Materials development for additive manufacturing
Dr Kambiz Kalantari examines the benefits of Lucideon’s MIDAR technology for high-temperature additive manufactured materials.
A technology that is finding increasing use, additive manufacturing (AM) is not without its challenges. Product quality and performance depend upon many factors, not least of which are the materials chosen and used in the process. AM is used in many industries to create complex components with minimal material wastage and high customisation. Various techniques, such as laser sintering, are used to create precise components out of selectively fused metal or polymer powders, and material deposition extrusion, which deposits a predefined path to build a 3D structure in layers.
With the increasing use of additive manufacturing in industries such as healthcare, where tailored implants can be produced, and construction, materials and their development must be a key focus if AM is to reach its predicted growth of US$21bln by 2020.
Ceramics, metals and polymers are all used in AM, either singularly, or combined with composites to give new and improved properties. While ceramics have many advantageous properties, such as extreme hardness and wear-resistance, using them in the AM process, alongside other materials, can pose problems.
Specifically, the high temperatures required in the sintering of ceramics limits the materials that can be combined with them – additional materials must also be able to withstand these extremely high temperatures. If they cannot do so, failures can occur because of thermal mismatch as the materials cool and contract at different rates, causing stress and, ultimately, fractures.
A new proprietary technology, MIDAR, designed by Lucideon, uses a low-temperature geopolymer chemical reaction to produce chemically activated amorphous ceramics that do not require sintering and are stable at room temperature. The technology can be varied to accommodate different areas of forming, extrusion, moulding and casting. MIDAR uses a low temperature (<100oC) chemical reaction to consolidate aluminosilicate materials including coal fly ash, waste glass and blast furnace slags into a robust inorganic material with high-strength and chemical stability.
Early experiments into the ability of MIDAR-produced materials to create components using AM have been successful. The manufacturing process and ingredient concentrations were altered to adapt the material using an iterative process to obtain the optimum rheology conditions. The working time is crucial, as too fast and the material mixture will set within the nozzle, yet too slow and the ability for the material to be layered up would be compromised. The standard MIDAR formulation was refined to increase viscosity and structural stability, while reducing the setting time, allowing the material to be extruded soon after mixing, yet setting solid quickly enough to support subsequent layers. The parameters of set time and extrusion rates are crucial for successful AM and accurate optimisation of these is necessary. This can then be used to encapsulate waste or alternatively for construction products and ceramics.
One of the advantages of MIDAR technology lies in the high level of control it provides over the processing and subsequent performance through the ability to vary the material composition. By controlling the nature and amounts of individual components, the properties of the product may be accurately tailored to required specifications. In this instance, rapid-setting, high-early-strength MIDAR products, suitable for the AM of large objects in construction applications, were developed. Additionally, where other properties, such as high temperature resistance or increased chemical resistance, are important, new materials can be developed to meet these requirements.
The development process involved a series of laboratory trials, starting with printing of simple objects, such as lines and circles. Additional tests included measuring parameters such as the extrudable timing nozzle velocity and levels of entrapped air. The processing parameters were shown to be highly dependent upon the amount of water in the mix. Trials to produce simple multi-layered structures indicated the potential of multi-layer extrusion. After formulating the material with suitable processing properties, the equipment was calibrated to optimise the results, as parameters including the extrusion pressure and nozzle velocity had a significant effect on the output.
The result is a reproducible small-scale hollow cylinder created using MIDAR material, which, with further optimisation, can be scaled up to form larger structures. Attempts to create more complex shapes have advanced in recent work, but the balance between extrudability and working time continues to be refined. Improvements to the additive layer manufacturing apparatus and the use of the support structures during the printing process offer avenues to refine the current methodology.
By combining MIDAR ceramics with other materials, such as metals or polymers, new composites can be created that are also 3D-printable. For example, metallic particles can be incorporated to alter the electrical or thermal conductivity or to improve ballistic performance in materials for armour applications. Foamed structures may also be made by AM of a suitably modified MIDAR material, as these printed inorganic foams do not require elevated temperatures for hardening. It allows the formation of highly porous low density materials with high early green strength, allowing handling and processing.
In line with the new BSI standard (PAS 8820), the MIDAR materials comply with material and product performance specifications to give manufacturers of construction materials the opportunity to develop products based on alkali activated cement technologies, safe as stated by the European standard of approval. The standard refers to, and benchmarks against, a number of well-used cement requirements, ensuring consumer confidence in the new technology.
In supporting component manufacturers, materials suppliers and machine producers working in aerospace, healthcare and power generation industries, aimed at optimising materials and processes to ensure that components and materials are validated, yields are improved and waste is reduced. The future of AM looks bright, but there is much to be done in both product and process development, with materials and their advancement the linchpin of that future.
Dr Kambiz Kalantari is Operations Manager in Materials at Lucideon. Kambiz holds a BSc in Materials Science and Engineering, an MSc degree on Microwave Materials and a PhD in Electroceramics