Led by Professor Peter Lee from the University of Manchester, the team used an X-ray ‘beamline’ at the Diamond Light Source in Harwell, Oxfordshire, to analyse the extremely rapid surface-level changes that happen during laser additive manufacturing (Lam).
The Lam process uses a laser to fuse together metallic, ceramic or other powders into complex 3D shapes that would be impossible to make using traditional manufacturing techniques. A potential downside, however, is that the Lam technique creates objects with unusual material properties.
Freshly printed layers cool down extremely quickly, said the researchers, meaning operators do not know the optimal conditions to get the best results. This has delayed the uptake of Lam for safety-critical parts such as turbine blades, energy storage and biomedical devices.
The researchers used the Diamond Light Source’s synchrotron, which accelerates electrons to close to the speed of light to generate useful radiation.
“You need to be able to take tens of thousands of radiographs per second… only at the synchrotron do you have sufficient flux of X-rays,” said Professor Lee.
The team built a 3D printer, known as Lampr, to operate on the facility’s Joint Engineering, Environmental and Processing (Jeep) beamline.
The set-up let the team follow the process from powder, through melting and then solidification.
The cooling rate of Lam-produced components is 10^5 degrees per second, compared to normal cooling rates of degrees per second. “These are such high rates, you need to understand how these changes can affect the final properties,” said Professor Lee. “It may be the properties are better, but you need to quantify that.”
The experiments revealed characteristics of newly printed superalloys, important information that could be crucial for their development. Researchers previously believed surface porosity on finished objects was because of incomplete melting of the powder or insufficient liquid-feeding, but the Diamond Light Source’s short, sharp burst of X-rays revealed it is actually caused by pores near the surface bursting and leaving behind depressions.
The X-ray blasts also revealed other potential causes of defects. The results showed that surface tension causes “pre-melting” ahead of the laser’s path, causing metal vapour and heating of inert gas, which can form plumes and eject powder and molten droplets away from the main track.
The experiment “captured the heart of the process,” said Professor Lee. “You get very different properties, which you need to be able to understand and use to get the lowest cost and highest quality components.”
Information from the Diamond Light Source research has already informed the manufacturing of better artificial knee replacements. The team hopes further trials will deepen understanding of the hidden processes within newly-formed components.
Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.