Superalloy CM247LC Research Articles

Microstructure and Tensile Properties of Hot Isostatically Pressed Nickel-base Superalloy CM247LC

Microstructure and Tensile Properties of Hot Isostatically Pressed Nickel-base Superalloy CM247LC

Crack inhibition and mechanical property enhancement of a CM247LC alloy fabricated by laser powder bed fusion through remelting strategy

High cracking susceptibility arising from the high thermal gradients is a critical issue of high γ'-fraction Ni-base superalloys fabricated by laser powder bed fusion (LPBF). Here in, laser remelting was used to inhibit cracking and enhance the strength-ductility synergy of additively manufactured CM247LC alloy by LPBF. The effects of laser remelting with different energy densities and scan strategies on microstructure, cracking behavior and mechanical properties of LPBF-processed CM247LC superalloy were systematically investigated. The crack inhibition mechanisms of laser remelting during the LPBF process were revealed. Low energy density of remelting and scan strategy with a rotation of 90° between remelting and prior melting were promising for decreasing the crack density. The crack inhibition caused by laser remelting could be attributed to the partial elimination of defects (e.g., lack-of-fusion pores), the mitigation of the segregation of elements, the backfilling of the grain boundaries and liquid film, and the reduction of the residual stress and the high-angle grain boundaries (HAGBs). Enhanced mechanical strength and ductility (ultimate tensile strength of 1280 MPa, yield strength of 885 MPa, and elongation of 11.9%) of LPBF-processed CM247LC samples were obtained by optimized-remelting parameters. This work demonstrated the potential of laser remelting in improving the printability of high γ'-fraction Ni-base superalloys fabricated by LPBF.

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Microstructural characteristics, hardness and tribological behavior of additive manufactured CM247LC nickel super alloy

Nickel superalloy CM247LC was fabricated by laser powder bed fusion (LPBF) and its microstructure, hardness and tribological properties were investigated. X-ray diffraction analysis showed the presence of γ, γ′ and metastable MC carbides. Microstructure revealed cellular structure with fine MC carbides at cell boundaries. Higher hardness of 408 HV was attributed to grain refinement from rapid solidification and strengthening from MC carbides. Wear testing showed lowest wear rate of 1.46x10-9 mm3/m at 10 N load and predominant wear mechanisms were abrasive and adhesive wear. This study recommends LPBF as a promising method for manufacturing CM247LC superalloy for load bearing and tribological applications.

Tensile strength analysis of additively manufactured CM 247LC alloy specimen by employing machine learning classifiers.

Using a cutting-edge net-shape manufacturing technique called Additive Layer Manufacturing (ALM), highly complex components that are not achievable with conventional wrought and cast methods can be produced. As a result, the aerospace sector is paying closer attention to using this technology to fabricate superalloys based on nickel to develop the holistic gas turbine. Because of this, there is an increasing need for the mechanical characterisation of such material. Conventional mechanical testing is hampered by the limited availability of material that has been processed, especially given the large number of process factors that need to be assessed. Thus, the present study focuses on manufacturing CM247LC Ni-based superalloy with exceptional mechanical characteristics by laser powder bed fusion (L-PBF). This study evaluates the effect of input process variables such as laser power, scan speed, hatch distance and volumetric energy density on the mechanical performance of the LPBF CM247LC superalloy. The maximum value of as-built tensile strength obtained in the study is 997.81 MPa. Plotting Pearson's heatmap and the Feature importance (F-test) was used in the data analysis to examine the impact of input parameters on tensile strength. The accuracy of the tensile strength data classification by machine learning algorithms, such as k-nearest neighbours, Naïve Baiyes, Support vector machine, XGBoost, AdaBoost, Decision tree, Random forest, and logistic regression algorithms, was 92.5%, 83.75%, 83%, 85%, 87.5%, 90%, 91.25%, and 77.5%, respectively.

Effect of High Temperature Oxidation on Surface Pit and Recrystallization of Directionally Solidified CM247LC Superalloy in Creep Gauge

This study analyzed a recrystallization phenomenon that occurred simultaneously with hightemperature oxidation on the surface of a directionally solidified CM247LC creep specimen, using optical and scanning electron microscopy. After heat treatment, the surface of the specimen subjected to the creep test at 982oC was oxidized by exposure to high temperature and underwent microstructural changes due to high temperature stress. The outermost layer of the oxidized surface pits was found to consist of an oxide of the Cr/Co component, and the lower layer contained an oxide of the Al component. The area adjacent to the surface oxide layer is a precipitated free zone (PFZ) depleted of the y’ precipitated phase. The PFZ is caused by the diffusion of the Al component from this area to the surface oxide layer, resulting in the depletion of y'-Ni3Al as the main Al component. The area adjacent to the PFZ is a y' coarsening layer, which is the result of increasing y' phase fraction and coarsening as the Cr/Co component of this region diffuses into the PFZ of the y phase composition. This y' coarsening and y' rafting occurs in the direction perpendicular to the creep stress. In the EBSD analysis, the PFZ and y' coarsened layer were observed to be recrystallized regions, with the recrystallization composed of a single grain including the PFZ and y' coarsened layer. It is presumed that this recrystallization is caused by the residual stress of one-way solidification or the residual stress of the specimen surface processing. Accordingly, high-temperature oxidation in DS CM247LC creep gauge caused surface pits associated with recrystallization.

Cracking mechanisms and effect of extensive preheating in CM247LC and IN713LC Ni-base superalloy processed by Laser Powder Bed Fusion

Laser powder bed fusion processing of non-weldable γ’-strengthened Ni-based superalloys is challenging due to various cracking mechanisms that occur during the solidification and cooling stages of the materials. The current study offers an insight into the failure mechanisms of the CM247LC and IN713LC alloys, identifying the pre-heating temperatures which can mitigate cracking phenomena during processing. The adopted methodology involved the alloy processing by a standard Laser powder bed fusion system followed by microstructural characterization. It was confirmed that the development of cracks with a length in the range of 100–500 µm became evident in both alloys. High-temperature preheating strategies were then investigated using a simplified approach based on single laser track remelting. At pre-heating levels up to about 900 °C, corresponding to measured cooling rates of the order of 104 °C/s, hot cracks were still observed in the alloys due to γ’ precipitation and segregation of carbide former elements at the cell boundaries according to both microstructure observations and thermodynamic predictions. Preheating conditions at higher temperatures targeting levels above the γ’-solvus temperature, exceeding 1200 °C could promote slower cooling rates, leading to full suppression of cracks in the investigated γ’-strengthened alloys.

γ'-Precipitation Free Zone and γ' Rafting Related to Surface Oxidation in Creep Condition of Directionally Solidified CM247LC Superalloy

γ'-Precipitation Free Zone and γ' Rafting Related to Surface Oxidation in Creep Condition of Directionally Solidified CM247LC Superalloy

The behavior of anisotropic fatigue short crack initiation and propagation for a directionally solidified superalloy CM247LC at room temperature

AbstractThe effects of mechanical and microstructural anisotropy on short fatigue crack initiation and propagation behaviors of a directionally solidified superalloy have been studied. An unusual result was found where the fatigue lives of specimens with grains longitudinally aligned along the loading direction fail at lower lifetimes than specimens with transversely loaded grains when the applied stress is close to the yield stress. This is mainly attributed to the lower Young's modulus of the longitudinal specimen, which induces more local plastic strain (at stress concentration features) leading to earlier crack initiation and faster crack propagation under the applied test stress.

Microstructural mechanism of local dynamic recrystallization around cavity during tertiary creep in directionally solidified superalloy CM247LC

This study investigated the mechanism of nucleation and growth of dynamic recrystallization (DRX) locally occurring around a cavity depending on the creep strain at 982 °C under 180 MPa in directionally solidified superalloy CM247LC. Misorientation increased when the creep strain reached 15%, and low-angle boundaries along rafted γ–γ’ around the cavity formed a high-angle boundary, which is assumed to be the initial nucleation mechanism of recrystallization. Further deformation increased the dislocation density in the recrystallized grains, resulting in post-DRX via fast dynamic recovery. The γ matrix was coarsened by the atomic diffusion of Co and Cr along the dislocation line in the γ’ phase of the recrystallized grains.

Tensile and creep-rupture response of additively manufactured nickel-based superalloy CM247LC

Tensile and creep-rupture response of additively manufactured nickel-based superalloy CM247LC

Effect of Pt on diffusion-controlled growth characteristics of interdiffusion zone between CM247LC superalloy and Ni(Pt)Al bond coat

Effect of Pt on diffusion-controlled growth characteristics of interdiffusion zone between CM247LC superalloy and Ni(Pt)Al bond coat

Cracking inhibition behavior and the strengthening effect of TiC particles on the CM247LC superalloy prepared by selective laser melting

CM247LC superalloy has high cracking sensitivity, which limits its wide application in additive manufacturing field. In this work, we propose to improve the cracking sensitivity of CM247LC alloy by adding nanoscale TiC particles. The TiC/CM247LC nanocomposite with low crack defects was successfully prepared by selective laser melting (SLM), and the cracking inhibition behavior and strengthening mechanisms were systematically studied. The results show that the optimal SLM processing of TiC/CM247LC nanocomposite is scanning spacing of 80 μm, scanning speed of 1000 mm/s and laser power of 200 W. Under this processing, the defects ratio of the nanocomposite is 1.95%, which is about 2% lower than that of CM247LC, and the ultimate tensile strength and yield strength were obviously improved. The TiC particles can refine the nanocomposite grain and reduce local strain concentration, and the uniformly distributed TiC particles at grain boundaries also can strengthen grain boundaries, which may be the main reasons for the low crack defects of the nanocomposite. Load-bearing strengthening of particles and Orowan strengthening are the main strengthening mechanisms of the TiC/CM247LC nanocomposite.

Liquation cracking susceptibility of remelted and reheated weld metals in CM247LC superalloy using multi-bead Varestraint testing

The liquation cracking temperature range (LCTR) of CM247LC was evaluated, especially in remelted and reheated weld metals produced during multi-pass repair welding. The LCTR of the remelted weld metal was 282 K, and those of the reheated weld metals were 455–805 K depending on the testing position. In the multi-bead within the reheated weld metal, the LCTR increased from 455 K far from the inter-pass boundary to 805 K near this boundary. The mechanism underlying the varying LCTR for the remelted and reheated weld metals was the amount of MC carbide at each weld position. The single-mode fibre laser welding suppressed the carbide within both areas, and lowered the liquation cracking susceptibility.

Influence of Hf and MC Carbide on Transverse Platform in Single Crystal Grain Selection of 2D Grain Selector.

CM247LC Ni-based components have been widely used in developing hot ends in aero-engines and gas industrial turbines, and these have exhibited promising directional solidification (DS) results. However, the superalloy CM247LC shows defects after adding carbon (C) and hafnium (Hf). In this study, the effects of adding C and Hf on grain selection have been explored to enhance the 2D grain selector’s performance and reduce casting costs. The experimental results reveal that the final region of carbide formation is where the dendrite is pushed into the paste region and finally solidifies. The performance requirements of carbide on the alloy can be controlled by changing the paste region and solidification sequence.

Microtwinning in the nickel-based superalloy CM247LC during compression tests

ABSTRACT We have studied the variations in the compressive yield strength and deformation modes of the directionally solidified nickel-based superalloy CM247LC with temperature. It is found that the yield strength of the alloy only fluctuates in a small range with temperature when the deformation temperature is below 800 °C. Whereas, as the temperature is raised further, the strength decreases dramatically. After a small plastic strain, the substructures in the alloy are investigated using transmission electron microscope. It is startling to find large numbers of APB (anti-phase boundary)-coupled a/2 < 011 > dislocation pairs, isolated stacking faults and extended stacking faults as well as microtwins in the alloy at temperatures between 25 and 950 °C. Whereas, at 1000 °C, although dislocations bypassing γ′ particles via climb dominates the plastic deformation, two matrix dislocations with different Burgers vectors collectively penetrating γ′ particles, APB shearing and shearing of γ′ particles by partial dislocations also take place during plastic deformation. On the ground of the microstructural observations, correlation between the flow stress and deformation modes is discussed.

Quantifying Susceptibility to Solidification Cracking in Oscillated CM247LC Superalloy Welds via Varestraint Testing

Quantifying Susceptibility to Solidification Cracking in Oscillated CM247LC Superalloy Welds via Varestraint Testing

High Temperature Oxidation Property of Ni Based Superalloy CM247LC Produced Via Selective Laser Melting Process

High Temperature Oxidation Property of Ni Based Superalloy CM247LC Produced Via Selective Laser Melting Process

Evaluation of Liquation Cracking Susceptibility of CM247LC Superalloy Repair Welds via Pre-Weld Varestraint Test

In this study, the liquation cracking susceptibility in the heat-affected zone of CM247LC superalloy gas turbine blades during repair welding was quantitatively evaluated using a newly developed pre-weld Varestraint test method. The repair welding geometry was replicated through Varestraint tests for the pre-weld bead. The liquation cracking susceptibility, that is, the liquation cracking temperature range (LCTR), could be evaluated through temperature visualization at the time of crack formation during the Varestraint test. The LCTR of CM247LC alloy repair welds (heat-affected zone of the second layer weld) was 280 K. Compared to the LCTR of as-cast (620 K) and aged (65 K) CM247LC, metallurgical mechanisms for controlling the LCTR of repair welds were examined based on the microstructural characterization and Scheil’s solidification calculations for the pre-weld. The LCTR of the CM247LC alloy repair weld was influenced by the MC carbide fraction and the segregated concentration of trace and impurity elements, such as B and S in pre-weld solidification path. A process design capable of reducing the fraction of MC carbides and solidification segregation of trace impurity elements is required. Based on the experimental and theoretical results, the proposed modified Varestraint testing method for dissimilar welds is expected to analyze the solidification cracking behavior effectively in manufacturing high-soundness CM247LC superalloy welds.

Effects of hot isostatic pressing treatment on the microstructure and tensile properties of Ni-based superalloy CM247LC manufactured by selective laser melting

This study manufactured CM247LC Ni-based superalloy, which has outstanding mechanical properties, using the laser powder bed fusion (L-PBF) method and selective laser melting (SLM). Furthermore, the effect of hot isostatic pressing (HIP) post treatment on the microstructure and mechanical properties of SLM-built CM247LC was investigated. The defect characteristics of SLM-built superalloy changed significantly according to scan speed and hatch space, and it achieved the optimal microstructure at an intermediate scan speed (600 mm/s). SEM, EBSD, and ECCI analyses of the as-built alloy identified grains that developed in the build direction, sub-grains consisting of dislocation networks and MC carbides. In addition, γ’ phase formed evenly throughout the sample after HIP post treatment. The as-built CM247LC showed a yield strength of 933.5 MPa and tensile strength of 1144.0 MPa. In the case of the HIP alloy, the yield strength remained at a similar level, but tensile strength increased significantly up to 1457.1 MPa, and tensile elongation increased four times compared to the as-built alloy. Based on the above findings, this study discussed the correlations between microstructure, improved tensile properties and deformation mechanism.