- Email: [email protected]
Fatigue and fracture toughness characteristics of laser rapid manufactured Inconel 625 structures
Materials Science and Engineering A 527 (2010) 7490–7497
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Fatigue and fracture toughness characteristics of laser rapid manufactured Inconel 625 structures P. Ganesh a,∗ , R. Kaul a , C.P. Paul a , Pragya Tiwari b , S.K. Rai b , R.C. Prasad c , L.M. Kukreja a a b c
Laser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Indore (MP) 452013, India Indus Synchrotrons Utilization Division, Raja Ramanna Centre for Advanced Technology, Indore (MP) 452013, India Metallurgy and Materials Science Department, IIT Bombay, Mumbai 400 076, India
a r t i c l e
i n f o
Article history: Received 24 March 2010 Received in revised form 5 July 2010 Accepted 11 August 2010
Keywords: Laser rapid manufacturing Inconel 625 Fatigue crack growth Fractography Transgranular fracture J-integral test
a b s t r a c t Fatigue crack growth and fracture toughness characteristics of laser rapid manufactured (LRMed) Inconel 625 compact tension specimens of thickness 12 and 25 mm were investigated. Fatigue crack propagation √ in all the specimens investigated in the stress intensity range (K) of 14–38 MPa m, exhibited stage II crack growth in Paris’ regime with nearly same slopes of crack growth per cycle versus K plot. Fatigue crack growth rates in the LRMed specimens of present study were found to be lower than the reported √ values for wrought Inconel 625 in the K range of 14–24 MPa m and above this range they tended to coincide. X-ray diffraction patterns of the fractured surfaces revealed that the crack propagated along the growth direction of the specimens which was predominantly along the (1 1 1) plane. The fracture toughness values (J0.2 ) for LRMed Inconel 625 specimens were found to be in the range of about 200–255 kJ/m2 . The LRMed specimens exhibited stable crack growth during the J-integral test. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years laser rapid manufacturing (LRM) has emerged as an efficient tool for fabrication of engineering and prosthetic components, starting from computer-aided design (CAD) model. LRM represents an additive manufacturing process involving layer-bylayer fabrication of 3D components in a pre-determined fashion [1]. The process involves laser deposition by injecting desired metal/alloy powder into the interaction zone of a scanning laser beam. The injected powder melts and re-solidifies into a dense clad deposit. Selective laser deposition, in a pre-determined manner, generates component of a desired geometry. The process is ideally suited for low volume manufacturing, particularly for high cost materials and has the flexibility to engineer microstructure, chemical composition and its gradient in the resultant component [2–10]. This technology is being deployed for the fabrication of a variety of small to medium sized highly complex components in aerospace, nuclear, dental, medical, and other industries. This technique is particularly suited for fabricating multi-component and multi-material structures to accomplish functionalities, which are unobtainable through conventional methodologies. This technol-
∗ Corresponding author. Tel.: +91 731 2488381; fax: +91 731 2488380. E-mail addresses: [email protected], ganesh [email protected] (P. Ganesh). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.08.034
ogy is being developed at various laboratories in the world under different names, viz. laser engineered net shaping (LENSTM ) [3], free form laser consolidation [4], selective laser powder re-melting (SLPR) [5], direct metal deposition (DMD) [6], automated laser fabrication [7] and laser additive manufacturing [8], etc. To date, a number of metals, including stainless steel, cobalt base alloys, titanium alloys, nickel base alloys, etc., have been explored for fabrication of functional metal parts by LRM [11]. Blackwell and Wisbey [10] reported low ductility, but acceptable fracture toughness values, of direct laser-fabricated parts of high strength titanium alloy. Keicher et al. [12] Keicher and Smugeresky [13] attributed excellent mechanical properties (strength and ductility) of laser rapid manufactured parts of type 316L stainless steel and Inconel 625 alloys to their fine grain size. Zheng et al. addressed the effect of volume fraction of TiC on the mechanical properties of Ti6Al4V-TiC composite [14]. In spite of extensive work on LRM, bulk of research work, pertaining to characterization of laser rapid manufactured parts, is largely limited to microstructural analysis, hardness and tensile property characterization [2–9]. LRM, being a layer-by-layer deposition process, generates a typical cast microstructure with relatively finer grain size and associated solidification texture [15]. Moreover, laser rapid manufactured parts may carry defects like lack of fusion regions between adjacent clad tracks [16], inclusions due to entrapment of oxidized powder particles, gas porosities,
P. Ganesh et al. / Materials Science and Engineering A 527 (2010) 7490–7497
7491
Table 1 Chemical composition (weight%) of the powder used for laser cladding. Material
C
Cr
Ni
Si
Mo
Fe
Ti
Al
Inconel 625
0.1
22.5
62.2
0.5
9.2
4.7
0.36
0.4
segregation at inter-dendritic/grain boundaries. In addition rapid solidification associated with LRM may result in high level of residual stresses, orientation dependent strength properties [17] and cracking in susceptible materials [18] or brittle phase formation (e.g. martensite in ferritic steels). Presence of these defects in the laser rapid manufactured parts may adversely affect their fatigue and fracture performance. At present there is no published literature available on the study of fatigue crack propagation and fracture toughness of laser-fabricated Inconel 625. The present work has been undertaken with an objective to study fracture toughness and fatigue crack growth behavior of laser rapid manufactured Inconel 625 alloy. 2. Material background Inconel 625 (IN-625), because of its high strength, excellent fabricability (including joining), outstanding corrosion resistance and wide service temperature range (from cryogenic to 1255 K), finds wide applications in aeronautical, aerospace, chemical, petrochemical, marine and nuclear industries. Inconel 625 is a solid-solution strengthened face centered cubic Ni–Cr–Mo alloy. It may contain alloy carbides, in the form of MC and M6 C (rich in Ni, Nb, Mo and C), which are inherent in this type of alloy. Strength of IN-625 alloy is derived from the stiffening effect of Mo and Nb on Ni–Cr matrix, thus precipitation-hardening treatments are not required [19]. Inconel 625, because of its high oxidation resistance, is a suitable material for LRM [15]. A recent study performed in authors’ laboratory has demonstrated that, defect free components of IN-625 can be fabricated by LRM. The study focused on the optimization of processes parameters and characterization of microstructure and mechanical properties (like hardness, tensile strength and impact toughness) of the resultant part [20].
Fig. 1. Schematic of experimental set-up.
2.1. Experimental The LRM setup consisted of an indigenously developed 3.5 kW CO2 laser system [21], integrated with a beam delivery system, co-axial powder-feeding nozzle and a 3-axis CNC work station, as shown schematically in Fig. 1. Raw laser beam, emanating out of the laser system, was folded with a 45◦ water-cooled gold-coated plane copper mirror and the folded laser beam was subsequently focused with a 127 mm focal length ZnSe lens, housed in a water-cooled co-axial copper nozzle. LRM process involved scanning the substrate with a defocused laser beam of about 3 mm diameter along with simultaneous injection of Inconel 625 alloy powder (particle size range: 45–106 m) into the resultant melt pool through the co-axial copper nozzle. During the course of LRM, argon gas was used as a shielding as well as powder carrier gas. Over-lapping clad tracks, with 50% overlap, were made to build the specimens. Chemical composition of IN625 alloy powder (in weight%) and process parameters used, in the present study, are presented in Tables 1 and 2, respectively.
Fig. 2. Schematic illustration of (A) tensile specimen extraction and (B) specimen’s dimensions in mm.
Optimization of these process parameters is reported elsewhere [20]. In order to characterize tensile properties of IN-625 in as-LRMed condition, tensile test specimens were extracted from as-laser deposited pads of Inconel 625 (as shown in Fig. 2) in such a way that tensile loading axis of the resultant specimen was normal to both growth and laser scanning directions. Fig. 2A shows the schematic of the tensile specimen extraction along with the scanning and built-up directions marked. These tensile specimens constitute laser-deposited region in the entire reduced section, as shown in Fig. 2B along with the dimensions. The orientation of the speci-
Table 2 Experimental parameters for laser cladding. Laser power (kW)
Laser interaction zone width
Powder feed rate (gm/min)
Scan speed (mm/s)
Carrier gas flow rate (Ar)
Overlap between successive clad tracks
2.8–3.0
3 mm
6–8
6.7–8.3
12 lpm
50%
7492
P. Ganesh et al. / Materials Science and Engineering A 527 (2010) 7490–7497
Fig. 3. Sequence of steps followed for laser rapid manufacturing (LRM) of compact tension test samples of Inconel 625. (A): Stainless steel 304L block with “V” groove; (B) LRM process with co-axial powder-feeding nozzle; (C) Stainless steel block with laser deposit on central region. Arrows indicating the direction of laser scanning in each layer. S represents the plane parallel to laser scanning direction and top surface; L represents the plane parallel the laser scanning direction and normal to top surface (S-plane); T represents the transverse plane normal to top surface (S) and the laser scanning direction. (D): Compact tension specimen extracted from laser-deposited block shown in (C).
men was chosen to provide tensile properties normal to the growth direction, which were important for selection of parameters and data analysis of fatigue crack propagation and fracture toughness tests (in which direction of loading was also normal to the growth and laser scanning directions). Three numbers of specimens were subjected to tensile test. LRM being a layer-by-layer deposition process may induce anisotropy in the fabricated components because of solidification texture. Anisotropy due the crystallographic texture plays a vital role in the stable crack extension under fatigue cycling, because the slip takes place in certain preferred crystallographic planes, which offer low resistance to fracture. In the present investigation attempt is made to orient the crack growth in the direction of least resistance to deformation by slip, so that the data of maximum crack growth rate and minimum toughness can be evaluated for the LRM IN-625 alloy. It is expected that LRMed IN625 may exhibit higher toughness and lower fatigue crack growth
rate (FCGR), when the orientation of the crack growth is perpendicular to growth direction. The fabrication of the Compact Tension (CT) test specimens involved machining of a V-groove in a block of type 304L stainless steel (SS) of 75 × 65 × 40 mm3 size (refer Fig. 3A), followed by laser deposition of IN-625 in the V-grooved region (refer Fig. 3B). The resultant laser-deposited blocks (refer Fig. 3C) were sliced normal to direction of laser scanning and subsequently machined to obtain 12 and 25 mm thick CT specimens (three numbers each) in such a way the tip of the machined notch lay at the bottom of the laser-deposited region (refer Fig. 3D). Dimensions of the all the CT samples extracted from the laser-fabricated blocks conform to ASTM E 647 and E 1820 standards [22,23]. The direction of crack growth in the resultant CT samples was parallel to the growth direction of the laser deposit. This fabrication methodology was chosen to avoid the tedious job of fabricating entire specimen by
P. Ganesh et al. / Materials Science and Engineering A 527 (2010) 7490–7497
LRM, while at the same time it served the purpose of evaluating mechanical properties viz. FCGR and fracture toughness of the laser deposit. Fatigue and fracture toughness measurements were carried out using 100 kN servo-hydraulic testing machine at room temperature (about 300 K). The fatigue tests were performed on 12 and 25 mm thick CT specimens (width, W = 50 mm) at room temperature, as per ASTM E 647 [22]. FCGR tests were conducted in both K-decreasing and K-increasing modes with stress ratios (R = minimum stress/maximum stress) of 0.1 and 0.3 at 25 Hz frequency. The higher stress ratio (R) of 0.3 in K-decreasing mode was √ used, particularly for testing in low K regime (below 20 MPa m), to obtain crack growth data in a reasonable period of time and minimize the crack closure related issues. In specimens tested in K-decreasing mode, low applied K at the end of the test provided small plastic zone at the crack tip, which served the purpose of precracking for J-test. However, in specimens tested in K-increasing mode, pre-cracking was conducted after fatigue testing to comply with the standard test procedure in low load conditions before conducting J-test. Determination of fatigue crack growth threshold (Kth ) is not included in the scope of present study. After conducting FCGR tests from initial a/W of 0.4 to about 0.6 (actual crack growth of about 8–10 mm), J-integral fracture toughness tests were conducted on the same specimens, as per ASTM E 1820 standard [23]. In addition to mechanical testing, laser rapid manufactured deposits of IN-625 were also characterized by X-ray diffraction (XRD), ␥-ray radiography, optical microscopy, and micro-hardness measurements. X-ray diffraction (XRD) studies were carried out to determine phase information from the laser deposit of IN-625. A Bruker D8-Discover system consisting of – goniometer along with 3 circle eulerian cradle was used for X-ray diffraction measurements. In this system incident beam has parabolic mirror to give parallel beam along with LiF flat monochromator in diffraction beam arm to collect diffracted beam. X-ray diffraction was con˚ while using ducted with CuK␣ characteristic radiation ( = 1.54 A) a step size of 0.025◦ and one second step time. All the machined CT samples were examined by ␥-ray radiography using a Co-60 ␥ radiation source (20–30 Curie). Single wall single image (SWSI) technique has been used for radiography with exposure time of about 3 min. The radiation was aligned perpendicular to the transverse plane (T) (refer Fig. 3C) so that the discontinuities between successive clad layers/tracks (e.g. cracks, lack of fusion), if any, could be detected. Micro-hardness measurements were carried out with Leitz miniload-2 micro-hardness tester while using a test force of 0.981 N and a dwell time of 30 s. Fatigue fracture surfaces have also been examined by scanning electron microscopy and XRD to analyze nature of crack propagation.
3. Results and discussion Visual examination of outer surface of LRM specimens revealed multiple clad tracks, giving it a layered appearance. Fig. 4 presents magnified view of the top and transverse surfaces of the one of the laser-fabricated test blocks used for the fabrication of CT specimens. The test block carried about 22 clad layers while the thickness of each clad layer was about 1.6 mm. The specimens in the as laserdeposited condition, did not exhibit any surface defects like cracks or lack of bonding at substrate/clad or interlayer interfaces. Radiographic examination of machined CT specimen did not reveal any defects like cracks and porosities. However, microscopic defects beyond the sensitivity of radiography may be present in the specimen. Optical microscopic examination carried out on the transverse cross-sections of laser-deposited IN-625 revealed fine dendritic
7493
Fig. 4. Stereo photographs of top and transverse sections of as laser-deposited stainless blocks with loosely adhered oxide layer and partially fused metal particles on top and transverse surfaces respectively.
microstructure with columnar grains parallel to the growth direction, as shown in Fig. 5. The specimens were associated with some isolated micro-porosities, with average diameter of about 2–5 m. Extensive cross-sectional examination revealed few lack of fusion (LOF) regions (measuring about 0.5–1 mm) at some locations near the edge of laser deposit at substrate clad interface (between stainless steel block and LRM deposit). However no such (LOF) defects were seen at the central region of the specimen close to the line of crack propagation. Micro-hardness measured at various locations on transverse cross-section (refer Fig. 3C; T the transverse plane normal to top surface (S) as well as to the scanning direction) revealed hardness in the range of 270–300 HV0.1 . The longitudinal cross-section (refer Fig. 3C; L the plane normal to top surface (S) and parallel the scanning direction) registered slightly higher hardness in the range of 290–320 HV0.1 . Fig. 6 presents the microhardness profiles obtained on different planes viz. top surface (S), longitudinal plane (L) and the transverse plane (T). The scatter in the micro-hardness is due to the fact that location of the indentation is some times at the interface of the two clad layers associated with highly refined structure (fine cells) resulting in slightly higher hardness. This has been correlated by etching the samples after micro-hardness measurements. X-ray diffraction (XRD), conducted on laser-deposited specimens with different orientations (refer Fig. 3C) confirmed that ␥ phase (face centered cubic (fcc) crystal structure) of IN-625 powder, was retained in the as-deposited LRM specimen shown in Fig. 7. No additional peaks of any new phase were seen in the XRD patterns. LRM specimens exhibited strong texturing effect with respect to random texture of the powder. The plane parallel to the top surface (refer Fig. 3C; S-plane) of LRM deposit exhibited enhanced (2 0 0) texture relative to random pattern of the powder which is in line with the fact that directional solidification took place along easy growth 1 0 0 direction [24]. On the other hand, transverse cross-section (refer Fig. 3C; T) exhibited strong (1 1 1) texture.
7494
P. Ganesh et al. / Materials Science and Engineering A 527 (2010) 7490–7497
Fig. 5. Microstructure of laser-deposited Inconel 625: (A) multiple clad tracks on the cross-section normal to the scanning direction; (B) magnified view of the microstructure showing columnar dendrites/cells.
Fig. 7. Comparison of XRD patterns of laser-deposited Inconel 625 with powder form. Refer Fig. 3C for S- and T-plane orientations. Fig. 6. Micro-hardness profiles obtained on the top (S), transverse (T) and longitudinal (L) planes of laser-deposited Inconel 625 alloy (refer Fig. 3C for plane orientations).
Table 3 Tensile properties of laser consolidated, cast and wrought Inconel 625 alloy. Conditions
Tensile properties of laser consolidated IN-625 alloy are presented in Table 3, along with corresponding data for cast and wrought materials for comparison [25–27]. Ultimate tensile strength (UTS) of IN-625 alloy in the as laser-deposited condition was found to be 690 MPa, which is about 10% lower than that reported by Xue et al. [25] and it is close to the UTS of as cast IN625 [26]. However yield strength and percentage elongation of as laser-deposited alloy were found to be 540 MPa and 36% (for 12 mm gauge length) respectively which are close to the published results. LRM specimens exhibited higher yield strength and lower ductility with respect to those in cast and wrought conditions. Fig. 8 presents variation of fatigue crack growth rate (da/dN) as a function of stress intensity factor range (K) for LRM CT specimens of IN-625. FCGR of LRM CT specimens, examined in the K
Tensile properties
LRM – Present studya LRM – Reported [25] Cast [26] Wrought [27] a
Yield strength, 0.2 (MPa)
Tensile strength, uts (MPa)
% Elongation (Gauge length = 12 mm)
540
690
36
477–518
744–797
31–48
350 490
710 855
48 50
Average of three tests.
√ range of 14–38 MPa m, showed consistently steady crack growth with increase in K. In this region, fatigue crack growth data follows Paris’ law: da/dN = C (K)m , where da/dN is in mm/cycle, K √ is in MPa m and “C” and “m” are empirical material constants. The
Table 4 Material constants in relation to K for IN-625. Material
IN-625 LRM IN-625 LRM IN-625 LRM IN-625 Wrought [28] IN-625 Wrought [28]
Test conditions CT specimen thickness (mm)
√ K range (MPa m)
Stress ratio (R)
25 12 12 – –
14–25 21–31.1 27.6–36.6 21.11–54.35 22.96–41
0.3 0.1 0.1 0.05 0.05
C
m
3.23 E-12 1.48 E-12 1.95 E-12 8.55 E-10 4.48 E-10
5.21 5.42 5.33 3.73 3.8
P. Ganesh et al. / Materials Science and Engineering A 527 (2010) 7490–7497
Fig. 8. Fatigue crack growth rate data of laser rapid manufactured Inconel 625 CT specimens with different thickness, tested under different stress intensity range.
results of the FCGR tests revealed almost similar slopes of (da/dN) vs. K plots (in logarithmic scales) for both 12 and 25 mm thick specimens tested in different K ranges for both K increasing and K decreasing test conditions. The effect of stress ratio on FCGR could not be established from the present investigation as the specimens were tested in different stress intensity range (K). Comparison of crack growth data obtained from laser rapid manufactured CT specimens and that reported in literature [28] (refer Fig. 9) shows that in general, FCGR of LRM specimens is lower than reported or predicted values for wrought IN-625. However, the difference in FCGRs of LRM and wrought (reported) IN-625 diminishes with increase in applied stress intensity range (K). Table 4 compares empirical constants of Paris’ equation (“C” and “m”) for LRM IN-625, obtained from the experimental data, with the corresponding reported data for wrought IN-625 [28]. The improved fatigue crack growth resistance of the LRM of IN-625 samples in the low stress intensity range could be due to the higher yield strength and residual stresses in the as LRMed condition. The FCGR results obtained in the present study are in line with the results reported by Heung et al. [29] wherein the FCGR increased with increasing plastic zone size for 25 mm thick 304 austenitic stainless steel CT specimens. Cyclic plastic deformation is the main mechanical driving force for the crack propagation. During crack propagation, the crack passes through a plastic zone
7495
Fig. 9. Comparison of fatigue crack growth rate data obtained from laser rapid manufactured Inconel 625 CT specimens with that reported in literature and simulated data for IN-625.
whose size depends upon variables such as yield stress of material, applied stress, crack length and the specimen thickness. With increasing applied stress intensity range the crack tip plastic zone size should increase, resulting in reduced FCGR, but the results of the present study are contrary to this. The above model is applicable if, increase in crack tip plastic zone size results in transition from plane strain to plane stress conditions. Therefore it can be inferred that, plane strain conditions prevailed in both the 12 and 25 mm thick specimens during fatigue tests in the applied stress intensity range, as FCGRs of all the specimens exhibited similar slopes. Fractographic examination of the initial part of the fracture surface, involving fatigue crack propagation (FCP), exhibited columnar facets along the macroscopic direction of crack propagation as seen in Fig. 10A. These long facets represent columnar grains of the LRM deposit. The fracture surface, presented in Fig. 10(B), shows crack propagation across several clad layers with transition in orientation of columnar grains across the interface between successive clad layers. SEM examination of the fracture surface revealed variation in the microscopic direction of crack propagation as different segments of the advancing crack front propagate in different columnar grains (refer Fig. 11). Crack propagation on different planes on the two sides of the grain boundaries resulted in the formation of
Fig. 10. Low magnification views of fracture surfaces of LRMed IN-625 CT samples. “FCP” and “J” (A) represent regions of crack extension during fatigue test and J-integral test respectively and PC (B) represents the crack growth during pre-cracking stage.
7496
P. Ganesh et al. / Materials Science and Engineering A 527 (2010) 7490–7497
Fig. 11. Fatigue fracture surface of 12 mm thick LRMed CT sample of IN-625 at a √ stress intensity factor range of K ∼ 20 MPa m and R = 0.1. Predominantly transgranular mode of crack propagation with arrows indicating the local crack front orientation.
Fig. 12. Fatigue fracture surface of 12 mm thick LRMed CT sample of IN-625 at a √ stress intensity factor range of K ∼ 31 MPa m and R = 0.1.
step at the grain boundary region (Fig. 12), which provided faceted appearance to the resultant fracture surface. Examination of the fracture surface at a higher magnification revealed presence of fatigue striations as shown in Fig. 13, which is suggestive of the transgranular fracture mechanism being operative at room tem-
Fig. 13. Magnified view of the fatigue fracture surface with uniform striations in the 25 mm thick LRMed CT sample of IN-625 at a stress intensity factor range of √ K ∼ 25 MPa m.
Fig. 14. Comparison of XRD patterns obtained from the fatigue fracture surfaces of 25 and 12 mm thick CT specimens and a polished specimen with surface orientation parallel to fracture surface of 25 mm thick specimen.
Fig. 15. J–resistance curve (J–R curve) of a 12 mm thick IN-625 LRMed CT specimen, with corresponding Jq value marked with arrow.
perature [30]. Occurrence of ductile striations on fracture surfaces suggests that crack growth was dominated by the crystal slip-based mechanism. X-ray diffraction patterns obtained on the fatigue fracture surfaces of 12 and 25 mm thick CT specimens shown in Fig. 14 revealed strong (1 1 1) texture indicating preferential presence of (1 1 1) planes on the fracture surface. It is also confirmed from the XRD patterns shown in Figs. 7 and 14 (L-plane, XRD pattern at bottom) that the slip plane (1 1 1) mainly existed parallel to the growth direction due to solidification texture. From these observations it is evident that the crack grew in the growth direction mainly along the (1 1 1) planes. Similar observations have been reported by Irawan et al. on the crack propagation along the rolling direction in cold-rolled aluminum sheets [31]. Fig. 14 compares XRD patterns of the fatigue fracture surfaces with the polished specimen of longitudinal cross-section (ref Fig. 3C, L) of LRM deposit, which was parallel to the fracture surface. After FCGR testing, same CT specimens, with the a/w of about 0.6, were subjected to J-integral fracture toughness testing by single specimen unloading compliance technique (refer Fig. 10A). Load versus load line displacement (COD measurements) data for all the specimens was analyzed to plot J versus a graphs. Fig. 15 presents J–resistance curve for a 12 mm thick IN-625 LRM CT specimen, with corresponding Jq value marked in the figure. Out of the three tests conducted on the 25 mm thick CT specimens only one test resulted
P. Ganesh et al. / Materials Science and Engineering A 527 (2010) 7490–7497
7497
Acknowledgements Authors thankfully acknowledge the support provided by Shri Harish Kumar, Shri T. Reghu, Shri C.H. Prem Singh, Shri C.S. Mandloi and Shri Ram Nihal Ram in preparation of specimens. Authors are also grateful to the staff members of laser materials processing division of RRCAT Indore, India for their kind support during the course of this work. References
Fig. 16. Fracture surface of 12 mm thick IN-625 LRMed CT specimen with dimples, representing ductile crack propagation during J-integral test.
in qualified Jq value and in other tests, qualified Jq could not be obtained as the crack did not grow in the plane normal to loading axis, due to crack branching and the load versus load-line displacement did not show any change in compliance with increasing load and unload cycles. The J integral fracture toughness (J0.2 ) for IN625 LRM CT specimens were found to be 225–255 and 196 kJ/m2 for 12 and 25 mm thick specimens, respectively. SEM examination of the resultant fracture surface exhibited equi-axed dimples, representing stable crack propagation during J-test, as shown in Fig. 16. 4. Conclusion In the light of the results obtained during present investigation, it is found that fatigue crack propagation in laser rapid manufactured specimens of IN-625 was characterized by steady crack growth (Paris’ regime) in the investigated K range of √ 14–38 MPa m. Both 12 and 25 mm thick CT specimens, tested in different K range, exhibited stage-II crack growth with nearly same slopes of log–log plot of da/dN versus K. Transgranular fatigue crack propagation in LRM specimens was associated with slip on predominantly (1 1 1) planes. In the investigated √ K range of 14–38 MPa m, FCGR in laser rapid manufactured specimens of IN-625 was lower than that reported for wrought IN-625. The difference between FCGRs in LRM and wrought specimens (reported value) narrowed down with increase in K. IN-625 LRMed CT specimens exhibited stable crack growth during the J-integral test without pop-in behavior. The J integral fracture toughness values (J0.2 ) for 12 and 25 mm thick specimens were found to be 225–255 and 196 kJ/m2 , respectively.
[1] W.M. Steen, Laser Material Processing, 2nd ed., Springer-Verlag, London, 2001, pp. 272–290. [2] C.P. Paul, A. Jain, P. Ganesh, J. Negi, A.K. Nath, Opt. Lasers Eng. 44 (2006) 1096–1109. [3] C. Atwood, M. Griffith, L. Harwell, E. Schlienger, M. Ensz, J. Smugeresky, T. Romero, D. Greene, D. Reckaway, Laser engineered net shaping (LENSTM ): a tool for direct fabrication of metal parts, in: ICALEO’98: Proceedings, vol. 85e, Laser Institute of America, Orlando, FL, 1998, pp. 1–7. [4] L. Xue, M. Islam, Free-form laser consolidation for producing functional metallic components, in: ICALEO’98: Proceedings, vol. 85e, Laser Institute of America, Orlando, FL, 1998, pp.15–24. [5] W. Meiners, K. Wissenbach, R. Poprawe, Direct generation of metal parts and tools by selective laser powder remelting (SLPR), in: ICALEO’98: Proceedings, vol. 85e, Laser Institute of America, Orlando, FL, 1998, pp. 31–37. [6] J. Majumder, J. Cjoi, K. Nagaratnam, J. Koch, D. Hetzner, JOM 49 (1997) 55–60. [7] C.P. Paul, A. Khajepour, Opt. Laser Technol. 40 (2008) 735–741. [8] P.A. Kobryn, S.L. Semiatin, JOM 53 (9) (2001) 40–42. [9] G.K. Lewis, E. Schlienger, Mater. Des. 21 (2000) 417–423. [10] P.L. Blackwell, A. Wisbey, J. Mater. Process. Technol. 170 (2005) 268–276. [11] R.P. Mudge, N.R. Wald, WJ 86 (1) (2007) 44–48. [12] D.M. Keicher, et al., Free Form Fabrication Using the Laser Engineered Net Shaping (LENS TM) Process, in: Paper presented at 1996 PivI2TEC, Washington, DC, 1996. [13] D.M. Keicher, J.E. Smugeresky, JOM 49 (1997) 51–54. [14] B. Zheng, J.E. Smugeresky, Y. Zhou, D. Baker, E.J. Lavernia, Metall. Mater. Trans. A 39A (2008) 1196–1205. [15] G.P. Dinda, A.K. Dasgupta, J. Majumder, Mater. Sci. Eng. 509A (2009) 98–104. [16] W.M. Steen, V.M. Weerasinghe, P. Monson, Proc. SPIE 650 (1986) 226–234. [17] Franz-Josef Kahlen, Aravinda Kar, J. Laser Appl. 13 (2) (2001) 60–68. [18] M. Zhong, W. Liu, J.-C. Goussian, C. Mayer, J. Fritz, Proc. SPIE 3888 (2000) 307–311. [19] http://www.specialmetals.com/documents/Inconel%20alloy%20625.pdf. [20] C.P. Paul, P. Ganesh, S.K. Mishra, P. Bhargava, J. Negi, A.K. Nath, Opt. Laser Technol. 39 (2007) 800–805. [21] A.K. Nath, L. Abhinandan, P. Choudhary, Opt. Eng. 33 (6) (1994) 1889–1893. [22] Standard test method for measurement of fatigue crack growth rates, ASTM E 647-00, ASTM International, West Conshohocken, PA, USA. [23] Standard Test Method for Measurement of Fracture Toughness, ASTM E 182006, ASTM International, West Conshohocken, PA, USA. [24] R.E Reed-Hill, Physical Metallurgy Principles, Van Nostrand, New York, 1973, p. 581. [25] L. Xue, M.U. Islam, J. Laser Appl. 12 (4) (2000) 160–165. [26] A.S.M. International, Metal Handbook, vol. 1, 9th ed., ASM International, Metals Park, OH, 1990, p. 984. [27] A.S.M. International, Metal Handbook, vol. 3, 9th ed., ASM International, Metals Park, OH, 1980, p. 143, 219. [28] http://www.ascgenoa.com/main/newsletter/9/%5B2%5D607 AIAA 2007 2381-metal-Fatigue.pdf. [29] H.-B. Park, K.-M. Kim, B.-W. Lee, Int. J. Pres. Ves. Pip. 68 (1996) 279–285. [30] D.G.L. Prakash, M.J. Walsh, D. Maclachlan, A.M. Korsunsky, Int. J. Fatigue (2009), doi:10.1016/j.ijfatigue.2009.01.023. [31] Y.S. Irawan, Y. Hagiwara, S.-I. Ohya, J. Soc. Mater. Sci. Jpn. 55 (4) (2006) 402–408.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Fatigue and fracture toughness characteristics of laser rapid manufactured Inconel 625 structures P. Ganesh a,∗ , R. Kaul a , C.P. Paul a , Pragya Tiwari b , S.K. Rai b , R.C. Prasad c , L.M. Kukreja a a b c
Laser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Indore (MP) 452013, India Indus Synchrotrons Utilization Division, Raja Ramanna Centre for Advanced Technology, Indore (MP) 452013, India Metallurgy and Materials Science Department, IIT Bombay, Mumbai 400 076, India
a r t i c l e
i n f o
Article history: Received 24 March 2010 Received in revised form 5 July 2010 Accepted 11 August 2010
Keywords: Laser rapid manufacturing Inconel 625 Fatigue crack growth Fractography Transgranular fracture J-integral test
a b s t r a c t Fatigue crack growth and fracture toughness characteristics of laser rapid manufactured (LRMed) Inconel 625 compact tension specimens of thickness 12 and 25 mm were investigated. Fatigue crack propagation √ in all the specimens investigated in the stress intensity range (K) of 14–38 MPa m, exhibited stage II crack growth in Paris’ regime with nearly same slopes of crack growth per cycle versus K plot. Fatigue crack growth rates in the LRMed specimens of present study were found to be lower than the reported √ values for wrought Inconel 625 in the K range of 14–24 MPa m and above this range they tended to coincide. X-ray diffraction patterns of the fractured surfaces revealed that the crack propagated along the growth direction of the specimens which was predominantly along the (1 1 1) plane. The fracture toughness values (J0.2 ) for LRMed Inconel 625 specimens were found to be in the range of about 200–255 kJ/m2 . The LRMed specimens exhibited stable crack growth during the J-integral test. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years laser rapid manufacturing (LRM) has emerged as an efficient tool for fabrication of engineering and prosthetic components, starting from computer-aided design (CAD) model. LRM represents an additive manufacturing process involving layer-bylayer fabrication of 3D components in a pre-determined fashion [1]. The process involves laser deposition by injecting desired metal/alloy powder into the interaction zone of a scanning laser beam. The injected powder melts and re-solidifies into a dense clad deposit. Selective laser deposition, in a pre-determined manner, generates component of a desired geometry. The process is ideally suited for low volume manufacturing, particularly for high cost materials and has the flexibility to engineer microstructure, chemical composition and its gradient in the resultant component [2–10]. This technology is being deployed for the fabrication of a variety of small to medium sized highly complex components in aerospace, nuclear, dental, medical, and other industries. This technique is particularly suited for fabricating multi-component and multi-material structures to accomplish functionalities, which are unobtainable through conventional methodologies. This technol-
∗ Corresponding author. Tel.: +91 731 2488381; fax: +91 731 2488380. E-mail addresses: [email protected], ganesh [email protected] (P. Ganesh). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.08.034
ogy is being developed at various laboratories in the world under different names, viz. laser engineered net shaping (LENSTM ) [3], free form laser consolidation [4], selective laser powder re-melting (SLPR) [5], direct metal deposition (DMD) [6], automated laser fabrication [7] and laser additive manufacturing [8], etc. To date, a number of metals, including stainless steel, cobalt base alloys, titanium alloys, nickel base alloys, etc., have been explored for fabrication of functional metal parts by LRM [11]. Blackwell and Wisbey [10] reported low ductility, but acceptable fracture toughness values, of direct laser-fabricated parts of high strength titanium alloy. Keicher et al. [12] Keicher and Smugeresky [13] attributed excellent mechanical properties (strength and ductility) of laser rapid manufactured parts of type 316L stainless steel and Inconel 625 alloys to their fine grain size. Zheng et al. addressed the effect of volume fraction of TiC on the mechanical properties of Ti6Al4V-TiC composite [14]. In spite of extensive work on LRM, bulk of research work, pertaining to characterization of laser rapid manufactured parts, is largely limited to microstructural analysis, hardness and tensile property characterization [2–9]. LRM, being a layer-by-layer deposition process, generates a typical cast microstructure with relatively finer grain size and associated solidification texture [15]. Moreover, laser rapid manufactured parts may carry defects like lack of fusion regions between adjacent clad tracks [16], inclusions due to entrapment of oxidized powder particles, gas porosities,
P. Ganesh et al. / Materials Science and Engineering A 527 (2010) 7490–7497
7491
Table 1 Chemical composition (weight%) of the powder used for laser cladding. Material
C
Cr
Ni
Si
Mo
Fe
Ti
Al
Inconel 625
0.1
22.5
62.2
0.5
9.2
4.7
0.36
0.4
segregation at inter-dendritic/grain boundaries. In addition rapid solidification associated with LRM may result in high level of residual stresses, orientation dependent strength properties [17] and cracking in susceptible materials [18] or brittle phase formation (e.g. martensite in ferritic steels). Presence of these defects in the laser rapid manufactured parts may adversely affect their fatigue and fracture performance. At present there is no published literature available on the study of fatigue crack propagation and fracture toughness of laser-fabricated Inconel 625. The present work has been undertaken with an objective to study fracture toughness and fatigue crack growth behavior of laser rapid manufactured Inconel 625 alloy. 2. Material background Inconel 625 (IN-625), because of its high strength, excellent fabricability (including joining), outstanding corrosion resistance and wide service temperature range (from cryogenic to 1255 K), finds wide applications in aeronautical, aerospace, chemical, petrochemical, marine and nuclear industries. Inconel 625 is a solid-solution strengthened face centered cubic Ni–Cr–Mo alloy. It may contain alloy carbides, in the form of MC and M6 C (rich in Ni, Nb, Mo and C), which are inherent in this type of alloy. Strength of IN-625 alloy is derived from the stiffening effect of Mo and Nb on Ni–Cr matrix, thus precipitation-hardening treatments are not required [19]. Inconel 625, because of its high oxidation resistance, is a suitable material for LRM [15]. A recent study performed in authors’ laboratory has demonstrated that, defect free components of IN-625 can be fabricated by LRM. The study focused on the optimization of processes parameters and characterization of microstructure and mechanical properties (like hardness, tensile strength and impact toughness) of the resultant part [20].
Fig. 1. Schematic of experimental set-up.
2.1. Experimental The LRM setup consisted of an indigenously developed 3.5 kW CO2 laser system [21], integrated with a beam delivery system, co-axial powder-feeding nozzle and a 3-axis CNC work station, as shown schematically in Fig. 1. Raw laser beam, emanating out of the laser system, was folded with a 45◦ water-cooled gold-coated plane copper mirror and the folded laser beam was subsequently focused with a 127 mm focal length ZnSe lens, housed in a water-cooled co-axial copper nozzle. LRM process involved scanning the substrate with a defocused laser beam of about 3 mm diameter along with simultaneous injection of Inconel 625 alloy powder (particle size range: 45–106 m) into the resultant melt pool through the co-axial copper nozzle. During the course of LRM, argon gas was used as a shielding as well as powder carrier gas. Over-lapping clad tracks, with 50% overlap, were made to build the specimens. Chemical composition of IN625 alloy powder (in weight%) and process parameters used, in the present study, are presented in Tables 1 and 2, respectively.
Fig. 2. Schematic illustration of (A) tensile specimen extraction and (B) specimen’s dimensions in mm.
Optimization of these process parameters is reported elsewhere [20]. In order to characterize tensile properties of IN-625 in as-LRMed condition, tensile test specimens were extracted from as-laser deposited pads of Inconel 625 (as shown in Fig. 2) in such a way that tensile loading axis of the resultant specimen was normal to both growth and laser scanning directions. Fig. 2A shows the schematic of the tensile specimen extraction along with the scanning and built-up directions marked. These tensile specimens constitute laser-deposited region in the entire reduced section, as shown in Fig. 2B along with the dimensions. The orientation of the speci-
Table 2 Experimental parameters for laser cladding. Laser power (kW)
Laser interaction zone width
Powder feed rate (gm/min)
Scan speed (mm/s)
Carrier gas flow rate (Ar)
Overlap between successive clad tracks
2.8–3.0
3 mm
6–8
6.7–8.3
12 lpm
50%
7492
P. Ganesh et al. / Materials Science and Engineering A 527 (2010) 7490–7497
Fig. 3. Sequence of steps followed for laser rapid manufacturing (LRM) of compact tension test samples of Inconel 625. (A): Stainless steel 304L block with “V” groove; (B) LRM process with co-axial powder-feeding nozzle; (C) Stainless steel block with laser deposit on central region. Arrows indicating the direction of laser scanning in each layer. S represents the plane parallel to laser scanning direction and top surface; L represents the plane parallel the laser scanning direction and normal to top surface (S-plane); T represents the transverse plane normal to top surface (S) and the laser scanning direction. (D): Compact tension specimen extracted from laser-deposited block shown in (C).
men was chosen to provide tensile properties normal to the growth direction, which were important for selection of parameters and data analysis of fatigue crack propagation and fracture toughness tests (in which direction of loading was also normal to the growth and laser scanning directions). Three numbers of specimens were subjected to tensile test. LRM being a layer-by-layer deposition process may induce anisotropy in the fabricated components because of solidification texture. Anisotropy due the crystallographic texture plays a vital role in the stable crack extension under fatigue cycling, because the slip takes place in certain preferred crystallographic planes, which offer low resistance to fracture. In the present investigation attempt is made to orient the crack growth in the direction of least resistance to deformation by slip, so that the data of maximum crack growth rate and minimum toughness can be evaluated for the LRM IN-625 alloy. It is expected that LRMed IN625 may exhibit higher toughness and lower fatigue crack growth
rate (FCGR), when the orientation of the crack growth is perpendicular to growth direction. The fabrication of the Compact Tension (CT) test specimens involved machining of a V-groove in a block of type 304L stainless steel (SS) of 75 × 65 × 40 mm3 size (refer Fig. 3A), followed by laser deposition of IN-625 in the V-grooved region (refer Fig. 3B). The resultant laser-deposited blocks (refer Fig. 3C) were sliced normal to direction of laser scanning and subsequently machined to obtain 12 and 25 mm thick CT specimens (three numbers each) in such a way the tip of the machined notch lay at the bottom of the laser-deposited region (refer Fig. 3D). Dimensions of the all the CT samples extracted from the laser-fabricated blocks conform to ASTM E 647 and E 1820 standards [22,23]. The direction of crack growth in the resultant CT samples was parallel to the growth direction of the laser deposit. This fabrication methodology was chosen to avoid the tedious job of fabricating entire specimen by
P. Ganesh et al. / Materials Science and Engineering A 527 (2010) 7490–7497
LRM, while at the same time it served the purpose of evaluating mechanical properties viz. FCGR and fracture toughness of the laser deposit. Fatigue and fracture toughness measurements were carried out using 100 kN servo-hydraulic testing machine at room temperature (about 300 K). The fatigue tests were performed on 12 and 25 mm thick CT specimens (width, W = 50 mm) at room temperature, as per ASTM E 647 [22]. FCGR tests were conducted in both K-decreasing and K-increasing modes with stress ratios (R = minimum stress/maximum stress) of 0.1 and 0.3 at 25 Hz frequency. The higher stress ratio (R) of 0.3 in K-decreasing mode was √ used, particularly for testing in low K regime (below 20 MPa m), to obtain crack growth data in a reasonable period of time and minimize the crack closure related issues. In specimens tested in K-decreasing mode, low applied K at the end of the test provided small plastic zone at the crack tip, which served the purpose of precracking for J-test. However, in specimens tested in K-increasing mode, pre-cracking was conducted after fatigue testing to comply with the standard test procedure in low load conditions before conducting J-test. Determination of fatigue crack growth threshold (Kth ) is not included in the scope of present study. After conducting FCGR tests from initial a/W of 0.4 to about 0.6 (actual crack growth of about 8–10 mm), J-integral fracture toughness tests were conducted on the same specimens, as per ASTM E 1820 standard [23]. In addition to mechanical testing, laser rapid manufactured deposits of IN-625 were also characterized by X-ray diffraction (XRD), ␥-ray radiography, optical microscopy, and micro-hardness measurements. X-ray diffraction (XRD) studies were carried out to determine phase information from the laser deposit of IN-625. A Bruker D8-Discover system consisting of – goniometer along with 3 circle eulerian cradle was used for X-ray diffraction measurements. In this system incident beam has parabolic mirror to give parallel beam along with LiF flat monochromator in diffraction beam arm to collect diffracted beam. X-ray diffraction was con˚ while using ducted with CuK␣ characteristic radiation ( = 1.54 A) a step size of 0.025◦ and one second step time. All the machined CT samples were examined by ␥-ray radiography using a Co-60 ␥ radiation source (20–30 Curie). Single wall single image (SWSI) technique has been used for radiography with exposure time of about 3 min. The radiation was aligned perpendicular to the transverse plane (T) (refer Fig. 3C) so that the discontinuities between successive clad layers/tracks (e.g. cracks, lack of fusion), if any, could be detected. Micro-hardness measurements were carried out with Leitz miniload-2 micro-hardness tester while using a test force of 0.981 N and a dwell time of 30 s. Fatigue fracture surfaces have also been examined by scanning electron microscopy and XRD to analyze nature of crack propagation.
3. Results and discussion Visual examination of outer surface of LRM specimens revealed multiple clad tracks, giving it a layered appearance. Fig. 4 presents magnified view of the top and transverse surfaces of the one of the laser-fabricated test blocks used for the fabrication of CT specimens. The test block carried about 22 clad layers while the thickness of each clad layer was about 1.6 mm. The specimens in the as laserdeposited condition, did not exhibit any surface defects like cracks or lack of bonding at substrate/clad or interlayer interfaces. Radiographic examination of machined CT specimen did not reveal any defects like cracks and porosities. However, microscopic defects beyond the sensitivity of radiography may be present in the specimen. Optical microscopic examination carried out on the transverse cross-sections of laser-deposited IN-625 revealed fine dendritic
7493
Fig. 4. Stereo photographs of top and transverse sections of as laser-deposited stainless blocks with loosely adhered oxide layer and partially fused metal particles on top and transverse surfaces respectively.
microstructure with columnar grains parallel to the growth direction, as shown in Fig. 5. The specimens were associated with some isolated micro-porosities, with average diameter of about 2–5 m. Extensive cross-sectional examination revealed few lack of fusion (LOF) regions (measuring about 0.5–1 mm) at some locations near the edge of laser deposit at substrate clad interface (between stainless steel block and LRM deposit). However no such (LOF) defects were seen at the central region of the specimen close to the line of crack propagation. Micro-hardness measured at various locations on transverse cross-section (refer Fig. 3C; T the transverse plane normal to top surface (S) as well as to the scanning direction) revealed hardness in the range of 270–300 HV0.1 . The longitudinal cross-section (refer Fig. 3C; L the plane normal to top surface (S) and parallel the scanning direction) registered slightly higher hardness in the range of 290–320 HV0.1 . Fig. 6 presents the microhardness profiles obtained on different planes viz. top surface (S), longitudinal plane (L) and the transverse plane (T). The scatter in the micro-hardness is due to the fact that location of the indentation is some times at the interface of the two clad layers associated with highly refined structure (fine cells) resulting in slightly higher hardness. This has been correlated by etching the samples after micro-hardness measurements. X-ray diffraction (XRD), conducted on laser-deposited specimens with different orientations (refer Fig. 3C) confirmed that ␥ phase (face centered cubic (fcc) crystal structure) of IN-625 powder, was retained in the as-deposited LRM specimen shown in Fig. 7. No additional peaks of any new phase were seen in the XRD patterns. LRM specimens exhibited strong texturing effect with respect to random texture of the powder. The plane parallel to the top surface (refer Fig. 3C; S-plane) of LRM deposit exhibited enhanced (2 0 0) texture relative to random pattern of the powder which is in line with the fact that directional solidification took place along easy growth 1 0 0 direction [24]. On the other hand, transverse cross-section (refer Fig. 3C; T) exhibited strong (1 1 1) texture.
7494
P. Ganesh et al. / Materials Science and Engineering A 527 (2010) 7490–7497
Fig. 5. Microstructure of laser-deposited Inconel 625: (A) multiple clad tracks on the cross-section normal to the scanning direction; (B) magnified view of the microstructure showing columnar dendrites/cells.
Fig. 7. Comparison of XRD patterns of laser-deposited Inconel 625 with powder form. Refer Fig. 3C for S- and T-plane orientations. Fig. 6. Micro-hardness profiles obtained on the top (S), transverse (T) and longitudinal (L) planes of laser-deposited Inconel 625 alloy (refer Fig. 3C for plane orientations).
Table 3 Tensile properties of laser consolidated, cast and wrought Inconel 625 alloy. Conditions
Tensile properties of laser consolidated IN-625 alloy are presented in Table 3, along with corresponding data for cast and wrought materials for comparison [25–27]. Ultimate tensile strength (UTS) of IN-625 alloy in the as laser-deposited condition was found to be 690 MPa, which is about 10% lower than that reported by Xue et al. [25] and it is close to the UTS of as cast IN625 [26]. However yield strength and percentage elongation of as laser-deposited alloy were found to be 540 MPa and 36% (for 12 mm gauge length) respectively which are close to the published results. LRM specimens exhibited higher yield strength and lower ductility with respect to those in cast and wrought conditions. Fig. 8 presents variation of fatigue crack growth rate (da/dN) as a function of stress intensity factor range (K) for LRM CT specimens of IN-625. FCGR of LRM CT specimens, examined in the K
Tensile properties
LRM – Present studya LRM – Reported [25] Cast [26] Wrought [27] a
Yield strength, 0.2 (MPa)
Tensile strength, uts (MPa)
% Elongation (Gauge length = 12 mm)
540
690
36
477–518
744–797
31–48
350 490
710 855
48 50
Average of three tests.
√ range of 14–38 MPa m, showed consistently steady crack growth with increase in K. In this region, fatigue crack growth data follows Paris’ law: da/dN = C (K)m , where da/dN is in mm/cycle, K √ is in MPa m and “C” and “m” are empirical material constants. The
Table 4 Material constants in relation to K for IN-625. Material
IN-625 LRM IN-625 LRM IN-625 LRM IN-625 Wrought [28] IN-625 Wrought [28]
Test conditions CT specimen thickness (mm)
√ K range (MPa m)
Stress ratio (R)
25 12 12 – –
14–25 21–31.1 27.6–36.6 21.11–54.35 22.96–41
0.3 0.1 0.1 0.05 0.05
C
m
3.23 E-12 1.48 E-12 1.95 E-12 8.55 E-10 4.48 E-10
5.21 5.42 5.33 3.73 3.8
P. Ganesh et al. / Materials Science and Engineering A 527 (2010) 7490–7497
Fig. 8. Fatigue crack growth rate data of laser rapid manufactured Inconel 625 CT specimens with different thickness, tested under different stress intensity range.
results of the FCGR tests revealed almost similar slopes of (da/dN) vs. K plots (in logarithmic scales) for both 12 and 25 mm thick specimens tested in different K ranges for both K increasing and K decreasing test conditions. The effect of stress ratio on FCGR could not be established from the present investigation as the specimens were tested in different stress intensity range (K). Comparison of crack growth data obtained from laser rapid manufactured CT specimens and that reported in literature [28] (refer Fig. 9) shows that in general, FCGR of LRM specimens is lower than reported or predicted values for wrought IN-625. However, the difference in FCGRs of LRM and wrought (reported) IN-625 diminishes with increase in applied stress intensity range (K). Table 4 compares empirical constants of Paris’ equation (“C” and “m”) for LRM IN-625, obtained from the experimental data, with the corresponding reported data for wrought IN-625 [28]. The improved fatigue crack growth resistance of the LRM of IN-625 samples in the low stress intensity range could be due to the higher yield strength and residual stresses in the as LRMed condition. The FCGR results obtained in the present study are in line with the results reported by Heung et al. [29] wherein the FCGR increased with increasing plastic zone size for 25 mm thick 304 austenitic stainless steel CT specimens. Cyclic plastic deformation is the main mechanical driving force for the crack propagation. During crack propagation, the crack passes through a plastic zone
7495
Fig. 9. Comparison of fatigue crack growth rate data obtained from laser rapid manufactured Inconel 625 CT specimens with that reported in literature and simulated data for IN-625.
whose size depends upon variables such as yield stress of material, applied stress, crack length and the specimen thickness. With increasing applied stress intensity range the crack tip plastic zone size should increase, resulting in reduced FCGR, but the results of the present study are contrary to this. The above model is applicable if, increase in crack tip plastic zone size results in transition from plane strain to plane stress conditions. Therefore it can be inferred that, plane strain conditions prevailed in both the 12 and 25 mm thick specimens during fatigue tests in the applied stress intensity range, as FCGRs of all the specimens exhibited similar slopes. Fractographic examination of the initial part of the fracture surface, involving fatigue crack propagation (FCP), exhibited columnar facets along the macroscopic direction of crack propagation as seen in Fig. 10A. These long facets represent columnar grains of the LRM deposit. The fracture surface, presented in Fig. 10(B), shows crack propagation across several clad layers with transition in orientation of columnar grains across the interface between successive clad layers. SEM examination of the fracture surface revealed variation in the microscopic direction of crack propagation as different segments of the advancing crack front propagate in different columnar grains (refer Fig. 11). Crack propagation on different planes on the two sides of the grain boundaries resulted in the formation of
Fig. 10. Low magnification views of fracture surfaces of LRMed IN-625 CT samples. “FCP” and “J” (A) represent regions of crack extension during fatigue test and J-integral test respectively and PC (B) represents the crack growth during pre-cracking stage.
7496
P. Ganesh et al. / Materials Science and Engineering A 527 (2010) 7490–7497
Fig. 11. Fatigue fracture surface of 12 mm thick LRMed CT sample of IN-625 at a √ stress intensity factor range of K ∼ 20 MPa m and R = 0.1. Predominantly transgranular mode of crack propagation with arrows indicating the local crack front orientation.
Fig. 12. Fatigue fracture surface of 12 mm thick LRMed CT sample of IN-625 at a √ stress intensity factor range of K ∼ 31 MPa m and R = 0.1.
step at the grain boundary region (Fig. 12), which provided faceted appearance to the resultant fracture surface. Examination of the fracture surface at a higher magnification revealed presence of fatigue striations as shown in Fig. 13, which is suggestive of the transgranular fracture mechanism being operative at room tem-
Fig. 13. Magnified view of the fatigue fracture surface with uniform striations in the 25 mm thick LRMed CT sample of IN-625 at a stress intensity factor range of √ K ∼ 25 MPa m.
Fig. 14. Comparison of XRD patterns obtained from the fatigue fracture surfaces of 25 and 12 mm thick CT specimens and a polished specimen with surface orientation parallel to fracture surface of 25 mm thick specimen.
Fig. 15. J–resistance curve (J–R curve) of a 12 mm thick IN-625 LRMed CT specimen, with corresponding Jq value marked with arrow.
perature [30]. Occurrence of ductile striations on fracture surfaces suggests that crack growth was dominated by the crystal slip-based mechanism. X-ray diffraction patterns obtained on the fatigue fracture surfaces of 12 and 25 mm thick CT specimens shown in Fig. 14 revealed strong (1 1 1) texture indicating preferential presence of (1 1 1) planes on the fracture surface. It is also confirmed from the XRD patterns shown in Figs. 7 and 14 (L-plane, XRD pattern at bottom) that the slip plane (1 1 1) mainly existed parallel to the growth direction due to solidification texture. From these observations it is evident that the crack grew in the growth direction mainly along the (1 1 1) planes. Similar observations have been reported by Irawan et al. on the crack propagation along the rolling direction in cold-rolled aluminum sheets [31]. Fig. 14 compares XRD patterns of the fatigue fracture surfaces with the polished specimen of longitudinal cross-section (ref Fig. 3C, L) of LRM deposit, which was parallel to the fracture surface. After FCGR testing, same CT specimens, with the a/w of about 0.6, were subjected to J-integral fracture toughness testing by single specimen unloading compliance technique (refer Fig. 10A). Load versus load line displacement (COD measurements) data for all the specimens was analyzed to plot J versus a graphs. Fig. 15 presents J–resistance curve for a 12 mm thick IN-625 LRM CT specimen, with corresponding Jq value marked in the figure. Out of the three tests conducted on the 25 mm thick CT specimens only one test resulted
P. Ganesh et al. / Materials Science and Engineering A 527 (2010) 7490–7497
7497
Acknowledgements Authors thankfully acknowledge the support provided by Shri Harish Kumar, Shri T. Reghu, Shri C.H. Prem Singh, Shri C.S. Mandloi and Shri Ram Nihal Ram in preparation of specimens. Authors are also grateful to the staff members of laser materials processing division of RRCAT Indore, India for their kind support during the course of this work. References
Fig. 16. Fracture surface of 12 mm thick IN-625 LRMed CT specimen with dimples, representing ductile crack propagation during J-integral test.
in qualified Jq value and in other tests, qualified Jq could not be obtained as the crack did not grow in the plane normal to loading axis, due to crack branching and the load versus load-line displacement did not show any change in compliance with increasing load and unload cycles. The J integral fracture toughness (J0.2 ) for IN625 LRM CT specimens were found to be 225–255 and 196 kJ/m2 for 12 and 25 mm thick specimens, respectively. SEM examination of the resultant fracture surface exhibited equi-axed dimples, representing stable crack propagation during J-test, as shown in Fig. 16. 4. Conclusion In the light of the results obtained during present investigation, it is found that fatigue crack propagation in laser rapid manufactured specimens of IN-625 was characterized by steady crack growth (Paris’ regime) in the investigated K range of √ 14–38 MPa m. Both 12 and 25 mm thick CT specimens, tested in different K range, exhibited stage-II crack growth with nearly same slopes of log–log plot of da/dN versus K. Transgranular fatigue crack propagation in LRM specimens was associated with slip on predominantly (1 1 1) planes. In the investigated √ K range of 14–38 MPa m, FCGR in laser rapid manufactured specimens of IN-625 was lower than that reported for wrought IN-625. The difference between FCGRs in LRM and wrought specimens (reported value) narrowed down with increase in K. IN-625 LRMed CT specimens exhibited stable crack growth during the J-integral test without pop-in behavior. The J integral fracture toughness values (J0.2 ) for 12 and 25 mm thick specimens were found to be 225–255 and 196 kJ/m2 , respectively.
[1] W.M. Steen, Laser Material Processing, 2nd ed., Springer-Verlag, London, 2001, pp. 272–290. [2] C.P. Paul, A. Jain, P. Ganesh, J. Negi, A.K. Nath, Opt. Lasers Eng. 44 (2006) 1096–1109. [3] C. Atwood, M. Griffith, L. Harwell, E. Schlienger, M. Ensz, J. Smugeresky, T. Romero, D. Greene, D. Reckaway, Laser engineered net shaping (LENSTM ): a tool for direct fabrication of metal parts, in: ICALEO’98: Proceedings, vol. 85e, Laser Institute of America, Orlando, FL, 1998, pp. 1–7. [4] L. Xue, M. Islam, Free-form laser consolidation for producing functional metallic components, in: ICALEO’98: Proceedings, vol. 85e, Laser Institute of America, Orlando, FL, 1998, pp.15–24. [5] W. Meiners, K. Wissenbach, R. Poprawe, Direct generation of metal parts and tools by selective laser powder remelting (SLPR), in: ICALEO’98: Proceedings, vol. 85e, Laser Institute of America, Orlando, FL, 1998, pp. 31–37. [6] J. Majumder, J. Cjoi, K. Nagaratnam, J. Koch, D. Hetzner, JOM 49 (1997) 55–60. [7] C.P. Paul, A. Khajepour, Opt. Laser Technol. 40 (2008) 735–741. [8] P.A. Kobryn, S.L. Semiatin, JOM 53 (9) (2001) 40–42. [9] G.K. Lewis, E. Schlienger, Mater. Des. 21 (2000) 417–423. [10] P.L. Blackwell, A. Wisbey, J. Mater. Process. Technol. 170 (2005) 268–276. [11] R.P. Mudge, N.R. Wald, WJ 86 (1) (2007) 44–48. [12] D.M. Keicher, et al., Free Form Fabrication Using the Laser Engineered Net Shaping (LENS TM) Process, in: Paper presented at 1996 PivI2TEC, Washington, DC, 1996. [13] D.M. Keicher, J.E. Smugeresky, JOM 49 (1997) 51–54. [14] B. Zheng, J.E. Smugeresky, Y. Zhou, D. Baker, E.J. Lavernia, Metall. Mater. Trans. A 39A (2008) 1196–1205. [15] G.P. Dinda, A.K. Dasgupta, J. Majumder, Mater. Sci. Eng. 509A (2009) 98–104. [16] W.M. Steen, V.M. Weerasinghe, P. Monson, Proc. SPIE 650 (1986) 226–234. [17] Franz-Josef Kahlen, Aravinda Kar, J. Laser Appl. 13 (2) (2001) 60–68. [18] M. Zhong, W. Liu, J.-C. Goussian, C. Mayer, J. Fritz, Proc. SPIE 3888 (2000) 307–311. [19] http://www.specialmetals.com/documents/Inconel%20alloy%20625.pdf. [20] C.P. Paul, P. Ganesh, S.K. Mishra, P. Bhargava, J. Negi, A.K. Nath, Opt. Laser Technol. 39 (2007) 800–805. [21] A.K. Nath, L. Abhinandan, P. Choudhary, Opt. Eng. 33 (6) (1994) 1889–1893. [22] Standard test method for measurement of fatigue crack growth rates, ASTM E 647-00, ASTM International, West Conshohocken, PA, USA. [23] Standard Test Method for Measurement of Fracture Toughness, ASTM E 182006, ASTM International, West Conshohocken, PA, USA. [24] R.E Reed-Hill, Physical Metallurgy Principles, Van Nostrand, New York, 1973, p. 581. [25] L. Xue, M.U. Islam, J. Laser Appl. 12 (4) (2000) 160–165. [26] A.S.M. International, Metal Handbook, vol. 1, 9th ed., ASM International, Metals Park, OH, 1990, p. 984. [27] A.S.M. International, Metal Handbook, vol. 3, 9th ed., ASM International, Metals Park, OH, 1980, p. 143, 219. [28] http://www.ascgenoa.com/main/newsletter/9/%5B2%5D607 AIAA 2007 2381-metal-Fatigue.pdf. [29] H.-B. Park, K.-M. Kim, B.-W. Lee, Int. J. Pres. Ves. Pip. 68 (1996) 279–285. [30] D.G.L. Prakash, M.J. Walsh, D. Maclachlan, A.M. Korsunsky, Int. J. Fatigue (2009), doi:10.1016/j.ijfatigue.2009.01.023. [31] Y.S. Irawan, Y. Hagiwara, S.-I. Ohya, J. Soc. Mater. Sci. Jpn. 55 (4) (2006) 402–408.