Characterisation and monitoring of TI-6AL-4V Eli power used for the qualification of medical implants produced through additive manufacturing

Abstract

The characterisation and monitoring of Ti-6Al-4V extra low interstitial (ELI) powder is an important requirement for full qualification of medical implants and aerospace components produced through Selective Laser Melting (SLM) and Laser Engineered Net Shaping (LENS) systems. This is based on levels of degradation of Ti-6Al-4V (ELI) powder observed during successive additive manufacturing (AM) cycles in these systems. Such powder degradation is due to the heating effect of the laser beam and the interaction of the powder with the atmosphere in the build chamber and could lead to compromised properties of the built part. The powder quality must be compared with the ASTM F3001 specification and this quality must be maintained throughout all production runs. It is, therefore, important to understand the impact of various powder characteristics in the as-received state and after reuse during successive AM build cycles. In this study the properties of the virgin, reused and stored Ti-6Al-4V (ELI) powder were determined and compared with the powder specification. The primary virgin powder batch was divided into three secondary virgin batches of different particle size distributions by the supplier, TLS Technik GmbH. These secondary virgin powder batches had the following particle size distributions: Direct Metal Laser Sintering (DMLS) system of the Central University of Technology, Free State (CUT), size range: <40 μm, Concept Laser (LaserCusing) system of Stellenbosch University (SU), size range: 25 μm – 55 μm and Laser Engineered Net Shaping (LENS) system of the Council for Scientific and Industrial Research (CSIR), size range: 40 μm – 100 μm. The purpose of this work was to characterise the powder properties of the as-received virgin powder for each of these batches and to subsequently monitor the characteristics after selected reuse cycles in the three AM systems. The study also included characterisation of properties of stored virgin powder (stored for 18 months) as this was added in the DMLS machine during the recycling process. Topping up of the reused powder with virgin powder after 10 cycles in the DMLS system had a positive effect on maintaining the powder particle size distribution and morphology. It was found that the powder could be recycled in the DMLS up to 35 cycles. From the results it was concluded that the Ti-6Al-4V (ELI) powder could be reused indefinitely in the DMLS system, provided the powder in the dispenser bin of the machine was topped up with virgin powder as required. The frequency of topping up would depend on the sizes of parts built in the different cycles and the number of parts built per cycle. Typically, topping up could be necessary after every 10 build cycles. In the LaserCusing system, the powder was reused for only 10 build cycles without any addition of virgin powder. This led to an unacceptable increase in the oxygen level of the powder, indicating that further reuse of the powder in its current state would not be feasible. At the time of this study the effect of reuse of powder in the LENS system was not studied further than analysis of the powder after one build cycle. Therefore, no assessment of reuse of the powder in the LENS system could be made.