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Effects of deep cryogenic treatment on the residual stress and mechanical properties of electron-beam-welded Ti–6Al–4V joints
Author’s Accepted Manuscript Effectsof deep cryogenic treatment on the residual stress and mechanical properties of electron-beamwelded Ti–6Al–4V joints L.Y. Xu, J. Zhu, H.Y. Jing, L. Zhao, X.Q, Lv, Y.D. Han www.elsevier.com/locate/msea
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S0921-5093(16)30876-0 http://dx.doi.org/10.1016/j.msea.2016.07.101 MSA33932
To appear in: Materials Science & Engineering A Received date: 30 June 2016 Revised date: 24 July 2016 Accepted date: 25 July 2016 Cite this article as: L.Y. Xu, J. Zhu, H.Y. Jing, L. Zhao, X.Q, Lv and Y.D. Han, Effectsof deep cryogenic treatment on the residual stress and mechanical properties of electron-beam-welded Ti–6Al–4V joints, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.07.101 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.
Effects of deep cryogenic treatment on the residual stress and mechanical properties of electron-beam-welded Ti– 6Al–4V joints L.Y. Xu1,2，J. Zhu1,2, H.Y. Jing1,2, L. Zhao1,2, X.Q, Lv1,2, Y.D. Han1,2* 1
School of Materials Science and Engineering, Tianjin University, Tianjin 300354, China
2
Tianjin Key Laboratory of Advanced Joining Technology, Tianjin 300354, China
[email protected]
Abstract The effects of deep cryogenic treatment (DCT) on the residual stress and mechanical properties of electron beam (EB)-welded Ti-6Al-4V (TC4) joints were investigated. In addition, the effect of different DCT soaking times was analyzed to obtain the optimal time. The results indicated that with a soaking time of 24 h, perpendicular to the welding direction, the residual stress declined by 31.2% for the longitudinal stress and 46.5% for the transverse stress, and along the welding direction, the residual longitudinal and transverse stress declined by 25.1% and 17.2%, respectively. More than 24h, the effect of increasing the soaking time on eliminating residual stress is not obvious. Meanwhile, the effect of DCT on the tensile strength of the EB-welded TC4 joint was also investigated, which showed that the strength increased by 4.3% and the elongation rate increased by 46.8% when the soaking time was longer than 24 h. The hardness test indicated that values at the weld were higher than that in the base metal, and the average values of hardness increased with DCT. Particular focus was given to the changes in the quantity, size, and morphology of the α and β phases as the main reasons for the above results. Keywords: Deep cryogenic treatment; EB-welded joints; Residual stress; Mechanical
properties
1. Introduction Titanium and its alloys exhibit a better strength-to-weight ratio, toughness, resistance to corrosion, and fatigue behavior than those of steel, aluminum, copper, or magnesium [1-4], and they are now widely utilized in aviation, spaceflight, ships, chemicals, weapons, metallurgy, and the power and automobile industries, as well as in biomedical fields. At present, TC4 is the most widely used titanium alloy by virtue of its good mechanical and physical properties, occupying a leading position of total titanium products used around the world [2]. Among all the welding techniques, electron beam (EB) welding has a unique advantage over other traditional fusion-welding methods in terms of its high energy density, large depth-to-width ratio, deep penetration, very small heat-affected zone, and vacuum environment [5]. These characteristics suggest that electron beam welding is more suitable and reliable for Ti-6Al-4V sheet welding because it introduces few defects during the welding process [6]. However, in spite of the various advantages of EB welding technology, residual stress at the welding joints remains problematic. In recent years, large numbers of cracking accidents have been observed in welded joints in chemical, power, and automobile applications due to stress corrosion and fatigue. The two phenomena act as mutual accelerating factors and are considered to be caused by the synergistic roles of the service environment and the residual stress from welding [7-9]. To prevent premature structural failure or removal from service, a variety of methods have been applied to reduce the residual stress, such as heat treatment and the hammer and vibration methods
[10-12]. New effective methods are constantly being explored and developed. Recently, deep cryogenic treatment (DCT) has been proposed as a way to simultaneously reduce the welding residual stress and improve the mechanical properties. The word “cryogenics” comes from the word “cold,” and it is simply the study of materials at low temperatures at which the properties of materials significantly change. DCT supplements traditional heat-treatment processes in order to improve the mechanical and physical properties of materials [13]. Some researchers have studied the wear resistance [14-25], tensile properties [26-28], hardness, impact toughness [29,30], fatigue properties [31-33], fracture toughness [34], and residual stress [35,36] in materials subjected to DCT. Surveying the current literature reveals that a large number of studies have been conducted to investigate the effects of DCT on the microstructure, mechanical properties, wear properties, and residual stress state of various steels that are used for tools, dies, gears, etc. Some assumptions about DCT are proposed in the literature: (1) the transformation of retained austenite into martensite increases hardness [24, 37], (2) DCT induces changes in the residual stress state [35], and (3) fine carbide precipitation increases wear resistance [38, 39]. In terms of the residual stress, Bensely et al. [35] studied the effect of cryogenic treatment on the distribution of residual stress for carburized En 353 steel. Experimental investigation revealed that when subjected to tempering, deep-cryogenically treated steel undergoes a reduction in residual compressive stress. Senthilkumar et al. [36] investigated the influence of shallow cryogenic treatment, conventional heat treatment, and DCT on the residual stress of 4140 steel, and the results indicated that in DCT samples, the residual stress is compressive,
while the residual stress is tensile in both shallow cryogenic samples and conventional heat treatment samples. However, little research has been done on the effects of DCT on the residual stress and mechanical properties of EB-welded TC4. It is thus necessary to investigate this phenomenon to make TC4 more widely applicable. Accordingly, in the present study, the residual stress of a TC4 sheet processed using EB welding and DCT was measured by the blind-hole method. The mechanical properties were investigated through tensile testing and hardness testing. The effects of DCT on the residual stress and mechanical properties of EB-welded TC4 joints were investigated by optical microscopy (OM), scanning electron microscope (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). 2. Experimental Details 2.1 Materials and Welding Procedure The test materials were TC4 titanium alloy sheets (thickness: 8 mm, length: 100 mm, width: 100 mm). Its chemical composition is shown in Table 1. The titanium alloy sheets were then welded by electron beam welding. The following welding parameters were optimized based on a set of preliminary experiments and were employed for obtaining the final welded joint properties as follows: (a) accelerating voltage of 60 kV; (b) electron beam current of 53 mA; (c) welding speed of 1000 mm/min; and (d) focus current of 2550 mA. Prior to welding, the specimens were cleaned to prevent including impurities in the fusion zone. The appearance of the weld line is shown in Fig. 1. 2.2 Deep cryogenic treatment (DCT) The test specimens used in this study were EB-welded TC4 sheets. They were divided into five groups: groups B, C, D, and E were subjected to DCT for 2 h, 15 h, 24 h
and 48 h, respectively. These groups were then slowly brought to room temperature, i.e., between 293 K and 298 K; group A was used as a reference without DCT. The overall treatment group scheme is summarized in Table 2. 2.3 Microstructural analyses The microstructures of the TC4 titanium alloy welded joints after DCT were analyzed using OM (OLYMPUS-TOKYO) and SEM (Hitachi S-4800). The types of phases were identified using an X-ray diffractometer (RIGAKU/DMAX2500, Japan). Before the SEM and XRD analyses, the samples were cleaned in acetone to remove surface contamination. Then, the dislocations in samples with different soaking times were analyzed by TEM (Tecnai G2 F20 S-TWIN, FEI). 2.4 Measurement of the residual stress by the blind-hole method The ASTM E837 standard test method, also known as the blind-hole method, is used to evaluate residual stresses. In the present study, a Model BE120-3CA strain gauge rosette was selected to measure the residual stress. The drill diameter was 1.5 mm, and the depth of the holes was 1.8 mm. The strain was measured in three directions according to the standard formula in order to calculate the longitudinal and transverse residual stress, both perpendiculars to and along the welding direction. The measurement positions are shown in Fig. 2. 2.5 Mechanical Testing The mechanical properties of plate tensile samples were measured according to ASTM E8-04 standard method. Three specimens were tested for each group in order to obtain a precise value. Vickers microhardness tests were performed on specimens of all five groups with a load of 1 kg and a dwell time of 10 s. The average of three
measurements was reported for each Vickers microhardness measurement. All tests were conducted between 293 K and 298 K. 3. Results 3.1 Structure of the welding joints The cross-sectional micrograph of one joint from group A is shown in Fig. 3. The joint was nail-shaped, and its microstructure could be divided into four zones: the weld metal (WM), the fusion zone (FZ), the heat-affected zone (HAZ), and the base metal (BM). The microstructure of the BM at room temperature consists of the close-packed hexagonal α phase and the body-centered cubic β phase, and the grains are fine and uniform, as shown in Fig. 4. The microstructures of the WM of EB-welded TC4 with different DCT soaking times are shown in Fig. 5. The microstructure of the WM of the control sample (DCT soaking time 0 h, Fig. 5a) is composed of acicular martensite within the prior β-grain structure, and the interlaced arrays of acicular martensite form a basketweave structure, which is similar to the observations of EB-welded TC4 alloys reported by Barreda et al. [41] and Babu et al. [42]. This basketweave structure showed good plasticity. Figs 5b–e show the samples subjected to DCT for 2 h, 15 h, 24 h, and 48 h, respectively. These images show that the size of grains was refined after DCT. The prior β-grain sizes in the welding zone with different DCT times are shown in Fig. 6. Figure 7 shows the X-ray diffraction pattern of the WM in EB-welded TC4 joints with different DCT soaking times. The diffraction peaks did not shift position after DCT, but the peaks changed in intensity compared to the diffraction peaks of the weld without DCT. In addition, no new peaks appeared, which indicates that no new phases formed
after DCT. Figure 8 shows the SEM micrographs of EB-welded TC4 joints after the various DCT soaking times. As shown in Fig. 8, the microstructure of the weld zone is the typical basketweave structure. With the increasing DCT soaking time, the amount of acicular martensite increased. Meanwhile, the degree of interlacing became more complex. The change in the volume fraction of acicular martensite in the welding zone after DCT is clearly shown in Fig. 9. After DCT for 24 h, the acicular martensite increased by 47.47% compared to group A. To further investigate the microstructure and substructure of each group, TEM was performed, as shown in Fig. 10. Some dislocations can be observed in the α grains. With the increasing soaking time, the dislocation density also increased. During the process of DCT, the structure is subject to high stress due to structural shrinkage and the different expansion coefficients of the α and β phases [43]. The high stress gives a sufficiently large driving force for the movement of dislocations. These dislocations will interact with other dislocations, interstitial atoms, and grain boundaries and generate dislocation networks. The high magnification images show that dislocations were gradually intertwined and became more complex. This process improved the resistance of welded joints to microplastic deformation, which can contribute to the improvement of plasticity after DCT [28]. 3.2 Residual stress distribution Figure 11 shows the distribution of residual stress after DCT for 0 h, 2 h, 15 h, 24 h, and 48 h. With longer DCT soaking times, the longitudinal and transverse residual stress values decreased correspondingly, both perpendiculars to and along the welding direction.
Perpendicular to the welding direction, the longitudinal residual stresses near the weld centers of groups B, C, D, and E were reduced by approximately 9.5%, 20.6%, 31.2%, and 34.1%, respectively, compared to group A, whereas the transverse residual stresses were reduced by approximately 20.4%, 32.1%, 46.5%, and 47.4%, respectively. At the same time, along the weld direction, the longitudinal residual stresses of groups B, C, D, and E were reduced by 9.9%, 18.6%, 25.1%, and 23.1%, respectively, and the transverse residual stresses were reduced by approximately 8.4%, 13.6%, 17.2%, and 18.4%, respectively, compared to group A. It can be concluded that the effect of DCT on residual stress is significant. 3.3 Mechanical properties of the welded joints 3.3.1 Hardness The results of the Vickers hardness tests are presented in Fig. 12. The results confirm that the Vickers hardness (HV1) values at the weld are significantly higher than that at the base metal. When the DCT time was 24 h, the hardness value increased in the BM by 6.87% and in the WM by 1.61% compared to the hardness of the untreated group. When the DCT time was increased to 48 h, the HV1 values basically remained unchanged. These results indicate that 24 h is the optimal soaking time to improve the hardness. 3.3.2 Tensile strength The results obtained from the tensile tests are reported in Table 3. The elongations observed in groups B, C, D, and E increased by approximately 21.3%, 26.6%, 40.4%, and 46.8%, respectively, compared to group A. Despite the differences in elongation, the tensile strength values showed no significant changes related to DCT. The biggest increase of tensile strength was approximately 2.28% after DCT.
4. Discussion EB welding has a high peak heating temperature (~2700 K) and a fast cooling rate after welding. The microstructural transformation of EB-welded TC4 joints is mostly dependent on the initial microstructure of the TC4 alloy base metal and the heat cycle during the EB-welding process. In the EB-welded TC4 alloy, as the distance increases between the center of the WM and the fusion line, the peak heating temperature and cooling rate decreases. The formation of WM’s martensite basketweave microstructure can be attributed to the high cooling rate. The fast cooling gives the atoms insufficient time to diffuse, and prevents the β phases from transforming into the stable α phases. In addition, because of the fast heating rate of EB welding, the α phase may not completely transform into the β phase. Thus, during the fast cooling process, the retained α phases do not transform, whereas the β phases transformed at high heating temperatures will transform into acicular martensite during cooling. However, for the FZ near the base metal, the peak heating temperature is lower than the β transus temperature. Therefore, during the cooling process, no transformation occurs for the primary α phases, and there are acicular α phases formed at the boundary and interior of the β grains owing to the fast cooling rate [42]. Deep cryogenics is the treatment of materials at extremely low temperatures, and it is considered environmentally friendly. Cryogenics refines and stabilizes the crystal lattice structure, resulting in a stronger and more durable material [44]. Since the DCT refines and stabilizes the crystal lattice structure (Fig. 6), grain refinement takes place in order to relieve internal stresses. With increasing DCT soaking time, the longitudinal and transverse residual stress values near the weld center decrease
correspondingly. The mechanical properties of materials are known to depend on their microscopic structure. In general, the hardness of the existing phases in the present joints is in the following order: martensite > α phase > β phase [45]. The microstructure of the WM is martensite basketweave. The base metal consists of a primary α phase and intergranular β phases. Thus, the HV1 values for the WM are higher than that of the BM [42, 46]. However, martensite cannot significantly improve the hardness of titanium alloys like it can in steel alloys, and it is only slightly harder than the α phase. As a result, the HV1 values of the five groups did not show significant changes at the weld. The tensile strength of the EB-welded TC4 joint was not significantly affected by DCT. The biggest difference was ~4.3% after the 48 h DCT soaking time. The measured change consists of an increase in elongation for the B, C, D, and E groups. The elongation is an indicator of the plasticity. Although some researchers believe that the plasticity of Ti-Al alloys does not depend on the grain size but rather on the chemical composition and microstructure [47], most studies have shown that the polycrystalline plasticity is improved with decreasing grain size [48]. Meanwhile, DCT induced complicated dislocation networks and improved the resistance of the joints to microplastic deformation. As a result, the plasticity of the EB-welded TC4 increased after DCT. 5. Conclusions (1) DCT reduced the residual stress of the EB-welded TC4 joint. Perpendicular to the welding direction, the residual stress declined by 31.2% for the longitudinal stress and 46.5% for the transverse stress, and along the welding direction, the residual stress
declined by 25.1% for the longitudinal stress and 17.2% for the transverse stress after 24 h. Moreover, when the soaking time was longer than 24 h, the residual stress decreased slowly. Therefore, 24 h was considered an optimal time for TC4 deep cryogenics treatment. (2) The XRD analysis indicated that the WM mainly consisted of an α′ martensitic phase and a small quantity of primary α and intergranular β phases. So, the hardness of the WM was higher than that of the BM owing to the formation of the martensitic α′ phase during the EB welding. (3) The TEM analysis showed that during the DCT process, dislocation networks formed due to the movement and interaction of dislocations at low temperature, which improved the plasticity of the EB-welded TC4 joint. (4) The decrease of the β phases and the increase of the martensitic phase after DCT led to a more refined and stable crystal lattice structure, which resulted in releasing residual stress and an improvement in the mechanical properties.
Acknowledgements The authors acknowledge the research funding by Natural Science Foundation of Tianjin (Grant No. 13JCYBJC18200).
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Fig. 1 The appearance of the weld line.
Fig. 2 The size of the specimen and the measurement point position of residual stress.
Fig. 3 Cross-sectional macrograph of the joint
Fig. 4 The microstructure of base meal
Fig. 5 Microstructure of the WM of ED-welded TC4 joint with different DCT soaking times: (a) 0, (b) 2, (c) 15, (d) 24, and (e) 48 h.
Fig. 6 The grain size of prior β in welding zone with different DCT time.
Fig. 7 XRD patterns of the WM of EB-welded TC4 joint of under different DCT soaking times.
Fig. 8 SEM micrographs of EB-welded TC4 joint under different DCT soaking times: (a) 0, (b) 2, (c) 15, (d) 24, and (e) 48 h.
Fig. 9 The volume fraction of acicular martensite in welding zone with different DCT time
Fig. 10 TEM micrographs of EB-welded TC4 joint different DCT soaking times: (a, a´) 0, (b, b´) 2, (c, c´) 15, (d, d´) 24, and (e, e´) 48 h.
(a)
(b)
(c)
(d) Fig. 11 The longitudinal (a, c) and transverse (b, d) residual stress of along (a, b) and perpendicular (c, d) to the welding direction.
Fig. 12 Hardness test results.
Table 1 Chemical composition of TC4 titanium alloy (wt. %). Al 5.6
V 4.2
Fe 0.17
C 0.02
N ＜0.0005
H 0.0015
O ＜0.0005
Ti others
Table 2 Treatment groups and parameters. Group index A B C D E
DCT temp. (K) 77 77 77 77
DCT soaking time (h) 2 15 24 48
Table 3 Tensile test results. Group index A
B
C
D
E
Sample index 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
Tensile strength（MPa） value mean 854.63 895.26 883.52 880.66 868.23 864.07 881.69 890.77 917.08 910.01 912.59 920.68 891.30 882.35 912.62 925.22 892.23 915.15 921.47 930.02
Elongation (%) value 10.2 8.5 9.5 10.7 11.5 12 12.3 12 11.4 13.5 12.2 14 13 14.5 13.8
mean 9.4
11.4
11.9
13.2
13.8
PII: DOI: Reference:
S0921-5093(16)30876-0 http://dx.doi.org/10.1016/j.msea.2016.07.101 MSA33932
To appear in: Materials Science & Engineering A Received date: 30 June 2016 Revised date: 24 July 2016 Accepted date: 25 July 2016 Cite this article as: L.Y. Xu, J. Zhu, H.Y. Jing, L. Zhao, X.Q, Lv and Y.D. Han, Effectsof deep cryogenic treatment on the residual stress and mechanical properties of electron-beam-welded Ti–6Al–4V joints, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.07.101 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.
Effects of deep cryogenic treatment on the residual stress and mechanical properties of electron-beam-welded Ti– 6Al–4V joints L.Y. Xu1,2，J. Zhu1,2, H.Y. Jing1,2, L. Zhao1,2, X.Q, Lv1,2, Y.D. Han1,2* 1
School of Materials Science and Engineering, Tianjin University, Tianjin 300354, China
2
Tianjin Key Laboratory of Advanced Joining Technology, Tianjin 300354, China
[email protected]
Abstract The effects of deep cryogenic treatment (DCT) on the residual stress and mechanical properties of electron beam (EB)-welded Ti-6Al-4V (TC4) joints were investigated. In addition, the effect of different DCT soaking times was analyzed to obtain the optimal time. The results indicated that with a soaking time of 24 h, perpendicular to the welding direction, the residual stress declined by 31.2% for the longitudinal stress and 46.5% for the transverse stress, and along the welding direction, the residual longitudinal and transverse stress declined by 25.1% and 17.2%, respectively. More than 24h, the effect of increasing the soaking time on eliminating residual stress is not obvious. Meanwhile, the effect of DCT on the tensile strength of the EB-welded TC4 joint was also investigated, which showed that the strength increased by 4.3% and the elongation rate increased by 46.8% when the soaking time was longer than 24 h. The hardness test indicated that values at the weld were higher than that in the base metal, and the average values of hardness increased with DCT. Particular focus was given to the changes in the quantity, size, and morphology of the α and β phases as the main reasons for the above results. Keywords: Deep cryogenic treatment; EB-welded joints; Residual stress; Mechanical
properties
1. Introduction Titanium and its alloys exhibit a better strength-to-weight ratio, toughness, resistance to corrosion, and fatigue behavior than those of steel, aluminum, copper, or magnesium [1-4], and they are now widely utilized in aviation, spaceflight, ships, chemicals, weapons, metallurgy, and the power and automobile industries, as well as in biomedical fields. At present, TC4 is the most widely used titanium alloy by virtue of its good mechanical and physical properties, occupying a leading position of total titanium products used around the world [2]. Among all the welding techniques, electron beam (EB) welding has a unique advantage over other traditional fusion-welding methods in terms of its high energy density, large depth-to-width ratio, deep penetration, very small heat-affected zone, and vacuum environment [5]. These characteristics suggest that electron beam welding is more suitable and reliable for Ti-6Al-4V sheet welding because it introduces few defects during the welding process [6]. However, in spite of the various advantages of EB welding technology, residual stress at the welding joints remains problematic. In recent years, large numbers of cracking accidents have been observed in welded joints in chemical, power, and automobile applications due to stress corrosion and fatigue. The two phenomena act as mutual accelerating factors and are considered to be caused by the synergistic roles of the service environment and the residual stress from welding [7-9]. To prevent premature structural failure or removal from service, a variety of methods have been applied to reduce the residual stress, such as heat treatment and the hammer and vibration methods
[10-12]. New effective methods are constantly being explored and developed. Recently, deep cryogenic treatment (DCT) has been proposed as a way to simultaneously reduce the welding residual stress and improve the mechanical properties. The word “cryogenics” comes from the word “cold,” and it is simply the study of materials at low temperatures at which the properties of materials significantly change. DCT supplements traditional heat-treatment processes in order to improve the mechanical and physical properties of materials [13]. Some researchers have studied the wear resistance [14-25], tensile properties [26-28], hardness, impact toughness [29,30], fatigue properties [31-33], fracture toughness [34], and residual stress [35,36] in materials subjected to DCT. Surveying the current literature reveals that a large number of studies have been conducted to investigate the effects of DCT on the microstructure, mechanical properties, wear properties, and residual stress state of various steels that are used for tools, dies, gears, etc. Some assumptions about DCT are proposed in the literature: (1) the transformation of retained austenite into martensite increases hardness [24, 37], (2) DCT induces changes in the residual stress state [35], and (3) fine carbide precipitation increases wear resistance [38, 39]. In terms of the residual stress, Bensely et al. [35] studied the effect of cryogenic treatment on the distribution of residual stress for carburized En 353 steel. Experimental investigation revealed that when subjected to tempering, deep-cryogenically treated steel undergoes a reduction in residual compressive stress. Senthilkumar et al. [36] investigated the influence of shallow cryogenic treatment, conventional heat treatment, and DCT on the residual stress of 4140 steel, and the results indicated that in DCT samples, the residual stress is compressive,
while the residual stress is tensile in both shallow cryogenic samples and conventional heat treatment samples. However, little research has been done on the effects of DCT on the residual stress and mechanical properties of EB-welded TC4. It is thus necessary to investigate this phenomenon to make TC4 more widely applicable. Accordingly, in the present study, the residual stress of a TC4 sheet processed using EB welding and DCT was measured by the blind-hole method. The mechanical properties were investigated through tensile testing and hardness testing. The effects of DCT on the residual stress and mechanical properties of EB-welded TC4 joints were investigated by optical microscopy (OM), scanning electron microscope (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). 2. Experimental Details 2.1 Materials and Welding Procedure The test materials were TC4 titanium alloy sheets (thickness: 8 mm, length: 100 mm, width: 100 mm). Its chemical composition is shown in Table 1. The titanium alloy sheets were then welded by electron beam welding. The following welding parameters were optimized based on a set of preliminary experiments and were employed for obtaining the final welded joint properties as follows: (a) accelerating voltage of 60 kV; (b) electron beam current of 53 mA; (c) welding speed of 1000 mm/min; and (d) focus current of 2550 mA. Prior to welding, the specimens were cleaned to prevent including impurities in the fusion zone. The appearance of the weld line is shown in Fig. 1. 2.2 Deep cryogenic treatment (DCT) The test specimens used in this study were EB-welded TC4 sheets. They were divided into five groups: groups B, C, D, and E were subjected to DCT for 2 h, 15 h, 24 h
and 48 h, respectively. These groups were then slowly brought to room temperature, i.e., between 293 K and 298 K; group A was used as a reference without DCT. The overall treatment group scheme is summarized in Table 2. 2.3 Microstructural analyses The microstructures of the TC4 titanium alloy welded joints after DCT were analyzed using OM (OLYMPUS-TOKYO) and SEM (Hitachi S-4800). The types of phases were identified using an X-ray diffractometer (RIGAKU/DMAX2500, Japan). Before the SEM and XRD analyses, the samples were cleaned in acetone to remove surface contamination. Then, the dislocations in samples with different soaking times were analyzed by TEM (Tecnai G2 F20 S-TWIN, FEI). 2.4 Measurement of the residual stress by the blind-hole method The ASTM E837 standard test method, also known as the blind-hole method, is used to evaluate residual stresses. In the present study, a Model BE120-3CA strain gauge rosette was selected to measure the residual stress. The drill diameter was 1.5 mm, and the depth of the holes was 1.8 mm. The strain was measured in three directions according to the standard formula in order to calculate the longitudinal and transverse residual stress, both perpendiculars to and along the welding direction. The measurement positions are shown in Fig. 2. 2.5 Mechanical Testing The mechanical properties of plate tensile samples were measured according to ASTM E8-04 standard method. Three specimens were tested for each group in order to obtain a precise value. Vickers microhardness tests were performed on specimens of all five groups with a load of 1 kg and a dwell time of 10 s. The average of three
measurements was reported for each Vickers microhardness measurement. All tests were conducted between 293 K and 298 K. 3. Results 3.1 Structure of the welding joints The cross-sectional micrograph of one joint from group A is shown in Fig. 3. The joint was nail-shaped, and its microstructure could be divided into four zones: the weld metal (WM), the fusion zone (FZ), the heat-affected zone (HAZ), and the base metal (BM). The microstructure of the BM at room temperature consists of the close-packed hexagonal α phase and the body-centered cubic β phase, and the grains are fine and uniform, as shown in Fig. 4. The microstructures of the WM of EB-welded TC4 with different DCT soaking times are shown in Fig. 5. The microstructure of the WM of the control sample (DCT soaking time 0 h, Fig. 5a) is composed of acicular martensite within the prior β-grain structure, and the interlaced arrays of acicular martensite form a basketweave structure, which is similar to the observations of EB-welded TC4 alloys reported by Barreda et al. [41] and Babu et al. [42]. This basketweave structure showed good plasticity. Figs 5b–e show the samples subjected to DCT for 2 h, 15 h, 24 h, and 48 h, respectively. These images show that the size of grains was refined after DCT. The prior β-grain sizes in the welding zone with different DCT times are shown in Fig. 6. Figure 7 shows the X-ray diffraction pattern of the WM in EB-welded TC4 joints with different DCT soaking times. The diffraction peaks did not shift position after DCT, but the peaks changed in intensity compared to the diffraction peaks of the weld without DCT. In addition, no new peaks appeared, which indicates that no new phases formed
after DCT. Figure 8 shows the SEM micrographs of EB-welded TC4 joints after the various DCT soaking times. As shown in Fig. 8, the microstructure of the weld zone is the typical basketweave structure. With the increasing DCT soaking time, the amount of acicular martensite increased. Meanwhile, the degree of interlacing became more complex. The change in the volume fraction of acicular martensite in the welding zone after DCT is clearly shown in Fig. 9. After DCT for 24 h, the acicular martensite increased by 47.47% compared to group A. To further investigate the microstructure and substructure of each group, TEM was performed, as shown in Fig. 10. Some dislocations can be observed in the α grains. With the increasing soaking time, the dislocation density also increased. During the process of DCT, the structure is subject to high stress due to structural shrinkage and the different expansion coefficients of the α and β phases [43]. The high stress gives a sufficiently large driving force for the movement of dislocations. These dislocations will interact with other dislocations, interstitial atoms, and grain boundaries and generate dislocation networks. The high magnification images show that dislocations were gradually intertwined and became more complex. This process improved the resistance of welded joints to microplastic deformation, which can contribute to the improvement of plasticity after DCT [28]. 3.2 Residual stress distribution Figure 11 shows the distribution of residual stress after DCT for 0 h, 2 h, 15 h, 24 h, and 48 h. With longer DCT soaking times, the longitudinal and transverse residual stress values decreased correspondingly, both perpendiculars to and along the welding direction.
Perpendicular to the welding direction, the longitudinal residual stresses near the weld centers of groups B, C, D, and E were reduced by approximately 9.5%, 20.6%, 31.2%, and 34.1%, respectively, compared to group A, whereas the transverse residual stresses were reduced by approximately 20.4%, 32.1%, 46.5%, and 47.4%, respectively. At the same time, along the weld direction, the longitudinal residual stresses of groups B, C, D, and E were reduced by 9.9%, 18.6%, 25.1%, and 23.1%, respectively, and the transverse residual stresses were reduced by approximately 8.4%, 13.6%, 17.2%, and 18.4%, respectively, compared to group A. It can be concluded that the effect of DCT on residual stress is significant. 3.3 Mechanical properties of the welded joints 3.3.1 Hardness The results of the Vickers hardness tests are presented in Fig. 12. The results confirm that the Vickers hardness (HV1) values at the weld are significantly higher than that at the base metal. When the DCT time was 24 h, the hardness value increased in the BM by 6.87% and in the WM by 1.61% compared to the hardness of the untreated group. When the DCT time was increased to 48 h, the HV1 values basically remained unchanged. These results indicate that 24 h is the optimal soaking time to improve the hardness. 3.3.2 Tensile strength The results obtained from the tensile tests are reported in Table 3. The elongations observed in groups B, C, D, and E increased by approximately 21.3%, 26.6%, 40.4%, and 46.8%, respectively, compared to group A. Despite the differences in elongation, the tensile strength values showed no significant changes related to DCT. The biggest increase of tensile strength was approximately 2.28% after DCT.
4. Discussion EB welding has a high peak heating temperature (~2700 K) and a fast cooling rate after welding. The microstructural transformation of EB-welded TC4 joints is mostly dependent on the initial microstructure of the TC4 alloy base metal and the heat cycle during the EB-welding process. In the EB-welded TC4 alloy, as the distance increases between the center of the WM and the fusion line, the peak heating temperature and cooling rate decreases. The formation of WM’s martensite basketweave microstructure can be attributed to the high cooling rate. The fast cooling gives the atoms insufficient time to diffuse, and prevents the β phases from transforming into the stable α phases. In addition, because of the fast heating rate of EB welding, the α phase may not completely transform into the β phase. Thus, during the fast cooling process, the retained α phases do not transform, whereas the β phases transformed at high heating temperatures will transform into acicular martensite during cooling. However, for the FZ near the base metal, the peak heating temperature is lower than the β transus temperature. Therefore, during the cooling process, no transformation occurs for the primary α phases, and there are acicular α phases formed at the boundary and interior of the β grains owing to the fast cooling rate [42]. Deep cryogenics is the treatment of materials at extremely low temperatures, and it is considered environmentally friendly. Cryogenics refines and stabilizes the crystal lattice structure, resulting in a stronger and more durable material [44]. Since the DCT refines and stabilizes the crystal lattice structure (Fig. 6), grain refinement takes place in order to relieve internal stresses. With increasing DCT soaking time, the longitudinal and transverse residual stress values near the weld center decrease
correspondingly. The mechanical properties of materials are known to depend on their microscopic structure. In general, the hardness of the existing phases in the present joints is in the following order: martensite > α phase > β phase [45]. The microstructure of the WM is martensite basketweave. The base metal consists of a primary α phase and intergranular β phases. Thus, the HV1 values for the WM are higher than that of the BM [42, 46]. However, martensite cannot significantly improve the hardness of titanium alloys like it can in steel alloys, and it is only slightly harder than the α phase. As a result, the HV1 values of the five groups did not show significant changes at the weld. The tensile strength of the EB-welded TC4 joint was not significantly affected by DCT. The biggest difference was ~4.3% after the 48 h DCT soaking time. The measured change consists of an increase in elongation for the B, C, D, and E groups. The elongation is an indicator of the plasticity. Although some researchers believe that the plasticity of Ti-Al alloys does not depend on the grain size but rather on the chemical composition and microstructure [47], most studies have shown that the polycrystalline plasticity is improved with decreasing grain size [48]. Meanwhile, DCT induced complicated dislocation networks and improved the resistance of the joints to microplastic deformation. As a result, the plasticity of the EB-welded TC4 increased after DCT. 5. Conclusions (1) DCT reduced the residual stress of the EB-welded TC4 joint. Perpendicular to the welding direction, the residual stress declined by 31.2% for the longitudinal stress and 46.5% for the transverse stress, and along the welding direction, the residual stress
declined by 25.1% for the longitudinal stress and 17.2% for the transverse stress after 24 h. Moreover, when the soaking time was longer than 24 h, the residual stress decreased slowly. Therefore, 24 h was considered an optimal time for TC4 deep cryogenics treatment. (2) The XRD analysis indicated that the WM mainly consisted of an α′ martensitic phase and a small quantity of primary α and intergranular β phases. So, the hardness of the WM was higher than that of the BM owing to the formation of the martensitic α′ phase during the EB welding. (3) The TEM analysis showed that during the DCT process, dislocation networks formed due to the movement and interaction of dislocations at low temperature, which improved the plasticity of the EB-welded TC4 joint. (4) The decrease of the β phases and the increase of the martensitic phase after DCT led to a more refined and stable crystal lattice structure, which resulted in releasing residual stress and an improvement in the mechanical properties.
Acknowledgements The authors acknowledge the research funding by Natural Science Foundation of Tianjin (Grant No. 13JCYBJC18200).
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Fig. 1 The appearance of the weld line.
Fig. 2 The size of the specimen and the measurement point position of residual stress.
Fig. 3 Cross-sectional macrograph of the joint
Fig. 4 The microstructure of base meal
Fig. 5 Microstructure of the WM of ED-welded TC4 joint with different DCT soaking times: (a) 0, (b) 2, (c) 15, (d) 24, and (e) 48 h.
Fig. 6 The grain size of prior β in welding zone with different DCT time.
Fig. 7 XRD patterns of the WM of EB-welded TC4 joint of under different DCT soaking times.
Fig. 8 SEM micrographs of EB-welded TC4 joint under different DCT soaking times: (a) 0, (b) 2, (c) 15, (d) 24, and (e) 48 h.
Fig. 9 The volume fraction of acicular martensite in welding zone with different DCT time
Fig. 10 TEM micrographs of EB-welded TC4 joint different DCT soaking times: (a, a´) 0, (b, b´) 2, (c, c´) 15, (d, d´) 24, and (e, e´) 48 h.
(a)
(b)
(c)
(d) Fig. 11 The longitudinal (a, c) and transverse (b, d) residual stress of along (a, b) and perpendicular (c, d) to the welding direction.
Fig. 12 Hardness test results.
Table 1 Chemical composition of TC4 titanium alloy (wt. %). Al 5.6
V 4.2
Fe 0.17
C 0.02
N ＜0.0005
H 0.0015
O ＜0.0005
Ti others
Table 2 Treatment groups and parameters. Group index A B C D E
DCT temp. (K) 77 77 77 77
DCT soaking time (h) 2 15 24 48
Table 3 Tensile test results. Group index A
B
C
D
E
Sample index 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
Tensile strength（MPa） value mean 854.63 895.26 883.52 880.66 868.23 864.07 881.69 890.77 917.08 910.01 912.59 920.68 891.30 882.35 912.62 925.22 892.23 915.15 921.47 930.02
Elongation (%) value 10.2 8.5 9.5 10.7 11.5 12 12.3 12 11.4 13.5 12.2 14 13 14.5 13.8
mean 9.4
11.4
11.9
13.2
13.8