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Fatigue behaviour at elevated temperature of friction stir channelling solid plates of AA5083-H111 aluminium alloy
International Journal of Fatigue 62 (2014) 85–92
Contents lists available at ScienceDirect
International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue
Fatigue behaviour at elevated temperature of friction stir channelling solid plates of AA5083-H111 aluminium alloy Catarina Vidal a,b,⇑, Virgínia Infante a,d, Pedro Vilaça c,d a
ICEMS – Institute of Materials and Surfaces Science and Engineering, Av. Rovisco Pais, 1049-001 Lisbon, Portugal Escola Superior de Tecnologia de Setúbal, Instituto Politécnico de Setúbal, Campus do IPS, Estefanilha, 2910-761 Setúbal, Portugal c IDMEC – Institute of Mechanical Engineering, Av. Rovisco Pais, 1049-001 Lisbon, Portugal d Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal b
a r t i c l e
i n f o
Article history: Received 31 October 2012 Received in revised form 6 February 2013 Accepted 10 October 2013 Available online 22 October 2013 Keywords: Friction stir channelling AA5083-H111 Bending tests Fatigue at elevated temperature
a b s t r a c t Friction stir channelling (FSC) is an innovative solid-state manufacturing technology able to produce continuous internal channels in a monolithic plate in a single step that can be used to produce conformal cooling/heating systems. During FSC a controlled amount of workpiece material flow-out from the processed zone producing the internal channel. The heat energy that softens the workpiece material is generated from dissipation during plastic deformation, internal viscous dissipation during the material flow and dissipation from frictional work between the tool and the workpiece. The purpose of this study is to investigate the fatigue behaviour of friction stir channelling specimens at elevated temperature. Fatigue tests were carried out at room temperature, 120 °C and 200 °C in a servo-hydraulic testing machine coupled with a furnace. The specimens were tested until fracture or up to 3 106 cycles. It was found that the fatigue resistance is dependent on the testing temperature. The results indicated that the fatigue life of an aluminium alloy friction stirred channel was reduced with increasing the testing temperature. Based on the fracture surfaces observations, the developing fatigue-crack always initiated at the advancing side, namely on the boundary between the nugget and the thermo-mechanically affected zone into the interior of the specimen. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Friction Stir Channelling (FSC) is an innovative process within solid-state manufacturing technologies able to produce continuous, integral channels in a monolithic plate in a single step that was proposed by Mishra as a method of manufacturing heat exchanging devices [1]. During Friction Stir Welding (FSW) a defect referred to as a ‘‘cavity’’ is generated if the processing parameters are not optimal. Arbegast [2] has discussed the formation of different defects during FSW, including ‘‘cavity’’ defects generation. The study presents a model based on the flow partition deformational zones for defect formation. The occurrence of cavities in the FSW nugget has been attributed to flow defects because of the non-optimal processing conditions or geometry of the tool features. The model applies the principle of mass balance to address void formation in the nugget. The FSC process was initially based on converting this defect formation into a manufacturing technique where all the material ⇑ Corresponding author at: Escola Superior de Tecnologia de Setúbal, Instituto Politécnico de Setúbal, Campus do IPS, Estefanilha, 2910-761 Setúbal, Portugal. Tel.: +351 218417643; fax: +351 218474045. E-mail address: [email protected] (C. Vidal). 0142-1123/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijfatigue.2013.10.012
extracted from the metal workpiece laid on the processed zone bellow the shoulder [3], within a clearance between the shoulder and the metal workpiece. Recent developments made by Vidal et al. [4], allow promoting distinct material flow, where a controlled amount of material from the metal workpiece, flow out from the processed zone producing the internal channel. Thus, the material flowing from the interior of the solid metal workpiece is not deposited on the processed surface but directed outside from the processed zone in the form of flash self-detachable or easy to extract. The position and size of the channels can therefore be controlled and the processed surface can be left at the same initial level. It is also possible to integrate in the tool, a surface finishing feature [4]. Balasubramanian et al. have discussed and demonstrated the applicability of the FSC concept to create continuous channels along linear and curved profiles, as well as the possibility of manufacturing Mini Channel Heat Exchangers (MCHX) [3]. The FSC process results from the application of the correct combination between the tool rotation direction and the orientation of the probe threads and shoulder scrolls. The features of the FS channels produced can be controlled by the processing parameters and the tool geometry. An upward action directed to the tool shoulder, along the threaded probe, combined with an outward/centrifugal
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C. Vidal et al. / International Journal of Fatigue 62 (2014) 85–92
Ω
FSC Tool
Table 1 Chemical composition of 5083-H111 aluminium alloy, % weight. Al
Mg
Mn
Fe
Cr
93.38
5.26
1.02
0.19
0.15
Table 2 Mechanical properties of 5083-H111 aluminium alloy. E (GPa)
YS (MPa)
UTS (MPa)
Elongation (%)
70.3
161
302
20
Shoulder Probe
Cylindrical probe d
Plane shoulder
t
Channel depth
Workpiece thickness Channel
Tool body
v
Fig. 3. Modular tool with internal refrigeration used during FSC trials.
Fig. 1. Schematic representation of a cross-section (above) and a plan view (below) of the friction stir channelling (FSC) process.
action along a spiral scrolled shoulder forces part of the viscous material to flow out from the processing zone. The FSC process relies, mainly, on the heat energy generated from dissipation during plastic deformation and internal viscous dissipation during the material flow, similarly to heat generation during FSW process [5], although there is frictional heat generation between the tool and the metal workpiece [6]. Fig. 1 shows a schematic representation of the FSC process. During FSC, a non-consumable rotating tool with a specific shoulder and probe profile is inserted into the solid component, where the channel will be produced, and forced to traverse along a predetermined path, creating a fine grained recrystallized microstructure around the new channel [7]. Fig. 2 shows a schematic representation of a cross-section of a FS channel where the advancing and retreating sides are identified and the main microstructural regions in the vicinity of the channel depicted. The high level of adaptability of FSC makes it possible to apply to many different technical field domains and can bring significant advantages for already existent and new industrial applications. HAZ
Meanwhile some aluminium alloys namely, 5 mm thick plates of AA6061-T6 [3], 13 mm thick plates of AA7178-T6 [7–8] and 15 mm thick plates of AA5083-H111 [9], have already been subjected to this new technological process. FSC is able to manufacture conformal cooling/heating channels for prototypes, which signifies the ability to substitute rapid prototyping in the mould production procedure. The performance of the cooling/heating channel affects the quality of the moulded parts and the productivity of the process. Moulds are subject to temperature variations during the injection moulding process. To assess the behaviour of the friction stir channels at different temperatures, namely the working temperatures of the moulds is mandatory to develop the FSC process as a conformal channel manufacturing technology. In this study the authors investigated the bending strength and the fatigue behaviour at elevated temperature of friction stir channelling specimens and it was possible to conclude that the bending strength varies with the process parameters and there is a temperature effect on the fatigue lifetime of the FSC specimens.
2. Experimental procedure 2.1. Material and specimens The non-heat treatable aluminium alloy strain hardened AA5083-H111 was used in this work. Chemical composition and
Flow arm
HAZ
TMAZ
Nugget Channel BM
BM
Advancing Side
Retreating Side
Fig. 2. Main microstructural regions in the transverse cross-section of a FS channel showing the channel, the nugget, the thermo mechanically affected zone (TMAZ), the heat affected zone (HAZ), the unaffected or base material (BM), the advancing and retreating sides’ localization.
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C. Vidal et al. / International Journal of Fatigue 62 (2014) 85–92 Table 3 Friction stir channelling parameters. Parameter
Set 1
Set 2
Set 3
Tool travel speed (mm/min.) Tool rotation speed (rpm) Vertical force Plunge speed Dwell time Process control mode
50 1000 3920 N 0.1 mm/s 3s Vertical force
50 1100
100 1100
Furnace
mechanical properties of this alloy are presented in Tables 1 and 2, respectively. Friction stir channels were produced on 15 mm thick plates, along the rolling direction, using an ESAB Legio FSW 3UL numeric control equipment. Plunge and dwell periods (v = 0) were performed under vertical position control and processing period (v > 0) was carried out under vertical force control. A patented modular concept of a FSW tool that enables internal forced refrigeration was used to perform all channels. This tool is based on three main components: body; shoulder and probe. It was selected, for all the runs, a cylindrical probe with an 8 mm diameter and left handed threads along its length and a plane shoulder with one spiral striates scrolling an angle of 360° with inner and outer diameters of 8 mm and 19 mm, respectively (Fig. 3). The probe was penetrated to a depth of 5.5 mm (without any gap between the shoulder and the workpiece). The tool tilt angle was 0° for all the runs. The tool was rotated in the counter clockwise direction. In order to obtain integral and continuous channels, after preliminary experimental tests, three sets of FSC parameters were selected and implemented as shown in Table 3. Cross section samples were prepared for macro and microscopic analysis. Samples were etched with Keller’s reagent and observed in a Leica DMI 5000M inverted optical microscope. Vickers hardness tests were performed using a Struers Duramin Vickers Hardness Testing Machine. Cross section hardness profiles were obtained according to the standard ISO 6507-1 [10], with a load of 1 kg. Specimens for bending and fatigue tests were manufactured according to the standard E 290-97a [11] as shown in Fig. 4. Bending tests were carried out in order to select the best set of FSC parameters which was used to produce the specimens to the fatigue tests. Bending tests were performed under a loading speed
Specimen
Channel
Fig. 6. Schematic representation of the experimental set up.
of 1 mm/min. Fatigue tests procedure is presented in Section 2.2. After fatigue testing the fracture surfaces of the specimens were observed and compared. 2.2. Apparatus Fatigue tests were carried out at room temperature, 120 °C and 200 °C under a sinusoidal four-point bending constant amplitude loading using an Instron 8502 servo-hydraulic testing machine 180
30
19
2
15
8
Fig. 4. Geometry and dimensions of the fatigue specimens.
h P/2
w 3w
P/2
Fig. 5. Schematic representation of a FSC specimen under four-point bending.
b
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C. Vidal et al. / International Journal of Fatigue 62 (2014) 85–92
350 300
Stress [MPa]
250 200 150 100
Set 1 (1000rpm; 50mm/min.) Set 2 (1100rpm; 50mm/min.)
50 Set 3 (1100rpm; 100mm/min.) 0 0
1
2
3
4
Displacement [mm] Fig. 7. Four-point bending tests results at room temperature for FSC parameters sets 1, 2 and 3.
with a load cell of 100 kN. A load-controlled mode was used, and a stress ratio R = 0.1 at a frequency of f = 10 Hz. The applied load was calculated by Eq. (1):
P¼
rbh2
ð1Þ
3w
which variables are described in the schematic representation presented in Fig. 5. b and h values, width and thickness respectively, are given in Fig. 4. The experimental set up was designed such that w = b according to the standard E 290-97a [11]. The specimens were tested with the processed surface under tensile stress and loads were applied out of the processed zone. Tests were performed with the maximum stress levels ranging from 30 to 125 MPa, in order to determine the traditional fatigue S–N curves and the fatigue limit rfSN. A furnace was used to heat the specimens to 120 °C and 200 °C, prior to test on the servo-hydraulic testing machine (Fig. 6). The specimens were tested until complete failure or to an endurance of 3 million cycles if there was no evidence of fatigue cracking. 3. Results and discussion 3.1. Bending tests A typical load-displacement curve for each four-point bending test was obtained. In order to compare the results, the nominal stress was calculated by Eq(2):
r¼
3P h
ð2Þ
2
where P is the load during the test and h the specimen thickness. Two four-point bending specimens were taken, and tested, from each plate friction stirred channel with FSC parameters sets 1, 2 and 3, respectively. Due to the low scattering of the results obtained for each set of parameters, it was selected a four-point bending test for each FSC parameters set as a representative test. Fig. 7 shows the four-point bending tests results at room temperature for the three sets of FSC parameters as defined in Table 3. From the analysis of Fig. 7 it is possible to verify that FSC specimens bending strength varies with the process parameters. For the same tool, when the tool rotation speed increases the maximum bending load increases. The channel area decreases with an increase in the tool rotation speed and for the same tool rotation speed increases with an increase in the tool travel speed [7]. As such, the reduction in the channel area increases the resistant section, namely the closing layer thickness (Fig. 8) increases. At higher heat indices (/X/v), the material ahead of the probe is pre-heated and softened due to frictional heat that is generated at the toolworkpiece interface which leads to the closure of the channels, thereby reducing its total area. From the four-point bending tests results and analysis, it was selected the FSC parameters Set 2 to produce the fatigue specimens; i.e. a tool travel speed of 50 mm/min. and a tool rotation speed of 1100 rpm. 3.2. Microstructure analysis Fig. 9 presents a cross section macrograph of a friction stir channel produced using Set 2 of FSC parameters. Detailed channel microstructure per zone is depicted in Fig. 9(a–d). In the macrograph presented in Fig. 9 three main regions are visible: (i) channel; (ii) stir zone (nugget), and the (iii) unprocessed base material. The channel nugget (Fig. 9a) presents a fine equiaxed recrystallized grain, with a tail heading to the shoulder periphery, at the advancing side (Fig. 9b). In details (b) and (d) of Fig. 9 it is possible to identify an additional layer surrounding the nugget, referred to as a thermo-mechanically affected zone (TMAZ). Due to tool rotation and linear movement combination, the probe shears the material from the advancing side and flows it around the retreating one, resulting in an asymmetric processed zone. It can be observed in the macrograph that the stir zone (nugget) is more extensive in the retreating side than in the advancing one. The channel geometry can be attributed to the volume of processed material that it is displaced out of the visco-plasticized region by the FSC tool per unit of rotation and also the compacting force that is applied on the channel top during the travel forward movement by the rotating shoulder. Analysing the macrograph presented in Fig. 9 is it possible to conclude that the nugget diameter and height are equal to the cylindrical probe diameter and length used to perform the channel. 3.3. Hardness profile
Closing layer thickness
Advancing side
Channel area
Retreating side
Shear angle Fig. 8. Schematic representation of a friction stir channel cross-section view.
Fig. 10 shows the hardness profile measured in the same crosssection presented in Fig. 9. From Fig. 10 it is possible to conclude that FSC process changes the material hardness in the vicinity of the channel. The base material is a non-heat treatable alloy which is very sensitive to strain hardening. The hardness increase is most significant in thermomechanically affected zone with emphasis for the channel nugget which grain refinement contributes to the increase of hardness. The higher values were measured below the channel inside the nugget. On the advancing side and outside the nugget the hardness values are similar to the base material one which is about 92 HV1.
C. Vidal et al. / International Journal of Fatigue 62 (2014) 85–92
(a)
(b)
(c)
(d)
89
Fig. 9. Metallographic results of a cross section of a friction stir channel.
Fig. 10. Hardness profile across the FSC processed zone.
3.4. Fatigue tests
rmax as the independent variable and r as the correlation coefficient. Results are presented in Table 4.
S–N curves were obtained by measuring the number of cycles to failure that the specimen supported under a sinusoidal waveform as described in Section 2.2. The S–N fatigue curves are presented in log–log scale. Fig. 11 shows the results obtained for FSC specimens at three different testing temperatures. In Fig. 12 is presented a comparison between base material and FSC specimens at room temperature. Each point represents an experimental test and the lines were obtained by fitting a linear regression Eq(3), assuming
N r rm max ¼ K 0
ð3Þ
The scatter of the results is small (Table 4), and the line for the room temperature lies above the results for 120 °C and 200 °C (Fig. 11). The fatigue tests at elevated temperature show that there is a testing temperature effect on the FSC specimens’ strength. From the analysis of Fig. 12 it is possible to verify that base
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C. Vidal et al. / International Journal of Fatigue 62 (2014) 85–92
160 Room temperature 120 ºC 200 ºC
σmax [MPa]
80
40
20 1.00E+04
1.00E+05
1.00E+06
1.00E+07
Fatigue life N, cycles Fig. 11. S–N fatigue curves at R = 0.1 for FSC specimens at room temperature, 120 °C and 200 °C. Four-point bending.
160 FSChannelling
σ máx [MPa]
Base material
80
40 1.00E+04
1.00E+05
1.00E+06
1.00E+07
Fatigue life N, cycles Fig. 12. S–N fatigue curves at R = 0.1 for base material and FSC specimens at room temperature. Four-point bending.
Table 4 Equations’ parameters of the mean fatigue S–N curves presented. m BM at Room temperature FSC at Room temperature 120 °C 200 °C
3.911 4.720 3.650 3.133
K0
r 12
6.11 10 1.26 1014 6.70 1011 4.33 1010
0.9842 0.9947 0.9979 0.9826
material has higher fatigue strength at higher stress values in comparison with the FSC specimens, at room temperature. For the same value of the maximum stress, fatigue life decreases with the testing temperature as shown in Fig. 13. Table 5 summarizes the fatigue limit values determined by extrapolating the S–N curves down to the specified cycle number 3 106. The results indicate that the fatigue limit for the room temperature is close to 40 MPa, for the 120 °C closes to 30 MPa and for the 200 °C condition is 20 MPa. The experimental tests confirm the value obtained for a testing temperature of 120 °C but show that, at
room temperature, there is no evidence of fatigue cracking in a FSC specimen tested at a maximum stress of 50 MPa. At room temperature, for the base material tested, a friction stir channel does not change the fatigue limit.
3.5. Fracture surfaces In order to show the most common fatigue initiation locals, crack propagation and final fracture types, some of the most important fracture surfaces are shown in Fig. 14 and Fig. 15. A schematic representation of a FSC fatigue specimen fracture surface is presented in Fig. 16. The developing fatigue-crack always initiated at the advancing side, namely on the boundary between the nugget and the thermo-mechanically affected zone into the interior of the specimen. The crack propagates through the channel nugget with a path tangential to the advancing side. After the crack reaches the processed surface a second crack initiates at the corner between the advancing side and the channel bottom. The fracture surfaces show a semi-elliptical shape crack front. This second crack propagates
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C. Vidal et al. / International Journal of Fatigue 62 (2014) 85–92
200
100MPa 65MPa 50MPa
Temperature [ºC]
160
120 T 50(N) = -0.0003N + 248.9 r² = 0.9964 80
40 T100 (N) = -0.0064N + 359.62 r² = 0.8624
T 65(N) = -0.0006N + 228.66 r² = 0.9049
0 1.00E+04
1.00E+05
1.00E+06
Fatigue life N, cycles Fig. 13. Temperature versus fatigue life at R = 0.1 for FSC specimens under a maximum stress of 100 MPa, 65 MPa and 50 MPa.
Table 5 Fatigue limit values at room temperature, 120 °C and 200 °C.
rfSN (MPa) MB at Room temperature FSC at Room temperature 120 °C 200 °C
41 41 29 21
uniformly through the base material. The fracture surfaces show a semi-elliptical shape crack front. The fracture process is mainly striation dominated with an increase in surface roughness, as the crack increase. Three different zones are observed in most of the fracture surfaces. The fast-fracture surface roughness is considerably lower at a testing temperature of 200 °C.
Room temperature σmax=65MPa R=0.1 N= 385977cycles
120ºC σmax=65MPa R=0.1 N= 152389cycles
200ºC σmax=65MPa R=0.1 N= 80924cycles
Fig. 14. Macros of fracture surfaces of FSC fatigue specimens tested at room temperature, 120 °C and 200 °C. rmax = 65 MPa.
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C. Vidal et al. / International Journal of Fatigue 62 (2014) 85–92
120ºC σmax=50MPa R=0.1 N= 433096cycles
200ºC σmax=50MPa R=0.1 N= 177173cycles
Fig. 15. Macros of fracture surfaces of FSC fatigue specimens tested at 120 °C and 200 °C. rmax = 50 MPa.
Acknowledgements
Nugget Channel Zone I Zone II Fast-fracture surface Fig. 16. Schematic representation of a FSC fatigue specimen fracture surface.
4. Conclusions With this study the authors investigated the fatigue behaviour at elevated temperature and the influence of friction stir channelling parameters on the bending strength of specimens with a transversal channel produced by FSC. This work shows an innovative process, based on the friction stir welding principles, of manufacturing conformal channels in monolithic plates of AA5083-H111 in a single step. It was observed a temperature effect on the fatigue lifetime of FSC specimens. For the same maximum stress value, fatigue life decreases with the increase of the testing temperature. The results point to a fatigue limit at room temperature about 50 MPa and at 120 °C about 30 MPa. It would be interesting to investigate the lifetime of the studied specimens at 200 °C. The FSC specimens bending strength varies with the process parameters. For the same tool when the tool rotation speed increases the maximum bending load increases. The channel area decreases with an increase in the tool rotation speed and for the same tool rotation speed increases with an increase in the tool travel speed. As such, the reduction in the channel area increases the resistant section, namely the closing layer thickness increases. All specimens fractured at the advancing side, namely in the boundary between the nugget and the thermo-mechanically affected zone.
The authors would like to acknowledge Portuguese Foundation for the Science and Technology (FCT) for its financial support through the PhD scholarship FCT SFRH/BD/62963/2009. References [1] Mishra, R. S., Patent No. US 6923362 B2. United States (2005). [2] Arbegast W. A flow-partitioned deformation zone model for defect formation during friction stir welding. Scripta Materialia 2008;58:372–6. http:// dx.doi.org/10.1016/j.scriptamat.2007.10.031. [3] Balasubramanian N et al. Friction stir channeling: characterization of the channels. J Mater Process Technol 2009;209:3696–704. http://dx.doi.org/ 10.1016/j.jmatprotec.2008.08.036. [4] Vidal, C., Vilaça, P., Processo de abertura de canais internos contínuos em componentes maciços sem alteração da cota de superfície processada e respectiva ferramenta modular ajustável. Patent No. PT 105628 (2011, April 15). [5] Vilaça P, Quintino L, Santos JD, Zettler R, Sheikhi S. Quality assessment of friction stir welding joints via analytical thermal model. Mater Sci Eng A 2007;445–446:501–8. http://dx.doi.org/10.1016/j.msea.2006.09.091. [6] Balasubramanian N, Mishra R, Krishnamurthy K. Process forces during friction stir channeling in an aluminum alloy. J Mater Process Technol 2011;211:305–11. http://dx.doi.org/10.1016/j.jmatprotec.2010.10.005. [7] Vidal C, Infante V, Vilaça P. Mechanical characterization of friction stir channels under internal pressure and in-plane bending. Key Eng Mater 2012;488–489:105–8. http://dx.doi.org/10.4028/www.scientific.net/ KEM.488-489.105. [8] Vidal C, Infante V, Vilaça P. Metallographic characterization of friction stir channels. Mater Sci Forum 2013;730–732:817–22. http://dx.doi.org/10.4028/ www.scientific.net/MSF.730-732.817. [9] Vidal, C., Infante, V., Vilaça, P., Assessment of Performance Parameters for Friction Stir Channelling, Proceedings of the IIW 2011 International Conference on Global Trends in Joining, Cutting and Surfacing Technology, Chennai, India, 21–22 July, 2011, ISBN 978-81-8487-152-4, paper IC_99. [10] ISO 6507–1, Mettalic materials – Vickers hardness test – Part 1: Test Method, 1997. [11] E 290–97a, Standard Test Methods for Bend Testing of Material for Ductility, American Society for Testing and Materials, 1998.
Contents lists available at ScienceDirect
International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue
Fatigue behaviour at elevated temperature of friction stir channelling solid plates of AA5083-H111 aluminium alloy Catarina Vidal a,b,⇑, Virgínia Infante a,d, Pedro Vilaça c,d a
ICEMS – Institute of Materials and Surfaces Science and Engineering, Av. Rovisco Pais, 1049-001 Lisbon, Portugal Escola Superior de Tecnologia de Setúbal, Instituto Politécnico de Setúbal, Campus do IPS, Estefanilha, 2910-761 Setúbal, Portugal c IDMEC – Institute of Mechanical Engineering, Av. Rovisco Pais, 1049-001 Lisbon, Portugal d Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal b
a r t i c l e
i n f o
Article history: Received 31 October 2012 Received in revised form 6 February 2013 Accepted 10 October 2013 Available online 22 October 2013 Keywords: Friction stir channelling AA5083-H111 Bending tests Fatigue at elevated temperature
a b s t r a c t Friction stir channelling (FSC) is an innovative solid-state manufacturing technology able to produce continuous internal channels in a monolithic plate in a single step that can be used to produce conformal cooling/heating systems. During FSC a controlled amount of workpiece material flow-out from the processed zone producing the internal channel. The heat energy that softens the workpiece material is generated from dissipation during plastic deformation, internal viscous dissipation during the material flow and dissipation from frictional work between the tool and the workpiece. The purpose of this study is to investigate the fatigue behaviour of friction stir channelling specimens at elevated temperature. Fatigue tests were carried out at room temperature, 120 °C and 200 °C in a servo-hydraulic testing machine coupled with a furnace. The specimens were tested until fracture or up to 3 106 cycles. It was found that the fatigue resistance is dependent on the testing temperature. The results indicated that the fatigue life of an aluminium alloy friction stirred channel was reduced with increasing the testing temperature. Based on the fracture surfaces observations, the developing fatigue-crack always initiated at the advancing side, namely on the boundary between the nugget and the thermo-mechanically affected zone into the interior of the specimen. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Friction Stir Channelling (FSC) is an innovative process within solid-state manufacturing technologies able to produce continuous, integral channels in a monolithic plate in a single step that was proposed by Mishra as a method of manufacturing heat exchanging devices [1]. During Friction Stir Welding (FSW) a defect referred to as a ‘‘cavity’’ is generated if the processing parameters are not optimal. Arbegast [2] has discussed the formation of different defects during FSW, including ‘‘cavity’’ defects generation. The study presents a model based on the flow partition deformational zones for defect formation. The occurrence of cavities in the FSW nugget has been attributed to flow defects because of the non-optimal processing conditions or geometry of the tool features. The model applies the principle of mass balance to address void formation in the nugget. The FSC process was initially based on converting this defect formation into a manufacturing technique where all the material ⇑ Corresponding author at: Escola Superior de Tecnologia de Setúbal, Instituto Politécnico de Setúbal, Campus do IPS, Estefanilha, 2910-761 Setúbal, Portugal. Tel.: +351 218417643; fax: +351 218474045. E-mail address: [email protected] (C. Vidal). 0142-1123/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijfatigue.2013.10.012
extracted from the metal workpiece laid on the processed zone bellow the shoulder [3], within a clearance between the shoulder and the metal workpiece. Recent developments made by Vidal et al. [4], allow promoting distinct material flow, where a controlled amount of material from the metal workpiece, flow out from the processed zone producing the internal channel. Thus, the material flowing from the interior of the solid metal workpiece is not deposited on the processed surface but directed outside from the processed zone in the form of flash self-detachable or easy to extract. The position and size of the channels can therefore be controlled and the processed surface can be left at the same initial level. It is also possible to integrate in the tool, a surface finishing feature [4]. Balasubramanian et al. have discussed and demonstrated the applicability of the FSC concept to create continuous channels along linear and curved profiles, as well as the possibility of manufacturing Mini Channel Heat Exchangers (MCHX) [3]. The FSC process results from the application of the correct combination between the tool rotation direction and the orientation of the probe threads and shoulder scrolls. The features of the FS channels produced can be controlled by the processing parameters and the tool geometry. An upward action directed to the tool shoulder, along the threaded probe, combined with an outward/centrifugal
86
C. Vidal et al. / International Journal of Fatigue 62 (2014) 85–92
Ω
FSC Tool
Table 1 Chemical composition of 5083-H111 aluminium alloy, % weight. Al
Mg
Mn
Fe
Cr
93.38
5.26
1.02
0.19
0.15
Table 2 Mechanical properties of 5083-H111 aluminium alloy. E (GPa)
YS (MPa)
UTS (MPa)
Elongation (%)
70.3
161
302
20
Shoulder Probe
Cylindrical probe d
Plane shoulder
t
Channel depth
Workpiece thickness Channel
Tool body
v
Fig. 3. Modular tool with internal refrigeration used during FSC trials.
Fig. 1. Schematic representation of a cross-section (above) and a plan view (below) of the friction stir channelling (FSC) process.
action along a spiral scrolled shoulder forces part of the viscous material to flow out from the processing zone. The FSC process relies, mainly, on the heat energy generated from dissipation during plastic deformation and internal viscous dissipation during the material flow, similarly to heat generation during FSW process [5], although there is frictional heat generation between the tool and the metal workpiece [6]. Fig. 1 shows a schematic representation of the FSC process. During FSC, a non-consumable rotating tool with a specific shoulder and probe profile is inserted into the solid component, where the channel will be produced, and forced to traverse along a predetermined path, creating a fine grained recrystallized microstructure around the new channel [7]. Fig. 2 shows a schematic representation of a cross-section of a FS channel where the advancing and retreating sides are identified and the main microstructural regions in the vicinity of the channel depicted. The high level of adaptability of FSC makes it possible to apply to many different technical field domains and can bring significant advantages for already existent and new industrial applications. HAZ
Meanwhile some aluminium alloys namely, 5 mm thick plates of AA6061-T6 [3], 13 mm thick plates of AA7178-T6 [7–8] and 15 mm thick plates of AA5083-H111 [9], have already been subjected to this new technological process. FSC is able to manufacture conformal cooling/heating channels for prototypes, which signifies the ability to substitute rapid prototyping in the mould production procedure. The performance of the cooling/heating channel affects the quality of the moulded parts and the productivity of the process. Moulds are subject to temperature variations during the injection moulding process. To assess the behaviour of the friction stir channels at different temperatures, namely the working temperatures of the moulds is mandatory to develop the FSC process as a conformal channel manufacturing technology. In this study the authors investigated the bending strength and the fatigue behaviour at elevated temperature of friction stir channelling specimens and it was possible to conclude that the bending strength varies with the process parameters and there is a temperature effect on the fatigue lifetime of the FSC specimens.
2. Experimental procedure 2.1. Material and specimens The non-heat treatable aluminium alloy strain hardened AA5083-H111 was used in this work. Chemical composition and
Flow arm
HAZ
TMAZ
Nugget Channel BM
BM
Advancing Side
Retreating Side
Fig. 2. Main microstructural regions in the transverse cross-section of a FS channel showing the channel, the nugget, the thermo mechanically affected zone (TMAZ), the heat affected zone (HAZ), the unaffected or base material (BM), the advancing and retreating sides’ localization.
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C. Vidal et al. / International Journal of Fatigue 62 (2014) 85–92 Table 3 Friction stir channelling parameters. Parameter
Set 1
Set 2
Set 3
Tool travel speed (mm/min.) Tool rotation speed (rpm) Vertical force Plunge speed Dwell time Process control mode
50 1000 3920 N 0.1 mm/s 3s Vertical force
50 1100
100 1100
Furnace
mechanical properties of this alloy are presented in Tables 1 and 2, respectively. Friction stir channels were produced on 15 mm thick plates, along the rolling direction, using an ESAB Legio FSW 3UL numeric control equipment. Plunge and dwell periods (v = 0) were performed under vertical position control and processing period (v > 0) was carried out under vertical force control. A patented modular concept of a FSW tool that enables internal forced refrigeration was used to perform all channels. This tool is based on three main components: body; shoulder and probe. It was selected, for all the runs, a cylindrical probe with an 8 mm diameter and left handed threads along its length and a plane shoulder with one spiral striates scrolling an angle of 360° with inner and outer diameters of 8 mm and 19 mm, respectively (Fig. 3). The probe was penetrated to a depth of 5.5 mm (without any gap between the shoulder and the workpiece). The tool tilt angle was 0° for all the runs. The tool was rotated in the counter clockwise direction. In order to obtain integral and continuous channels, after preliminary experimental tests, three sets of FSC parameters were selected and implemented as shown in Table 3. Cross section samples were prepared for macro and microscopic analysis. Samples were etched with Keller’s reagent and observed in a Leica DMI 5000M inverted optical microscope. Vickers hardness tests were performed using a Struers Duramin Vickers Hardness Testing Machine. Cross section hardness profiles were obtained according to the standard ISO 6507-1 [10], with a load of 1 kg. Specimens for bending and fatigue tests were manufactured according to the standard E 290-97a [11] as shown in Fig. 4. Bending tests were carried out in order to select the best set of FSC parameters which was used to produce the specimens to the fatigue tests. Bending tests were performed under a loading speed
Specimen
Channel
Fig. 6. Schematic representation of the experimental set up.
of 1 mm/min. Fatigue tests procedure is presented in Section 2.2. After fatigue testing the fracture surfaces of the specimens were observed and compared. 2.2. Apparatus Fatigue tests were carried out at room temperature, 120 °C and 200 °C under a sinusoidal four-point bending constant amplitude loading using an Instron 8502 servo-hydraulic testing machine 180
30
19
2
15
8
Fig. 4. Geometry and dimensions of the fatigue specimens.
h P/2
w 3w
P/2
Fig. 5. Schematic representation of a FSC specimen under four-point bending.
b
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350 300
Stress [MPa]
250 200 150 100
Set 1 (1000rpm; 50mm/min.) Set 2 (1100rpm; 50mm/min.)
50 Set 3 (1100rpm; 100mm/min.) 0 0
1
2
3
4
Displacement [mm] Fig. 7. Four-point bending tests results at room temperature for FSC parameters sets 1, 2 and 3.
with a load cell of 100 kN. A load-controlled mode was used, and a stress ratio R = 0.1 at a frequency of f = 10 Hz. The applied load was calculated by Eq. (1):
P¼
rbh2
ð1Þ
3w
which variables are described in the schematic representation presented in Fig. 5. b and h values, width and thickness respectively, are given in Fig. 4. The experimental set up was designed such that w = b according to the standard E 290-97a [11]. The specimens were tested with the processed surface under tensile stress and loads were applied out of the processed zone. Tests were performed with the maximum stress levels ranging from 30 to 125 MPa, in order to determine the traditional fatigue S–N curves and the fatigue limit rfSN. A furnace was used to heat the specimens to 120 °C and 200 °C, prior to test on the servo-hydraulic testing machine (Fig. 6). The specimens were tested until complete failure or to an endurance of 3 million cycles if there was no evidence of fatigue cracking. 3. Results and discussion 3.1. Bending tests A typical load-displacement curve for each four-point bending test was obtained. In order to compare the results, the nominal stress was calculated by Eq(2):
r¼
3P h
ð2Þ
2
where P is the load during the test and h the specimen thickness. Two four-point bending specimens were taken, and tested, from each plate friction stirred channel with FSC parameters sets 1, 2 and 3, respectively. Due to the low scattering of the results obtained for each set of parameters, it was selected a four-point bending test for each FSC parameters set as a representative test. Fig. 7 shows the four-point bending tests results at room temperature for the three sets of FSC parameters as defined in Table 3. From the analysis of Fig. 7 it is possible to verify that FSC specimens bending strength varies with the process parameters. For the same tool, when the tool rotation speed increases the maximum bending load increases. The channel area decreases with an increase in the tool rotation speed and for the same tool rotation speed increases with an increase in the tool travel speed [7]. As such, the reduction in the channel area increases the resistant section, namely the closing layer thickness (Fig. 8) increases. At higher heat indices (/X/v), the material ahead of the probe is pre-heated and softened due to frictional heat that is generated at the toolworkpiece interface which leads to the closure of the channels, thereby reducing its total area. From the four-point bending tests results and analysis, it was selected the FSC parameters Set 2 to produce the fatigue specimens; i.e. a tool travel speed of 50 mm/min. and a tool rotation speed of 1100 rpm. 3.2. Microstructure analysis Fig. 9 presents a cross section macrograph of a friction stir channel produced using Set 2 of FSC parameters. Detailed channel microstructure per zone is depicted in Fig. 9(a–d). In the macrograph presented in Fig. 9 three main regions are visible: (i) channel; (ii) stir zone (nugget), and the (iii) unprocessed base material. The channel nugget (Fig. 9a) presents a fine equiaxed recrystallized grain, with a tail heading to the shoulder periphery, at the advancing side (Fig. 9b). In details (b) and (d) of Fig. 9 it is possible to identify an additional layer surrounding the nugget, referred to as a thermo-mechanically affected zone (TMAZ). Due to tool rotation and linear movement combination, the probe shears the material from the advancing side and flows it around the retreating one, resulting in an asymmetric processed zone. It can be observed in the macrograph that the stir zone (nugget) is more extensive in the retreating side than in the advancing one. The channel geometry can be attributed to the volume of processed material that it is displaced out of the visco-plasticized region by the FSC tool per unit of rotation and also the compacting force that is applied on the channel top during the travel forward movement by the rotating shoulder. Analysing the macrograph presented in Fig. 9 is it possible to conclude that the nugget diameter and height are equal to the cylindrical probe diameter and length used to perform the channel. 3.3. Hardness profile
Closing layer thickness
Advancing side
Channel area
Retreating side
Shear angle Fig. 8. Schematic representation of a friction stir channel cross-section view.
Fig. 10 shows the hardness profile measured in the same crosssection presented in Fig. 9. From Fig. 10 it is possible to conclude that FSC process changes the material hardness in the vicinity of the channel. The base material is a non-heat treatable alloy which is very sensitive to strain hardening. The hardness increase is most significant in thermomechanically affected zone with emphasis for the channel nugget which grain refinement contributes to the increase of hardness. The higher values were measured below the channel inside the nugget. On the advancing side and outside the nugget the hardness values are similar to the base material one which is about 92 HV1.
C. Vidal et al. / International Journal of Fatigue 62 (2014) 85–92
(a)
(b)
(c)
(d)
89
Fig. 9. Metallographic results of a cross section of a friction stir channel.
Fig. 10. Hardness profile across the FSC processed zone.
3.4. Fatigue tests
rmax as the independent variable and r as the correlation coefficient. Results are presented in Table 4.
S–N curves were obtained by measuring the number of cycles to failure that the specimen supported under a sinusoidal waveform as described in Section 2.2. The S–N fatigue curves are presented in log–log scale. Fig. 11 shows the results obtained for FSC specimens at three different testing temperatures. In Fig. 12 is presented a comparison between base material and FSC specimens at room temperature. Each point represents an experimental test and the lines were obtained by fitting a linear regression Eq(3), assuming
N r rm max ¼ K 0
ð3Þ
The scatter of the results is small (Table 4), and the line for the room temperature lies above the results for 120 °C and 200 °C (Fig. 11). The fatigue tests at elevated temperature show that there is a testing temperature effect on the FSC specimens’ strength. From the analysis of Fig. 12 it is possible to verify that base
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160 Room temperature 120 ºC 200 ºC
σmax [MPa]
80
40
20 1.00E+04
1.00E+05
1.00E+06
1.00E+07
Fatigue life N, cycles Fig. 11. S–N fatigue curves at R = 0.1 for FSC specimens at room temperature, 120 °C and 200 °C. Four-point bending.
160 FSChannelling
σ máx [MPa]
Base material
80
40 1.00E+04
1.00E+05
1.00E+06
1.00E+07
Fatigue life N, cycles Fig. 12. S–N fatigue curves at R = 0.1 for base material and FSC specimens at room temperature. Four-point bending.
Table 4 Equations’ parameters of the mean fatigue S–N curves presented. m BM at Room temperature FSC at Room temperature 120 °C 200 °C
3.911 4.720 3.650 3.133
K0
r 12
6.11 10 1.26 1014 6.70 1011 4.33 1010
0.9842 0.9947 0.9979 0.9826
material has higher fatigue strength at higher stress values in comparison with the FSC specimens, at room temperature. For the same value of the maximum stress, fatigue life decreases with the testing temperature as shown in Fig. 13. Table 5 summarizes the fatigue limit values determined by extrapolating the S–N curves down to the specified cycle number 3 106. The results indicate that the fatigue limit for the room temperature is close to 40 MPa, for the 120 °C closes to 30 MPa and for the 200 °C condition is 20 MPa. The experimental tests confirm the value obtained for a testing temperature of 120 °C but show that, at
room temperature, there is no evidence of fatigue cracking in a FSC specimen tested at a maximum stress of 50 MPa. At room temperature, for the base material tested, a friction stir channel does not change the fatigue limit.
3.5. Fracture surfaces In order to show the most common fatigue initiation locals, crack propagation and final fracture types, some of the most important fracture surfaces are shown in Fig. 14 and Fig. 15. A schematic representation of a FSC fatigue specimen fracture surface is presented in Fig. 16. The developing fatigue-crack always initiated at the advancing side, namely on the boundary between the nugget and the thermo-mechanically affected zone into the interior of the specimen. The crack propagates through the channel nugget with a path tangential to the advancing side. After the crack reaches the processed surface a second crack initiates at the corner between the advancing side and the channel bottom. The fracture surfaces show a semi-elliptical shape crack front. This second crack propagates
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200
100MPa 65MPa 50MPa
Temperature [ºC]
160
120 T 50(N) = -0.0003N + 248.9 r² = 0.9964 80
40 T100 (N) = -0.0064N + 359.62 r² = 0.8624
T 65(N) = -0.0006N + 228.66 r² = 0.9049
0 1.00E+04
1.00E+05
1.00E+06
Fatigue life N, cycles Fig. 13. Temperature versus fatigue life at R = 0.1 for FSC specimens under a maximum stress of 100 MPa, 65 MPa and 50 MPa.
Table 5 Fatigue limit values at room temperature, 120 °C and 200 °C.
rfSN (MPa) MB at Room temperature FSC at Room temperature 120 °C 200 °C
41 41 29 21
uniformly through the base material. The fracture surfaces show a semi-elliptical shape crack front. The fracture process is mainly striation dominated with an increase in surface roughness, as the crack increase. Three different zones are observed in most of the fracture surfaces. The fast-fracture surface roughness is considerably lower at a testing temperature of 200 °C.
Room temperature σmax=65MPa R=0.1 N= 385977cycles
120ºC σmax=65MPa R=0.1 N= 152389cycles
200ºC σmax=65MPa R=0.1 N= 80924cycles
Fig. 14. Macros of fracture surfaces of FSC fatigue specimens tested at room temperature, 120 °C and 200 °C. rmax = 65 MPa.
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120ºC σmax=50MPa R=0.1 N= 433096cycles
200ºC σmax=50MPa R=0.1 N= 177173cycles
Fig. 15. Macros of fracture surfaces of FSC fatigue specimens tested at 120 °C and 200 °C. rmax = 50 MPa.
Acknowledgements
Nugget Channel Zone I Zone II Fast-fracture surface Fig. 16. Schematic representation of a FSC fatigue specimen fracture surface.
4. Conclusions With this study the authors investigated the fatigue behaviour at elevated temperature and the influence of friction stir channelling parameters on the bending strength of specimens with a transversal channel produced by FSC. This work shows an innovative process, based on the friction stir welding principles, of manufacturing conformal channels in monolithic plates of AA5083-H111 in a single step. It was observed a temperature effect on the fatigue lifetime of FSC specimens. For the same maximum stress value, fatigue life decreases with the increase of the testing temperature. The results point to a fatigue limit at room temperature about 50 MPa and at 120 °C about 30 MPa. It would be interesting to investigate the lifetime of the studied specimens at 200 °C. The FSC specimens bending strength varies with the process parameters. For the same tool when the tool rotation speed increases the maximum bending load increases. The channel area decreases with an increase in the tool rotation speed and for the same tool rotation speed increases with an increase in the tool travel speed. As such, the reduction in the channel area increases the resistant section, namely the closing layer thickness increases. All specimens fractured at the advancing side, namely in the boundary between the nugget and the thermo-mechanically affected zone.
The authors would like to acknowledge Portuguese Foundation for the Science and Technology (FCT) for its financial support through the PhD scholarship FCT SFRH/BD/62963/2009. References [1] Mishra, R. S., Patent No. US 6923362 B2. United States (2005). [2] Arbegast W. A flow-partitioned deformation zone model for defect formation during friction stir welding. Scripta Materialia 2008;58:372–6. http:// dx.doi.org/10.1016/j.scriptamat.2007.10.031. [3] Balasubramanian N et al. Friction stir channeling: characterization of the channels. J Mater Process Technol 2009;209:3696–704. http://dx.doi.org/ 10.1016/j.jmatprotec.2008.08.036. [4] Vidal, C., Vilaça, P., Processo de abertura de canais internos contínuos em componentes maciços sem alteração da cota de superfície processada e respectiva ferramenta modular ajustável. Patent No. PT 105628 (2011, April 15). [5] Vilaça P, Quintino L, Santos JD, Zettler R, Sheikhi S. Quality assessment of friction stir welding joints via analytical thermal model. Mater Sci Eng A 2007;445–446:501–8. http://dx.doi.org/10.1016/j.msea.2006.09.091. [6] Balasubramanian N, Mishra R, Krishnamurthy K. Process forces during friction stir channeling in an aluminum alloy. J Mater Process Technol 2011;211:305–11. http://dx.doi.org/10.1016/j.jmatprotec.2010.10.005. [7] Vidal C, Infante V, Vilaça P. Mechanical characterization of friction stir channels under internal pressure and in-plane bending. Key Eng Mater 2012;488–489:105–8. http://dx.doi.org/10.4028/www.scientific.net/ KEM.488-489.105. [8] Vidal C, Infante V, Vilaça P. Metallographic characterization of friction stir channels. Mater Sci Forum 2013;730–732:817–22. http://dx.doi.org/10.4028/ www.scientific.net/MSF.730-732.817. [9] Vidal, C., Infante, V., Vilaça, P., Assessment of Performance Parameters for Friction Stir Channelling, Proceedings of the IIW 2011 International Conference on Global Trends in Joining, Cutting and Surfacing Technology, Chennai, India, 21–22 July, 2011, ISBN 978-81-8487-152-4, paper IC_99. [10] ISO 6507–1, Mettalic materials – Vickers hardness test – Part 1: Test Method, 1997. [11] E 290–97a, Standard Test Methods for Bend Testing of Material for Ductility, American Society for Testing and Materials, 1998.