ANALYTICAL POSSIBILITIES OF ESTIMATION OF THE DEGREE OF HOMOGENEITY OF THE CAST ALUMINUM ALLOYS
DOI:
https://doi.org/10.14529/met180208Keywords:
aluminum alloys, alloy B95, alloy 7475, homogenizing annealing, degree of homogeneityAbstract
Homogenization of castings from cast aluminum alloys is a priority for improving their technological parameters. The mode of homogenizing annealing is regulated by two parameters – temperature and holding time. Traditionally, one or two-stage annealing modes are used.
To assess the degree of homogeneity of the alloy after such heat treatments at the enterprises, as a rule, various methods of mechanical testing are used. The paper proposes a new method for calculating the criterion for estimating the homogeneity of the structure of cast alloys using the aluminum alloy B95 as an example.
The criterion of homogeneity (γ) can be the degree of approximation of compositions of local volumes of cast alloys to branded composition. Experience shows that it is more convenient to measure the content of alloying elements along the axes of dendrites. Dendrites, as a rule, contain a smaller number of basic alloying elements, which are usually localized in interdendritic volumes.
Taking the value characterizing the degree of inhomogeneity of the cast alloy, the deviation of the composition from the branded (Δ), the homogeneity criterion can be represented as: γi = 1 – Δ.
For a perfect homogeneous alloy, the value of the homogeneity criterion should approach 1. The average homogeneity coefficient for the cast alloy B95 calculated according to this formula is equal to γΣ = 0.503.
Comparing the content of alloying elements in the center of grains and along the boundaries of homogenized grains, it should be noted that the local composition of grains, both in the center and at the boundaries, became close to the brand composition of alloy B95. This is indicated by the conditional homogeneity coefficient γ that is close to 1, introduced earlier. Indeed, the average homogeneity coefficient became equal to γΣ = 0.94.
The proposed method is recommended as an analytical method for evaluating the degree of homogeneity of aluminum cast alloys, and as a criterion of homogeneity (γ), a generalized calculated index based on the analysis of local compositions along the dendrites and interdendritic regions can be proposed. Homogenization of castings from cast aluminum alloys is a priority for improving their technological parameters. The mode of homogenizing annealing is regulated by two parameters – temperature and holding time. Traditionally, one or two-stage annealing modes are used. To assess the degree of homogeneity of the melt after such heat treatments at the enterprises, as a rule, various methods of mechanical testing are used. The paper proposes a new method for calculating the criterion for estimating the homogeneity of a structure using the aluminum alloy B95 as an example. The proposed method consists in comparing the chemical composition in the dendritic and interdendritic areas and calculating on the basis of the obtained values of the homogeneity criterion (γ).
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