Joining light materials have become an important way to achieve the lightweight of automotive, rail transportation, and other fields. Carbon fiber reinforced nylon composites (CF/PA) are thought to a promising application due to high specific strength and modulus. However, a hybrid joint of lightweight metals and CF/PA is necessary when considering the cost of CF/PA. Moreover, joining aluminum alloys with high thermal conductivity and low price to CF/PA plays a significant role in saving energy and reducing emission. To overcome the shortcomings of traditional mechanical connection and adhesive bonding, various welding methods have been investigated for joining aluminum alloys to CF/PA such as laser welding, friction stir welding, and induction welding. Thereinto, the laser as a heat source possesses a flexible controllability and a less heat affected area to base materials. A high-quality joint can be obtained via optimizing the process parameters and strengthening the interface bonding though some differences in physical and chemical properties exist between two materials. The melting point of CF/PA66 is higher than those of the other PA composites, which causes a narrow process window. So far, few researches are performed to investigate the laser joining of aluminum alloys to CF/PA66. Therefore, in the current study, laser power is chosen as a variable to confirm the possibility of direct joining aluminum alloys to CF/PA66, and the bonding mechanism is finally revealed.
The 1.5 mm-thick 6061 aluminum alloy and a 3 mm-thick carbon fiber reinforced composite (CFRTP) are selected to perform the laser direct joining. The resin substrate of CFRTP is polyhexamethylene adipamide (PA66). The traveling speed and defocus distance are kept constant, and the laser power is changed from 900 W to 1300 W with an interval of 100 W to investigate the effect of heat input on the interface bonding and the corresponding joint characteristics. Tensile shear tests with a stretch speed of 2 mm/s are adopted to evaluate the mechanical properties. The melting width of CFRTP and the actual contact area between the aluminum alloy and the CFRTP are extracted by ImageJ software. Interface bonding is observed by optical digital microscope (OM) and scanning electron microscope (SEM). Fracture surfaces are also detected by SEM. Chemical bonding is determined by combining the analysis of energy spectrum (EDS) with X-ray photoelectron spectroscopy (XPS).
An excellent weld formation without defects is obtained under the condition of the selected parameters (Fig. 4). The melting width of CFRTP is increased from 9.3 mm to 14.8 mm when the laser power increases from 900 W to 1300 W (Fig. 5), which could expand the contact area between the aluminum alloy and the CFRTP. Interface bonding under different laser powers is comparatively observed (Fig. 6). Enlarged SEM morphologies are also presented to further observe the interface (Fig. 7). Pores occur at the interface when the temperature exceeds the decomposition point of the CFRTP, which is detrimental to improve the strength of joints. Therefore, when the tensile shear force of joints is discussed, a tendency of first increasing and then decreasing is found, and the maximum value is obtained at a laser power of 1100 W (Fig. 8). The tensile shear force is first increased from 2083.9 N (tensile shear strength of 6.4 MPa) to 2571.6 N (tensile shear strength of 10.2 MPa) when the laser power increases from 900 W to 1100 W, and subsequently is decreased to 2114.7 N (tensile shear strength of 7.8 MPa) when the laser power increases to 1300 W. Interface failure is determined by analyzing the fracture surfaces under different laser powers (Fig. 10). A diffusion of Al, C and O elements at the interface is detected (Fig. 12). Based on this result, the XPS analysis is performed to confirm the existence of new chemical bonds including Al—O—C bond and Al—C bond (Fig. 13). This provides the evidence for the direct bonding of aluminum alloys and CFRTP by a laser.
To investigate the process and bonding mechanism of aluminum alloys and CFRTP during the laser direct joining, laser power is changed to reveal the macroscopic morphologies of the joints, the melting width of CF/PA66, and interface bonding under different heat inputs. The relationship between laser power and tensile shear properties of joints is thus established. Results indicate that the melting width of CF/PA66 increases as the laser power increases. The bearing capacity of joints is thus improved because of expanded actual contact areas. However, when the heat input is excessively high, some pores will be produced at the interface due to the decomposition of resin, which will weaken the joint properties. The maximum tensile shear force of 2571.6 N (tensile shear strength of 10.2 MPa) is obtained when laser power is 1100 W. The formation of new chemical bonds including Al—O—C bond and Al—C bond is identified during the laser direct joining. A tight metallurgical bonding at the interface between the aluminum alloy and the CF/PA66 forms, and the direct-bonding joints with high quality are thus obtained.
