Picture: UCL
Researchers at University College London (UCL) and the University of Greenwich have developed a method that could significantly improve the quality of laser-based 3D printing of metallic components. Through the targeted use of magnetic fields during the printing process, the team succeeded in reducing the formation of pores in metal alloys by up to 80 percent. In the long term, this could improve the safety and durability of components for the aviation and automotive industries.
The research is based on high-resolution X-ray imaging, which enabled the scientists to analyze the mechanisms behind defects in additively manufactured metal components in real time. The synchrotron X-ray imaging used at the Advanced Photon Source (APS) in Chicago showed that laser irradiation causes vapor-driven melt pool instability, which leads to the formation of voids. These defects can compromise the mechanical integrity of the components.
Dr Xianqiang Fan, first author of the study from UCL Mechanical Engineering, said: “When the laser heats up the metal it becomes liquid, but also produces vapour. This vapour forms a plume that pushes the molten metal apart, forming a J-shaped depression. Surface tension causes ripples in the depression and the bottom of it breaks off, resulting in pores in the finished component. When we apply a magnetic field to this process, thermoelectric forces cause a fluid flow that helps to stabilise the hole so that it resembles an ‘I’ shape, with no tail to break off when it ripples.”
By applying a magnetic field during the printing process, this effect could be significantly reduced. The magnetic fields influence the thermodynamic forces in the melt pool and stabilize the so-called keyhole formation. In the process, thermal gradients convert electrical currents that control the flow movement of the molten metal.
Professor Peter Lee, senior author of the study from UCL Mechanical Engineering, said: “Though keyhole pores in these types of components have been known about for decades, strategies to prevent their formation have remained largely unknown. One thing that has been shown to occasionally help is applying a magnetic field, but the results have not been repeatable and the mechanism by which it works is disputed. In this study we’ve been able to watch the manufacturing process in unprecedented detail by capturing images over 100,000 times a second, both with and without magnets, to show that thermoelectric forces can be used to reduce keyhole porosity significantly. In real terms, this means that we have the knowledge we need to create higher-quality 3D printed components that will last much longer and expand use into new safety critical applications, from aerospace to Formula 1.”
The challenge now is to optimize the integration of magnetic fields into industrial production processes. In the long term, this could contribute to the more efficient and durable production of complex and safety-critical components. The researchers are optimistic that this technology can be applied to various industries, including aviation, medical technology and electromobility.
Professor Andrew Kao, a senior author of the study from the University of Greenwich, said: “Our research sheds light on the physical forces involved in this type of manufacturing, where there are intricate dynamics between surface tension and viscous forces. Applying the magnetic field disrupts this and further introduces electromagnetic damping and thermoelectric forces and, in this work, the latter acts to beneficially stabilise the process. With this new powerful tool, we can control the melt flow without the need of modifying feedstock materials or laser beam shape. We are very excited to see how we can apply this tool to develop unique microstructures tailored for a range of end-use applications. Whether it is fabricating artificial hips or battery packs for electric vehicles, improvements in additive manufacturing will make it quicker and cheaper to produce 3D printed components that are also of higher quality.”
