Influence of pores on crack initiation in monotonic tensile and cyclic loadings in lost foam casting A319 alloy by using 3D in-situ analysis

Abstract

Lost Foam Casting (LFC) process is replacing the conventional gravity Die Casting (DC) process in automotive industry for the purpose of geometry optimization, cost reduction and consumption control. However, due to lower cooling rate, LFC produces in a coarser microstructure that reduces fatigue life. In order to study the influence of the casting microstructure of LFC Al-Si alloy on damage micromechanisms under monotonic tensile loading and Low Cycle Fatigue (LCF) at room temperature, an experimental protocol based on the three dimensional (3D) in-situ analysis has been set up and validated. This paper focuses on the influence of pores on crack initiation in monotonic and cyclic tensile loadings. X-ray Computed Tomography (CT) allowed the microstructure of material being characterized in 3D and damage evolution being followed in-situ also in 3D. Experimental and numerical mechanical fields were obtained by using Digital Volume Correlation (DVC) technique and Finite Element Method (FEM) simulation respectively. Pores were shown to have an important influence on strain localization as large pores generate enough strain localization zones for crack initiation both in monotonic tensile and cyclic loadings.

Fatigue of Additive Manufacturing Specimens: A Comparison with Casting Processes

“…Such unmelted zones are observed in the bulk or in sub-surface areas. As the position of the defects respectively to the surface is an influencing parameter on the crack initiation time, as shown in the case of shrinkage in casting alloys [4,5], this has an impact on the criticality and the lifetimes. In the case of unmelted zones, one can observe in Figure 3 that the fatigue properties are qualitatively slightly better for the specimens built in the XY direction.…”

“…In fact, in the case of casting alloys, it has been shown that a simple 2D analysis (size, morphology) from data extracted from surface images of such complex 3D defects is generally not sufficient to understand and quantify their impact on crack initiation mechanisms and on lifetime, even if some statistical corrections are possible [11]. Therefore, a systematic 3D quantitative analysis of the specimens microstructure by CT-tomography (as conducted in [6]) could be a direct perspective of this work, associated to in-situ fatigue tests as in [4,5].…”

“…Depending on the post-treatment applied to the specimens, parts coming from additive manufacturing with these both processes show similar fatigue properties than cast or wrought parts. The qualitative conclusions about crack initiation mechanisms could be challenged by performing 3D CT-tomography associated to in situ tensile tests on several specimens; real-time analysis could then be performed in order to observe the crack initiation and propagation phenomenon as alreadydone on casting alloys [4][5][6].…”

“…At the microstructure scale, it is shown that surface defects and unmelted zones seem to be the dominant features. As this type of defect is close to casting ones, these fatigue crack initiation processes are then compared and discussed with the ones identified in specimens obtained from casting processes [4][5][6]. In these previous studies, the main interest was the possibility to observe, qualify and quantify on in-situ tests, the three dimensional mechanisms responsible of the crack initiation and the microcrack growth.…”

Digital Volume Correlation: Review of Progress and Challenges

“…In particular, the local crack propagation law was extracted from the kinematic measurements [132]. Likewise, crack initiation was observed in cast aluminum alloy with kinematic and residual fields [234]. Short fatigue cracks were analyzed in cast magnesium alloy with the measured bulk displacement fields [147].…”