Objective Light metal and thermoplastic polymer have excellent material properties, and the mixed structure formed by the combination of light metal and thermoplastic polymer can meet the requirements of structural performance and lightweight design in automotive industry and aerospace field. The combined application of these two materials is an important way to improve structural performances and reduce energy consumption, so it can be seen that it is very important to realize the connection between them. Laser direct jointing technology has attracted extensive attention in the industrial sector due to the advantages of high welding efficiency, less defects, and flexibility. In existing studies, there are still some shortcomings of laser direct connecting of metal and polymer. On the one hand, when welding aluminum alloy materials with a high reflection property, most of laser energy is reflected which leads to low energy efficiency, and the reflected light is easy to damage laser optical elements. On the other hand, in order to improve the bonding strength, some scholars have proposed to pretreat the metal material surfaces, but the process is complex, the cost is high, and there is still the problem of chemical reagent pollution. More importantly, the treatment area of the material surface is not easy to control, which influences the appearance and performance of the unconnected area. Laser direct jointing technology has a broad application prospect. We hope that through the research in this paper, one can explore a more efficient, high-quality, low-cost laser direct connection and pretreatment process.
Methods The research objects are A5052 aluminum alloy and PBT, which are widely used. Firstly, the upper surface of aluminum alloy is blackened with a black marker, and then the joint of the lower surface of aluminum alloy is oxidized locally by a nanosecond pulse laser. Then, the effect of oxidation power on the surface morphology of aluminum alloy is observed by the scanning electron microscope, and the oxygen content on the surface of aluminum alloy is analyzed by the EDS system. The surface roughnesses of aluminum alloys after oxidation are detected by laser confocal microscope under different laser oxidation powers, and the changes in the surface contact angles of aluminum alloys under different oxidation powers are measured by the contact angle measuring instrument, so as to further analyze the effect of laser oxidation treatment on the surface wettability of aluminum alloy. The effect of oxidation treatment on the laser bonding strength of aluminum alloy and PBT is tested by the tensile test, and the changes in the surface chemical compositions of aluminum alloys before and after oxidation are analyzed by X-ray photoelectron spectroscopy (XPS) to explore the effect of oxidation treatment on the surface chemical composition of aluminum alloy and whether a new chemical bond is formed at the bonding interface. Through these means, one can reveal the mechanism of connection.
Results and Discussions A micro-nano structure is formed on the surface of aluminum alloy oxidized by laser, and with the increase of laser oxidation power, the more attachments appear on the surface (Fig. 3), the roughness gradually increases (Fig.6), and the oxygen content on the surface also gradually increases (Fig. 4). When the laser oxidation power is 19 W, the oxygen content (mass fraction) is 13.64% and the oxygen content on the untreated aluminum alloy surface is only 0.9%, which is increased by about 14 times. The surface energy of untreated aluminum alloy is 74.61 mN/m. With the increase of laser oxidation power,the surface energy of aluminum alloy first decreases to 44.68 mN/m, and then increases rapidly to 83.13 mN/m (Fig. 10). Laser surface oxidation treatment obviously improves the surface energy of aluminum alloy, which greatly improves the surface wettability of aluminum alloy. Through the XPS analysis, it is found that a large amount of Al2O3 is formed on the surface of aluminum alloy after laser oxidation, which is also the reason for the increase of oxygen content on the surface of aluminum alloy, and the thickness of the new Al2O3 layer is obviously thicker than that of the naturally formed Al2O3 film (Fig. 11 and Fig. 12). Based on the analysis of the interface of the stripped joint, it is found that the Al2O3 layer formed on the surface of aluminum alloy by laser surface oxidation treatment promotes the chemical reaction between aluminum alloy and PBT during welding, resulting in new bonds, which are Al—O—C and Al—C (Fig. 14). Chemical bonding is one of the key factors to improve the strength of the joint.
Conclusions After laser surface oxidation treatment, the micro-nano structure is formed on the surface of aluminum alloy, and the surface roughness and the oxygen content increase obviously, which increase with the increase of laser oxidation power. During welding, the melted PBT material flows into the micro-nano structure of the surface, forming a strong anchoring effect. When the welding power is too large, the PBT material near the weld decomposes and produces large bubbles, and the existence of air bubbles adversely affects the strength of the joint. Laser surface oxidation treatment can effectively improve the surface wettability of aluminum alloy, which is conducive to the wetting and spreading of molten PBT material on the aluminum alloy surface, promote the anchoring connection between aluminum alloy and PBT, and effectively improve the strength of the two welded joints. Through the XPS analysis, it is found that a large amount of Al2O3 is formed on the surface of aluminum alloy after the laser oxidation treatment, which promotes the chemical reaction between aluminum alloy and PBT at the interface during welding, resulting in new bonds, namely Al—O—C and Al—C. Chemical bonding effectively increases the strength of the welded joint. Laser oxidation treatment of aluminum alloy and PBT in the laser direct connection results in mechanical connection, physical connection, and chemical connection, so it can effectively improve the strength of the joint.
