While 3D printing technology does allow for the efficient production of complex metal parts, these items tend to warp when stressed and heated. However, thanks to a new technology developed at MIT, this embarrassment may soon no longer be the case. The problem with existing 3D printed metal parts is a phenomenon known as “creep,” in which constant mechanical stress and high heat cause permanent deformation of the metal. Creep is especially likely to occur when the metal is composed of tiny grains, which is the case with 3D-printed metals.
A team at MIT, led by Professor Zachary Cordero, has developed a heat treatment process that makes these grains larger and thus less prone to creep. This is a variation of an existing technique known as directional recrystallization.
In laboratory tests, 3D-printed nickel alloy rods were initially placed in a room-temperature water bath directly beneath an induction coil and then slowly pulled upward through the coil at varying speeds. Doing so heated a portion of each rod to temperatures ranging from 1200 ºC to 1245 ºC (2192 ºF to 2273 ºF), which created a steep thermal gradient within the metal between the coil and the water.
This gradient in turn causes the microscopic grains of the metal to transform into much larger “columnar” grains. As the term implies, the new grains take the form of columns, aligned with the axis of maximum stress within the metal.
The best results occur at a temperature of 1235 ºC (2255 ºF) and a stretching rate of 2.5 millimeters per hour, and scientists are working to increase this rate, and other combinations may work better for other metals. In fact, depending on the intended use of the 3D printed part, the grain structure can be changed in a single project by varying the temperature and speed during processing.
The researchers now plan to test the technique on structures similar to gas turbine or jet engine blades, which must be subjected to constant mechanical stress and high heat. If they do prove less prone to creep, it could pave the way for better, more efficient designs.
The new blade and weathervane geometry will make land-based gas turbines and eventually aero engines more energy efficient,” Cordero said. From a baseline perspective, this could lead to lower CO2 emissions and improved efficiency of these devices.”
A paper on this research was recently published in the journal Additive Manufacturing.
