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WEB Influence of casting process, cooling rate and grain refining additives on the microstructure and the mechanical properties of an AlSi8.4MgMn cast alloy

Friday (25.09.2020)
11:35 - 11:50 S: Structural Materials 1
Part of:

Hypoeutectic AlSi cast alloys are used for numerous lightweight applications, e.g. automotive body parts [1]. Since there is a common interest of extending the application fields and simplify alloy systems, a comprehensive understanding of the correlation between casting practice (like casting process, cooling rate, grain refining additives), microstructure and mechanical properties is indispensable.
Currently - next to other additives - grain refiners are added to the melt before die casting with the aim to positively influence the microstructure by a finer grain size, a higher homogeneity as well as less and smaller porosity and thus the mechanical properties [2,3]. In the case of high pressure die casting (HPDC), where the solidification rate is much higher compared to low pressure (LPDC) or gravity die casting (GDC), the microstructure is thoroughly much finer. Nevertheless, the need of grain refiners in HPDCs has not yet been investigated properly.

In the presenting work the influence of casting process (GDC, HPDC), cooling rate and the grain refining additives AlTi1.5B1.3 and AlNb2B2 (concentrations 0.1 and 0.3/1 wt.-%) on the properties of an AlSi8.5Mg0.4Mn0.4 alloy with small amounts of  Fe, Ti and Zr was investigated. The microstructure was analyzed by means of light optical microscopy (LOM) and scanning electron microscopy (SEM) techniques. Mechanical data were determined by hardness-, compression- and tensile testing.

Through EBSD measurements it could be confirmed that the grain size in HPDC parts is much smaller compared to GDC parts. While in the GDC parts one grain consist of several primary dendrites in the HPDC parts one grain consists of just one to a very few equiaxed primary dendrites. In the case of GDC through the addition of 0.3 wt.-% AlTi1.5B1.3 and 1 wt.-% AlNb2B2 an obvious grain refinement was achieved. Despite the changes in grain size the mechanical properties remained almost unaffected, what leads to the assumption that the deformation behavior is dominated by the structure of primary phases and the dendrite arm spacing in the as cast state. Further, it was observed that the addition of grain refiners to the HPDC parts - for a wall thickness of 3 mm or, respectively, 5 mm - has no decisive positive influence on the mechanical properties. Instead, a reduction of necking deformation with increasing grain refiner concentration was found.

[1] F. Casarotto, A. J. Franke, R. Franke, High-pressure die cast (HPDC) aluminum alloys for automotive applications, In Advanced Materials in Automotive Engineering, ed. J. Rowe, pp. 109-149, Woodhead Publishing, Cambridge/Philadelphia, 2012.
[2] A.L. Greer, Overview: Application of heterogeneous nucleation in grain-refining of metals, The Journal of Chemical Physics 145 (2016) 211704-1-211704-14.
[3] G.K. Sigworth, T.A. Kuhn, Grain refinement of aluminium casting alloys, International Journal of Metalcasting (2007) 31–40.

M.Sc. Nina Pfeffer
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Additional Authors:
  • Dr. Heinz Werner Höppel
    Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)