The Effect of Multi-inclined Holes on the Creep Properties of Nickel-Based Superalloy

Introduction

The nickel-based superalloy GH3536 is widely used in the combustion chamber of aircraft engines and honeycomb structure under the conditions of high stresses and extreme temperatures because of their good creep, oxidation and corrosion resistance [1]. The basic creep properties of nickel-based superalloys have been extensively carried out [2, 3, 4, 5]. S.K. Yerra and V. Tvergaard, et al. [6, 7] studied the relationship between growth, coalescence of voids and the creep behavior of single-crystal superalloy. MacLachlan et al. [8, 9] modeled the creep performance of nickel-based single-crystal superalloy CMSX-4 by damage mechanics technique.

It had been extensively reported that many cooling blades were broken near film cooling holes [10]. In these years, the relative researches focus on the cooling efficiency, flow-field characteristics with different shapes and different arrangements of film cooling holes. Based on thin plate specimen with one central hole to simulate the air-cooled turbine blade, Yu [11, 12, 13] calculated the distributions of resolved shear stresses and strains of the octahedral slip systems and found that the cooling hole has a remarkable effect on both von Mises stress and resolved shear stress distributions. Mehdi Zolfagharian et al. [14] concluded that the laidback hole provides better film cooling performance than the other holes. Lu et al. [15] presented five arrangements of cooling holes and introduced the effect of the shape on different arrangements. Gao [16] listed four common kinds of film-cooling hole configurations and investigated film-cooling effectiveness along the axis and with a compound angle.

However, the structure integrity of the combustion chamber is destroyed by cooling holes, and high multi-axial stress state near the cooling holes leads to the crack initiation. What is worse, the cooling holes are not always perpendicular to the surfaces because of complex geometry shape. The inclined angle plays an important role on the strength and life of combustion chamber [17]. To improve the work temperature of engine combustion chamber, the air-cooled flame tubes are widely used in aeroengineering. Unfortunately, neither the effect of drill process nor multi-inclined holes on creep properties of GH3536 have been studied.

In this paper, the plate specimens with multi-inclined holes were used to model the air-cooled flame tubes. To investigate the effect of multi-inclined holes on the creep behavior of air-cooled combustion chamber flame tubes, the creep tests under applied stress 80/90/100 MPa at 850 ℃ were carried out in both plate specimens with/without multi-inclined holes. Meanwhile, two drilling processes, that is, both laser drilling and electric spark drilling, were tested to study the effect of drill process on creep properties of GH3536. In order to understand the creep damage mechanism, finite element analysis was also performed to demonstrate the stress distribution of plate specimens with the damage mechanics equations of Kachanov [18] and Rabotnov [19].

Experiments

The nickel-based superalloy of GH3536 was melted in a vacuum induction furnace, then poured into bars, and the chemical composition of GH3536 is shown as in Table 1 [20]. The system of heat treatment of the superalloy is given as follows: 1175 ℃+15 ℃/6 h+AC. After the heat treatment, the superalloy was machined into the plate specimens, then a set of multi-inclined holes with 1 mm diameter and inclined angle of 15° were processed by laser drilling and electric spark drilling, respectively. The plate specimens contain three rows of multi-inclined holes, of which the adjacent

Other elements ≤

inclined holes spacing is p = 5 mm and q=2 mm. The dimensions of the modeling specimen with multi-inclined holes and the specimen map are shown in Figures 1 and 2, respectively. According to the actual loading environment of the combustion chamber, to study the effect of multi-inclined holes as well as drill process on creep properties, we use the plate specimens to do the creep tests under the applied stress of 80/90/100 MPa at 850 ℃, respectively. After the creep tests, the creep fracture morphology was observed by the scanning electron microscope (SEM) analysis.

Figure 1:

Sketch map of modeling specimen with multi-inclined holes.

Figure 2:

The specimen map: (a) electric spark drilling and (b) laser drilling.

The creep tests were all carried out on the CSS-2910 high-temperature creep machine, and the loading rate is set as 1 kN/min. The test temperature (850℃) is controlled by K-type thermocouple, and the temperature control precision is +3 ℃. When the temperature was raised to the target value, it was kept for 30 min insulation. The whole experiment process was referred to the Metal Mechanical Properties Test National Standard: Metallic materials creep and stress rupture test in tension (GB/T 2039-1997).

Results and discussion

The creep properties

To investigate the creep properties of plate specimens with/without multi-inclined holes, a series of creep tests were done to analyze the creep properties of different plate specimens under applied stress of 80/90/100 MPa at 850 ℃, and the results of average creep life and partial fracture elongation are listed in Table 2.

Table 2:

Creep properties of plate specimens under different applied stresses at 850 ℃.

No.	Stress/MPa	Average creep life/h	Fracture elongation/%	Multi-inclined holes
				Laser drilling	Electric spark drilling	Without
P1	80	119	14.35	√
P2	90	39.33	30.5	√
P3	100	16.7	24.05	√
P4	80	95.07			√
P5	90	37.43			√
P6	100	11.07			√
P7	80	223.55	31.3			√
P8	90	124.08	43.4			√
P9	100	64.27	68.4			√

From Figure 3(a) we can see, on one hand, regardless of the holes, the average creep life decreasing with the increase of stress in three different specimens, which imply that stress level has evident effect on the creep life. On the other hand, the creep life of plate specimens with multi-inclined holes is clearly shorter than plate ones, which indicates that the multi-inclined holes shorten the creep life in large scale. What is more, the creep life of laser drilling specimens are slightly longer than that of the electric spark drilling ones. This may provide a proof that proper improvements of drill process make for the creep performance.

Figure 3:

Creep test results: (a) the curve of average creep life versus stress and (b) the curve of fraction elongation versus stress.

Suppose the small distinction in different drilling process could be ignored, or in view of the better creep performance of the laser drilling specimens, comparison and fracture morphology will be only focused later on the laser drilling ones. The curve between fraction elongation and stress of laser drilling specimens is shown in Figure 3(b). It is strange that the fraction elongation of laser drilling specimens achieves the maximum value under 90 MPa, which is different from the ones without holes. We will discuss this interesting phenomenon later. All in all, the stress relaxation phenomenon of specimens with multi-inclined holes causes the bad extension properties at high temperature [21].

Select a typical creep test curve, of which the curve of creep strain versus creep time is shown in Figure 4. From Figure 4(a), apparent second and third stages of creep are observed in the specimens with multi-inclined holes under three stress states, and the creep of the first stage is very short. When stress increases from 80 to 90 MPa, the maximum creep strain increases from 0.19 % to 0.33 %, and then decreases to 0.25 % when stress increases to 100 MPa, as shown in Figure 4(b). Based on the inflection point shown in Figure 4(b), the comparison of creep curves under 90 MPa between specimens with multi-inclined holes and without is shown in Figure 4(c). The maximum creep strain of specimen with holes is 0.33 %, which is lower than that of contrast one (approximately 0.5 %). We can conclude that the multi-inclined holes and applied stress are the main reasons on the creep life and creep strain rate under high temperature.

Figure 4:

The creep curves of GH3536 plate specimens: (a) specimens with multi-inclined holes by laser drilling; (b) the creep strain versus stress of specimens with multi-inclined holes; and (c) the comparison of creep curves between specimens with multi-inclined holes and without.

The finite element simulation

The damage level of materials can only be determined by the damage divisor D in this damage model, which the damage divisor D can be understood by the volume fraction of micro-crack and void in the whole material structures. It means that material has no damage if D=0, and material structure has been partially destroyed when the 0 < D < 1, whereas the material is ruptured if the damage divisor D=1. Imaging the initial effective area F₀, then the effective area is F₀ (1 – D), which can obtain the effective stress as

(1)σ∗=σ1−D

The creep damage constitutive relation equation is as follows:

(2)ε˙c=Bσeqn(1−D)n

where

(3)D˙=Aασ1+(1−α)σeqx(1−D)k

Here, ε˙c and D˙represent the creep strain rate and damage rate; σeq and σ1 are, respectively, equivalent stress and maximum primary stress; A, B, n, k and x are all material parameters, from which the upper parameters can be obtained by creep curves. α represents the three-axis stress (0<α<1), and the normal demarcate methods can refer to references.

When ignoring the influence of the first primary stress and keeping constant n=x, the damage equations can be simplified as

(4)ε˙=Aσeqn(1−ω)n

(5)ω˙=Bσeqn(1−ω)n

To study the creep behaviors of GH3536 Ni-based superalloy specimens with multi-inclined holes, the K-R damage model is introduced into commercial software ABAQUS to calculate the creep properties. However, in order to validate the accuracy of those calculating parameters, we calculate the creep curve and the simulated creep curve of specimens without multi-inclined holes under 90 MPa at 850 ℃ to agree well with the test one, as shown in Figure 5.

Figure 5:

The simulation and test creep curves for specimen without multi-inclined holes under 90 MPa at 850 ℃.

The finite element model was built according to the gauge dimensions of modeling specimen with multi-jet holes shown in Figure 6. The boundary conditions were as follows: x=0, u_x=u_y = u_z = 0; x = L,σx = 90 MPa. The upper model has been implemented as user subroutine into commercial software ABAQUS to calculate the von Mises stress distribution around the jet holes. The results of contour plots are shown as follows:

Figure 6:

Distribution of equivalent stress when calculation started.

The multi-inclined holes can not only lead to the stress concentration but also cause the so-called interference phenomenon, both of which result in the complex stress environment in the regions near those inclined holes. However, the maximum stress σxmax in the multi-inclined holes region is the main reason that contributes to the decreasing creep life.

In Figure 6, the dangerous areas appear in both sides of ellipse axis end, and maximal stress reaches 201.4 MPa, where the stress is approximately 2.24 times larger than that of non-stress concentration regions. Stress gradient is also noted near the inclined holes. Figures 6 and 7(a) show that stress redistribution and stress relaxation take place with the increase of creep time. The contrast between Figure 7（b）and 7(C) shows that stress distribution for different number of holes exhibits obvious different interference phenomenon. Figure 7 shows the distribution of the equivalent von Mises stress around the multi-inclined holes when the calculation stopped. It is clear that the stress concentration occurs near the inclined holes. So the maximal equivalent stress exists there. As the holes “shield” some of the load, a low Mises stress area appears between two neighboring holes along the loading direction, with its value much less than the maximal. The equivalent stress distributions in A–A and B–B sections show that the stress around the hole is obviously larger than that in the area with a short distance from the holes. The stress distribution is more uniformly distributed in the inner holes (Figure 7 (d)) parallel to the direction of inclined angle, which is in accordance with the specimens’ fracture picture as shown in Figure 8.

Figure 7:

Distribution of equivalent stress when calculation stopped: (a) the whole appearance; (b) the section A–A; (c) the section B–B; and (d) the stress distribution of inner hole.

Figure 8:

Laser drilling specimens’ fracture appearance.

Creep characters and fracture mechanism

To figure out the inflection point shown in both Figures 3(b) and 4(b), the SEM technology was introduced to analyze the rupture appearances of both kinds of plate specimens.

Figure 9 shows the macroscopic photos of fracture appearances for specimens with multi-inclined holes by laser drilling and without holes specimens under the condition of 850 ℃/90 MPa, respectively. The fracture appearance of specimen with multi-holes presents the necking phenomenon caused by stress concentration and the rupture surface is flat, and the surface is full of micro-holes and shear lip areas. Whereas the fracture surface of specimen without holes is very rough, uneven and covered with a lot of large voids.

Figure 9:

The macroscopic SEM photos: (a) multi-inclined holes specimen and (b) specimen without holes.

The microscopic photos of fracture appearances are shown in Figure 10. The fracture appearance of specimen with multi-inclined holes is covered with a large number of shallow dimples [22, 23], as shown in Figure 10(a). From Figure 10(a), the surface is full of a lot of dimples, and the accumulation of voids leads to secondary cracks, which indicates that the creep rupture mechanism is dimple fracture. In fact, the rupture of specimens initiates from the inner hole of specimens and expands out along the cracks’ path until the specimen is totally destroyed. However, the rupture surface of the specimen without holes is covered with all shaded holes and shear ridges, as is shown in Figure 10(b). A large fluctuation surface and different sizes of holes were observed in Figure 10(b), which reveals that the failure mode is void coalescence fracture.

Figure 10:

The microscopic fracture appearances: (a) the gathering of dimples for multi-inclined holes specimen and (b) the formation of voids for specimen without holes.

From Table 2 and Figure 10, it can be understood that different specimens have different creep resistances and rupture mechanisms. The gathering of tiny holes and rupture layers are the main morphology on the fracture appearance, as shown in Figure 11(a). Figure 11(b) shows the gathering and growth of micro-voids at high temperature. It is noted that the formation of micro-voids appears in the first and second creep stages, and the growth of micro-voids is the result of motion of dislocation source under the high temperature and applied driving force in the third creep stage. Meanwhile, the formation and growth of micro-holes can lead to the separation of matrix and micro-cracks. The high temperature and applied stress can damage the atom bond of micro-crack tip, then leading to further expansion of cracks. In this way, the inner matrix is slowly divided into many small regions with the expansion of cracks until specimens come to fracture.

Figure 11:

(a) The accumulation of tiny holes and rupture layers and (b) the growth and coalescence of voids.

The creep of alloys can be considered as a diffusion of elements and dislocation motion [24] under high temperature and applied stress condition. The temperature provides thermal activation energy, which accelerates the diffusion of atoms. Meanwhile, the applied stress contributes to accelerating the thermal motion, and then induces the formation of dislocation climb and slip. The dislocation pileup and dislocation group will come into being when the dislocation motion meets the obstacles, which will arouse high stress regions. And those micro-voids will spread around obstacles. Under the effect of shear stress and diffusion at high temperature, the number of plastic holes will be gradually improved. Stress field of the growing voids will interact with each other, which leads to the link of holes and cracks.

Conclusions

1. The multi-inclined holes have very important effect on the creep properties of Ni-based superalloy GH3536, which leads to the decrease of creep life, the maximum creep strain and creep rate during the creep. Meanwhile, different drilling process has effect on the creep life (see Figure 3(a)), which may provide a proof that the good creep performance and high firing temperature could be achieved at the same time if we can successfully develop a better drilling process.

2. The von Mises stress of plate specimens with multi-inclined holes are not of uniformity (see Figure 7(a)), and high stress distribution gradient and high stress concentration are observed near the holes. According to the FEA results of plate specimens with multi-inclined holes, the dangerous areas appear in the regions near the end of ellipse axis along the inclined angle orientation, which is similar to the actual fracture appearances.

3. Under the condition of 850 ℃/90 MPa, the creep fracture appearances of specimens with multi-inclined holes are noted different from those for specimens without holes (see Figure 9). The tiny holes and dimples are the main characters in the creep fracture face, which leads to the damage fracture of specimens with multi-inclined holes; whereas the growth and coalescence of voids is the reason of damage fracture for specimens without holes (see Figure 11).