Damage mechanisms in an aluminium-silicon alloy with a controlled defect
Résumé
With new economic and environmental challenges in the automotive industry, the cylinder head is affected by the reduction of manufacturing costs and the system mass optimization due to the “downsizing” trend.
Al-7Si-3Cu aluminum alloy meets the specifications of this application. In the 80’s, the Lost Foam Casting process (LFC) with sand mold was developed. It competes with the traditional gravity Die Casting (DC) process thanks to cost reduction and better part accuracy. Nevertheless, the significantly slower cooling rates for cylinder heads produced by LFC, result in coarser aluminum dendrites and more numerous and large micro-shrinkage.
Several studies on castable Al-Si alloy have been done to understand the impact of microstructure phases and defects on the product lifetime under cyclic thermos-mechanical stresses including in our laboratory, particularly on LFC Al-7Si-3Cu cylinder heads specimens. Yet, the high variability of distribution and characteristics of the defects observed complicates the cracking mechanism analysis.
The new approach is to create a model material with a representative cylinder heads microstructure, ie.size of the microstructure and defects location. Controlling the defect position allows to know the crack initiation location. Hence, the observation area is limited during the mechanical test and it helps to ease quantitative measurement at a very fine scale information. Two strategies are considered for the model. The first is to control the cooling of a cast tensile sample prepared in our laboratory’s foundry with the aim of placing an internal pore in the middle of the gauge length. Optical microscopy and X-ray micro-tomography observations were carried out in order to quantify the microstructure features and the defects location. The results show that it is possible to repeatedly obtain the specific microstructure of the engine part with the same interdendritic size. The second method, that will be presented here, is to close pores of the LFC alloy take a part from the engine with the Hot Isostatic Pressure (HIP) processing and drill an inclined hole in the center of the machined specimen.
Instrumented cyclic mechanical tests were performed on the specimen with a drilled hole using a small size tensile stage and high resolution cameras. The displacement fields around the defect on the surface were measured and analyzed with digital image correlation. These fields are used as boundary conditions in a finite element method simulation for experimental – numerical dialogue. The test was stopped at different stages to characterize the 3D morphology of the micro-cracks by X-ray micro-tomography. The deformations of von Mises calculated on the surface and those calculated in the volume allow an understanding of the damage mechanisms throughout the interest area of the specimen. It reveals cracks initiation and propagation along the silicon eutectic phase and intermetallics close to the hole.