186/1

Prediction of the influence of microstructure, porosity and residual stresses on strength properties of aluminum castings R Baehr *, M Todte * and H Stroppe **. * University of Magdeburg, Casting Technologies, Germany, ** Institute of High Technologies and Education e.V. Magdeburg, Germany Abstract It is necessary for a design engineer of castings to precisely know the me-chanical properties (yield strength, tensile strength, elongation to fracture, strain hardening exponent and so on) of every part of the casting. These are very closely connected to the locally very different cooling- and solidifi-cation preconditions of the melt. Estimating strength properties through all the varying characteristics of the cast structure (dendrite structure, mor-phology of pores, eutectic phases, residual stresses and so forth) via nu-meric simulation is yet an unsolved task of great theoretical and practical importance. The introduced research results have created a possibility to already pre-dict achievable properties of the casting during the construction phase. By taking the primary simulation results and calculating their mechanical properties and quality index, several production techniques, casting alloys and geometrical variants of castings will virtually be analyzed on a com-puter. Results of own analyses will be referred to in this article. Key words Simulation, mechanical properties, microstructure, residual stresses, fa-tigue life.

186/2

Introduction Simulation of casting processes has become an important tool in foundry technology worldwide. They make it possible to numerically calculate complex activities of flow and numerous phenomena of solidification and cooling of castings. In order to achieve a practical realization of the results, adequate criteria functions have to be used. By evaluating the calculated primary temperature- and flow fields including their mathematical-physical regularities, certain casting properties can be estimated. Regarding casting products, especially in the automotive industry, there’s a trend recognizable towards the production of highly specialized parts that reach the limit of their strength. This requires the development and use of qualitative novel simulation tools. Therefore, the faculty of foundry technology of the Otto-von-Guericke University Magdeburg, in close col-laboration with industrial partners (like NEMAK, Porsche), is intensively engaged in the estimation of mechanical properties that appear during the solidification of the melt (yield strength, tensile strength, elongation to frac-ture, strain hardening exponent and so on), whereas, so far, we were only able to simulate the residual stress of the cast part, as well as certain structure properties such as dendrite arm spacing, porosity and so on. One cannot assume that the properties of castings are homogeneous throughout the entire part. This holds true especially for geometrically complicated cast parts, due to the different local cooling- and solidification conditions of the melt (Figure 1). Furthermore, inner tensions, which can take pretty high values, develop within the casting and may influence the casting’s strength behavior. Since not only specific structure properties (dendrite arm spacing, poros-ity), but also residual stress of the casting can be determined via simula-tion, we have the opportunity to predict the impact of the casting process on the static and cyclic strength properties of a casting via virtual reality. Correlations between mechanical properties, structure properties and solidification parameters The above mentioned mechanical properties of a casting are significantly dependant on its structural properties. These are, in turn, influenced by the chemical composition, the metallurgic treatment of the melt (decrease of grain size, refinement and so on), the different preconditions and parame-ters for the process during the casting- and solidification activities, as well as on the after-treatments (heat treatment, HIP) of the casting. The structure of AlSi casting alloys is basically characterized by the pri-mary dendritic growth of the α-phase in aluminum, as well as by the for-mation of an interdendritic eutectic. The secondary dendrite spacing (DAS), as well as the formation structure and distribution of hard eutectic Silicon particles and intermetallic phases respectively are the substantial structural properties, which have an effect on ductility- and strength prop-

186/3

erties of the solidified casting alloy. Additionally, there are the feeding- and solidification properties. All the mentioned structure properties are dependant on each other; they correlate with each other via local solidification time. As the solidification time increases, both dendrite arm spacing (Figure 2), and the eutectic or intermetallic particles and pore volume increase as well. These structure properties in turn have a quantifiable impact on the solidification properties (Figure 3). Thus, the solidification time turns out to be a basic decisive pa-rameter for the quality of the casting structure, with which a prediction of mechanical properties in a simulation of the casting- and solidification pro-cess can be made; provided that the correlations between these proper-ties and the structure parameters are known (Figure 4). Influence of porosity on mechanical properties The physical behavior of cellular structures is, next to the structure build-ing material, characterized by its porosity, which is characterized by por-tion, arrangement, size and shape of each pore. Porosity in casting parts is, next to dimensional deviation and surface irregularities, one of the most frequent reasons for rejection. A degradation of the mechanical properties of the casting is, as known, adherent to this porosity. Therefore, this phe-nomenon was included in the simulation model. Research results from former works [1] formed the basis. If one applies the derived mathematic-physical relations between mechanical properties and porosity to the re-search results, you receive a high concordance of experimental and the actual, calculated values (Figure 5). Implementation of criteria-functions into the simulation system Appropriate correlation functions between solidification parameters, struc-tural properties and mechanical casting properties were deduced by Todte (with porosity involved) and integrated to the known simulation program SIMTEC/WinCast® of the RWP GmbH Germany [2]. That way we are able to predict the mechanical properties, as well as the quality index in de-pendence on the alloying additions and the solidification process [3 to 5] Aiming at the verification of the newly developed correlations, locally cal-culated characteristic values have been compared to values estimated by simulations. Figure 6 shows an example: The tensile strength of two cho-sen measuring points of a cylinder head. Despite possible errors (such as subjectively conditioned tolerances, systematic errors), all results show high and even very high correlations between experiment and theory [2]. Estimation of fatigue life of engine components As the power output of modern combustion engines increases, the mate-rial-appropriate design and the dimensioning of cylinder heads made of aluminum casting alloys grow more and more important. On the one hand, the installation- and operation preconditions (like pretensioning force, cyl-inder pressure and oscillating mass-forces) have a higher-frequented,

186/4

largely isotherm fatigue of the material. On the other hand, temporally and locally inhomogeneous fields of temperature and stress, which develop in, for example, start-stop- and load changes, result in a low cycle fatigue, as well as in creep resistance. Those aren’t only dependent on temperature and level of stress, but also to a great extent on the structural properties of the material. Calculating the fatigue life of castings, especially of cylinder heads, during cyclic stress in dependence on material properties and the amount of load, is yet a problem that has to be solved. Most of the theories that have been developed so far base on macroscopic-phenomenological points of view, in which the material’s characteristics, such as residual stress, porosity and structure formation, plus their impact on locally static and dynamic strength properties, are disregarded. [6, 7] Highly stressed cylinder heads usually consist of a hypoeutectic AlSi-alloy. The structure’s fineness, which significantly influences the fatigue strength, is primarily conditioned by the secondary dendrite arm spacing. Since the dendrite growth correlates with the local solidification time of the melt, it is controllable through the casting process. When considering the influence the structure has on the analysis of the operational stability, there has to be used a material model in dependence on the dendrite arm spacing for every FE-structure element. In order to realize this, complex tests to calculate the material-S-N curve on test items are necessary for every alloy in different states of solidification. Based on an analytic model of structure-dependent material fatigue devel-oped by Stroppe [8] for aluminum alloys, calculation methods are being developed that allow to calculate S-N curves of hypoeutectic AlSi-casting alloys not only within the time strength range, but also in the area of the low cycle fatigue for any tension depending on the dendrite arm spacing (Figure 7). The new theoretical model also allows us to take the influence of porosity and residual stresses on material fatigue into consideration. So far, no analytic approach in casting simulations is known that makes it possible to calculate S-N curves for different kinds of stress from structure parameters (like dendrite arm spacing) via a model based on material-physics. An attempt to estimate dynamic characteristic values for a cylin-der head is shown in Figure 8. By introducing specific model theories into the simulation system SIMTEC/WinCast®, it is possible, for instance, to es-timate the permanent stress cycles in dependence on dendrite arm spac-ing (and therefore on casting- and solidification processes). Summary Based on new mathematic-physical models to describe the developing structure- and cast part properties during the solidification of the melt, it is possible to already predict mechanical properties of cast parts during the development phase via numeric simulation. In order to get these predic-

186/5

tions, one used to be dependent on long-winded and cost-intensive analy-sis of the material. Furthermore, not all areas of the casting have the geo-metrical preconditions for taking samples. Therefore, “trial and error-loops” still have to be run trough until new components can be produced. With the presented research results, the numeric simulation of casting proc-esses has been enhanced by the creation of a physically based calcula-tion of mechanical properties and the quality index. Due to the increasing competitive pressure on the one hand and the pre-sent development status of hard- and software on the other hand, basic innovation in the process of product design is to be expected within the coming years. In the nearer future it’s going to be possible to make realis-tic statements concerning essential product characteristics already during an early state of development. References 1. Stroppe H, Einfluss der Porosität auf die mechanischen Eigen-

schaften von Gusslegierungen, Gießereiforschung 52 (2000) 2, pp 58 – 60.

2. Todte M, Prognose der mechanischen Eigenschaften von Alumi-nium-Gussteilen durch numerische Simulation des Erstarrungs-prozesses, Dissertation (2003), Otto-von-Guericke University Magdeburg.

3. Bähr R; Scheel B; Mnich F; Stroppe H; Ambos E and Todte M, Optimization of the mechanical properties of highly stressed cast-ings through direct control of the casting process, Proceedings of the 65th World Foundry Congress, Gyeongju, Korea 2002.

4. Stroppe H; Todte M; Ambos E and Bähr R, Einfluss von Erstar-rung und Gefüge auf den Qualitätsindex von Aluminiumguss, Gie-ßerei Forschung, 55 (2003) 4, pp 151 – 155.

5. Todte M; Stroppe H and Honsel C, Prognose der mechanischen Eigenschaften von Aluminium-Gussteilen durch numerische Simu-lation des Erstarrungsprozesses, Gießerei-Praxis, (2003) 6, pp 263 – 269.

6. Nefischer P; Steinparzer F and Kratachwill H, Neue Ansätze bei der Lebensdauerberechnung von Zylinderköpfen ,12. Aachener Kolloquium für Fahrzeug- und Motorentechnik 2003.

7. Minichmayr R and Eichlseder W, Lebensdauerberechnung von Gussbauteilen unter Berücksichtigung des lokalen Dendritenarm-abstandes und der Porosität, Giesserei 90 (2003) 5, pp 70 – 75.

8. Stroppe H, Lebensdauerberechnung von Gussbauteilen unter Be-rücksichtigung des lokalen Dendritenarmabstandes und der Po-rosität, Magdeburger Forschungsinstitut für Fertigungsfragen MFF e.V., (2004) previously unreleased.

186/6

Figures

Figure 1: Experimentally determined, locally different mechanical proper-

ties within a cylinder head made of an Al-alloy (DAS = secondary dendrite arm spacing)

Figure 2: Experimentally determined correlation between dendrite arm

spacing DAS and the local solidification time tS in an Al-alloy

186/7

Figure 3: Experimentally determined correlation between yield strength,

tensile strength and dendrite arm spacing (DAS)

186/8

Figure 4: Prediction of mechanical properties of a casting through results

of numeric simulation during the solidification process

186/9

Figure 5: Experimentally determined and theoretically calculated de-

pendency of the tensile strength Rm on porosity P (s = shape factor)

Figure 6: Comparison of the calculated* and experimentally determined**

values of the yield strength

186/10

Figure 7: Dependency of the cycles of fracture on the dendrite arm spac-

ing DAS for the alloy GK-AlSi7Mg0,3 wa (measuring point taken from [7], curve progression calculated according to Stroppe [8])

Figure 8: Fatigue life (cycles of fracture) of a cylinder head, calculated

with the program SIMTEC/WinCast®