Rene 125
Rene125 alloy is a directionally solidified nickel-based superalloy with excellent high temperature mechanical properties, outstanding oxidation resistance and remarkable creep resistance. These properties make it particularly suitable for high temperature components in aerospace applications, such as turbine blades. Although traditional directional solidification technology (DS) has advantages in preparing large-scale directional crystal structures, it is costly and complex. Laser powder bed melting (LPBF) provides an alternative through high-energy laser point-by-point scanning and ultra-high cooling rates, enabling similar microstructure possible.
The Rene125 powder used in this study was provided by Truer Technology Co., Ltd.. The powder was prepared by gas atomization method with a particle size range of 15-53 μm.
| SEM images of Rene125 powder with PSD 15-53um |
Experimental Method:
Before the LPBF process, the powders were dried in an oven at 130 °C for 2 h to remove moisture and improve their fluidity. All samples were fabricated on single crystal substrates prepared by DS. The layer thickness (30 μm), scanning pitch (100 μm), and interlayer scanning angle (67°) were kept constant. Different laser powers (180/240/300/360 W) and scanning speeds (400/500/600/700/800/900 mm/s) were selected for parameter optimization to determine the best combination. Cubic specimens with dimensions of 8 mm (length) × 8 mm (width) × 8 mm (height) were prepared for microstructure, and tensile specimens with a length of 30 mm were prepared, parallel to the build direction.
| LPBF specimen and building direction |
The hot isostatic pressing (HIP) was set at a constant pressure of 120 MPa under argon atmosphere, and three different temperatures were selected – 1050 ° C, 1150 ° C and 1230 ° C to treat the samples, which were marked as HIP-1050, HIP-1150 and HIP-1230 respectively. During the HIP treatment, the heating rate was maintained at 10 ° C / min. After reaching the target temperature, it was isothermally maintained for 3 hours and then cooled to room temperature with the furnace.
Solidification crack:
| Solidification cracks of LPBF specimen of Rene 125 |
Consistent with previous studies on LPBF-treated nickel-based superalloys, the internal cracks in the original sample are mainly composed of solidification cracks cracks. The sample fabricated with 360 W laser power and 700 mm/s scanning speed was selected to investigate the crack formation mechanism. The microstructure of the typical solidification crack exhibits intergranular fracture characteristics along the dendrite boundary. Obvious primary dendrite and secondary dendrite arm morphology were observed inside the crack.
| EPMA element distribution picture |
EPMA results compare cracks and intact areas near the crack tip. The obvious segregation of solute elements (Hf, Ti, Ta) on the grain boundaries produces the characteristic smooth solidification morphology of the liquid film. During the rapid solidification process, the local melting of the low-melting eutectic at the grain boundaries leads to the rupture of the liquid film under thermal stress. Subsequent thermal cycles from overlapping molten pools intensify the crack propagation through repeated liquefaction-solidification processes.
Effect of HIP temperature on microstructure evolution:
| Microstructure under different HIP temperature |
The HIP treated samples were all manufactured under the optimal process parameters of 300 W/900 mm/s. The above figure shows the microstructure of Rene125 alloy HIP treated at 1050°C, 1150°C and 1230°C, respectively. On the macro scale (300 μm), the 1050°C-HIP sample retains some unhealed cracks and irregular pores, indicating that the diffusion-driven pore elimination mechanism is insufficiently activated at this temperature. As the HIP temperature rises to 1150°C, the number of pores decreases significantly, and the residual pores are spherical (diameter <5 μm), indicating that there is sufficient thermal energy for plastic flow and diffusion bonding. Under 1230°C HIP treatment, the sample achieves nearly complete densification and the pores are completely eliminated, indicating that bulk diffusion and grain boundary sliding dominate the densification process at high temperatures.
the 1050°C-HIP sample retains columnar grain structures during the LPBF process, although there is obvious nucleation of recrystallized grains at the grain boundaries. The 1150°C-HIP sample shows more obvious recrystallization, forming a dual-phase microstructure of equiaxed grains and residual columnar grains. Abundant white precipitates are observed along the grain boundaries. After 1230°C HIP treatment, nearly complete recrystallization occurs. The microstructure of the HIP-1230 sample transforms from coarse columnar crystals to equiaxed grains with straight grain boundaries, which is consistent with the accelerated grain boundary migration at high temperature. The number of white grain boundary precipitates is significantly reduced compared with the 1150°C condition.
Conclusions
Crack formation in LPBF-treated Rene125 alloy is mainly attributed to grain boundary stress concentration and segregation of low-melting-point eutectic phase in the melt pool overlap region. Increasing the scanning speed promotes the transition from keyhole mode to conduction mode, which promotes directional grain growth and alleviates crack formation.
HIP treatment effectively heals cracks and pores retained during LPBF. Nearly complete densification is achieved under 1230°C HIP, accompanied by complete recrystallization and significant reduction in dislocation density.
