Experimental support for yield strength calculation in aluminium cast material A226

Abstract

As the recent development of thermo kinetic software MatCalc has made it possible to successfully model the mechanical properties of wrought aluminium alloys, it is a natural step to go further in order to predict yield strength properties of aluminium cast alloys. The aluminium cast structure can be considered as a soft continuous matrix (aluminium solid solution), containing hard precipitates of silicon of different size and morphology. This structure is in much respect, like a metal matrix composite (MMC) for which numerous micromechanical models for tensile response have been proposed. The total yield strength of the cast can be considered as being given by the effect of the strength contribution of the pre-eutectic matrix and the eutectic matrix in which the effect of silicon is being considered.<br />In the present study, a squeeze cast material, type A226, was given for the yield strength calculation and simulation via MatCalc and the present study is to provide necessary experimental activities to support the modelling and simulation of yield strength calculations of the A226 squeeze cast material. Solidification in the squeeze cast process occurs at high pressure.<br />Because the thermodynamic database, which is used for the simulation process by MatCalc is based on atmospheric pressure, the simulation output for the A226 squeeze cast can differ from the properties of the real A226 squeeze cast material. Therefore a comparison between A226 squeeze cast and gravity cast materials has been done by material characterization of both materials. An experimental comparison of A226 squeeze- and gravity cast material has shown that in the squeeze cast samples, which were in the as-delivered and artificial aged condition, the yield strength and hardness is higher in comparison to the gravity cast samples. Further, quenching the A226 squeeze and gravity cast material with a higher cooling rate after the solution heat treatment and artificial aging leads to improved mechanical properties and finally, higher maximum hardness values were obtained in comparison to those, which were subjected to lower cooling rates after the solution heat treatment and artificial aging.<br />By running a SEM analysis on the A226 squeeze cast and gravity cast materials, in both, the same phases [theta]-Al2Cu, AlSi, Q-Al5Mg8Cu2Si6, [alpha]-AlSiFeMnCu and [beta]-AlFeSiMn were recognized, which differed in shape. Further, tensile tests on the gravity cast material (in heat treated and non-heat treated condition) have been carried out and the results can be used to validate the final simulation of the cast. To be able to simulate and calculate the yield strength of the cast, Scheil simulation of the A226 cast alloy was carried out to find the chemical composition of the pre-eutectic and eutectic part of the matrix. The chemical composition of the pre-eutectic part obtained to be Al-Si-Cu (Si, 1,3; Cu, 0,4) and the chemical composition of the metal eutectic part is near to the chemical composition of the alloy AA2014.<br />The metal eutectic part of the matrix (AA2014) represents the eutectic strength contribution of the cast material. Several experimental investigations on AA2014 were carried out in order to calibrate the precipitation kinetics for the eutectic part of the matrix. Differential scanning calorimetry (DSC) of the AA2014 was carried out with two different quenching medium, quenching in water and cooling in air. The experimental DSC curves were compared with DSC simulation to optimize the precipitation kinetic parameters.