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Joining of 14YWT and F82H by friction stir welding
Accepted Manuscript Joining of 14YWT and F82H by Friction Stir Welding D.T. Hoelzer, K.A. Unocic, M.A. Sokolov, Z. Feng PII: DOI: Reference:
S0022-3115(13)00627-2 http://dx.doi.org/10.1016/j.jnucmat.2013.04.027 NUMA 47392
To appear in:
Journal of Nuclear Materials
Please cite this article as: D.T. Hoelzer, K.A. Unocic, M.A. Sokolov, Z. Feng, Joining of 14YWT and F82H by Friction Stir Welding, Journal of Nuclear Materials (2013), doi: http://dx.doi.org/10.1016/j.jnucmat.2013.04.027
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Joining of 14YWT and F82H by Friction Stir Welding D.T. Hoelzer, K.A. Unocic, M.A. Sokolov and Z. Feng, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Abstract Friction stir welding was investigated for joining specimens of the ODS 14YWT ferritic alloy together and to an F82H tempered martensitic steel plate. The FSW run was performed using a polycrystalline boron nitride tool and resulted in good bonding between 14YWT/14YWT and 14YWT/ F82H. Joints and interfaces were observed by light microscopy and SEM analysis to be narrow in width. The ultra-small grain size of 14YWT increased by a factor up to 4 while that of F82H decreased by a considerable amount in the weld zones. The TEM analysis showed no significant changes in the size of the oxygen-enriched nanoclusters in the weld zone of 14YWT. However, defects such as a wormhole on the advancing side of the weld zone in 14YWT and small pores associated with joints and interfaces were observed in the FSW sample. The hardness measurements from unaffected zone into weld zones showed ~20% decrease in hardness for 14YWT (from ~500 VH to ~380VH) and ~100% increase in hardness of F82H (from ~220 VH to ~440VH). Keywords: J0100 E0300 M0300 M0500 S0800
Joining (includes Welding, Brazing and Soldering) Electron Microscopy Mechanical Properties (not listed elsewhere) Microstructure and Texture (excludes by Irradiation) Steels, Ferritic/Martensitic
Introduction Friction stir welding (FSW) is an innovative solid-state joining technique that was originally developed within the aluminum industry in 1991 [1]. The basic concept of FSW involves plunging a specially designed tool which rotates at high speeds into the seam between work pieces and joins them by solid-state mixing as the tool travels along the seam. Heat produced by friction from contact between the rotating tool and work pieces allows material to flow around the tool during the translation of the rotating tool. As the tool travels along the seam, a cavity forms at the rear but is filled with material redistributed by the rotating tool to form the weld. The redistribution process causes the material to experiences extreme levels of plastic deformation and thermal exposure that, in turn, can have a significant effect on microstructural changes in the weld zone. A recent review of the friction stir welding and related friction stir processing methods has been published [2]. Friction stir welding has recently been applied to joining high performance alloys having complex microstructures such as oxide dispersion strengthened (ODS) ferritic alloys, (FA; >12Cr), and tempered martensitic steels (TMS; <12Cr with C). These alloys are produced by mechanical alloying (MA) to greatly improve the strength and creep properties at elevated temperatures by the dispersion of thermally stable oxide particles. Although there have been a limited number of FSW studies that have been conducted on ODS alloys in the recent past, these studies demonstrated that joints could be produced with INCO MA957 FA [3, 4], INCO MA956 [5, 6], ODS Eurofer 97 TMS [7] and Plansee PM2000 FA [8]. However, further research to optimize FSW conditions and characterize resulting microstructures is needed to improve such joints that are produced in ODS FA and TMS alloys. Recent advances in understanding the MA process have led to development of the advanced ODS 14YWT FA during the past 10 years. The microstructural features of 14YWT are the high number density of 2-4 nm dia. Ti-, Y-, and O-enriched particles, or nanoclusters (NC), and nanosize (<500 nm) grains that provide high strength up to 1073 K [9]. The purpose of this study was to investigate the joining of 14YWT specimens by FSW without degrading the highly tailored microstructure and also the joining of dissimilar 14YWT to F82H specimens. Experimental Procedure The FSW experiment was conducted on specimens of ODS 14YWT FA and plate of F82H TMS. The 14YWT specimens were from the SM6 heat that was used in the doctoral research project of McClintock [10-12]. This heat was produced by ball milling pre-alloyed Fe-14Cr-3W-0.4Ti powder with 0.3Y2O3 powder (wt. %) followed by degassing the powder in a sealed steel can at 673 K and extruding at 1123 K into a rod. The rod was annealed at 1273 K for 1 h and fabricated into plate by cutting the top and bottom sections off and rolling to 40% reduction in thickness at 1123 K. The F82H TMS was supplied to ORNL by JAEA (Japan Atomic Energy Agency) as a 2.86 mm thick plate with nominal composition of Fe-7.7Cr-2.0W-0.16V-0.16Mn0.11C [13]. The fabricated sample of 14YWT and F82H is shown in Figure 1. The 14YWT specimens were prepared by cutting the end section off of dual-notch bend bar (DNBB) fracture toughness specimens that had been fabricated from the 14YWT plate in the LT orientation [11, 12]. The dimension of the specimens was 15 mm long, 6 mm wide and 2.86 mm thick. A 12 x 30 mm slot
was then removed from the F82H plate (63 mm long, 25 mm wide and 2.86 mm thick) by Electro Discharge Machining (EDM) and four 14YWT specimens were arranged in the slot. A plate of F82H (30 mm long, 25 mm wide and 2.86 mm) was placed next to the slotted F82H plate and all joints between the 14YWT specimens and F82H plates were spot welded. Finally, this section was then spot welded to the backing plate of F82H. The FSW experiment was performed with the lap joint approach using a Polycrystalline Cubic Boron Nitride (PCBN) pin tool. The tool shoulder was 16 mm diameter, with a step scroll convex profile. The pin was 6 mm diameter nominal and threaded. The length of the pin was 3mm. The FSW run was performed with tool rotation speed of 200 to 350 rpm and translation welding speed of ~7.62 cm per minute. The rotating pin tool traveled parallel to the extrusion and rolling directions of the 14YWT specimens, i.e. from left to right in Figure 1. The microstructural analysis of the FSW sample was conducted on 3 specimens that were cut from the weld zone and mounted in cross sectional orientation and polished. One specimen was cut at the leading edge (~0.0 mm) of the 14YWT DNBB specimens and the next 2 specimens were cut ~7.5 mm and ~15 mm from the leading edge. For light microscopy, the polished specimens were chemically etched in a solution of 70% H2O - 20% NNO3 - 10% HF. The specimens were re-polished with a final step using colloidal silica and investigated using the JEOL 6500FEG (Field Emission Gun) Scanning Electron Microscope (SEM). A specimen was prepared for Transmission Electron Microscopy (TEM) analysis from the joint between two 14YWT DNBB specimens (7.5 mm specimen) by the lift-out/FIB (Focused Ion Beam) method. Detailed microstructural analysis was conducted using the Philips CM200 FEG-TEM/STEM (Scanning Transmission Electron Microscope) and the JEOL 2200FS aberration-corrected STEM/TEM (ACEM) instrument. The Vickers Hardness (VH) was measured across joints and interfaces in the FSW sample using a 500 g load and acquisitions every 0.1 mm for lengths of 3 to 4 mm. Results and Discussion The FSW run performed on the 14YWT/F82H sample was completed successfully without failure of the PCBN pin tool. The weld profile observed for the sample after the FSW run is shown in Figure 2. The FSW run initiated on the left side of the sample and proceeded to the right where the location of the pin tool exit from the weld is marked as concentric rings. The results indicated that the width of the weld zone included the 14YWT DNBB specimens that were spot welded together at the center of the sample. Cross sectional views of the microstructures observed by light microscopy for specimens at 0.0, 7.5 and 15 mm distances in the FSW sample are shown in Figure 3. The 14YWT specimens are observed on the top side of the sample as featureless while the F82H plates are observed on the bottom and right sides with microstructural contrast due to chemical etching. The lack of microstructural contrast for the 14YWT specimens was due to the high (14%Cr) content combined with the 387 +/- 80 nm grain size, which is below the resolution of the light microscope. Together, the micrographs revealed an apparent weld nugget, or weld zone, associated with the 14YWT specimens with tapered interfaces extending from the bottom to the top of the weld zone. No discernible interface was observed near the center of the weld zone where the two 14YWT DNBB specimens were spot welded together or between the apparent weld zone and heat affected zone (HAZ) of 14YWT on the retreating side. However, the
interface between the weld zone and HAZ of 14YWT on the advancing side was clearly observed as was the large cavity on the advancing side of the weld zone of 14YWT that continued the length of the FSW sample. The pin tool penetrated to a sufficient depth in the FSW sample that resulted in formation of the joint between 14YWT and H82H. Overall, the results showed that good joints were established between the 14YWT specimens and between the 14YWT specimens and the F82H plate. The microstructures on the advancing and retreating sides of the FSW sample at 7.5 mm were observed by SEM and are shown in Figure 4. For the 14YWT specimens, the analysis revealed variations in microstructural contrast emanating from the top of the cavity that spiraled towards the surface in the weld zone on the advancing side (Figure 4a), but virtually no microstructural contrast between the weld zone and HAZ on the retreating side (Figure 4b). However, there was a distinct interface observed between these two regions on the retreating side. The joint between F82H and 14YWT contained a narrow gap on the retreating side of the weld zone (Figure 4b); indicating that plasticized material extruded on the trailing side of the pin tool was not consolidated. However, this joint showed good consolidation on the advancing side of the weld zone (Figure 4a). The interfacial regions observed between the weld zone and HAZ of 14YWT on the advancing and retreating sides of the FSW sample are shown in Figure 5. The analysis of the interface on the advancing side (Figure 5a) revealed ultra-small grains in the HAZ and both larger grains plus non-uniformly distributed pores mostly in the weld zone. The grain size in the weld zone was ~2 to 4 times larger than the grain size in the HAZ, indicating that the weld zone near the interface was the thermomechancially affected zone (TMAZ), which is the transition zone between the weld nugget and HAZ, or parent material [2]. For the retreating side (Figure 5b), the grain size differences between the TMAZ and HAZ were very small. However, porosity was also observed on the retreating side, but was mostly associated with the interface. Interestingly, the width of the interface was fairly narrow for both the advancing and retreating sides. The analysis of the joints between the two 14YWT specimens and the 14YWT specimens with F82H are shown in Figure 6. The SEM analysis revealed no disguisable joint near the center of the weld zone where the two 14YWY DNBB specimens were located (Figure 6a). Plus, the grain size was ultra-small and fairly uniform in size, indicating that recrystallization had not occurred in the weld zone during the FSW run, which is more consistent with the characteristics of TMAZ. The analysis of the joint between the TMAZ of 14YWT and F82H (Figure 6b) showed small pores distributed along the joint that was relatively narrow in width. The small pores showed no correlation with grain boundaries. The grains observed in the TMAZ of 14YWT were sub-micron in size and slightly elongated in shape compared to those in the unaffected zone. The grains present in the TMAZ of F82H were equiaxed in shape, but were significantly smaller in size compared to that of the unaffected zone. Encouraging results obtained by TEM analysis revealed that both the nano-size grains and Ti-, Y- and O-enriched nanoclusters present in 14YWT were fairly resistant to the effects of extreme plastic deformations and increases in temperature due to FSW. The Bright-Field STEM image shown in Figure 7a showed grains that were nearly equiaxed and mostly 1 m or less in size. Dislocations were also present in the grains, but a quantitative assessment of the density was not obtained in this study. The High-Angle Annular Dark Field (HAADF) STEM image shown in
Figure 7b showed numerous small particles, ~2 to 10 nm in size, with dark contrast relative to the surrounding matrix. Nanoclusters in 14YWT have previously been shown to exhibit darker contrast than the surrounding matrix when imaged by HAADF, or commonly known as Zcontrast, since the intensity scales with ~Z2 (atomic number) [14]. Therefore, the results indicated that both the grains and nanoclusters were resistant to coarsening kinetics promoted by high temperatures during FSW. Hardness changes occurred in the weld zones of 14YWT and F82H resulting from FSW. The LM micrograph showing the two VH line profile measurements on the advancing side of the FSW sample and the corresponding measured values are shown in Figure 8. Starting from the unaffected region of 14YWT (Horizontal Line), the hardness decreased from 499 +/-17 VH to ~300 VH across the HAZ/TMAZ interface and then to 376 +/-3 VH crossing into the weld zone. The decrease in hardness was ~20%, which is reasonably consistent with FSW results for MA957 and PM2000 [3,4,8]. For the joint between 14YWT and F82H (Vertical Line), the hardness of F82H in the unaffected zone increased from 221 +/-6 VH to 443 +/-20 VH in the TMAZ, which was an increase of ~100%. Passing into the weld zone of 14YWT, the hardness eventually decreased to ~370 VH. The significant hardening observed for F82H was attributed to refinement of primary austenite grains and martensitic laths in the reference material [13]. Several mechanisms have been shown to affect the temperature-dependence of strengthening for 14YWT, including factors such as the oxide dispersion, grain boundaries and dislocation densities [15]. Since FSW results in intense plastic deformations and temperature increases, it is reasonable that each strengthening factor could have be affected, resulting in the decrease in hardness of ~20%. Interestingly, the HAADF results (Figure 7b) suggested that the size of the nanoclusters was not increased significantly by FSW, which implies that the decrease in hardness was not influenced too a great extent by coarsening. However, the number density of the nanoclusters was not determined, which will require further research. Dislocation forest strengthening most likely had little influence on the decrease in hardness since the BF STEM analysis (Figure 7a) did not show any abnormalities in dislocation densities in the grains. This was somewhat surprising since the grains in the weld zone experienced severe plastic deformation, but the elevated temperatures from friction heating must have caused dynamic recovery of dislocations. The results showed that FSW caused the grain size to increase by a factor of 2 to 4 in sections of the weld zone, such as TMAZ, compared to that of the unaffected zone (387 +/- 80 nm). Using Hall-Petch parameters recently determined for 14YWT [15], the estimated grain boundary strengthening is: y
= Ky • d-1/2 = 338 [MPa√ m] • d-1/2 [ m-1/2] (at 298 K)
where Ky is the strengthening coefficient and d is the mean grain size. Strengthening from 387 nm size grains is estimated to be 543 MPa. For an increase in grain size by a factor of 2 to 4, the strengthening is 384 to 272 MPa. Thus, the loss of strength due to grain size increase is 159 to 271 MPa, which corresponds to 53 to 91 VHN. Compared to the measured hardness decrease of ~123 VH (Figure 8b), these results suggested that the increase in grain size by FSW had the most significant effect on decreasing the hardness of 14YWT in the weld zone. Porosity was observed in the FSW sample in the form of a cavity (Figures 3 and 4a) in 14YWT that extended the length of the weld on the advancing side and smaller size pores distributed primarily in the TMAZ of 14YWT on the advancing side (Figure 5a) and along the interface
between the HAZ and TMAZ of 14YWT on the retreating side (Figure 5b). The cavity observed in 14YWT was consistent with that of worm hole, which has been observed in FSW studies of other alloys such as aluminum [16]. The worm hole typically forms in the lower region of the weld nugget on the advancing side due to poor vertical movement of material by the rotating pin. The temperatures obtained by frictional heating will influence the plastic flow of material. During FSW, plasticized material flows from the retreating to the advancing side and is forged in the cavity behind the advancing pin tool to form the weld. If the tool travels too rapid and/or the tool rotation speed is not high enough, then the plasticized material may be relatively cool and may not be forged adequately to form the consolidated bond. The presence of small pores along the interface between the HAZ and TMAZ of 14YWT plus the gap between 14YWT and F82H on the retreating side of the weld zone (Figure 5b) indicated that inadequate forging of material occurred with tool travel and rotation speeds used in the FSW experiment. Thus, further optimization of the FSW parameters may reduce the propensity to form the worm hole and the small pores along the interfaces and improve the joint between 14YWT and F82H. However, the small pores associated with the joint between F82H and 14YWT (Figure 6b) may be difficult to eliminate with adjustment of FSW parameters. These pores most likely formed from the air gap that existed between the F82H plate and the 14YWT DNBB specimens prior to FSW. For this joint, the interface between the F82H plate and 14YWT specimens was in plane with the rotating tool pin. Thus, the air gap would mainly be redistributed into pores along the interface during FSW. Conclusion The results showed that specimens of ODS 14YWT were successfully joined together and with F82H by FSW. No discernible joint and no appreciable increase in grain size was observed between the 14YWT specimens in the weld zone. The TEM analysis of a specimen prepared from the joint region revealed that no significant increase in size of the oxygen-enriched nanoclusters occurred during FSW. The joint between 14YWT and F82H showed a narrow gap on the retreating side that evolved into good bonding along the joint towards the advancing side. However, small pores were distributed along the joint region. On the advancing side, a large worm hole formed in the weld zone of 14YWT that extended the length of the sample. Small pores combined with an increase in grain size of 2 to 4 times was observed in the TMAZ of 14YWT on the advancing side, in contrast to the retreating size, which showed small pores at the interface between HAZ and TMAZ of 14YWT and no appreciable changes in grain size in the weld zone. The hardness decreased from 499 to 376 VHN (~20%) across the HAZ into TMAZ of 14YWT on the advancing side and increased from 221 to 443 VHN (~100%) across the HAZ into TMAZ of F82H. Changes in grain size caused by FSW were attributed as the main factor for the observed decrease in hardness for 14YWT and increase in hardness for F82H. Acknowledgement Research at Oak Ridge National Laboratory (ORNL) was primarily sponsored by the Office of Fusion Energy Sciences, U.S. Department of Energy and research at the ORNL SHaRE Facility was supported in part by the Division of Scientific User Facilities, Office of Basic Energy Sciences, U.S. Department of Energy. ORNL is managed by UT-Battelle, LLC for the U.S. Department of Energy.
References [1] W.M. Thomas, E.D Nicholas, J.C. Needham, M.G. Murch, P. Templesmith and C.J. Dawes, G.B. Patent Application No. 9125978.8, 1991. [2] R.S. Mishra and Z.Y. Ma, Material Science and Engineering R, Vol. 50, 2005, p. 1-78. [3] S.M. Howard, B.K. Jasthi, W.J. Arbegast, G.J. Grant, S. Koduri, D.R. Herling and D.S. Gelles, DOE/ER-0313/37, 2004, p. 55-60. [4] P. Miao, G.R. Odette, J. Gould, J. Bernath, R. Miller, M. Alinger, and C. Zanis, Journal of Nuclear Materials, Vol. 367–370, (2007), p. 1197-1202. [5] Z. Feng and W. Ren, Initial Development in Joining of ODS Alloys Using Friction Stir Welding, ORNL/GEN4/LTR-06-021, (2006). [6] Z. Feng and W. Ren, 2007 ASME PVP Conference, paper no PVP200726663. [7] G.J. Grant, D.S. Gelles, R.J. Steel and R. Lindau, DOE/ER-0313/38, 2005, p. 47-53. [8] F. Legendre, S. Poissonnet, P. Bonnaillie, L. Boulanger and L. Forest, Journal of Nuclear Materials, Vol. 386-388, 2009, p. 537-539. [9] D.T. Hoelzer, J. Bentley, M.A. Sokolov, M.K. Miller, G.R. Odette and M.J. Alinger, Journal of Nuclear Materials, Vol. 367-370, (2007), p. 166-172. [10] D.A. McClintock, Ph.D. Dissertation, Univ. of Texas, Austin, 2008. [11] D.A. McClintock, D.T. Hoelzer, M.A. Sokolov and R.K. Nanstad, Journal of Nuclear Materials, Vol. 386–388, (2009), p. 307-311. [12] D.A. McClintock, M.A. Sokolov, D.T. Hoelzer and R.K. Nanstad, Journal of Nuclear Materials, Vol. 392, (2009), p. 353-359. [13] H. Tanigawa, K. Shiba, M.A. Sokolov and R.L. Klueh, Fusion Science and Technology, Vol. 44, No. 1, (2003), p. 206-210. [14] M.C. Brandes, L. Kovarik, M.K. Miller and M.J. Mills, Journal of Materials Science, Vol. 47, (2012), p. 3913-3923. [15] J.H Kim, T.S. Byun, D.T. Hoelzer, C.H. Park, J.T. Yeom and J.K. Hong, Materials Science & Engineering A, Vol. 559, (2013), p. 111-118. [16] P. Sinclair, Master of Science, Vanderbilt University, Tennessee, 2009.
Joining of 14YWT and F82H by Friction Stir Welding D.T. Hoelzer, K.A. Unocic, M.A. Sokolov and Z. Feng, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Figure Captions Figure 1. Digital image showing 4 DNBB specimens of 14YWT spot welded in the slot that was cut in the F82H plate. Figure 2. Digital image showing the completed friction stir weld run that was performed on the 14YWT/F82H sample. Figure 3. Light micrographs showing the cross sectional view of the FSW joint located at (a) 0.0 mm, (b) 7.5 mm and (c) 15 mm from the starting edge of the 14YWT specimens (Note: each micrograph consists of two light images merged together). Figure 4. BSE micrographs showing the microstructures in the joint and interface regions between 14YWT and F82H on the (a) advancing and (b) retreating sides of the FSZ weld. Figure 5. BSE micrographs of the interface between the TMAZ and HAZ of 14YWT on the (a) advancing and (b) retreating sides of the FSZ weld. Figure 6. BSE micrograph of the joint near (a) center of weld zone between two 14YWT DNBB specimens and (b) the TMAZ between 14YWT and F82H. Figure 7. (a) BF STEM image of the grain structure and (b) HAADF STEM image of the nanoclusters observed in the specimen prepared from the weld zone near the joint between the two 14YWT specimens. Figure 8. Vickers Hardness measurements of the FSW sample. (a) Light microscope image showing the hardness indentation profiles and (b) plot of the VH values for the interface between the TMAZ and HAZ of 14YWT (Horizontal Line) and the joint between TMAZ of F82H and 14YWT (Vertical Line).
Figure 1. Digital image showing 4 DNBB specimens of 14YWT spot welded in the slot that was cut in the F82H plate.
Figure 2. Digital image showing the completed friction stir weld run that was performed on the 14YWT/F82H sample.
(a)
(b)
(c)
Figure 3. Light micrographs showing the cross sectional view of the FSW joint located at (a) 0.0 mm, (b) 7.5 mm and (c) 15 mm from the starting edge of the 14YWT specimens (Note: each micrograph consists of two light images merged together).
(a)
Weld Zone 14YWT Interface Cavity HAZ 14YWT
Joint
HAZ F82H
TMAZ F82H (b)
Weld Zone 14YWT HAZ 14YWT
Interface Joint
TMAZ F82H HAZ F82H
Figure 4. BSE micrographs showing the microstructures in the joint and interface regions between 14YWT and F82H on the (a) advancing and (b) retreating sides of the FSZ weld.
(a)
TMAZ 14YWT
HAZ 14YWT
Interface
(b)
HAZ 14YWT
TMAZ 14YWT
Interface
Figure 5. BSE micrographs of the interface between the TMAZ and HAZ of 14YWT on the (a) advancing and (b) retreating sides of the FSZ weld.
(a) (b)
TMAZ 14YWT
TMAZ 14YWT
Joint
TMAZ F82H Figure 6. BSE micrograph of the joint near (a) center of weld zone between two 14YWT DNBB specimens and (b) the TMAZ between 14YWT and F82H.
(a)
(b)
2.8 nm
7.5 nm
Figure 7. (a) BF STEM image of the grain structure and (b) HAADF STEM image of the nanoclusters observed in the specimen prepared from the weld zone near the joint between the two 14YWT specimens.
(a)
(b)
Interface TMAZ/HAZ 14YWT Joint F82H/14YWT Interface HAZ/TMAZ F82H
Figure 8. Vickers Hardness measurements of the FSW sample. (a) Light microscope image showing the hardness indentation profiles and (b) plot of the VH values for the interface between the TMAZ and HAZ of 14YWT (Horizontal Line) and the joint between TMAZ of F82H and 14YWT (Vertical Line).
S0022-3115(13)00627-2 http://dx.doi.org/10.1016/j.jnucmat.2013.04.027 NUMA 47392
To appear in:
Journal of Nuclear Materials
Please cite this article as: D.T. Hoelzer, K.A. Unocic, M.A. Sokolov, Z. Feng, Joining of 14YWT and F82H by Friction Stir Welding, Journal of Nuclear Materials (2013), doi: http://dx.doi.org/10.1016/j.jnucmat.2013.04.027
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Joining of 14YWT and F82H by Friction Stir Welding D.T. Hoelzer, K.A. Unocic, M.A. Sokolov and Z. Feng, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Abstract Friction stir welding was investigated for joining specimens of the ODS 14YWT ferritic alloy together and to an F82H tempered martensitic steel plate. The FSW run was performed using a polycrystalline boron nitride tool and resulted in good bonding between 14YWT/14YWT and 14YWT/ F82H. Joints and interfaces were observed by light microscopy and SEM analysis to be narrow in width. The ultra-small grain size of 14YWT increased by a factor up to 4 while that of F82H decreased by a considerable amount in the weld zones. The TEM analysis showed no significant changes in the size of the oxygen-enriched nanoclusters in the weld zone of 14YWT. However, defects such as a wormhole on the advancing side of the weld zone in 14YWT and small pores associated with joints and interfaces were observed in the FSW sample. The hardness measurements from unaffected zone into weld zones showed ~20% decrease in hardness for 14YWT (from ~500 VH to ~380VH) and ~100% increase in hardness of F82H (from ~220 VH to ~440VH). Keywords: J0100 E0300 M0300 M0500 S0800
Joining (includes Welding, Brazing and Soldering) Electron Microscopy Mechanical Properties (not listed elsewhere) Microstructure and Texture (excludes by Irradiation) Steels, Ferritic/Martensitic
Introduction Friction stir welding (FSW) is an innovative solid-state joining technique that was originally developed within the aluminum industry in 1991 [1]. The basic concept of FSW involves plunging a specially designed tool which rotates at high speeds into the seam between work pieces and joins them by solid-state mixing as the tool travels along the seam. Heat produced by friction from contact between the rotating tool and work pieces allows material to flow around the tool during the translation of the rotating tool. As the tool travels along the seam, a cavity forms at the rear but is filled with material redistributed by the rotating tool to form the weld. The redistribution process causes the material to experiences extreme levels of plastic deformation and thermal exposure that, in turn, can have a significant effect on microstructural changes in the weld zone. A recent review of the friction stir welding and related friction stir processing methods has been published [2]. Friction stir welding has recently been applied to joining high performance alloys having complex microstructures such as oxide dispersion strengthened (ODS) ferritic alloys, (FA; >12Cr), and tempered martensitic steels (TMS; <12Cr with C). These alloys are produced by mechanical alloying (MA) to greatly improve the strength and creep properties at elevated temperatures by the dispersion of thermally stable oxide particles. Although there have been a limited number of FSW studies that have been conducted on ODS alloys in the recent past, these studies demonstrated that joints could be produced with INCO MA957 FA [3, 4], INCO MA956 [5, 6], ODS Eurofer 97 TMS [7] and Plansee PM2000 FA [8]. However, further research to optimize FSW conditions and characterize resulting microstructures is needed to improve such joints that are produced in ODS FA and TMS alloys. Recent advances in understanding the MA process have led to development of the advanced ODS 14YWT FA during the past 10 years. The microstructural features of 14YWT are the high number density of 2-4 nm dia. Ti-, Y-, and O-enriched particles, or nanoclusters (NC), and nanosize (<500 nm) grains that provide high strength up to 1073 K [9]. The purpose of this study was to investigate the joining of 14YWT specimens by FSW without degrading the highly tailored microstructure and also the joining of dissimilar 14YWT to F82H specimens. Experimental Procedure The FSW experiment was conducted on specimens of ODS 14YWT FA and plate of F82H TMS. The 14YWT specimens were from the SM6 heat that was used in the doctoral research project of McClintock [10-12]. This heat was produced by ball milling pre-alloyed Fe-14Cr-3W-0.4Ti powder with 0.3Y2O3 powder (wt. %) followed by degassing the powder in a sealed steel can at 673 K and extruding at 1123 K into a rod. The rod was annealed at 1273 K for 1 h and fabricated into plate by cutting the top and bottom sections off and rolling to 40% reduction in thickness at 1123 K. The F82H TMS was supplied to ORNL by JAEA (Japan Atomic Energy Agency) as a 2.86 mm thick plate with nominal composition of Fe-7.7Cr-2.0W-0.16V-0.16Mn0.11C [13]. The fabricated sample of 14YWT and F82H is shown in Figure 1. The 14YWT specimens were prepared by cutting the end section off of dual-notch bend bar (DNBB) fracture toughness specimens that had been fabricated from the 14YWT plate in the LT orientation [11, 12]. The dimension of the specimens was 15 mm long, 6 mm wide and 2.86 mm thick. A 12 x 30 mm slot
was then removed from the F82H plate (63 mm long, 25 mm wide and 2.86 mm thick) by Electro Discharge Machining (EDM) and four 14YWT specimens were arranged in the slot. A plate of F82H (30 mm long, 25 mm wide and 2.86 mm) was placed next to the slotted F82H plate and all joints between the 14YWT specimens and F82H plates were spot welded. Finally, this section was then spot welded to the backing plate of F82H. The FSW experiment was performed with the lap joint approach using a Polycrystalline Cubic Boron Nitride (PCBN) pin tool. The tool shoulder was 16 mm diameter, with a step scroll convex profile. The pin was 6 mm diameter nominal and threaded. The length of the pin was 3mm. The FSW run was performed with tool rotation speed of 200 to 350 rpm and translation welding speed of ~7.62 cm per minute. The rotating pin tool traveled parallel to the extrusion and rolling directions of the 14YWT specimens, i.e. from left to right in Figure 1. The microstructural analysis of the FSW sample was conducted on 3 specimens that were cut from the weld zone and mounted in cross sectional orientation and polished. One specimen was cut at the leading edge (~0.0 mm) of the 14YWT DNBB specimens and the next 2 specimens were cut ~7.5 mm and ~15 mm from the leading edge. For light microscopy, the polished specimens were chemically etched in a solution of 70% H2O - 20% NNO3 - 10% HF. The specimens were re-polished with a final step using colloidal silica and investigated using the JEOL 6500FEG (Field Emission Gun) Scanning Electron Microscope (SEM). A specimen was prepared for Transmission Electron Microscopy (TEM) analysis from the joint between two 14YWT DNBB specimens (7.5 mm specimen) by the lift-out/FIB (Focused Ion Beam) method. Detailed microstructural analysis was conducted using the Philips CM200 FEG-TEM/STEM (Scanning Transmission Electron Microscope) and the JEOL 2200FS aberration-corrected STEM/TEM (ACEM) instrument. The Vickers Hardness (VH) was measured across joints and interfaces in the FSW sample using a 500 g load and acquisitions every 0.1 mm for lengths of 3 to 4 mm. Results and Discussion The FSW run performed on the 14YWT/F82H sample was completed successfully without failure of the PCBN pin tool. The weld profile observed for the sample after the FSW run is shown in Figure 2. The FSW run initiated on the left side of the sample and proceeded to the right where the location of the pin tool exit from the weld is marked as concentric rings. The results indicated that the width of the weld zone included the 14YWT DNBB specimens that were spot welded together at the center of the sample. Cross sectional views of the microstructures observed by light microscopy for specimens at 0.0, 7.5 and 15 mm distances in the FSW sample are shown in Figure 3. The 14YWT specimens are observed on the top side of the sample as featureless while the F82H plates are observed on the bottom and right sides with microstructural contrast due to chemical etching. The lack of microstructural contrast for the 14YWT specimens was due to the high (14%Cr) content combined with the 387 +/- 80 nm grain size, which is below the resolution of the light microscope. Together, the micrographs revealed an apparent weld nugget, or weld zone, associated with the 14YWT specimens with tapered interfaces extending from the bottom to the top of the weld zone. No discernible interface was observed near the center of the weld zone where the two 14YWT DNBB specimens were spot welded together or between the apparent weld zone and heat affected zone (HAZ) of 14YWT on the retreating side. However, the
interface between the weld zone and HAZ of 14YWT on the advancing side was clearly observed as was the large cavity on the advancing side of the weld zone of 14YWT that continued the length of the FSW sample. The pin tool penetrated to a sufficient depth in the FSW sample that resulted in formation of the joint between 14YWT and H82H. Overall, the results showed that good joints were established between the 14YWT specimens and between the 14YWT specimens and the F82H plate. The microstructures on the advancing and retreating sides of the FSW sample at 7.5 mm were observed by SEM and are shown in Figure 4. For the 14YWT specimens, the analysis revealed variations in microstructural contrast emanating from the top of the cavity that spiraled towards the surface in the weld zone on the advancing side (Figure 4a), but virtually no microstructural contrast between the weld zone and HAZ on the retreating side (Figure 4b). However, there was a distinct interface observed between these two regions on the retreating side. The joint between F82H and 14YWT contained a narrow gap on the retreating side of the weld zone (Figure 4b); indicating that plasticized material extruded on the trailing side of the pin tool was not consolidated. However, this joint showed good consolidation on the advancing side of the weld zone (Figure 4a). The interfacial regions observed between the weld zone and HAZ of 14YWT on the advancing and retreating sides of the FSW sample are shown in Figure 5. The analysis of the interface on the advancing side (Figure 5a) revealed ultra-small grains in the HAZ and both larger grains plus non-uniformly distributed pores mostly in the weld zone. The grain size in the weld zone was ~2 to 4 times larger than the grain size in the HAZ, indicating that the weld zone near the interface was the thermomechancially affected zone (TMAZ), which is the transition zone between the weld nugget and HAZ, or parent material [2]. For the retreating side (Figure 5b), the grain size differences between the TMAZ and HAZ were very small. However, porosity was also observed on the retreating side, but was mostly associated with the interface. Interestingly, the width of the interface was fairly narrow for both the advancing and retreating sides. The analysis of the joints between the two 14YWT specimens and the 14YWT specimens with F82H are shown in Figure 6. The SEM analysis revealed no disguisable joint near the center of the weld zone where the two 14YWY DNBB specimens were located (Figure 6a). Plus, the grain size was ultra-small and fairly uniform in size, indicating that recrystallization had not occurred in the weld zone during the FSW run, which is more consistent with the characteristics of TMAZ. The analysis of the joint between the TMAZ of 14YWT and F82H (Figure 6b) showed small pores distributed along the joint that was relatively narrow in width. The small pores showed no correlation with grain boundaries. The grains observed in the TMAZ of 14YWT were sub-micron in size and slightly elongated in shape compared to those in the unaffected zone. The grains present in the TMAZ of F82H were equiaxed in shape, but were significantly smaller in size compared to that of the unaffected zone. Encouraging results obtained by TEM analysis revealed that both the nano-size grains and Ti-, Y- and O-enriched nanoclusters present in 14YWT were fairly resistant to the effects of extreme plastic deformations and increases in temperature due to FSW. The Bright-Field STEM image shown in Figure 7a showed grains that were nearly equiaxed and mostly 1 m or less in size. Dislocations were also present in the grains, but a quantitative assessment of the density was not obtained in this study. The High-Angle Annular Dark Field (HAADF) STEM image shown in
Figure 7b showed numerous small particles, ~2 to 10 nm in size, with dark contrast relative to the surrounding matrix. Nanoclusters in 14YWT have previously been shown to exhibit darker contrast than the surrounding matrix when imaged by HAADF, or commonly known as Zcontrast, since the intensity scales with ~Z2 (atomic number) [14]. Therefore, the results indicated that both the grains and nanoclusters were resistant to coarsening kinetics promoted by high temperatures during FSW. Hardness changes occurred in the weld zones of 14YWT and F82H resulting from FSW. The LM micrograph showing the two VH line profile measurements on the advancing side of the FSW sample and the corresponding measured values are shown in Figure 8. Starting from the unaffected region of 14YWT (Horizontal Line), the hardness decreased from 499 +/-17 VH to ~300 VH across the HAZ/TMAZ interface and then to 376 +/-3 VH crossing into the weld zone. The decrease in hardness was ~20%, which is reasonably consistent with FSW results for MA957 and PM2000 [3,4,8]. For the joint between 14YWT and F82H (Vertical Line), the hardness of F82H in the unaffected zone increased from 221 +/-6 VH to 443 +/-20 VH in the TMAZ, which was an increase of ~100%. Passing into the weld zone of 14YWT, the hardness eventually decreased to ~370 VH. The significant hardening observed for F82H was attributed to refinement of primary austenite grains and martensitic laths in the reference material [13]. Several mechanisms have been shown to affect the temperature-dependence of strengthening for 14YWT, including factors such as the oxide dispersion, grain boundaries and dislocation densities [15]. Since FSW results in intense plastic deformations and temperature increases, it is reasonable that each strengthening factor could have be affected, resulting in the decrease in hardness of ~20%. Interestingly, the HAADF results (Figure 7b) suggested that the size of the nanoclusters was not increased significantly by FSW, which implies that the decrease in hardness was not influenced too a great extent by coarsening. However, the number density of the nanoclusters was not determined, which will require further research. Dislocation forest strengthening most likely had little influence on the decrease in hardness since the BF STEM analysis (Figure 7a) did not show any abnormalities in dislocation densities in the grains. This was somewhat surprising since the grains in the weld zone experienced severe plastic deformation, but the elevated temperatures from friction heating must have caused dynamic recovery of dislocations. The results showed that FSW caused the grain size to increase by a factor of 2 to 4 in sections of the weld zone, such as TMAZ, compared to that of the unaffected zone (387 +/- 80 nm). Using Hall-Petch parameters recently determined for 14YWT [15], the estimated grain boundary strengthening is: y
= Ky • d-1/2 = 338 [MPa√ m] • d-1/2 [ m-1/2] (at 298 K)
where Ky is the strengthening coefficient and d is the mean grain size. Strengthening from 387 nm size grains is estimated to be 543 MPa. For an increase in grain size by a factor of 2 to 4, the strengthening is 384 to 272 MPa. Thus, the loss of strength due to grain size increase is 159 to 271 MPa, which corresponds to 53 to 91 VHN. Compared to the measured hardness decrease of ~123 VH (Figure 8b), these results suggested that the increase in grain size by FSW had the most significant effect on decreasing the hardness of 14YWT in the weld zone. Porosity was observed in the FSW sample in the form of a cavity (Figures 3 and 4a) in 14YWT that extended the length of the weld on the advancing side and smaller size pores distributed primarily in the TMAZ of 14YWT on the advancing side (Figure 5a) and along the interface
between the HAZ and TMAZ of 14YWT on the retreating side (Figure 5b). The cavity observed in 14YWT was consistent with that of worm hole, which has been observed in FSW studies of other alloys such as aluminum [16]. The worm hole typically forms in the lower region of the weld nugget on the advancing side due to poor vertical movement of material by the rotating pin. The temperatures obtained by frictional heating will influence the plastic flow of material. During FSW, plasticized material flows from the retreating to the advancing side and is forged in the cavity behind the advancing pin tool to form the weld. If the tool travels too rapid and/or the tool rotation speed is not high enough, then the plasticized material may be relatively cool and may not be forged adequately to form the consolidated bond. The presence of small pores along the interface between the HAZ and TMAZ of 14YWT plus the gap between 14YWT and F82H on the retreating side of the weld zone (Figure 5b) indicated that inadequate forging of material occurred with tool travel and rotation speeds used in the FSW experiment. Thus, further optimization of the FSW parameters may reduce the propensity to form the worm hole and the small pores along the interfaces and improve the joint between 14YWT and F82H. However, the small pores associated with the joint between F82H and 14YWT (Figure 6b) may be difficult to eliminate with adjustment of FSW parameters. These pores most likely formed from the air gap that existed between the F82H plate and the 14YWT DNBB specimens prior to FSW. For this joint, the interface between the F82H plate and 14YWT specimens was in plane with the rotating tool pin. Thus, the air gap would mainly be redistributed into pores along the interface during FSW. Conclusion The results showed that specimens of ODS 14YWT were successfully joined together and with F82H by FSW. No discernible joint and no appreciable increase in grain size was observed between the 14YWT specimens in the weld zone. The TEM analysis of a specimen prepared from the joint region revealed that no significant increase in size of the oxygen-enriched nanoclusters occurred during FSW. The joint between 14YWT and F82H showed a narrow gap on the retreating side that evolved into good bonding along the joint towards the advancing side. However, small pores were distributed along the joint region. On the advancing side, a large worm hole formed in the weld zone of 14YWT that extended the length of the sample. Small pores combined with an increase in grain size of 2 to 4 times was observed in the TMAZ of 14YWT on the advancing side, in contrast to the retreating size, which showed small pores at the interface between HAZ and TMAZ of 14YWT and no appreciable changes in grain size in the weld zone. The hardness decreased from 499 to 376 VHN (~20%) across the HAZ into TMAZ of 14YWT on the advancing side and increased from 221 to 443 VHN (~100%) across the HAZ into TMAZ of F82H. Changes in grain size caused by FSW were attributed as the main factor for the observed decrease in hardness for 14YWT and increase in hardness for F82H. Acknowledgement Research at Oak Ridge National Laboratory (ORNL) was primarily sponsored by the Office of Fusion Energy Sciences, U.S. Department of Energy and research at the ORNL SHaRE Facility was supported in part by the Division of Scientific User Facilities, Office of Basic Energy Sciences, U.S. Department of Energy. ORNL is managed by UT-Battelle, LLC for the U.S. Department of Energy.
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Joining of 14YWT and F82H by Friction Stir Welding D.T. Hoelzer, K.A. Unocic, M.A. Sokolov and Z. Feng, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Figure Captions Figure 1. Digital image showing 4 DNBB specimens of 14YWT spot welded in the slot that was cut in the F82H plate. Figure 2. Digital image showing the completed friction stir weld run that was performed on the 14YWT/F82H sample. Figure 3. Light micrographs showing the cross sectional view of the FSW joint located at (a) 0.0 mm, (b) 7.5 mm and (c) 15 mm from the starting edge of the 14YWT specimens (Note: each micrograph consists of two light images merged together). Figure 4. BSE micrographs showing the microstructures in the joint and interface regions between 14YWT and F82H on the (a) advancing and (b) retreating sides of the FSZ weld. Figure 5. BSE micrographs of the interface between the TMAZ and HAZ of 14YWT on the (a) advancing and (b) retreating sides of the FSZ weld. Figure 6. BSE micrograph of the joint near (a) center of weld zone between two 14YWT DNBB specimens and (b) the TMAZ between 14YWT and F82H. Figure 7. (a) BF STEM image of the grain structure and (b) HAADF STEM image of the nanoclusters observed in the specimen prepared from the weld zone near the joint between the two 14YWT specimens. Figure 8. Vickers Hardness measurements of the FSW sample. (a) Light microscope image showing the hardness indentation profiles and (b) plot of the VH values for the interface between the TMAZ and HAZ of 14YWT (Horizontal Line) and the joint between TMAZ of F82H and 14YWT (Vertical Line).
Figure 1. Digital image showing 4 DNBB specimens of 14YWT spot welded in the slot that was cut in the F82H plate.
Figure 2. Digital image showing the completed friction stir weld run that was performed on the 14YWT/F82H sample.
(a)
(b)
(c)
Figure 3. Light micrographs showing the cross sectional view of the FSW joint located at (a) 0.0 mm, (b) 7.5 mm and (c) 15 mm from the starting edge of the 14YWT specimens (Note: each micrograph consists of two light images merged together).
(a)
Weld Zone 14YWT Interface Cavity HAZ 14YWT
Joint
HAZ F82H
TMAZ F82H (b)
Weld Zone 14YWT HAZ 14YWT
Interface Joint
TMAZ F82H HAZ F82H
Figure 4. BSE micrographs showing the microstructures in the joint and interface regions between 14YWT and F82H on the (a) advancing and (b) retreating sides of the FSZ weld.
(a)
TMAZ 14YWT
HAZ 14YWT
Interface
(b)
HAZ 14YWT
TMAZ 14YWT
Interface
Figure 5. BSE micrographs of the interface between the TMAZ and HAZ of 14YWT on the (a) advancing and (b) retreating sides of the FSZ weld.
(a) (b)
TMAZ 14YWT
TMAZ 14YWT
Joint
TMAZ F82H Figure 6. BSE micrograph of the joint near (a) center of weld zone between two 14YWT DNBB specimens and (b) the TMAZ between 14YWT and F82H.
(a)
(b)
2.8 nm
7.5 nm
Figure 7. (a) BF STEM image of the grain structure and (b) HAADF STEM image of the nanoclusters observed in the specimen prepared from the weld zone near the joint between the two 14YWT specimens.
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(b)
Interface TMAZ/HAZ 14YWT Joint F82H/14YWT Interface HAZ/TMAZ F82H
Figure 8. Vickers Hardness measurements of the FSW sample. (a) Light microscope image showing the hardness indentation profiles and (b) plot of the VH values for the interface between the TMAZ and HAZ of 14YWT (Horizontal Line) and the joint between TMAZ of F82H and 14YWT (Vertical Line).