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Investigation on mechanical properties and microstructure of Ti-5Al-5V-5Mo-1Cr-1Fe Titanium alloy butt welded EBW joints
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ScienceDirect Materials Today: Proceedings 5 (2018) 28061–28070
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Investigation on mechanical properties and microstructure of Ti-5Al-5V-5Mo-1Cr-1Fe Titanium alloy butt welded EBW joints Manjeet Rania,*, R. K. Guptab, Sharad K. Pradhana, V. Anil Kumarb, Rishi Gaurc a
National Institute of Technical Teachers’ Training & Research, Bhopal,462002, India b Vikram Sarabhai Space Centre (ISRO), Trivandrum, 695022, India c National Institute of Technical Teachers’ Training & Research, Chandigarh, 160019, India
Abstract Welding of annealed plates with thickness 8.5 mm of metastable β Ti-55511 alloys has been carried out through Electron beam Welding (EBW) technique in this research effort. Weldment has been characterized by microscopy, microhardness and tensile test at room temperature with controlled strain rates (i.e. 0.01s-1, 0.001s-1and 0.0001s-1). The Fusion zone (FZ) found is ‘crown shaped’ or ‘hour-glass shaped’. The microstructure of FZ, Heat Affected Zone (HAZ) and Parent Material (PM) consists of long columnar β-grains, coarse equiaxed β-grains and near globular/equiaxed α in transformed β matrix respectively. Lower UTS, lower percentage elongation and lower microhardness are observed in EB weldments as compared to PM (annealed). Minimum 80% weld efficiency has been noticed in welded condition. The fracture surface of PM shows flute as well as fine dimples which confirm the ductile fracture while ‘as-welded’ fracture shows facets and ridges with very fine dimples and micro-pores indicating relatively brittle nature. © 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International Conference on Composite Materials: Manufacturing, Experimental Techniques, Modeling and Simulation (ICCMMEMS-2018). Keywords: Ti-5Al-5V-5Mo-1Cr-1-Fe Ti alloy; EBW; PM; FZ; HAZ
This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. * Corresponding author. Tel.: +919412608558 E-mail address: [email protected] 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International Conference on Composite Materials: Manufacturing, Experimental Techniques, Modeling and Simulation (ICCMMEMS-2018).
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Nomenclature AR As-Received AW As-Welded 1. Introduction Table. 1. Chemical composition of Ti-55511 alloy (mass fraction, %) used for EB welding studies [22]. Al
Mo
V
Cr
Fe
C
4.5-5.9 Si <0.15
4.0-5.5 Zr <0.30
4.0-5.5 O2 <0.20
0.5-2.0 N2 <0.05
0.5-1.5 H2 <0.015
<0.15 Ti 79.4-86.3
Impurities 0.3
Metastable β-Titanium alloy Ti-5Al-5V-5Mo-1Cr-1Fe (Ti-55511) is a high strength Titanium alloy. Chemical composition of the used alloy is shown in Table 1. It has attractive features like high strength-to-density ratio, good hardenability, substantial ductility and excellent crack propagation resistance and hence is suitable for aerospace and automotive industries. The alloy has been used for landing gear components, detachable joint components, frames, runways and wing slides of aircraft [1-4]. Fabrication plays an important role and fusion welding is mostly used in the production of complex structures and assemblies with a high material utilization factor. But in case of welding of titanium alloys, one should consider the effects of thermal cycle which comes into picture during welding and its effects on the phase of the weld zone and on the mechanical properties as well [5]. Moreover, being a reactive material additional precautions are required during the welding of Titanium. The welded joints of Titanium alloys by traditional fusion welding methods like argon-shielded tungsten electrode arc welding [5-7] with filler wire SPT-2 [8], advanced welding techniques like friction stir welding [9], Electron Beam Welding (EBW) [6-15], and Laser Beam Welding [6,16] with conduction regime [17] have already been reported. Among all, EBW is strongly used because of its capability of producing narrow weld zone, deep penetration, complete weld in a single pass, very high welding speed, rapid and precise processing capabilities. Through EBW, contamination free and highly consistent weld can be obtained [6-12]. It is well known that, in the Titanium and its alloys (except metastable β-titanium alloys), weldments have mechanical properties equivalent to parent or base metal when welded through EBW. The reason of limited weldability of metastable β-titanium alloys may be due to the higher volume content of β-stabilizer elements (i.e. Mo, V, Cr, Si, Fe etc.) [18,19]. Most of the existing investigations focus on the weld pool shape, size, microstructure and mechanical properties of β-Ti alloy weldments. Schwab et al. [5] conducted TIG welding on 8 mm thick VT22 Ti-alloy plates using an external control magnetic field which resulted to properly shaped weld with improved quality. Zamkov et al. [8] study showed low ductility in the weld zone (HAZ and FZ) during TIG welding of VT22 Ti-alloy with filler wire SPT-2. X-shaped weld symmetry was obtained in FSW whereas in EBW an ‘Hourglass’ shape was obtained with 4mm FZ top and 2mm bottom surface width. Pasang et al. [6] studied and compared the weldability of Ti-5Al-5V-5Mo-3Cr (Ti-5553) Ti-alloy joints using EBW, GTAW and LBW. Results showed that the FZ consists of long columnar β-grains and HAZ with retained equiaxed β-phase and minimum FZ width (≈1.7mm) with EBW. Fracture takes place in weld zone due to lower micro-hardness and lower strength as compared to base metal. FZ of EBW and LBW was of hour-glass shaped while V-shaped in case of GTAW. Similar microstructure and mechanical properties were also reported by Liu et al. [16] and Maria et al. [17]. Sabol et al. [20] confirmed the presence of ω-phase during EBW of Ti-5553 alloy by TEM. Becker et al. [15] showed that the metastable β-Ti alloys (Ti-8V-4Cr-2Mo-2Fe-3Al, Ti-15V-3Cr-3Al-3Sn and Ti-8V-7Cr-3Al-4Sn-1Zr) are weldable by EBW and also studied the mechanical properties and microstructure after heat treatment. Baeslack et al. [21] confirmed the epitaxial growth from HAZ to FZ with columnar dendritic β-grains in TIG welding. From the reported literature it can be generalized that the β-Ti alloys or metastable β-Ti alloys are readily weldable using advanced welding techniques. Numbers of investigations have been reported on the welding of a β-Ti alloy which revealed the microstructure and mechanical properties of the weldments during different welding processes. From the literature it is found that the EBW study of the particularly Ti-55511 β-Ti alloy is still an area of research. Hence an attempt has been made
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to investigate the weldability, microstructure and mechanical properties of the EB weldments of the selected Tialloy. In this paper, microstructure and mechanical properties of β-Titanium alloy Ti-5Al-5V-5Mo-1Cr-1Fe, designated as Ti-55511 have been investigated in ‘As-Received’ and ‘As-Welded’ conditions. 2.
Experimental Setup
In the present work, annealed plates of Ti-55511 alloy have been used. EBW parameters are optimized by the bead-on-plate welding trials (with varying current, voltage and welding speed) on 8.5 mm thick Ti-55511 alloy plates. Radiography test is conducted to confirm the absence of weld defects, lack of fusion and porosity. Welding coupons before welding and in ‘As-Welded’ conditions are shown in Figure 1. After cleaning of EB welded samples in an aqueous solution of 3% HF+35% HNO3, EB welding was performed with the help of 60 kV, EO Paton-make, model KL-134 EB welding machine at a vacuum level of 2.1 x 10-4 Torr. The selected weld parameters are shown in Table 2.
a
b
Fig. 1. Weld coupons (a) before welding ‘as-received’; (b) ‘as-welded’ Table. 2. EBW parameters used for welding Ti-55511 alloy. Current/mA (I)
Voltage/kV (V)
Speed/mm*s-1 (v)
Oscillation/mm
Focus/IF
(circular) 125-132
60
25-32
0.2
Q = ηVI/v J/mm
532
257
EB butt welded coupons are examined through digital X-radiography to identify the presence of weld defects (i.e. lack of fusion, porosity, lower beam penetration etc.). X-radiography is conducted according to ASTM-2680 and to conduct the tensile test, rectangular cross section tensile specimens are prepared according to the ASTME8M standard (as depicted through Figure 2) using conventional milling machine from annealed (‘As-Received’) condition and ‘As-Welded’ condition. The weld position is perpendicular to the loading and at the centre of the
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gauge length (25 mm). Tensile testing at room temperature is conducted at different strain rates (0.01, 0.001 and 0.0001s-1) on INSTRON 4602 Universal Testing Machine (UTM). Prepared and fractured tensile specimens are shown in Figure 3.
Fig. 2. Schematic of tensile test specimens (as per ASTM E8M) used for testing.
a
b Fig. 3. a) Prepared tensile specimen; b) Fractured tensile specimen.
Optical Microscopy is done to observe the microstructure and properties of ‘As-Received’ and ‘As-Welded’ condition through OLYMPUS make GX71 optical microscope. After sample preparation by metallographic technique, etching is done using Kroll’s reagent (100ml H2O + 6ml HNO3 + 3ml HF) for 10 seconds to reveal the microstructure. Microhardness across the weldments is measured at an interval of 0.5 mm and at two level (2 mm apart from the centre line on both sides) from the base metal, HAZ and fusion zone are measured using Wilson automated digital microhardness tester having a load range of 10g-30kg with a magnification level of 30X-2000X. Fracture analysis of tensile tested specimens is carried out using Environmental Scanning Electron Microscope (ESEM) of Carl Zeiss EVO 50 series model attached with INCA EDS. 3.
Results and Discussion
In the present study, the alloy Ti-55511 has been characterized in two conditions viz. ‘As-Received’ and ‘AsWelded’. Ti-55511 alloy has been characterized using different characterization techniques and results have been discussed in this section. 3.1. Microstructural Examination Figure 4 shows optical micrographs of the different zone of weldments of Ti-55511 alloys welded with EBW. PM microstructure of Ti-55511 alloy is shown in Figure 4(a), which consists of near globular/equiaxed alpha (α) (grain size 4 μm – 8 μm) in fine transformed beta (β) matrix. There is no evidence of any grain boundary α. Same has been observed by Kofstad [23] and Ankem et al. reported [24] that the presence of equiaxed alpha phase with enough amount leads to the good ductility of the alloy and small β-grains helps in increasing the crack nucleation resistance. Weld Parent interface or HAZ is depicted in Figure 4(b), which shows the fine transformed β matrix along with coarser β matrix and Figure 4 (c-e) shows the fusion zone (FZ) microstructure which is composed of large columnar dendrites within the β-grains. The shape of FZ is of “crown-shaped” [17] or “hour-glass-shaped” [6,
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16] with widening weld pool area at the top and bottom surface as compared to the mid-surface. This behaviour is due to the existence of Marangoni convective currents which helps to move the molten metal from the heat source area which is also in good agreement with the earlier reported literature [25]. Moreover, HAZ constituted recrystallized equiaxed β-grains. Dendrites arm length and width is reported for this particular weld with maximum dendrite arm length of 1000 μm at bottom of the weld and 296 μm & 111 μm with at head & tail respectively. At the top, dendrite arm length were 1600 μm with a width of head and tail 228 μm and 57 μm respectively. The larger dendritic arms with irregular shape and size near the weld centre line at the top and bottom indicated the peak temperature during welding that helps grain growth. The weld bead width at top and bottom surface is observed about 3.85mm and 3.5mm respectively. Minimum weld width above bottom surface observed is of 2 mm approximately. In Figure 4 (d, e), weld profile micrograph of EBW shows grain growth mode type from FZ to HAZ is epitaxial and it is observed that the epitaxial mode of solidification is limited near the boundary region of weld fusion and
Near globular/equiaxed α HAZ
Fine transformed β
a
b
3.85mm
c
d
e
Fig. 4. EB Welded microstructure (a) parent material; (b) parent weld interface region; (c-e) weld zone; (c) top; (d) mid; and (e) bottom of the weld pool.
FZ
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HAZ due to relatively fast cooling of melt in the EB weldments. A remarkable change has been noted in the microstructure of FZ and PM/BM (Base Metal) zone. The disappearance of the globular α in FZ leads to the brittle nature of the weldments. From the micrographs, it can be observed that the grain size is gradually decreasing from the fusion boundary to PM and grain growth from FZ to HAZ. In Figure 4 (b), PM is dark as compared to HAZ and FZ. 3.2. Mechanical Properties Mechanical properties (Microhardness, UTS, YS and %El) of ‘As-Received’ and ‘As-Welded’ specimens have been tested. Microhardness values of PM, weld-parent interface and weld FZ are shown in Figure.5. Measurement of microhardness has been taken across the EB weldment. Microhardness values of weld FZ and HAZ are lesser as compared to parent metal zone. It is observed from plot shown in Figure.5 that, the weld (HAZ and FZ) is having the least value of microhardness in the range of 321-339 HV which reveals coarse grains of β formed during welding and the parent metal is having microhardness value in the range of 373-389 HV. From the Figure 5, it has been observed that lower microhardness is 2 mm away from the weld centre line on the top side (i.e. HAZ), and on the bottom side lower hardness is in the weld zone or FZ and HAZ of the weldments. It may be noted that the most probable failure location will be this boundary region due to lower hardness and it shows good agreement with the reported literature [10]. The difference in the microhardness curve of the top and the bottom region is due to the weld profile width at top and bottom weld width (Figure 4c and 4e).The trend which is followed by the microhardness curve may result in the variation of β-grain size in the FZ and HAZ across the EB welded specimens [26].
Fig. 5. Microhardness plot for EB welded specimens at top and bottom locations.
Tensile testing has been done at room temperature for both the conditions at different strain rates. Engineering stress-strain curves for ‘As-Received’ specimen tested at strain rate 0.01s-1 is shown in Figure 6, which indicates ductile behaviour with UTS of 1288 MPa, YS of 1260 MPa (0.2% Proof Stress) and with an elongation of 10.7 %. The alloy exhibited similar behaviour showing an excellent combination of UTS, YS with optimum ductility. The
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Table. 3. Mechanical properties of ‘as-received’ and EB welded specimens. Processing History
Strain Rate
0.2% YS MPa
UTS MPa
% El
Annealed condition (AR)
10-2
1260
1288
10.7
As-Welded condition (AW)
10-2
--
1094
1.3
10-3
--
1049
1.3
10-4
--
1037
1.2
results of tensile testing in ‘As-Welded’ conditions of Ti-55511 EBW butt welds are shown in Table 3. Significant reduction in tensile strength and elongation values is observed in ‘as-welded’ as compared to the annealed condition. A similar trend has been observed in EB weldments by the other researchers for different alloys [10,27]. Figure 7 shows the engineering stress-strain diagrams for ‘As-Welded’ specimens and the transverse tensile test was conducted at room temperature at different strain rates with the weld located in the transverse direction of the applied load. EB weld limits the specimen’s elongation which may be due to the microstructural changes of alloy in the FZ and HAZ. ‘As-Welded’ condition of present alloy shows the minimum joint efficiency of around 80% and a a significant reduction in ductility compared to the ‘As-Received’ condition.
Fig. 6. Engineering stress-strain diagram of ‘As-Received condition at strain rate of 0.01 s-1.
Fig. 7. Engineering stress-strain diagram ‘As-Welded’ condition at different strain rates.
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3.3. Fractographs The fracture surface is observed to understand the failure mechanism during the tensile testing of ‘As-Received’ and ‘As-Welded’ specimens. Fractograph of the tensile specimen in ‘As-Received’ condition is shown in Figure 8 reveals flute dimples as well as fine dimples along the direction of loading the specimen. The fracture is predominantly ductile. Fractographs of Ti-55511 alloy in ‘As-Welded’ condition are shown in Figure 9, which shows the formation of facets giving information about the lower ductility of the FZ and HAZ or it shows the fracture mode as relatively brittle (Figure 9). It may be attributed to the change in the microstructure of weld zone which affects the tensile properties and fracture phenomenon critically.
Flute Dimples
Fine Dimples
Fig. 8. Representative Fractograph of alloy for ‘As-Received’ condition.
a
b
c
d
Fig. 9. Representative Fractographs of alloy for ‘As-Welded’ condition.
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Conclusion
According to experimental results and discussions in ‘As-Received’ and ‘As-Welded’ condition of metastable β Ti-55511 alloy, following conclusions are drawn. 1. 2.
3. 4. 5.
Microstructure in ‘As-Received’ condition consists of near globular/equiaxed α (grain size 4 μm – 8 μm) in the transformed β matrix. The tensile testing reveals high 1288 MPa (UTS), 1260 MPa (0.2% YS) with 10.77 %El and microhardness values vary from 373 HV to 389 HV. The microstructure of EB butt welded joint consists of long columnar β-grains and coarse equiaxed β grains in FZ and HAZ respectively which leads to lower strength and ductility in the weld zone. Epitaxial growth is observed from FZ to HAZ with long irregular shape columnar dendrites with maximum arm length 1600 μm at the top and 1000 μm at the bottom. The shape of FZ is looked like “crown-shaped” or “hour-glass-shaped”. EB butt welded joint exhibits lower microhardness and tensile strength with a drastic reduction in ductility as compared to ‘As-Received’ condition and minimum 80% welding efficiency has been obtained. EB welded joint shows increment in UTS (1037-1094 MPa) with a small change in ductility (1.2-1.3%El) with an increase in strain rate from 0.0001s-1 to 0.01s-1. By fracture analysis, transition from ductile (i.e. Flute and fine dimples) behaviour in ‘As-Received’ to brittle like (i.e. cleavage facets and ridges) behaviour in ‘As-Welded’ condition is observed. Fracture location in ‘AsWelded’ specimen is observed in the weld zone near to fusion line and this statement can be supported by the lower microhardness, UTS and ductility.
Acknowledgement Authors are thankful to DD, MME, VSSC for providing support during this work and Director, VSSC for kind permission to publish the work. References [1] O.M. Ivasishin, P.E. Markovsky, Yu. V. Matviychuk, and S.L. Semiatin, Metallurgical and Materials Transactions A, 34A (2003) 147-158. [2] V. Anil Kumar, S.V.S.N. Murthy, R.K. Gupta, R. R. Babu, and M.J.N.V. Prasad, Trans Indian Inst Met (2015) 68 (Suppl 2): S207-S215. [3] O.M.Ivasyshyn, P.E. Markov Kyi, and I.M. Havrysh, Materials Science, 51(9), ( 2015) 158-164. [4] Aman Kaur, Colin Ribton, and W. Balachandran, Journal of Materials Processing Technology, 221, (2015) 225-232. [5] S.L. Schwab, I.K. Petrychenko, and S.V. Akhonin, Biuletyn Instytutu Spawalnictwa, 6 (2017). [6] T. Pasang, J.M. Sanchez Amaya, Y. Tao, M.R. Amaya-Vazquez, F.J. Botana, J.C. Sabol, W.Z. Misiolek, and O. Kamiya, The Manufacturing Engineering Society International Conference, MESIC 2013, Procedia Engineering 63 (2013) 397-404. [7] K. Balachander, V. Subramanya Sharma, Bhanu Pant, and G. Phanikumar, The Minerals, Metals & Materials Society and ASM International 2009, Metallurgical And Materials Transactions A, Volume 40A(11), (2009) 2685-2693. [8] V.N. Zamkov, S.L. Antonyuk, and A.G. Molyar, Metallic Materials with High Structural Efficiency, 2004 Kluwer Academic Publishers, Netherlands, 269-278. [9] R. Pederson, F. Niklasson, F. Skystedt, and R. Warren, Material Science and Engineering A 552, (2012) 555-565. [10] V. Anil Kumar, R.K. Gupta, Sushant K. Manwatkar, P. Ramkumar, and P.V. Venkitakrishnan, Journal of Materials Engineering and Performance, 25(6), (2016), 2147-2156. [11] G. Wang, Z. Chen, J. Li, J. Liu, Q. Wang, and R. Wang, Journals of Material Science and Technology (2016), http://dx.doi.org/doi:10.1016/j.jmst.2016.10.007. [12] W. F. Chambers and R.L. Apps, Cranfield Report Material 7(6), (1972). [13] HU Mei-Juan, and LIU Jin-he, Transactions of Nonferrous Metals of Society China 19 (2009) 324-329. [14] F. Gao, Q. Gao, P. Jiang, Z. Liu, and Z. Liao, Vacuum (2017), doi: 10.1016/j.vacuum.2017.09.038. [15] Becker, D.W., Baeslack III, W.A., Welding Journal,59, Research Supplement (1980) 85s-92s. [16] Liu, P.S., Hou, K.H., Baeslack, W.A., III, J. Hurley, In The Minerals, Metals and Materials Society in Titanium-World Conference, Titanium ’92; Fores, F.H., Caplan, I., Eds., TMS: Pittsburgh, PA, USA, (1993) 1477. [17] Jose Maria Sanchez-Amaya, T. Pasang, M.R. Amaya-Vazquez, J.D.D. Lopez-Castro, C. Churiaque, Y. Tao and F.J.B. Pademonte, MPDI, Metals, article 7 (269),(2017), doi:10.3390/met070269. [18] Mathew J. Donachie, Jr. Titanium: A Technical Guide, ASM International, Second Edition (2000) 65-78. [19] M.A. Greenfield, and D.S. Duvall, Welding Research Supplement, 55th AWS Annual Meeting, Houston, Texas (1975) 73-80.
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[20] J.C. Sabol, C.J. Marvel, M. Watanabe, T. Pasang and W.Z. Misiolek, Science Direct, Scripta Materialia 92 (2014) 15-18. [21] Baeslack III, W.A, P.S., Liu, Barbies, D.P., Schley, J.R., and J.R. Wood, Titanium ’92, Science and Technology, TMS (1993) 1469-1476. [22] Valentin N. Moiseyev. Titanium Alloys, Russian Aircraft and Aerospace Applications, Taylor and Francis Group, (2006). [23] P. Kofstad, “High-Temperature Oxidation of Metals”, New York, 1996, 168. [24] S. Ankem and C.A. Greene, Material Science and Engineering, A263 (1999) 127-131. [25] R. Rai, P. Burgardt, J.O. Milewski, T.J.Lienert, T.DebRoy, Journal of Physics: Applied Physics, 42, (2), (2009) 1-12. [26] Zhang, Peng He, J. Feng and H. Wu, Elsevier, Material Science and Engineering, A425, (2006) 255-259. [27] R.K. Gupta and V. Anil Kumar, Journal of Materials Engineering Performance, 26, (2017).
ScienceDirect Materials Today: Proceedings 5 (2018) 28061–28070
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ICCMMEMS_2018
Investigation on mechanical properties and microstructure of Ti-5Al-5V-5Mo-1Cr-1Fe Titanium alloy butt welded EBW joints Manjeet Rania,*, R. K. Guptab, Sharad K. Pradhana, V. Anil Kumarb, Rishi Gaurc a
National Institute of Technical Teachers’ Training & Research, Bhopal,462002, India b Vikram Sarabhai Space Centre (ISRO), Trivandrum, 695022, India c National Institute of Technical Teachers’ Training & Research, Chandigarh, 160019, India
Abstract Welding of annealed plates with thickness 8.5 mm of metastable β Ti-55511 alloys has been carried out through Electron beam Welding (EBW) technique in this research effort. Weldment has been characterized by microscopy, microhardness and tensile test at room temperature with controlled strain rates (i.e. 0.01s-1, 0.001s-1and 0.0001s-1). The Fusion zone (FZ) found is ‘crown shaped’ or ‘hour-glass shaped’. The microstructure of FZ, Heat Affected Zone (HAZ) and Parent Material (PM) consists of long columnar β-grains, coarse equiaxed β-grains and near globular/equiaxed α in transformed β matrix respectively. Lower UTS, lower percentage elongation and lower microhardness are observed in EB weldments as compared to PM (annealed). Minimum 80% weld efficiency has been noticed in welded condition. The fracture surface of PM shows flute as well as fine dimples which confirm the ductile fracture while ‘as-welded’ fracture shows facets and ridges with very fine dimples and micro-pores indicating relatively brittle nature. © 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International Conference on Composite Materials: Manufacturing, Experimental Techniques, Modeling and Simulation (ICCMMEMS-2018). Keywords: Ti-5Al-5V-5Mo-1Cr-1-Fe Ti alloy; EBW; PM; FZ; HAZ
This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. * Corresponding author. Tel.: +919412608558 E-mail address: [email protected] 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International Conference on Composite Materials: Manufacturing, Experimental Techniques, Modeling and Simulation (ICCMMEMS-2018).
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Nomenclature AR As-Received AW As-Welded 1. Introduction Table. 1. Chemical composition of Ti-55511 alloy (mass fraction, %) used for EB welding studies [22]. Al
Mo
V
Cr
Fe
C
4.5-5.9 Si <0.15
4.0-5.5 Zr <0.30
4.0-5.5 O2 <0.20
0.5-2.0 N2 <0.05
0.5-1.5 H2 <0.015
<0.15 Ti 79.4-86.3
Impurities 0.3
Metastable β-Titanium alloy Ti-5Al-5V-5Mo-1Cr-1Fe (Ti-55511) is a high strength Titanium alloy. Chemical composition of the used alloy is shown in Table 1. It has attractive features like high strength-to-density ratio, good hardenability, substantial ductility and excellent crack propagation resistance and hence is suitable for aerospace and automotive industries. The alloy has been used for landing gear components, detachable joint components, frames, runways and wing slides of aircraft [1-4]. Fabrication plays an important role and fusion welding is mostly used in the production of complex structures and assemblies with a high material utilization factor. But in case of welding of titanium alloys, one should consider the effects of thermal cycle which comes into picture during welding and its effects on the phase of the weld zone and on the mechanical properties as well [5]. Moreover, being a reactive material additional precautions are required during the welding of Titanium. The welded joints of Titanium alloys by traditional fusion welding methods like argon-shielded tungsten electrode arc welding [5-7] with filler wire SPT-2 [8], advanced welding techniques like friction stir welding [9], Electron Beam Welding (EBW) [6-15], and Laser Beam Welding [6,16] with conduction regime [17] have already been reported. Among all, EBW is strongly used because of its capability of producing narrow weld zone, deep penetration, complete weld in a single pass, very high welding speed, rapid and precise processing capabilities. Through EBW, contamination free and highly consistent weld can be obtained [6-12]. It is well known that, in the Titanium and its alloys (except metastable β-titanium alloys), weldments have mechanical properties equivalent to parent or base metal when welded through EBW. The reason of limited weldability of metastable β-titanium alloys may be due to the higher volume content of β-stabilizer elements (i.e. Mo, V, Cr, Si, Fe etc.) [18,19]. Most of the existing investigations focus on the weld pool shape, size, microstructure and mechanical properties of β-Ti alloy weldments. Schwab et al. [5] conducted TIG welding on 8 mm thick VT22 Ti-alloy plates using an external control magnetic field which resulted to properly shaped weld with improved quality. Zamkov et al. [8] study showed low ductility in the weld zone (HAZ and FZ) during TIG welding of VT22 Ti-alloy with filler wire SPT-2. X-shaped weld symmetry was obtained in FSW whereas in EBW an ‘Hourglass’ shape was obtained with 4mm FZ top and 2mm bottom surface width. Pasang et al. [6] studied and compared the weldability of Ti-5Al-5V-5Mo-3Cr (Ti-5553) Ti-alloy joints using EBW, GTAW and LBW. Results showed that the FZ consists of long columnar β-grains and HAZ with retained equiaxed β-phase and minimum FZ width (≈1.7mm) with EBW. Fracture takes place in weld zone due to lower micro-hardness and lower strength as compared to base metal. FZ of EBW and LBW was of hour-glass shaped while V-shaped in case of GTAW. Similar microstructure and mechanical properties were also reported by Liu et al. [16] and Maria et al. [17]. Sabol et al. [20] confirmed the presence of ω-phase during EBW of Ti-5553 alloy by TEM. Becker et al. [15] showed that the metastable β-Ti alloys (Ti-8V-4Cr-2Mo-2Fe-3Al, Ti-15V-3Cr-3Al-3Sn and Ti-8V-7Cr-3Al-4Sn-1Zr) are weldable by EBW and also studied the mechanical properties and microstructure after heat treatment. Baeslack et al. [21] confirmed the epitaxial growth from HAZ to FZ with columnar dendritic β-grains in TIG welding. From the reported literature it can be generalized that the β-Ti alloys or metastable β-Ti alloys are readily weldable using advanced welding techniques. Numbers of investigations have been reported on the welding of a β-Ti alloy which revealed the microstructure and mechanical properties of the weldments during different welding processes. From the literature it is found that the EBW study of the particularly Ti-55511 β-Ti alloy is still an area of research. Hence an attempt has been made
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to investigate the weldability, microstructure and mechanical properties of the EB weldments of the selected Tialloy. In this paper, microstructure and mechanical properties of β-Titanium alloy Ti-5Al-5V-5Mo-1Cr-1Fe, designated as Ti-55511 have been investigated in ‘As-Received’ and ‘As-Welded’ conditions. 2.
Experimental Setup
In the present work, annealed plates of Ti-55511 alloy have been used. EBW parameters are optimized by the bead-on-plate welding trials (with varying current, voltage and welding speed) on 8.5 mm thick Ti-55511 alloy plates. Radiography test is conducted to confirm the absence of weld defects, lack of fusion and porosity. Welding coupons before welding and in ‘As-Welded’ conditions are shown in Figure 1. After cleaning of EB welded samples in an aqueous solution of 3% HF+35% HNO3, EB welding was performed with the help of 60 kV, EO Paton-make, model KL-134 EB welding machine at a vacuum level of 2.1 x 10-4 Torr. The selected weld parameters are shown in Table 2.
a
b
Fig. 1. Weld coupons (a) before welding ‘as-received’; (b) ‘as-welded’ Table. 2. EBW parameters used for welding Ti-55511 alloy. Current/mA (I)
Voltage/kV (V)
Speed/mm*s-1 (v)
Oscillation/mm
Focus/IF
(circular) 125-132
60
25-32
0.2
Q = ηVI/v J/mm
532
257
EB butt welded coupons are examined through digital X-radiography to identify the presence of weld defects (i.e. lack of fusion, porosity, lower beam penetration etc.). X-radiography is conducted according to ASTM-2680 and to conduct the tensile test, rectangular cross section tensile specimens are prepared according to the ASTME8M standard (as depicted through Figure 2) using conventional milling machine from annealed (‘As-Received’) condition and ‘As-Welded’ condition. The weld position is perpendicular to the loading and at the centre of the
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gauge length (25 mm). Tensile testing at room temperature is conducted at different strain rates (0.01, 0.001 and 0.0001s-1) on INSTRON 4602 Universal Testing Machine (UTM). Prepared and fractured tensile specimens are shown in Figure 3.
Fig. 2. Schematic of tensile test specimens (as per ASTM E8M) used for testing.
a
b Fig. 3. a) Prepared tensile specimen; b) Fractured tensile specimen.
Optical Microscopy is done to observe the microstructure and properties of ‘As-Received’ and ‘As-Welded’ condition through OLYMPUS make GX71 optical microscope. After sample preparation by metallographic technique, etching is done using Kroll’s reagent (100ml H2O + 6ml HNO3 + 3ml HF) for 10 seconds to reveal the microstructure. Microhardness across the weldments is measured at an interval of 0.5 mm and at two level (2 mm apart from the centre line on both sides) from the base metal, HAZ and fusion zone are measured using Wilson automated digital microhardness tester having a load range of 10g-30kg with a magnification level of 30X-2000X. Fracture analysis of tensile tested specimens is carried out using Environmental Scanning Electron Microscope (ESEM) of Carl Zeiss EVO 50 series model attached with INCA EDS. 3.
Results and Discussion
In the present study, the alloy Ti-55511 has been characterized in two conditions viz. ‘As-Received’ and ‘AsWelded’. Ti-55511 alloy has been characterized using different characterization techniques and results have been discussed in this section. 3.1. Microstructural Examination Figure 4 shows optical micrographs of the different zone of weldments of Ti-55511 alloys welded with EBW. PM microstructure of Ti-55511 alloy is shown in Figure 4(a), which consists of near globular/equiaxed alpha (α) (grain size 4 μm – 8 μm) in fine transformed beta (β) matrix. There is no evidence of any grain boundary α. Same has been observed by Kofstad [23] and Ankem et al. reported [24] that the presence of equiaxed alpha phase with enough amount leads to the good ductility of the alloy and small β-grains helps in increasing the crack nucleation resistance. Weld Parent interface or HAZ is depicted in Figure 4(b), which shows the fine transformed β matrix along with coarser β matrix and Figure 4 (c-e) shows the fusion zone (FZ) microstructure which is composed of large columnar dendrites within the β-grains. The shape of FZ is of “crown-shaped” [17] or “hour-glass-shaped” [6,
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16] with widening weld pool area at the top and bottom surface as compared to the mid-surface. This behaviour is due to the existence of Marangoni convective currents which helps to move the molten metal from the heat source area which is also in good agreement with the earlier reported literature [25]. Moreover, HAZ constituted recrystallized equiaxed β-grains. Dendrites arm length and width is reported for this particular weld with maximum dendrite arm length of 1000 μm at bottom of the weld and 296 μm & 111 μm with at head & tail respectively. At the top, dendrite arm length were 1600 μm with a width of head and tail 228 μm and 57 μm respectively. The larger dendritic arms with irregular shape and size near the weld centre line at the top and bottom indicated the peak temperature during welding that helps grain growth. The weld bead width at top and bottom surface is observed about 3.85mm and 3.5mm respectively. Minimum weld width above bottom surface observed is of 2 mm approximately. In Figure 4 (d, e), weld profile micrograph of EBW shows grain growth mode type from FZ to HAZ is epitaxial and it is observed that the epitaxial mode of solidification is limited near the boundary region of weld fusion and
Near globular/equiaxed α HAZ
Fine transformed β
a
b
3.85mm
c
d
e
Fig. 4. EB Welded microstructure (a) parent material; (b) parent weld interface region; (c-e) weld zone; (c) top; (d) mid; and (e) bottom of the weld pool.
FZ
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HAZ due to relatively fast cooling of melt in the EB weldments. A remarkable change has been noted in the microstructure of FZ and PM/BM (Base Metal) zone. The disappearance of the globular α in FZ leads to the brittle nature of the weldments. From the micrographs, it can be observed that the grain size is gradually decreasing from the fusion boundary to PM and grain growth from FZ to HAZ. In Figure 4 (b), PM is dark as compared to HAZ and FZ. 3.2. Mechanical Properties Mechanical properties (Microhardness, UTS, YS and %El) of ‘As-Received’ and ‘As-Welded’ specimens have been tested. Microhardness values of PM, weld-parent interface and weld FZ are shown in Figure.5. Measurement of microhardness has been taken across the EB weldment. Microhardness values of weld FZ and HAZ are lesser as compared to parent metal zone. It is observed from plot shown in Figure.5 that, the weld (HAZ and FZ) is having the least value of microhardness in the range of 321-339 HV which reveals coarse grains of β formed during welding and the parent metal is having microhardness value in the range of 373-389 HV. From the Figure 5, it has been observed that lower microhardness is 2 mm away from the weld centre line on the top side (i.e. HAZ), and on the bottom side lower hardness is in the weld zone or FZ and HAZ of the weldments. It may be noted that the most probable failure location will be this boundary region due to lower hardness and it shows good agreement with the reported literature [10]. The difference in the microhardness curve of the top and the bottom region is due to the weld profile width at top and bottom weld width (Figure 4c and 4e).The trend which is followed by the microhardness curve may result in the variation of β-grain size in the FZ and HAZ across the EB welded specimens [26].
Fig. 5. Microhardness plot for EB welded specimens at top and bottom locations.
Tensile testing has been done at room temperature for both the conditions at different strain rates. Engineering stress-strain curves for ‘As-Received’ specimen tested at strain rate 0.01s-1 is shown in Figure 6, which indicates ductile behaviour with UTS of 1288 MPa, YS of 1260 MPa (0.2% Proof Stress) and with an elongation of 10.7 %. The alloy exhibited similar behaviour showing an excellent combination of UTS, YS with optimum ductility. The
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Table. 3. Mechanical properties of ‘as-received’ and EB welded specimens. Processing History
Strain Rate
0.2% YS MPa
UTS MPa
% El
Annealed condition (AR)
10-2
1260
1288
10.7
As-Welded condition (AW)
10-2
--
1094
1.3
10-3
--
1049
1.3
10-4
--
1037
1.2
results of tensile testing in ‘As-Welded’ conditions of Ti-55511 EBW butt welds are shown in Table 3. Significant reduction in tensile strength and elongation values is observed in ‘as-welded’ as compared to the annealed condition. A similar trend has been observed in EB weldments by the other researchers for different alloys [10,27]. Figure 7 shows the engineering stress-strain diagrams for ‘As-Welded’ specimens and the transverse tensile test was conducted at room temperature at different strain rates with the weld located in the transverse direction of the applied load. EB weld limits the specimen’s elongation which may be due to the microstructural changes of alloy in the FZ and HAZ. ‘As-Welded’ condition of present alloy shows the minimum joint efficiency of around 80% and a a significant reduction in ductility compared to the ‘As-Received’ condition.
Fig. 6. Engineering stress-strain diagram of ‘As-Received condition at strain rate of 0.01 s-1.
Fig. 7. Engineering stress-strain diagram ‘As-Welded’ condition at different strain rates.
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3.3. Fractographs The fracture surface is observed to understand the failure mechanism during the tensile testing of ‘As-Received’ and ‘As-Welded’ specimens. Fractograph of the tensile specimen in ‘As-Received’ condition is shown in Figure 8 reveals flute dimples as well as fine dimples along the direction of loading the specimen. The fracture is predominantly ductile. Fractographs of Ti-55511 alloy in ‘As-Welded’ condition are shown in Figure 9, which shows the formation of facets giving information about the lower ductility of the FZ and HAZ or it shows the fracture mode as relatively brittle (Figure 9). It may be attributed to the change in the microstructure of weld zone which affects the tensile properties and fracture phenomenon critically.
Flute Dimples
Fine Dimples
Fig. 8. Representative Fractograph of alloy for ‘As-Received’ condition.
a
b
c
d
Fig. 9. Representative Fractographs of alloy for ‘As-Welded’ condition.
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Conclusion
According to experimental results and discussions in ‘As-Received’ and ‘As-Welded’ condition of metastable β Ti-55511 alloy, following conclusions are drawn. 1. 2.
3. 4. 5.
Microstructure in ‘As-Received’ condition consists of near globular/equiaxed α (grain size 4 μm – 8 μm) in the transformed β matrix. The tensile testing reveals high 1288 MPa (UTS), 1260 MPa (0.2% YS) with 10.77 %El and microhardness values vary from 373 HV to 389 HV. The microstructure of EB butt welded joint consists of long columnar β-grains and coarse equiaxed β grains in FZ and HAZ respectively which leads to lower strength and ductility in the weld zone. Epitaxial growth is observed from FZ to HAZ with long irregular shape columnar dendrites with maximum arm length 1600 μm at the top and 1000 μm at the bottom. The shape of FZ is looked like “crown-shaped” or “hour-glass-shaped”. EB butt welded joint exhibits lower microhardness and tensile strength with a drastic reduction in ductility as compared to ‘As-Received’ condition and minimum 80% welding efficiency has been obtained. EB welded joint shows increment in UTS (1037-1094 MPa) with a small change in ductility (1.2-1.3%El) with an increase in strain rate from 0.0001s-1 to 0.01s-1. By fracture analysis, transition from ductile (i.e. Flute and fine dimples) behaviour in ‘As-Received’ to brittle like (i.e. cleavage facets and ridges) behaviour in ‘As-Welded’ condition is observed. Fracture location in ‘AsWelded’ specimen is observed in the weld zone near to fusion line and this statement can be supported by the lower microhardness, UTS and ductility.
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