Metallographic Investigation and Solidification-Structure Modelling of Al Micro Castings J-F. Charmeux a , R. Minev a , S. Dimov a , E. Minev a a Manufacturing Engineering Center, Cardiff University, Cardiff, CF24 3AA, UK Abstract Producing micro-castings trough vacuum investment casting is known to be associated with high cooling rates due to small scale of the castings. High cooling rates together with alloy composition might be the main factors affecting the final metallographic structure of castings’ alloys during the solidification process. When using Al-Si-Mg casting alloys, the size of the dendritic structure can be used for a non-destructive test to assess the mechanical properties and overall quality of the castings. Also the ability of the alloys to be structured by different mechanical and energy assisted processes is highly dependant on their metallographic structure. This paper investigates the structural changes occurring when shifting from conventional size castings to micro-castings. An empirical model describing the degree of dendrite cell refinement as a function of micro-castings aspect ratio is proposed. Additionally, the paper reports the changes in porosity and Si particles morphology observed on micro-castings produced with different flask temperatures. MHV measurements were also performed for different aspect ratio of the cast micro-features. Keywords: micro-casting, dendrite cell refinement, investment casting. 1. Introduction The capability of the Vacuum Rapid Investment Casting (VRIC) to produce complex micro and meso- micro components was studied in details in [1-3]. It was shown that features with size less than 150 µm, ribs with aspect ratio (AR) higher than 50, and surface roughness in the range of 5 µm could be reproduced with accuracy of 10% or less. This makes the technology quite promising in Rapid Prototyping (RP) micro features and components especially by applying RP techniques to produce accurate sacrificial patterns or casting clusters through direct shell processes. However, our knowledge is still limited in regards to microstructure and properties of these castings. It is not clear how the extreme cooling conditions associated with this technology affect the structure and mechanical properties of the components. Also, it is important to verify to what extent the existing solidification theories are valid when micro size casting channels are utilised. The importance of answering those questions arises from the emergence of new application areas, e.g. the use of aluminium (Al-Si-Mg) castings in microelectronics cooling devices, and heat exchangers [4]. Since the material morphology of the castings affects their machining response [5-6], it is also important to obtain information on as-cast microstructure of micro components and thus reduce uncertainty that this introduces to their further structuring with mechanical and energy assisted processes. It is well known that the solidification conditions of cast metals have a direct relationship to their metallographic structure and mechanical properties [7- 8]. Over the years, extensive research has been done to assess the influence of casting parameters on the mechanical properties of Al alloys [9-11]. The work of Spear & Gardner [10] showed that the dendrite cell size measurement could be used to describe their structural refinement. They concluded that both alloy composition and solidification rate were major factors influencing the dendrite cell refinement. The dendrite cell size of Al casting alloys could be related to the solidification rate using the following equation: φ λ R C ⋅ = , (1) where: λ - dendrite cell size (DCS) in [µm]; R - K/s- solidification rate; C and φ - constants. They also concluded that a decrease of dendrite cell size leads to an increase of the tensile ductility of the castings. This work has been more recently re-examined by Wang & Cáceres [11] in the light of the current understanding of the relationship between microstructure and plastic deformation of Al–7Si–0.4Mg alloys. It was concluded that although DCS provided a means of linking mechanical properties to solidification conditions, they played only a limited role in determining the tensile ductility of these alloys. This was attributed to the fact that the size, shape and distribution of the eutectic Si particles is also an important factor affecting the tensile behaviour of these alloys [11-12]. In general, for unmodified alloys under normal cooling conditions, Si particles are polyhedral and are in the form of coarse acicular needles [12]. These Si particles are large and brittle and act as crack initiators that consequently lower the ductility of the castings [11- 13]. However, when fast cooling rates are applied (>10 K/s), eutectic Si particles undergo a morphological change and are present in the microstructure as small individual particles [14-15]. Such eutectic type is called fibrous eutectic [16]. Due to the poor thermal conductivity of the ceramic shells in investment casting, the cooling rates are low during the solidification process, which often produces equiaxed and coarse structures that lower the mechanical properties of the castings [7-8]. With the decrease of the casting channels, especially at micro scale, the cooling rates increase even further and extreme values can be achieved [17]. In this research, an experimental study is conducted to investigate the degree of dendrite refinement when producing micro components using Al-Si-Mg casting alloys. An empirical model is proposed to describe the evolution of DCS in walls with different thicknesses as a function of their AR for a given mould/metal materials and processing parameters.