Researchers have used a combination of imaging techniques to increase their fundamental understanding of metal-based laser powder bed fusion (LBPF), enabling them to fine-tune the process and increase its quality.
During LPBF, a high-powered laser creates a small pool of liquid metal as the particles fuse together. During this stage, a minute amount of metal evaporates and presses against the liquid, creating a cavity at the centre of the melt pool. This cavity, often referred to as a keyhole, can become unstable and collapse, leading to pores in the material. At the same time, the vapour shoots upwards from the keyhole, forming a plume, which interacts with the particles and can potentially disturb the spread layer.
Such events create tiny imperfections scattered throughout the component and, consequently, an unacceptable level of material porosity to many manufacturers.
Scientists from Heriot-Watt University, in collaboration with Carnegie-Mellon University and Argonne National Laboratory, have therefore simultaneously used x-ray and Schlieren imaging to analyse the interplay between gas, vapour, liquid and solid phases present during LPBF.
They found a direct link between the behaviour of the vapour plume released through the evaporation of metal and the overall stability of the molten material. The more dynamic the plume, the more unstable and porous the material.
By fine-tuning the laser parameters – adjusting its power, focused spot size and scan speed – the team discovered they could control the stability of the plume and melt pool, making the printed structure far more consistent.
According to the researhcers, using the plume as a ‘process signature’ that can be visualised and monitored holds exciting new potential for a variety of industries that rely on high-performance components such as in aerospace, automotive, healthcare and defence.
'Despite showing great promise, defects in printed parts still prevent metal additive manufacturing from fulfilling its potential,’ said Professor Andrew Moore, leader of the Optical Diagnostics group in the Institute of Photonics and Quantum Sciences at Heriot-Watt University. ‘Research has so far focused on detecting and predicting defects based on the behaviour of the liquid metal or particles, often overlooking the effects of the vapour jet and plume generated above the melt pool.
'What we found has exciting new prospects for 3D printing: we can vastly reduce these imperfections and produce components that are far less likely to fail. We believe that this work will enable the creation of improved process monitoring and analytic tools that identify and prevent defects in the additive manufacturing of metals. Additionally, it will underpin a more predictive class of multiphysics models which include atmospheric effects and powder motion, allowing accurate a priori calculations of process maps.'
The work, published in Nature Communications, was partly supported by Renishaw, a UK manufacturer of LPBF systems, under its Strategic Alliance with Heriot-Watt University. The team will continue collaborating with Renishaw to use these new insights to improve the 3D printing machines of the future.