Aluminum Casting Defect

Background

Special Thanks to Titan Metallurgy for their Technical Services.


Explico’s stereoscope experienced a failure in its support frame which rendered the stereoscope unusable. The failure occurred in an aluminum component of the support frame which was manufactured using a casting process. Visual examination of the failed component revealed the appearance of surface casting defects at a critical threaded connection. 

Stereoscopy is a critical examination step in the failure analysis process, that comes between visual examination and more sophisticated examination techniques, such as scanning electron microscopy. Explico’s stereoscope can provided magnifications of 2X to 90X and offers a binocular view at these magnifications. Objects of a wide range of sizes can be examined and a digital camera is attached for documenting failure features. The stereoscope with the replaced component is presented in the figure below.


Process

Explico’s Eugene Lopez-Ona investigated the failure to characterize the defects observed in the casting. A metallurgical analysis was conducted using technical services provided by Titan Metallurgy. The analysis consisted of visual examination, scanning electron microscopy, energy dispersive spectroscopy, and metallographic preparation and examination. 

A stitched image of the metallographic specimen shows both surface and internal casting defects and poorly formed threads. The internal defects appear to be composed of a mixture of gas porosity and shrinkage voids throughout the materials. The threads’ cross-sections appear to indicate an issue in passes of the thread cutting operation which resulted in incompletely formed threads. 

Energy dispersive spectroscopy (EDS) is a technique for determining the relative elemental composition of deposits on the surface of components. EDS is a technique used to determine the presence of elements as an indicator of the compounds that may be present. EDS results can be provided in the form of a table or a map which presents the elements in an overlay for visual reference. The figure below presents a view of the thread surfaces; notably surface defects can be seen in the threads. The bright white areas in the image on the left are due to charging by paint which had been deposited into the casting defects prior to the thread cutting operation. The image on the right contains an EDS map presenting the locations of color-coded elements.


Results

The manufacturing defects reduced the cross-sectional area of threads which reduced their ability to bear stress. Metallographic analysis revealed gas porosity and shrinkage voids throughout the casting and it appears that successive passes cut through the threads.


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