Study of AlSi7Mg T6, AlSi7Mg AND AlSi11Mg alloys applications as an automobile wheel material

AlSi7Mg and AlSi11Mg hypoeuthectic alloys are most common alloys for the aluminium wheels production with low pressure die casting method. Since these alloys contain less than %12.7 Si, these are called hypoeuthectic alloys. Production of AlSi7Mg is possible with or without heat treatment (T6, artificial age hardening). Production of AlSi7Mg alloy without heat treatment ise not common, even though there are products produced with AlSi7Mg alloy without heat treatment. However, AlSi11Mg alloy is not proper for heat treatment. The information which alloy has to be chosen for the given product has been defined in specification sheet by the original equipment manufacturer (OEM) with the styling of the wheel. Besides, the common target of original equipment manufacturer is to design products which are lighter and less expensive in the mean of material and manufacturing without a decrease in the strength level. Because of this, in order to be compatible in the aluminium alloy market, it is necessary to be able to design and produce light, robust and less expensive products, which are designed and optimized according to the market demand. Even though original equipment manufacturer defines which alloy has to be used for a given product, a solution where the strengh of the product is not decreasing with a decrease in price will be acceptable from the OEM, as long as it can be proved with tests and simulations. OEM's request these studies where the product design and manufacturing know-how is strongly needed from their suppliers. The suppliers can compete aggresive in the market, which can present low-cost and robust products and valuable solutions in the mean of technical know-how. If we compare AlSi11Mg, AlSi7Mg with heat treatment (T6) and without heat treatment, we see that all have advantages and disadvantages in the mean of producibility, robustness and price. Because of this, by defining the alloy for a given product, the design and the size of the wheel is very important. The most robust products are produced with heat treated AlSi7Mg alloy. Because of this, big wheels, which have high wheel load, have to be produced with AlSi7Mg T6 alloy in order to sustain the high wheel load, since there is no other alloy available for such wheel loads for the low pressure die casting applications. Generally, the cars, where big wheel have been used, require big wheel loads. However, heat treated alloys are more expensive because of the heat treatment process and time necessary for heat treatment process and higher ratio of the rejected parts because of the distortion coming from heat treatment process. Because of the higher %Si ratio by AlSi11Mg, AlSi11Mg alloy's casting process is easier in comparion with AlSi7Mg alloy. Additionally, solidification time of AlSi11Mg alloy is %20 shorter than AlSi7Mg alloy. Accordingly, number of production of AlSi11Mg alloy is %20 more than AlSi7Mg alloy for a given time. However, since the time necessary for solidification is very short by the AlSi11Mg, so that the solidification control cannot be done very well, AlSi11Mg alloy's elongation values are lower than AlSi7Mg alloy's elongation values. Besides, yield and tensile strength values of AlSi11Mg alloys are lower than AlSi7Mg alloy's yield and tensile strength values. Because of this, AlSi11Mg alloys can be used for a limited product range and size, but there is no exact limit size for the usage of alsi11Mg alloy. This limit will be defined not only by the size of the wheel, also the design of wheel defines whether it is proper to use AlSi11Mg alloy or not. In order to make the AlSi11Mg alloy for the alloys which require high wheel loads, it is necessary to design the wheel heavier than the limit weight, which would be not allowed. In order to make AlSi11Mg alloys, heat treatment application have been tried; however, because of the unadequate %Mg ratio of AlSi11Mg alloy was the increase of the strength not enough to make heat treatment application feasible for this alloy. Additionally, diamond wheels production is not possible with AlSi11Mg alloys, which market demand increases every year. The reason is, since AlSi11Mg casting actualize in a lower temperature than AlSi7Mg, the density index of the alloy will be increased by adding additional hydrogen to the melt alloy, which increases the hydrogen porosity level of the wheel. The discontinuity with the increased hydrogen ratio is not a reason for X-Ray rejection, however, after machining of styling surface for diamond cut production, the pores became visible which couldn't be covered with the painting process. This is also an another limitation for the production with AlSi11Mg alloy. AlSi7Mg alloy is proper for the heat treatment process, that's why the yield and tensile strength values of the product produced with AlSi7Mg alloy is not as high as heat treated AlSi7Mg alloy. AlSi7Mg alloy's density index is lower than AlSi11Mg alloy, so that its' progress in the tool as a molten metal, in other words, its' castability is not as good as AlSi11Mg alloy. If we compare the strength of the AlSi7Mg alloy with AlSi11Mg alloy, it can be seen that the yield and tensile strength of the AlSi7Mg alloy is higher than AlSi11Mg alloy, however, the impact behaviour of these alloys cannot be predicted, since it is mostly up to the alloy's design. The target of this study is; to produce a wheel, which is given with a size 6,5 J x 16" and AlSi7Mg heat treated alloy, with AlSi7Mg alloy without heat treatment and AlSi11Mg alloy, in order to make a comparison in the mean of castability, metallurgical structure and mechanical behaviour and select the most low-cost solution for this wheel. The properties of AlSi11Mg and AlSi7Mg alloys and their advantages and disadvantages are well-known in the literature; however, it cannot be decided whether the low-cost alloy is suitable for a given wheel without doing the mould design verification and product design verification studies. First, it has to be proved whether the product and mould design is proper for casting two different alloys. Within this scope, it has to be controlled whether there is a need to change the mould or product design. These controls will be done by execution of casting simulation using finite volume analysis program Magma 5.3 and verified with real production. After this study, product design verifications and tests has been be done. Within the scope of product verification analysis, ISO7141 13° impact simulation has been performed dynamically using Abaqus Explicit 6.9 and fatigue simulation has been performed statically using Ansys Workbench 14.5. Within the scope of product verification tests; impact test, dynamic fatigue test, tensile test, hardness test, chemical analysis and microstructure analysis have been performed and compared. At the end of this study; it is observed, how the mechanical and metallurgical behaviour of the wheel is changing by comparing finite element analysis simulation results and test results. Additionally, it is aimed to grow the technical know-how for the future projects by the selection of alloy.