Abstract
Laser additive forming is considered to be one of the promising techniques to repair single crystal Ni-based superalloy parts to extend their life and reduce the cost. Preservation of the single crystalline nature and prevention of thermal mechanical failure are two of the most essential issues for the application of this technique. Here we employ synchrotron X-ray microdiffraction to evaluate the quality in terms of crystal orientation and defect distribution of a Ni-based superalloy DZ125L directly formed by a laser additive process rooted from a single crystalline substrate of the same material. We show that a disorientation gradient caused by a high density of geometrically necessary dislocations and resultant subgrains exists in the interfacial region between the epitaxial and stray grains. This creates a potential relationship of stray grain formation and defect accumulation. The observation offers new directions on the study of performance control and reliability of the laser additive manufactured superalloys.
Highlights
Can preserve the single crystalline nature and how effectively one can avoid the thermal effects such as hot cracking[5,9]
Taking advantage of the micron scale spatial resolution, high orientation resolution, as well as the significant penetration depth of high energy X-ray beam, we studied in depth the microstructural evolution, including both crystal orientation, subgrain boundary distribution, and defect density gradient, over a millimeter size region including the single crystalline substrate, the epitaxial columnar dendrite layers directly formed by laser additive manufacturing, and stray grains
We have shown that lattice distortion exists in the epitaxial Ni-based superalloy manufactured by laser additive forming, but the disorientation gradient throughout the bulk part of the scanned 500 μ m × 1300 μ m area is tiny and almost homogeneous
Summary
Can preserve the single crystalline nature and how effectively one can avoid the thermal effects such as hot cracking[5,9]. Limited by the probe depth of EBSD and the poor spatial resolution of HRXRD and RSM (usually at the scale of hundreds of microns or even millimeters), the orientation and defect distribution and gradient in the laser deposited layers, especially from the substrate to the stray grain region, are not easy to be characterized quantitatively. Taking advantage of the micron scale spatial resolution, high orientation resolution, as well as the significant penetration depth of high energy X-ray beam, we studied in depth the microstructural evolution, including both crystal orientation, subgrain boundary distribution, and defect density gradient, over a millimeter size region including the single crystalline substrate, the epitaxial columnar dendrite layers directly formed by laser additive manufacturing, and stray grains. A high density of defects was detected near the epitaxy-stray interface, indicating that the epitaxial-to-stray transition may be related to the defect-assisted heterogeneous nucleation
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