The effect of porosity on the fatigue strength of cast aluminum alloys

Summary It is known that the fatigue strength of cast aluminum alloys is affected by various types of defects related to the manufacturing process, especially the micro-shrinkage pores created during the solidification stage of the casting process. Even though specific classification methods are provided in the ASTM E155-15 standard [1], the presence of defects is not easily related to the capacity of a component or a structure to meet the requirements. And fatigue is moderate strength - strength. This is possible by searching for the average size of the critical defects and using the relevant statistical analysis. More precisely, Murakami's approach is based on extreme statistics. The main originality of this work is the application of this approach to a real structure subjected to high cycle fatigue damage: the engine cylinder head, used in the automotive industry. In fact, both fatigue tests and microstructural properties are performed on cylindrical specimens and real structures. Samples subjected to uniaxial and multiaxial loading conditions [2]. The original fatigue test, developed by PSA for loading during critical service areas, is performed on the cylinder head. A systematic analysis of the fatigue failure surface is performed to obtain the statistics of critical defects at the origin of failures for specimens and structures. In parallel, the critical regions and associated local loading modes in the structure are characterized by an appropriate high-cycle fatigue analysis of the latter, together with fatigue test data and critical failure statistical analysis, leading to a discussion of the size effect. and an approach for related fatigue design is proposed. 1. Introduction Cast aluminum alloys are widely used in the automotive industry due to their combination of good relative strength with low density and excellent conductivity. These alloys are commonly used for chassis and engine components that are in most cases subjected to mechanical cyclic loads and are at serious risk for mechanical fatigue, in order to prevent fatigue crack initiation and ensure the strength of the component, appropriate design methods should be developed. They are mostly based on a suitable fatigue criterion, which is identified by fatigue tests performed on test specimens, which are usually quite different in size. The size of the critical fatigue zones found in real manufactured parts. The fatigue strength of cast aluminum alloys is strongly influenced by different types of microstructure. Inhomogeneities caused by the manufacturing process Due to this process and the subsequent treatment, the microstructural characteristics can be significantly changed in terms of eutectic components (silicon particles) and between iron-based metals) and casting defects, especially pores. microshrinkage and gas porosity. Despite considerable effort to describe and understand the effect of these microstructural heterogeneities on the high cycle fatigue behavior (HCF) of these alloys [2][7], the development of design methods considering the effect of defects on fatigue life prediction of structures is still in progress [ 8][10]. However, despite the presence of defects, car manufacturers must ensure a targeted level of reliability of their components structures. In the case of holes, manufacturers set up control procedures through X-ray inspection to detect defects and reject components containing defects that pose an unacceptable risk of failure. Even though some classification procedures are provided in the ASTM E155-15 [1] standard, the presence of defects cannot be easily attributed to the capacity of a component or a structure to meet the requirements of mechanical specifications. SoThe definition of material defect requirements is in most cases based on the manufacturer's experience, without quantitative justification. With the annual production of 3 million engines, the optimization of production costs is a major challenge for the PSA group. This justifies the need for development methods to define a strong acceptance index. . The first step is to make a clear connection between failure characteristics and fatigue strength. This requires careful understanding of the fatigue damage mechanisms associated with microstructural features under high cycle fatigue loading conditions. This has been done for three different aluminum alloys in previous work by the authors [2][11]. The aim of the present study is to establish a relationship between microshrinkage porosity characteristics and the average fatigue strength. This work is done using the classical approach introduced by Murakami [12] and formalized in the ASTM international standard [13]. This applies to both prototypes and the actual structure: the engine cylinder head. The use of a real structure is an opportunity to highlight the possible size effect between the test specimens used to identify the fatigue criterion and the component to be designed. The capacity of the method to calculate different defect distribution parameters is tested using the approach to two aluminum alloys described by different. Casting processes: gravity casting and gravity loss foam casting. During this exercise, the method proposed by Murakami to extrapolate the average size of critical defects of objects to other dimensions with the alternative method proposed by Makkonen [14]. Finally, based on joint analyzes of average critical size defects in specimens and structures, a corresponding fatigue design method is proposed to account for the size effect. 2. Test materials and conditions 2.1. The microstructure and material properties of two cast aluminum alloys named alloys A and B are investigated in this study. These denominations are consistent with the results used in other published results [2][9][15]. These alloys were made by either gravity die casting or lost foam casting and both were treated with T7. The characteristics of each casting process, including important differences in solidification time, result in two materials with different microstructures. Typical microstructures of these two alloys are presented in Figure 1. As a preliminary approach, these differences can be characterized in terms of secondary dendrite arm spacing (SDAS). This value is determined by identifying individual aluminum dendrites. The linear intercept method is then used to measure SDAS. Forty dendrites from each material were analyzed to assess whether alloy A fully represents what is found in industrial components. In fact, all samples used for both microstructural analysis and fatigue testing were extracted from the cylinder head. In order to have a much larger volume from which fatigue samples can be extracted, the industrial casting process was slightly modified to produce negligible microstructural changes in the alloy. Then the test specimens were extracted close to the component close to the critical areas for fatigue failure. Faced with the problem of modifying the mold used to produce the lost foam casting cylinder heads, a plate-shaped mold was preferred. Consequently, and due to the fundamental difference in geometry and volume between the case of the cylinder heads and the mold used, it should be noted that the microstructure and defect population of Alloy B are not representative of the materials in the fabricated components. 2.2. Population of pores and identification of critical defects The population of pores and critical defects were identified using two different methods. Analysis of pores distribution by methodMetallography is presented by Murakami [12]. This analysis has been done through optical microscopic observations of polished samples. On each sample, several observations are made, using a fixed inspection area S0. Only the largest pores of each optical micrograph are considered for morphological analysis (Figure 2.a), in order to extract the extreme value distribution of pore sizes. The description of the critical failure is based on the analysis of the fatigue failure levels of the specimens tested in it. Each sample is examined to identify and measure the size of the casting defect at the origin of fatigue. Fracture for alloys A and B (Figure 2.b). When multiple cracks are found on the fatigue failure surface, only the defect at the origin of the main crack is considered. In cases where several defects can be found for the same crack, defects located near the surface are considered. The main reason for this is that cracks tend to start at the beginning. 2.3. Experimental conditions 2.3.1. Fatigue testing on specimens All fatigue tests shown below were performed at ambient temperature on a Rumul resonance testing machine at 90-110 Hz. All fatigue tests were performed with the same specimen geometry as in [2] in order to remove The effect of each size of two loading conditions, combined stress-torsion with a biaxial ratio of 0.5 and uniaxial loading, are investigated in this contribution. Between 15 and 20 specimens were tested for each loading condition. All fatigue tests under symmetric alternating load (R=-1), using the staircase technique, to evaluate the fatigue strength at 2.106 cycles. A 10 MPa staircase was used for the staircase protocol. The stopping criterion was chosen to ensure the presence of a fatigue crack approximately 3 mm long. In order to determine the effect of defects on fatigue strength, all specimens subjected to the staircase process at a higher step stress range were retested for an additional 2x106 cycles. This was repeated until failure was observed and the final stage stress level was considered as the fatigue strength [16]. This method is regularly used to investigate the effect of defects on fatigue strength. 2.3.2. Fatigue test on the cylinder head Fatigue tests were performed at ambient temperature on the cylinder head made of alloy A. These tests are done. For each new engine development in order to evaluate the average part fatigue strength. This effective loading test is available under in-service loading conditions, especially combustion pressure loading. Even if the test conditions are far from in-service conditions, this loading test will bring the critical areas as close as possible to in-service conditions, especially in terms of Size allows. of loaded areas and stress gradients. The integration of such tests into the cylinder head validation protocol is of particular importance because it is an efficient tool to validate the numerical design method and to avoid large design errors of the four metal blocks on the contact surfaces of the cylinder mounting bolts (highlighted in green) Figure 3-b ) is used as a support. The load is applied by a vertical force in the center of the fire deck to each cylinder using a template in the center of the candle hole (see Figure 3-a). Although this boundary condition is simple, it leads to real stress states in the critical regions located on the water side of a cooling passage (see Fig. 3.c). All fatigue tests with positive load ratio (R=0.1) using the staircase technique to evaluate the fatigue strength at 2 x 106 cycles. Since the presence of fatigue cracks cannot be detected during the test, all tests are carried out until 2x106 cycles have been performed. The presence of a fatigue crack after the test is analyzed by cutting the cylinder head, then by checking the paint penetrant and opening the cracks if necessary. References:[1] ASTM E155-15, Standard Reference Radiographs for Inspection of Aluminum and Magnesium Castings, ASTM International, West Conshohocken, PA, 2015, www.astm.org [2] Le, V. D., Morel, F., Bellett, D., Saintier, N., & Osmond, P. (2016). Multiaxial high cycle fatigue damage mechanisms associated with the different microstructural heterogeneities of cast aluminium alloys. Materials Science and Engineering: A, 649, 426-440. [3] Imade Koutiri, Daniel Bellett, Franck Morel, Louis Augustins, and Jérome Adrien. High cycle fatigue damage mechanisms in cast aluminium subject to complex loads. International Journal of Fatigue, 47(0):44 – 57, 2013. [4] Q.G. Wang, D Apelian, and D.A Lados. Fatigue behavior of a356-t6 aluminum cast alloys. part i. effect of casting defects. Journal of Light Metals, 1(1):73 – 84, 2001. editor:a.salemi