Zum Einfluss interdendritischer Erstarrungsporen auf die mechanischen Eigenschaften von Al-Legierungen
Final Report Abstract
The objective of this project was to investigate and quantify the influence of microporosity on the mechanical behaviour of A356 (AlSi7Mg0.3) experimentally and by numerical simulations. Two alloy variants were prepared by sand casting: a “pure” ternary AlSi7Mg0.3 alloy from pure base materials and a “technical” A356 alloy according to industrial standards. Gas porosity was varied by the addition of hydrogen during melt preparation. Microstructure and porosity were investigated by optical microscopy and computer tomography. The porosity varied largely even for samples from the same casting. This was attributed to a limited observation area and degassing during solidification. Averaging over all samples with nominally the same density index (DI) revealed an increase of porosity with the DI according to Boyle’s law. The “pure” and “technical” alloy showed no difference regarding porosity. Stress-strain curves from tensile testing showed also a large scatter. Brittle fracture characteristics were observed in both alloys and confirmed by failure surface observations. Heat treatment resulted in an approximately twofold increase in strength and reduced ductility. After heat treatment no effect from porosity was identified in the stress-strain curves for the different castings. Both materials showed the same strength levels and stress-strain curves differed only in the strain to failure without any systematic dependency on porosity. Compact tension tests (CTT) have been performed for both alloys to obtain J-R curves as well as the fracture toughness Jic. Both alloys show an increase in fracture toughness with increasing porosity. For a the A356 technical alloy the fracture toughness is a little bit higher with the same volume fracture of porosity as for pure alloy, That means that with decreasing size of pores the fracture toughness increases which in accordance with our simulations of microscopic crack propagation features in the microstructures. The phase-field method has been used to simulate solidification microstructures and extended towards the simulation of hydrogen porosity. The dendritic morphology could be reproduced in simulations as well as the coarse eutectic microstructure found in the pure alloy. Scale bridging simulations are possible with eutectic Si and fcc-Al lumped into one “effective” eutectic phase, representing the correct phase fractions but not resolving the eutectic structure. Two different methods have been developed for the simulation of porosity formation. The main difficulty to overcome is the large volume change connected to gas pore formation. In a first approach the volume expansion is modelled as “volume diffusion” and simulations showed that the relation between external pressure and hydrogen content in the melt fulfil Sieverts law. A second, computationally more efficient approach describes the gas phase as a “second liquid phase” with enhanced hydrogen solubility equal to the hydrogen density in a gas pore. This concept enabled 3D-simulations, simulated pore morphologies showed fair agreement to experimental gas pores. Simulations of failure behaviour with the Rousselier model (macro-level) incorporate the data from metallographic observations and tensile test experiments. The crack paths were predicted for microstructures with embedded voids using the element elimination technique (EET), here also microstructures from phase-field simulations have been investigated. Without pores cracks were found to initiate at silicon lamellae elongated along the tensile axis and propagate into the fcc-matrix phase, perpendicular to the loading axis. In microstructures with porosity the stress concentrations from pores change the crack path in the later stage of loading (deformation around 3%) in a way that cracks propagate in directions connecting pores, leading to an increase in the length of the crack and correspondingly to the increase in fracture toughness in agreement with experiment. CTT were simulated with the Cohesive Zone Model (CZM) combining macro and microscale. Measured values for fracture energy and cohesive strength were used to define the Traction-Separation Law (TSL) employed in the CZM. The simulations with measured values for fracture energy and cohesive strength and with variation in the shape of TSL (triangle and trapezoid) didn’t reveal exact matching with experimental Force-COD-curve. The physical reason behind the discrepancy could lay in the variation of the microstructure along the crack path. Another possible reason is an alteration of the mechanical properties of the material at the crack tip caused by the loading-reloading procedure during the test. The experimental Force-COD curve has been successfully reproduced in simulations by variations of the fracture energy along the crack path. The variation of cohesive strength along the crack path would be the next step in a modification of the simulated Force-COD-curve in the direction of experimental values. CZM simulations for variable pore sizes keeping the volume fraction constant showed higher fracture toughness for smaller pores. The performed technique of the simulation of crack propagation in the microstructure could be applied for the analysis of the fracture toughness of the material with different pore distributions.
Publications
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“Simulation of the Microstructure Formation in Technical Aluminum Alloys using the Multi-Phase-Field Method”, Transactions of the Indian Institute of Metals 62 4-5 (2009) 299
B. Böttger, A. Carré, G. J. Schmitz, J. Eiken, M. Apel
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“Phase-field modelling of gas porosity formation during the solidification of aluminum“, International Journal of Materials Research 101 (2010) 4
A. Carré, B. Böttger, M. Apel
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“Effect of microstructure on the mechanical behaviour of Al7%Si0.3%Mg alloy”, 21th International Workshop on Computational Mechanics of Materials (IWCMM21), Limerick, Ireland, August 21st-24d 2011
G. Lasko, M. Apel, A. Carré, U. Weber, S. Schmauder
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“Effect of Microstructure and Hydrogen Pores on the Mechanical Behavior of an AlSi7%Mg0.3% alloy Studied by a Combined Phase-Field and Micromechanical Approach”, Adv. Eng. Mat. 14, pp. 236-247 (2012)
G. Lasko, M. Apel, A. Carré, U. Weber, S. Schmauder
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“Experimental and numerical study of the influence of the porosity on mechanical properties of AlSi-casting alloy”, Tomsk International conference ”Multiscale hierarchically built systems of organic and inorganic nature”, September 9-13, 2013, Tomsk, Russia
G. Lasko, U. Weber, S. Schmauder, M. Apel, R. Berger
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“Finite element simulations of damage in AlSi7%Mg0.3% casting alloys on micro and macrolevel”, 21st Annual international conference on composites or nano engineering (ICCE-21), Tenerife, Spain, July 21-27, 2013
G. Lasko, U. Weber, S. Schmauder
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“Finite Element Study of crack propagation in AlSi-cast alloys on macro- and microscale level”, 24th International Workshop on Computational Mechanics of Materials (IWCMM24) in Madrid, Spain, October 1-3, 2014
G. Lasko, U. Weber, S. Schmauder, M. Apel, R. Berger
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“Finite Element Analysis of Crack Propagation in AlSi7%Mg0.3% Cast Alloys using Macro and Micro-scale Levels”, Adv. Eng. Mat. 17, pp. 1536-1546 (2015)
G. Lasko, U. Weber, M. Apel, R. Berger, S. Schmauder
