Statistical Relationship Between Strontium Content and Cooling Rate on A356 Alloy by Using Regression Analysis

Eutectic silicon modification is an important casting parameter on Al-Si alloys on the aspect of mechanical capability and energy absorption of the cast part. Chemical modifier element strontium has been used to obtain eutectic modification on Al-Si alloy commercially. On the other hand, high cooling rate on Al-Si alloys both refine dendrites and silicon phase which enhances mechanical characteristic. In order to find a statistical relationship between strontium amount and cooling rate, a special mold was designed in order to obtain different range of cooling rates in same cast part, then tensile test data of A356 alloy were analyzed in Minitab software. Therefore, after regression and analysis of variance tests have been proceeded, it was found that strontium amount is only dominant for lower cooling rates of < 0.9 oC.

Effects of Eutectic Modification and Grain Refinement on Microstructure and Properties of PM AlSi7 Metallic Foams

For AlSi7 foams, microstructure modification by variation of solidification rates and addition of Sr, B and TiB2/TiAl3 was investigated and its transfer to powder metallurgical metal foaming processes demonstrated. Microstructural characterization focused on grain size and morphology of the eutectic phase. Cooling rates during solidification were linked to secondary dendrite arm spacing, establishing a microstructure-based measure of solidification rates. Effects of refining and modification treatments were compared and their influence on foam expansion evaluated. Studies on foams focused on comparison of micro- and pore structure using metallographic techniques as well as computed tomography in combination with image analysis. Reference samples without additives and untreated as well as annealed TiH2 as foaming agent allowed evaluation of pore and microstructure impact on mechanical performance. Evaluation of expansion and pore structure revealed detrimental effects of Sr and B additions, limiting the evaluation of mechanical performance to the TiB2 samples. These, as well as the two reference series samples, were subjected to quasi-static compression testing. Stress-strain curves were gained and density-dependent expressions of ultimate compressive strength, plateau strength and tangent modulus derived. Weibull evaluation of density-normalized mechanical properties revealed a significant influence of grain size on the Weibull modulus at densities below 0.4 g/cm3.

Determination of temperature-dependent crystallographic parameters of Al–Si alloys using in situ neutron diffraction

Accurate evaluation of temperature-dependent lattice spacings of individual phases in complex engineering alloys enables analysis and design of enhanced components for high-temperature applications. However, typical characterization techniques are limited to measurements at temperatures at which the alloy is completely solid state. In this study, in situ neutron diffraction was performed to determine the crystallographic parameters of several phases in unmodified and Sr-modified Al–6 wt% Si and A319 alloys from the start of solidification to 423 K. Approximately linear correlations were found for the interplanar spacings and lattice parameters of Al, Si, Al5Mg8Cu2Si6 and Al15(Fe,Mn)3Si2 phases with temperature for the entire measured range. The greater concentration of lattice-contracting elements dissolved in the A319 solid solutions resulted in smaller lattice spacings and larger coefficients of thermal expansion than the same phases in Al–6 wt% Si, yet eutectic modification with Sr did not produce noticeable changes in the crystallographic parameters. The Si phase was found to have the largest thermal expansion coefficient misfit with the Al matrix. These findings offer significant insight into the development and mitigation of thermal stresses in the processing and operation of engineering components.

Effect of cooling rates of production process of Al-3%B-3%Sr master alloy on grain refinement and eutectic modification efficiency in cast A356 alloy

In this paper, size and morphology of the grain refiner and modifier particles in the Al-3%B-3%Sr master alloy production by using different cooling rates were investigated. Two Al-3%B-3%Sr master alloys were produced with 0.2 and 10°C/s, respectively. The grain refinement and eutectic modification efficiency of the Al-3%B-3%Sr master alloy were tested in casting process of A356 alloy by addition of 4wt.% and holding times for 10-120 min. The experimental result showed that microstructure of the M1 alloy (Slow cooling) consisted of larger solidified particles of AlB2, SrB6 and Al4Sr in the matrix of α-Al compared to the M2 alloy (Rapid cooling). The addition of the M1 alloy in cast A356 alloy, it was found that small grain size and fully modify eutectic silicon were obtained from the holding time in a range of 10-60 min. While the addition of M2 alloy, a small grain size was achieved in shorter holding time in a range of 10-30 min but the eutectic silicon was partly modify. From the thermal analyzed result, solidification of un-modified A356 alloy was changed after addition of Al-3%B-3%Sr master alloy. It was clearly observed that both the undercooling of nucleation and eutectic reaction was reduced and the solidification time was shifted to longer.

Taguchi Approach for Optimization of Parameters that Effect Grain Size of Cast A357 Alloy

Grain refining and modification are common foundry practice for improving properties of cast Al-Si alloys. In general, these types of treatments provide better fluidity, decreased porosity, higher yield strength and ductility. However, in practice, there are still some discrepancies on the reproducibility of the results from grain refining and effect of the refiner’s additions. Several factors include the fading effect of grain refinement and modifiers, inhomogeneous dendritic structure and non-uniform eutectic modification. In this study, standard ALCAN test was used by considering Taguchi’s experimental design techniques to evaluate grain refinement and modification efficiency. The effects of five casting parameters on the grain size have been investigated for A357 casting alloy. The results showed that the addition of the grain refiner was the most effective factor on the grain size. It was found that holding time, casting temperature, alloy type and modification with Sr were less effective over grain refinement.