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The effect of hot isostatic pressing on crack healing, microstructure, mechanical properties of Rene88DT superalloy prepared by laser solid forming
Materials Science and Engineering A 504 (2009) 129–134
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
The effect of hot isostatic pressing on crack healing, microstructure, mechanical properties of Rene88DT superalloy prepared by laser solid forming Xiaoming Zhao, Xin Lin, Jing Chen, Lei Xue, Weidong Huang ∗ State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, PR China
a r t i c l e
i n f o
Article history: Received 25 April 2008 Received in revised form 16 October 2008 Accepted 11 December 2008 Keywords: Rene88DT Laser solid forming Crack healing HIP Mechanical properties
a b s t r a c t Crack characterization in Rene88DT superalloy prepared by laser solid forming (LSF) was investigated; the effect of hot isostatic pressing (HIP) on crack healing, microstructure and mechanical properties was also analyzed. It was found that cracks in LSFed Rene88DT mainly nucleate and propagate in the overlap zone of laser-deposited passes, and can be attributed to the liquation cracks. The size and amount of the cracks in LSFed Rene88DT are significantly affected by the overlap of as-deposited passes. Through HIP processing, crack healing occurred, which leads to a substantial improvement of the mechanical properties of LSFed Rene88DT superalloy. However, the insufficient overlap will result in very large cracks in the deposit, which makes the diffusion bonding during HIP not completely eliminate the local segregation of Ti and Nb in the crack healing region. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The nickel-based superalloy Rene88DT, with a significantly improved balance of creep, damage tolerance and tensile properties, was developed for using in advanced aeroengine turbine disk in 1983 [1]. Due to severe composition segregation and poor workability for nickel-based superalloy turbine disk fabricated by traditional casting or forging processing, powder metallurgy (P/M) processing have been employed as a standard manufacturing route for nickel-based superalloy turbine disk, including alloy atomization, hot compaction, extrusion and superplastic isothermal forging [2,3]. P/M processing route has been proved to be an indispensable method for the fabrication of nickel-based superalloy turbine disk except the long time and high cost consumed in process. Laser solid forming (LSF), also referred to as laser engineered net shaping (LENSTM ) [4], direct light fabrication (DLF) [5], etc. is a promising technology based on a new additive manufacturing principle, which combines laser cladding with rapid prototyping into a solid freeform fabrication process for manufacturing full dense metallic parts with high performance. During LSF, by moving the laser beam and CNC working table to generate certain trajectories, fine metal powders are deposited onto the substrate to directly fabricate three-dimensional components layer by layer in near-net shape without die. This leads to the savings of the delivery time and manufacturing cost. Meanwhile, a fine microstructure, which fur-
∗ Corresponding author. Tel.: +86 29 88494001; fax: +86 29 88494001. E-mail address: [email protected] (W. Huang). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.12.024
ther leads to the superior mechanical properties, can be obtained due to the non-equilibrium near-rapid solidification process during LSF. Thus, LSF provides an innovative way for the preparation of hot-end turbine engine components. It should be indicated that the weldability of materials has an important effect on the forming characteristic of LSF process. Based on the understanding on the traditional welding process, the superalloy, containing substantial concentration of Al + Ti (>6 wt.%), will present high susceptibility to cracks happening in heat affected zone (HAZ) during welding [6,7]. As a precipitate-hardened P/M superalloy, Rene88DT is strengthened by the precipitation of ordered L12 intermetallic Ni3 (Al, Ti) ␥ phase. The total amount of Al and Ti in Rene88DT is approximately 6 wt.%. So it is a crucial issue to investigate the weldability of Rene88DT in order to realize the preparation of hot-end turbine engine components by LSF route. Meantime, hot isostatic pressing (HIP) process has been extensively used in the healing of casting superalloy defects such as cavities, voids and hot cracking [8–11]. In this paper, the crack characterization, and the effect of HIP treatment on crack healing behaviors, microstructure and mechanical properties of LSFed Rene88DT were investigated.
2. Experimental The experiments were carried out on a LSF equipment, which consists of a Rofin-Sinar RS850 5 kW continuous wave CO2 laser, a LPM-408 CNC working table, a DPSF-1 powder feeding system and an off-axial nozzle. Pure argon was used as shielding gas, which constrains the powder particles into a jet stream and simultaneously
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used to characterize the cracks and microstructure. Energy dispersive X-ray spectrometer (EDS) was utilized to analyze chemical composition of the samples. The density of the LSFed Rene88DT was measured with the accuracy of ∼0.001 g/cm3 according to the Germany standard of DIN EN 725-7. 3. Results and discussion 3.1. Crack characterization of LSFed Rene88DT
Fig. 1. LSFed Rene88DT bulk sample.
prevents both the molten pool and the HAZ from oxidization and oxide contamination. In the LSF process, the Rene88DT powders with the sizes of 44–150 m, were laser-deposited on the stainless steel substrate with the size of 100 mm × 50 mm × 6 mm. The chemical composition of Rene88DT superalloy was (wt.%) 17 Cr, 14 Co, 4.2 W, 4 Mo, 3.3 Ti, 2.2 Al, 0.7 Nb, 0.04 C, 0.03 O, 0.02 N and balance nickel. The LSF processing parameters were as follows: laser power of 2.0–3.1 kW, scanning speed of 5–10 mm/s, beam diameter of 2–4 mm, powder feeding rate of 5–7 g/min, overlap of 30–50% and shielding gas flow of 5–10 L/min. The substrate surface was ground with SiC paper and cleaned with acetone prior to be laser-deposited. The dimension of the LSFed bulk samples is 85 mm × 15 mm × 15 mm, as shown in Fig. 1. After LSF, as-deposited Rene88DT samples were HIP treated (1160 ◦ C, 2 h/200 MPa) to investigate the effect of HIP treatment on crack healing, microstructure and mechanical properties of LSFed Rene88DT. All tensile testing samples were further subjected to the following heat treatments: solution treatment (1160 ◦ C, 2 h/AC) + aging (760 ◦ C, 8 h/AC). The tensile testing bars have a nominal gauge length of 25 mm with a diameter of 5 mm. The samples for microstructure observation were sectioned from the LSFed bulk samples by wire electrolytic-discharge machine. For overall view of the cracks distribution in the transverse section of the LSFed Rene88DT bulk samples, the dye penetrant inspection reagent of DPT-5 was used to reveal the cracks distribution. The samples for microstructure analysis were prepared by the mechanical grounding, polishing and chemical etching. The microstructure of the LSFed Rene88DT samples was revealed using an etchant of HCl and H2 O2 with the volume ratio of 10:3. Optical microscope (OM) and scanning electron microscopy (SEM) were
During LSF of Rene88DT, the overlap between the adjacent laserdeposited passes has a significant effect on the size and amount of the cracks. In present experiments, the overlap of 30–50% was used. The overlap can be divided into two groups: the insufficient (Fig. 2a) and sufficient (Fig. 2b), according to the morphology at the top surface of laser deposit. When the overlap is about 30%, the groove with a sharp angle can be observed between the adjacent laser-deposited passes (Fig. 2a), which should be a weak bonded zone in LSF. As the overlap reaches 40%, the groove is not observed and a smooth top surface can be obtained as shown in Fig. 2b. Fig. 3 presents the cracks distribution when the overlap is 30%. The cracks generally nucleate and propagate in the overlap zone between two adjacent laser-deposited passes (Fig. 3a). Both the long cracks with the length of 3–10 mm (Fig. 3b) and short cracks with the length of 100–300 m (Fig. 3c) occur in the transverse section of the LSFed Rene88DT bulk sample with insufficient overlap. There are some re-solidified products within the cracks as shown in Fig. 3b. For sufficient overlap (about 40–50%), only 100–300 m length crack can be found. It can also be noted that the cracks are totally embedded in the top portion of the sample, which do not propagate across the top surface of the sample. In the laser-deposited or welding process, it is generally recognized that there are two competing factors influencing the occurrence of cracking: the thermal/shrinkage stress and the intrinsic resistance of materials to cracking. Thermal/shrinkage stress was induced by the rapid heating/cooling processing during LSF. The overlap zone usually experienced the remelting and solidification process repeatedly, which results in a higher stress concentration in overlap zone. Meanwhile, LSFed sample exhibits a typical directional solidification columnar structure [12]. Generally, there are low melting eutectics (like (␥ + ␥ )) in the interdendritic region and along the columnar grain boundaries. Thus, the grain boundary will become a weak bonded source, which may initiate cracking in the laser-deposited layer. The fracture surface in Fig. 4 does show the formation of re-solidified products along grain boundaries in the HAZ, which further indicates that HAZ cracking in LSFed Rene88DT results from the liquation cracking. Fig. 4a shows an
Fig. 2. The top zone for the different overlap between two adjacent laser-deposited passes of LSFed Rene88DT: (a) insufficient overlap (about 30%) and (b) sufficient overlap (about 40%).
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Fig. 3. HAZ cracks in LSFed Rene88DT sample: (a) overall view of the cracks distribution in the transverse section of the sample; (b) the long crack (3–10 mm); (c) the short crack (100–300 m).
obvious liquation characteristic for the HAZ cracking with a typical brittle intergranular fracture mode. EDS results near the crack in Fig. 3b (marked A) and the fracture surface of the liquation crack in Fig. 4b (marked B and C), are listed in Table 1. It can be deduced that the re-solidified products within the cracks, containing high amount of Ti, Al, Cr, Co and Ni, are the re-solidified (␥ + ␥ ) eutectics.
During LSF, the cooling rate near the substrate is higher than that of the top part. In lower part of the deposit, the HAZ temperature, which gradually decreases from the solid/liquid interface of molten pool to the substrate, is low totally. If the re-solidified products (like (␥ + ␥ ) eutectics) in the interdendrite are remelted near the molten pool interface, or solidification cracks initiate, it can be healed due to easily liquid feeding from the molten pool. While in the top
Fig. 4. Fracture surface of LSFed Rene88DT: (a) the intergranular fracture surface; (b) magnification of (a) showing liquation feature.
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Fig. 5. The crack healed microstructure of LSFed Rene88DT after HIP treatment: (a) The typical short crack healed microstructure and (b) the typical long crack healed microstructure. Table 1 The composition in Fig. 3b (marked A) and the fracture surface of the liquation crack in Fig. 4b (marked B and C) (at.%).
Table 2 The composition of precipitated particles (marked D) and the surrounding matrix (marked E) after HIP treatment in Fig. 6b (at.%).
Element
Al
Ti
Cr
Co
Ni
Mo
W
Element
Al
Ti
Cr
Co
Ni
Mo
Nb
W
A B C
4.61 4.62 5.19
3.59 6.49 7.38
18.41 18.65 16.77
13.31 12.35 11.62
56.27 53.79 55.88
2.12 2.84 2.08
1.69 1.26 1.08
D E
2.20 4.82
23.32 5.12
12.71 17.94
9.30 12.11
35.76 56.57
4.63 2.22
9.12 –
2.96 1.22
portion of the deposit, the heat accumulation with the increase of the laser-deposited layers will raise the HAZ temperature, which makes the liquation zone within the HAZ broader and longer, in which liquid feeding becomes difficult due to rapid solidification of molten pool during LSF. Thus, liquation cracks will occur under the tensile stress introduced by the rapid heating/cooling processing. Meanwhile, columnar dendrites grow epitaxially from the substrate during LSF, which makes the cracks develop along the growth orientation of the columnar dendrites layer by layer. So cracks are mainly in the top portion of the final deposit, and concentrated in the interdendritic regions of the overlap zone. The sharp step in the overlap zone means a higher structural stress raiser, which make the cracks occur more easily in HAZ when the subsequent remelting of the post-deposited layers happens in this zone. However, the equiaxed crystal at the top of molten pool will hinder the development of the crack between the columnar dendrites at the bottom of the molten pool, even if the cracks in HAZ also “grow” epitaxially during solidification of the molten pool. The crack will be totally embedded in the final deposit. Thus, the sufficient overlap between two adjacent laser-deposited passes
will promote a higher resistance to stress, and even finally eliminate the long cracks (3–10 mm). As for short cracks (100–300 m), it is found that it is difficult to eliminate all the cracks by merely adjusting laser processing parameters. The short cracks can still initiates due to the low melting point eutectics existing in the weak bonded grain boundary in the overlapping zone, even if the overlap is suitable (about 40%) for obtaining the smooth top surface. 3.2. Effect of HIP treatment on crack healing and microstructure of LSFed Rene88DT In view of the above analysis, it is indicated that the further post-treatment should be employed to ensure good mechanical performance of LSFed Rene88DT. Thus, The HIP treatment was introduced to heal the two typical kinds of cracks, especially short cracks in LSFed Rene88DT. The HIP processing parameters of 1160 ◦ C, 2 h/200 MPa was applied. Through imposing the high uniform stresses on LSFed Rene88DT samples at elevated temperature, the crack healing occurred. Fig. 5a shows the typical healed microstructure of the HIPed samples. All the short cracks in the as-deposited sample are healed through HIP treatment. The long
Fig. 6. SEM–EDS analysis of the discrete particles: (a) SEM image of the precipitates trace along the healed crack zone and (b) magnification of the precipitates trace and the surrounding matrix showing EDS analyses region.
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Table 3 The tensile properties of LSFed Rene88DT before and after HIP treated. Process (25 ◦ C)
b (MPa)
0.2 (MPa)
Elongation ı (%)
Reduction in area
LSF + heat treated LSF + HIP + heat treated P/M Rene88DT
1250–1350 1400–1440 1520–1600
990–1000 1010–1030 1080–1210
8–11 16.5–17.5 17–25
13–15 17.5–18 –
(%)
Fig. 7. ␥ precipitates morphology and distribution of LSFed Rene88DT: (a) before and (b) after HIP treatment.
cracks can also be successfully healed through HIP treatment as shown in Fig. 5b. However, some discrete second phase particles with blocky-like morphology precipitate along the healed crack zone, which form a healed crack trace. During HIP treatment, the LSFed Rene88DT samples were subjected to the high temperature and pressure simultaneously, the fracture surfaces of the cracks were first mechanically closed by the high temperature creep, then bonded together, and finally diffusion homogenized. Therefore, the short cracks in the LSFed Rene88DT can be perfectly healed by diffusion bonding during HIP processing (Fig. 5a). There is no evidence of the healed short crack traces in HIPed samples. Meanwhile, the HIP treatment was performed at 1160 ◦ C, which is obviously above the supersolvus temperature of ␥ (1130 ◦ C) for Rene88DT superalloy. Hence, the recrystallization is expected, which may be beneficial to the mechanical properties according to Hall–Petch relationship. But for the long cracks, the healed crack trace with discrete second phase particles formed. The compositions of these particles (marked D in Fig. 6) and the surrounding matrix (marked E in Fig. 6) are listed in Table 2. It can be seen that the particles contain the excessive amount of Nb and Ti, as compared with that of the surrounding matrix. Thus, these precipitates should be (Ti, Nb)-riched MC type carbides. The source of Nb and Ti in MC type carbides that formed during HIP treatment, should be the re-solidified (␥ + ␥ ) eutectics and Nb segregation in the interdendrite, which contain the excessive amount of Ti and Nb. The existence of MC type carbides (Fig. 6a) also suggests that the diffusion bonding of HIP cannot completely eliminate the local segregation of MC-forming elements. Thereby the MC carbides should also be a weak bonded zone for mechanical test. 3.3. Effect of HIP treatment on mechanical properties of the LSFed Rene88DT The effect of the HIP treatment on the room temperature tensile properties of the LSFed Rene88DT is presented in Table 3. For comparison, the typical mechanical properties for P/M Rene88DT are also listed. It can be seen that tensile properties of nonHIPed LSFed Rene88DT are weaken to a great extent due to the presence of short cracks, and its ductility (elongation: 8–11%,
reduction in area: 13–15%) is nearly half lower than that of the P/M Rene88DT. Meanwhile, the variable range of tensile strength of non-HIPed LSFed Rene88DT reaches 100 MPa. For HIPed samples, there exhibits a considerable improvement in the tensile strength, and the variable range of 40 MPa is smaller than that of non-HIPed samples. It is noticeable that there is a remarkable increase in ductility (elongation: 16.5–17.5%, reduction in area: 17.5–18%) for HIPed LSFed Rene88DT on comparison with the P/M Rene88DT. It can be deduced that the cracks in the LSFed Rene88DT not only decrease tensile properties but also increase the variable range of tensile properties. HIP treatment can enhance the mechanical properties of LSFed Rene88DT and reduce the scatter of them. This is mainly attributed to HIP’s ability to heal the cracks, as seen in Fig. 5. Fig. 7 shows the morphology and distribution of ␥ precipitates in LSFed Rene88DT before and after HIP treatment. It can be seen the ␥ precipitates in HIPed samples coarse a little but distribute more homogeneously on comparison with those in the non-HIPed samples. In addition, HIP treatment is also a densification process. The density of the LSFed Rene88DT measured has an increase of 0.06 g/cm3 (from 8.24 g/cm3 to 8.30 g/cm3 ) after HIP treatment. The variable range of the measured tensile strength and ductility are also decreased through HIP treatment. By the integration of these effects, the ductility will increase remarkably after HIP treatment. 4. Conclusions 1. The cracks in LSFed Rene88DT mainly nucleate and propagate in the overlap zone between two adjacent laser-deposited passes. The overlap has a significant effect on the size and amount of the cracks. Two typical kinds of cracks, long cracks (3–10 mm) and short cracks (100–300 m), occur based on the corresponding overlap. The cracks were attributed to the liquation cracking. It is difficult to eliminate all the short cracks through merely adjusting LSF processing parameters. 2. HIP is an effective way to heal the cracks within the LSFed Rene88DT. Short cracks in the LSFed Rene88DT samples can be perfectly healed by HIP diffusion bonding. When there are the
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long cracks in LSFed Rene88DT samples, the discrete MC carbides will precipitate along the healed crack trace after HIPed treatment. 3. Cracks in the LSFed Rene88DT not only decrease tensile properties but also increase their variability. Through HIP treatment, the mechanical properties of LSFed Rene88DT can be enhanced remarkably. Meanwhile, the scatter of the mechanical properties can also be reduced. Acknowledgements This work was funded by National Natural Science Foundation of China under Grant No. 50331010 and the National High Technology Research and Development Program (“863” Program) of China (No. 2006AA03Z0449). The work was also supported by Program for New Century Excellent Talents in University (Grant No. NCET06-0879).
References [1] D.D. Krueger, R.D. Kissinger, R.G. Menzies, in: S.D. Antolovich, R.W. Stusrud, R.A. MacKay (Eds.), Superalloys 1992, TMS, Warrendale, PA, 1992, pp. 277–286. [2] S.T. Wlodek, M. Kelly, D.A. Alden, in: R.D. Kissinger, D.J. Deye, D.L. Anton (Eds.), Superalloys 1996, TMS, Warrendale, PA, 1996, pp. 129–136. [3] M.J. Caton, S.K. Jha, A.H. Rosenberger, J.M. Larsen, in: K.A. Green, T.M. Pollock, H. Harada (Eds.), Superalloys 2004, TMS, Warrendale, PA, 2004, p. 305. [4] D.M. Keicher, J.E. Smugeresky, J.A. Romero, M.L. Griffith, L.D. Harwell, Proc. SPIE 2293 (1997) 91. [5] G.K. Lewis, R.B. Nemec, J.O. Milewski, D.L. Thoma, M.R. Barbe, D.A. Cremers, in: T.D. McCay, A. Matsunawa, H. Hugel (Eds.), Proceedings of ICALEO 94, Laser Institute of America, Orlando, FL, 1994, p. 17. [6] O.A. Ojo, N.L. Richards, M.C. Chaturvedi, Scripta Mater. 51 (2004) 141. [7] M.L. Zhong, H.Q. Sun, W.J. Liu, X.F. Zhu, J.J. He, Scripta Mater. 53 (2005) 159–164. [8] R.W. Hatala, J.J. Schirra, in: R.D. Kissinger, D.J. Deye, D.L. Anton (Eds.), Superalloys 1996, TMS, Warrendale, PA, 1996, p. 137. [9] P.G. Bailey, W.H. Schweikert, in: D.R. Muzyka, W.H. Couts, G.E. Wasielewski (Eds.), Superalloys 1976, TMS, Warrendale, PA, 1976, pp. 451–462. [10] H.Y. Bor, C. Hsu, C.N. Wei, Mater. Chem. Phys. 84 (2004) 284–290. [11] M.T. Kim, S.Y. Chang, J.B. Won, Mater. Sci. Eng. A 441 (2006) 126–134. [12] W.D. Huang, J. Chen, Y.M. Li, X. Lin, Proc. SPIE 5629 (2005) 72–73.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
The effect of hot isostatic pressing on crack healing, microstructure, mechanical properties of Rene88DT superalloy prepared by laser solid forming Xiaoming Zhao, Xin Lin, Jing Chen, Lei Xue, Weidong Huang ∗ State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, PR China
a r t i c l e
i n f o
Article history: Received 25 April 2008 Received in revised form 16 October 2008 Accepted 11 December 2008 Keywords: Rene88DT Laser solid forming Crack healing HIP Mechanical properties
a b s t r a c t Crack characterization in Rene88DT superalloy prepared by laser solid forming (LSF) was investigated; the effect of hot isostatic pressing (HIP) on crack healing, microstructure and mechanical properties was also analyzed. It was found that cracks in LSFed Rene88DT mainly nucleate and propagate in the overlap zone of laser-deposited passes, and can be attributed to the liquation cracks. The size and amount of the cracks in LSFed Rene88DT are significantly affected by the overlap of as-deposited passes. Through HIP processing, crack healing occurred, which leads to a substantial improvement of the mechanical properties of LSFed Rene88DT superalloy. However, the insufficient overlap will result in very large cracks in the deposit, which makes the diffusion bonding during HIP not completely eliminate the local segregation of Ti and Nb in the crack healing region. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The nickel-based superalloy Rene88DT, with a significantly improved balance of creep, damage tolerance and tensile properties, was developed for using in advanced aeroengine turbine disk in 1983 [1]. Due to severe composition segregation and poor workability for nickel-based superalloy turbine disk fabricated by traditional casting or forging processing, powder metallurgy (P/M) processing have been employed as a standard manufacturing route for nickel-based superalloy turbine disk, including alloy atomization, hot compaction, extrusion and superplastic isothermal forging [2,3]. P/M processing route has been proved to be an indispensable method for the fabrication of nickel-based superalloy turbine disk except the long time and high cost consumed in process. Laser solid forming (LSF), also referred to as laser engineered net shaping (LENSTM ) [4], direct light fabrication (DLF) [5], etc. is a promising technology based on a new additive manufacturing principle, which combines laser cladding with rapid prototyping into a solid freeform fabrication process for manufacturing full dense metallic parts with high performance. During LSF, by moving the laser beam and CNC working table to generate certain trajectories, fine metal powders are deposited onto the substrate to directly fabricate three-dimensional components layer by layer in near-net shape without die. This leads to the savings of the delivery time and manufacturing cost. Meanwhile, a fine microstructure, which fur-
∗ Corresponding author. Tel.: +86 29 88494001; fax: +86 29 88494001. E-mail address: [email protected] (W. Huang). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.12.024
ther leads to the superior mechanical properties, can be obtained due to the non-equilibrium near-rapid solidification process during LSF. Thus, LSF provides an innovative way for the preparation of hot-end turbine engine components. It should be indicated that the weldability of materials has an important effect on the forming characteristic of LSF process. Based on the understanding on the traditional welding process, the superalloy, containing substantial concentration of Al + Ti (>6 wt.%), will present high susceptibility to cracks happening in heat affected zone (HAZ) during welding [6,7]. As a precipitate-hardened P/M superalloy, Rene88DT is strengthened by the precipitation of ordered L12 intermetallic Ni3 (Al, Ti) ␥ phase. The total amount of Al and Ti in Rene88DT is approximately 6 wt.%. So it is a crucial issue to investigate the weldability of Rene88DT in order to realize the preparation of hot-end turbine engine components by LSF route. Meantime, hot isostatic pressing (HIP) process has been extensively used in the healing of casting superalloy defects such as cavities, voids and hot cracking [8–11]. In this paper, the crack characterization, and the effect of HIP treatment on crack healing behaviors, microstructure and mechanical properties of LSFed Rene88DT were investigated.
2. Experimental The experiments were carried out on a LSF equipment, which consists of a Rofin-Sinar RS850 5 kW continuous wave CO2 laser, a LPM-408 CNC working table, a DPSF-1 powder feeding system and an off-axial nozzle. Pure argon was used as shielding gas, which constrains the powder particles into a jet stream and simultaneously
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used to characterize the cracks and microstructure. Energy dispersive X-ray spectrometer (EDS) was utilized to analyze chemical composition of the samples. The density of the LSFed Rene88DT was measured with the accuracy of ∼0.001 g/cm3 according to the Germany standard of DIN EN 725-7. 3. Results and discussion 3.1. Crack characterization of LSFed Rene88DT
Fig. 1. LSFed Rene88DT bulk sample.
prevents both the molten pool and the HAZ from oxidization and oxide contamination. In the LSF process, the Rene88DT powders with the sizes of 44–150 m, were laser-deposited on the stainless steel substrate with the size of 100 mm × 50 mm × 6 mm. The chemical composition of Rene88DT superalloy was (wt.%) 17 Cr, 14 Co, 4.2 W, 4 Mo, 3.3 Ti, 2.2 Al, 0.7 Nb, 0.04 C, 0.03 O, 0.02 N and balance nickel. The LSF processing parameters were as follows: laser power of 2.0–3.1 kW, scanning speed of 5–10 mm/s, beam diameter of 2–4 mm, powder feeding rate of 5–7 g/min, overlap of 30–50% and shielding gas flow of 5–10 L/min. The substrate surface was ground with SiC paper and cleaned with acetone prior to be laser-deposited. The dimension of the LSFed bulk samples is 85 mm × 15 mm × 15 mm, as shown in Fig. 1. After LSF, as-deposited Rene88DT samples were HIP treated (1160 ◦ C, 2 h/200 MPa) to investigate the effect of HIP treatment on crack healing, microstructure and mechanical properties of LSFed Rene88DT. All tensile testing samples were further subjected to the following heat treatments: solution treatment (1160 ◦ C, 2 h/AC) + aging (760 ◦ C, 8 h/AC). The tensile testing bars have a nominal gauge length of 25 mm with a diameter of 5 mm. The samples for microstructure observation were sectioned from the LSFed bulk samples by wire electrolytic-discharge machine. For overall view of the cracks distribution in the transverse section of the LSFed Rene88DT bulk samples, the dye penetrant inspection reagent of DPT-5 was used to reveal the cracks distribution. The samples for microstructure analysis were prepared by the mechanical grounding, polishing and chemical etching. The microstructure of the LSFed Rene88DT samples was revealed using an etchant of HCl and H2 O2 with the volume ratio of 10:3. Optical microscope (OM) and scanning electron microscopy (SEM) were
During LSF of Rene88DT, the overlap between the adjacent laserdeposited passes has a significant effect on the size and amount of the cracks. In present experiments, the overlap of 30–50% was used. The overlap can be divided into two groups: the insufficient (Fig. 2a) and sufficient (Fig. 2b), according to the morphology at the top surface of laser deposit. When the overlap is about 30%, the groove with a sharp angle can be observed between the adjacent laser-deposited passes (Fig. 2a), which should be a weak bonded zone in LSF. As the overlap reaches 40%, the groove is not observed and a smooth top surface can be obtained as shown in Fig. 2b. Fig. 3 presents the cracks distribution when the overlap is 30%. The cracks generally nucleate and propagate in the overlap zone between two adjacent laser-deposited passes (Fig. 3a). Both the long cracks with the length of 3–10 mm (Fig. 3b) and short cracks with the length of 100–300 m (Fig. 3c) occur in the transverse section of the LSFed Rene88DT bulk sample with insufficient overlap. There are some re-solidified products within the cracks as shown in Fig. 3b. For sufficient overlap (about 40–50%), only 100–300 m length crack can be found. It can also be noted that the cracks are totally embedded in the top portion of the sample, which do not propagate across the top surface of the sample. In the laser-deposited or welding process, it is generally recognized that there are two competing factors influencing the occurrence of cracking: the thermal/shrinkage stress and the intrinsic resistance of materials to cracking. Thermal/shrinkage stress was induced by the rapid heating/cooling processing during LSF. The overlap zone usually experienced the remelting and solidification process repeatedly, which results in a higher stress concentration in overlap zone. Meanwhile, LSFed sample exhibits a typical directional solidification columnar structure [12]. Generally, there are low melting eutectics (like (␥ + ␥ )) in the interdendritic region and along the columnar grain boundaries. Thus, the grain boundary will become a weak bonded source, which may initiate cracking in the laser-deposited layer. The fracture surface in Fig. 4 does show the formation of re-solidified products along grain boundaries in the HAZ, which further indicates that HAZ cracking in LSFed Rene88DT results from the liquation cracking. Fig. 4a shows an
Fig. 2. The top zone for the different overlap between two adjacent laser-deposited passes of LSFed Rene88DT: (a) insufficient overlap (about 30%) and (b) sufficient overlap (about 40%).
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Fig. 3. HAZ cracks in LSFed Rene88DT sample: (a) overall view of the cracks distribution in the transverse section of the sample; (b) the long crack (3–10 mm); (c) the short crack (100–300 m).
obvious liquation characteristic for the HAZ cracking with a typical brittle intergranular fracture mode. EDS results near the crack in Fig. 3b (marked A) and the fracture surface of the liquation crack in Fig. 4b (marked B and C), are listed in Table 1. It can be deduced that the re-solidified products within the cracks, containing high amount of Ti, Al, Cr, Co and Ni, are the re-solidified (␥ + ␥ ) eutectics.
During LSF, the cooling rate near the substrate is higher than that of the top part. In lower part of the deposit, the HAZ temperature, which gradually decreases from the solid/liquid interface of molten pool to the substrate, is low totally. If the re-solidified products (like (␥ + ␥ ) eutectics) in the interdendrite are remelted near the molten pool interface, or solidification cracks initiate, it can be healed due to easily liquid feeding from the molten pool. While in the top
Fig. 4. Fracture surface of LSFed Rene88DT: (a) the intergranular fracture surface; (b) magnification of (a) showing liquation feature.
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Fig. 5. The crack healed microstructure of LSFed Rene88DT after HIP treatment: (a) The typical short crack healed microstructure and (b) the typical long crack healed microstructure. Table 1 The composition in Fig. 3b (marked A) and the fracture surface of the liquation crack in Fig. 4b (marked B and C) (at.%).
Table 2 The composition of precipitated particles (marked D) and the surrounding matrix (marked E) after HIP treatment in Fig. 6b (at.%).
Element
Al
Ti
Cr
Co
Ni
Mo
W
Element
Al
Ti
Cr
Co
Ni
Mo
Nb
W
A B C
4.61 4.62 5.19
3.59 6.49 7.38
18.41 18.65 16.77
13.31 12.35 11.62
56.27 53.79 55.88
2.12 2.84 2.08
1.69 1.26 1.08
D E
2.20 4.82
23.32 5.12
12.71 17.94
9.30 12.11
35.76 56.57
4.63 2.22
9.12 –
2.96 1.22
portion of the deposit, the heat accumulation with the increase of the laser-deposited layers will raise the HAZ temperature, which makes the liquation zone within the HAZ broader and longer, in which liquid feeding becomes difficult due to rapid solidification of molten pool during LSF. Thus, liquation cracks will occur under the tensile stress introduced by the rapid heating/cooling processing. Meanwhile, columnar dendrites grow epitaxially from the substrate during LSF, which makes the cracks develop along the growth orientation of the columnar dendrites layer by layer. So cracks are mainly in the top portion of the final deposit, and concentrated in the interdendritic regions of the overlap zone. The sharp step in the overlap zone means a higher structural stress raiser, which make the cracks occur more easily in HAZ when the subsequent remelting of the post-deposited layers happens in this zone. However, the equiaxed crystal at the top of molten pool will hinder the development of the crack between the columnar dendrites at the bottom of the molten pool, even if the cracks in HAZ also “grow” epitaxially during solidification of the molten pool. The crack will be totally embedded in the final deposit. Thus, the sufficient overlap between two adjacent laser-deposited passes
will promote a higher resistance to stress, and even finally eliminate the long cracks (3–10 mm). As for short cracks (100–300 m), it is found that it is difficult to eliminate all the cracks by merely adjusting laser processing parameters. The short cracks can still initiates due to the low melting point eutectics existing in the weak bonded grain boundary in the overlapping zone, even if the overlap is suitable (about 40%) for obtaining the smooth top surface. 3.2. Effect of HIP treatment on crack healing and microstructure of LSFed Rene88DT In view of the above analysis, it is indicated that the further post-treatment should be employed to ensure good mechanical performance of LSFed Rene88DT. Thus, The HIP treatment was introduced to heal the two typical kinds of cracks, especially short cracks in LSFed Rene88DT. The HIP processing parameters of 1160 ◦ C, 2 h/200 MPa was applied. Through imposing the high uniform stresses on LSFed Rene88DT samples at elevated temperature, the crack healing occurred. Fig. 5a shows the typical healed microstructure of the HIPed samples. All the short cracks in the as-deposited sample are healed through HIP treatment. The long
Fig. 6. SEM–EDS analysis of the discrete particles: (a) SEM image of the precipitates trace along the healed crack zone and (b) magnification of the precipitates trace and the surrounding matrix showing EDS analyses region.
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Table 3 The tensile properties of LSFed Rene88DT before and after HIP treated. Process (25 ◦ C)
b (MPa)
0.2 (MPa)
Elongation ı (%)
Reduction in area
LSF + heat treated LSF + HIP + heat treated P/M Rene88DT
1250–1350 1400–1440 1520–1600
990–1000 1010–1030 1080–1210
8–11 16.5–17.5 17–25
13–15 17.5–18 –
(%)
Fig. 7. ␥ precipitates morphology and distribution of LSFed Rene88DT: (a) before and (b) after HIP treatment.
cracks can also be successfully healed through HIP treatment as shown in Fig. 5b. However, some discrete second phase particles with blocky-like morphology precipitate along the healed crack zone, which form a healed crack trace. During HIP treatment, the LSFed Rene88DT samples were subjected to the high temperature and pressure simultaneously, the fracture surfaces of the cracks were first mechanically closed by the high temperature creep, then bonded together, and finally diffusion homogenized. Therefore, the short cracks in the LSFed Rene88DT can be perfectly healed by diffusion bonding during HIP processing (Fig. 5a). There is no evidence of the healed short crack traces in HIPed samples. Meanwhile, the HIP treatment was performed at 1160 ◦ C, which is obviously above the supersolvus temperature of ␥ (1130 ◦ C) for Rene88DT superalloy. Hence, the recrystallization is expected, which may be beneficial to the mechanical properties according to Hall–Petch relationship. But for the long cracks, the healed crack trace with discrete second phase particles formed. The compositions of these particles (marked D in Fig. 6) and the surrounding matrix (marked E in Fig. 6) are listed in Table 2. It can be seen that the particles contain the excessive amount of Nb and Ti, as compared with that of the surrounding matrix. Thus, these precipitates should be (Ti, Nb)-riched MC type carbides. The source of Nb and Ti in MC type carbides that formed during HIP treatment, should be the re-solidified (␥ + ␥ ) eutectics and Nb segregation in the interdendrite, which contain the excessive amount of Ti and Nb. The existence of MC type carbides (Fig. 6a) also suggests that the diffusion bonding of HIP cannot completely eliminate the local segregation of MC-forming elements. Thereby the MC carbides should also be a weak bonded zone for mechanical test. 3.3. Effect of HIP treatment on mechanical properties of the LSFed Rene88DT The effect of the HIP treatment on the room temperature tensile properties of the LSFed Rene88DT is presented in Table 3. For comparison, the typical mechanical properties for P/M Rene88DT are also listed. It can be seen that tensile properties of nonHIPed LSFed Rene88DT are weaken to a great extent due to the presence of short cracks, and its ductility (elongation: 8–11%,
reduction in area: 13–15%) is nearly half lower than that of the P/M Rene88DT. Meanwhile, the variable range of tensile strength of non-HIPed LSFed Rene88DT reaches 100 MPa. For HIPed samples, there exhibits a considerable improvement in the tensile strength, and the variable range of 40 MPa is smaller than that of non-HIPed samples. It is noticeable that there is a remarkable increase in ductility (elongation: 16.5–17.5%, reduction in area: 17.5–18%) for HIPed LSFed Rene88DT on comparison with the P/M Rene88DT. It can be deduced that the cracks in the LSFed Rene88DT not only decrease tensile properties but also increase the variable range of tensile properties. HIP treatment can enhance the mechanical properties of LSFed Rene88DT and reduce the scatter of them. This is mainly attributed to HIP’s ability to heal the cracks, as seen in Fig. 5. Fig. 7 shows the morphology and distribution of ␥ precipitates in LSFed Rene88DT before and after HIP treatment. It can be seen the ␥ precipitates in HIPed samples coarse a little but distribute more homogeneously on comparison with those in the non-HIPed samples. In addition, HIP treatment is also a densification process. The density of the LSFed Rene88DT measured has an increase of 0.06 g/cm3 (from 8.24 g/cm3 to 8.30 g/cm3 ) after HIP treatment. The variable range of the measured tensile strength and ductility are also decreased through HIP treatment. By the integration of these effects, the ductility will increase remarkably after HIP treatment. 4. Conclusions 1. The cracks in LSFed Rene88DT mainly nucleate and propagate in the overlap zone between two adjacent laser-deposited passes. The overlap has a significant effect on the size and amount of the cracks. Two typical kinds of cracks, long cracks (3–10 mm) and short cracks (100–300 m), occur based on the corresponding overlap. The cracks were attributed to the liquation cracking. It is difficult to eliminate all the short cracks through merely adjusting LSF processing parameters. 2. HIP is an effective way to heal the cracks within the LSFed Rene88DT. Short cracks in the LSFed Rene88DT samples can be perfectly healed by HIP diffusion bonding. When there are the
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long cracks in LSFed Rene88DT samples, the discrete MC carbides will precipitate along the healed crack trace after HIPed treatment. 3. Cracks in the LSFed Rene88DT not only decrease tensile properties but also increase their variability. Through HIP treatment, the mechanical properties of LSFed Rene88DT can be enhanced remarkably. Meanwhile, the scatter of the mechanical properties can also be reduced. Acknowledgements This work was funded by National Natural Science Foundation of China under Grant No. 50331010 and the National High Technology Research and Development Program (“863” Program) of China (No. 2006AA03Z0449). The work was also supported by Program for New Century Excellent Talents in University (Grant No. NCET06-0879).
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