3.1 Weld formation and microstructure of joints
The weld bead characteristics of the LFB joints under different groove angles are exhibited in Fig. 2(a-d). Large spatters are observed on the top surface of LFB-90 joint due to the unstable keyhole. Both the size and number of spatters decrease with the decrease of groove angle. Since the laser beam offset distance from the lower edge of the titanium alloy groove to the aluminum alloy side is kept same in all experiments, reducing the groove angle results in diminishing the heat input to the titanium alloy side under same laser power. Large heat input may cause the titanium alloy to be melted and then to enter the aluminum alloy molten pool, which causes instability of the keyhole and spatters, as shown in Fig. 2(a). But too low heat input may cause the defect of unwetting of molten aluminum alloy on the top surface of titanium alloy, as exhibited in Fig. 2(d). The appearance of weld bottom surface is satisfactory except for the LFB-60 joint, a regular wetting of aluminum alloy occurs without uneven hump defect. Unwetting defects at the bottom surface are observed when the groove angle is 60°, the majority of laser energy is used to melt the aluminum alloy base metal and to fill the groove in this case, resulting in insufficient heat input to the back surface.
Figure 3(a-d) shows the cross-sections of the LFB joints obtained under conditions of four different groove angles. The interface between the aluminum weld metal and TA31 base metal remains straight except for the LFB-90 joint. Minor melting of titanium alloy is observed at the top and middle region, which is marked in Fig. 3(a). Cracks in the aluminum weld metal occurs when the groove angle is 60°, as can be seen in Fig. 3(d). Acceptable Al/Ti fusion brazed joints are obtained when the groove angles are 80° and 70°, no obvious defects such as porosity and cracks are found on a macroscopic scale, as shown in Fig. 3(b-c). Slightly concave top surface and convex back surface of the weld metal can be found due to the low surface tension coefficient and viscosity of aluminum alloy. The spreading and wetting of aluminum alloy on the bottom surface of titanium alloy is beneficial to improve the convex phenomenon of the weld seam, which provides additional force to resist the gravity and recoil pressure in the keyhole.
To better understand the influence of heat input on the interface and weld microstructure under different groove angles, the typical metallographic structures in different zones of the fusion brazed joints are shown in Fig. 4. A large difference in the metallographic structure of varied joints can be distinguished, especially near the interfacial zone. As shown in Fig. 4(e), the Al/Ti interface zone can be partitioned into three areas at the macroscopic scale: the aluminum weld metal, the TA31 base metal and the HAZ of TA31 alloy side. The effect of heat input on the interfacial microstructure can be reflected partly by the shape and width of the HAZ. As displayed in Fig. 4(a-c), the HAZ is wide, and even the bulk titanium alloy base metal melting occurs at the top and bottom interface region in the LFB-90 joint. The HAZ width decreases dramatically with the decrease of groove angle. A narrow HAZ with uniform width at three typical zones is obtained in the LFB-60 joint, as shown in Fig. 4 (m-o), however, the unwetting defects of aluminum weld metal are found at the bottom of joint interface, accompanied by continuous cracks near the interface. As displayed in Fig. 4(e-g) and (i-k), the HAZ with relatively uniform width at three typical locations can be obtained in the LFB-80 and LFB-70 joints, which indicates a relatively uniform heat input distribution along the interface. Besides, the microstructure of weld metal is also critical in determining joint strength. Numerous acicular IMCs are formed in the weld metal of LFB-90 joints, and fine black precipitates were distributed in the dendrite base metal, as shown in Fig. 4(d). Dendrite grains with fine precipitates along the grain boundaries are formed in the other three remaining groups of joints.
The morphology, thickness and uniformity of the distribution in different locations of IMC are prime factors for determining the mechanical properties of the fusion brazed butt joints. So the interfacial microstructure at three regions (top, middle and bottom) are selected to compare the effect of groove angle, as shown in Fig. 5. As previously discussed, the melting of bulk titanium base metal occurs due to the excessive heat input at the interface in the LFB-90 joint, resulting in the formation of rod-like and cellular IMC along the interface, as shown in Fig. 5(a-c). Cracks appear at the root of TiAl3 IMC in the middle and bottom interface, which will significantly weaken the joint properties. As exhibited in Fig. 5 (d-f) and (g-i), the IMC morphology of the LFB-80 and LFB-70 joints are similar, characterized by the transition from rod-like to serrated and then to lamellar, from the top to the bottom interface. As shown in Fig. 5(j-l), extremely thin interfacial IMC is formed in the LFB-60 joint. Furthermore, it is characterized by serrated IMC at the top interface and lamellar IMC at the bottom interface.
To better compare the effect of groove angles on the thickness and uniformity of IMC along the interface, the averaged thickness of IMCs is counted and displayed in Fig. 6. Combined with the morphology characteristics shown in Fig. 5, it could be concluded that the rod-shaped IMCs are usually thicker and the lamellar IMCs are thinner than the serrated IMCs. The homogeneity of interfacial microstructure is observed in each group of joints, and it gets improved with the decrease of groove angles. The IMC thickness of the LFB-90 joint reaches a maximum of up to 5.54 µm at the middle interface due to the local melting of TA31 base metal (Fig. 4(b)), and the thickness of IMC at the top and bottom interface is comparatively small, but its minimum thickness still reaches 3.15 µm. The IMC thickness distribution of three groups of joints with smaller groove angles shows a better regularity, and the IMC thickness decreased from the top interface to bottom interface at different rates. The IMC of LFB-60 joint is the thinnest and most uniform in thickness, with only 0.27 µm at the bottom interface.
As shown in Fig. 5(b), the TiAl3 IMCs grow mainly perpendicular to the interface, and there are also blocky IMCs floating in the weld metal. We deduce that the element diffusion plays a major role in the formation of IMCs, whereas the convection in molten pool plays a role at specific locations. The element diffusion phenomenon at the interface is illustrated in Fig. 7 and Fig. 8. The main elements Al, Mg (in AA5083) and Ti (in TA31) diffuse into titanium base metal and aluminum weld metal, respectively. As shown in Fig. 7(a-d), the rod-like TiAl3 IMC is mainly composed of Al and Ti elements, and minor Mg element is also detected. The Al element is distributed on both sides of the interfacial crack, and the content of Al is low near the left side of the crack. No noticeable diffusion of Al element into the TA31 base metal occurs at the interface of LFB-80 joint and other joints with smaller groove angles. The element diffusion curves of three main elements at the middle interface of the joints are displayed in Fig. 8(a-d), approximately linear elemental change occurs at the interface. The stage that the elemental content changes without stability is defined as the thickness of diffusion layer. The diffusion layer thickness is reduced from 11.98 µm for LFB-90 joint to 3.65 µm for LFB-60 joint. Reducing the groove angle of the joints has a considerable impact on lowering the heat input at the interface, which in turn limits the diffusion of elements.
3.2 Interfacial crack and intergranular crack in weld
As shown in Fig. 9(a) and (d), the interfacial crack at the bottom interface of the LFB-90 joint and the continuous cracks in the weld of LFB-60 joint are formed with different characteristics. The presence of cracks will have a fatal effect on the mechanical properties of the fusion brazed joint. Therefore, the formation mechanism of interfacial cracks and intergranular cracks in the weld is emphasized in this part. The weld microstructure can partially reflect the growth characteristics of IMCs of the LFB-90 joint. Acicular IMCs consisting of numerous dispersed particles were formed in the weld, as displayed in Fig. 9(b-c). However, the acicular IMCs are integral blocks in the weld metal near the interface (marked in Fig. 9(a)), which means that the acicular IMCs are formed at the interface then float into the weld and react with the liquid aluminum metal during the fusion brazing process. Irregular-shaped pores are formed in the weld metal of LFB-90 joint, probably due to the keyhole instability aggravated by mixing IMCs into the molten pool. The intergranular cracks initiate from the bottom interface near the unwetting defects and propagate along the vicinity of the interface to the upper part of the weld, as displayed in Fig. 9(d-f). Apparent tracks of grain detachment can be observed in Fig. 9(f), but the shapes of the boundaries on both sides of the crack are not fully overlapped. It can be deduced that the intergranular hot cracks are formed when the aluminum weld metal is in mushy state, the aluminum grains on both sides of the cracks continue to deform even after cracking.
The phase composition, crystallographic orientation statistics and deformation of phases near the cracks are performed through EBSD analysis. Figure 10 (c, f) display the phase composition near the cracks in different colors. The continuous cellular TiAl3 phase in red color forms at the right side of interfacial cracks, and small quantities of aluminum atoms diffuse across the IMC layer into the titanium base metal. The floating TiAl3 IMCs are filled with distributed aluminum metal. As for the intergranular cracks in the weld, there is single aluminum phase on both sides of the crack. Figure 10(a, d) show the inverse pole Fig. (IPF) maps of Ti-TiAl3-Al near the interfacial cracks and Al phase near the intergranular cracks. The IMCs dispersed in the weld have different orientations and are distributed between the aluminum grains or inside the grains, while most of the IMCs grown perpendicular to the interface of the titanium alloy are oriented in (001). The intergranular cracks in the aluminum weld appear at the junction between the coarse grain region (zone A) inside the weld and the fine grain region (zone B) growing along the titanium alloy wall. Figure 10(b) and (e) show the kernel average misorientation (KAM) maps near the interface of cracks, revealing the deformation degree of different phases or neighboring grains. Deformation is localized in the titanium alloy matrix and TiAl3 IMCs near the interfacial crack. Similar deformation distribution conditions can be observed near the intergranular crack. The grain size and texture orientation of zone A and zone B are analyzed in Fig. 11. The coarse grain region and fine grain region exhibit a huge grain size mismatch, the average grain size of aluminum alloy in zone A and zone B are 130 µm and 31 µm, as shown in Fig. 11(a) and (b). Here we use Tmax to represent the texture orientation of microstructure in inverse pole Fig.s. The aluminum grains of zone A show random texture with Tmax of only 1.86, while the fine cellular grains of zone B exhibit strong texture orientation with Tmax of 6.75 (Tmax=1.0 represents a random texture). The microstructure mismatch including grain size misfit and texture orientation mismatch contributes to the formation of intergranular hot cracks.
A nano hardness test was implemented to reveal the difference in micromechanical properties of different phases near the cracks. Vibratory polishing was conducted to remove the residual stress on the surface before the test. Figure 12(a) displays the indentation marks at the top interface of the LFB-90 joint, the typical loading and unloading curves of different phases and the counted results of the mechanical properties are shown in Fig. 12(b, c). A noticeable difference in the indention size of Ti, TiAl3 and Al phase can be identified. The TiAl3 phase experienced the slightest deformation under the same loading force, thus it has the highest nano hardness and modulus of elasticity. The average nano hardness of TiAl3 phase is 6.33 GPa, almost five times that of the Al phase and twice that of the Ti phase. Residual stresses are introduced to the interface of Ti/TiAl3 and Al/TiAl3 since the vast difference in hardness and elastic modulus between dissimilar phases. The Al phase with good ductility and deformability (nano hardness of 1.40 GPa and elastic modulus of 99.29 GPa) can relieve the residual stress at the interface of Al/TiAl3. Therefore, the interfacial cracks tend to initiate at the interface of Ti/TiAl3 owing to the poor deformability of both under the combined effect of residual stress and thermal strain. It should be mentioned that the coarse and fine grains on two sides of the intergranular cracks have a difference in nanomechanical properties. The fine grains near the interface possess higher nano hardness and elastic modulus due to the solid solution effect of titanium element diffused from TA31 base metal and adjacent TiAl3 IMCs. The nanomechanical properties mismatch between the fine grain areas and coarse grain regions also aggravates the development of intergranular hot cracks.
3.3 Mechanical properties and fracture analysis
Tensile test was performed to compare the influence of groove angles on the mechanical properties of joints. The stress of fusion brazed joints was calculated through load value divided by the original cross-section dimension (15 mm ⋅ 6 mm) of the tensile specimens due to different fracture locations of joints. As shown in Fig. 13(b), The LFB-60 joints show extremely low strength due to the presence of continuous intergranular cracks in the weld. The LFB-90 joints possess the strength of nearly 90 MPa, and the upper part of the joint without interfacial cracks bears the load during deformation. The stress-strain curves of LFB-70 and LFB-80 joints exhibit similar shapes with typical plastic deformation characteristics. But the tensile strength and elongation of LFB-70 joints are much higher than LFB-80 joints, and show a typical Portevin–Le Chatelier effect that was also observed in the AA5083 base metal [24]. The LFB-70 joints show the highest tensile strength of 268.76 MPa, which is nearly 88% of that of the AA5083 base material (305 MPa). However, the elongation of LFB-70 joints is only 37% of the Al alloy base metal, the interfacial microstructure needs to be further modified to allow larger deformation of the aluminum alloy weld. Chen [25] performed laser oscillation welding brazing 5 mm thick AA6061 and 4.5 mm thick Ti6Al4V plates with an oscillating amplitude of 2 mm and frequency of 25–30 Hz to optimize the energy distribution. The maximum tensile strength of dissimilar Al/Ti joints reaches 173 MPa at the laser offset distance of 1.2 mm and oscillation frequency of 28 Hz. The heat input on the titanium alloy side decreases with increasing oscillation frequency, but inducing instability of the keyhole and producing porosity defects in the aluminum weld. The significant improved mechanical resistance of LFB-70 joints indicates that uniformly distributed interfacial microstructure and aluminum weld with few defects can be obtained by optimizing the groove angles of dissimilar plates, which works effectively even in the joining process of 6 mm thick plates.
The fracture morphology maps at the titanium alloy side of fusion brazed joint obtained at four different groove angles are shown in Fig. 14(a-d). Typical cleavage plane and cleavage steps are observed except for the LFB-60 joint. The marked phases in Fig. 14(a-c) are confirmed as TiAl3 + α-Ti phase, implying that the fracture locations of joints are inside the brittle TiAl3 IMCs. Cleavage planes with uneven size distribution are observed at the fracture surface of the LFB-90 joint, large size and uneven distribution of interfacial IMCs weaken the mechanical properties of joints. Similar cleavage characteristics are observed at the fracture surface of LFB-80 and LFB-70 joint, but the broken TiAl3 IMC grains are finer due to its thinner IMC layer thickness. Smooth fracture surface occurs at the LFB-60 joints. The formation of hot cracks causes the adjacent weld metal not to bear the load. A small part of effective connecting parts of aluminum weld metal bear the load and form the dimple features, as displayed in Fig. 14(d).
Phase compositions on the fracture surface of titanium alloy side were characterized through XRD analysis, and the results are shown in Fig. 15(a-d). Phases at the Ti alloy side of LFB-90 joint are full of TiAl3, TiAl and Ti3Al IMCs, while only TiAl3 IMCs are detected at the fracture surface of LFB-80 and LFB-70 joint. Combined with the fracture morphology in Fig. 14(a-c) and detected Ti phase in Fig. 15(a-c), fracture happens at the interface of IMCs and Ti alloy base metal for the three LFB joints obtained with larger groove angles. It should be mentioned that minor aluminum diffraction peaks are also detected, but they should not be counted in the scope of interfacial fracture. Because the samples for XRD test are taken from the entire fracture of tensile specimens, and there will be a small amount of residual aluminum at the top and bottom of the joint. The phase analysis results of fracture surface for the LFB-60 joint are displayed in Fig. 15(d), the diffraction peaks of Al from aluminum alloy weld metal and Ti from unwetted titanium alloy surface are detected, which is consistent with the fracture surface shown in Fig. 14(d).