This paper aims to assess the role of Cu on Al-Si-Mg alloys, in a range of 0 - 5 wt%, qualitatively on microstructure, defect formation, in terms of porosity, and strength in the as-cast conditions. The ternary system of Al-Si-Mg, using the A356 alloy as a base material, were cast using the gradient solidification technique; applying three different solidification rates to produce directional solidified samples with a variety of microstructure coarsenesses. Microstructural observations reveal that as the Cu levels in the alloys are increased, the amounts of intermetallic compounds as well as the Cu concentration in the α-Al matrix are increased. Furthermore, the level of porosity is unaffected and the tensile strength is improved at the expense of ductility.
The alloy and cooling conditions govern to a large extent the mechanical properties of Al-Si cast alloys. The cooling rate determines the coarseness of the microstructure including the Secondary Dendrite Arm Spacing, SDAS, which is often used as measure of the coarseness of the microstructure, the fraction, size and distribution of intermetallic phases and the segregation profiles of solute in the α-Al phase. Si particles in the microstructure, acting as stress initiating sites, could be modified by employing proper amounts of Sr, altering their shape from needle-like into fibrous morphology; the outcome is a more ductile material. The strength of components is furthermore governed by proper additions of Cu and Mg, which will be done at the expense of ductility [1,2]. The plastic deformation behaviour will be depending upon factors such as whether the Cu and Mg are found as coarse intermetallic compounds, the level of Cu and Mg in solid solution, or if Cu and Mg are found as GP zones formed at room temperature and/or as precipitates due to a post solidification treatment. Cu and Mg present in Al-Si cast alloys lead also to the formation of bands of coarse Si particles as they also enlarge the solidification interval, leading to an increased risk in forming shrinkage porosity, leading to premature failures [
Many studies have been carried out in order to investigate the effect of single variables on the properties of cast Al-Si-Mg alloys and especially the Cu content. Some studies [4,5] emphasize that due to ternary eutectic reaction at about 525˚C, the Cu content in the eutectic melt is high which increases volumetric shrinkage during solidification and porosity and thus decreasing the strength of components. Other studies [6,7] reveal that the strength of these alloys are improved as Cu is added up to level of 5%.
Based on the design of castings used in these studies, the samples could be differently solidified and fed causing contradictory results. Therefore, this paper seeks to assess the solely influence of Cu and microstructure on the porosity level and mechanical properties of Al-Si-Mg alloys with Cu levels of 0% - 5% using well-fed gradient solidified samples. The study aims to impart knowledge and recommendations on selecting Al-Si-Mg alloys with proper Cu levels for attaining high strength components and casting qualities.
Seven Al-7%Si-0.4%Mg alloys, based on A356 master alloy, modified with approximately 200 - 250 ppm Sr, were cast having Cu concentrations as shown in
The samples produced using the gradient solidification technique have generally a low defect content. The solidification is directional giving a good feeding and gas and oxides are preferably pushed in front of the solidification front. Average SDAS (center-to-center distance between the dendrites) values from 10 measurements have been conducted to assure that the targeted microstructures are obtained.
Tensile test bars with a gauge length of 50 mm and a diameter of 7 mm were machined from the gradient solidified rods. Tensile tests were performed at a constant strain rate of 0.5 mm/min using a Zwick/Roell Z100 machine equipped with a 100 kN load cell and a clip-on 20 mm gauge length extensometer. Samples were tested until fracture, using three tensile test bars for each condition. Since the proposed gradient solidification technology has proven to deliver optimal tensile test results, revealing the potential of studied alloys, only the sample that performed the optimum quality is presented. Fracture surfaces and microstructures, including element segregation profiles, were studied using optical as well as scanning electron microscopy, SEM, equipped with energy and wavelength dispersive spectroscopy, EDS and WDS respectively. WDS measurements have been conducted on at least 3 dendrites, including centre and edge.
Using the Archimedes’s principle, the density was calculated using the relation, Equation (1),
where Wair and Wwater are the weights of the sample, measured in air and distilled water respectively. The accuracy of the analytical balance was approximately 0.0001 g, and the temperature ranged from 20˚C to 22˚C. The size of samples was 9 mm in diameter and 5 mm in length; the values presented are an average based upon 4 samples/condition.
Solidification of these alloys begins with the development of primary aluminium dendrite that are directionally grown along with cooling direction as demonstrated in
As the Cu levels are increased, area fractions and coarsenesses of phases such the Al2Cu phase, embedded in-between the dendrites, both as blocky and as eutectic, and the Q-Al5Mg8Si6Cu2 phase seem to be increased, see
agreement with Pedersen et al. [
The segregation profiles for Mg and Cu are shown in Figures 3(b) and (c). The Mg concentration of the samples with larger SDAS are on a higher level than the finer ones, which probably is a result of back diffusion due to the longer solidification time. Similar concentrations of Mg in the centre of dendrites in the as-cast condition have been reported by Sjölander et al. [
The Cu concentrations in the dendrites seem to be slightly influenced by the solidification rate, with a higher Cu concentration at the dendrite centre and edge for coarser microstructures, which is also supposed to be a result of back diffusion due to the longer solidification time. Cu concentrations in the centre of the dendrites have been reported in the literature, but these results show some variations. Qian et al. [
Possible reasons for deviations in the reported results and as compared to what is available in the literature are differences in measurement method used, EDS or WDS, and the difficulty of finding a dendrite that is cut through its centre. The spot size, being a larger part of the dendrite for the finest microstructure, influences the result by giving a too high concentration in the centre of the dendrite as a larger part of the dendrite is included in the measurement.
Second phase constituents such as Si and Cu-bearing particles are dispersed more finely and evenly as the cooling rate is increased, which in turn also has been observed to govern the length as well as the percentage area fraction of porosity. It is obvious that as the solidification time is short, small SDAS, less time will be available for the diffusion of the hydrogen into the interdendritic regions which results in small sizes of pores. Theories of oxide films [
Generally, as Cu is added, a ternary eutectic reaction at about 525˚C will occur leading to shrinkage that will not be compensated. Besides, the sample hydrogen activity coefficient might decrease with increasing Cu content and hydrogen solubility decreases, leading to increased porosity [4,5,21]. Due to the mode of solidification, the current study cannot confirm that increased Cu content lead to increased levels of porosity. Gradient solidified materials, directionally solidified, resulted on well fed samples, especially for SDAS 10 and 50 µm. Theoretical density calculations of the alloys confirm higher density values than the measured ones, as
Worth to indicate is that the melt hydrogen content has not been measured and assumed in this case to be constant for all the alloys since they have been produced under similar conditions
Cu is normally added to enhance the strength of Al-SiMg alloys, which is normally at the expense of ductility. Literature recommends lower levels of Cu, approximately 2% in order to avoid porosity formations which has a counter effect on the overall properties [
Furthermore, fracture surfaces of the Cu free tensile test samples reveal dimples and the mode of fracture is mostly transgranular. The coarser the structure becomes, the fracture seems to be shifted toward an intergranular mode of failure which is reasonably due to coarser intermetallics such as Cu-bearing and Si particles, which are once cracked, they easily link the cracks due to an increased levels of stresses at particles tips. As the level of Cu is increased, the ductility is decreasing and brittle Cu-intermetallics are found like “brittle layers” in between the secondary dendrite arms. Fracture surfaces of Cu rich alloys displays dimples for the finer microstructures and the higher Cu levels accompanied coarser
structures the mode of failure is nearly always transgranular.
The following conclusions can be drawn from this study:
1) Al-Si-Cu-Mg alloys contain several interdendritic phases and the more Cu is added, the larger the area fractions of Cu-rich compounds. The size of the particles is largely influenced by the solidification rate, SDAS.
2) The solidification rate impacts on the segregation profiles of Si and Cu but seem to be stronger for Mg 3) Pore fractions and sizes of the alloys are not depending on Cu content but influenced by the mode of solidification.
4) The tensile strength seems to be favourably influenced by the addition of Cu on the expense of ductility which is lowered due to the increased levels of intermetallics.
5) The tensile strength decreases simultaneously by decreasing the cooling rate, SDAS, of the alloys.
6) Depending on the component design, castings that are directionally solidified and well fed, are beneficially alloyed with higher levels of Cu.
The authors would like to thank the Swedish Knowledge Foundation, Stena Aluminium AB and CompTech AB for the financial support.