Shaafi Shaikh, Materials and Manufacture

Opponent:

Professor K. A. Christofidou, University of Sheffield, United Kingdom 

Link to dissertation

Most metal alloys are strong at room temperature but lose strength as the temperature rises. Superalloys, however, retain their strength for thousands of hours at temperatures close to their melting point. These materials are crucial for aeroplane engines and turbines for power generation. Engineers are exploring 3D printing of superalloys to improve the performance and efficiency of these machines. However, 3D printing superalloys is challenging due to crack formation, loss of high-temperature durability, and cost. Understanding and solving these metallurgical issues is the focus of this thesis. 

Superalloys are complex, often containing over ten elements. We found that the chemical elements boron and zirconium, which make up only 1 out of every 1000 atoms, significantly affect crack formation during 3D printing. But these elements are essential for maintaining high-temperature strength and cannot be removed from the alloy. By experimenting with different compositions and studying melting and freezing behaviours, we found a way to prevent cracks while preserving strength.

Our research showed that the size and shape of the small grains that make up the superalloy are critical for long-term durability at high temperatures. The first heat treatment applied directly after 3D printing is especially important as it can alter the size, shape, and boundary character of the grains. In 3D printing, a laser melts layers of metal powder to build parts. Our tests showed that increasing the thickness of the layers can reduce costs but may lead to defects. Despite this, the properties remain competitive with conventionally produced materials.

In summary, 3D printing is becoming a valuable method for manufacturing superalloys, with the potential to enhance sustainability and performance in energy and aerospace applications.