Sliver Defects Research Articles

An insight into the creep failure mechanism of sliver defect in the second-generation nickel-based single crystal superalloy CMSX-4

Sliver defects are common casting imperfections in the preparation of single crystal blades, significantly impacting the high-temperature mechanical performance of blades and potentially leading to blade failure. A detailed study was conducted on the microstructure of sliver in the second-generation nickel-based single crystal superalloy CMSX-4, focusing on its influence on creep performance at 980 °C/250 MPa. Results indicate a substantial impact of sliver on the creep performance of single crystal superalloys, with a 26.37 % reduction in creep rupture time for samples with sliver compared to those without. Fracture analysis reveals that sliver primarily contributes to performance deterioration by promoting the extension of grain boundary microcracks through grain boundary sliding during the creep process, accelerating sample fracture. Utilizing crystal plasticity theory, a successful fit was achieved for the creep rupture life of samples with sliver, allowing the prediction of fracture life for samples with different orientations of slivers. Predictive results suggest that, for CMSX-4 alloy, the orientation of the sliver should be restricted to within 8°.

A Study of Sliver in C-Shaped Grain Selectors during Investment Casting of Single-Crystal Superalloy

In this study, the formation mechanism of sliver defects in C-shaped 2D grain selectors during investment casting of single-crystal superalloy was investigated by analyzing the effects of the C-shaped 2D grain selectors on the solidification behavior of superalloy. The physical field properties of the sliver formation and solidification characteristics of CM247LC nickel-based superalloy were determined. The temperature and stress fields were simulated using ProCAST. The results showed that the stress fields of solidification played an essential role in the formation of sliver, indicating that the solidification interval characteristics of the alloy and stress based on the geometry of the C-shaped 2D grain selector are crucial for the formation of sliver. In addition, the origin of sliver depended upon tensile stress during solidification, relying on the constraints of dendrite boundaries. The findings suggested that the joint sections of the starter block—i.e., selector and selector-casting joint of C-shaped selector sections—are stress-sensitive areas where sliver can form readily. Furthermore, sliver is formed in the final stages of solidification and especially originates in the grain selection part where the accumulated thermal stress is high, and there is only a small quantity of liquid phase with a low melting point between the dendrites. Therefore, the solidification and stress conditions generate thermal cracks, which can also cause sliver defects.

Effect of Al2O3 Inclusions or Mold Flux Particles on Their Surrounding Microstructures of Sliver Defects on the Surface of Automobile Exposed Panel

The Al2O3 inclusions and mold flux particles are the initial causes of the sliver defects on the surface of automobile exposed panels. During the rolling process, the smashed Al2O3 inclusions or mold flux particles will hinder the growth of recrystallized grains. Compared with mold flux particles, the smashed Al2O3 inclusions have a smaller size, a denser distribution, and a larger number density, so the smashed Al2O3 inclusions have a stronger ability to hinder grain boundary migration. Therefore, the average grain size is small in the following sequence: Al2O3 defect zone with Al2O3 inclusions (Al2O3 DZ with Al2O3), mold flux defect zone with mold flux particles (MFDZ with MFP), mold flux defect zone without mold flux particles (MFDZ without MFP), Al2O3 defect zone without Al2O3 inclusions (Al2O3 DZ without Al2O3), and non-defect zone (NDZ). The influence of particles on the grain orientation of the defect zones results in the microtexture of Al2O3 DZ without Al2O3 is {111}<313>, which is close to the {111}<101> microtextures of NDZ and MFDZ without MFP, while the {001}<114> microtexture on Al2O3 DZ with Al2O3 and the {313}<111> microtexture on MFDZ with MFP are quite different from that of NDZ. Due to differences in the inclusions, orientation, and microtexture of the defect zones and NDZ, dark-gray or bright white sliver defects on the surface of the automobile exposed panel are eventually formed.

Generation mechanism and motion behavior of sliver defect in single crystal Ni-based superalloy

Sliver is a common but easily neglected defect in single crystal Ni-based superalloy castings. To date, there is still no unified viewpoint on its formation mechanism and generation causes. In this work, the orientation discontinuity and motion behavior of sliver defects were studied through experiments and numerical simulations. The ultrathin wedge-shaped specimen containing the grain boundary of the sliver and the matrix was prepared at the initial position of the sliver defect for the observation of equal thickness fringes. The discontinuity of equal thickness fringes on both sides of the grain boundary was observed through a transmission electron microscope, which directly confirms the abrupt change in the orientation between the sliver and matrix from the nanoscale. The crystal lattices at the smooth area and the bulging area of the grain boundary were found to have unusually different arrangements. The irregular lattice arrangement at the bulging area shows that the grain boundary has experienced high-stress deformation. Statistical results of sliver orientation deviation with a further composition analysis show the micro protuberance of the mold shell has a noticeable inductive effect on the sliver generation. Furthermore, a self-developed three-dimensional phase-field simulation model coupled with the spatial topology algorithm is established to simulate the orientation deflection behavior and orientation deviation threshold of fractured dendrites. The simulation results indicated that there is an upper limit of the cross-section solid fraction at the fracture position for the motion of the fractured dendrites. When the cross-section solid fraction at the fracture position is higher than this upper limit, it will be difficult to produce large deviation slivers due to the structural limitation of surrounding dendrites. This upper limit does not change with the solidification temperature gradient.

Influence of Fluorine and CaO/Al2O3 Ratio on the Wetting Behavior Between CaO–Al2O3‐Based Mold Flux and Steel Substrate

The wetting behavior of flux melt on the initial solidified shell greatly determines the slag entrapment in the casting product and the sliver defect in the rolled product. Herein, the wetting behavior of CaO–Al2O3‐based mold flux with various fluorine (F) contents and CaO/Al2O3 (C/A) ratio on steel substrate is studied using the sessile drop method. Results show that the contact angle decreases from 71.35° to 59.86° and from 65.96° to 22.73°, meanwhile, the interfacial tension decreases from 1743.02 to 1669.69 mN m−1 and from 1705.40 to 1448.53 mN m−1, with the addition of F and the increase of C/A ratio, respectively. X‐ray photoelectron spectroscopy analysis indicates that the addition of F and increase of C/A ratio lead to positive adsorption () of non‐bridging oxygen (O−) and free oxygen (O2−) at the interface, which is the main reason why the interfacial tension between the mold flux and steel is reduced and the wettability of flux droplet on the steel is enhanced.

Water modeling on slag entrainment in the slab continuous casting mold

A water model is built to investigate the transport phenomena in the slab continuous casting mold. The gas flow rate, casting speed, and slab width on the slag entrainment are studied. The sliver defect in the IF steel plate is analyzed by the scanning electron microscope. The slag entrainment is recorded by the high-speed camera and the velocity is measured by the PIV. The results show that sliver defects contain Al, Ca, Si, Mg, Na, and O, which are mainly derived from the mold slag entrainment. With the casting speed increase, the fluid velocity from the submerged entry nozzle rises clearly and the slag droplets detach from slag layer. As the gas flow rate increases, the liquid moves upward with bubble floating and attacks the slag-steel interface. It is suggested the gas flow rate should be less than 3.3 NL/min for the slab width of 1300 mm and the casting speed of 1.2 m/min. With the slab width increasing from 1300 mm to 1700 mm, no slag entrainment zone is reduced. The gas flow rate should be below 2.2 NL/min with the casting speed increasing to 1.4 m/min, both for the slab width of 1300 mm and 1700 mm.

Improved YOLOV4-CSP Algorithm for Detection of Bamboo Surface Sliver Defects With Extreme Aspect Ratio

Bamboo surface defect detection provides quality assurance for bamboo product manufacture in industrial scenarios, an integral part of the overall manufacturing process. Currently, bamboo defect inspection predominantly relies on manual operation, but manual inspection is very time-consuming as well as labor-intensive, and the quality of inspection is not guaranteed. A few visual inspection systems based on traditional image processing have been deployed in some factories in recent years. However, traditional machine vision algorithms extract features in tedious steps and have poor performance along with poor adaptability in the face of complex defects. Accordingly, many scholars are committed to seeking deep learning methods to accomplish surface defect detection. However, existing deep learning object detectors struggle with specific industrial defects when directly applied to industrial defect detection, such as sliver defects, especially for ones with extreme aspect ratios. To this end, this paper proposes an improved algorithm based on the advanced object detector YOLOV4-CSP, which introduces asymmetric convolution and attention mechanism. The introduction of asymmetric convolution enhances the feature extraction in the horizontal direction of the bamboo strip surface, improving the performance in detecting sliver defects. In addition, convolutional block attention module(CBAM), a hybrid attention module, which combines channel attention with spatial attention, is utilized to promote the representation ability of the model by increasing the weights of crucial channels and regions. The proposed model achieves outstanding performance in the general categories and excels in the hard-to-detect categories. Some enterprise’s bamboo strip dataset experiments verify that the model can reach 96.74% mAP for the typical six surface defects. Meanwhile, we also observe significant improvements when extending our model to aluminum datasets with similar characteristics.

Orientation Discontinuity and Motion Behavior of Sliver Defects in Single Crystal Ni-Based Superalloy

Orientation Discontinuity and Motion Behavior of Sliver Defects in Single Crystal Ni-Based Superalloy

Characteristics of sliver defects caused by mould flux entrainment on the surface of the hot-dip galvanised automobile exposed panel

ABSTRACT The characteristics of sliver defects caused by entrained mold fluxes were analyzed after nonaqueous electrolysis. There are three types of entrained mold fluxes, namely the granular, flaky and blocky. When the entrained mold fluxes are small, they are granular or flaky. When the entrained mold fluxes are large, they are blocky or flaky. From the start part of the defect zone, the area fraction of the entrained mold fluxes per unit area decreases exponentially with the length of defect. After the mold fluxes are entrained into the molten steel, SiO2 will react with Al and Ti, resulting in the changes in compositions of mould powders. On the cross-section, the zinc layer thickness in the defect zone, is 22–35 μm, thicker than that in the non-defect zone. Near the entrained mold fluxes, the randomness of grain orientation and the inhomogeneity of grain size are increased and the local grains are refined.

Investigating the evolution of freckles into sliver defects in Ni-based single-crystal superalloy castings

Freckles are detrimental solidification defects in Ni-based single-crystal superalloy castings that are formed during directional solidification. However, the evolution of freckles into sliver defects has rarely been reported, and the mechanism of this evolution is unclear. In this study, the evolution of freckles into slivers in the extended zone of casting was established to be the deformation of dendrites induced by the thermal contraction stress in the freckle channel. The morphology of the mushy zone was simulated using ProCAST, which indicated that freckles were formed at the highest position of the mushy zone. During the initiation of the slivers, deformation occurred at the freckle channel, indicating that the thermal contraction stress was a result of the solidification shrinkage of solute-rich liquid. The misorientation from the initial freckle grains to the slivers ranged from 15° to 57°. In addition, the orientation and ability of dendrite branching are crucial for the extension of the sliver dendrites.

Sliver defects in an ultra-low carbon Al-killed steel caused by low steel level in the tundish

ABSTRACT Sliver defects were detected in galvanized and uncoated cold-rolled strips, and their causes were investigated. Most of these strips were produced from transition ladle slabs, which were obtained in a sequence of twenty heats of an ultra-low carbon Al-killed steel. A significant reduction in the steel level in tundish occurred during these slabs’ casting. The strip defects, and as a comparative, a mould flux inclusion found in a first slab of the sequence, were analysed with a scanning electron microscope (SEM). Also, each of the twenty heats was sampled in tundish, quenched in water, and manually analysed with SEM to characterize its dominant inclusions. Regarding the sliver defects, mainly silica together with alumina or Al–Ti–O systems particles were found in them. The primary sources for these particles are likely tundish flux and reoxidation products. Finally, a mechanism was proposed for the formation of these inclusions.

Distribution characteristics of inclusions along with the surface sliver defect on the exposed panel of automobile: A quantitative electrolysis method

The specific distribution characteristics of inclusions along with the sliver defect were analyzed in detail to explain the formation mechanism of the sliver defect on the automobile exposed panel surface. A quantitative electrolysis method was used to compare and evaluate the three-dimensional morphology, size, composition, quantity, and distribution of inclusions in the defect and non-defect zone of automobile exposed panel. The Al2O3 inclusions were observed to be aggregated or chain-like shape along with the sliver defect of about 3–10 µm. The aggregation sections of the Al2O3 inclusions are distributed discretely along the rolling direction, with a spacing of 3–7 mm, a length of 6–7 mm, and a width of about 3 mm. The inclusion area part is 0.04%–0.16% with an average value of 0.08%, the inclusion number density is 40 mm−2 and the inclusion average spacing is 25.13 µm. The inclusion spacing is approximately 40–160 µm, with an average value of 68.76 µm in chain-like inclusion parts. The average area fraction and number density of inclusions in the non-defect region were reduced to about 0.002% and 1–2 mm−2, respectively, with the inclusion spacing of 400 µm and the size of Al2O3 being 1–3 µm.

Evolution of Sliver Defect in Ni-Based Single Crystal Superalloy

Evolution of Sliver Defect in Ni-Based Single Crystal Superalloy

Microstructure refinement of single crystal Ni-based superalloy by improvement of thermal radiation shielding in the industrial-scale Bridgman solidification process

Refinement of dendritic microstructure in CMSX-4 Ni-based single crystal casting blade, manufactured using standard adjusted (ARB) or novel inner radiation baffles (IRBs) in the industrial-scale Bridgman solidification process, was investigated in the paper. Temperature gradient in the blade was mainly controlled by baffle geometry and by changing local solidification rate along casting height. Results showed that the use of relatively simple modification of mold, by introducing IRBs, resulted in increase of temperature gradient and noticeable refinement of dendritic microstructure. Temperature gradient in airfoil of blade increased by 30 and 95%, while primary dendrite arm spacing (PDAS) was favorably reduced by 7.8 and 21% for ARB and IRBs, respectively, compared with standard ring radiation baffle. The use of IRBs and ARB resulted in increase of tendency to form sliver defect in airfoil and low angle grain boundary in blade platforms. However, length of freckle chains decreased at root edge of blades when IRBs were used. New design and a proposal of the implementation of IRBs technique for the production of large IGT blades using Bridgman method were also discussed in this article.

Sliver defect formation in single crystal Ni-based superalloy castings

Sliver is a typical grain defect in single crystal Ni-based superalloy castings produced by Bridgman directional solidification. However, its formation mechanism has yet to be fully understood. In this study, the formation of sliver defects was investigated with respect to the defect morphological features, the microstructure of defect initiation points, and the solidification conditions. Experiment results showed that sliver defects were originated from fragmentation of dendrite trunks and could developed to various morphologies. The misorientation range of sliver grains was between 3.2° to 12°. Microstructural analysis results indicated that the dendrites fragmented abruptly without significant plastic deformation. And the dendrite fragmentation could be facilitated by oxide inclusions and solidification pores. It was also found that the sliver defects were prone to occur at the interface where primary dendrite converge to the mold inner wall. Based on the low misorientation level of sliver grains in the experimental observation, the critical solid volume fraction range for fragmentation was suggested to be 0.6–0.8. The dendrite fragmentation was attributed to the non-uniform stress distribution in dendrite during solidification. In addition, the effect of airfoil geometry of a typical blade casting and defect prevention methodology were discussed.

Fluid flow and heat transfer with nail dipping method in mould during continuous casting process

ABSTRACTThe character and cause of sliver defect on IF steel sheet surface are studied by means of SEM, unstable flow in mould could induce surface velocity and level fluctuations, leading to surface defects during continuous casting of steel. The nozzle clogging is a serious problem during the continuous casting of steel, due to its influence on the casting operations and products quality. In this study, the nail dipping method for measuring surface velocity and flow direction in molten steel were employed. The fluid flow in mould of whole casting sequence was investigated, especially during the nozzle clogging conditions. The results showed that when nozzle clogging occurred in the 7th heat, the flow velocities on R and L side of nozzle were 0.280 and 0.402 m/s, respectively. The surface defect ratio of hot-rolled and cold-rolled plates increases with the increase of heat flux deviation on both sides of the mould copper. The different clogging per cent on both sides of the nozzle will lead to asymmetry flow, the surface velocity is higher with the small clogging per cent side compared to that of relatively large per cent clogging side.

Formation of Sliver Defects in Single CrystalCastings of Superalloys

Formation of Sliver Defects in Single CrystalCastings of Superalloys

Formation of Sliver Defect in Ni-Based Single Crystal Superalloy

The evolution of sliver defect in a Ni-based single crystal superalloy was explored by casting samples with a similar geometry of the airfoil section of a turbine blade using optical microscope and electron backscattered diffraction. It was found that the sliver generally initiated from 1 to 2 tertiary dendrites at the diverging boundary between the mold wall and matrix. All slivers were tilt from matrix along the [001] and some of them rotated relative to the matrix on the plane perpendicular to the solidification direction. During solidification sliver in most castings grew with a fixed orientation. All slivers can extend along the solidification direction to the top of the casting. Most of the sliver defects showed constant width and did not extend on the casting surface. However, the sliver with large misorientation to matrix on the cross section perpendicular to the solidification direction extended on the casting surface and into the casting quickly.

Metallurgical Analyses of Surface Defects in Cold-Rolled Steel Sheets

Six cold-rolled steel sheet samples having surface defects like slivers and dents have been metallurgically analyzed to identify the reasons for generation of such defects. In all the cases, large inclusions/entrapments were located at the defect site. Energy-dispersive spectroscopy (EDS) of the entrapments in samples 1 and 2 having dents revealed incidences of O, Mg and Ca, confirming that these entrapments are essentially complex oxides of probable slag genesis. The dents in samples 1 and 2 must have occurred due to heavy cold rolling load on the preexisting cracks in the coil. Apart from dents, sliver defects were also observed in some of the samples. Slivers are, however, of various types and appearances, and there could be many reasons for occurrences of these slivers in the cold-rolled sheets. In the present study, the SEM–EDS analyses of the subsurface entrapment/deposit observed in sample 3 revealed complex oxides of CaO-MgO of possible slag genesis and could be the reason for incidences of slivers in this sample. On the other hand, the wavelength-dispersive spectroscopy of samples 4 and 5 having sliver defects carried out in an electron probe microanalyzer (EPMA) revealed subsurface chains of dark and angular brittle inclusions of either alumina or silicate of possible deoxidation product genesis. EPMA analysis of sample 6 having pencil line sliver defects, however, revealed entrapments of possible casting powder origin.

Quantitative Evaluations of Surface Cleanliness in IF Steel Slabs at Unsteady Casting

Macro-inclusions in the surface layer of slabs are mainly responsible for the sliver defects in IF steel sheets. In the present work, macro-inclusions in six kinds of unsteady cast IF steel slabs involving four unsteady states (casting start, ladle change, nozzle change and casting end) in one casting sequence were quantitatively characterized using an automatic inclusion analyzer with a total detection area of 238,048 mm2. The cleanliness of corresponding normal slabs was also evaluated as the benchmark study, and the number density (ND) of inclusions > 50 µm was used to describe the surface slab cleanliness. Generally, alumina-based and mold powder inclusions were detected, and the former type was further subdivided into alumina clusters, alumina associated with bubbles, refractory-related alumina, Al-Ti complex oxides and CaO-Al2O3-MgO. Several large-sized alumina-based inclusions were detected in the normal slabs with an average ND of 0.31 N/cm2, although they had very low total oxygen (TO) contents (10~13 ppm). No mold powder inclusions were found in normal slabs. For all the unsteady slabs, alumina-based and mold powder inclusions were both observed, and the percentage of the former kind was ≥ 90 pct. The average density of macro-inclusions for the first slab was 1.621 N/cm2, whose cleanliness was much worse than any of the other unsteady slabs. The first slab should be downgraded. For the second slab, the ND of macro-inclusions was 0.385 N/cm2, which was still larger than the normal slabs because of the lasting effect of the re-oxidation. For the penultimate, cap-off and SEN changed slabs, the density of macro-inclusions fluctuated from 0.25~0.35 N/cm2, which was close to the average level of normal slabs. However, mold flux-originated inclusions were frequently observed in these slabs because of flow pattern variations due to the casting speed change. In terms of the slab during the ladle change process, mold powder inclusions were frequently detected even though the casting speed remained constant. A mold powder entrapment mechanism was proposed, which showed the entrapment occurred owing to the flow pattern change induced by forming/dropping of SEN clogs due to re-oxidation by FeO in the ladle slag and tundish steel temperature variations during the ladle change practice. The severity of powder entrapment and re-oxidation was also given for different kinds of unsteady slabs, which provides a direct guide to treating these slabs in the steel plant.