Quantifying porosity in AM50 die cast tensile bars
EMSL Project ID
45695
Abstract
Magnesium alloys are finding increasing applications in lightweight vehicles because Mg and its alloys are the lightest metallic structural materials. High strength and low density make Mg alloys prospective alternatives to aluminum alloys. However, Mg's limited ductility presents a critical hurdle to more widespread use. It is well established that microstructural features such as amount and distribution of porosity, grain size, and brittle eutectic phase and precipitates can significantly influence the ductility of Mg alloys. Unfortunately, these features vary spatially throughout a casting and from specific alloy to alloys. The specific casting process used and the processing parameters can also influence the final microstructure.The factors limiting the ductility of Mg castings can generally be categorized into two types: intrinsic and extrinsic. Intrinsic factors include features intrinsic to the specific Mg alloy such as the phase composition, grain size, morphology, volume fraction and mechanical properties of the alpha-Mg matrix and the eutectic beta phase. Extrinsic factors come from the external processes applied to the alloy such as casting and heat treatment processes, and they include: porosity, segregation, incomplete fill, hot tear and cold shut. The specific alloy and casting process determine both factors, which in turn influence the ductility of the final cast.
Previous studies have demonstrated that extrinsic defects, i.e., porosity, are the dominant ductility limiting factor for locations with porosity volume fraction exceeding a critical value. When porosity volume fraction is less than the critical value, fracture brought by the intrinsic factors, such as heterogeneous grain size and the brittleness of the beta eutectic phase, is dominant. Once the critical value of porosity is reached, researchers have found that the areal fraction of porosity on the final failure plane, not just the average porosity of the sample, has a direct effect on the ultimate tensile strength and ductility.
To develop and validate the ductility prediction models, a number of Mg alloy castings are being examined experimentally to correlate initial porosity to the final failure location. First, sample will be characterized using X-ray tomography the determine the internal porosity of each sample. The high resolution image provided by EMSL are need to resolve pores down to 80 microns in size. This non-destructive method of imaging the internal microstructure is necessary so that the sample may next be subjected to tensile testing. Finally, this data will be used to validate computational models of ductility.
Project Details
Project type
Limited Scope
Start Date
2011-08-03
End Date
2011-10-04
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members