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Research Papers

Influence of Porosity in the Fatigue Behavior of the High-Pressure Die-Casting AZ91 Magnesium Alloys

[+] Author and Article Information
M. Preciado

University of Burgos,
Avenida Cantabria s/n.,
Burgos 09006, Spain
e-mail: mpreciado@ubu.es

P. M. Bravo

University of Burgos,
Avenida Cantabria s/n.,
Burgos 09006, Spain
e-mail: pmbravo@ubu.es

D. Cárdenas

University of Burgos,
Avenida Cantabria s/n.,
Burgos 09006, Spain
e-mail: dcardenas@ubu.es

1Corresponding author.

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received January 18, 2016; final manuscript received April 8, 2016; published online June 13, 2016. Assoc. Editor: Antonios Kontsos.

J. Eng. Mater. Technol 138(4), 041006 (Jun 13, 2016) (5 pages) Paper No: MATS-16-1020; doi: 10.1115/1.4033466 History: Received January 18, 2016; Revised April 08, 2016

The fatigue properties of high-pressure die-casting (HPDC) magnesium (Mg) alloys AZ91 exhibit a high variability, due primarily to the porosity that is inherent in the injection process. In the 94% of the studied samples, the porosity in which crack nucleation originates is at the surface or adjacent to the surface. The threshold stress intensity factor amplitude and the limit of fatigue have been calculated following the classical models of parameterization of defects. A new set of samples were prepared by machining the surface slightly, in order to conserve the microstructure, and the fatigue behavior at low level of stress was improved. All the samples were produced in molds with the final shape by HPDC process, which allowed a realistic study of the surface effect and the influence of grain size variation from the edge to the center of the samples.

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Figures

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Fig. 1

Microstructure of the sample. Grains of Mg with eutectic grain boundaries.

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Fig. 2

The SEM image of the eutectic: Mg17Al12–Mg

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Fig. 3

Decrease in grain size from the edge to the center of the samples

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Fig. 4

HPDC specimens for fatigue testing

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Fig. 5

The SEM image of a pore at the surface as the origin of fatigue fracture in the sample

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Fig. 6

The SEM image of a big pore at the center of the sample as origin of the fatigue fracture

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Fig. 7

The SEM image of cracking at the grain boundaries

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Fig. 8

S–N test of the machine and nonmachined samples. (The curves are obtained according to the ASTM E739 Standard).

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Fig. 9

Stress intensities factor amplitude of broken specimens. The vertical line separates the failed and unfailed specimens.

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Fig. 10

The SEM image of a defect near the surface in a machined sample

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