Statistical analysis of micropore size distributions in Al–Si castings evaluated by X-ray computed tomography

Abstract

In general, micropore size acts as one of the most significant influencing factors on the fatigue strength of aluminium castings. Hence, an in-depth knowledge of the occurrence of micropore sizes and their local distributions in different locations in complexly-shaped lightweight components is of great interest to the casting industry. In this work, the local properties of AlSi8Cu3 and AlSi7Cu0.5Mg cylinder heads and AlSi8Cu3 crankcases were analyzed. Extensive X-ray computed tomography (CT) scans of three specimen positions revealed significant differences in micropore size and distribution. Two CT scan resolutions were selected, with respect to different micropore size populations in the cast components, to enable accurate detection of the microporosity, in addition to an adequate scanning volume, in order to achieve a statistically approved parameter study. Thereby, specimen positions exhibiting smaller mean micropore sizes were scanned at 3 μm/voxel scanning resolution and ones with larger micropore sizes at 8 μm/voxel. A statistical assessment of all of the alloy specifications and specimen positions indicates that the general extreme value and lognormal distribution appropriately describe the micropore size distributions. Finally, an extensive sensitivity study is presented, aimed at examining micropore size characteristics, such as the porosity, sphericity, maximum and mean values and standard deviation, and to investigate their relationships in the investigated cast specimens.

Modification of a Defect-Based Fatigue Assessment Model for Al-Si-Cu Cast Alloys

“…Hence, after initiation of a crack at the most-stressed inhomogeneity, the free surface of the flaw Apore is greatly enhanced, whereas the very pore volume Vpore is increased by only a small amount. For comparison, representative flaws of aluminium castings with varying sphericity ψ can be found in [39].…”

“…On the one hand, preliminary studies [38] reveal a proper conformance with Murakami’s empirical approach, but the present model does not invoke the defect size distribution itself. As presented in preliminary studies [39,40,41,42], the statistical distribution of flaw sizes can be evaluated by non-destructive investigation of the defect population, such as X-ray computed tomography scanning. However, the statistical distribution of the most extreme values evaluated from the XCT does not always represent the distribution of fatigue fracture initiating defects, due to the fact that cracks may initiate at the surface near heterogeneities even in the presence of larger flaws within the bulk volume (see [43,44,45]).…”

Application of a area -Approach for Fatigue Assessment of Cast Aluminum Alloys at Elevated Temperature

“…Thus, a circle-shaped flaw would possess a shape factor φ = 1. The shape of representative defects with varying sphericity is presented in detail in [53].…”

Characterization and Analysis of Porosities in High Pressure Die Cast Aluminum by Using Metallography, X-Ray Radiography, and Micro-Computed Tomography

“…Characterization of the defects may be performed non-destructively by X-ray micro-computed tomography (μCT) [ 32 , 33 , 34 ] or destructively using metallography examination [ 35 , 36 ]. X-ray μCT has the capability of evaluating the pores in a three-dimensional scale, but it is costly and not always accessible.…”