
FSW. They reported that the microstructure of both constituent phases of austenite and ferrite evolved through the continuous dynamic recrystallization (CDRX). They also reported that the microstructure of austenite went through static recrystallization (SRX) evidencing the cube texture in orientation distribution functions (ODFs). Santos et al. [10,11] indicated that ferrite with high stacking fault energy (SFE) softens through CDRX mechanism accompanied by the formation of dislocation substructures which in turn results in the formations of low angle grain boundaries (LAGBs) and austenite with lower SFE tends to soften discontinuously through grain boundary bulging mechanism. Emami et al. [12] also investigated FS welds of a SAF 2205 DSS with single welding parameter. These researchers demonstrated that the austenite and ferrite softened through the CDRX mechanism. Furthermore, they showed that SRX occurred in austenite evidencing common recrystallization texture components of face centered cubic (FCC) materials in ODF sections. These recently deep studies have achieved some valuable knowledge on the microstructural evolution of duplex stainless steels during the FSW at a constant welding and rotational speeds. In the following, the present study aims to go one step further and to investigate the effect of FSW parameters on the weld microstructure and microtexture development of SAF 2205 DDS. 2. Material and methods 2.1. Base material and the welding procedure A commercial SAF 2205 DSS with chemical composition of Fe-22.31Cr-5.48Ni-3.34Mo1.42Mn-0.03C-0.023P-0.005S (wt. %) was used. The dimensions of the samples used for the study was 300mm×100mm×2mm. The FSW process was conducted along the rolling direction of the samples. The welding tool was designed from a WC-based material with simple
3
cylindrical pin geometry. The diameters of the shoulder, pin, and the length of the pin were 16 mm, 5 mm, and 1.5 mm, respectively. The tilt angle of the welding tool with respect to the normal direction of the plate was 3º. Ar shielding gas with a flow rate of 18 L/min was used to avoid the formation of any surface oxides. Fifteen welds were made with rotational speeds of 400, 600, and 800 rpm and welding speeds ranging 50-350 mm/min. The insertion depth of tool pin in to the samples was kept constant at 1.7 mm during welding. The normal force to the plate surface was controlled at 14 kN. 2.2. Weld thermal history K-type thermocouples with the diameter of 2 mm were used to record the thermal cycles from beneath the samples in the centerline of the welds. Thermocouples were located in the predesigned holes with the diameter of 2.5 mm in the carbon steel backing plate. A copper paste was used to fix the located thermocouples in the holes so as not to be disconnected while welding procedure. 2.3. Macro and microstructure studies Visual inspection along with the X-ray radiography tests were carried out on the trial joints to reveal the existence of any weld defect developed on the surface and/or inside of the joints. The microstructure of the resultant joints were mainly observed and characterized using an electron back-scattered diffraction analysis (EBSD). EBSD samples were prepared by mechanical grounding, then pre-polishing with 1 and 0.25 µm diamond pastes and finally electro-polishing with 700 mL ethanol, 120 mL distilled water, 100 mL glycerol, and 80 mL perchloric acid at ambient temperature with a voltage of 35 V for 10 s.
4
2.4. Mechanical evaluation of the weldments Hardness measurements were performed on the cross-section of the welds normal to the welding direction along the centerline parallel with transverse direction (TD) using a 4.9 N load for 10 s. Longitudinal tensile specimens were prepared from the joints in accordance with the ASTM E8 standard. Tensile tests were conducted at ambient temperature with a crosshead rate of 0.05 mm/s. 3. Results and discussion 3.1. Welds appearance Figure 1 shows the process window of FS welded DSS samples along with the X-ray radiography results. The dashed line on the figure indicates the conditions at which sound welds are obtained. Unsuccessful welding conditions are related to pin fracture and groove-like defects. These kinds of defects can deteriorate the mechanical properties of the joints, such as ultimate tensile strength and elongation percentage. These defects happen when the welding parameter is not associated with enough heat input to flow the material easily and smoothly which may leave the generated gap by the stirring pin partially filled or unfilled [13,14].
5
Figure 1. Process window and radiography results of FS welded specimens indicating the defect free welds.
3.2. Thermal history of the resultant welds Results of temperature measurements obtained from the sound welds are depicted in Fig. 2. Temperature measurements from beneath the pin showed that the maximum temperatures are ranging 400 – 953 ºC wherein, the highest value is related to the sample welded at 50 mm/min of welding speed and 800 rpm of rotational speed. It can be seen that maximum temperature increases with increasing the rotational speed and/or with decreasing the welding speed [5].
6
Figure 2. Welds thermal histories: a) constant rotational speed of 800 rpm, b) constant welding speed of 50 mm/min.
3.3. Microstructure evaluation using EBSD 3.3.1. The microstructure of SZ at different welding conditions Fig. 3 indicates the microstructure of resultant SZs under different welding and rotational speeds. It is apparent from the figure that the FSW process has developed fine equiaxed microstructures in the SZs. Concurrence of elevated temperature and severe plastic straining in the SZ stimulated the occurrence of the dynamic recrystallization through which fine and equiaxed grains are formed in the microstructure of the processed material [1-3,5,12]. Statistical analysis of the phase fraction of the resultant SZs showed that the FSW process effectively preserved the desired ratio 7
(1:1) between the constituent phases of ferrite and austenite in the center of welded samples. These microstructural observations are supplementary evidences for the temperature measurements and approve that the maximum temperature never exceed the transformation temperature of γ to α. Previous studies [15,16] have showed that the desirable ratio of the constituent phases (1:1) in a SAF 2205 duplex stainless steel changes when the temperature of the material increases more than 1100 ºC. Therefore, the trivial changes in phase fraction indicated that the peak temperature of the welded material never exceeded 1100 ºC.
Figure 3.Phase maps from the base metal and the center of the resultant SZs.
Fig. 4 depicts the distribution of the average grain size in the center of the resultant SZs. It is seen that the average grain size of the developed microstructure is decreased when the welding speed is increased and/or the rotational speed is decreased. It can be seen that the smallest grains developed through the SZ microstructure of the sample welded with welding and rotational
8
speeds of 150 mm/min and 400 rpm, respectively (0.48 µm and 0.42 µm in ferrite and austenite, respectively), and the largest grains formed in the SZ of sample welded with 50 mm/min and 800 rpm (1.55 µm and 1.39 µm in ferrite and austenite, respectively). Based on the temperature measurements, higher ratios of rotational speed (ω)/ welding speed (υ) increased the amount of heat input and the peak temperature and decreased the cooling rate during the weld thermal cycles. It is well accepted that the kinetics of the recrystallization mainly depends on the time and the temperature. When the cooling time and the temperature are increased, grain growth can also takes place beside the recrystallization [17].
Figure 4. Grain size variation in the resultant SZs: a) austenite, and b) ferrite.
3.4. Textural characterization using EBSD 3.4.1. BM Figs. 5 and 6 represent the texture of the BM using pole figures and the orientation distribution functions (ODFs). The texture analysis shows that the ferrite phase is composed of H 9
{001}<110>, I {211}<011>, and K {110}<110> textural components which are considered as common rolling texture components in BCC materials [18, 19].
= 45° ODF section (Fig. 5a)
shows that the ferrite phase is strongly textured near the {001}<110> and {211}<011> which is parallel with α fiber. Regarding the fact that the occurrence of recrystallization in BCC structured materials decreases the intensity of the textural components specially those ranging between the {001}<110> and {211}<011>, therefore, the texture of ferrite phase in the BM is predominantly deformational. But for the austenite,
= 0° ODF section (Fig. 6a) shows that the texture is
composed of the brass (B) ({110}<112>), copper (C) ({112}<111>), Goss (G) ({110}<001>), and the cube ({001}<100>) components. The texture components of B, C, and G are related to the rolling texture whereas the cubic one is related to common component of recrystallization texture of the FCC structured materials [17,18]. It is seen that the intensity of the texture in ferrite phase is effectively higher than that of the austenite. High stacking fault energy (SFE) of ferrite stimulates the occurrence of dynamic recovery which tends to preserve the deformation texture. But in the austenite, low SFE hinder the occurrence of recovery process and facilitate the recrystallization process which decreases the intensity of the deformation texture [17,20].
10
Figure 5. = ° ODF section and 100 pole figure: a) obtained from the investigated BM, and b) standard = ° ODF section and 100 pole figureindicating the position of some ideal deformational orientations in BCC materials [17].
Figure 6. = ° ODF section and 100 pole figure: a) obtained from the investigated BM, and b) a three dimensional Euler space and 111 pole figure indicating the position of some ideal deformational orientations in FCC materials [17].
11
3.4.2. SZs Figs. 7 and 8 depict the ferrite 110 and austenite 111 pole figures obtained from the center of the resultant SZs. It is to be mentioned that these pole figures with simple shear coordinate system obtained after proper rotations on common welding coordinate system. The conversion of the coordinate systems has been completely described in the previous work by Emami et al. [12]. It is seen that the stirring action of FSW tool has developed a simple shear texture in both constituent phases in the SZ of the welded samples. During the FSW, individual grains tend to be rotated along with the direction of applied strain. This kind of adaptation occurs through the activation of easy slip systems. Slip usually tends to occur along the plane with shortest Burgers vector which is available in the most close-packed planes. Therefore, 111 planes as the most closely packed planes of the FCC structured materials facilitate the dislocation movements and finally the plastic deformation in the austenite. In the BCC structured materials, however there is not any truly close-packed plane, and so, slip tends to occur on the most densely packed 110 plane [18]. The results shows that the intensity of the resultant pole figures is increased with increasing the rotational speed and/or decreasing the welding speed. It is also seen that simple shear texture are gradually disappeared and replaced by a complex texture with increasing the welding speed or with decreasing the rotational speed. Therefore, the formation of simple shear texture in the SZs is mainly related to the stirring action of the tool pin. Since the intensity of the resultant texture is directly related to the amount of applied deformation, thus it can be inferred from the intensities that the amount of applied strain is decreased with decreasing the rotational speed and/or with increasing the welding speed. This behavior also implies that the contribution of stirring action of the tool pin in flow of the material decreases with decreasing the rotational speed and or with increasing the welding speed.
12
Figure 7. Ferrite 110 pole figures obtained from the center of the SZs.
Figure 8. Austenite 110 pole figures obtained from the center of the SZs.
13
3.5. Mechanical properties Fig. 9 indicates the influence of welding and rotational speeds on the hardness and the ultimate strength of the produced joints. The results of hardness measurements showed that the hardness values for the SZs were higher than that of the base materials (260 Hv). This behavior is contributed to the generation of dislocations, development of the LAGBs and the reduction of the grain size in the SZs [5]. Grain boundaries are considered as effective barriers against the movement of the dislocations [17]. Therefore, fine microstructures of the SZs with high density of dislocations along with high fraction of HAGBs and LAGBs show higher values of hardness with respect to the BM [5]. It is also apparent from the figure that the hardness and the tensile strength of the joints improve with increasing the welding speed and/or decreasing the rotational speed. This finding are in a good agreement with those of the previous studies which have been conducted on the carbon steels [21,22] and austenitic stainless steel [23]. As it mentioned previously, lower rotational speeds and/or higher welding speeds result in the formation of fine microstructures in the SZ which improves the mechanical properties of the joints.
Figure 9. Influence of welding and rotational speeds on the mechanical properties of the SZs.
14
4. Conclusions
Influence of frictions stir welding parameters on the microstructure and the texture of SAF 2205 duplex stainless steels was investigated. The grain size of the resultant microstructure in the SZs is decreased by increasing the welding speed and/or with decreasing the rotational speed. Simple shear texture has been developed in the SZs due to the strain applied by the rotating welding tool. Decreasing the texture intensities with increasing the welding speed implied that the formation of the simple shear is mainly contributed to the stirring action of the pin. References
[1] L.E. Murr, “Handbook of materials structures, properties, processing and performance”, Springer, Switzerland, 2015. [2] R.S. Mishra, Z.Y. Ma, Friction stir welding and processing, Materials Science and Engineering R 50 (2005) 1–78. [3] T. Saeid, A. Abdollah-zadeha, T. Shibayanagi, K. Ikeuchi, H. Assadi, On the formation of grain structure during friction stir welding of duplex stainless steel, Materials Science and Engineering A 527 (2010) 6484–6488. [4] K. Okamoto, S. Hirano, I. Masahisa, Metallurgical and mechanical properties of friction stir welded stainless steels, Proceedings of the 4th International FSW Symposium,Park City, UT, USA, 2003 [5] T. Saeid, A. Abdollah-zadeh, H. Assadi, F. Malek Ghaini, Effect of friction stir welding speed on the microstructure and mechanicalproperties of a duplex stainless steel, Materials Science and Engineering A 496 (2008) 262–268.
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[6] T.F.A. Santos,E.A. Torres, A.J. Ramirez, Friction stir welding of duplex stainless steels, Welding International, 21 (1) (2016) 59–69. [7] T.F.A. Santos, H.S. Idagawa, A.J. Ramirez, Thermal history in UNS S32205 duplex stainless steel friction stir welds, Science and Technology of Welding and Joining 19 (2014) 150-156. [8] F.C. Liu, T.W. Nelson, In-situ grain structure and texture evolution during friction stir welding of austenite stainless steel, Materials & Design 115 (2017) 467–478. [9] Y.S. Sato, T.W. Nelson, C.J. Sterling, R.J. Steel, C.O. Pettersson, Microstructure and mechanical properties of friction stir welded SAF 2507 super duplex stainless steel, Materials Science and Engineering A 397 (2005) 376–384. [10] T.F.A. Santos, E.A. Torres, J.C. Lippold, and A.J. Ramirez, Detailed Microstructural Characterization and Restoration Mechanisms of Duplex and Super duplex Stainless Steel Friction-Stir-Welded Joints, Journal of Materials Engineering and Performance 25 (2016) 51735188. [11] T.F.A Santos, E.A.T. López, E.B. Fonseca, A.J. Ramirez, Friction stir welding of duplex and superduplex stainless steels and some aspects of microstructural characterization and mechanical performance Materials Research 19(1) (2016) 117-131. [12] S. Emami, T. Saeid, R. Azari Khosroshahi, Microstructural evolution of friction stir welded SAF 2205 duplexstainless steel, Journal of Alloys and Compounds 739 (2018) 678-689. [13] H.B. Chen, K. Yan, T. Lin, S.B. Chen, C.Y. Jiang, Y. Zhao, The investigation oftypical welding defects for 5456 aluminum alloy friction stir welds, MaterialsScience and Engineering A 433 (2006) 64–69. 16
[14] Y.G. Kim, H. Fujii, T. Tsumura, T. Komazaki, K. Nakata, Three defect types infriction stir welding of aluminum die casting alloy, Materials Science andEngineering A 415 (2006) 250– 254. [15] ASM Specialty Handbook, Stainless Steels, ASM International, Materials Park, 1994, 383– 388. [16] H. Sieurin, R. Sandstrom, Austenite reformation in the heat-affected zone ofduplex stainless steel 2205, Materials Science and Engineering A, 418 (2006) 250–256. [17] F.J. Humphreys, M. Hatherly, Recrystallization and related annealing phenomena, Elsevier (2004). [18] P. Cizek, J. A. Whiteman, W. M. Rainforth, J. H. Beynon, EBSD and TEM investigation of the hot deformation substructure characteristics of a type 316L austenitic stainless steel, Journal of Microscopy, 213 (2003) 285-295. [19] U.F. Kocks, C.N. Tome, H.R. Wenk, Texture and Anisotropy: preferredorientations in polycrystals and their effect on materials properties, CambridgeUniversity Press, 1998. [20] J. Keichel, J. Foct, and G. Gottstein, Deformation and annealing behavior of nitrogen alloyed duplex stainless steels. part II: annealing, ISIJ International, 43(2003) 1788–1794. [21] H. Fujii, L. Cui, N. Tsuji, M. Maeda, K. Nakata, K. Nogi, Friction stir welding ofcarbon steels, Materials Science and Engineering A, 429 (2006) 50–57. [22] L. Cui, H. Fujii, N. Tsuji, K. Nakata, K. Nogi, R. Ikeda, and M. Matsushita,Transformation in stir zone of friction stir welded carbon steels with differentcarbon contents, ISIJ International, 47 (2007), 299–306. 17
[23] A.P. Reynolds, W. Tang, T.G. Herold, H. Prask, Structure, properties, and residualstress of 304L stainless steel friction stir welds, Scripta Materialia, 48 (2003)1289–1294.
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Highlights 1. Defect-free FS welds (50–350 mm/min and 400–800 rpm) of 2205 DSS were obtained. 2. Effect of FSW parameters was studied on the texture and microstructure of the SZs.
S0925-8388(19)33030-0
DOI:
https://doi.org/10.1016/j.jallcom.2019.151797
Reference:
JALCOM 151797
To appear in:
Journal of Alloys and Compounds
Received Date: 4 June 2019 Revised Date:
5 August 2019
Accepted Date: 9 August 2019
Please cite this article as: S. Emami, T. Saeid, A. Abdollah-zadeh, Effect of friction stir welding parameters on the microstructure and microtexture evolution of SAF 2205 stainless steel, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.151797. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
Effect of friction stir welding parameters on the microstructure and microtexture evolution of SAF 2205 stainless steel S. Emamia, T. Saeida, A. Abdollah-zadeh*b1∗ a) Faculty of Materials Engineering, Sahand University of Technology, Tabriz, Iran. b) Department of Materials Engineering, Tarbiat Modares University, Tehran, Iran.
Abstract The present study investigates the influence of different welding parameters of welding and rotational speeds on the microstructure and the texture in the stir zone (SZ) during friction stir welding (FSW) of a SAF 2205 duplex stainless steel. The FSW process was conducted using a WC– based tool with welding speeds ranging 50 – 350 mm/min and rotational speeds ranging 400 – 800 rpm. Microstructure characterization and textural studies showed that fine and dynamically recrystallized grains developed through the microstructure of the SZs and the grain size of the developed microstructures in the SZs is decreased with increasing the welding speed and/or with decreasing the rotational speeds. The intensity of the resultant simple shear texture developed in the SZs increased with increasing the rotational speed and/or with decreasing the welding speeds. Increasing the welding speed tends to weaken the intensity of shear texture and finally randomize it. Therefore, the contribution of rotational speed in formation of simple shear texture in the SZs of the processed materials is greater than that of welding speed. Keywords: FSW, duplex stainless steel, microstructure, grain size, shear texture.
∗
Corresponding Author Tel.: +98 2182883522 E-mail address: [email protected] (A. Abdollah-zadeh)
1
1. Introduction Friction stir welding, as a solid-state joining technique has become increasingly prominent in joining and welding of duplex stainless steels (DSS). This is more probably contributed to the solid state nature of the FSW which efficiently retain the desired ratio of the constituent phases and produces welds without any significant changes in phase balance especially at welding parameters associated with low heat input [1-3]. In this regard, there have been conducted several studies on FSW of duplex stainless steels. Okamoto et al. [4] investigated the feasibility of the FSW on 6 mm thick plates of 329 DSS. Saeid et al. [5] examined the microstructure and the mechanical properties FS welded joints of a SAF 2205 DSS produced with different welding parameters. Santos et al. [6] also investigated the feasibility of FSW process on four types of superduplex stainless steels with different welding parameters. Santos et al. [7] recorded the thermal history of an UNS S32205 DSS. Recorded FSW thermal cycles exposed the welded material at high temperatures for a short time which disapproved the formation of any detrimental second phase precipitation. It is clear from the open literature that these early cited studies focused on the feasibility of FSW on the DSSs. Since an essential perception of microstructural evolution during FSW can provide useful information for both academic research and industrial applications, recent studies are mainly limited to microstructure development during the FSW of DSSs [8]. For example, Sato et al. [9] studied the evolved microstructure and the resultant mechanical properties of a SAF 2507 super DSS in the FSW process. They showed that the microstructures of both constituent phases of ferrite and austenite were dynamically recrystallized and very fine microstructures were developed. Saeid et al. [3] investigated the softening behavior of a SAF 2205 DSS subjected to 2
FSW. They reported that the microstructure of both constituent phases of austenite and ferrite evolved through the continuous dynamic recrystallization (CDRX). They also reported that the microstructure of austenite went through static recrystallization (SRX) evidencing the cube texture in orientation distribution functions (ODFs). Santos et al. [10,11] indicated that ferrite with high stacking fault energy (SFE) softens through CDRX mechanism accompanied by the formation of dislocation substructures which in turn results in the formations of low angle grain boundaries (LAGBs) and austenite with lower SFE tends to soften discontinuously through grain boundary bulging mechanism. Emami et al. [12] also investigated FS welds of a SAF 2205 DSS with single welding parameter. These researchers demonstrated that the austenite and ferrite softened through the CDRX mechanism. Furthermore, they showed that SRX occurred in austenite evidencing common recrystallization texture components of face centered cubic (FCC) materials in ODF sections. These recently deep studies have achieved some valuable knowledge on the microstructural evolution of duplex stainless steels during the FSW at a constant welding and rotational speeds. In the following, the present study aims to go one step further and to investigate the effect of FSW parameters on the weld microstructure and microtexture development of SAF 2205 DDS. 2. Material and methods 2.1. Base material and the welding procedure A commercial SAF 2205 DSS with chemical composition of Fe-22.31Cr-5.48Ni-3.34Mo1.42Mn-0.03C-0.023P-0.005S (wt. %) was used. The dimensions of the samples used for the study was 300mm×100mm×2mm. The FSW process was conducted along the rolling direction of the samples. The welding tool was designed from a WC-based material with simple
3
cylindrical pin geometry. The diameters of the shoulder, pin, and the length of the pin were 16 mm, 5 mm, and 1.5 mm, respectively. The tilt angle of the welding tool with respect to the normal direction of the plate was 3º. Ar shielding gas with a flow rate of 18 L/min was used to avoid the formation of any surface oxides. Fifteen welds were made with rotational speeds of 400, 600, and 800 rpm and welding speeds ranging 50-350 mm/min. The insertion depth of tool pin in to the samples was kept constant at 1.7 mm during welding. The normal force to the plate surface was controlled at 14 kN. 2.2. Weld thermal history K-type thermocouples with the diameter of 2 mm were used to record the thermal cycles from beneath the samples in the centerline of the welds. Thermocouples were located in the predesigned holes with the diameter of 2.5 mm in the carbon steel backing plate. A copper paste was used to fix the located thermocouples in the holes so as not to be disconnected while welding procedure. 2.3. Macro and microstructure studies Visual inspection along with the X-ray radiography tests were carried out on the trial joints to reveal the existence of any weld defect developed on the surface and/or inside of the joints. The microstructure of the resultant joints were mainly observed and characterized using an electron back-scattered diffraction analysis (EBSD). EBSD samples were prepared by mechanical grounding, then pre-polishing with 1 and 0.25 µm diamond pastes and finally electro-polishing with 700 mL ethanol, 120 mL distilled water, 100 mL glycerol, and 80 mL perchloric acid at ambient temperature with a voltage of 35 V for 10 s.
4
2.4. Mechanical evaluation of the weldments Hardness measurements were performed on the cross-section of the welds normal to the welding direction along the centerline parallel with transverse direction (TD) using a 4.9 N load for 10 s. Longitudinal tensile specimens were prepared from the joints in accordance with the ASTM E8 standard. Tensile tests were conducted at ambient temperature with a crosshead rate of 0.05 mm/s. 3. Results and discussion 3.1. Welds appearance Figure 1 shows the process window of FS welded DSS samples along with the X-ray radiography results. The dashed line on the figure indicates the conditions at which sound welds are obtained. Unsuccessful welding conditions are related to pin fracture and groove-like defects. These kinds of defects can deteriorate the mechanical properties of the joints, such as ultimate tensile strength and elongation percentage. These defects happen when the welding parameter is not associated with enough heat input to flow the material easily and smoothly which may leave the generated gap by the stirring pin partially filled or unfilled [13,14].
5
Figure 1. Process window and radiography results of FS welded specimens indicating the defect free welds.
3.2. Thermal history of the resultant welds Results of temperature measurements obtained from the sound welds are depicted in Fig. 2. Temperature measurements from beneath the pin showed that the maximum temperatures are ranging 400 – 953 ºC wherein, the highest value is related to the sample welded at 50 mm/min of welding speed and 800 rpm of rotational speed. It can be seen that maximum temperature increases with increasing the rotational speed and/or with decreasing the welding speed [5].
6
Figure 2. Welds thermal histories: a) constant rotational speed of 800 rpm, b) constant welding speed of 50 mm/min.
3.3. Microstructure evaluation using EBSD 3.3.1. The microstructure of SZ at different welding conditions Fig. 3 indicates the microstructure of resultant SZs under different welding and rotational speeds. It is apparent from the figure that the FSW process has developed fine equiaxed microstructures in the SZs. Concurrence of elevated temperature and severe plastic straining in the SZ stimulated the occurrence of the dynamic recrystallization through which fine and equiaxed grains are formed in the microstructure of the processed material [1-3,5,12]. Statistical analysis of the phase fraction of the resultant SZs showed that the FSW process effectively preserved the desired ratio 7
(1:1) between the constituent phases of ferrite and austenite in the center of welded samples. These microstructural observations are supplementary evidences for the temperature measurements and approve that the maximum temperature never exceed the transformation temperature of γ to α. Previous studies [15,16] have showed that the desirable ratio of the constituent phases (1:1) in a SAF 2205 duplex stainless steel changes when the temperature of the material increases more than 1100 ºC. Therefore, the trivial changes in phase fraction indicated that the peak temperature of the welded material never exceeded 1100 ºC.
Figure 3.Phase maps from the base metal and the center of the resultant SZs.
Fig. 4 depicts the distribution of the average grain size in the center of the resultant SZs. It is seen that the average grain size of the developed microstructure is decreased when the welding speed is increased and/or the rotational speed is decreased. It can be seen that the smallest grains developed through the SZ microstructure of the sample welded with welding and rotational
8
speeds of 150 mm/min and 400 rpm, respectively (0.48 µm and 0.42 µm in ferrite and austenite, respectively), and the largest grains formed in the SZ of sample welded with 50 mm/min and 800 rpm (1.55 µm and 1.39 µm in ferrite and austenite, respectively). Based on the temperature measurements, higher ratios of rotational speed (ω)/ welding speed (υ) increased the amount of heat input and the peak temperature and decreased the cooling rate during the weld thermal cycles. It is well accepted that the kinetics of the recrystallization mainly depends on the time and the temperature. When the cooling time and the temperature are increased, grain growth can also takes place beside the recrystallization [17].
Figure 4. Grain size variation in the resultant SZs: a) austenite, and b) ferrite.
3.4. Textural characterization using EBSD 3.4.1. BM Figs. 5 and 6 represent the texture of the BM using pole figures and the orientation distribution functions (ODFs). The texture analysis shows that the ferrite phase is composed of H 9
{001}<110>, I {211}<011>, and K {110}<110> textural components which are considered as common rolling texture components in BCC materials [18, 19].
= 45° ODF section (Fig. 5a)
shows that the ferrite phase is strongly textured near the {001}<110> and {211}<011> which is parallel with α fiber. Regarding the fact that the occurrence of recrystallization in BCC structured materials decreases the intensity of the textural components specially those ranging between the {001}<110> and {211}<011>, therefore, the texture of ferrite phase in the BM is predominantly deformational. But for the austenite,
= 0° ODF section (Fig. 6a) shows that the texture is
composed of the brass (B) ({110}<112>), copper (C) ({112}<111>), Goss (G) ({110}<001>), and the cube ({001}<100>) components. The texture components of B, C, and G are related to the rolling texture whereas the cubic one is related to common component of recrystallization texture of the FCC structured materials [17,18]. It is seen that the intensity of the texture in ferrite phase is effectively higher than that of the austenite. High stacking fault energy (SFE) of ferrite stimulates the occurrence of dynamic recovery which tends to preserve the deformation texture. But in the austenite, low SFE hinder the occurrence of recovery process and facilitate the recrystallization process which decreases the intensity of the deformation texture [17,20].
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Figure 5. = ° ODF section and 100 pole figure: a) obtained from the investigated BM, and b) standard = ° ODF section and 100 pole figureindicating the position of some ideal deformational orientations in BCC materials [17].
Figure 6. = ° ODF section and 100 pole figure: a) obtained from the investigated BM, and b) a three dimensional Euler space and 111 pole figure indicating the position of some ideal deformational orientations in FCC materials [17].
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3.4.2. SZs Figs. 7 and 8 depict the ferrite 110 and austenite 111 pole figures obtained from the center of the resultant SZs. It is to be mentioned that these pole figures with simple shear coordinate system obtained after proper rotations on common welding coordinate system. The conversion of the coordinate systems has been completely described in the previous work by Emami et al. [12]. It is seen that the stirring action of FSW tool has developed a simple shear texture in both constituent phases in the SZ of the welded samples. During the FSW, individual grains tend to be rotated along with the direction of applied strain. This kind of adaptation occurs through the activation of easy slip systems. Slip usually tends to occur along the plane with shortest Burgers vector which is available in the most close-packed planes. Therefore, 111 planes as the most closely packed planes of the FCC structured materials facilitate the dislocation movements and finally the plastic deformation in the austenite. In the BCC structured materials, however there is not any truly close-packed plane, and so, slip tends to occur on the most densely packed 110 plane [18]. The results shows that the intensity of the resultant pole figures is increased with increasing the rotational speed and/or decreasing the welding speed. It is also seen that simple shear texture are gradually disappeared and replaced by a complex texture with increasing the welding speed or with decreasing the rotational speed. Therefore, the formation of simple shear texture in the SZs is mainly related to the stirring action of the tool pin. Since the intensity of the resultant texture is directly related to the amount of applied deformation, thus it can be inferred from the intensities that the amount of applied strain is decreased with decreasing the rotational speed and/or with increasing the welding speed. This behavior also implies that the contribution of stirring action of the tool pin in flow of the material decreases with decreasing the rotational speed and or with increasing the welding speed.
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Figure 7. Ferrite 110 pole figures obtained from the center of the SZs.
Figure 8. Austenite 110 pole figures obtained from the center of the SZs.
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3.5. Mechanical properties Fig. 9 indicates the influence of welding and rotational speeds on the hardness and the ultimate strength of the produced joints. The results of hardness measurements showed that the hardness values for the SZs were higher than that of the base materials (260 Hv). This behavior is contributed to the generation of dislocations, development of the LAGBs and the reduction of the grain size in the SZs [5]. Grain boundaries are considered as effective barriers against the movement of the dislocations [17]. Therefore, fine microstructures of the SZs with high density of dislocations along with high fraction of HAGBs and LAGBs show higher values of hardness with respect to the BM [5]. It is also apparent from the figure that the hardness and the tensile strength of the joints improve with increasing the welding speed and/or decreasing the rotational speed. This finding are in a good agreement with those of the previous studies which have been conducted on the carbon steels [21,22] and austenitic stainless steel [23]. As it mentioned previously, lower rotational speeds and/or higher welding speeds result in the formation of fine microstructures in the SZ which improves the mechanical properties of the joints.
Figure 9. Influence of welding and rotational speeds on the mechanical properties of the SZs.
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4. Conclusions
Influence of frictions stir welding parameters on the microstructure and the texture of SAF 2205 duplex stainless steels was investigated. The grain size of the resultant microstructure in the SZs is decreased by increasing the welding speed and/or with decreasing the rotational speed. Simple shear texture has been developed in the SZs due to the strain applied by the rotating welding tool. Decreasing the texture intensities with increasing the welding speed implied that the formation of the simple shear is mainly contributed to the stirring action of the pin. References
[1] L.E. Murr, “Handbook of materials structures, properties, processing and performance”, Springer, Switzerland, 2015. [2] R.S. Mishra, Z.Y. Ma, Friction stir welding and processing, Materials Science and Engineering R 50 (2005) 1–78. [3] T. Saeid, A. Abdollah-zadeha, T. Shibayanagi, K. Ikeuchi, H. Assadi, On the formation of grain structure during friction stir welding of duplex stainless steel, Materials Science and Engineering A 527 (2010) 6484–6488. [4] K. Okamoto, S. Hirano, I. Masahisa, Metallurgical and mechanical properties of friction stir welded stainless steels, Proceedings of the 4th International FSW Symposium,Park City, UT, USA, 2003 [5] T. Saeid, A. Abdollah-zadeh, H. Assadi, F. Malek Ghaini, Effect of friction stir welding speed on the microstructure and mechanicalproperties of a duplex stainless steel, Materials Science and Engineering A 496 (2008) 262–268.
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[6] T.F.A. Santos,E.A. Torres, A.J. Ramirez, Friction stir welding of duplex stainless steels, Welding International, 21 (1) (2016) 59–69. [7] T.F.A. Santos, H.S. Idagawa, A.J. Ramirez, Thermal history in UNS S32205 duplex stainless steel friction stir welds, Science and Technology of Welding and Joining 19 (2014) 150-156. [8] F.C. Liu, T.W. Nelson, In-situ grain structure and texture evolution during friction stir welding of austenite stainless steel, Materials & Design 115 (2017) 467–478. [9] Y.S. Sato, T.W. Nelson, C.J. Sterling, R.J. Steel, C.O. Pettersson, Microstructure and mechanical properties of friction stir welded SAF 2507 super duplex stainless steel, Materials Science and Engineering A 397 (2005) 376–384. [10] T.F.A. Santos, E.A. Torres, J.C. Lippold, and A.J. Ramirez, Detailed Microstructural Characterization and Restoration Mechanisms of Duplex and Super duplex Stainless Steel Friction-Stir-Welded Joints, Journal of Materials Engineering and Performance 25 (2016) 51735188. [11] T.F.A Santos, E.A.T. López, E.B. Fonseca, A.J. Ramirez, Friction stir welding of duplex and superduplex stainless steels and some aspects of microstructural characterization and mechanical performance Materials Research 19(1) (2016) 117-131. [12] S. Emami, T. Saeid, R. Azari Khosroshahi, Microstructural evolution of friction stir welded SAF 2205 duplexstainless steel, Journal of Alloys and Compounds 739 (2018) 678-689. [13] H.B. Chen, K. Yan, T. Lin, S.B. Chen, C.Y. Jiang, Y. Zhao, The investigation oftypical welding defects for 5456 aluminum alloy friction stir welds, MaterialsScience and Engineering A 433 (2006) 64–69. 16
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Highlights 1. Defect-free FS welds (50–350 mm/min and 400–800 rpm) of 2205 DSS were obtained. 2. Effect of FSW parameters was studied on the texture and microstructure of the SZs.