New technique, old material
The technique known as “additive manufacture” (also known as “3D printing”) saves material and offers good opportunities to manufacture complicated designs. The method is used in, for example, the energy and aerospace industries to produce parts for turbines and aircraft engines.
For metals, the two most common techniques used in additive manufacture (AM) are selective laser melting (SLM) and electron beam melting (EBM). In his thesis, On the Microstructures and Anisotropic Mechanical Behaviours of Additively Manufactured IN718, Dunyong Deng describes how these two methods influence the properties of a nickel-based superalloy.
Dunyong Deng with one of many samples from the tests.
This alloy, Iconel 718 (IN718), has good mechanical properties and high weldability, while at the same time being cheaper than other alloys. Its high weldability makes it particularly suitable for 3D printing.
“You could say that it is an old material, with a new use, additive manufacture. AM is gaining ground rapidly, and will be widely used in the future”, says Dunyong Deng.
Compares traditional methods
In the thesis, he presents examinations of both the microstructure and the anisotropic mechanical properties of IN718 for the two manufacturing methods, SLM and EBM. The term “anisotropic” describes materials that have different physical properties along different directions. He has also compared the AM materials with IN718 manufactured in tradition ways, such as casting.
The tests have been carried out at room temperature and at 550 °C, which is a commonly used operating temperature for many high-temperature components.
“Material manufactured by AM has totally different properties. There are large differences – surprising large – between the AM material and that manufactured by traditional means”, says Dunyong Deng.
At the microstructural level, the grain structure of EBM IN718 is inhomogeneous and changes from the surface of the piece and in towards its centre. The structure of SLM IN718, in contrast, is more homogenous, with more evenly shaped grains and a clear cellular structure. Furthermore, the AM material reacts differently to heat than IN718 manufactured by conventional means.
The mechanical properties of both materials are clearly anisotropic, but the causes for this at the microscopic level are different. The creep resistance (the ability of the material to resist changes of shape under high loads) at high temperatures of EBM IN718 is, however, significantly higher.
Different use, different qualities
So which material is best? Dunyong Deng avoids answering the question.
“It depends on the application, what the material is to be used for. Different materials are suitable for different fields of use. You could say that the conventional manufacturing method is best, simply because it is cheaper. But I’m sure that the cost of AM will fall.”
The thesis not only contributes new knowledge about the different materials: it also increases our understanding of additive manufacture, or 3D printing, as a production method. Dunyong Deng points out that this is an equally important result.
“Indeed, and our better understanding increases the possibilities of using AM for completely other materials in the future. The method at the moment is both expensive and time-consuming, but that may change.”