Laser-induced forward transfer of pure metals

Abstract

Additive manufacturing offers several advantages compared to conventional methods of production, such as an increased freedom of design and a toolless production suited for variable lot sizes. In particular the printing concept has gained momen- tum for rapid prototyping and manufacturing, since it allows for unprecedented freedom in the design of novel products, and is not limited by the drawback of powder bed fusion methods, such as selective laser melting. However, nozzle based printing of metals remains challenging, since the melting temperature of most metals is similar to components within the printing nozzle. Therefore, metal printing has been limited to low-melting point metals and metal containing inks, which are generally not optimized for other material properties (e.g. strength, conductivity, and corrosion rate) and cost. Laser-induced forward transfer (LIFT), is a direct-write method allowing for direct deposition of a wide range of materials including metals like aluminum, chromium, copper, gold, nickel and tungsten. In LIFT of metals, the energy of the laser pulse is absorbed by the metal, resulting in the generation of a thermal stress wave or evaporation of a part of the metal film, which subsequently leads to the ejection of a metal micro-droplet. Subsequently the droplet is deposited on the receiving substrate, on which the product is to be printed. However, the exact ejection mechanisms of these droplets are still under debate. Therefore, this thesis provides a detailed study on the ejection mechanisms during picosecond and nanosecond LIFT of copper and gold. High-speed imaging experiments were performed in order to visualize fluence dependent ejection dynamics and hence to identify and characterize ejection regimes during LIFT. To interpret those ejection regimes, the physical conditions in the metal film were assessed. To that end, the response of the material to the absorbed laser pulse was computed using a numerical two-temperature model. From that, two driving mechanisms, namely laser-induced stress relaxation and the vaporization of the metal film were studied and discussed. It was found that the generated stress distribution is key to the interpretation of the observed ejection dynamics and to the explanation of the ejection fluence threshold. Apart from the ejection dynamics, also the deposition process as well as the production of complex metal parts with dimensions in the micrometer scale were addressed. Hereunto, the presented work provides first studies towards full 3D printing capabilities demonstrated by the manufacturing of high-aspect ratio pillars of copper and gold. In addition to the printing of liquid phase metal droplets, also the transfer of thin metal films in solid phase was studied. To this end, an advanced LIFT setup was developed and successfully employed to demonstrate the solid phase transfer of thin films.