Laser powder bed fusion additive manufacturing is a form of LAM that where a spread layer of powder particles are selectively fused into a solid form using a focused laser beam, layer-by-layer, to build up complex 3D objects.
LAM is a key part of Digital Manufacturing and the Industry 4.0 revolution, helping UK Industry to develop an entirely new range of designs methods ranging from the production of components and repair processes for the aerospace, biomedical, naval, and nuclear fusion sectors.
A limitation of the application of LAM is the ultra fast cooling rates involved produce unique, but poorly understood, microstructural features which may enhance or degrade the properties. Work led by Prof Peter Lee and Dr. Chu Lun Alex Leung (University College London) in collaboration with Mike Towrie in the CLF ULTRA Facility devised experiments to better characterize and understand how and why these features form. Such features include porosity (which may potential initiate fatigue failure or affect pressure tightness) and balling (leading to the incomplete formation of a solid part from the powder), ejection of powder and liquid metal particles (affecting metrology), and severe thermal stresses which may lead to delamination or cracking.
Although the link between powder chemistry and defect formation is widely acknowledged, there are many hypotheses on the causes and formation of closed pores including 1) powder contamination, 2) coating defects, 3) the presence of carbon, hydrogen and oxide inclusions in the molten pool, 4) internal gas porosity from the powder, 5) keyhole collapse, and 6) gas entrapment during laser melting. However, which of these mechanisms that induce pores in the build parts remains unclear. The in situ experiments at Harwell Campus are revealing when each of these mechanisms is active, and methods by which the undesirable features can be avoided. This is enabling components with higher performance to be produced more reliably.
Another key challenge with additive manufacturing is the reuse of the powder, which oxidises further during each re-use. Understanding the re-use and recycling of the powders used in LAM is key to minimizing its environmental impact. Scientists at Harwell Campus investigated the effect of oxidation of nickel superalloy powders on porosity formation during LAM using ultra-fast X-ray imaging. They observed the evolution of pores while forming multi-layer tracks for different levels of oxidation. The chemical states of the virgin and oxidised powders were characterised using the EPSRC National X-ray photoelectron spectroscopy (XPS) facility at Harwell, revealing that the powder simulating re-use had 6 times the oxygen content of the virgin powder.
The results suggest that the metal hydroxide formed on the surface of re-used powder thermally decomposes into metal oxide and soluble hydrogen during LAM, with both being released into the melt pool. The presence of the iron and nickel oxides in the molten pool alters the change in surface tension with temperature from negative to positive, reversing the fluid flow and resulting in a deeper melt depth and increased pore formation. This study confirms that excessive oxygen in the powder feedstock may cause defect formation in laser additive manufacturing. With this understanding of how defect formations can occur, future applications of LAM can be optimised to avoid the pitfalls that lead to manufacturing defects.
If you would like to see more, click link to the video: https://www.sciencedirect.com/science/article/pii/S1359645418309698?via%3Dihub#!
Here is the link to Alex's youtube channel:
https://www.youtube.com/watch?v=qFMJVVw8UN4
