Technique transforms metals microscopic structure

An MIT-developed heat treatment transforms the microscopic structure of 3D-printed metals, making the materials stronger and more resilient in extreme thermal environments. The technique could allow 3D printing of high-performance blades and vanes for jet engines, which would enable new designs with improved fuel consumption and energy efficiency.

Gas turbine blades are manufactured by casting, where molten metal is poured into molds and directionally solidified. These heat-resistant metal alloy components are designed to rotate at high speeds in extremely hot gas.

But efforts to 3D-print turbine blades haven’t cleared a big hurdle: creep – a metal’s tendency to permanently deform in persistent mechanical stress and high temperatures. Researchers found the printing process produces fine grains especially vulnerable to creep.

“In practice, this would mean a gas turbine would have a shorter life or less fuel efficiency,” says Zachary Cordero, the Boeing Career Development Professor in Aeronautics and Astronautics at MIT.

Cordero and his colleagues found a way to improve the structure of 3D-printed alloys by adding a heat-treating step, transforming the as-printed material’s fine grains into much larger columnar grains – a sturdier microstructure that should minimize the material’s creep potential, since the columns are aligned with the axis of greatest stress.

Their method is a form of directional recrystallization – a heat treatment passing a material through a hot zone at a precisely controlled speed to meld a material’s many microscopic grains into larger, sturdier, and more uniform crystals.

The team tested the method on 3D-printed nickel-based superalloys, placing 3D-printed samples of rod-shaped superalloys in a room-temperature water bath placed just below an induction coil. They drew each rod out of the water and through the coil at various speeds, dramatically heating the rods to temperatures between 1,200°C and 1,245°C.

Drawing the rods at 2.5mm/hour at 1,235°C transformed the material’s printed, fine-grained microstructure.

After cooling the heat-treated rods, the researchers found the material’s printed microscopic grains replaced with columnar grains, or long crystal-like regions significantly larger than the original grains.

“We can increase the grain size by orders of magnitude, to massive columnar grains, which theoretically should lead to dramatic improvements in creep properties,” says study lead author Dominic Peachey.

The team could also create regions of specific grain size and orientation, allowing manufacturers to print turbine blades with site-specific microstructures that are resilient to specific operating conditions.

This research was supported, in part, by the U.S. Office of Naval Research.

Massachusetts Institute of Technology (MIT)