Scientists at the Massachusetts Institute of Technology (MIT) have innovated a unique heat treatment technique that enhances the strength of 3D printed metals.
The innovative procedure modifies the microscopic structures of the 3D printed substance, facilitating the creation of parts with superior durability and resilience to thermal shock.
It’s reported that this method opens up fresh opportunities in 3D printing high-performance blades and vanes for jet engines and gas turbines, paving the way for revolutionary designs that minimize fuel usage and maximize energy efficiency.
The heat treatment technique, which utilizes AM IN738LC, an additively-manufactured super-alloy, substitutes the fine grain structure inside the as-printed material with larger, more robust columnar grains.
Enhanced metal structures with larger grains and condensed pores can be created using existing post-processing treatments such as hot isostatic pressing (HIP). Nevertheless, such processes may potentially lead to metals that deform under constant mechanical stress and high temperature, referred to as creep. The novel post-processing treatment developed by the MIT team minimizes the tendency of materials to creep.
The study, named Directional recrystallization of an additively manufactured Ni-base superalloy, is published in the journal Additive Manufacturing.
Zachary Cordero, the Boeing Career Development Professor in Aeronautics and Astronautics at MIT, anticipates that in the near future, gas turbine makers will print their blades and vanes at vast additive manufacturing plants and then post-process them using their heat treatment.
“3D-printing will facilitate new cooling architectures that can enhance the thermal efficiency of a turbine; thereby it can generate the same power while consuming less fuel and consequently, emitting less carbon dioxide.”
Anti-creep heat treatment
The research team’s new heat treatment technique is a form of directed recrystallization. This heat treatment technique passes a material, in this case laser powder bed fusion 3D printed IN738LC rods, through an induction coil at controlled speeds to meld the materials’ microscopic grains into larger, stronger, and more uniform crystalline structures.
During testing, each rod was slowly drawn from a water bath and through the coil at varying speeds, heating the metal to between 1,200℃ and 1,245℃ in the process. Each sample was drawn through the ‘hot zones’ at rates ranging from 1 mm/hr to 100 mm/hr.
The outcomes of the research indicated that heating the rods at a rate of 2.5 mm/hr at a temperature of 1,235℃ instigated an ideal change in the 3D metal’s microstructure. This process was helped by a sharp thermal gradient presented at the specific temperature.
The research team delved into the microscopic surface analysis of the 3D printed metal part post heating using both electron and optical microscopy techniques. The exploration affirmed that the surface of the 3D printed metal element contains columnar grains, duly enhancing the metal’s protection against creep.
The manipulations begin with grains carrying defects termed as dislocations, likened to a jumble of spaghetti. Cordero stated, “the disordered mass is then subjected to heat, allowing the defects merely to disappear, reposition themselves, and lead to grain enlargement. We essentially stretch the grains by consuming the defective material and smaller grains through a method called recrystallization.”
Besides, it has been inferred that by altering the draw speed and temperature during the sample rod tests, the specific grain size and orientation can be achieved. This level of manipulation proves highly useful for industrial manufacturers desiring to 3D print metal components with certain structural insights.
Indeed, the researchers argue that this process will allow manufacturers to fabricate new, optimized blade and vane geometries for more energy-efficient land-based gas turbines and jet engines.
Post-processing in metal 3D printing
One of the most widely used metal 3D printing processes, Laser powder bed fusion 3D printing can produce metal parts that possess pores and keyholes in the metal part. These structural characteristics weaken the strength of the 3D printed metal part.
“Why did the 3D printer go to therapy? Because it had too many layers of unresolved issues!”
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