Effect of friction stir processing conditions on fatigue behavior and texture development in A356-T6 cast aluminum alloy

International Journal of Fatigue 80 (2015) 192–202

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International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue

Effect of friction stir processing conditions on fatigue behavior and texture development in A356-T6 cast aluminum alloy Akiko Tajiri a, Yoshihiko Uematsu b,⇑, Toshifumi Kakiuchi b, Yasunari Tozaki c, Yosuke Suzuki d, Angga Afrinaldi e a

Murata Machinery Ltd., 2 Nakajima, Hashizume, Inuyama, Aichi 484-8502, Japan Department of Mechanical Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan Industrial Research Institute of Gifu Prefecture, 1288 Oze, Seki, Gifu 501-3265, Japan d Okuma Corp., Oguchi-cho, Niwa-gun, Aichi 480-0193, Japan e Mechanical Engineering, Andalas University, Kampus Limau Manis, Padang 25163, Indonesia b c

a r t i c l e

i n f o

Article history: Received 30 January 2015 Received in revised form 28 May 2015 Accepted 4 June 2015 Available online 12 June 2015 Keywords: Cast aluminum alloy Friction stir processing Fatigue crack initiation Fatigue crack path Texture

a b s t r a c t Friction stir processing (FSP) was applied to A356-T6 cast aluminum alloy to modify the microstructure and to eliminate casting defects under two different tool rotational speeds. Plane bending fatigue tests had been conducted, revealing that FSP could enhance the fatigue strength where the lower rotational speed condition gave better results. The enhancement of fatigue strength was attributed to the elimination of casting defects. Crystallographic analysis by EBSD revealed that the texture induced by FSP had detrimental effect on growth resistance. The lower rotational speed condition resulted in the weaker texture, and consequently, further increase of fatigue strength was achieved compared with the higher rotational speed condition. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction A356-T6 cast aluminum (Al) alloy is a light weight structural material, and widely used for mechanical components especially in transportation industries because near-net-shape manufacturing is easier than wrought Al alloys. To use this alloy for mechanical components, understanding of fatigue performance is mandatory. There have been many studies on fatigue performances and fatigue fracture mechanisms of this alloy, revealing that fatigue crack very frequently initiated and propagated from casting defects leading to final fracture [1–6]. Consequently, it is important to eliminate casting defects to improve fatigue properties of cast Al alloys. Friction stir welding (FSW) had been developed as a butt joining technique of light weight alloy plates with low melting points such as Al and magnesium (Mg) alloys [7,8]. In this joining procedure, a rotating tool consisting of shoulder and probe is plunged into the butt surfaces and travels along joint line. Severe plastic deformation around the probe results in the joining of two plates. In addition, it is well known that grain refinement occurs in the weld zone due to the dynamic recrystallization [7,8]. In a friction stir processing (FSP) technique, a rotating tool is plunged into a bulk plate just ⇑ Corresponding author. Tel.: +81 58 293 2501; fax: +81 58 293 2491. E-mail address: [email protected] (Y. Uematsu). http://dx.doi.org/10.1016/j.ijfatigue.2015.06.001 0142-1123/Ó 2015 Elsevier Ltd. All rights reserved.

for the microstructural modification induced by severe plastic deformation. FSP had been applied to some cast Al alloys, revealing that casting defects were successfully eliminated by the severe plastic deformation [9–12]. For the application of FSPed cast Al alloys for structural components, it is important to understand fatigue behavior from the view point of reliability. The effects of FSP on fatigue behavior in cast Al alloys had been investigated and it was revealed that fatigue strengths [13–15] and small fatigue crack growth resistance [16] were improved by the elimination of casting defects and microstructural modification. In the previous studies [10,11], the authors had shown that the fatigue strength of A356-T6 was improved by the application of FSP, but the fatigue crack path was strongly affected by the texture induced by FSP. It indicates that the fatigue properties could be affected by the texture because fatigue is sensitive to the microstructure. Furthermore, the development of texture during severe plastic deformation should be strongly dependent on FSP conditions, while the effect of FSP conditions on fatigue fracture mechanism is unclear from the view point of texture and crystallographic orientations. In this study, FSP was applied to A356-T6 under two different processing conditions, where the tool rotational speed was changed at the fixed tool traveling speed. Subsequently, fully reversed plane bending fatigue tests had been performed to investigate

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(a)

(b)

100μm

300μm

(c)

10μm Fig. 1. Microstructure of as-received material: (a) macroscopic appearance, (b) magnified view of the rectangular area in figure (a) showing casting defect. (c) Magnified view showing eutectic Si particles.

R34.02

t=4

10.5×2

5

18

9

40

20

4

120

Fig. 2. Fatigue specimen configuration.

the effect of processing conditions on the crack initiation and growth behavior. As mentioned above, FSP will induce texture depending on the processing parameters, thus electron back scattered diffraction (EBSD) technique was used to figure out the effect of texture on fatigue behavior. 2. Experimental details

Specimen blanks with the thickness of 5 mm were cut from the ingots and subsequently friction stir processed (FSPed), from which fatigue specimens (Fig. 2) were machined. The FSP direction corresponds to the longitudinal direction of the specimen. To remove stress concentration sites, both the upper and lower surfaces were removed 0.3 and 0.7 mm in depth, respectively. Before fatigue test, the surface of the gauge section was mechanically polished using #2000 grade emery paper followed by buff-finishing.

2.1. Materials and specimen configuration 2.2. FSP conditions The material used is A356-T6 cast Al alloy, whose chemical composition (wt.%) is Si: 6.66, Mg: 0.383, Fe: 0.153, Ni: 0.009, Cr: 0.002, Sn: 0.002, Al: balance. The size of the as-received ingot is 190, 33 and 39 mm in length, width and height, respectively, according to the ingot geometry in Japanese Industrial Standard (JIS) H 5202. The ingots had been T6 heat treated where the solution treatment at 808 K for 4 h followed by the water quenching and aging at 453 K for 3 h. The microstructure of as-received material is shown in Fig. 1. Typical dendrite structures are seen (Fig. 1(a) and 1(b)), where fine eutectic Si particles disperse (Fig. 1(c)). The secondary dendrite arm spacing (DAS) of this alloy was measured near the center of ingot which corresponds to the gage section of the fatigue specimen, and defined as 26.3 lm. It should be noted that a large casting defect is recognized in the microstructure (Fig. 1(b)).

The FSP tool was made from alloy-tool steel, SKD61 (JIS G 4404), and consists of concave shoulder with a diameter of 14 mm and M6-threaded probe with a length of 4.7 mm. The tool was rotated clockwise and tilted 3° opposite to the processing direction. The tool traveling speed was fixed at 150 mm/min. It is considered that the development of texture will be affected by the strain rate. Accordingly, the tool rotational speed was set to be 500 and 1000 rpm to investigate fatigue properties in the FSPed specimens fabricated under low and high strain rates. Hereafter, the specimens are designated as L (Low strain rate) and H (High strain rate) samples. It is known that FSW and FSP would reduce the hardness of T6-treated Al alloys after processing because of the resolution of hardening precipitates due to the heat input [11,12,17,18]. Thus

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L specimen

R-side (a)

H specimen

R-side (a)

A-side

B

B

SZ

SZ

C TMAZ

TMAZ

1mm

1mm (d)

(b)

(d)

D

Si

D

20 m

100 m

100 m (c)

C TMAZ

TMAZ

(b)

A-side

10 m

(c)

(e)

E

(e)

E

100 m

20 m

10 m

100 m

Fig. 3. Microstructure of L specimen: (a) macroscopic appearance, (b) magnified view of area ‘‘B’’ in figure (a), (c) magnified view of area ‘‘C’’ in figure (a), (d) magnified view of area ‘‘D’’ in figure (b), (e) magnified view of area ‘‘E’’ in figure (c).

Fig. 4. Microstructure of H specimen: (a) macroscopic appearance, (b) magnified view of area ‘‘B’’ in figure (a), (c) magnified view of area ‘‘C’’ in figure (a), (d) magnified view of area ‘‘D’’ in figure (b), (e) magnified view of area ‘‘E’’ in figure (c). Dotted lines in figure (d) and (e) are the trace of onion ring structure.

(a)

L specimen

(b)

(c)

5m m

4 mm

Tool plunge direction

on cti ire d ing vel a r T

R-s ide

Asid

e

Fig. 5. Microstructure of L sample in fatigue specimen: (a) macroscopic appearance, (b) microstructure and (c) Schematic illustration of ring patterns.

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(a)

H specimen

(b)

(c)

5m m

4 mm

Tool plunge direction

ion ect r i d ng eli v a Tr

R-s id

e

Asid

e

Fig. 6. Microstructure of H sample in fatigue specimen: (a) macroscopic appearance, (b) microstructure and (c) schematic illustration of onion rings.

R-side (a)

A-side

R-side (a)

A-side B

SZ SZ

B

TMAZ

TMAZ

1mm (b)

TMAZ

TMAZ

1mm (c)

(b)

(c) D

C 500 m

200 m

50 m

500 m

(d)

Fig. 7. Microstructure of PHL specimen: (a) macroscopic appearance, (b) magnified view of area ‘‘B’’ in figure (a), (c) magnified view of area ‘‘C’’ in figure (b).

post heat treatment was applied to the samples after FSP. The solution treatment at 808 K for 12 h and the following aging at 428 K for 4 h were applied to the FSPed samples as post heat treatment. Post-heat-treated L and H samples are designated as PHL and PHH samples, respectively.

50 m Fig. 8. Microstructure of PHH specimen: (a) macroscopic appearance, (b) magnified view of area ‘‘B’’ in figure (a), (c) Grain boundaries in figure (b) are traced by the dotted lines, (d) magnified view of area ‘‘D’’ in figure (c).

2.3. Procedures Fatigue tests had been conducted using resonance-type plane bending fatigue testing machine, SIMADZU TB-10, at a test

frequency f = 33.3 Hz and load ratio R = 1 (fully reversed bending). The hardness was measured by micro-Vickers hardness tester at a load of 2.49 N and dwell time of 30 s.

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Table 1 Tensile properties.

R side 0.2% proof stress r0.2 (MPa)

Elongation d (%)

Elastic modulus E (GPa)

As-received L: 500 rpm H: 1000 rpm PHL: Postheattreated L PHH: Postheattreated H

218 199 215 243

180 138 159 180

11 17 18 14

64 65 63 66

251

198

14

65

3. Results 3.1. Microstructural characterization 3.1.1. L and H specimens The microstructures of L sample observed on the cross section are shown in Fig. 3. Dendrite structures were completely broken-up by the stirring action during FSP in the stir zone (SZ). The thermos-mechanically affected zones (TMAZ) exist next to SZ, and the boundary between SZ and TMAZ was identified by the broken line. The magnified views in the areas B (Fig. 3(b) and (d)) and C (Fig. 3(c) and (e)) reveal that small Si particles and equiaxed grains with the average grain size of 4 lm are recognized in the SZ. The size of Si particles was nearly the same with that of the as-received specimen (Fig. 1(c)), because eutectic Si could not be dissolved by heat input during FSP. It should be noted that some clusters of Si particles are seen in Fig. 3(c), depending on the complex material flow in the SZ. The microstructural features in H sample (Fig. 4) are similar to those in L sample, i.e. small Si particles and equiaxed grains in the SZ as shown by the magnified views in the areas B (Fig. 4 (b) and (d)) and C (Fig. 4(c) and (e)). The average grain size in the SZ is 8 lm, and slightly larger than that of L sample. It should be noted that the onion ring structures are more prominent in H sample especially indicated in Fig. 4(d) and (e). Fatigue specimens were sampled, where the specimen cross section should be included within the area of broken lines, namely within the SZ. 3-dimentional microstructure in the actual fatigue specimen is shown in Fig. 5 and 6 for L and H samples, respectively. The onion ring patterns were identified in Fig. 5(b) and 6(b) and schematically traced as shown in Fig. 5(c) and 6(c). It should be noted that onion ring patterns on the specimen surface are clearer in H sample than in L sample. 3.1.2. Post-heat-treated specimens Microstructures of PHL and PHH samples are shown in Figs. 7 and 8, respectively. Macroscopic appearances (Figs. 7(a) and 8(a)) are similar to those of L and H samples without post heat treatments (Figs. 3(a) and 4(a)). That is because eutectic Si particles were not changed even at the highest temperature of the post heat treatment (808 K). But magnified views (Figs. 7(c) and 8(c) and (d)) reveal that grain coarsening occurred in the SZ, where the grain size was 200–300 lm and 1–2 mm for PHL and PHH samples, respectively. The grain coarsening was recognized only in the SZ. In wrought Al alloys, it is known that abnormal grain growth took place in SZ of FSW joints by post heat treatment [19,20]. In the cast Al alloy, A356, similar grain coarsening occurred indicating that the distribution of small Si particles cannot prevent grain growth during post heat treatment [15]. Shibayanagi et al. had reported that the abnormal grain growth occurred due to severe residual strain in the SZ [20]. More prominent grain growth in PHH than in PHL implies that the plastic deformation was severer in PHH sample due to the higher tool rotational speed.

103HV: as received

100

Vickers Hardness HV

Tensile strength rB (MPa)

80

60

L specimen PHL H specimen PHH 40

2

+2 mm Middle 2 mm 0

2

Distance from centre of SZ (mm) Fig. 9. Vickers hardness profiles.

3.2. Mechanical properties The tensile properties are summarized in Table 1. The proof stress, r0.2, of L and H specimen was decreased 23% and 12% by FSP, respectively, compared with the as-received material. Subsequently, the tensile strength, rB, was decreased 8.7% and 1.4% for L and H specimens, respectively. It could be concluded that the proof stress was more sensitive to the softening by FSP, and the lower rotational speed gives slightly lower r0.2 and rB than the higher speed. But rB seems to be insensitive to the tool rotational speeds. Post heat treatment could improve both proof stress and tensile strength. Fig. 9 shows the hardness profiles of the specimens, where the average hardness of the as-received material is indicated by the solid line. The hardness profiles were measured along middle line and ±2 mm from the middle line on the cross section. Sillapasa

200

A356 Plane bending R= 1

Stress amplitude σa (MPa)

Material

A side

120

150

100

as received L PHL H PHH

50 10

4

10

5

10

6

10

7

Number of cycles to failure Nf Fig. 10. S–N diagram.

10

8

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100 m

200 m

Fig. 11. Fracture surfaces near crack initiation cite in the as-received material (ra = 140 MPa, Nf = 2.81  105).

50 m

100 m

Fig. 12. Fracture surfaces near crack initiation cite in L specimen (ra = 170 MPa, Nf = 1.62  105).

50 m

100 m

Fig. 13. Fracture surfaces near crack initiation cite in H specimen (ra = 180 MPa, Nf = 4.23  104).

N=5000 (0.12Nf)

et al. showed higher hardness in the advancing side (A-side) than in the retreating side (R-side) in the friction stir welds of 6N01 Al alloy [21]. However, the hardness was symmetric on both sides in the present cast Al alloy. The hardness profiles along those three lines on the cross section are nearly the same, indicating that the hardness profile is insensitive to the depth from the surface. L and H specimens exhibit lower hardness than the as-received one, because of the resolution of hardening precipitates by heat input. Severer plastic deformation under high tool rotational speed resulted in the slightly higher hardness in H specimen than in L specimen. The post heat treatment had successfully increased the hardness in the SZ.

Si

N=13000 (0.31Nf)

N=21000 (0.50Nf)

3.3. Fatigue properties

50 m Fig. 14. Typical small crack growth behavior in H specimen observed by a plastic replication technique (ra = 180 MPa, Nf = 4.23  104). Loading axis is vertical to the crack path.

3.3.1. Fatigue strength The S–N diagram is indicated in Fig. 10. It is clear that high cycle fatigue strengths were highly improved by FSP compared with the as-received material, where FSP under lower rotational speed could give further increase. Post-heat-treated materials exhibit higher fatigue strength than the as-received one, but post heat

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4

300 m (b)

Crack legth 2c (mm)

(a)

(a) as received: σa=140MPa L PHL σa=180MPa H PHH

3

coalescence of cracks 2

1

0 3 10

300 m

10

4

5

10

Number of cycles N (c)

(b)

Fig. 15. Crack growth paths just before final fracture: (a) as-received, ra = 140 MPa, N = 0.78Nf, (b) L specimen, ra = 180 MPa, N = 0.98Nf, (c) H specimen, ra = 180 MPa, N = 0.92Nf.

treatment had detrimental effect on the fatigue strengths of the FSPed materials. Fig. 11 reveals the fracture surface near crack initiation site in the as-received material. Large casting defect is recognized at the crack initiation site, indicating the typical fatigue crack initiation mechanism of A356 cast Al alloy. On the contrary, casting defects were not found in L and H specimens as revealed in Figs. 12 and 13, showing fracture surfaces near crack initiation sites. It could be concluded that casting defects were successfully eliminated by both FSPs under low and high tool rotational speeds. Consequently, the increase of fatigue strengths in FSPed specimens could be mainly attributed to the transition of crack initiation mechanism from defect-dominated to cyclic-slip-dominated crack initiation. In the post-heat-treated specimens, severe grain growth occurred (Figs. 7 and 8), resulting in the lower fatigue strengths than the FSPed specimens without post heat treatments. PHH specimens exhibit slightly lower fatigue strengths than PHL ones in high cycle fatigue region due to the severer grain coarsening in PHH specimens. The fatigue strengths of post-heat-treated specimens were sensitive to the grain size. But the hardness of them was comparable to that of the as-received material (Fig. 9). It indicates that the hardness is mainly dominated by the hardening precipitates because this Al alloy is heat treatable. The reason why L specimen exhibits higher fatigue strength than H specimen will be discussed from the view point of texture in the next section. 3.3.2. Small fatigue crack growth behavior Small fatigue crack growth behavior was monitored by a plastic replication technique, as typically shown in Fig. 14 (H specimen). Plastic replication films of the specimen surface were periodically sampled by terminating fatigue test, and time-series estimation of crack length was performed by observing replication films using an optical microscope. The fatigue crack paths before final fatigue fracture in the as-received, L and H specimens are shown in Fig. 15(a): ra = 140 MPa, (b): ra = 180 MPa and (c): ra = 180 MPa,

as received: σa=140MPa L PHL σa=180MPa H PHH

3

coalescence of cracks 2

1

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Cycle ratio N/Nf Fig. 16. Fatigue crack growth curves: (a) in terms of number of cycles, N, (b) in terms of cycle ratio, N/Nf.

10

Crack growth rate da/dN (mm/cycle)

300 m

Crack legth 2c (mm)

4

4

as received σa=140MPa L PHL H PHH

10

10

σa=180MPa

5

A356 Plane bending R= 1

6

1

2

3

4

5

6 7

Maximum stress intensity factor Kmax (MPa m) Fig. 17. Relationship between crack growth rate, da/dN, and maximum stress intensity factor, Kmax.

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A-side

R-side

(a)

(A)

(B) (C) (D)

(E)

(F)

1mm

(A)

(B)

(C)

(D)

(E)

(F)

ND TD RD

{110} pole figure

{111} pole figure

{001} pole figure

Figure of fcc unit cell

Region in Fig.18(a)

(b)

Fig. 18. EBSD analysis results along crack path in L specimen (ra = 180 MPa): (a) fatigue crack growth path and (b) pole figures at the rectangular regions in figure (a).

respectively, where the numbers of cycles are 0.78Nf–0.98Nf (Nf: Number of cycles to failure). Fatigue crack, initiated from casting defect, grew macroscopically perpendicular to the loading direction in the as-received specimen (Fig. 15(a)). On the contrary, the crack paths are macroscopically curved in L and H specimens. Especially, in H specimen, the shape of fatigue crack path corresponds well with the onion ring patterns in Fig. 6. Crack length 2c is shown as a function of number of cycle, N, in Fig. 16(a) and cycle ratio, N/Nf, in Fig. 16(b). The coalescence of two long fatigue cracks was observed in the as-received material as shown by the arrow in the figure. The coalescence was also observed in the PHL specimen, but that was the coalescence of small crack to the long main crack near the final fracture (at N/Nf  70%). It should be noted that small fatigue cracks initiated where N/Nf was smaller than 0.1 in the as-received, H, PHL and PHH specimens, while N/Nf at the crack initiation was about 0.3 only in L specimen. It indicates that the crack initiation resistance was the highest in L specimen. Lower crack initiation resistances in the post-heat-treated specimens are attributed to the grain coarsening. The stress amplitude

for the as-received specimen (ra = 140 MPa) was set to be lower than the other stress levels for the L and H specimens (ra = 180 MPa), because the as-received specimen has the lowest fatigue strength as shown in the S–N diagram (Fig. 10). It should be noted that L specimen has longer crack initiation life (N/Nf = 0.3) than the as-received one (N/Nf < 0.1) even at the higher stress amplitude. That supports the fact that L specimen exhibits higher crack initiation resistance. The relationship between fatigue crack growth rate, da/dN, and maximum stress intensity factor, Kmax, is shown in Fig. 17. da/dN– Kmax relationships were obtained from Fig. 16(a). Kmax was calculated using Newman–Raju equation [22]. The effect of different stress amplitudes among the as-received, L and H specimens on the crack growth resistance could be eliminated using linear elastic fracture mechanics parameter, Kmax. H specimen has the highest crack growth rates followed by L and as-received specimens in the decreasing order, indicating that the as-received material has the highest crack growth resistance. Fatigue strength is generally dominated by both fatigue crack initiation and growth resistances.

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A-side

R-side

(a) (C) (A)

(D)

(B) (E)

1mm

(A)

(B)

(C)

(D)

(E)

{001} pole figure

Figure of fcc unit cell

Region in Fig.19(a)

(b)

{111} pole figure

ND TD

{110} pole figure

RD

Fig. 19. EBSD analysis results along crack path in H specimen (ra = 180 MPa): (a) fatigue crack growth path and (b) pole figures at the rectangular regions in figure (a).

Although the as-receive material has the highest crack growth resistance, the fatigue strengths was the lowest. It means that the fatigue strength of the as-received material was mainly dominated by weaker crack initiation resistance. It is reasonable because as shown in Fig. 11 and 15(a), large defects were found at the crack initiation sites in the as-received specimens. Consequently, it is concluded that the increase of fatigue strengths in the FSPed specimens could be attributed to the higher crack initiation resistances than the as-received one due to the elimination casting defects. Post heat treatment has smaller effect on the crack growth rates of the FSPed specimens. The difference of crack growth rates between PHL and L, or PHH and H specimens was much smaller than the difference between FSPed and as-received specimens. But small fatigue crack growth rates of PHL or PHH specimens were slightly lower than those of L or H ones. That could

be attributed to the larger grain sizes [23] of post-heat-treated materials. 4. Discussion 4.1. Heat input and material flow during FSP Frigarrd et al. proposed that the heat input, Q, during FSP could be estimated by the following equation [19].

Q¼

4 2 3 p R Pl  N 3

ð1Þ

where R; tool radius, P; tool presser, l; friction coefficient, N; tool rotational speed. The tool geometry and work material are the same in the present study, so that the heat input is mainly dominated by

A. Tajiri et al. / International Journal of Fatigue 80 (2015) 192–202

near the end of the probe on the advancing side in L specimen. Asadi et al. analyzed local strain and temperature distribution during FSP, and showed that the effective strain was very large near the end of probe on the advancing side [25]. It is considered that the heat input and strain late were low when the tool rotational speed was low, but locally high strain near the end of probe had resulted in the partial onion ring structure in L specimen as shown in Fig. 5(b).

Travelling direction Material flow

A-side

R-side

4.2. Effect of texture on fatigue properties

Probe

Texture behind tool Fig. 20. Schematic illustration of material flow around the tool and texture behind tool in Al–Mg–Si FSW joint [26].

Travelling direction

A-side

R-side

Material flow

Probe

(E) (D) (C)

(B) (A)

Fig. 21. Schematic illustration of texture behind tool in A356 and fatigue crack path.



the tool rotational speed. In addition, the strain rate, e, by FSP could be simply estimated by the following equation as indicated by Cheng et al. [24] 

e¼

Rm  2p r e Le

201

To investigate the effect of texture induced by FSP on the fatigue properties, EBSD analyses were performed along the fatigue crack paths using specimens tested at the stress amplitude, ra = 180 MPa. Fig. 18(a) indicates the macroscopic fatigue crack path of L specimen. EBSD analyses were conducted in the areas (A)  (F) in Fig. 18(a), and corresponding pole figures are shown in Fig. 18(b). FCC unit cells in accordance with pole figures are also indicated in Fig. 18(b). The macroscopic fatigue crack path of H specimen, pole figures and FCC unit cells are shown in Fig. 19. Texture evolution is clear along the crack path in H specimen (Fig. 19(b)). Morita et al. investigated texture evolution in wrought Al–Mg–Si alloy FSW joint, and proposed the rotation of unit cells around the probe as shown in Fig. 20 [26]. Fig. 21 indicates the schematic illustration of the rotation of unit cells around the probe, resulting in the texture evolution in A356. The unit cell rotation in A356 is slightly different from that in wrought Al– Mg–Si alloy (Fig. 20), due to the different processing conditions and chemical composition. It is clear that the fatigue crack path is dependent on the orientations of unit cells. In L specimen, however, unit cell rotations are less clearer as shown in Fig. 18(b) than in H specimen, indicating that the texture evolution is weaker under lower heat input and strain rate. Consequently, it could be concluded that H specimen has stronger texture. As shown in Fig. 10, H specimen exhibited lower fatigue strengths than L specimen due to lower crack initiation (Fig. 16(b)) and crack growth (Fig. 17) resistances. The lower resistances could be attributed to the stronger texture induced by FSP under higher heat input and strain rate. It is concluded that the elimination of casting defects is mandatory to improve fatigue strength of the as-received cast Al alloy. In addition, FSP condition with weaker texture could give further increase of fatigue strength because weaker texture could give higher crack initiation (Fig. 16(b)) and growth (Fig. 17) resistances. 5. Conclusions FSP was applied to T6-treated A356 Al alloy under two different FSP conditions with low and high tool rotational speeds. The plane bending fatigue tests were conducted to investigate the effect of FSP conditions on fatigue properties. EBSD analyses were performed to figure out the texture evolution. The conclusions are as follows.

ð2Þ

where Rm is average material flow rate, re and Le are the effective (or average) radius and depth of SZ, respectively. Cheng et al. proposed that Rm was roughly equal to a half of tool probe rotational speed. In the present study, re and Le are roughly the same as shown in Figs. 3(a) and 4(a), thus the strain rate is mainly controlled by the tool rotational speed. Consequently, FSP under low tool rotational speed results in the lower heat input and lower strain rate. Onion ring structures in L and H specimens are schematically shown in Figs. 5(b) and 6(b), respectively. Onion ring patterns were uniformly formed in H specimen, while repeated patterns are only formed

(1) Casting defects were successfully removed by both FSP conditions. Clear onion ring patterns were fully developed under high tool rotational speed, while onion rings were partially formed near the top of the probe under low speed. (2) The fatigue strengths of FSP specimens were improved compared to the as-cast specimen, due to the elimination of casting defects. FSP under lower tool rotational speed could further improve fatigue strength than the higher speed. Post heat treatments induced remarkable grain growth resulting in the reduction of fatigue strengths of the FSPed specimens.

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(3) L specimens exhibited higher crack initiation and growth resistances than H specimen. The higher resistances could be attributed to the weaker texture evolution under lower heat input and strain rate.

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