2D and 3D Characterization of Damage Mechanisms in A319 Alloy

Abstract

Background: Al-7Si-3Cu (A319) is used for automotive engine cylinder heads and the risk of cracking in the inter valve bridge of the cylinder head is high [1]. Due to the very slow cooling rate with Lost Foam Casting (LFC) foundry process, the alloy microstructure is coarse with shrinkage defects [2 -7]. Previous studies show the key role of microshrinkage cavities in the initiation and the propagation of cracks: pores induce stress concentration in the close microstructure [8 -20]. Objective: the aim of this work is to determine alloy damage at the microstructure scale in a model material with a simplified defect during a cyclic mechanical test. Methods: High Isostatic Pressing treatment and specific drilled hole were made on the sample test to control the crack initiation. A new speckle pattern was used for digital image correlation. The mechanical test was interrupted to couple 2D observations on the two sample surfaces with optical cameras and 3D observations with X-ray microtomography. Results: the resulting von Mises strain fields acquired in situ at the surface or ex situ in bulk show similarities and allow to study the strain evolution during a cyclic test. The study reveals crack initiation around the artificial hole in the gauge length of the specimen while propagation is mainly driven by the surrounding microstructure. Measured strain also allowed to identify the areas of damage and even to anticipate the cracking path. Conclusions: due to the microstructural complexity of a cast aluminum-silicon-copper alloy, the proposed approach based on a simplified defect material and two simultaneous observation methods with digital and volume image correlation is a strong advantage for quantitatively characterizing damage mechanisms at the microstructure scale.