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Microstructure Evolution and Mechanical Properties of Linear Friction Welded TiAl Alloy Joint with Online Stress Relieving  PDF

  • Chen Xi 1 ✉
  • Zhang Zhao 1
  • Xie Faqin 1
  • Ma Tiejun 2
  • Li Wenya 2
  • Wu Xiangqing 1
1. School of Civil Aviation, Northwestern Polytechnical University, Xi'an 710072, China； 2. State Key Laboratory of Solidification Processing, Shaanxi Key Laboratory of Friction Welding Technologies, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi 'an 710072, China

Updated：2021-09-06

DOI：XX.XXXX/j.issn.1002-185X.2021.08.015

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Abstract

Crack-free TiAl joints were welded by linear friction welding with online stress relieving process. Microstructural examination demonstrates that the joint has a clearly identified weld zone which consists mainly of equiaxed γ grains with a few α2 phase. Lamellar grains in thermo-mechanically affected zones near the weld margin are oriented along with the flow direction of the plasticized material. The microhardness of the joint presents a global increase from the base metal to the weld interface by about 1700 MPa; the tensile strength of the joints ranges from 683 MPa to 717 MPa at ambient temperature, which is comparable to that of the base metal. The refinement strengthening contributes to enhancing the joint strength.

Science Press

TiAl-based intermetallic alloy, a new class of advanced lightweight high-temperature structural material, is suitable to be used around 700~1000 °C[

1,2]. It has attracted widespread attention for potential application in turbine blades of aircraft engine due to its high specific strength, excellent oxidation resistance, high specific modulus, good creep resistance and high temperature mechanical properties. TiAl-based alloys have been applied in manufacturing turbine blades of advan-ced aircraft engines produced by GE[3-5]. In traditional gas turbines, blades are assembled at disks applying a fir-tree arrangement. However, the fir-tree blade-disk assembly is a costly and time-consuming process[6]. In addition, fretting fatigue damage, leakage flow caused by the slotted blade to disk assembly and the large mass of the components restrict the development of high thrust-weight ratio aeroengines[7]. Because of the increasing needs to manufacture blisks (integrally bladed disks) which have lower mass, improved product quality and ease of maintenance over existing conventional slotted blade/disk assemblies, a substitution of this mechanical joining technique, linear friction welding (LFW), has been introduced[8,9].

LFW as a rather new process aiming at extending the current applications for rotary friction welding to non-axisymmetric components such as blades and disks, has attracted more and more attention due to its outstanding characteristics such as high reliability, low cost, refined microstructure and almost no weld defects[

10]. In LFW, a metallurgically sound solid state joint is usually formed with the help of frictional heat generated from the reciprocating movement of one component (as the oscillation side) relative to another (as the extrusion side) under an axial force. Typical solidification defects, such as pores, pinholes, shrinkage cracks, segregation, grain coarsening and cast structure can be avoided by the absence of a liquid phase during LFW. LFW has already been widely used to join a range of materials including steel, aluminum, copper, nickel-based superalloys and titanium alloys[11-20].

To date, although LFW is of great attraction for manufacturing blisks with TiAl-based alloys, research on LFW of TiAl-based alloys is rare. Threadgill[

21] in The Welding Institute (TWI) mentioned the prospects of LFW for joining dissimilar TiAl/Ti3Al alloys; low ductility of base metal, high welding residual stress and fast cooling rate after welding will contribute to the initiation of cracks on weld interface, and further study of microstructural evolution and mechanical properties of LFWed TiAl alloys is not satisfying. There have been some researches on rotating friction welding between TiAl-based alloys and steels. According to Lee, rotating friction welded joints between TiAl alloy and AISI 4140 were achieved using Cu as insert layer[22]. Dong et al have reported the rotating friction welding of dissimilar TiAl/40Cr[23] and TiAl/42CrMo[24] joints. The results show that the as-welded specimens are failed through the interface, while the heat-treated specimens are failed on TiAl alloy side, indicating that PWHT is necessary to improve the joint strength. Therefore, due to the intrinsic properties such as brittleness and low ductility, it is indispensable to use inset layer or post-weld heat treatment (PWHT) to relive high residual stress and to avoid weld crack in TiAl joint.

To sum up, cracks are easily formed at the interface after welding due to the high residual stress and fast cooling rate, which lead to the deterioration of joint strength. It is still a challenge to achieve satisfactory TiAl joints by direct welding. Therefore, for the sake of integrity of joint, the online stress relieving process by induction heating was employed after linear friction welding immediately in this study. Micro-structural examination and mechanical properties tests were carried out to examine the welding quality.

1 Experiment

The welding experiments were carried out using the LFW machine (XMH-160) developed by Northwestern Polyte-chnical University (China). The as-extruded TiAl alloy was used for linear friction welding and its nominal chemical composition is given in Table 1. The microstructure revealed by scanning electron microscope (SEM) shows a typical lamellar γ structure, as shown in Fig.1. Specimens were cut into the following dimensions: width of 10 mm, length of 17 mm and height of 40 mm, while the welding interface had dimensions of 170 mm2.

Table 1 Nominal chemical composition of the TiAl alloy (wt%)
AlCrVNiONHTi
31.86 1.32 3.27 0.45 0.047 0.0079 0.0008 Bal.

Fig.1 Microstructure of parent TiAl alloy

On the basis of preliminary experiments and characteristics of XMH-160, the welding parameters were selected as follows: amplitude of oscillation of 2.5 mm, frequency of oscillation of 30 Hz, friction pressure of 60 MPa and friction time of 4 s. Prior to welding, the welding surface of the samples was ground and cleaned in an acetone bath. The online stress relieving device (induction heating coil around the joint) employed in this study is shown in Fig.2, which was used immediately after welding.

Fig.2 Online stress relieving device

The joint was cut into two parts along the direction of oscillation, as demonstrated in Fig.3, one part was used for metallurgical examination, and the other part was used for tensile strength testing which was carried out at ambient temperature according to GB/6397-86 (China). The polished metallographic specimens were etched with a solution of 10 mL HF, 30 mL HNO3 and 70 mL H2O. The microstructure was examined by optical microscope (OM, Olympus PMG3) and SEM (Tescan MIRA3 XMU). The tensile test was conducted at ambient temperature using a universal testing machine (Shimadzu AG-X) with a cross-head speed of 1 mm/min. The microhardness was measured along the length direction from base metal to weld line with a hardness tester (model: Duramin-A300, 500 g load). Three test lines were adopted and an average profile was obtained disregarding doubtful values.

Fig.3 Metallographic specimen cut from welded specimens and dimension of tensile specimen

2 Results and Discussion

2.1 Appearance of joint

As shown in Fig.4, the morphology of flash of LFWed TiAl joint is different from that of LFWed Ti joint, which can be divided into two parts: the upper flash and the lower flash[

8,18-20]. The two layers of flash are not mutually adhesive and the surface has metallic luster, which is similar to that of Ni-based superalloy[15,16]. The quantity of the extruded flash of the joint is much fewer compared with that of Ti LFWed joint. As the flash layer consists of plastically deformed material extruded during the welding process, the difference in the high-temperature strength between TiAl alloys and Ti alloys is almost certainly responsible for the difference in flash morphology[25].

Fig.4 Typical post-weld LFWed TiAl joints

2.2 Joint microstructure

Fig.5a shows an overall view of the cross-sectional microstructure of the joint along the direction of oscillation. The narrow weld zone (<500 μm in width) can be clearly identified. No weld cracks are observed at weld interface, indicating that online stress relieving process used in this study is feasible to eliminate the weld cracks caused by high weld residual stress and fast cooling rate. On both sides of the joint, the orientations of γ grains near the weld zone have changed appreciably because of the oscillation and friction pressure during the welding process, which shows the flow direction of the plasticized material.

Typical microstructures in different locations (zone A, B and C) of the thermo mechanical affected zone (TMAZ) are shown in Fig.5b~5d, respectively. Compared to the base metal, zones A, B and C possess more equiaxed grains. And the closer to the weld zone, the more lamellar structures are transformed to equiaxed structures. Zone D presents the weld center zone, as shown in Fig.5d, in which all the lamellar structures are transformed to equiaxed grains.

The microstructure at high magnification of weld zone is illustrated in Fig.6a, where faint indications of grain boun-daries can be observed, indicating that grains in this area are not effectively revealed through chemical etching. The low etching response is ascribed to great deformation and dynamic recrystallization happening in weld zone. A few more equiaxed grains are visible in the area adjacent to weld zone as shown in Fig.6b. Fig.6c represents the microstructure at about 0.6 mm from the weld line, and acicular and equiaxed structures can be revealed. Finally, from the micrograph of Fig.6d, 1.2 mm from the weld line, more lamellar structures similar to the base metal (Fig.1) can be found clearly. This characteristic of weld microstructure is also found in a LFWed near-β Ti joint[

26].

Fig.6 SEM microstructures of the joint: (a) weld zone, (b) 200 μm from weld line, (c) 600 μm from weld line, and (d) 1.2 mm from weld line

In order to reveal the details of the weld zone, including the grain characteristics and phase composition, EBSD technique was performed on this area, as shown in Fig.7. Fig.7a presents the grain misorientation angle map: black and red lines represent the high-angle grain boundaries (HGBs, >15°) and low-angle grain boundaries (LGBs, 2°~15°), respectively. Eq-uiaxed refined grains around 10 μm can be found in weld zone. As shown in Fig.7b, the weld zone features primary HGBs with number percentage of 91.4%, revealing that weld zone undergoes complete dynamic recrystallization under the thermomechanical conditions during linear friction welding process. It has been reported that hot working above the eutectoid temperature (above 1125 °C) can totally transform the lamellar structure in TiAl alloy to equiaxed grains due to dynamic recrystallization[

27,28]. Generally, the peak value of the interface temperature can easily overtake 1100 °C and reach 1200 °C during linear friction welding process[29]. In addition, the combination of elevated temperatures and high strain rates also promote the dynamic recrystallization during the LFW process. Thus, it is reasonable for the transformation from lamellar structures to equiaxed structures in weld zone. Nevertheless, in TMAZ, the temperature of TMAZ adjacent to weld zone is around the eutectoid temperature so some lamellar microstructures turn into equiaxed microstructures. But along with away from the weld zone, the effect of elevated temperature and high strain rates on microstructure in TMAZ decreases, so the lamellar structures change little.

The phase composition of weld zone is shown in Fig.7c, where the α2 phase and γ phase are indicated by red and blue color, respectively. It is seen that the weld zone is mainly composed of γ phase while the quantity of α2 phase is small. It should be noted that no retained α phase is found. For TiAl alloys, when the hot working temperature enters the α+γ phase region and the cooling rate is faster than 250 K/s, a portion of α phase can remain in the alloy and develop into Widmanstatten structure due to insufficient decomposi-tion[

23,30]. In this experiment, the peak value of temperature is supposed to be above the α+γ phase transition line, and the cooling rate of LFW is about 350~400 K/s, faster than the critical cooling rate of 250 K/s[29, 31]. Thus, it is supposed that the weld zone should consist of some retained α phases. However, the online stress relieving process used after welding can offer more time for the transformation from α to α2+γ phase. As a result, the high-temperature α phase in the weld zone fully decomposes into the γ and α2 phase[32].

2.3 Mechanical properties

Fig.8 presents the microhardness profile obtained for welded sample with online stress relieving. It shows the variation of microhardness from the base metal to the weld line on both oscillation side and extrusion side. The curves of both oscillation side and extrusion side have the same variation trend, and an overall increase of strength from base metal (around 3100 MPa) to weld zone (around 4800 MPa) is observed. As mentioned above, the equiaxed microstructure formed during dynamic recrystallization in weld zone contri-

butes to improve the microhardness by refined grain streng-thening.

The tensile properties at ambient temperature of the joint are shown in Table 2. The fracture locations of sample 1 and sample 2 are far away from the weld line in the base metal area, which indicates that the tensile property of the weld is at least comparable to that of the base metal. The sound tensile strength of the welded samples can be explained by the presence of a hardened region in the weldment with a refined microstructure. However, sample 3 fails in TMAZ, while its tensile strength is comparable to that of sample 1 (702 MPa) and sample 2 (717 MPa). Sample 3 was cut from the edge of the joint, where lamellar structures in TMAZ adjacent to weld zone are nearly parallel to the plastic flow direction, as shown in Fig.5d. It will cause the degradation of the tensile strength of the sample and result in the fracture in TMAZ[

23]. The average tensile strength of the three tensile samples at ambient temperature is about 694 MPa, with the elongation of 0.35%. The corresponding fracture morphology of sample 3 exhibits quasicleavage fracture pattern, as shown in Fig.9a.The mor-phology of fracture sample is composed of cleavage planes and river pattern, as shown in Fig.9b and 9c, revealing the low ductility of TiAl alloy at ambient temperature.

Table 2 Tensile properties of joints at ambient temperature
SampleUTS/MPaYS/MPaElongation/%Failed location
1 702 663 0.39 Base metal
2 717 680 0.37 Base metal
3 683 629 0.32 TMAZ

Fig.9 SEM fractographs of the joint after tensile test at ambient temperature: (a) over view, (b) river pattern and (c) cleavage plane

3 Conclusions

1) The sound linear friction welding (LFW) joints free of cracks and pores can be obtained for TiAl alloy by online stress relieving process. Microstructure examination of weld zone and TMAZ presents different features. The orientation of lamellar structure changes drastically and is parallel to the flow direction of the plasticized material formed in thermo mechanical affected zone (TMAZ) near the weld margin.

2) In the weld zone, equiaxed refined grains form due to the interaction of elevated temperatures and high strain rates during the welding process. The closer to the weld zone, the more lamellar microstructures in TMAZs are transformed into equiaxed structure. The weld zone consists mainly of γ phase with a small portion of α2 phase.

3) The microhardness presents an overall decrease from the interface to base metal. This trend is in accordance with the microstructural evolution. The tensile strength of the joints ranges from 683 MPa to 717 MPa at ambient temperature, which is comparable to that of the base metal. Only one specimen fails at the TMAZ due to the orientations of lamellar structure near the weld zone. The fracture morphology exhibits a quasi-cleavage pattern.

References

1

Wu Yulun, Hu Rui, Yang Jieren. Journal of Alloys and Compounds[J], 2019, 790: 42 [Baidu Scholar] 

2

Song Y L, Dou Z H, Zhang T A et al. Journal of Alloys and Compounds[J], 2019, 789: 266 [Baidu Scholar] 

3

Eylon D, Keller M M, Jones P E. Intermetallics[J],1998, 6(7-8): 135 [Baidu Scholar] 

4

Saari H, Beddoes J, Seo D Y et al. Intermetallics[J], 2005, 13(9): 937 [Baidu Scholar] 

5

Peng Zichao, Tian Gaofeng, Jiang Jun et al. Materials Science & Engineering A[J], 2016, 676: 441 [Baidu Scholar] 

6

Aschenbruck J, Adamczuk R, Seume J R. Procedia CIRP[J], 2014, 22: 256 [Baidu Scholar] 

7

Chamanfar A, Jahazi M, Gholipour J et al. Materials Science & Engineering A[J], 2012, 555: 117 [Baidu Scholar] 

8

Li W Y, Vairis A, Preuss M et al. International Materials Reviews[J], 2016, 61(2): 71 [Baidu Scholar] 

9

Kelly Matthew R, Schmid Steven R, Adams Daniel C et al. Journal of Manufacturing Processes[J], 2019, 39: 26 [Baidu Scholar] 

10

McAndrew A R, Colegrove P A, Bühr C et al. Progress in Materials Science[J], 2018, 92: 225 [Baidu Scholar] 

11

Bhamji I, Preuss M, Threadgill P L et al. Materials Science & Engineering A[J], 2010, 528: 680 [Baidu Scholar] 

12

Buff G, Cammalleri M, Campanell D et al. Materials & Design[J], 2015, 82: 238 [Baidu Scholar] 

13

Mogami H, Matsuda T, Sano T et al. Materials & Design[J], 2018, 139: 457 [Baidu Scholar] 

14

Bhamji I, Moat R J, Preuss M et al. Science & Technology of Welding & Joining[J], 2012, 17(4): 314 [Baidu Scholar] 

15

Geng P H, Qin G L, Zhou J et al. International Journal of Mechanical Sciences[J], 2019, 152: 420 [Baidu Scholar] 

16

Ma T J, Chen X, Li W Y et al. Materials & Design[J], 2016, 89: 85 [Baidu Scholar] 

17

Wang X Y, Li W Y, Ma T J et al. Materials & Design[J], 2017, 116: 115 [Baidu Scholar] 

18

Vairis A, Frost M. Materials Science & Engineering A[J], 2000, 292(1): 8 [Baidu Scholar] 

19

Vairis A, Frost M. Wear[J], 1998, 217: 117 [Baidu Scholar] 

20

Wang X Y, Li W Y, Ma T J et al. Materials Science & Engineering A[J], 2019, 743: 12 [Baidu Scholar] 

21

Threadgill P L. Materials Science & Engineering A[J], 1995, 192-193(2): 640 [Baidu Scholar] 

22

Lee W B, Kim Y J, Jung S B. Intermetallics[J], 2004, 12: 671 [Baidu Scholar] 

23

Dong H G, Yu L Z, Gao H M et al. Transactions of Nonferrous Metals Society of China[J], 2014, 24: 3126 [Baidu Scholar] 

24

Dong H G, Yu L Z, Deng D W et al. Advanced Manufacturing Processes[J], 2014, 30(9): 1104 [Baidu Scholar] 

25

Wang L, Liu Y, Zhang W et al. Intermetallics[J], 2011, 19: 68 [Baidu Scholar] 

26

Dalgaard E, Wanjara P, Gholipour J et al. Acta Materialia[J], 2012, 60: 770 [Baidu Scholar] 

27

Schwaighofer E, Clemens H, Mayer S et al. Intermetallics[J], 2014, 44: 128 [Baidu Scholar] 

28

Dey S R, Hazotte A, Bouzy E. Intermetallics[J], 2009, 17: 1052 [Baidu Scholar] 

29

McAndrew A R, Colegrove P A, Addison A C et al. Materials & Design[J], 2015, 66: 183 [Baidu Scholar] 

30

Wang G, Xu L, Cui Y Y et al. Acta Metallurgica Sinica[J], 2016, 52(9): 1079 (in Chinese) [Baidu Scholar] 

31

Li W Y, Ma T J, Li J L. Materials & Design[J], 2010, 31: 1497 [Baidu Scholar] 

32

Chen G Q, Zhang B G, Liu W et al. Intermetallics[J], 2011, 19: 1857 [Baidu Scholar] 

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