Effects of thermal undercooling and thermal cycles on the grain and microstructure evolution of TC17 titanium alloy repaired by wire arc additive manufacturing

Wire arc additive manufacturing (WAAM) can be used to repair blades or blisk made of titanium alloy with the advantage of high efficiency and low-cost. In this work, the finite element model of repairing the blade based on the arc heat source was established to investigate it. Results showed that the maximum effect of thermal undercooling appeared when the peak current transformed to the base current (1 Hz or 5 Hz), which will promote the grains refinement with the combination of sufficient constitutional supercooling. Compared to the single-layer deposition, the microstructure in the near-heat affected zone (near-HAZ) of multi-layer deposition changes from the metastable β phases to the extremely fine α phases, which was caused by the repeated thermal cycles.


Introduction
The development process of aero-engine is mainly the improvement process of thrust-weight ratio [1]. Aero-engine blades are the core component of aero-engine. To reduce the weight of blades, designers use the blisk which leaving out the tenon and mortice to replace the traditional blades. However, the blisk are frequently subjected to kinds of damages due to the poor work condition, which results in a serious decrease in engine operating efficiency [2][3][4]. The replacement of damaged blades is extremely expensive. Therefore, the research of repairing blisk is necessary and will bring huge economic benefit.
For different types of damages, the corresponding repairing methods are different. For the damaged blade with surface cracks, conventional brazing [5,6] and wide gap braze [7][8][9] are recommended. These methods have the advantages of no jigging, no pressure required, and batch processability. Ye et al. [9] successfully repaired the tapered slots defects artificially fabricated in IN738LC superalloy by diffusion brazing with a Ni-Cr-Co-Al-Ta-B filler alloy at 1150 °C and then heat treatment was employed. For the fractured blades, linear friction welding [10][11][12] can achieve the replacement of the whole damaged blade within the damaged blisk instead of discarding it. Meanwhile, additive manufacturing has received a lot of attention in recent years, which has much advantage to repair the damage type of partial defects. According to the different of heat sources, AM technology can be divided into three categories: laser additive manufacturing (LAM) [13][14][15][16], electron beam additive manufacturing (EBAM) [17][18][19], and wire arc additive manufacturing (WAAM) [20][21][22][23][24]. LAM has the advantage of excellent flexibility controllable heat input and high forming accuracy. Zhao et al. [15] repaired wrought Ti17 titanium alloy with small surface defects by LAM technology with powder feeding, and the tensile fracture of repaired specimen occurred in the repaired zone with a mixed dimple and cleavage mode. However, the improvement of density and the cost reduction are still the huge challenge for LAM technology. For EBAM technology, the biggest advantage is the vacuum environment, which brings the high quality protection effect to avoid the harmful gas. Meanwhile, the preheating effect on powder is thought to be beneficial to forming brittle materials, for example TiAl, nickel base superalloy. However, the high cost and the forming size restrict should be further improved in the further. In the study by Wanjara et al. [19], an "extensively eroded" fan blade leading edge was repaired by the wirefeed electron beam additive manufacturing technology. For WAAM technology, it has the unique advantage of low cost and high deposition efficiency (up to 10 kg/h) [23]. Meanwhile, WAAM technology has low equipment requirements, which is very suitable for the war state. The Collaborative Research Centre (CRC) 871 focused on the overhaul of compressor blisk since 2014, and added the research subjects "Repair Methods of High-performance Titanium-alloy Components by Arc Welding Process" [24]. Therefore, it is necessary and valuable to investigate the application of WAAM on repairing the damaged blades.
According to our previous study [25], the feasibility of repairing the damaged blade by the WAAM technology was verified. Meanwhile, the columnar grains can be transformed to equiaxed grains by the combination of pulsed arc and boron addition in our previous work [26]. However, the thermal cycle feature of the multi-layer deposition based on the pulsed arc is not clear, which is closely related to the evolution of the microstructure and grains. Therefore, the thermal processing needs to be further studied. In this work, the finite element model of repairing the blade based on the arc heat source was established. The corresponding relationship of the single-layer and multi-layer deposition between the temperature cycle and the microstructure was investigated. Meanwhile, the user subroutine was developed to investigate the effect of pulsed arc on the grain evolution. The triggering condition and effect position of thermal undercooling were also discussed in detail.

Experimental setup
As shown in Fig. 1, the WAAM equipment used for repairing TC17 titanium alloy consists of a gas tungsten arc welding (GTAW) torch equipped with a wire feed unit, a trailing shield, and CNC controlled table. The filler material was TC11 titanium alloy wire with a diameter of 1.2 mm, and the base metal was a TC17 titanium alloy plate with the size of 100 mm in length, 10 mm in width, and 40 mm in height. The single-layer deposition was employed with the welding parameters of 100A in welding current, 3 mm/s in welding speeding, 15 mm/s in wire speeding. The multi-layer position in our previous work [25] was cited here to explain the relationship of the thermal cycle and microstructure. Meanwhile, the parameters of deposition fabricated by pulsed arc were same as our previous work [26]. This work mainly focused on the simulation computation, which promotes the understanding of grain and microstructure evolution.

Characterization
After the repairing deposition, the metallographic samples were prepared by the standard mechanical polishing method and then etched by the corrosive liquid of HF: HNO 3 : H 2 O with a volume ratio of 1: 6: 7. The macrostructures were observed by an OLYMPUS-SZX21 optical microscope (OM), and the microstructure was analyzed by a Quanta 200FEG scanning electron microscope (SEM).

Model
The finite element software package, MSC. Marc was used to investigate the grain and microstructure evolution of titanium alloy fabricated by the WAAM technology. The thermophysical properties of TC11 and TC17 were calculated by JMatPro software. The geometry of the single-layer was measured from the experiments separately, and the three-dimensional thermal model and finite element mesh of the single-layer and multi-layer deposition are shown in Fig. 2. Linear brick elements with 8 nodes were used for the thermal simulation. To save the calculation time, dense meshes were used for the deposition, and the meshes became coarser in the −z direction away from the fusion line.
Element birth technique was used to simulate the material deposition procedure [27]. To apply this method, all elements constituting the whole mesh including the deposition layer elements must be created. All the deposition layer elements will be deactivated at the first step of the analysis. As the analysis goes on, the deposition layer will be activated gradually. the revised sentences. The user subroutine in the Fortran code was developed to simulate the effect of pulsed arc on the thermal cycle. Meanwhile, the double ellipsoidal heat source [28,29] was used in this study. All the modeling parameters were identical to the experimental conditions, including the Fig. 1 The schematic diagram of repairing titanium alloy by wire arc additive manufacturing dimension of the models, welding speed, and the cooling time between subsequent layers. The values of convection coefficient and radiation coefficient were determined by running a series of numerical trials based on the experiments. Meanwhile, the thermocouple was attached on the surface of the base metal to record the temperature during the deposition process.

The thermal cycles feature of the single-layer deposition fabricated by the pulsed arc
As shown in Fig. 3, the measured curve and the simulated curve of single-layer deposition fabricated by direct current  Fig. 4. The overall trend of the pulsed arc has little different from that of current arc. However, the peak temperature of pulsed arc (1 Hz or 5 Hz) at the measured point is higher than that of DC arc. It might be caused by the expansion of peak arc, which promotes the heat flux conducting to the measured point. Meanwhile, it is worth noting that there appears an inflection point in the thermal curve of 1 Hz pulsed arc, which was caused by the thermal undercooling effect of pulsed arc. Due to the relatively long distance between the arc center and the measured point, the inflection of temperature under the 5 Hz pulsed arc is not obvious.
As shown in Figs. 5 and 6, the thermal cycle at different depositions during one pulsed period of 1 Hz and 5 Hz was presented. The values of thermal undercooling at the different positions are different. The maximum values of thermal undercooling appear at the position of the peak current transforming to the base current whatever the frequency is 1 Hz or 5 Hz. As shown in the grain morphology within Figs. 5 and 6, with the combination of the constitutional supercooling [26], the fine equiaxed grains at the position with the large value of thermal undercooling were formed. It is worth noting that the fine equiaxed grains can be only formed at the position calculated by the center of arc minus the distance between the arc center and the margin of the welding pool when the peak current transforms to the base current. Fine equiaxed grains were hard to be formed at other positions. Since this numerical model only considers the temperature field (without the influence of

Effect of the thermal cycle on the microstructure evolution of the deposition layer fabricated by the WAAM technology
As shown in Fig. 7, horizontal and parallel heat affected bands (HABands) were formed in the deposition layer, which was also observed in other AM process of titanium alloy [30][31][32]. The position of the HABands is different from that of the fusion line, which indicates the HABands were not formed by the effect of the fusion line, but repeated thermal cycles. The microstructure and corresponding thermal cycle of different positions are shown in Fig. 7b, c, d. The microstructure between the adjacent HABands is the typical lamellar microstructure with thin and long α phases. The HABands were caused by the coarse and grown α phase (Fig. 7c). According to the thermal cycles extracted at corresponding positions, the dwell time of thermal cycles with the peak temperature above the β transus temperature between the HABands and other positions is different. The phase transformation process of α phases to β phases happens when the temperature range is about 600-977 °C calculated by JMatPro software (Fig. 8). Therefore, the increased dwell time at this temperature range promotes the growth of α phases, which causes the most coarse α phases with the most dwell time.

Effect of the thermal cycle on the microstructure evolution of the heat affected zone
The heat affected zone (HAZ) has a significant influence on the mechanical properties of the repairing component. Therefore, it is necessary to investigate the microstructure evolution in this zone. As shown in Fig. 9, the microstructure and corresponding thermal cycles at different positions of the HAZ are presented. The microstructure of  (Fig. 9c), the secondary α phase (α s ) within the basketweave microstructure dissolved due to the peak temperature of thermal cycle exceeding the β transus temperature, as shown in Fig. 9c1. With the position closing to the interface, the peak temperature of thermal cycle increased, as shown in Fig. 9e1. The relatively longer dwell time caused more dissolution of α s , and primary α phases (α p ) started to dissolve (Fig. 9d, e). TC17 titanium alloy is one type of rich β stable element titanium alloy, which promotes β phase retained at room temperature [33]. At the near-HAZ, metastable β phases were retained due to the fast cooling rate, the higher peak temperature and more longer dwell time above the β transus temperature, which caused α phases to dissolve completely, as shown in Fig. 9f. Meanwhile, thin and long α phases distributed crisscross within the single-layer deposition, which was the base of the microstructure evolution of the subsequent multi-layer deposition.
Compared to the single-layer deposition, the microstructure of the near-HAZ in the multi-layer deposition has the obvious difference, and other positions of the HAZ have little difference. As shown in Fig. 10b, the numbers of acicular α phases formed in the near-HAZ in the multi-layer deposition, which was different from the metastable β phases in the single-layer deposition. The thermal cycle at the near-HAZ of the multi-layer deposition was extracted to understand the microstructure evolution (Fig. 10c). The near-HAZ underwent three thermal cycles with the peak temperature above the β transus temperature, which caused the dissolution and formation of α phases. Meanwhile, the subsequent multiple thermal cycles kept the temperature of deposits in the range of 600-977 °C for a long time, which promotes the transform of β phases to α phase. Therefore, the metastable β phases translated to acicular α phases due to repeated rapid heating and cooling thermal cycling.

Conclusions
In this work, the finite element model of repairing the blade based on the arc heat source was established to investigate the effect of thermal cycles on the grain and microstructure evolution. The following conclusions can be drawn from this work: 1. Compared to the thermal cycle of direct current arc, the thermal cycle of pulsed arc has an obvious inflection point, which was caused by the thermal undercooling effect of pulsed arc. Meanwhile, the maximum thermal undercooling appears when the peak current transforms to the base current, which will promote the grain refinement. 2. The microstructure of heat affected bands (HABands) is the lamellar microstructure with partial α phase coarsening, which is caused by the longest dwell time of thermal cycle at the temperature range for α phase growth. 3. Compared to the metastable β phases in the near-heat affected zone (near-HAZ) of single-layer deposition, the microstructure in the near-HAZ of multi-layer deposition becomes extremely fine α phases, which was caused by the repeated thermal cycles.

Declarations
Ethical approval Not applicable.

Consent to participate Not applicable.
Consent for publication Not applicable.

Competing interests
The authors declare no competing interests.

Fig. 10
The heat affected zone (HAZ) morphology and corresponding microstructure and thermal cycle: a the macrostructure morphology at the HAZ, modified by our previous work [25], b the microstructure at the near-HAZ, and c the thermal cycle at the near-HAZ