Fatigue Crack Propagation Behavior of as-Extruded AZ31B Mg Alloy Welded Joint

Fatigue Crack Propagation Behavior of as-Extruded AZ31B Mg Alloy Welded Joint

Rare Metal Materials and Engineering Volume 41, Issue 6, June 2012 Online English edition of the Chinese language journal Cite this article as: Rare M...

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Rare Metal Materials and Engineering Volume 41, Issue 6, June 2012 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2012, 41(6): 0967-0972.

ARTICLE

Fatigue Crack Propagation Behavior of as-Extruded AZ31B Mg Alloy Welded Joint Zhang Hongxia, Li Juan,

Pei Feifei,

Yan Zhifeng,

Wang Wenxian,

Liang Peiyang,

Wei Yinghui

Taiyuan University of Technology, Taiyuan 030024, China

Abstract: The fatigue crack propagation behavior of as-extruded AZ31B magnesium alloy welded joint and heat affected zone have been studied. Compact tensions [C(T)] of welded joint [L-T(W)] and heat affected zone(welded joint is parallel [T-L(H)] and vertical [L-T(H)] to the extruded direction) were researched. Results indicate that the crack propagation direction is parallel to the extrusion direction for L-T(W); fatigue crack propagation rate is a rapid-slow process. T-L(H) is parallel to the notch direction and L-T(H) can be divided into two states (i.e., parallel to or angularly deflected towards the notch direction); crack propagation rate initially goes through a rapid course before it slows down. The fracture modes of crack tip remain a mixed-mode of transgranular and intergranular fractures. The fatigue fracture consists of quasi-cleavage and is a brittle fracture. Key words: AZ31B magnesium alloy; welded joint; fatigue crack propagation rate; brittle fracture

Magnesium alloys are the lightest structural materials, their low density, high specific strength[1], good electromagnetic shielding capability and reclamation property have made them appealing materials to designers[2]. Their applications have rapidly increased in recent years[3-5]. For the applications to the load-bearing components, it is necessary to evaluate the fatigue properties of the magnesium alloys. Therefore, the accumulation of various fatigue data is now of particular importance. One of the important criterions in evaluating the resistant ability of the alloy under cyclic loading is to determinate the fatigue crack propagation behavior[6]. Casting and die-casting methods are used for several magnesium alloy products, and are inadequate for numerous other applications and large-scale structures. As-extended AZ31B magnesium alloy has superior performance compared with cast and die-cast magnesium alloys. Many studies have focused on the joining of magnesium alloys using welding procedures. Fatigue fracture is a major failure mode in metal struc-

tures, and 70% to 90% of failure accidents involving welded structures are caused by the fatigue fracture of welded joints. Welded structures are used in most buildings and construction projects, and accidents stemming from fatigue damage often result in catastrophic loss of lives and property. The weldability, welding technology adaptability, and conventional mechanical properties of magnesium alloys are widely studied[7]. However, fewer studies have dealt with the fatigue properties of the welded joints of magnesium alloys. Studies show that under dynamic load, the fatigue crack propagation (FCP) rate of the welded joints of magnesium alloys is of theoretical and practical significance for engineering applications. A vertically dynamic load is usually dangerous. Welded joints are affected by the tensile load and compact tension [C(T)] of specimens because tests are conducted through axial loading. In this paper, the FCP rate of welded joints and heat affected zone (HAZ) are tested, and FCP behavior and fracture mechanism are studied.

Received date: June 16, 2011 Foundation item: NSFC (50675148, 51175364) Corresponding author: Zhang Hongxia, Ph. D., Associate Professor, College of Material Science and Engineering of Taiyuan University of Technology, Taiyuan 030024, P. R. China, Tel: 0086-351-6010076, E-mail: [email protected] Copyright © 2012, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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1

Experiment

1.1

Materials and preparation of welded joint

As-extruded AZ31B magnesium alloy welded joint specimen with a 10 mm thick C(T) was used. Manual TIG welding method, 300 GP AC/DC TIG welding equipment and Φ2.8 mm welding wire were used to weld the AZ31B magnesium alloy. The welding parameters are listed in Table 1. Fatigue tests were conducted using a PLG-200D high frequency fatigue testing machine with a stress ratio of 0.02~0.03. The fatigue frequency of the magnesium alloy specimens varied from 98~100 Hz.

1.2

Table 1 Joint style Butt joint

ΔP (2 + α ) ΔK = (0.886 + 4.64α − 13.32α 2 + 14.72α 3 3/ 2 B W (1 − α ) − 5.6α 4 ) (1)

Where α=ā/W; ΔP is applied load amplitude; B is specimen thickness; W is specimen width; ā is crack average length=(ai+1+ai)/2. The variables da/dN and ΔK were calculated by da/dN=C(ΔK)m, where C and m were material constants. Stress ratio is 0.02~0.03 under invariable load ΔP.

Specimen process and test parameters

The weld width was about 11 to 12 mm, reinforcement was milled, and the welded joint was in the same plane as the base metal. The specimens were processed according to GB/T 6398-2000, adopting the standard C(T) specimen. The FCP rate specimen is shown in Fig.2. The notch was 14 mm, and linear cutting was used to process the notch. The notch was in the welded joints. Fig.2a shows the notch in the weld center line, and Fig.2b shows the notch in the HAZ.

2

Results and Discussion

2.1

X

170~180

12~15

2.8~3.0

Fig.1

Schematic of the welded joint C(T) specimens

a

b

FCP test and data analysis method

Specimens were pre-fabricated with a definite crack length by loading consecutively with an invariable load. When the crack propagated, the crack length (a) and cycle times (N) were recorded and fitted into an a-N curve. The slope of two adjoining points was calculated using the secant law. The ΔK for the C(T) specimens was calculated from Eq.(1):

1.4

Groove Welding current, Arc voltage, Welding speed, style I/A U/V v/mm·s-1

Specimen preparation

The cyclic loading typically exhibits diverse directions for metals, and is parallel or vertical to the direction of the extrusion deformation. Thus, the FCP direction differs and has different propagation regions. The welded joint specimens were intercepted, and the notch direction is shown in Fig.1

1.3

Welding parameters

a-N and da/dN-ΔK curves of L-T(W)

The applied load (ΔP) was decreased stepwise with 4.0

Fig.2

C(T) specimen size parameters: (a) notch in the weld center line and (b) notch in the HAZ

kN, and the test was carried out with ΔP=2.9 kN. The testing frequency was 83~79 Hz, and fatigue pre-cracking length was 1 mm. The stress circulation times(N) and corresponding FCP length (a) were recorded every 0.5 mm of the FCP. The a-N and da/dN-ΔK curves of the L-T(W) specimens are shown in Fig.3 and 4. The da/dN-ΔK curve of the L-T(W) specimens have one inflection point. The da/dN-ΔK curve can be divided into two segments: da/dN=2.67×10-17(ΔK)15.9, 5.52<ΔK≤6.12 MPa·m½; and da/dN=9.33×10-7(ΔK)2.53, 6.12<ΔK≤12.0 MPa·m½. The FCP of the L-T(W) specimen initially goes through a rapid course before it slows down. It begins to slowly propagate at about ΔK=6.12 MPa·m½ when ΔP is vertical to the extrusion— direction. — The— grain of the {1012} <1120> texture composite shows a {1012} pyramidal twin while the material is deformed. In the course of deformation, the leading roles of the twin are to alter grain orientation, advance the plasticity of the material’s entirety, and release partial stress. The twin releases more stress concentration and reduces plastic deformation resistance; consequently, rapid and then slow crack propagation occur.

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35 30 25 20 15 0.0

Fig.3

Crack Propagation Rate -1 da/dN/mm·cycle

Crack Length, a/mm

40

Result of L-T(W)1 Result of L-T(W)2 Result of L-T(W)3

0.5

1.0 1.5 2.0 5 N/×10 cycle

a-N curves of L-T(W) specimens

1E-3

1E-4

1E-5

39 36 33 30 27 24 21 18 15

2.5

Result of L-T(W)1 Result of L-T(W)2 Result of L-T(W)3

Result of T-L(H)1 Result of T-L(H)2 Result of T-L(H)3 Result of T-L(H)4

1 2 3 4 5 6 7 5 Number Cycles, N/×10 cycle

Fig.5 Crack Propagation Rate, -1 da/dN/mm·cycle

Crack Length, a/mm

45

Result of T-L(H)1 Result of T-L(H)2 Result of T-L(H)3 Result of T-L(H)4

1E-3

1E-4 10 Stress Intensity Factor Range, ΔK/MPa·m1/2

10 1/2 Stress Intensity Factor Range,ΔK/MPa·m Fig.4

da/dN-ΔK curve of L-T(W) specimens

Fig.6

2.2

a-N and da/dN-ΔK curves of the notch in the HAZ 2.2.1 a-N and da/dN-ΔK curves of T-L(H)

2.2.2

a-N and da/dN-ΔK curves of L-T(H)

The applied load (ΔP) was decreased stepwise with 4.0 kN, and the test was carried out with ΔP=3.2 kN except that specimen 1 was tested with 2.9 kN. The frequency was 88~81 Hz and fatigue pre-cracking length was 1 mm. The a-N and da/dN-ΔK curves of the L-T(H) specimens are shown in Fig.7 and 8. Two crack propagation directions occur in L-T(H), and one direction is parallel to the notch direction. The da/dN-ΔK curve of the L-T(H) specimens has one inflection point. The da/dN-ΔK curve can be divided into two segments: da/dN=1.12×10 -13 (ΔK) 9.91 , 5.7<ΔK≤8.2 MPa·m ½ ; and da/dN =1.32×10-7(ΔK)3.49, 8.2<ΔK≤12.6 MPa·m½. The initial crack propagation of the L-T(H) specimen is rapid. At later cycles, the FCP slowly propagates. It begins to

da/dN-ΔK curve of T-L(H) specimens Result of L-T(H)1 Result of L-T(H)2 Result of L-T(H)3 Result of L-T(H)4 Result of L-T(H)5

45

a/mm

40 35 30 25 20 15 0

2

Fig.7 Crack Propagation Rate -1 da/dN/mm·cycle

The applied load (ΔP) was decreased stepwise with 5.0 kN, and the test was carried out with ΔP=3.2 kN. The frequency was 83~79 Hz, and fatigue pre-cracking length was 1 mm. The a-N and da/dN-ΔK curves of the T-L(H) specimens are shown in Fig.5 and 6. The da/dN-ΔK curves of T-L(H) disperses and the FCP does not possess a steady stage because the HAZ exhibits brittleness, and there is less plastic diversification in the HAZ in the course of FCP. The FCP rate is invariable.

a-N curve of T-L(H) specimens

4

6 8 10 12 14 16 18 5 N/×10 cycle

a-N curve of L-T(H) specimens

1E-3 1E-4 1E-5

Result of L-T(H)1 Result of L-T(H)2 Result of L-T(H)3

1E-6 Stress Intensity Factor Range, Δ K/MPa·m1/2 Fig.8

da/dN-ΔK curve of L-T(H) specimens

slowly propagate at about ΔK=8.2 MPa·m½ when ΔP is vertical to the extrusion direction; another direction is propagated from the HAZ to the weld center line. Specimens 1, 2, and 3 are parallel to the notch direction (Fig.8) and speci-

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mens 4 and 5 deviate from the notch direction. The FCP parameters of L-T(W), T-L(H), and L-T(H) as-extruded AZ31B magnesium alloy specimens are shown in Table 2. The results show that the different FCP rates of the specimens vary, that is, rapidly propagating at first, and then slowly propagating for L-T(W) and L-T(H). After the crack reaches a steady state, the FCP of the notch in the HAZ slightly rapidly propagates compared with that in the weld center line. This is mainly because the HAZ is heated in the postweld, generating a coarse grain. The notch is parallel to the extrusion direction in the HAZ (T-L(H)), which has a stabilized expansion process. Its FCP is more rapid than that vertical to the extrusion direction.

b

a

10 μm Fig.9

Crack tip propagation metallograph of L-T(W): (a) transgranular fracture and (b) transgranular and intergranular fracture a

b

c

d

e

f

2.3 FCP mechanism 2.3.1 FCP metallographic analysis of L-T(W)

FCP direction

A CMM-20 optical microscope was used to determine the FCP behavior of the L-T(W) fatigue crack tip. The macroscopic cracks of L-T(W) are mostly parallel to the notch direction. 1) FCP of L-T(W) 3% oxalic acid solution was used to etch inspection AZ31B magnesium alloy welded joint and near the crack tip region. The crack propagation direction of L-T(W) is shown in Fig.9. The crack tip propagates mainly by transgranular fracture and remains a mixed-mode of transgranular and intergranular fracture along the notch direction for L-T(W).

2) Fracture analysis of L-T(W) The fracture of the L-T(W) specimen was analyzed by JSM-6700F Cold FE SEM (Fig.10 and 11). Fig10a, 10c, and 10e depict the fracture close to the extruded surface along the crack propagation direction. Fig.10b, 10d, and 10f illustrate the fracture surface center. The crack propagation direction is indicated by the arrow. Fig.10 and 11 show a quasi-cleavage fracture at the cleavage plane. Fig.10a and 10b show a cleavage fracture characterized by a river pattern; some secondary cracks can be seen as well. A brittle fracture is observed in Fig.10 on the fatigue crack edge position. The cleavage crack is nucleated and propagates along the difference plane in Fig.10a and 10c, in the adjacent plane, cleavage stripe inter-perforation, and formed cleavage step. Fig.10c is partially enlarged and is depicted in Fig.11. Table 2 AZ31B magnesium alloy welded joint FCP parameter m Corr. Style C ΔK/MPa·m½ coef. r Result deviation L-T(W 2.67×10-17 15.9

8.997

5.52<ΔK≤6.12 0.530

2.53

0.313

6.12<ΔK≤12.0 0.701

1.65

0.185

5.29<ΔK≤11.6 0.686

9.33×10

-7

T-L(H) 1.50×10

-5

-13

9.91

1.661

5.7<ΔK≤8.2

1.32×10-7 3.49

0.418

8.2<ΔK≤12.6 0.759

)

L-T(H)

1.12×10

0.700

100 μm

Fig.10

SEM photograph of the L-T(W) fracture: (a,c,e) fracture close to extruded surface along crack propagation direction and (b,d,f) fracture surface center

10 μm

Fig.11

Partial enlargement of Fig.10c

The figure indicates that the fracture occurs along the difference cleavage plane, and converges into the cleavage step. The L-T(W) fatigue fracture, which is brittle, is prone to cleavage fracture. The fracture surface morphology and da/dN-ΔK curves show that the FCP rate of L-T(W) ini-

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tially goes through a rapid course before it slows down.

2.3.2

FCP metallographic analysis of the HAZ

b

c

d

e

f

FCP direction

FCP mechanism of T-L(H) ① FCP of T-L(H) The macroscopic cracks of the T-L(H) are shown in Fig.12. The fracture modes of crack tips remain a mixed-mode of transgranular and intergranular fracture along the notch direction. ② Fracture analysis of T-L(H) The fracture of the T-L(H) specimen was analyzed by SEM (Fig.13 and 14). Fig.13 fracture position is similar to Fig.10. Fig.13a, 13c, and 13e show a brittle fracture characterized by a cleavage plane. Fig.13b, 13d, and 13f are characterized by a river pattern. The fatigue step in Fig.13e is partially enlarged in Fig.14. The crack propagates along the cleavage plane of the magnesium alloy. The defects in the crystal cause cleavages in the grain not only along one crystal face but also along a family of crystal faces parallel to each other (crystal faces having the same indices) and at different heights. In this manner, the cracks of the crystal faces at different heights coalesce into one cleavage step.

a

1)

100 μm

Fig.13

SEM photograph of T-L(H) fracture: (a, c, e) brittle fracture and (b, d, f) river pattern

2) FCP mechanism of L-T(H) ① FCP of L-T(H) The macroscopic cracks of L-T(H) are shown in Fig.15. Two prorogation directions are found. One crack tip is along the HAZ prorogation; the other is a deviation from the welded joint, and finally propagates along the welded joint. The FCP shows a deviation notch in the HAZ because the grain size is coarse and the crack propagates in an intergranular manner. In the course of crack propagation, the FCP direction has a deviation. The microscopic cracks of L-T(H) are shown in Fig.16. The fracture modes of crack tips remain a transgranular fracture along the notch direction, and secondary cracks are observed. ② Fracture analysis The fracture of the L-T(H) specimen was analyzed by SEM (Fig.17 and 18). Fig.17 depict the fracture is similar to Fig.10. Fig.17 shows a quasi-cleavage fracture of L-T(H). Fig.17c shows that the fatigue fracture is along the cleavage plane. Fig.17e is characterized by a river pattern. Fig.17 and 18 show that L-T(H) is char acterized by a quasi-cleavage fracture, which is a brittle fracture.

10 μm

Fig.14

Partial enlargement of Fig.13e

Welded joint

Fig.15

a

Crack propagation macrographies of L-T(H): (a) along with HAZ and (b) with a deviation from the welded joint a

50 μm 25 μm

b

10 μm

10 μm Fig.16

Fig.12

b

Welded joint

Crack tip propagation metallography of T-L(H)

Crack tip propagation metallographies of L-T(H): (a) crack tip and (b) near crack tip

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a

b

c

d

e

f

FCP direction

Zhang Hongxia et al. / Rare Metal Materials and Engineering, 2012, 41(6): 0967-0972

100 μm

Fig.17

SEM photograph of the L-T(H) fracture which is similar to Fig.10

rion. 2) The L-T(W) crack propagation direction is parallel to the weld center line. Fatigue crack rapidly occurs during the propagation stage when ΔK is up to about 6.12 MPa·m½. The fatigue fracture is a brittle fracture characterized by cleavage, and a secondary crack is observed in the fracture. The fracture modes of crack tips remain a transgranular fracture along the notch direction. 3) The T-L(H) crack propagation direction is parallel to the HAZ. The FCP is steady. The fatigue fracture is a brittle fracture characterized by cleavage. The fracture modes of crack tips remain a mixed-mode of transgranular and intergranular fracture along the notch direction. 4) The L-T(H) crack propagation direction has two directions, one along the HAZ and the other deviating from the welded joint. Fatigue crack rapidly occurs during the propagation stage when ΔK is up to about 8.2 MPa·m½. The fatigue fracture is a brittle fracture. The fracture modes of crack tips remain a transgranular fracture along the notch direction, and secondary cracks are observed.

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Keiro Tokaji, Masaki Nakajima, Yoshihiko Uematsu. International Journal of Fatigue[J], 2009, 31: 1137

2

Luo T J, Yang Y S, Tong W H et al. Materials and Design[J]. 2010, 31: 1617

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Chen Zhenhua. Wrought Magnesium Alloy[M]. Beijing: Chemical Industry Press, 2005: 1

4 10 μm

Science Engineering A[J], 2007, 468-470: 214 5

Fig.18

Partial enlargement of Fig.17c

Conclusions

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Sotomi Ishihara, Zhenyu Nan, Takahito Goshima. Materials

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1) L-T(W), T-L(H), and L-T(H) can meet the Paris crite-

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