Fatigue Properties of Aluminum Alloy(A6061-T6) with … Fatigue properties.… · (position 1: analysis in deep from the surface) ... UNSM treatment changed surface structure of A6061-T6

Advanced Material Lab. in Kyungpook National University

Fatigue Properties of Aluminum Alloy(A6061-T6)with Ultrasonic Nano-crystal Surface Modification

Ri-ichi, Murakami (The Univ. of Tokushima)


Contents

1. Introduction

2. Experimental procedure

3. Experimental results & discussion

4. Conclusions

5. Acknowledgments

6. References


General knowledge

Aluminum Alloy

Good effectiveness on light weight

Good thermal conductivityand non-magnetism

Good usage to the transportation fields (ex :Aircraft, Vehicle , Railroad etc..)

Good corrosion resistance, a low density and a high specific strength

01

100% recyclable

Good ductility and low melting point

Keyword : UNSM, Fatigue strength, VHCF (Very High Cycle Fatigue)


Introduction UNSM Technology

02

ftpPP stt π2sin+=(2.5~5 times of static load)

Pst : Static Load A : AmplitudeV : Speed, S : Feed (mm/rev)D : Specimen Diameterf : frequency (kHz)

Fig. 1 Model of UNSM


UNSM Device

03

Fig. 2 Configuration of the UNSM device Fig. 3 Distribution of stress and amplitude of the UNSM device

UNSM device


Comparison among UNSM and established technologies

04

Fig. 4 Comparison of “case" depths by cold work and fatiguestrength for different mechanical surface treatments

Surface plastic deformation

“case” depth

explosive hardening

shot peening

Ultrasonic shot peeningGravity peening

Water peening

Roller burnishing

UNSM

Fatigue strength

Laser shock peening

Deep rolling

Table 1 Results of various established technologies on mechanical properties

(5: excellent ~ 1: poor)


Experimental procedure

05

Element Si Fe Cu Mn Mg Cr Zn Ti AlContent(Wt. %) 0.6 0.24 0.33 0.12 1.0 0.21 0.01 0.02 Remainder

Table 2 Chemical composition of A6061-T6 Alloy.

Table 3 Mechanical properties of A6061-T6 Alloy.

MaterialYield

Strength(MPa)

UTS(MPa)

Elongation(%)

Young’sModulus(GPa)

VickersHardness

(Hv)

A6061-T6 276 310 20 68.9 117



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W : weight (Kgf)L : distance between the loading point and the critical

section of the specimen ( L=46.5mm)Kt : stress concentration factor (Kt = 1.08)d : diameter of the critical section of the specimen

Fig. 5 Fatigue specimen (dimension : ㎜)

100


07


Very High Cycle Fatigue (VHCF) test machine (Giga Cycle Fatigue)

Fig. 6 Dual-spindle rotating bending fatigue testing machine

(a) Motor

(b) Spindle

(c) Specimen

(d) Photo sensor & Micro switch

(e) Bearing & Spring

(f) Weight

(g) Counter


Result & Discussion

08

Fig. 7 Distribution of hardness ‘before’ and ‘after’ UNSM treatment

40% increase compared with before UNSM specimens


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Fig. 8 The variation of surface treatments between ‘before’ and ‘after’ UNSM

(a) before UNSM (b) after UNSM


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Fig. 9 Profiles of surface roughness test

Surface roughness

(a) ‘before UNSM’ specimen (b) ‘after UNSM’ specimen0

5

10

Ra(㎛) Rmax(㎛)

The results of roughness test

before UNSM after UNSM

A6061-T6Roughness Before UNSM after UNSM

Ra (㎛) 1.48 0.17 8.7 times

Rmax (㎛) 5.4 1.6 3.4 times

Table 4 Comparison of surface roughness ‘before’ and ‘after’ UNSM treatment.


S-N curve

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Fig. 10 S-N Curve

HCF

50% increased


Observations of fracture surface (before UNSM)

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(a) Low magnification (×100) of crackinitiation site (σa = 150MPa, Nf = 2.02×107)

(b) Enlarged photo of crack initiation site at high magnification (×1000)


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(c) A fracture surface indicates many crack initiation site at (σa = 250MPa, Nf =

1.59×106)

(d) Enlarged surface crack initiation site ( X 500)

Observations of fracture surface (before UNSM)


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(e) (σa = 250MPa, Nf = 1.59×106) (f) An example of ductile striation

Observations of fracture surface (before UNSM , striation)


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(h) magnification (×3000) of crack initiation site (σa = 250MPa, Nf=1.14×107)

Observations of fracture surface (after UNSM, striation)

(g) Surface structure changed by UNSM treatment(68.9 ~ 172㎛)


SEM & EDS analysis

16

Al-Mg-Si compounds were analyzed by EDS

Element Weight % Atomic %

Al K

Mg K

O K

C K

Si K

7.25

Totals

11.58

55.0445.88

0.67 0.53

45.74 32.53

0.46 0.31

100


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XRD experimental procedure

Fig. 11 Diagram of A6061-T6 specimen etching process

Fig. 12 XRD equipment


18

X-ray target Cu-Kα

Diffraction plane (311)

Filter Ni

Voltage 40kV

Current 20mA

Fig. 13 Schematic illustration of X-ray diffraction profile

Table 4 Conditions of X-ray diffraction


19

Texture observation

(a) Observation of the micro structure of ‘before UNSM’

(b) ‘after UNSM’ 1.5Kgf (c) ‘after UNSM’ 3.0Kgf


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XRD experiment results

(a) diagram shows specimen for ‘after UNSM’ 1.5Kgf (b) diagram shows specimen for ‘after UNSM’ 3.0Kgf

Fig. 14 2θ-sin2ψ diagram for (311) diffraction plane in A6061-T6


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specimen PositionCompressive

residual stress(MPa)1 (surface) -198 ± 4.32 (100μm) -188 ± 3.23 (200μm) -140.6 ± 3.74 (300μm) -88 ± 2.6

after UNSM 1.5Kgf

5 (400μm) -97 ± 6.71 (surface) -173.5 ± 1.92 (100μm) -158.9 ± 3.63 (200μm) -147.7 ± 6.24 (300μm) -141.9 ± 4.7

after UNSM 3.0Kgf

5 (400μm) -115 ± 3.8

Table 5 Compressive residual stresses of measured positions

Fig. 15 Relation between distance from specimen surface and compressive residual stress


22

(a) Analysis of stereo graphic projection pole figure (surface) (b) 2.5D for pole figure analysis of specimen (surface)

Fig. 16 Pole figure analysis of ‘after UNSM’ 3.0Kgf (position 1: analysis in deep from the surface)

XRD Pole-figure analysis


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(a) Analysis of stereo graphic projection pole figure(100μm) (b) 2.5D for pole figure analysis of specimen(100μm)

Fig. 17 Pole figure analysis of ‘after UNSM’ 3.0Kgf (position 2: analysis in 100μm deep from the surface)


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Fig. 20 Pole figure analysis of ‘after UNSM’ 3.0Kgf (position 5: analysis in deep 400μm from the surface)


Conclusions

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2. 40% surface hardness was improved by UNSM treatment. The surface roughness also decreased 3.4 ~ 8.7 times with the treatment.

1. The UNSM treatment increased the fatigue strength about 50%.

3. UNSM treatment changed surface structure of A6061-T6 material as nano-size of it about 69~ 172 ㎛ depth from surface by severe plastic deformation (SPD).

SPD layer which has low roughness hardened surface with compressive residual stress (CRD) improves fatigue strength and mechanical properties.

4. The maximum -198MPa was measured for compressive residual stress in ‘after UNSM’

1.5Kgf treatment condition by using X-RAY diffraction, and this compressive residual stress(CRD) would be caused by increase in fatigue strength of al material, and, the creation of texture could be confirmed through pole figure analysis sincethere was no change in slope according to depth in ‘after UNSM’ 3.0Kgf residualstress distribution.


This research was performed under a Double-Degree (DD) program at Kyungpook National University in Koreaand the University of Tokushima in Japan.I would like to deeply appreciate Prof. Chang-Min, Suhand Prof. Ri-ichi, Murakami and who have supervised me during this D.D program.

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Acknowledgments


29References

[1] C. M. Suh, G. H Song, Y. S. Pyoun, " A Study of the Mechanical Characteristics of Ultrasonic Cold Forged SKD 61," I. J. of Modem Physics B, Vo. 2(27), pp. 4541-4546, 2006.

[2] Y. S. Pyoun, K. G. Son, "Development of D2 tool steel trimming knives with nanoscalemicrostructure," AIS-Tech Vol 2, pp. 465~468, 2005.

[3] Y. S. Pyoun,"An application of micro cold forging technology using ultrasonic energy for the nano level surface texture and the improved mechanical properties," Korea-Japan international nano-system, Nov, 2003.

[4] C. S. Lee, I. G. Park, I. S. Cho, et al., Hong, "Ultrasonic Nano Crystal Surface Modification(UNSM) on Structure and Fatigue Characteristics of TI-6AL-4V Alloy," Proceedings of Third Asian Pacific Conference on Biomechanics. No. 126, 2007.

[5] Y. Watanabe, N. Hasegawa, and Duchazeaubeneix 1. M., "Effect of Ultrasonic Shot Peening on Fatigue Strength of High Strength Steel," Proc. 8th Int. Conf. on Shot Peening, Wiley-VCH, Germany, pp. 306-310, 2003.

[6] A. Otsuka, K. Tohgo, and H. Matsutama, "Fatigue Crack Initiation and Growth Under Mixed Mode Loading in Aluminum Alloy 2017-T3 and 7075-T6," Journal of the Society of Materials Science, Japan, Vol. 36, No. 405, pp. 38~44, Jun. 1987.


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[7] H. J. Rack, in Dispersion Strengthened Aluminum Alloys (edited by Y. W. Kim and W. M. Griffith). pp. 64~l. TMS (1988).

[8] S. I. Choi, "Studies on the prevention of damages on the carbody of Aluminum rolling stock," rans. of the KSR, Vol. 3, No. 5, PP. 181~186, 2002.

[9] Y. S. Pyoun, et al., "Application of Ultrasonic Nano-Crystal Surface Modification Technology Which realizes simultaneous improvement of fatigue strength with peening effect and improvement of wear resistance and friction loss with case hardening and micro dimples surface topology," Proceeding of the KSMTE Spring Conference pp. 346~354, 2007.

[10] G. E. Thomson and G.C. Wood, Corrosion Sci. 18, 721, 1978.

[11] J. H. Choi, Kang, D. I. Jang, Y. S. Kim, et al., " A study on the development of welding techique of aluminum alloy for rolling stock," Trans. of the KWS, pp. 220~223, 1997.

[12] W. B. Koster, "Effect of Residual Stress on Fatigue of Structural Alloys," Practical Applications of Residual Stress Technol., Proc. of the Third Int. Conf., Indianapolis, IN, USA. 1, 1991.

[13] L. Wanger, "Mechanical Surface Treatments on Titanium, Aluminum and Magnesium Alloys," Materials Science & Engineering A pp. 263~270, 1999.


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Fatigue Properties of Aluminum Alloy(A6061-T6) with … Fatigue properties.… · (position 1: analysis in deep from the surface) ... UNSM treatment changed surface structure of A6061-T6