3.1 the thermal cycles feature of the sing-layer deposition fabricated by the pulsed arc.
As shown in Fig. 3, the measured curve and the simulated curve of sing-layer deposition fabricated by direct current (DC) agrees well, which indicates the accuracy of the calculation model. Therefore, the calculation parameters of peak and base current of pulsed arc were chosen based on the direct current model. The thermal cycle at the same extracted position of the direct arc model of 1Hz and 5Hz is shown in Fig. 4. The overall trend of the pulsed arc has little different from that of current arc. However, the peak temperature of pulsed arc (1Hz or 5Hz) 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 1Hz 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 5Hz pulsed arc is not obvious.
As shown in Fig. 5 and Fig. 6, the thermal cycle at different depositions during one pulsed period of 1Hz and 5Hz was presented. The values of thermal undercooling at the different positions are different. The maximum values of thermal undercooling appears at the position of the peak current transforming to the base current whatever the frequency is 1Hz or 5Hz. As shown in the grain morphology within Fig. 5 and Fig. 6, with the combination of the constitutional supercooling , 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 molten flow), this model can only describe the process of thermal undercooling from a qualitative perspective. This calculation model is helpful to understanding the thermal undercooling and the effecting position of pulsed arc, which promotes the application of pulsed arc on the repairing technology of the damaged blisk.
3.2 Effect of thermal cycle on the microstructure evolution of titanium alloy fabricated by the WAAM technology.
As shown in Fig. 7, horizontal and parallel heat affected bands (HABands) was formed in the deposition layer, which was also observed in other AM process of titanium alloy [27–29]. The position of the HABands is different from that of the fusion line, which indicates the HABands was 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. 7(b,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. 7(c)). 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℃ 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.
3.3 Effect of thermal cycle on the microstructure evolution of titanium alloy fabricated by the WAAM technology.
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 base metal is a typical basketweave microstructure with the cross distribution of α phase. At the far-HAZ (Fig. 9(c)), the secondary α phase (αs) within the basketweave microstructure dissolved due to the peak temperature of thermal cycle exceeding the β transus temperature, as shown in Fig. 9(c1). With the position closing to the interface, the peak temperature of thermal cycle increased, as shown in Fig. 9(e1). The relatively longer dwell time caused more dissolution of αs, and primary α phases (αp) started to dissolve( Fig. 9(d,e) ).
TC17 titanium alloy is one type of rich β stable element titanium alloy, which promotes β phase retained at room temperature . 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. 9(f). Meanwhile, thin and long α phases distributed crisscross within the sing-layer deposition, which was the base of the microstructure evolution of the multi-layer deposition.
Compare to the sing-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. 10(b), 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 sing-layer deposition. The thermal cycle at the near-HAZ of the multi-layer deposition was extracted to understand the microstructure evolution (Fig. 10(c)). 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 deposits in the temperature range of 600–977℃ 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.