“Inks” for 3D Printing

Currently, the adoption of 3D printing based on inkjet printing has been limited to the lack of suitable materials with those available often tied to a specific process. In recent years, some progress has been made in developing printing materials, polyamides polyurethanes, polyureas and self-healing materials, metal nanoparticle loaded inks, polycaprolactone and polyethylene glycols. However, in each of these cases, a single material development has taken a significant time, often in the order of months.

Over the last years, CfAM has developed a suite of high throughput materials assessment systems, which has enabled the synthesis of libraries of suitable materials for the production of the next-generation biomedical devices (e.g. FDA approved) and electronic products (photovoltaic devices, wearable electronics, printed electrodes, etc.).

What makes a material “printable”?

Each additive manufacturing (AM) technology has specific requirements for the feedstock material that is used to build the final product. These requirements vary depending on the technology and can include factors such as the material's composition, shape, size, and physical properties. The process requirements define the so-called “printability” of the materials. For instance, in the context of inkjet 3D printing, inks must have the appropriate viscosity to flow through the printhead and be able to form a stable droplet, appropriate surface tension to create a stable droplet as it is ejected from the nozzle and suitable stability at the temperatures and pressures experienced in the nozzle to be reliably jetted. Minor changes in any one of these requirements can have a significant impact on the final print quality. Much of the research at CfAM has focussed on developing methodologies that would allow rapids identification of printable materials and then demonstrate their use in industrially relevant products. 

Design strategies for the formulation of novel functional inks for 3D printing

Developing “inks” based on nanoparticles suspensions for 3D printing requires the choice of appropriate metal particle granulometry and loading fraction, as well as the identification of an appropriate organic carrier.

Nanoparticles are used to design the functionality of the ink, but residues of the organic carrier can, in fact, concentrate in between stacked layers that are being printed, impacting negatively on the macroscopic properties of the printed parts.

By understanding the evolution of the residual organic carriers, it is possible to consider optimisation strategies to overcome the functional anisotropy and maximise material’s properties. 

Strategies for new material development currently investigated at CfAM include the design of inks with organic carrier of controlled volatility and break-down excitation upon light exposure. 

Metallic Materials

Similarly to the case of materials for inkjet 3D printing, the pallets of printable metallic materials is currently limited. Although material innovation in metal additive manufacturing is rapid evolving, only a handful of printed alloys are being comprehensibly characterised and report properties compatible to conventionally manufactured (cast or wrought) materials.

Research at CfAM focusses primarily on metallic materials for laser-based AM processes. As metallic materials of poor printability typically exhibit poor melt pool behaviour, stress concentration upon cooling or formation of unfavourable phases during solidification, research has been concentrated on the identification of those features which make one alloy amenable to laser processing and, where needed, alloy adaptations to enhance processability.

At the heart of this research lies a thorough understanding of the technological innovations in laser modulation and (multi-)beam path control, which is deemed to be crucial to enable the realisation of additively manufactured parts with microstructures by design and tailored properties.

Design strategies for novel alloys for laser powder bed fusion

The interest and demand of high-strength alloys,for example, alloys from the 2000 and 7000 series,in laser additive processes has increased steadily in the last decade. Nevertheless, this class of alloys remains challenging, as hot cracks are frequently observed, compromising part integrity. These hot cracks can be mitigated by unravelling the link between the thermo-mechanical aspects associated to the process (e.g. the dwell time and energy distribution of the beam) and the solidification characteristics of the alloy of interest.

High-fidelity modelling of the thermal conditions, associated to various laser processing regimes, as well as alloy design methods to control micro-segregation and grain structure formation are key tools to identify appropriate alloys and manufacturing regimes that avoid the formation of hot cracks.