3.1 Macro morphology of weld joint
The BM reacts strongly with laser and forms welding molten pool during the welding process. Therefore, the size and shape of welded joint is directly affected by the weld pool. The schematic diagrams of top WZ and HAZ width(named WT1, WT2), middle WZ and HAZ width(named WM1, WM2) and bottom WZ and HAZ width(named WB1, WB2) are shown in Fig. 3(a). Besides, the macro morphology of the cross-section profile of the bottom-locking welded joint under different welding parameters is shown in Fig. 3(b-d). It is easily seen from Fig. 3 that all welds are in the state of penetration and no obvious defects is found in the welding joints. As for the weld width, there is no doubt that the maximum value appears at the top of WZ, but the area where the minimum value appears cannot be determined. At the same time, it can be roughly observed from Fig. 3 that the HAZ shows two regions with different microstructure. According to the relevant literature, the one closed to WZ is called high temperature heat affected zone(HT-HAZ) and the other closed to BM is called low temperature heat affected zone(LT-HAZ).
The detailed characteristic parameter curves of weld macro morphology under different welding speeds are shown in Fig. 4. It is obliviously seen from Fig. 4 that the weld width is closely related to the welding speed. The specific performance is that with the increase of the welding speed, the weld width becomes smaller and smaller, especially the bottom of the weld seam. In addition, the width of HAZ also shows a certain trend that increases first and then decreases while the welding speed increasing from 1.0 m·min-1 to 1.5 m·min-1. Therefore, whether it is in WZ or HAZ, the macro morphology will change with the welding heat input according to what has been discussed above.
3.2 Micro morphology of welded joint
Figure 5 shows the microstructural characterization of laser welded bottom-locking joint of TA15 titanium alloy. The microstructure transformation of WZ and HAZ are closely related to the initial microstructure of BM and welding thermal cycle process. It should be mentioned that the redistribution of joint elements will be limited during the process from metal melting to cooling to room temperature, resulting in the formation of supersaturated martensite in the WZ. As is shown in Fig. 5, WZ is mainly composed of coarse β columnar crystal and fine α' acicular martensite. The β columnar crystal at the top of the WZ grows from the fusion line to the center of the weld. Differently, the β columnar crystal in the middle of the WZ grows perpendicular to the fusion line, and equiaxed crystal forms in the middle of the weld. This situation occurs due that the columnar crystal is formed by intergrowth crystallization and epitaxial growth during the solidification process of the weld. In addition, the grain grows preferentially and keeps consistent with the fastest heat dissipation direction until it grows to the center of the weld. The temperature and undercooling are relatively high in the middle of the weld, at the same time, the heat is dissipated to both sides at the same time. Before the columnar crystal grows to the center of the weld, the liquid metal in this area has nucleated. Besides, the α' acicular martensite precipitated along different directions in the β columnar crystal is distributed in basket shape and crisscross with columnar grains in the WZ according to the content of Fig. 5. Therefore, the α' acicular martensite is dense in the weld. But the microstructure in HAZ is complex and uneven, which changes along the fusion line and vertical direction.
Figure 6 shows the microstructure and morphology of weld metal at 1.0 m·min-1, 1.2m·min-1 and 1.5m·min-1, respectively. It is easily observed from Fig. 6 that there are some differences in the shape and size of β columnar crystal in the WZ under different welding speed. Specifically, the smaller the welding speed is, the coarser the shape and size is. Besides, the growth direction of β columnar crystal from the fusion line to the weld center in WZ also have a little difference under different parameters. The result is the difference in the amount of α' acicular martensite in the WZ. Specifically, with the increase of welding speed, i.e. the decrease of welding heat input, the number of α' acicular martensite decreases gradually, but it is still dispersed, and the directivity is not obvious from Fig. 6(b)(e)(h). Therefore, it can be found that there are more dense α' acicular martensites in the WZ in the case of 1.0 m·min-1. It should be noted that a lot of twins are existed in the acicular martensite α'. The increase of the amount of twins is equivalent to the refinement of grains, which can produce strengthening effect on the welded joint. As a result, compared with the welding speed of 1.2 m·min-1 and 1.5 m·min-1, the joint of welding speed at 1.0 m·min-1 has the highest strength and the best performance from the perspective of microstructure. Morever, there are mainly coarse β columnar crystals in the middle of the weld, and the growth zone of columnar crystal becomes smaller with the increase of welding speed from Fig. 6. However, the existence of acicular martensite is rarely seen in the middle of the weld where the temperature is not conducive to the transformation of β columnar crystal into acicular martensite.
Furthermore, there are certain initial martensitic α phase and α' phase in HT-HAZ where is close to the weld, but the size and amount of α' phase is smaller than that in WZ from Fig. 6(f); there are a small amount of α' and β and numerous initial α phase in LT-HAZ where is close to the BM from Fig. 6(I). As mentioned in Sect. 3.1, the width of HAZ will change to a certain extent under different welding speed. Accordingly, the width of HT-HAZ will also change to a certain extent and the number of various types of organizations in this region will also show certain differences. Besides, it is not different that the grain size of the whole HAZ increases gradually from BM to WZ. As a result, it will make some effect on the mechanical properties.
In order to further explain the characteristics of the microstructure in each region of the joint, the phase transformation is analyzed. In this experiment, the schematic diagram of phase transformation in WZ and HAZ region is shown in Fig. 7(c) and (d). The initial α phase is affected by the laser heat source, and it turns into liquid β phase when the phase transformation point is reached. The growth of β grains grow in the opposite direction of heat dissipation and the diffusionless transformation from β phase to acicular martensite α' phase occurs due to the high cooling rate, so the microstructure in WZ is composed of α' phase. The α phase in HT-HAZ is heated to above the β-transformation temperature but belower than the melting point, so the microstructure consists of martensitic α' phase and a few α phase. The LT-HAZ is heated to below the β-transformation temperature but higher than the minimum temperature required for microstructure transformation and the cooling rate is relatively slow. Therefore, this region with recrystallizationis a partial transformation zone, and the microstructure is composed of primary α phase, intergranular β phase and a small amount of martensitic α' phase.
3.3 Tensile test and fracture analysis
Tensile tests are conducted at room temperature for the welded joints in this study. However, due to the difference between the thickness of the two plates, a heel block should be added to make the left and right ends in the equal height position. The geometries of the tensile specimens, schematic diagram of tensile test, schematic diagram of fracture location and tensile strength and elongation are shown in Fig. 8.
After the test, the tensile data are sorted out. There are great differences in the fracture location, tensile strength and elongation of weld joint of bottom-locking structure under different welding parameters. It can be observed from Fig. 8(a) that Case 1 is fractured on the BM and the fracture direction is 45° to tensile direction, which indicates that the joint has high strength and good quality. Differently, Case 2 is fractured at the weld joint, and the fracture trace runs through the WZ and HAZ. The fracture position of Case 3 is the same as that of Case 2, which is also at the weld joint. But the fracture direction is different from that of Case 2, the surface trace line of which is almost perpendicular to the tensile direction. The fracture results show that the joint strength is the best when speed is 1.0m·min-1, which is related to the more acicular martensite α' in the WZ, as mentioned in Sect. 3.2. The effects of three different welding parameters on tensile strength and elongation are respectively as follows: with the increase of welding speed, the tensile strength first decreases and then increases while the elongation decreases all the time. Specifically, the tensile strength is the largest when the welding speed is 1.5m·min-1, which is 999 MPa, but the elongation is the smallest, which is 2.7%; the elongation is the largest when it is 1.0 m·min-1, which is 4.2%. The maximum tensile strength and elongation of the two Cases with fracture position in the weld can reach 89% and 24% of the BM of TA15 titanium alloy, which indicates that laser welding with the bottom locked titanium alloy has strength loss phenomenon.
The SEM fracture morphology and high multiple graph of local position of the welded joint with three different welding parameters are shown in Fig. 9. As is shown in Fig. 9(a), the fracture surface of Case 1 is relatively flat and the fracture surface is at an angle of 45 degrees to the direction of tensile stress, which shows obvious characteristics of ductile fracture. From the magnification diagram of the local position, as shown in Fig. 9(b), it can be seen that there are many small and deep dimples that show a state of aggregation state, indicating that the good toughness of the BM. Differently, the fracture surface of Case 2 shows obvious cleavage fracture characteristics and its surface is divided along the diagonal line, showing obvious differences between the top and the bottom according to the Fig. 9(c). The upper surface of the fracture is relatively flat and there are no obvious defects. Differently, there are a large number of porosity that appears in a certain aggregation state on the lower surface of the fracture. This phenomenon has a great impact on the mechanical properties of the joint and also explains that why the tensile strength of Case 2 is the lowest. Besides, from the view of the morphology of the fracture surface at high times of the local position, there are also some small and shallow dimples just as shown in Fig. 9(d). In addition, it can be seen from Fig. 9(e) that the fracture surface of Case 3 is relatively uniform, and compared with Case 2, the porosity defects are also reduced a lot. From the magnification of its local position, it can be found that the dimple shape and size distribution on the fracture surface are different. The specific performance is as follows: the size of dimple is larger than that of Case 2, and the dimple aggregation is less obvious than that of Case 1.
In order to further analyze the strength loss of joint, the element analysis is carried out on this part. The composition analysis of the fracture part of Case 2 is shown in Fig. 10. It is not difficult to see from the EDS analysis results that the contents of main elements are dissimilar at different positions of the fracture. And it can be seen from Fig. 10(a-b) that in the dimple of fracture, Al and V elements have certain burning loss, especially V element, which is almost completely burned. In addition, the content of Ti around the porosity is lower than that in TA15 titanium alloy, which indicates that the element is seriously burned, and the burning loss of V element is similar according to Fig. 10(c-d). However, there is an obvious difference that the relative content of Al element around the increases, even higher than that of the BM, which is mainly related to the characteristics of Al. Due to a large amount of burning loss of V, a stable element, the strength of the joint is affected to a certain extent.