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Research Papers

Fatigue Characterization and Modeling of Friction Stir Spot Welds in Magnesium AZ31 Alloy

[+] Author and Article Information
J. B. Jordon

Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL 35487

M. F. Horstemeyer

Department of Mechanical Engineering and Center for Advanced Vehicular Systems (CAVS), Mississippi State University, Mississippi State, MS 39762

S. R. Daniewicz

Department of Mechanical Engineering, Mississippi State University, Mississippi State, MS 39762

H. Badarinarayan

Automotive Products Research Laboratory, Hitachi America Limited, Farmington Hills, MI 48335

J. Grantham

Center for Advanced Vehicular Systems (CAVS), Mississippi State University, Mississippi State, MS 39762

J. Eng. Mater. Technol 132(4), 041008 (Sep 29, 2010) (10 pages) doi:10.1115/1.4002330 History: Received September 16, 2009; Revised July 23, 2010; Published September 29, 2010; Online September 29, 2010

The fatigue behavior of friction stir spot welds in magnesium AZ31 alloy is experimentally investigated and modeled. The friction stir spot welds employed in this study are representative of preliminary welds made in developing the joining process for potential use in automobile manufacturing. Load control cyclic tests were conducted on single weld lap-shear coupons to determine fatigue life properties. Optical fractography of the failed fatigue coupons revealed that fatigue cracks initiated from the interfacial “hook” and eventually failed by either nugget pullout or full width separation, depending on the cyclic load amplitude. The failure modes of the magnesium AZ31 alloy were similar to the aluminum alloys of comparable friction stir spot welds. To predict the fatigue life of the lap-joint coupons, a crack growth modeling approach based on a kinked crack stress intensity solution was used. The fatigue model predictions compared well to the experimental fatigue life results, despite an approximate stress intensity factor solution for this weld geometry. The experiments and modeling conducted in this study suggest that the size of the interfacial hook, which comes about from the speed, depth of plunge, dwell time, and tool configuration of the friction stir spot weld process, is a major contributor to the fatigue life of the joint.

Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Geometry of Mg AZ31 friction stir spot weld lap-shear coupon. Dimensions in millimeters.

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Figure 2

Schematic of friction stir spot weld tool geometry: cylindrical pin tool having a 10 deg concave shoulder. The shoulder has a diameter of 12 mm and the cylindrical pin has a diameter of 5 mm with a length of 1.6 mm and M5 threads.

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Figure 3

A sectioned macroscopic view of an untested friction stir spot weld coupon

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Figure 4

A magnified view of the highlighted region in Fig. 3 showing the primary and secondary interfacial hooks

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Figure 5

Optical microscopic images of the: (a) weld nugget, (b) TMAZ, (c) HAZ, and (d) base metal, as indicated in Fig. 3

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Figure 6

(a) A servohydraulic load frame shown with the friction stir spot weld coupon being tested under quasi-static tensile loading conditions. (b) A magnified view of the lower grips of the FSSW test setup illustrating the use of the shim to compensate for the coupon offset. A 1.96 mm thick shim was installed behind the lower wedge grips, as highlighted. An identical sized shim was also used in the upper grip (not shown).

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Figure 7

Schematic of a kinked crack stress intensity factor for a resistance spot weld (11,33)

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Figure 8

Schematic of a kinked crack stress intensity factor overlaid on a fiction stir spot weld crack

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Figure 9

Geometry of a propagating fatigue crack in a friction stir spot weld

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Figure 10

Fractured fatigue friction stir spot weld coupons. The maximum load applied and the number of cycles for complete failure (Nf) is denoted above each coupon. Arrows indicate the location of the through-crack initiation sites. These specimens were tested at R=0.

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Figure 11

Sectioned view of fractured friction stir spot weld coupons subjected to cyclic loading. The maximum load applied is denoted for each coupon (R=0). The arrows indicate the lap-shear leg and the direction of loading. For the five sectioned FSSW coupons, the following is denoted: (a) primary crack, (b) shear/tensile overload region, and (c) secondary crack.

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Figure 12

Scanning electron microscope fractography of a fatigued friction stir spot weld coupon. This specimen was tested at a load range of 3 kN at R=0.

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Figure 13

Through-crack initiation life results of friction stir spot welds fatigue tested at R=0, R=0.3, and R=0.7 compared with the crack growth model

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Figure 14

A magnified view of the interfacial hook of an untested friction stir spot weld via scanning electron microscopy

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Figure 15

A comparison of experimental fatigue life results of friction stir spot weld in magnesium and aluminum alloys

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