Experimental Investigation on Quasi-Freckle Phenomenon in Single-Crystal-Blade Castings of Superalloys

Abstract

During the production of single-crystal superalloy blades, a kind of channel-type defect, named “quasi-freckle”, was found on the casting surface, which is similar to typical freckles in macroscopic appearance but different in microstructure. In the as-cast microstructure of the quasi-freckle channels, the γ/γ’ eutectic is significantly accumulated and can be dissolved during the solution heat treatment. Since no disoriented grains were detected, the quasi-freckles have a basically identical crystal orientation with the matrix. The quasi-freckle channels actually appear as thermosolutal convection traces in the directional solidification process of single-crystal casting. Because the convection was not strong enough to break dendrite arms, the single-crystal consistency of the castings was not destroyed. However, with the deterioration of the solidification condition and the increase in solutal convection, quasi-freckles often develop into typical freckle defects.

1. Introduction

Nickel-based single-crystal (SC) superalloys have excellent high-temperature comprehensive performance and are the preferred material for fabricating turbine blades for aero-engines and industrial gas turbines [1,2,3,4,5]. In order to meet the increasing requirements of high-temperature strength, microstructural stability, and corrosion resistance, higher generations of SC superalloys with more content of refractory elements have been developed. The most important refractory elements added to the chemical composition are Re and Ru, whose additive amount determines the generation number of the SC superalloys. Due to the complex alloying system, it becomes more difficult to control the casting defects in the production of the SC turbine blades [6,7].

Freckle (FR) is a common chain-like grain defect occurring during SC solidification of the superalloy castings. Freckles contain small stray grains with random crystal orientations, which destroy the monocrystalline property of castings, leading to a high rejection rate of SC castings. Freckles severely damage the mechanical performance of SC superalloy castings at high temperatures, including the fatigue and creep properties, which are extremely sensitive to the grain boundaries. It is commonly believed that freckles are formed as a result of the thermosolutal convection induced by inverse density in the mushy zone. During upward directional solidification of superalloys, the inter-dendritic melt becomes gravitationally unstable because of the chemical segregation of alloying elements. The density difference between the less dense liquid in the mushy zone and the above bulk liquid ahead of the dendrite tip results in an unstable state of top-heavy and bottom-light conditions. The thermosolutal convection can then be generated when the driving force for the fluid flow exceeds the surrounding frictional forces. The vertical freckle chains are consequently formed due to the fragmentation of the dendrite arms in the convection channels [8,9,10,11,12]. However, due to the interlacing of dendrite arms inside the casting, the internal resistance to the liquid floating is very large. In comparison, the flow along the smooth inner wall of the mold shell is much easier. As a result, the lighter liquid in the mushy zone tends to flow laterally toward the shell wall first, and then flow upward along the shell wall in the form of one or more tunnels. This is the wall effect of solutal convection during the directional solidification of superalloys, resulting in the channel-type appearance of freckles as a surface defect. The experimental results of different generations of SC superalloys show that increasing the Re content can significantly increase the freckle defects in the castings [13].

Besides the alloy composition, solidification conditions are considered an important factor affecting freckle formation. Previous and recent studies have confirmed that under a lower temperature gradient and a slower solidification rate, the freckle formation becomes more serious [14,15,16], because a wider mushy zone and coarser dendrite structure are favorable for inter-dendritic liquid flow.

In recent works on the freckle problem, the authors of this article and other researchers have found that the geometric shape of castings also has a significant impact on freckle formation, in addition to alloy factors and solidification conditions [13,17,18,19]. The freckles were mainly observed on the casting edges instead of on the plane surface, although the cooling condition on the edges was unfavorable for the freckle formation. In the castings with stepwise increasing cross-sections, the freckle chains are not formed immediately at the bottom edge of the steps, but after an incubation distance, indicating a suppressing effect on the freckling. In contrast, contracting the dimension shows a promoting effect on the freckle onset. Freckle patterns on the sloping surface are not arranged in a vertical direction but have the same sloping angle as the surface gradient, because the mold/melt interface has limited the freckling flow, affected by gravity. Since the geometrical features of the components can strongly change the reservoir condition for thermosolutal convection, it can more effectively affect the freckle formation than the local solidification conditions [13,17]. On the basis of experimental research, some models were developed to simulate the freckle formation considering the influence of the geometry effect [20,21].

In recent years, due to the increasing addition of refractory elements in superalloys and the more complex design of blade geometry, freckle defects in SC castings have shown an increasingly serious trend. In the production process of SC blade castings of superalloys, another channel-type defect was observed. It is similar to typical freckles in macroscopic appearance but different in microstructure. This kind of solidification defect is referred to as “quasi-freckle” (QF) in this article and is experimentally examined in detail, in order to better understand its formation mechanism and to correctly evaluate its influence on the casting quality.

2. Experimental Materials and Methods

In the casting experiments for SC turbine blades, various Ni-based superalloys (

Table 1. Chemical composition of the Ni-based superalloys used in the work (wt.%).

	Cr	Co	W	Mo	Al	Ti	Ta	Re	Hf	Ni
DD419	6.44	9.57	6.38	0.61	5.60	1.02	6.47	2.94	0.11	Bal.
CMSX-4	6.50	9.00	6.00	0.60	5.60	1.00	6.50	3.00	0.10	Bal.
WZ30	3.50	6.00	6.50	0.40	5.80	0.15	8.00	4.95	-	Bal.

) were used, including the second-generation alloys DD419 (Jiangsu Longda Superalloys Co., Ltd., Wuxi, China) and CMSX-4 (Jiangsu Longda Superalloys Co., Ltd., Wuxi, China), as well as the third-generation one, WZ30 (Wedge Central South Research Institute Co., Ltd., Shenzhen, China). It has been found that alloy WZ30 has a significantly higher freckling tendency than CMSX-4 [13]. The ceramic mold shells assembled into cluster configurations were prepared by the conventional lost wax process. An industrial-type vacuum furnace (VIM-IC 5E/DS/SC, ALD Vacuum Technologies GmbH, Hanau, Germany) consisting of an upper mold-heating chamber and a lower withdrawal chamber was employed to fabricate various SC blade castings. After preheating to 1500 °C and pouring the alloy melt, the shell molds were withdrawn at a programmed rate of 3 mm/min from the heating zone through the baffle into the cooling zone of the furnace. After the casting experiments, the castings were knocked out of the ceramic mold and mechanically separated from the cluster. The castings were sand-blasted to remove any ceramic deposits attached to their surfaces and were then macro-etched in a solution of 50%H₂O₂ and 50%HCl (Guangzhou Chemical Reagent Factory, Guangzhou, China). By visual inspection, the grain defects, including freckles and quasi-freckles, on the casting surface were identified. The stereomicroscope was used to photograph the grain defects on the rough surface. For metallographic investigation, corresponding parts of the castings were cut off and sampled. The sample surface or sections were ground and polished. The etching solution used for microstructural inspection was 500 mL HCl + 500 mL H₂O + 300 g FeCl₃·6H₂O (Guangzhou Chemical Reagent Factory, Guangzhou, China). The microstructure beneath the castings was examined using an optical microscope (Nikon MM-400, Nikon Metrology Inc., Shanghai, China). The crystal orientation of the relevant parts was analyzed using the EBSD device (FEI XL30S, FEI Company, Hillsboro, OR, USA) equipped with a scanning electron microscope (SEM, NOVA NANO SEM 230, FEI Company, Hillsboro, OR, USA).

3. Experimental Results and Analysis

In order to ensure the integrity of the single crystal, all blade castings were macroscopically etched and visually inspected. Because the freckling tendency in the second- and third-generation SC superalloys is quite serious [13,17], the freckle defects in the castings were emphatically examined. As a result, the grain defects detected were classified into three types: typical freckles with disoriented grains, quasi-freckles without disoriented grains, and mixed-type defects, including both typical freckles and quasi-freckles.

3.1. Typical Freckles (FR)

Figure 1a is a surface photo of a part of a blade casting produced using alloy WZ30. Due to the addition of a large number of refractory elements Re and W, the alloy had severe element segregation during solidification and was especially prone to freckle defects [13], as shown in Figure 1a. In particular, multiple freckle defects occurred in the tenon area of the upper part of the blade, as this area was relatively thick and had a slow solidification rate, which was conducive to the occurrence of freckle defects. In addition, freckle chains also occurred both on the leading edge and on the trailing edge of the blade body, as freckle defects were more likely to occur at the casting edges due to the shape effect of freckle formation [13,17]. These typical freckles were elongated chains containing many randomly oriented fine grains.

After visual inspection, the freckle-containing surface of the blade tenon shown in Figure 1a was lightly ground to remove the oxide skin, and then polished and etched to obtain a metallographic image of the freckle structure (Figure 1b). It is obvious that there were many fine stray grains in each freckle chain. These grains had a distinct contrast with the matrix and showed a disordered crystal orientation different from that of the matrix. Figure 1c shows the EBSD result for the corresponding freckle chain, confirming the disarray of grain orientation in freckles. Because of the existence of these high-angle grain boundaries, the SC integrity of these blade castings was destroyed, making them unqualified products.

3.2. Typical Quasi-Freckles (QF)

Figure 2a is a photo of the tenon part of a SC blade casting produced using alloy CMSX-4. There were several typical quasi-freckle defects on the tenon surface, as indicated by a dashed circle, and no discernible broken grains were found. This indicates that the local solutal convection was not strong enough to produce typical freckles containing broken grains, such as those shown in Figure 1a. This is mainly because the second-generation SC alloy CMSX-4 had a less severe freckle tendency than the third-generation SC alloy WZ30 and, therefore, did not exhibit the typical freckle defect as severely as that shown in Figure 1a.

Compared with the macroscopic morphology of typical freckle defects, the contrast between quasi-freckle defects and the matrix was less significant. In the production, however, the SC castings with the quasi-freckles, as shown in Figure 2a, were sometimes simply judged as unqualified products, due to their freckle-like appearance. Therefore, it was necessary to detect and study the quasi-freckle structure to confirm the effect of this defect on the quality of SC castings.

The tenon part of the blade casting shown in Figure 2a was cut perpendicularly to the surface along a quasi-freckle, to obtain a longitudinal section of the quasi-freckle channel, as shown in the dashed box in Figure 2b. Figure 2b shows a metallographic image of the as-cast structure, where the white bright color represents the γ/γ′ eutectic structure enriched in the quasi-freckle channel. The depth of the quasi-freckle channel was about 400 μm, which is close to the width of the quasi-freckle on the casting surface and is approximately at the scale of the dendrite spacing. The eutectic fraction within the quasi-freckle channel was significantly higher than that in the matrix of the castings. This was because of the strong segregation of alloying elements during the directional solidification process of superalloys, which resulted in the enrichment of γ′-phase-forming elements, such as Al, Ti, and Ta, in the residual liquid in the mushy zone, leading to a decrease in density and an upward buoyancy force. The smooth inner wall of the mold shell had less resistance to liquid flow, so the upward convection occurred along the side surface, resulting in chain-like traces on the casting surface. Because of the enrichment of γ′-forming elements, more γ′ phase will be precipitated in the form of γ/γ′ eutectic when the residual liquid in the convection channel finally solidifies. In fact, it is the segregation of these elements that causes eutectic enrichment, which forms the channel-like structure of quasi-freckles with a different contrast to the surrounding matrix. It is worth noting that although the eutectic fraction increased, the orientation of all dendrites remained consistent, and there was no broken dendrite deviating from the crystal orientation of the matrix.

Figure 2c is the metallographic image of the sample shown in Figure 2b after solution heat treatment. It can be seen that most of the eutectic in the casting was dissolved, and only a small amount of residual eutectic remained due to the significant improvement in element distribution uniformity. In this state, the quasi-freckle characteristics completely vanished. In fact, whether in the as-cast state or in the heat-treated state, the crystal orientation of the dendrites in the quasi-freckle channel remained consistent with the matrix, without fragmented grains caused by the breaking of dendrite arms.

As shown in Figure 3a, the typical quasi-freckles were also observed on the side surface of a guide vane platform prepared using the alloy DD419. There was only a luminance difference between the chain defects and the matrix in the photographs, and no disoriented grains were found due to the fracture and deflection of the dendrite arms. Figure 3b is a metallograph of area A in Figure 3a after shallow grinding, which clearly shows the dendritic structure beneath the casting surface. It can be seen that the difference in brightness between the quasi-freckle defect and the matrix was due to the accumulation of more white eutectic. EBSD crystal orientation detection was performed on the sample in Figure 3a. The results showed that the crystal orientation of the whole sample, including area A, was identical (Figure 3c), and no grain boundary defect was introduced. This shows that the eutectic fraction in the quasi-freckles was obviously higher than that in the matrix, but it did not affect the crystal orientation and SC consistency of the related structure.

3.3. Mixed Freckles

Besides the individual appearance of typical freckle and quasi-freckle defects, as respectively shown in Figure 1a as well as in Figure 2a and Figure 3a, the coexistence of both kinds of defects was also found on the surface of some blade castings. Figure 4a shows a partial photo of a blade casting made of the CMSX-4 alloy. A thin quasi-freckle channel appeared at the lower part of the blade body, but it became a typical freckle defect when it grew to the upper part, showing many fine stray grains on the chain-like channel (Figure 4b). This is because in the production of superalloy castings using conventional directional solidification furnaces, the temperature gradient at the front of solidification decreases continually in the process of solidification, and the mushy zone becomes wider and wider. Therefore, the dendritic structure in the upper part of the SC castings became significantly coarser, and the tendency toward freckle defects increased continuously. In these mixed channel-like defects, quasi-freckle is actually a transitional form before freckle defects occur, and freckle defect is a further development of the quasi-freckle morphology.

As shown in Figure 5, a surface photo of a guide vane platform of the alloy DD419, there were two mixed freckle channels on the outer surface of the platform, with the lower half being a quasi-freckle structure and the upper half being freckle defects with disoriented grains. It should be noted that in order to ensure SC growth in this wide guide vane casting, a tilted mold cluster was assembled, where both the blade body and the platforms were inclined at an angle of approximately 45° from the vertical solidification direction. The use of seeding technology in production ensured that the [001] crystal orientation of the SC casting was perpendicular to the platform, i.e., parallel to the blade body axis. It is worth noting that the dendrite growth direction in the platform was oblique in the [001] orientation controlled by the seed, while the two mixed freckle channels were basically vertical. This is because the macroscopic solidification direction of the casting was vertical, causing vertical thermosolutal convection and vertical channel-like freckle or quasi-freckle defects. This indicates that the oblique growth direction of dendrites cannot change the vertical growth direction of freckle chains.

The metallograph in Figure 6a shows two mixed freckle channels beneath the side surface of a blade tenon of the alloy DD419, revealing typical freckles and quasi-freckles in the upper and lower parts of both channels, respectively. The lower part of one mixed freckle channel (area A in the dashed frame) was subjected to EBSD crystal orientation detection, and the result is shown in Figure 6b. Several grains with serious crystal orientation deviations are shown in the upper part of Figure 6b, while in the lower part a nearly single tone consistent with the matrix was observed, indicating the identical orientation. However, since the upper part was a typical freckle chain with high-angle grain boundaries, this type of casting was not accepted as a qualified SC product.

3.4. Discussion

The formation mechanisms of quasi-freckles and typical freckles are shown schematically in Figure 7a,b, respectively, both of which were caused by solutal convection resulting from the density inversion. Due to the wall effect, the lighter liquid in the mushy zone flowed laterally toward the shell wall first, and then flowed upward along the shell wall in the form of one or more channels. The dendrite arms will be scoured, affecting the normal development of dendrite morphology and even causing the deformation and deflection of dendrites. However, when the convection was relatively weak, the dendrite arms were not broken off to form misoriented grains, and only quasi-freckle defects were formed (Figure 7a), without formation of high-angle grain boundaries. These quasi-freckle channels were enriched with positively segregated γ′-forming elements, such as Al, Ti, and Ta, so that more γ/γ′ eutectic structures were eventually formed (Figure 2 and Figure 3).

When the solutal convection in the mushy zone becomes very strong, the dendrite arms and even the dendrite trunks will be broken, forming a typical freckle defect consisting of misoriented grains along the convection channels on the casting surface (Figure 7b).

During the conventional directional solidification in a production furnace, the solidification condition normally becomes worse and worse, leading to the increase in solutal convection and the successive formation of corresponding channel-like defects as a consequence. Besides the individual appearance of quasi-freckles (Figure 2 and Figure 3) and the typical freckles (Figure 1), the mixed channels consisting of both kinds of defects could also be observed, as shown in Figure 4, Figure 5 and Figure 6.

It should be pointed out that both the quasi-freckle and typical freckle defects in superalloy castings originated from the solutal convection in the mushy zone during directional solidification. The difference between the two lies in whether the dendrites are broken and whether the SC integrity of the castings is damaged. Both channel-type defects were enriched with the γ/γ′ eutectic structure, making the contrast between the channel and the matrix significantly different. The quasi-freckle structure is actually a trace of the solutal convection channel during directional solidification and has the potential to develop further into freckle defects.

It is also interesting to note that in this work, the quasi-freckle phenomenon was mainly observed in the SC castings of the second-generation alloys CMSX-4 (Figure 2) and DD419 (Figure 3), which could further develop to the mixed freckle channels, including quasi-freckles (Figure 4, Figure 5 and Figure 6). In comparison, the castings of the third-generation alloy WZ30 revealed only typical freckles (Figure 1) instead of quasi-freckles. This once again proves that higher-generation alloys have a more severe freckling tendency than the lower ones, as observed in the authors’ previous work [13].

4. Conclusions

Besides the typical freckle defect, the quasi-freckle on the surface of SC superalloy castings was observed, which was caused by relatively weak convection during directional solidification. Due to solute segregation, the quasi-freckle channels were enriched with γ/γ’ eutectic, exhibiting a morphology different from that of the matrix. After solution heat treatment, the eutectic structure could be dissolved, and the channel morphology of quasi-freckles disappeared. Due to the absence of dendrite fragments, the SC integrity of the castings was not damaged, because no grain boundary was introduced. For the sake of safety, however, further investigation should be carried out to confirm the mechanical properties of the castings with quasi-freckles by operational testing in real load conditions, before they are officially accepted as qualified products.