A new heat treatment makes 3D-printed metal suitable for use in extremely hot environments, such as gas turbines and jet engines. The treatment changes the microscopic structure of the material.
The technique was developed by researchers at MIT. They foresee a variety of applications. For instance, the heat treatment makes 3D-printed metal suitable for gas turbine and jet engine components, among others. According to the researchers, this paves the way for new designs leading to more favourable fuel consumption and higher energy efficiency.
High heat resistance
Both gas turbines and jet engines are very demanding applications. For instance, components rotate at very high speeds in extremely hot gas. In the production of contemporary gas turbine blades, manufacturers use conventional casting processes, pouring molten metals into complex moulds. Blades are made of metal alloys with very high heat resistance.
There has long been interest in 3D-printing blades for gas turbines. A major obstacle to this is a phenomenon called creep. In metallurgy, the term creep describes permanent deformations of the material due to long-term mechanical stress and high temperatures. Creep is an extra big problem with 3D-printed metal because the printing process creates tiny grains of a few tenths to hundredths of a micron in size. These grains are particularly prone to creep.
"In practice, this would mean a gas turbine would have a shorter lifespan or lower fuel efficiency," explains Zachary Cordero, Boeing Career Development Professor in Aeronautics and Astronautics affiliated with MIT. "These are costly, undesirable outcomes."
Transforming microstructure
Cordero and colleagues developed a heat treatment that provides the solution. The treatment transforms the tiny grains into much larger 'columnar' grains. This microstructure theoretically minimises the material's susceptibility to creep, as the 'columns' are aligned with the axis that experiences the greatest stress.
The treatment is a form of 'directional recrystallisation'. This is a heat treatment in which a material moves through a hot zone at a precisely controlled speed so that microscopic grains transform into larger, stronger and more uniform crystals. This technique was developed and applied more than eighty years ago for processing various materials.
1,235 degrees Celsius
The method was tested by the team on a 3D-printed nickel-based superalloy. This material is widely used in gas turbines. The researchers applied directional recrystallisation by placing 3D-printed 'rods' of the alloy in a room-temperature water bath. They then removed the rods one by one from the water and at varying speeds through an induction coil. In the process, the rods were heated to a temperature of 1,200 to 1,245 degrees Celsius.
This test showed that if the material is pulled through the coil at 2.5 millimetres per hour at a temperature of 1,235 degrees Celsius, the microstructure of the material transforms. "The material starts out as small grains with defects called dislocations, resembling interwoven spaghetti," Cordero explains. "If you heat material, these dislocations can disappear and reconfigure, and the grains grow. We continuously enlarge the grains by consuming defective material and smaller grains - a process called recrystallisation."
Industrial 3D printing of blades
The researchers expect their development to pave the way for industrial 3D-printing of blades for gas turbines. "We foresee gas turbine manufacturers printing their blades and vanes in large-scale additive manufacturing facilities in the near future and then post-processing them using our heat treatment," says Cordero.
"3D printing enables new cooling architectures that can improve the thermal efficiency of a turbine so that it produces the same amount of power while consuming less fuel and ultimately emitting less carbon dioxide."
Further information is available here.
Author: Wouter Hoeffnagel
Photo: PublicDomainArchive
