4.1 Groove configuration fabricated on Al alloy surface
The purpose of fabricating groove on Al alloy surface is to improve the adhesion between CFRTP and Al alloy during the laser joining process. The depth and width of the groove have an important influence on the joining effect and mechanical properties of CFRTP/Al laser joints. In the laser surface texturing process, the high energy pulse laser beam repeatedly scans the Al alloy plate, and finally forms the surface groove of Al alloy, as shown in Fig. 4. The groove with a length of 25 mm is arranged in parallel along the width direction of the sample, and the spacing between the groove is relatively uniform. The groove depth and width are controlled by processing times, which indicates the depth and width increase with the number of processing repetitions. As expected, groove configuration with various width and depth is achieved by laser surface texturing. At the same time, groove shape is found to vary with the number of processing repetitions.
The optical micrographs of the groove structure are shown in Fig. 5. When the pulse laser scanning is repeated 40, 55, 70 times, the depth of the groove on the Al alloy surface is 0.5, 0.6, 0.7 mm, respectively. As shown in Fig. 5(a), the phenomenon of mechanical interlocking at the interface is not observed when the Al alloy is untreated. After laser textured Al alloy, the melted resin can fill in the textured groove as shown in Fig. 6(b)-(f). When the groove width of the Al alloy surface is small, the groove presents a “triangle” shape, as described in Fig. 5(b), (e) and (f). This phenomenon is related to the degree of laser texturing on Al alloy surface. In the process of laser texturing, with the increase of textured depth, the molten Al alloy forms a “recasting zone” at the bottom of the groove. At the same time, the heat at the bottom of the groove diffuses faster, resulting in less molten of Al alloy, thus forming a “triangle” shape. Compared with Fig. 5(a) and (e), the groove depth with 0.7 mm indicates a larger width at the bottom of the groove in Fig. 5(f) due to increased texturing scanning and larger heat input. In addition, it can be seen from Fig. 5(b), (c) and (d) that with the increase of textured groove width, the groove shape gradually tends to be a “rectangular” shape.
4.2 Mechanical properties of CFRTP/Al laser joints
Figure 6 shows the tested shear strength of CFRTP/Al laser joint under various CFRTP/Al laser joint groove configuration on Al alloy surface. The shear strength of specimens without laser texturing is 18.51 MPa. As expected, the shear strength of CFRTP/Al laser joint is obviously increased when conducting laser textured Al alloy surface. As for the groove depth, the shear strength gradually increases as the laser textured groove depth increases from 0.5 mm to 0.6 mm. The maximum shear strength of 24.33 MPa is attained at the laser textured groove depth of 0.6 mm. Then, the shear strength decreases to 21.66 MPa at the laser textured groove depth of 0.7 mm as the laser textured groove depth further increases. As for the groove width, the shear strength is found to increase as the width of the groove increases from 0.3 mm to 0.5 mm. The maximum shear strength is 22.8 MPa at the laser textured groove width of 0.5 mm. With further increase of the laser textured groove width, the shear strength decreases to 21.73 MPa at the groove width of 0.7 mm. In short, with increasing groove depth or width, the tensile-shear strength of CFRTP/Al laser joints firstly increases and then decreases.
SEM results of fracture surface of CFRPT/Al laser joint with various groove configuration are exhibited in Fig. 7 to analyze the failure mechanism. For all studied cases, the residual resins adhered on the CFRTP surface can be detected and original CFRTP substrate morphology can be observed. It is noted that the residual amounts of resins on the fracture surface indicate the adhesion ability with various groove configuration, which is related to the shear strength of CFRPT/Al laser joint. As shown in Fig. 7(a) and (b), the failure occurs at the interface of CFRPT/Al laser joint, i.e., a large number of residual resins remain on the groove of Al alloy. This suggests that fracture mode of laser joints under the case of No. 1 is the mixed failure including interface failure and cohesive failure under the action of tensile-shear force. However, the raised resins adhered on the CFRTP surface are observed in the case of Nos. 2 and 3 in Fig. 7(c)-(f). This phenomenon reveals that the failure occurs on the interface and interior of CFRTP/Al laser joint during the tensile-shear test, which suggests that fracture mode of CFRTP/Al laser joint is also mixed failure, but cohesive fracture of CFRTP/Al laser joint accounts for a large proportion. When the width of the groove is large, it is not easy to crack in the root region with the tensile stress and shear stress. At this time, the crack initiation location appears in the middle of the filled resins, and the proportion of cohesive fracture of tensile specimen increases accordingly. Hence, the shear strength of CFRTP/Al laser joint rises with the increase of groove width and resin filling amount, which is consistent with the tensile-shear test results in Fig. 6. As the groove width further increases, the melted resins fill into the micro-textures ineffectually and decrease the actual joining area between resins and Al alloy. Therefore, the shear strength of No. 3 is lower than that of No. 2.
Figure 7(g) and (h) exhibits the SEM morphology of typical fracture surface in Al alloy side under the condition of laser textured Al alloy surface with 0.6 mm groove depth. A large number of melted resins are found to be filled into the textured groove. At the same time, the failure site occurs in the root of filled resins, indicating that the joint strength between resins and groove wall is large. This phenomenon implies that the fracture mode of the tensile specimen features mixed fracture and cohesive fracture accounts for a large proportion, which also further promote the interlocking of melted resins to textured groove of Al alloy substrate. When the groove depth increases to 0.7 mm, the fracture mode is also characterized by the mixed fracture between the resins and Al alloy surface. But there is still a small amount of cohesive fracture of the resin layer observed in local areas, and some carbon fibers are exposed, as indicated in Fig. 7(i) and (j). The above results suggest that joint strength correlates with fracture mode of CFRTP/Al laser joint at the joining region.
4.3 Simulation of interface temperature field
Figure 8 shows the comparison between the experimental results and simulation results of the temperature field in laser joining of CFRTP to Al alloy. The experimental dotted line was drawn along the isotherm of melting temperature. It can be observed that the predicted weld profile is well consistent with experimental results, which indicates that the thermal representation in the model is reasonably accurate. Besides, the heat source model is suitable for the structure of the CFRTP/Al lap laser joining process of finite element simulation.
Figure 9 depicts the calculated temperature field at the cross section of CFRTP/Al laser joint as a function of groove width and depth. The phenomenon of the different temperature profiles in the CFRTP/Al laser joint is discovered owing to varying groove configuration during the joining process. Within the bonded region of resins, it can be seen that the interfacial temperature of CFRTP/Al laser joint lies between its melting point (343℃) and gasification temperature (520℃). When the interfacial temperature is in the range of 340–460℃, the resins can closely attach to the Al alloy since they are melted at 340℃ and vaporized at 460℃ or higher. As shown in Fig. 9(b), (c) and (e), when Al alloy surface is textured by pulse laser, the temperature of the groove fusion zone and the interface fusion zone is below the decomposition temperature of resins, and its decomposition does not occur in this zone. Thus, resins filled in the groove are completely melted in the laser joining process. However, when the groove depth is 0.7 mm in Fig. 9(f), it is found that the temperature of the groove bottom in groove fusion zone is higher than resin decomposition temperature. In this case, the resin decomposition occurs, and the defects such as gaps and bubbles are easily formed in the groove, which can deteriorate the joint strength. When the groove width increases from 0.4 mm to 0.7 mm in Fig. 9(d), the temperature at the bottom of the groove is higher than the resin decomposition temperature due to the accumulation of heat in the area below the molten pool. As a result, the resins tend to decompose, forming bubbles and other defects, which is not conducive to forming good joints.
The thermal cycle curves of the groove bottom under the condition of various groove configuration are presented in Fig. 10. Overall, the peak temperature tends to surpass the melting temperature of resins for various groove configuration. It should be also noted that the maximum temperature of samples Nos. 1, 2, and 4 is all lower than the decomposition temperature of resins. Thus, the molten resins filled in the groove do not decompose during the laser joining process, theoretically reducing the possibility of forming bubbles, gaps and other microscopic defects at the interface. However, the maximum temperature of samples Nos. 3 and 5 is higher than the decomposition temperature of resins. So, the partially melted resin in the groove is decomposed into CO2 and other gases due to the high temperature, tending to produce non-fusion defects at the interface. This may reduce the bonding area of the molten resins and Al alloy surface. Macroscopically, the shear strength of CFRTP/Al laser joint is deteriorated.
4.4 Failure analysis of CFRTP/6061 Al alloy laser joints
In the laser joining of CFRTP to Al alloy, the melted resins at the interface fill into the micro-textures on the Al alloy surface under the compression of external pressure, where mechanical interlocking is achieved to fasten two joining components. The interface bonding between Al alloy and CFRTP largely depends on the van der Waals force and mechanical bonding force. In general, the mechanical binding force is featured by the interlocking force that is formed after cooling and curing of molten resin filling the textured structure on the Al alloy surface, which is dominant at the interface. Based on above results, for 6061 Al alloy and CFRTP laser joint, the fracture mode of lap tensile specimens mainly includes interface fracture, cohesive fracture, and mixed fracture. When the cohesive fracture of CFRTP/Al laser joint accounts for a larger proportion, it is indicated that the shear strength of the joint presents a higher value.
It can be seen from Fig. 11 that two kinds of interface morphology are observed at the resin/Al joint in the groove. One is the serrated recasting interface and the other is a relatively smooth interface. It should be known that the recasting material layer not only increases the contact surface area between the resins and Al alloy but also enhances the wettability and the mechanical anchoring effect between resins and Al alloy [34, 35]. As a result, the serrated recasting interface tends to improve the shear strength of CFRTP/Al laser joint, resulting in the formation of cohesive fracture. In comparison, the smooth interface enables mostly interfaced fracture due to weak binding force.
Figure 12 shows the schematic diagram of fracture modes of CFRTP/Al laser joint when the tensile shear load is conducted. When a material is subjected to tensile force on both ends, apart from the tensile shear force parallel to the surface of CFRTP, the root of the resins filling into the groove structure has to bear the higher bending moment caused by the height-distributed force on the surface of resin protrusion. Hence, the root of the resin protrusion endures severe shear and tensile stress at the same time, and a crack easily appears there first [36]. Specifically, under the condition of the higher bonding force between resins and Al alloy at the inner wall of groove structure, the resins tend to break from the root or inside and remains a lot in the groove structure, which presents a cohesive fracture. When the inner wall of the groove structure is relatively smooth and only a small amount of recasting material appears, it is easier for filling resins to be peeled out from the groove structure to generate interface fracture. As shown in Fig. 7, for various groove configuration, the fracture mode of CFRTP/Al laser joint has the feature of mixed fracture including cohesive fracture and interface fracture. It is worth noting that the fracture mode of CFRTP/Al laser joint converts from interface fracture to cohesive fracture, which can contribute to the improvement of joint strength. Therefore, it is suggested that the values of groove width and depth should be set an appropriate range towards the popularization and application of laser surface texturing according to our experiences.