The aim of present work is to study the influence of microstructure and defects on the mechanical properties of a structural high pressure die cast (HPDC) component of a commercial Al-Si based foundry alloy, EN AC-44300. The alloy which contains mainly 12% Si and 0.7% Fe, is a successful application of a die-casting alloy for the automotive market. Tensile test specimens were extracted from both high pressure die cast components and from ones with comparable microstructures produced through gradient solidification technique, which offers specimens with low levels of defects. The microstructure and defects available in the component were well mapped via X-ray inspection system, optical and scanning electron microscopy.  The results clearly confirmed the components’ performance dependency to configuration of defects and Si morphology as well as revealed the potential of the alloy in terms of ultimate tensile strength and ductility.

The aim of this paper is to assess the role of Sr modification on eutectic Si, Fe-rich intermetallic phases and porosity and their responses to the mechanical properties of a commercial high pressure die cast alloy Al-Si-Cu-Mg with Fe level up to 1%. Tensile test samples with a variety of coarsenesses, containing different Sr levels were cast using the gradient solidification technique, that enables a study of the solely influence of Sr on microstructure and tensile properties. The modification altered the morphology and size of eutectic silicon, while did not make a significant change on morphology of Fe-rich intermetallic and volume fraction distribution of porosity. The tensile test results indicate that eutectic Si modification is not a guarantee for improved mechanical properties due to the presence of a variety of intermetallics that tend to have a larger role on initiating and propagating cracks leading to premature failures in these commercial alloys.

The quantitative methods for controlling and predicting the level of Si modification in EN AC-46000 aluminum cast alloys were examined using thermocouples (thermal analysis) and optical microscopy (image analysis). A wide range of Sr, from 37 to 486 ppm, was added to the alloy. The alloys were cast using three different molds providing different cooling rate and consequently varied microstructure coarseness. Large difference in nucleation and growth temperature of unmodified and modified alloy was found irrespective of coarseness of microstructure. The depression in growth temperature of eutectic Si found to be strongly correlated to content of modification agent as well as modification level. Thermal analysis technique was realized as a non-biased, accurate and inexpensive approach for on-line prediction of Si modification level in the EN AC-46000 alloy cast under different cooling rate.

In this study the high temperature tensile deformation behavior of a commercial Al–Si–Cu–Mg cast alloy was investigated. The alloy was cast with two different cooling rates which resulted in average secondary dendrite arm spacing of 10 and 25 μm, which is typical of the microstructure scale obtained from high pressure die casting and gravity die casting. Tensile tests were performed at different strain rates (10^− 4 s^− 1 to 10^− 1 s^− 1) and over a wide temperature range from ambient temperature to 500 °C. The fine microstructure had superior tensile strength and ductility compared to the coarse microstructure at any given temperature. The coarse microstructure showed brittle fracture up to 300 °C; the fracture mode in the fine microstructure was fully ductile above 200 °C. The fraction of damaged particles was increased by raising the temperature and/or by microstructure coarsening. Cracks arising from damaged particles in the coarse microstructure were linked in a transgranular-dominated fashion even at 500 °C. However, in the fine microstructure alloy the inter-dendritic fracture path was more prevalent. When the temperature was raised to 300 °C, the concentration of alloying elements in the dendrites changed. The dissolution rates of Cu- and Mg-bearing phases were higher in the fine microstructure.

The flow stress of an as-cast Al-Si based alloy was modeled using a dislocation density based model. The developed dislocation density-based constitutive model describes the flow curve of the alloy with various microstructures at quite wide temperature range. Experimental data in the form of stress-strain curves for different strain rates ranging from 10⁻⁴ to 10⁻¹ s⁻¹ and temperatures ranging from ambient temperature up to 400 °C were used for model calibration. In order to model precisely the hardening and recovery process at elevated temperature, the interaction between vacancies and dissolved Si was included. The calibrated temperature dependent parameters for different microstructure were correlated to the metallurgical event of the material and validated. For the first time, a dislocation density based model was successfully developed for Al-Si cast alloys. The findings of this work expanded the knowledge on short strain tensile deformation behaviour of these type of alloys at different temperature, which is a critical element for conducting a reliable microstructural FE-simulation.