Substrate design and aluminium droplet morphology study of a new 3D printing technique

Abstract

In recent years, additive manufacturing (AM) techniques have been developed quickly, attracting industry and home user attention. The AM market also expands and creates much economic effectiveness. To avoid the drawbacks of common technologies (high cost and dangerous work environment) and allow home users to print metal products, a new technique named Metal Droplet Extrusion (MDE) was developed at the University of Nottingham Ningbo China (UNNC) and supervised by Dr Adam Rushworth. For this technique, the previous researchers have successfully designed a print head and produced the controllable aluminium 6061 alloy droplets in the water base. However, a substrate system, generally made by a heater, a plate and a support structure that allows the droplets to stay and remelt to form an object, is required to provide a complete printer prototype and study the droplet impact behaviour. When droplets impact the substrate, they will have different morphology of the final product because of the distance, temperature and roughness; the final product morphology will affect the further remelting of the droplets and, hence, influence the final surface quality and structure strength of the printed products.
To design the substrate system and have a good understanding of the droplets’ impact behaviour on the solid surface, this thesis will finish 2 main objectives – substrate system and prototype design, and the experiments about four parameters that affect the droplet contact angle.
At first, 8 requirements of designing a substrate system have been outputted based on the previous literature review and the recent equipment in the lab. The whole design of the substrate system has been developed and changed 3 times in digital software. Different substrate plate materials or material sets have been tested by conducting high-temperature experiments to reach the maximum temperature requirement. After the substrate was manufactured and the feasibility tested, a printer prototype was assembled for further behaviour study. The prototype has also been tested for the possibility of printing simple one-layer and multiple-layer products. As for the droplet behaviour study, three factors that will affect the impact behaviour of droplets were examined by conducting different experiments, including different distances between the nozzle and plate, different substrate surface temperatures and surface roughness. After the experiments have been done, the pictures of the droplets will be taken and calibrated in the MATLAB program. Then, the contact angles of droplets will be measured by a software called camera measure. The contact angle results will be analysed to show how these factors affect the contact angle of the droplets and determine the best material for the substrate plate for the technique.
According to all experiments in this thesis, 95 aluminium oxide ceramics with a Mika cover to help accumulate heat are the best materials for the future study of this technique. This set of materials produces the highest temperature in the substrate system temperature experiments (532.4℃). It can produce 90° contact angle droplets with 165℃ with a roughness of 1.241μm and 1μm of the roughness when the temperature is room temperature. 304 stainless steel cannot provide 90° contact angle droplets; all the droplets’ contact angles were larger than 120°. As for the copper plate, 150 degrees of plate temperature and 1.8μm surface roughness will bring 90° of the droplet’s contact angle. Meanwhile, the rise in distance between the nozzle and the substrate surface will cause more splashed droplets. The contact angle of impacting droplets will increase with the increase of the surface roughness but decrease with the rise of the surface temperature.

Date of Award	15 Jul 2025
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Adam Rushworth (Supervisor), Hao Chen (Supervisor), Guang Li (Supervisor) & Yi Nie (Supervisor)