To understand why the shadow effect matters, it helps to picture how single-crystal castings are made. Directional solidification technology works by withdrawing a ceramic mold from a hot furnace zone into a cooler region, so that molten alloy solidifies from the bottom upward along a carefully controlled temperature gradient. The goal is for a single crystal orientation to propagate through the entire component. But when a cluster of large-caliber castings is arranged together in the furnace, the components block each other’s view of the cooling surfaces. One casting literally casts a thermal shadow on its neighbor, disrupting the local heat flow and the temperature gradient that the solidification front depends on. The result is one of the primary causes of casting defects in Ni-base single-crystal superalloy castings, including stray grains, freckles, and solidification pores that force manufacturers to scrap otherwise valuable parts.

The research team, led by Donghan Fan, Naicheng Sheng, and Yizhou Zhou of the University of Science and Technology of China and the Institute of Metal Research, Chinese Academy of Sciences, in collaboration with AECC Shenyang Liming Aero-Engine Co., Ltd, investigated a novel central cooling and heating process as a way to fight back against this thermal shadowing. Instead of relying solely on the conventional withdrawal-based cooling from the outside, the CCH process introduces controlled cooling and heating from the center of the casting cluster. This central intervention changes the radiation heat transfer dynamics inside the furnace, so that each component in the cluster receives a more balanced share of the heat exchange. The approach effectively rewrites the thermal geometry of the casting environment, preventing any single casting from being thermally starved by its neighbors.

The experimental results are striking. In CCH-processed castings, the size of the S-pores—solidification pores that form during directional solidification and act as stress concentrators during service—is significantly reduced. Even more dramatically, the freckles are eliminated. Freckles are chains of equiaxed grains that form when segregated interdendritic liquid becomes buoyantly unstable in the mushy zone, the semi-solid region between fully liquid and fully solid material. These freckle chains are catastrophic for single-crystal integrity because they introduce grain boundaries into a component that is supposed to have none. Previous studies have linked freckle formation to geometric features such as edges, curvature, and abrupt cross-section changes in castings, and the mushy zone width plays a decisive role in whether the segregated liquid can destabilize. By narrowing the mushy zone, the CCH process removes the conditions that allow freckles to nucleate in the first place.

Simulation results provide quantitative backing for these experimental observations. The numerical analysis shows that the mushy zone width of CCH-processed castings is shortened by 47.9 percent—a nearly halving of the vulnerable semi-solid region where defects incubate. The temperature field distribution also becomes more uniform across the casting cluster, meaning that the solidification front advances more evenly and the local thermal gradients stay within the safe range for single-crystal growth. A narrower mushy zone reduces the time that segregated liquid spends in the buoyantly unstable state, suppressing the interdendritic fluid flow that drives freckle formation and the shrinkage feeding problems that generate pores.

To clarify exactly how much control the CCH process achieves over the shadow effect, the researchers performed an in-depth deduction of the view factor, a geometric quantity from radiation heat transfer theory that describes how efficiently two surfaces exchange thermal radiation. The view factor analysis demonstrates the radiation heat transfer efficiency inside the furnace and quantifies the level of control over the shadow effect achieved by the central cooling and heating process. In conventional cluster casting, the view factors between neighboring components and the cooling surfaces are distorted by shadowing, so some castings radiate heat inefficiently and solidify under degraded gradients. The CCH process corrects this distortion by inserting central thermal elements that rebalance the radiation exchange, ensuring that the heat extraction from each component stays within the controlled range needed for defect-free directional solidification.

The benefits extend beyond the casting stage into the mechanical performance of the finished crystals. During deformation, a hexagonal dislocation network forms at the deformed gamma/gamma-prime interface in CCH-processed castings. The gamma-prime phase is the ordered intermetallic precipitate that strengthens nickel-base superalloys, and the gamma/gamma-prime interface is where plastic deformation concentrates at high temperatures. Hexagonal dislocation networks at this interface are a well-known signature of effective creep resistance, because they lock the interface and impede dislocation motion through the crystal. The formation of this network enhances the rupture life of the castings, meaning that components produced with the CCH process can withstand high-temperature stress for longer before failing. This is a crucial advantage for turbine blades, which must survive thousands of hours of creep loading at temperatures approaching the melting point of the alloy.

The significance of this work becomes clear when considering the economics of superalloy casting. Single-crystal superalloy components contain expensive alloying elements such as rhenium, and every rejected casting represents a substantial loss of both material and furnace time. The shadow effect is inevitable when directional solidification technology is employed to fabricate a cluster of large-caliber castings, which means that manufacturers have historically accepted high rejection rates as the cost of doing business. A process that suppresses the shadow effect and improves the yield rate of the castings directly translates into lower production costs, less material waste, and more reliable supply chains for the aerospace industry. The CCH process achieves this without requiring changes to the alloy chemistry itself, making it compatible with existing superalloy compositions and casting infrastructure.

The research builds on a growing body of work on defect control in directional solidification. Earlier studies by the same group examined freckle formation during casting single crystal superalloys through the central cooling and heating process, published in the Journal of Alloys and Compd in 2025, and the effect of wall thickness on residual stress and microstructural evolution in multi-blade casting. Other researchers have developed thermal uniformity-based downward directional solidification technologies, non-uniform thickness molds for controlling temperature gradients, and digital twin models for single-crystal turbine blade solidification. The CCH process adds a new dimension to this toolkit by attacking the shadow effect at its geometric root—the distorted radiation exchange within casting clusters—rather than merely compensating for its downstream consequences.

Looking ahead, the combination of experimental validation, quantitative simulation, and view factor analysis demonstrated in this study offers a template for rational process design in superalloy casting. By deducing the radiation heat transfer efficiency from first principles, engineers can now predict how different furnace geometries and central thermal interventions will affect the shadow effect before committing to expensive trial castings. As the demand for single-crystal superalloy components grows with the expansion of commercial aviation and gas turbine power generation, processes like CCH that improve yield rates while enhancing rupture life will become increasingly valuable. The shadow effect has haunted superalloy casting for decades; with central cooling and heating, it may finally be losing its grip.

Subject of Research: Materials Science

Article Title: Suppression of the shadow effect during casting single-crystal superalloys through a central cooling and heating process

Article References: Fan, D., Sheng, N., Chu, Z., Liu, J., Sun, S., Fan, S., Hou, G., Li, J., Zhu, S., & Zhou, Y. (2026). Suppression of the shadow effect during casting single-crystal superalloys through a central cooling and heating process. Journal of Materials Science, 61(43), 33878-33895. https://doi.org/10.1007/s10853-026-13496-z

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13496-z

Keywords: single-crystal superalloys, directional solidification, shadow effect, central cooling and heating, freckles, S-pores, mushy zone, view factor, turbine blades, nickel-base superalloys, Suppression, shadow

That constraint has fueled the rise of miniature specimen testing, a family of techniques that extract creep data from tiny slivers of material harvested by non-destructive methods such as scoop sampling or in-situ trepanning. The real power of the approach lies in targeted sampling: instead of testing generic material, engineers can take specimens from the exact locations that have experienced actual service temperatures and stresses. Yet each established miniature technique carries a trade-off. Impression creep specimens have an equivalent gauge length of only about two millimeters, making them poorly suited to the low stress levels, often below 80 megapascals for 9 to 12 percent chromium steels at 600 to 650 degrees Celsius, that represent real operating conditions. Small punch tests can trace complete deformation-time curves, but the complex, multi-axial deformation they impose has prevented the emergence of a universally accepted data conversion method.

A new synthesis published in Results in Engineering argues that a class of bending-dominated miniature specimens deserves far more attention. Simply supported beams, fixed-end beams, cantilevers, and three-point bending plates share a crucial property: they are inherently flexible. Under load they undergo large, easily measurable deflections dominated by bending, which translates into a much larger equivalent gauge length than other miniature specimens of similar overall size. That gauge length is the fundamental key to sensitivity. A three-point bending specimen with an equivalent gauge length of roughly 8.6 millimeters produces nearly four times the displacement of an impression creep specimen of about 2.2 millimeters at the same equivalent strain, allowing accurate testing at low stresses where stiffer geometries would barely deform over practical test durations. The simple rectangular geometry is also easier to manufacture than rings or punch discs, and the pure bending mode permits closed-form analytical solutions grounded in classical beam theory.

The intellectual backbone of the framework is the reference stress method, pioneered by A. C. MacKenzie in 1968. The idea is elegantly simple: a complex structure creeping under load deforms at the same rate as a uniaxial specimen tested at a specific, constant reference stress. For a material obeying Norton’s power law, in which creep strain rate scales with stress raised to a power n, the steady-state displacement rate of a structure depends on both material constants and geometry. The reference stress method introduces a scaling factor that cleanly separates the two. Crucially, the reference stress depends only on the applied load and specimen geometry, not on the material’s creep properties, which makes the interpretation scheme transferable across steels, superalloys, and beyond.

Two dimensionless conversion parameters anchor the data reduction. The stress conversion factor eta relates the nominal stress of the bending specimen to the equivalent uniaxial reference stress, while the strain conversion factor beta, multiplied by a characteristic length, defines the equivalent gauge length: the length of a hypothetical uniaxial specimen that would experience the same strain as the measured displacement of the bending specimen. Both parameters are material-independent but geometry- and loading-mode-dependent, so once they are established for a given configuration they can be applied to any material tested with that geometry. For a simply supported three-point bending beam, eta is approximately 0.89, and a typical miniature specimen with a 10-millimeter span and 1.54-millimeter thickness yields an equivalent gauge length near 8.6 millimeters. Fixed-end and cantilever configurations follow the same mathematics with different moment distributions, while the three-point bending plate spans the continuum from plane stress in narrow beams to plane strain in wide plates, where the deformation rate drops by roughly a factor of three for a stress exponent of six. For intermediate width-to-span ratios, finite element analyses supplied piecewise polynomial correction functions with errors generally below one percent.

The derivations themselves rest on the complementary strain energy method, a classical technique that dates back nearly a century to MacCullough’s 1933 work on creep in bending and Popov’s later analyses. For a beam under pure bending, strain compatibility requires a linear strain distribution through the depth, and the bending moment relates to the curvature rate through a generalized moment of inertia that depends on the stress exponent. Castigliano’s second theorem then converts the total complementary strain energy into a load-line displacement rate, producing closed-form expressions for each configuration. The historical lineage runs through Kraus, Boyle and Spence, and more recently Altenbach and Naumenko’s creep-damage analyses of beams, plates, and shells, culminating in the present unified treatment.

Where the framework becomes genuinely forward-looking is in its extension beyond steady-state creep. By coupling the bending analysis with the Kachanov-Rabotnov continuum damage mechanics model, the method captures tertiary creep, the accelerating stage that precedes rupture. Damage accumulates through the beam cross-section, reducing the load-carrying capacity in a way that makes the governing equations nonlinearly coupled; no closed-form solution exists, but numerical implementation reproduces full deflection-time curves. Calibration against P91 steel data at 650 degrees Celsius, a material exhibiting insignificant primary creep, a clear secondary stage, and a long ductile tertiary stage with more than 20 percent strain to failure, showed that the analytical formulation accurately predicts center displacements across a range of loads when compared with finite element results. That opens the door to full creep curve characterization and rupture life assessment through inverse parameter identification from a single tiny beam.

The authors also confront the method’s limits honestly. Small-deformation assumptions underpin the conversion relationships, but under high loads and long durations specimens enter large-deformation regimes where regressed parameters can go badly wrong, producing artificial stress dependencies or incorrect activation energies. An error function based on the exact curvature expression defines a critical rotation angle of 14.37 degrees, corresponding to a 10 percent curvature error, and a finite-element-derived correction factor accounts for the transient stress redistribution from elastic to steady-state creep. Multi-layer systems such as coatings, thermal barrier layers, and weldments introduce further complexity: stress distributions become discontinuous at material interfaces and the neutral axis shifts from the geometric center as layers with different creep resistances redistribute load. Inverse procedures can nonetheless extract unknown layer properties, a capability that matters enormously where coating layers are orders of magnitude too thin for conventional uniaxial testing.

Experimental validation supports the approach for the configurations tested. Miniature three-point bending specimens of turbine rotor steel 23CrMoNiWV88, just 19.8 millimeters long and 1.54 millimeters thick, were tested at five load levels at 540 degrees Celsius on a custom-built system. When the measured displacement rates were converted through the reference stress framework, the resulting equivalent stress-strain rate data agreed well with conventional uniaxial creep tests on standard plate specimens, running slightly conservative. A comparative analysis across techniques, from small punch and impression creep to miniature rings and thin-plate tension, shows that bending tests occupy a sweet spot of material utilization, measurement sensitivity, and data-conversion capability, requiring only about 56 cubic millimeters of effective material while delivering both steady-state and rupture information. The authors close with a practical selection matrix and a research agenda: extending the reference stress framework to anisotropic single-crystal superalloys and additively manufactured materials, establishing minimum specimen dimensions against size effects, validating with tests of thousands of hours where oxidation becomes significant, and developing the test standards needed for industrial adoption. If those steps succeed, the humble bent beam may become the standard instrument for reading the remaining life of the machines that keep the lights on and the aircraft flying.

Subject of Research: Miniature bending-dominated creep testing methods for assessing remaining life of high-temperature components

Article Title: Bending-dominated miniature creep testing: A synthesis of theory and practice

Article References: Sun, W., Zhou, G.-Y., Li, M., Luo, R., Wen, Z., Yue, Z., & Tu, S.-T. (2026). Bending-dominated miniature creep testing: A synthesis of theory and practice. Results in Engineering, 32, Article 113468. https://doi.org/10.1016/j.rineng.2026.113468

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113468

Keywords: creep testing, miniature specimen testing, reference stress method, three-point bending, equivalent gauge length, Norton creep law, Kachanov-Rabotnov damage model, turbine blades, high-temperature materials, P91 steel, thermal barrier coatings, remaining life assessment

The challenge the researchers set out to solve is deceptively narrow but notoriously difficult. The cladding target on a turbine blade crown typically measures less than one square centimetre and accounts for less than five percent of the total three-dimensional scan data of the part. Conventional machine-vision techniques for welding and grinding robots, such as clustering algorithms or RANSAC-based plane fitting, assume either that the region of interest is large relative to the whole point cloud or that it can be approximated by regular mathematical shapes. Neither assumption holds for a blade crown, whose cladding zone is an irregular patch of discontinuous planes and step features. Team lead by researchers including Zhongqi Wang, Chengyuan Ma and corresponding author Bo Chen at the Harbin Institute of Technology and its Weihai campus, together with Xiaoguo Song and Caiwang Tan, therefore designed a dedicated pipeline built around template matching in three-dimensional space.

Their experimental platform pairs an industrial robot with an RVC-I540 area-scan structured light camera mounted on the robot arm in an eye-in-hand configuration. After hand-eye calibration, the camera sweeps over the blade and acquires a dense point cloud, which is streamed to a host computer over TCP/IP. Because blades are manually placed on the workbench, their positions shift slightly between scans, mostly in the horizontal plane. The raw scans contain more than a million points, so the team first filters out irrelevant data with a pass-through filter that exploits the known height separation between blade body and crown, reducing the cloud to roughly 182,000 points. An axis-aligned bounding box alignment then brings the scan and the template into coarse coincidence, and a voxel grid filter downsamples both datasets for efficient processing.

The core of the approach is a two-stage registration strategy. For coarse alignment, the researchers turned to particle swarm optimisation, a search technique inspired by the flocking behaviour of birds, in which candidate solutions move through the solution space guided by personal and collective experience. Standard particle swarm optimisation with fixed parameters is prone to getting trapped in local optima, a fatal flaw when the template is tiny and the search space is vast. The team therefore developed an adaptive variant, GA-APSO, that blends in genetic algorithm operations: elite particles are selected each generation, crossover creates offspring, mutation perturbs personal best positions with a decaying strength, and an elite injection strategy accelerates convergence. Crucially, they also crafted a geometry-tailored fitness function combining three weighted terms: root mean square error, the mean distance of the farthest two percent of points, and a plane normal vector angular difference score.

Each of these terms addresses a specific failure mode. Pure RMSE, it turned out, allowed the template to slide along edges, because the aggregate point-to-point distance metric barely constrains boundary regions. Adding the farthest-point term suppressed much of this edge sliding, yet occasional misalignment of side surfaces remained, since proximity alone does not capture orientation. The normal vector term, computed from dominant planar regions identified by a direction-constrained RANSAC variant, finally embedded orientation awareness into the optimisation. In thirty independent experiments, conventional PSO produced fitness values above 0.4 in half of its runs and adaptive APSO in several, whereas every GA-APSO run stayed below that threshold with minimal variation, confirming both superior accuracy and stability.

Coarse registration alone left errors around 0.2 millimetres, still too coarse for blade crown work, so the pipeline finishes with a new fine registration algorithm the team calls Robust Edge-Constrained Iterative Closest Point, or RE-ICP. Classic ICP refines alignment by repeatedly matching nearest points, but its convergence criterion based only on RMSE is blind to edge fidelity, which is exactly where cladding extraction fails. RE-ICP monitors, every five iterations, the distances of the worst-matched 0.2 percent of template points, deriving an edge distance metric from their median and 90th percentile values. This edge metric is combined with RMSE in a normalised, smoothed distance measure whose improvement rate serves as the stopping condition, with a restart mechanism triggered if RMSE rises over three consecutive iterations. Compared with standard ICP, RE-ICP cut RMSE by 28.4 percent and the final edge distance by 56.99 percent, at the cost of only marginally longer computation.

With template and scan precisely registered, extraction becomes straightforward. The convex hull of the registered template is reconstructed by iteratively adding the farthest external points, and a ray-casting test classifies every point of the blade scan as inside or outside that hull. The retained interior points form the segmented cladding area, which feeds into the path planning module. There, principal component analysis determines the cladding region’s principal, secondary and normal directions, allowing rectangular cutting planes spaced one millimetre apart to slice the region into coarse track segments. Singular value decomposition then fits a straight line to each segment, endpoints are trimmed inward by half the single-track cladding width to prevent edge collapse, and five equidistant waypoints per track are reordered by parity into a continuous zigzag trajectory that minimises travel and evens out thermal history. Finally, local surface normals computed from small neighbourhoods around each waypoint are converted into roll and yaw commands for the robot controller, keeping the cladding torch perpendicular to the surface throughout.

The experimental results are striking for a system with no human in the loop. Across thirty trials, the framework achieved an average root mean square error of 0.1960 millimetres with a total execution time of 5789 milliseconds, and it significantly outperformed Fast Global Registration and RANSAC combined with FPFH, both of which showed severe instability or outright failure on this geometry. The team also stress-tested the system with suboptimal optical conditions, specular reflections and random blade placements, and found that extraction and path generation remained exceptionally stable even under partial occlusion of the crown. On a desktop workstation with an Intel Core i7-12700KF processor and an RTX 3060 Ti GPU, the entire chain from scan to finished trajectory completes in under six seconds, with registration accounting for about 84 percent of that time. Compared with the traditional teach-playback workflow, which demands several minutes of skilled operator intervention per blade and exposes workers to hazardous fumes, the efficiency gains are transformative.

The implications reach well beyond a single repair shop. Turbine blades are standardised components, which makes template-based registration a broadly applicable strategy wherever the region of interest is small, irregular and buried in a large scan. The work also offers a template for how hybrid metaheuristics and geometry-aware fitness functions can rescue classical robotics pipelines from the local optima that plague generic algorithms on weakly featured surfaces. The authors acknowledge that GA-APSO coarse registration carries higher computational overhead than simpler alternatives and that the fitness weighting currently relies on empirical tuning; future work will target faster registration and adaptive parameter selection. If those refinements land, teaching-free robotic cladding of precision aero-engine parts could move from laboratory demonstration to routine industrial practice, marking another step in the quiet automation of some of manufacturing’s most demanding repair tasks.

The study appears in Advanced Materials Joining, a Springer journal, and is published as open access, meaning the full methods and results are freely available to researchers and engineers working on automated repair technologies. Open publication of this kind of work matters for the welding and surface engineering community, because reproducible pipelines for vision-guided cladding are still rare in the literature, and the detailed description of the fitness function and stopping criteria allows other groups to adapt the framework to different components.

Cladding itself is a well-established family of processes, typically carried out by laser deposition or arc-based techniques, in which a protective alloy layer is fused onto a worn or vulnerable surface. The quality of the deposited layer depends heavily on how consistently the heat source tracks the target area and how well the torch orientation follows the local surface geometry. This is why the path planning stage of the reported framework, with its one-millimetre track spacing and surface-normal-based posture control, is as important as the registration algorithms that precede it: even a perfectly recognised cladding zone would yield poor results if the deposited tracks overlapped unevenly or drifted off the boundary.

The timing of the research also reflects a broader industrial trend. Aero-engine maintenance providers face growing backlogs of blades requiring refurbishment, while skilled welders become scarcer. Automating the recognition and programming stages, rather than the deposition itself, addresses the true bottleneck in current practice. The authors’ stated future directions, faster registration and adaptive parameter selection, suggest the framework is intended as a foundation for production deployment rather than a one-off demonstration.

Subject of Research: Automated 3D vision-based recognition and robotic path planning for wear-resistant cladding on aero-engine turbine blade crowns

Article Title: A 3D vision-based method for turbine blade crown cladding area recognition and automatic path planning

Article References: Wang, Z., Ma, C., Meng, Z., Zhang, Y., Chen, B., Tan, C., & Song, X. (2026). A 3D vision-based method for turbine blade crown cladding area recognition and automatic path planning. Advanced Materials Joining, 1(1), Article 8. https://doi.org/10.1007/s44500-026-00010-3

Image Credits: AI Generated

DOI: 10.1007/s44500-026-00010-3

Keywords: turbine blades, 3D vision, point cloud registration, laser cladding, path planning, GA-APSO, iterative closest point, structured light camera, aero-engine repair, robotics, convex hull segmentation, zigzag trajectory