DETERMINATION OF THE ESSENTIAL EFFECT OF MODIFICATION IN THE COMPOSITION OF THE COMBINED VIBRATION TREATMENT TECHNOLOGY ON THE MECHANICAL PROPERTIES OF AK7 ALLOY

The object of research is AK7 alloy, which is subjected to the complex action of vibration and modification in the process of producing cylindrical billets by means of casting in a metal mold. One of the most problematic places is the determination of rational modes of influence of technological factors on the alloy, ensuring the satisfaction of a given level of properties in terms of their competitive nature. During the study, the methods of regression analysis were used and identified as promising methods of analysis of variance to improve the accuracy of the findings. In particular, the results of the regression analysis of experimental and industrial research data on the vibration treatment technology with a frequency of 100–150 Hz with an additional input of the ultradisperse modifier into the melt are presented. The results are obtained concerning the confirmation of the hypothesis about the significance of the influence of the modifier on the increase of the mechanical properties of the alloy: НВ, σ 0.2 , σ s , δ s , ψ. This is important because attempts to improve the properties of the alloy using the combined technological solutions invariably cause the expenditure of energy and material resources. This ensures the possibility of energy and resource savings in the process. The obtained results show that the use of modification as an additional component of the technological process increases only ψ, but reduces HB and δ s . In the vibration frequency range of 100–140 Hz, modification has a positive effect on σ 0.2 , and an increase in the frequency above this value leads to a deterioration in the result. The opposite effect is observed with respect to δ s – with an increase in the vibration frequency of more than 140 Hz, modifying increases this alloy characteristic, in the range of 100–140 Hz, the introduction of a modifier can be considered justified. Thus, it is found that the modification has an ambiguous effect on the properties of the alloy, therefore, in order to select a rational technological regime, a solution to the compromise problem is necessary. This is due to the revealed fact of competing properties. It is proposed to use the methods of analysis of variance to refine the obtained results.


Introduction
External effects on alloys are based on the use of different physical principles, for example: -complex treatment of liquid silumin in a magnetodynamic unit (MDU) [1], which allows to eliminate liquation, grind the structural components of pre-and hypereutectic silumin, increase the relative elongation of the alloys; -treatment by increasing pressure and modifying [2], providing a complex effect on the micro-and macrostructure of the alloy; -complex treatment by vibration and directional crystallization [3], which allows to increase the mechanical properties due to the grinding of grain. The possibilities of regulating the structure formation processes during the vibration process are noted in [4,5]. It is shown that when exposed to a metal with an electric vibrator, the economic aspects of the implementation of such technological solutions should be taken into account [6]. In [7,8], variants of technological implementations of the vibration process for pouring traction drive electric motors into the molds of rotors are described. It is noted that the use of selected modes in the practice of casting rotors minimizes the probability of formation of internal porosity, thus providing the best traction characteristics. Therefore, it is relevant to apply an integrated approach to the impact on the liquid metal. Thus, the object of research is AK7 alloy, which is subjected to the complex effect of vibration and modification during the production of cylindrical billets by means of casting in a casting mold. The aim of research is mathematically confirmation or denies the effect of modifying, as part of the combined treatment of a melt by vibration, on the mechanical properties of the alloy.

Methods of research
Experimental industrial data obtained under the conditions of the foundry shop of Dnipropetrovsk Aggregate Plant OJSC (Ukraine) [9] were taken as the basis for the analysis. Melting of the AK7 alloy was performed in a CAT-04 furnace. Filling was carried out in a steel turnout casting mold with an average diameter of 60 mm with ISSN 2226-3780 a wall thickness of 5 mm and a working cavity height of 150 mm. Previously, the working cavity of the casting mold was heated to 380-400 °С and covered with paint based on disten sillimanite. Heated casting mold with a protective coating was installed and fixed on the vibration installation, providing after pouring the mode of vibration exposure with amplitude of 0.7 mm at a frequency of 100 Hz, 150 Hz, 200 Hz. The melt was poured into the casting mold at a temperature of 720 ± 5 °C.
In the process of vibration at these frequencies, two series of experiments were carried out: Series 1: Filling of the unmodified and modified by the preparation «Typhoon-Z» (0.1 wt. %).
Series 2: Filling of unmodified and modified alloy in the casting mold without vibration.
After cooling the poured samples, the mechanical properties and density of the alloy were measured. The experimental method is described in detail in [9].
On the basis of the obtained data, a table of results was formed, on the basis of which a regression analysis was carried out using the Excel spreadsheet environment. The task was in determination of the effect of modifying on the properties of the alloy as an additional factor of influence, together with vibration.  Fig. 1-5 show that the answer to the question of the significant effect of modification as an additional factor is ambiguous. So, from Fig. 1 it is shown that the modification reduces the hardness of the alloy in comparison with the vibration treatment. The same conclusion can be drawn with respect to σ s (Fig. 3).   However, the modification increases σ 0.2 , but only up to a frequency of 140 Hz, the frequency excess leads to worse results compared to the usual vibration treatment. Modification has a positive effect on σ s only when the frequency exceeds 140 Hz (Fig. 4). In relation to ψ, it can be said that in the entire frequency range of vibration, the action of the modifier enhances this characteristic of the alloy (Fig. 5).

Research results and discussion
It should be noted that the graphs ( Fig. 1-5) were obtained for the average values of the properties of the alloy (mathematical expectations). Mathematical expectations were calculated as the average of parallel measurements (n = 2 or n = 3). The difference in the sample size and the characteristics of the object of research leads to the fact that the variance can't be determined with sufficient accuracy. It is possible to talk about the assessment of dispersions, which should be considered when comparing the two technological modes -treatment of the alloy with pure vibration and the combined treatment of vibration + modification. Therefore, the need for the use of an analysis of variance can be considered a prospect for the development of this study [10]. The well-known results of work [11], devoted to the use of an analysis of variance to assess the significance of the effect of modifying on the quality of an alloy, allow to consider it reasonable for the problem under consideration. The task in this case is to determine whether the effect of modification on the increase of mechanical properties and the density of the alloy in the process of vibration treatment ISSN 2226-3780 is significant. The procedure algorithm is as follows. The group average contents of the y (i) values for the alloy not treated with the modifier (y (i0) ) and treated with the modifier (y (i1) ) are calculated: where k -the number of levels; k = 2 (the first level is an unmodified alloy, the second level is a modified alloy); y j i i , The factor variance is calculated: the effect of the modifier on the i-th alloy property should be considered significant. Otherwise, the influence of the factor is considered insignificant. In this case, using the Student's t-criterion, the level from which its influence is manifested is determined. However, this requires additional experimental studies with different amounts of the introduced modifier.

Conclusions
It is shown that the use of an ultradisperse modifier as an additional technological step for vibration treatment of the AK7 system alloy by means of a casting in a metal mold increases the value of ψ over the entire range of vibration frequencies studied (100-150 Hz), but reduces the HB and σ s . In the frequency range of 100-140 Hz, modifying increases σ 0.2 , however, when the frequency exceeds 140 Hz, the positive effect of the modifier ceases and its use becomes unjustified in relation to this alloy characteristic. The opposite effect has been established with respect to the effect of the modifier on σ s -the use of the modifier is justified only in the frequency range of 140-150 Hz. To refine the obtained results, it is necessary to carry out an analysis of variance, which will require additional experiments.
The obtained results suggest that competing criteria in the properties of the alloy and the need for a compromise solution to the problem of choosing rational technological modes of AK7 alloys processing in the manufacture of cylindrical blanks by casting in a casting mold.