October AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 9 2 0 Rapid screening of high volume- fraction gamma prime ( γ′ ) nickel-base superalloys for LPBFAM . The intro- duction of high temperature-capable γ′ -strengthened Ni-base superalloys for LPBFAM has proven to be a significant challenge due to the inherent suscep- tibility of these superalloys to microc- racking during processing. Significant effort has been devoted toward de- veloping and optimizing process pa- rameters and scan strategies for what are generally categorized as “diffi- cult-to-weld” superalloys. Within this class of alloys, emphasis has been placed on producing a near crack- free microstructure in alloys such as CM247LC [1,18-20] , a low carbon content Ni-base superalloy similar in composi- tion to GE René 108. GE used the active learning framework described above to rapidly screen LPBFAM compatibili- ty of both commercial and unique alloy compositions designed to meet a vari- ety of performance targets at tempera- tures higher than 1600°F. In coupons similar to those in Fig. 3a, build chemistry was modified at incremental cylinder heights, using a fixed set of process parameters. While process parameters remained fixed within each multi-chemistry cylinder, they were varied between cylinders to map the effects of both chemistry and process parameters in a single build. Af- ter achieving each incremental 0.25 in. build height, the build chamber was cleaned, powder composition changed, and a new alloy powder was introduced into the chamber. Metallography and defect quantification were performed in the same manner as described pre- viously. To rapidly compare the LPB- FAM compatibility of these alloys, a selection of 24 different processing con- ditions spanning a range of typical val- ues of 70 to 360 W laser power, 25 to 2300 mm/s scan speed, hatch spacing, and beam focus were tested using the experimental framework described in Fig. 3. Note that only bulk parameters were varied in the study. To assess the crack resistance of al- loys tested, image analysis results were rank-ordered based on defect-concen- tration statistics and their response surfaces compared with those gener- ated on a commercial standard René 108. Figure 5 shows an experimentally determined response surface of a new process-robust alloy for LPBFAM com- pared with a similarly processed com- mercial René 108. Fig. 4 — (a) Box plot shows distribution of defects for different layer thicknesses and particle size distributions used in this study; (b) Scatter plots of normalized hatch spacing and laser scan speed suggest that higher laser power is needed to further increase the build rates while keeping the overall defect content low. The majority of low defect parts were obtained at near maximum laser power of the LPBFAM system. Fig. 5 — Experimentally determined response surface of (a) commercial alloy René 108 and (b) a new process-robust alloy for LPBFAM. Micrographs of as-built defects that (c) fail as-built defect targets and (d) pass defect targets. (a) (b) (a) (b) (c) (d)