Lightweight structure has become a focus in industrial fields. One of the research hotspots is the effective joining of dissimilar materials. Because of their high specific strength, titanium alloys have been widely used in aerospace and automotive engineering applications. Carbon fiber reinforced plastics (CFRP) have many advantages over traditional metals, such as higher specific strength, fatigue resistance, and corrosion resistance, and have a broad application prospect to further realize lightweight. The laser joining of titanium alloy and CFRP can combine the advantages of the two materials and broaden their application. However, because of the large differences in microstructure and physical properties between the two base materials, the joining strength is relatively low. The properties enhancement of metal/CFRP joint can be achieved by improving mechanical interlocking and chemical bonding. The laser texturing process can fabricate a microstructure to improve the surface roughness of metal, as well as change the chemical state of the metal surface. Thus, the laser joining process of TC4 to CFRP via laser texturing TC4 surface and the strengthening mechanism of the interface were studied by us.
Ti-6Al-4V alloy (TC4) and polyether ether ketone reinforced by 30% carbon fibers (CFRP) were selected as base materials. The resin matrix of the composite material was polyether ether ketone (PEEK). The TC4 sheets were treated by laser texturing before laser joining. To obtain ideal micron-scale grooves, the spacing between each grid was 1 mm, and each grid line was filled with multiple equidistant scan lines. The number of scan lines was adjusted to control different micro-texture widths, and the micro-texture width in this study were set to 0.1-0.5 mm, as shown in Fig. 1. To evaluate the mechanical properties, tensile shear tests with a stretch speed of 0.5 mm/s were used. A high-temperature wetting angle measurement system was used to characterize the wettability of melted PEEK under different TC4 surface states. The optical digital microscope (OM) and scanning electron microscope (SEM) were used to examine the three-dimensional morphology of the laser textured surface, interface, and fracture surface of TC4/CFRP joints. The chemical bonding at the TC4/CFRP joint was examined using an X-ray photoelectron spectroscopy (XPS) analysis system.
The introduction of micro-texture significantly increased the surface roughness of TC4, which first increased and then decreased with the increase of micro-texture width, when compared to the untreated TC4 surface. The canalization effect improved wettability significantly, as shown in Figs. 5 and 6. CFRP melted and completely filled the textured grid after laser joining with widths of 0.1 mm and 0.2 mm. When the width of the texturing grid was too wide, the molten CFRP could not be completely filled in the grid, as shown in Fig. 8. As shown in Fig. 10, new chemical bonding, such as Ti-C, was discovered at the treated interface, indicating that chemical bonding occurred. The shear force increased significantly after texturing compared with the untreated joint. The maximum tensile-shear force in the case of 0.2 mm micro-texture width was 2596 N, which was 154% higher than that of the untreated joint. The tensile-shear force of the TC4/CFRP laser joints increased first as the laser textured micro-texture width increased. The tendency was similar to that of TC4's surface roughness. A large amount of resin-carbon fiber mixture adhered to the fracture surface of the textured TC4 side, as shown in Figs. 12 and 13. The failure mode included interface failure, cohesive failure, and TC4 matrixes stripping from the substrate due to relatively high interfacial joining strength after laser texturing, indicating mechanical property enhancement.
Laser texturing was used in the laser joining TC4 and CFRP. The grid pattern was used as the texturing pattern, and the effect of micro-texture width on joint strength was investigated. After laser texturing, the surface roughness of the TC4 surface and the wettability of molten CFRP to TC4 were significantly improved. With a micro-texture width of 0.2 mm, the surface roughness of TC4 could be increased approximately 15 times compared with untreated TC4 and the contact angle reduced from nonwetting to 49.9°, demonstrating that laser texturing could improve the affinity of molten CFRP on TC4 substrate. The maximum tensile-shear force in the case of 0.2 mm micro-texture width was 2596 N, which was 154% higher than the untreated joint. The surface failure mode was a hybrid failure mode including interface failure and cohesive failure. After laser texturing, TC4 matrixes are stripped from the substrate, indicating that the texturing grid may promote mechanical interlocking and increase the tensile-shear force of joints. New chemical bonding was confirmed at the joining interface, implying that chemical bonding occurred at the interface due to the high interfacial temperature. The laser texturing process increased the contact area of the joining interface, improving mechanical interlocking. Surface modification of the TC4 substrate can be achieved using laser texturing, promoting the formation of chemical bonding between CFRP and TC4 to further strengthen joints.
