Ultrasound : aided laser joining of metals to plastics

Abstract

In recent years, much interest has been shown in joining dissimilar materials such as light metals and plastics using laser technology, especially in the automobile and biomedical industries. To this end, a direct laser bonding technique for joining metals to plastics, namely laser-assisted metal and plastic (LAMP) joining, has been developed over the last ten years. It uses laser light to heat up the metal-plastic interface, and the plastic at the interface is melted and partly decomposed. This results in the formation of gas bubbles in the molten plastic. It is generally accepted that the high pressure developed inside the laser-induced bubbles causes the molten plastic to have intimate contact with the metal surface, and as a result, a strong bonding between the metal and plastic parts is attained. Despite a number of successes that have been obtained in the LAMP process, the inherent features of laser-induced bubbles in the joint are still a main concern because the bubbles themselves are defects by nature, and therefore have a counter effect on the joint strength. In this research, a new ultrasound-aided laser joining process - using the conjoint action of a laser and ultrasonics - for joining metals to plastics (U-LAMP) has been successfully developed, and has been shown to be able to eliminate the laser-induced bubbles with increased joint strength. For the first phase of the research, an ultrasonic generation system with an appropriate specimen clamping tool was designed and fabricated for studying the U-LAMP process. The system allows the specimens to be joined to be ultrasonically vibrated while under laser irradiation. For the second phase of the research, the new U-LAMP process was employed to join polyethylene terephthalate (PET) to pure titanium (Ti) to show the superiority of U-LAMP over the conventional LAMP. The effects of ultrasonic vibration on the chemical bond formation at the joint interface, removal of bubbles in the joint, and the shear and fatigue properties of the joints were studied. Under a low laser power condition, where decomposition of the plastic was avoided and therefore the laser-induced bubble problem was not encountered, the benefit of ultrasonic action was found to be in the promotion of the formation of chemical bonds (TiO₂, Ti₂O₃, and Ti-C) at the joint interface. This was supported by the results of XPS analyses conducted across the joint interface. As a result, the joint strength can be increased by as much as seven times that produced by the conventional LAMP process. While joining at high laser power, where laser-induced bubbles are present, the primary function of ultrasonic vibration is to eliminate bubbles in the joint. With a proper ultrasonic tool design to create a differential pressure field in the joint specimen, bubbles in the molten plastic pool can be made to escape from the joint zone. The effects of ultrasonic vibration on the bubble dynamics, bubble translation path and bubble removal mechanism were analysed based on the ultrasonic vibration induced pressure field. The bubble translation path in the molten pool was also studied using a high speed camera, and the captured bubble movement path agrees well with the predicted path of the pressure field analysis. In regard to the mechanical properties, the joint strength of the U-LAMP specimens, measured in terms of the fracture load, was significantly higher than that of the LAMP specimens. Moreover, for the former, fracture occurred in the parent plastic material, whereas for the latter, almost all fractures occurred at the metal-plastic interface. This shows that the metal-plastic interfacial bond strength of U-LAMP specimens is higher than that of LAMP specimens. The superiority of U-LAMP over the conventional LAMP is also evident by the extended fatigue life of the U-LAMP specimens over that of the LAMP specimens, at least one order of magnitude higher. The improvement in fatigue life is due to the absence of porosity in the U-LAMP specimens which also exhibit strong interfacial bonding. The different fatigue crack propagation paths in both LAMP and U-LAMP joints were established.