In-situ monitoring of component integrity during additive manufacturing using optical coherence tomography

Additive manufacturing has been hailed as representing the latest industrial revolution and has captured the imagination of expert users and the general public alike. New manufacturing capabilities have permitted us to explore new design freedoms and produce optimised products which are customised to the needs of the user. These trends are set to increase as the technology grows in capability and gathers credibility.

There are an array of machine tools available on the market at the moment that can realise parts direct from digital. These make use of various energy sources (e.g Lasers, electron beams, IR lamps, heated nozzles etc) and material feedstocks (e.g metal/polymer powders, photocurable resins, plastic wires) to realise the design intent. Unlike more established machine tools there is a marked lack of process monitoring and feedback control of key process variables in these systems. This presents a significant problem since there is no method for ensuring that all is well within the build process. Therefore, in many cases it is only possible to identify defects after the process is complete assuming they are visible at the surface. Of significant concern, when integrity of parts is critical, are defects within the body of components. These can only be observed through cross sectioning or processes such as X-Ray Computed Tomography (CT). Naturally this comes at some considerable cost and only provides information once a part has been produced.

Therefore, there is a real need for new methods to provide 'in process' information about the quality of the pre-fused material layer and the quality post melting. Clearly some penetration into the part is required to create a full picture which can be reconstituted in a layerwise manner to create an integrity map of parts upon completion. This can be used to identify buried regions which exhibit de-lamination, pores, cracking and density variations. Furthermore analysis of the deposition material pre melting should be able to identify voids and give some information about surface roughness and ideally material properties. Optical Coherence Tomography (OCT) is an imaging technique which, if tuned to the specific requirements of plastic imaging and applied in-situ within the AM tool, could be used to meet this challenge.

This project will enable high-value additive manufacturing to come of age through implementation of sophisticated, in-situ, real-time process control based on novel non-contact optical techniques. OCT is a non-invasive imaging technique, which has the potential to revolutionise additive manufacturing technologies. It will bring additive manufacturing in line with established production processes with respect to product integrity, whilst also offering significant cost and resource efficiencies to support the widespread deployment of additive manufacturing tools throughout the manufacturing sector and to develop new and untapped applications.

Appropriate high-speed OCT configurations aimed specifically at distinguishing between polymers of use in additive manufacturing are not currently available. Such a system will be built, integrating novel mid-IR components within an external cavity laser configuration, allowing vibration independent imaging of a range of single and multi-polymer parts.

A successful outcome of this project will be the realization of an OCT system capable of rapid analysis of the sub-surface microstructure (e.g. voids and composition) of additively manufactured parts composed of multiple plastics, and a scheme for its incorporation into the additive process whereby in-situ monitoring of process integrity is enabled. Beyond this data sets will be gathered and post processed to evaluate and demonstrate the applicability of this new technology to additive manufacturing

Design and manufacturing organisations use additively manufactured parts for products in consumer, industrial, and military markets as constituents of digital cameras, mobile phones, automotive trims, aircraft parts and medical implants. They can serve as a pre-market vizualisation tool, or as a finished product, and can significantly reduce tooling, inventory maintenance and labour costs, and produce lighter parts. A growing number of industrial sectors and geographical locations are adopting AM technologies and sales of AM equipment is growing rapidly. The compound annual growth rate for AM products and services is >27% with a $10.8b global market forecast for 2021 [1]. At present the major markets are for consumer products in the USA, but with major markets being established in automotive, aerospace and medical/dental implants. Whilst the outcomes of this project are not intended to impact on the low-end consumer products that constitute the largest current market sector (22%), the introduction of in-situ process monitoring will promote uptake of AM parts in the high-end markets where part integrity and reliability can be guaranteed for safety critical applications. It is in these such markets where current uptake of presently available low integrity AM parts is poor, the barrier to uptake being the perceived low quality/reproducibility/reliability/part integrity. The highly competitive selective laser melting (SLM) tool market will allow future adopters of our concept to steal a lead in safety-critical and high-end applications. Many UK service providers and industrial users will benefit from application of AM-produced parts in their systems/products/tools, enhancing the competitiveness of UK industries, leveraged from the new capability in AM tools. There is also an opportunity for UK-based SLM tool manufacturers (e.g. Renishaw) to expand into the plastics AM market. Both routes allow for job and wealth creation in the UK. Further jobs and wealth creation are possible through growth of UK companies active in applied optics (e.g. OCT systems manufacturers) and optoelectronic component manufacturers (e.g. CST, M2) through realisation of a new mid-IR capability.

It is well-known that the aerospace industry requires extremely high confidence in part integrity, but impacts are also envisaged in the medical/dental industry, where personalised implants and prosthestics provide a better patient fit, whilst custom models for planning surgery result in shorter, more successful medical procedures (eg. knee and maxillofacial reconstructions) and theatre efficiency and enabling both improved quality of life and lower burden on healthcare professionals by aiding both a health workforce and an ageing population. More blue-sky impacts are predicted in healthcare component innovation such as scaffolds for tissue engineering and manufacture of organs.

Expanding the knowledge base in AM processes and materials and its application in a number of disparate and interdisciplinary industries and research projects will enhance the UK R&D standing and enable sophisticated characterization of novel materials, structures and processes and realisation of innovative parts that would otherwise not be possible. It will promote interdisciplinary working and realise PhD graduates with strong interdisciplinary awareness and skills, enabling both interdisciplinary research programmes and interdisciplinary-minded graduates and future industry leaders.

[1] Wohlers Report 2013 - Additive Manufacturing and 3D Printing State of the Industry Annual Worldwide Progress Report, Wohlers Associates, Inc. ISBN 0-9754429-9-6