Seth James Griffiths
Metal Additive Manufacturing (AM) technologies have enabled the manufacturing of parts with complex geometries that were previously not feasible with conventional manufacturing. Unfortunately, many commercial engineering alloys (with the exception of alloys for welding) were designed with conventional manufacturing in mind and can behave poorly in the rapid solidification and thermal conditions that go along with AM. Research is needed to optimize the higher performance engineering alloys, such as pre-cipitation hardened alloys, for AM. One important class of precipitates in both the nick-el-base and aluminum-base superalloys is the intermetallic L12 âstructured (Strukturbericht notation) intermetallics. Many scientific challenges must be overcome prior to widespread industrial usage of these alloys. AM of L12-strengthened Ni-base superalloys results in extensive micro-cracking and AM of L12-strengthened Al-base alloys has been successful but requires further research to understand the material be-havior. This thesis focuses on overcoming the scientific challenges for the AM of L12-strengthened alloys.
A new Al-Mg-Zr alloy strengthened via L12-structured Al3Zr precipitates, Addalloy®,was successfully fabricated for the first time with Laser-Powder Bed Fusion (L-PPF). The as-fabricated Al-Mg-Zr alloy displayed a unique bimodal grain structure: (i) the bottom of the melt pools consisted of fine equiaxed grains (~1 µm) and (ii) the top of the melt pool consisted of columnar grains (up to 40 µm long). The bimodal microstructure was at-tributed to the changing solidification conditions during the lifetime of the melt. A new microstructure modification strategy, laser rescanning, was shown to refine the micro-structure. The grain refinement was attributed to the reduced melt pool depths of the additional scans, thus breaking up the previously solidified columnar grains. Scanning transmission electron microscopy (STEM) investigations of peak-aged (400ËC, 8 h) samples reveal both continuous (~2 nm in diameter) and discontinuous (~5 nm wide and hundreds of nanometers long) coherent, secondary L12-Al3Zr precipitates. The Al-Mg-Zr in the peak aged (400ËC, 8 h) condition had a 385 MPa yield strength and a 19% elongation at fracture. The excellent combination of strength and ductility was attribut-ed to the bimodal microstructure. For short-term elevated temperature yield tests, as-fabricated samples displayed higher yield strengths than peak-aged samples at temper-atures above 150ËC (e.g., 87 vs 24 MPa at 260ËC). For longer term creep tests at 260 ËC, both as-fabricated and peak-aged samples displayed near-identical creep behavior. The reduced elevated temperature performance of the peak aged samples was attributed to coarsening of grain-boundary precipitates during aging, decreasing their ability to in-hibit grain-boundary sliding (GBS) of the fine equiaxed grains.
The micro-cracking of CM24LC, a Ni-base superalloy, was attributed to both solidifica-tion and liquation cracking. Based on experimental evidence, reduction in the solidifica-tion interval of CM247LC was investigated as a candidate for micro-crack mitigation and a new alloy was developed. As Hf is found to have a significant influence on the freezing range of the alloy, a new CM247LC without Hf was produced and tested. Sam-ples fabricated with the Hf-free CM247LC, CM247LC NHf, in combination with opti-mized processing conditions exhibit a reduction in crack density of 98%.
EPFL2020
