Microstructure Evolution of Inertia Friction Welded Joints of TC21 Titanium Alloy

Abstract

In this paper, TC21 titanium alloy welded joints were successfully formed through inertial friction welding (IFW) processes. Microstructure evolution of IFW joints was investigated by way of different analysis methods including optical microscope (OM), scanning electron microscope(SEM), Electron Back-Scattered Diffraction(EBSD), X-Ray Diffraction(XRD), and Energy Dispersive Spectrometer(EDS). The results indicate that large-sized equiaxial β grains, original α phases, and basketweave structure existing in the BM have completely disappeared in the WZ. Instead, fine equiaxial grains sized at 20–30 μm and very fine α + β lamellar microstructure are formed in the WZ. However, as transition zones, the microstructures of the TMAZ and HAZ are also in transition state while the microstructures existing in the BM partially remain in the TMAZ and completely remain in the HAZ. In addition, second α phases are precipitated and fine α + β lamellar microstructure are formed on the original β base in the TMAZ and HAZ. XRD and EBSD results reveal that the proportion of β phase in the WZ zone decreases greatly. EDS results show that there are aggregations of stabilizing elements of β phase in the BM, but no element aggregation in the WZ. Dynamic recrystallization during the IFW process and element distribution under the rapid cooling condition after the welding process are believed to be responsible for formation of the microstructure in the weld zone of IFW joints.

1. Introduction

Titanium alloys are important structural materials in aerospace applications due to their high strength-to-weight ratio, low elastic modulus, excellent corrosion resistance, high heat resistance, and good weldability [1,2,3,4]. With the development of the aerospace industry, performance of titanium alloys must be much higher. Damage tolerance has become a design criterion for advanced aircraft and engines to meet safety requirements. High damage tolerance means that a material has a lower crack growth rate and a higher crack growth threshold to ensure that the cracks in structural parts could not become critical cracks and would not result in fatigue failure during service lifetimes [5]. As a key structural material of aerospace application, titanium alloy is becoming a high damage tolerance type. TC21 (Ti-6Al-2Zr-2Sn-3Mo-1Cr- 2Nb-0.1Si) is a high damage-tolerance type of titanium alloy that was developed independently by China in 2003 [6]. Later, further research revealed that TC21 titanium alloys with basketweave microstructure obtained by quasi-β forging have the best damage tolerance performance compared with that of other microstructures [7]. TC21 titanium alloy has excellent comprehensive performance with good matching of strength, plasticity, fracture toughness, damage tolerance, and so on [8,9,10]. Based on its advantages, TC21 titanium alloy has become the key structural material for a new generation of aircraft, and is suitable for manufacturing aircraft frames, fuselages, engine shafts, and rotors.

Welding technology is very critical for titanium alloy to realize the manufacturing of large-scale structural parts and ensure weight-reduction efficiency. However, traditional fusion welding methods are not suitable for titanium alloy due to its properties such as high melting point and low thermal conductivity. In addition, defects such as oxidation slag inclusion, poor fusion, lack of penetration, and solidification cracks often occur in fusion welding methods. Current welding methods for titanium alloys include brazing, laser welding, friction stir welding, diffusion bonding, and so on [11,12,13,14]. Among them, properties of brazing joints are not so good because they needs filler metal to accomplish welding. The properties of laser welded joints are good, but are only suitable for the welding of thin parts. Friction stir welding and diffusion bonding are not suitable for the welding of shaft parts. Compared with the above technologies, inertia friction welding (IFW) is a solid-state joint technology and has many advantages including high quality and efficiency, environmental friendliness, low heat input, small deformation, and narrow welding seam [15,16,17]. IFW has been proven to be a suitable welding method for titanium alloy and has been widely used in aerospace and other high-tech fields. As titanium alloy is a relatively new material, research on IFW technology of TC21 titanium alloy is rarely reported. In this work, welding joints with good properties of TC21 alloy is successfully achieved using the IFW process under suitable parameters. Microstructure evolution of IFW joints is emphatically analyzed. The effects of friction mechanical force and heat on the microstructure of the IFW joint are revealed.

2. Materials and Methods

The materials used in IFW experiments are typical TC21 titanium alloys with basketweave microstructure achieved by quasi-β forging. Chemical composition of the experimental material of TC21 alloy is depicted in

Table 1. Chemical composition of TC21 alloy, wt.%.

Al	Mo	Nb	Sn	Zr	Cr	Fe	O	C	N	H	Si	Ti
6.35	2.75	2.09	2.03	2.06	1.48	0.098	0.099	0.020	0.017	0.002	≤0.13	Bal.

. Ring-shaped samples of TC21 titanium alloy with inner diameter of 65 mm, outer diameter of 135 mm, and length of 300 mm were prepared for IFW experiments, as shown in Figure 1. The welding process was conducted using HSMZ-130 axial and radial inertia friction welding equipment designed by Harbin Welding Institute Limited Company (China Harbin). Welding parameters are summarized in

Table 2. Inertia friction welding parameters of TC21 alloy.

Initial Rotating Speed (RPM)	Moment of Inertia (kg·m2)	Friction Pressure (MPa)	Upsetting Pressure (MPa)
700	388	76	102

. Welded samples were followed by post-welding heat treatment at 730 ℃ for 2 h and then air cooling to room temperature.

Original bar specimens with dimensions of ϕ15 mm and length of 70 mm were extracted from welded rings by wire electrical discharge machining (WEDM). The original specimens cover four zones: the weld zone (WZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ), and base metals (BM). Microscopic samples were acquired from original specimens by cutting longitudinally along the axial center, and followed by grinding, polishing, and corrosion using the solution of 1% HF + 6% HNO3+93% water. The samples used for the electron back-scatter diffraction (EBSD) test were prepared from microscopic samples further treated by argon ion polishing using FISCHIONE 1061 equipment.

Microstructure of TC21 base metal and the IFW joint was studied using a VHX-7000 optical metallographic microscope (OM) and an SU8100 scanning electron microscope (SEM). The phase analysis of base metal and the IFW joint was carried out by an X-ray diffractometer (XRD) of D/MAX-2500. Zeiss GMINI 300+symmetry S2 equipment was used for the EBSD test. EBSD analysis was performed in the BM, TMAZ, and WZ zones, respectively. Because observed grain size in different zones of the IFW joint is obviously different, EBSD test area and step distance were chosen in different zones of the IFW joint to ensure testing accuracy. Test points in the BM and TMAZ were scanned within an area of 200 um × 150 um with a step gap of 0.4 um × 0.4 um. However, test points in the WZ were scanned within an area of 50 um × 50 um with a step gap of 0.1 um × 0.1 um due to its fine grains. Finally, energy dispersive X-ray spectroscopy (EDS) was used to determine the chemical element compositions of the BM and WZ of IFW joints.

3. Results and Discussion

3.1. Appearance and Microstructure of the Overall IFW Joint

Figure 2 depicts optical micrographs of the IFW joint. It is shown that the IFW joint zone is darker than the base metal zone after corrosion by a chemical solution, indicating that the corrosion resistance of the IFW joint is worse than that of base metal. Figure 2a delineates different zones including the WZ, TMAZ, HZ, and BM. The total width of the IFW joint is about 5.1 mm. In the BM zone, the shapes of large-sized equiaxial β grains can be clearly observed with ranges of hundreds of micrometers. As shown in Figure 2a, the WZ is the central zone near the welding interface, with a width of about 1.7 mm. Figure 2b is the optical micrograph of the WZ with relatively higher microscope magnification, where a welding line with width of about 120 μm is observed in the middle of the WZ. The microstructure of the WZ has become uniform and the shapes of large-sized equiaxial β phase grains existing in the BM have completely disappeared. As shown in Figure 2a, the TMAZ zone with a width of about 0.6 mm is adjacent to the WZ. The TMAZ zone is relatively narrow, because it is a transitional zone where conditions of heat and mechanical force are unstable due to high heat conduction. Figure 2c shows an optical micrograph depicting part of the IFW joint with a larger range. As marked in the red box in Figure 2c, it is found that the shape of β phase in the TMAZ deforms due to the rotating friction force. The microstructure with deformed β phases also confirms the existence of the TMAZ in the IFW joint. Moreover, the HAZ, with a width of about 1.1 mm, is observed next to the TMAZ. In the HAZ, deformation of β phases does not occur, meaning that the HAZ is not affected by rotational friction, but only by heat.

3.2. SEM Results of IFW Joint Microstructure

Figure 3 shows the microstructure photos of different zones in the IFW joint obtained by SEM. In the BM shown in Figure 3d, a typical basketweave structure formed by quasi-β forging process of TC21 alloy and consisting of α + β two phases (black color α phase and white color β phase) is observed. After quasi-β forging of the TC21 alloy, the primitive β grains are destroyed in forging deformation, and the α phases at the β grain boundary are kinked or spheroidized and distributed in chain shape. Moreover, the lamellar α phases inside the β grain boundary are cross-distributed with each other and woven into a basket shape [18]. The width of most lamellar α phases (hereafter called “BM original α phase”) are measured with ranges of 1–4 μm. Figure 3a displays the microstructure of the WZ. It is observed that the basketweave structure and BM original α phase have completely disappeared. Moreover, the shape of fine equiaxial β grains with a size of 20–30 μm are found due to dynamic recrystallization. The size of recrystallized equiaxial β grains in the WZ is much smaller than the mean size of equiaxial β grains in the BM, which spans hundreds of micrometers. In the WZ, a great deal of heat is generated by friction with high-speed rotation during the IFW process and causes the temperature (≥1200 ℃) exceeding the β-transformation temperature (970 ± 20 ℃) of TC21 alloy [15,19]. As a result, dynamic recrystallization happens in the WZ. In addition, severe plastic deformation occurs in the WZ during the IFW process. Because of the large deformation and resultant large distortion energy, the number of nucleation sites increases greatly during recrystallization. According to recrystallization theory, the recrystallized grain size is determined by the short axis of the elongated grains [20]. Therefore, the size of recrystallized equiaxial β grains in the WZ is much smaller than that of the equiaxial β grains in the BM. Figure 3b,c show the microstructure of the TMAZ and HAZ, respectively. There is no fine equiaxial grain observed in the TMAZ and HAZ, indicating that no recrystallization occurred in the TMAZ and HAZ because the temperature is lower than β-transformation temperature [21,22]. It can be found that BM original α phases partially remain in the TMAZ and completely remain in the HAZ.

Figure 4 shows SEM microstructure pictures of different zones of the IFW joint with very high microscope magnification. As shown in Figure 4e, the microstructure of BM is relatively coarse, with lamellar α phases (black color) with width of 1 to 4μm distributed on the β phase base (white color). In contrast, a very fine α + β lamellar microstructure is formed in the WZ as shown in Figure 4a. The width of lamellar α phases of the WZ is even less than 0.1μm, which is much smaller than that of the BM original α phase (1–4 μm). Boundary α phases are also indicated in Figure 4a, because here, different morphology of α phases is observed. Based on the equiaxial grains with 20–30 μm width observed in Figure 3a, we believe they are boundary α phases located at the boundary of equiaxial grains in WZ. Because of the rapid heat conduction, the α phases exist in fine-acicular α’ martensite after welding [15], but the unstable fine-acicular α’ martensite becomes equilibrium α + β structure in the post welding heat treatment, and the equilibrium structure finally exists in fine lamellar α + β phases. Figure 4b displays the microstructure of the TMAZ. It is shown that BM original α phases have not completely disappeared because the degree of plastic deformation decreases and the temperature has not reached β-transformation temperature in this zone. However, some original lamellar α grains are interrupted due to the influence of friction mechanical force. Moreover, some secondary α phases are precipitated and fine α + β lamellar microstructure are formed on the original β base. Figure 4c shows the microstructure of the HAZ, where BM original α phases completely remain because the HAZ is only affected by heat. The only difference with the BM is that the second α phases are precipitated and fine α + β lamellar microstructure are formed on the original β base.

3.3. Results of Crystal Orientation Analysis of the IFW Joint

EBSD micrographs of different zones of the IFW joint are shown in Figure 5 to illustrate the feature of crystal orientation. As shown in Figure 5a, in the BM, each lamellar α phase does not exist independently. Instead, a series of lamellar α phases with the same crystal orientation exist together and form a cluster. However, the crystal orientations for different clusters are different. It is revealed that α phase clusters with different crystal orientations are cross-distributed with each other and woven into a basketweave structure. Due to this kind of cross-woven structure, TC21 base material has excellent comprehensive properties. Figure 5b displays the EBSD mapping of the TMAZ. It is observed that the grain boundaries between lamellar α phases in the same orientation cluster have become indistinct and the lamellar α phases in the same cluster adhere together, which means the basket weave microstructure existing in the BM has been destroyed in the TMAZ due to the high heat and friction force conditions. Because of the very fine grain size of the WZ, we could hardly obtain any useful information when using the same EBSD scan step with the BM and TMAZ, so we reduced EBSD scan step gap in the WZ. Figure 5c depicts the EBSD micrographs of the WZ after reducing the scanning step gap. It is found that a series of fine lamellar α phases formed by recrystallization also have the same crystal orientation, but the range of lamellar α phases with the same crystal orientation is obviously smaller than that in the BM, and no basket weave structure is formed in the WZ.

Figure 6 shows misorientation angle distribution the of grain boundary for the different zones of the IFW joint. On the right side of Figure 6, distribution mappings of the grain boundary in the different zones are also correspondingly displayed. It is shown that the number of grain boundaries of the WZ are the largest, revealing that the grain refinement degree is very high in the WZ. However, grain boundaries of the BM look more like those of the TMAZ, which is believed that the reason is the scanning step gap of 0.4 m is chosen too large for the refined phases of the TMAZ, and the fine α phase boundary cannot be recognized. From the misorientation angle distribution of β phase for different zones shown in the right part of Figure 6, it could be found that β phases with a large angle boundary of 30° and 60° appear in the WZ, but do not appear in the TMAZ and BM. This is observed because only dynamic recrystallization takes place in WZ and a lot of fine equiaxial grains are formed, which possess large angle grain boundaries. As shown in the left part of Figure 6, there is no significant difference for the misorientation angle distributions of α phase in the different three zones of the IFW joint. In the BM, α phases exist in the form of cross-woven basket structure, so there are large-angle grain boundaries of around 60° as well as small angle grain boundaries. However, the obvious difference for the three joint zones is that there are more large-angle grain boundaries near 90° in the WZ than in the TMAZ and BM. It is believed that the rapid temperature decreasing in the WZ results in more large-angle grain boundaries close to 90° to remain.

3.4. Phase Analysis Results of the IFW Joint

Figure 7a plots XRD patterns of the BM and WZ zones of the IFW joint. Most of the peaks in the XRD patterns are similar to the report from Wu and Yuan et al. [15,23]. From Figure 7a, we can find the XRD peaks in the WZ have obvious broadening trend comparing with the peaks in the BM, suggesting that the grain size of the WZ is smaller than that of the BM, and this result is also consistent with the SEM observed microstructure. The three strong peaks of α-phase in the BM are located at (101), (002) and (103) peak, instead, the three strong peaks in WZ are changed to (100), (101) and (103) peak. Moreover, the peak relative intensity of (100) α-phase increases the most in the WZ zone, indicating that (100) crystal orientation is dominant after IFW process. In addition, it can be found in the pattern of the WZ that there are two “Abnormal peaks” located at about 72°and 75°, respectively. The peak at around 72° is an abnormal broadening peak corresponding to the (103) α phase, which is believed to be caused by the peak superimposition of normal (103) α phase and residual (113) α” phase. Another peak at about 75° is a strange and very small peak that does not conform to any XRD standard peak of titanium alloy, and the closest peak in known titanium alloy is (211) α” phase. Therefore, this may be the residual α” phase with severe lattice distortion. Comparing the XRD peak of β phase in the BM and WZ, we can see that the peak relative intensity of (110) β phase becomes much lower, and the peak of (200) β phase disappears completely in WZ, suggesting that the proportion of β phase in WZ zone decreases greatly. Figure 7b,c show mappings of EBSD phase analysis for BM and WZ, respectively, where the blue color represents α phase and yellow color represents β phase. It could be found that the proportion of β phase in WZ zone decreases obviously. The calculated proportion of β phase decreases from 14.4% in the BM to 3.8% in the WZ. On the contrary, the proportion of α phase increases from 85.6% in the BM to 96.2% in the WZ.

3.5. Element Analysis Results of the IFW Joint

Figure 8 shows chemical element composition at different spots in the BM and WZ obtained by EDS. The EDS test spots are, respectively chosen at the white color area (β phase) and black color area (α phase) for the BM and WZ. The EDS spectrum distributions of the test spots are correspondingly displayed on the right side.

Table 3. Ratio of elements at different test spots in BM and WZ.

Element		Ti	Al	Mo	Zr	Sn	Nb	Cr
Spot 1 (BM β phase)	Weight(%)	76.4	5.1	6.5	2.4	2.3	3.0	4.3
	Atomic(%)	79.3	9.4	3.4	1.3	1.0	1.6	4.2
Spot 2 (BM α phase)	Weight(%)	86.3	7.4	0	1.9	2.3	1.6	0.6
	Atomic(%)	84.1	12.7	0	1.0	0.9	0.8	0.5
Spot 1 (WZ β phase)	Weight(%)	82.6	6.7	2.8	2.2	2.5	1.5	1.7
	Atomic(%)	82.3	11.8	1.4	1.1	1.0	0.8	1.6
Spot 2 (WZ α phase)	Weight(%)	82.8	6.4	2.8	2.1	2.5	1.6	1.7
	Atomic(%)	82.7	11.4	1.4	1.1	1.0	0.8	1.6

shows the ratio of elements at different test spots in the BM and WZ. Comparing the element percentage at the two spots of the BM, we can see there are many more elements of Mo, Nb and Cr at the β phase area than that at the α phase area. In addition, there is no Mo element and only slight Nb and Cr existing at α phase area. As shown in Table 3, at the BM β phase area, the weight percentage of Mo is even higher than that of Al, while the percentages of other elements with low content are slightly changed. It is known that Mo and Nb are stabilizing elements of eutectic-type β phase and that Cr is a stabilizing element of eutectoid-type β phase in titanium alloy. Therefore, it is also confirmed that the white area represents the β phase and the black area represents the α phase in the BM. In contrast, as shown in Table 3, the element percentages of the two spots (white and black color) in the WZ zone are almost the same. Even considering the accuracy of the test equipment, it could be found that the microstructure in the WZ cannot clearly distinguish the α phase from the β phase, and the microstructure in the WZ has changed from the coarse lamellar α phase and the residual β phase in the BM to the very fine α + β lamellar microstructure. The evolution of microstructure of weld zone of the IFW joint could be explained by the following: 1. As mentioned before, high force and heat during IFW process result in dynamic recrystallization and severe plastic deformation in WZ, giving rise to grain refinement. 2. The rapid cooling after the welding process causes stabilizing elements of β phase (such as Mo, Nb, and Cr) to be pinned in place and could not form aggregation. Moreover, the temperature of post welding annealing is lower than the β phase transition temperature, which also could not lead to diffusion and aggregation of stabilizing elements of β phase. Therefore, the β phase in weld zone clearly decreases when compared with that of the base metal. This would also explain the results of the decrease in the β phase and increase in the α phase proportion in the weld zone observed by EBSD.

4. Conclusions

Welding experiments of TC21 titanium alloy were successfully conducted using inertia friction welding (IFW) and relatively narrow joints (about 5.1 mm width) were achieved. Microstructure evolution of IFW joints was studied by OM, SEM, EBSD, XRD and EDS. From this study, the following conclusions could be drawn:

(1)

The shapes of ”large-size equiaxial β grains”, with ranges of hundreds of micrometers existing in the BM, have completely disappeared in the WZ of the IFW joint. While the shape of “large-size equiaxial β grains” still remains in the TMAZ and HAZ, but severe deformation of equiaxial β grains is found in the TMAZ due to the effect of rotating friction force.

(2)

Fine equiaxial grains with the size of 20–30 μm are formed in the WZ of the IFW joint. However, there are no fine equiaxial grains observed in the TMAZ and HAZ. It is believed that recrystallization in the WZ results in the formation of the fine equiaxial grains.

(3)

The BM zone consists of α + β two phases with original lamellar α phases with the width of 1–4 μm. In the WZ zone of joint, BM original α phases have completely disappeared. Instead, a very fine α + β lamellar microstructure is formed in the WZ and the width of fine lamellar α phases in the WZ is even less than 0.1 μm. Moreover, BM original α phases partially remain in the TMAZ and completely remain in the HAZ, and secondary α phases are precipitated on the BM original β base in the TMAZ and HAZ.

(4)

EBSD crystal orientation analysis shows that the basketweave microstructure has been destroyed in the TMAZ and WZ. XRD results indicate that (100) crystal orientation becomes dominant for α-phase in the WZ, and peak relative intensity of the β phase becomes much lower and some peaks of β phase even disappear completely in the WZ, revealing that the proportion of β phase in the WZ zone decreases greatly. The tested proportion of β phase decreases from 14.4% in the BM to 3.8% in WZ.

(5)

EDS results show there are aggregations of Mo, Nb, and Cr (stabilizing elements of β phase) at β phase area in the BM. However, in the WZ, element aggregation could not be found, and element distribution is homogeneous in the WZ, which is consistent with the decrease in β phase proportion in the WZ and very fine α + β lamellar microstructure of the WZ. It is believed that the recrystallization under high force and heat conditions, and no element aggregation under the rapid cooling condition after welding process, are responsible for the formation of fine grain microstructure and high α phase proportion in the weld zone of IFW joints.