QUALITY MANAGEMENT OF DISPERSION-STRENGTHENED ALUMINUM-BASED SAP-ISML COMPOSITE ALLOY

The article is devoted to the analysis of the composition and properties of dispersion-strengthened aluminum-based SAPISML composite materials, which are used in various industries, including the aviation. The properties of such materials have been analyzed with the aim of ensuring the management of their quality for rational use and subsequent disposal. Mathematical models of dependence of parameters of dispersed-hardened materials on the basis of aluminum of SAP-ISML type on the aluminum content and temperature are constructed.


Introduction
Nowadays, composite materials (CM) are often used in engineering.The use of these materials is aimed at obtaining such products, which will be resistant to loads of various types and, simultaneously, to reduce the total weight of the resulting product.Therefore, they are used for such industries as automotive, aircraft and space technology, building industry, etc.At the moment, there are 2 types of composite materials used for these purposes: 1) CMs based on a metal matrix; 2) CMs based on a nonmetallic matrix.
The most used CMs with a nonmetallic matrix are polymeric carbon fibers, boron fibers and organofibers [1].They are the materials for various parts of aviation equipment -chassis, bearings, disks of aircraft brakes, and so on.
As for CMs with a metal matrix, they are made on the basis of various metals: aluminum, beryllium, magnesium, nickel, cobalt, chromium, etc.Such materials are called dispersion-strengthened composite materials (DSCM) [2].DSCM belong to the group of composite materials, which are made, mainly, by methods of powder metallurgy.The microstructure of DSCMs consists of polycrystalline matrices in which the particles (mainly oxides, carbides and/or nitrides) are dispersed [3].Their peculiarity is that the matrix of metal or alloy is strengthened by fine-dispersed artificially introduced particles having a size of 0.1 to 15 μm in an amount from 0.1 to 15 %.As a strengthening phase, dispersed oxide particles, carbides, nitrides and other refractory compounds are used.
After formation and sintering, a hot plastic deformation is carried out to produce a composite material, resulting in a dense semi-finished product without pores (tape, strip, profile).
This research is devoted to aluminum-based DSCMs of SAP-ISML type.Such materials are used, for example, for cladding gondolas of power plants in the exhaust zone, piston rods, compressor blades and other elements that are not exposed to high temperatures As it is noted in [4], oxides are used most widely as compounds used as a strengthening phase in aluminum-based DSCMs and its alloys.At the moment there are 3 domestic brands of aluminum-based DSCMs: САП-1, САП-2 and САП-3.The difference of these materials differs from each other in the difference in the concentration of oxides (from 6 % to 9 % of Al 2 O 3 for САП-1, from 9 % to 13 % of Al 2 O 3 for САП-2, from 13 % to 17 % of Al 2 O 3 for САП-3) [4].Also, they contain up to 25 % silicon and up to 5 % iron.In addition to these materials, there are also Al-C DSCMs.In such materials the role of the strengthening phase is performed by Al 4 C 3 aluminum carbide [5].
The foreign counterparts of the above DSCMs are, respectively, for САП-1 -SAP-930, for САП-2 -SAP-895, for САП-3 -SAP-865.They are distinguished by increased structural stability and corrosion resistance due to the reduced iron content in the matrix (less than 0.1 %), which somewhat differs their properties from the properties of SAP-type materials [5].
Studies show that with an increase in Al 2 O 3 content in materials of this type, an increase in the hardness and strength parameters is observed, in contrast to the thermal and electrical conductivity, the coefficient of thermal expansion and plasticity, which decrease [6][7][8][9][10].
Studies have shown that at temperatures of 300-500 °С, aluminum-based DSCMs is superior in strength to all industrial aluminum alloys and is characterized by high strength and creep characteristics [4].
Until this time, the task of determining the parameters of aluminum-based DSCMs and modeling their properties for quality management in the process of creation, operation and disposal has not been fully reflected in the studies.In particular, the properties of the SAP type alloys are investigated in [11], while alloys of the SAP-ISML type are not fully investigated.The study of the parameters of aluminum-based DSCMs will make it possible to single out their main significant parameters, to assess the starting point from which values of the studied parameters will have a significant impact on the requirements imposed on them.This will optimize the composition of these materials in terms of both operation and subsequent disposal.
The aim of research is to analyze the properties and composition of dispersion-strengthened aluminum-based SAP-ISML composite materials used for aerospace engineering and other industries, as well as to select their optimal parameters to ensure the determination of the quality parameters applied to them.
To achieve this aim, the following tasks are solved: -development of a mathematical model for determination of relationship of the chemical composition and properties of the aluminum-based DSCMs, such as SAP-ISML; -analysis and identification of the possibility of clarifying the requirements for the composition and properties of the aluminum-based DSCMs, to ensure the management of their quality.

Materials and methods of research
Table 1 of the experimental material is taken on the basis of [4].

Note: t -the test temperature of the sample; YS -yield strength; US -ultimate strength
The analysis of the obtained data allows to use the plan of the full factorial experiment and determine the values of the output and input variables, taking into account the fact that the number of experiments corresponds to N=2 k .The model is obtained on the basis of the method of constructing of the second order complete central orthogonal central design, the principles of its use Material Science for analogous problems are described in [12,13].The values of the input variables are normalized according to the following formulas: (1) x , 2 x -the average values of the input variables 1 x =11 %, 2 x =260, I 1 , I 2 -the variation intervals of the input variables (I 1 =4, I 2 =240).
The mathematical model is described by a polynomial of the following form: where a i -the estimated coefficients, β -the parameter that is calculated depending on the number of kernel points of the composite plan 2 n-p of the shoulder of the "star" points α and the number of plan points according to the formula: A general view of the data for construction of a central orthogonal composite design is given in Table 2.The following formulas are applied to determine the coefficients a i :
For a polynomial of the second degree of the form ( 4), the values of these parameters are given in Table 3.
To estimate the accuracy of the obtained model, the sum of the squared deviations of the experimental values of the output variables from the calculated values obtained from the model (S R ) and the variance estimates (s 2 ) is calculated: where ϕ = − + N (k 1) -the number of degrees of freedom, N -the number of experiments (N=9), k -the number of estimated parameters (k=5).

Table 3
The values of parameters for calculation of the coefficients of a mathematical model 0,6667 0,1667 0,5 0,25 Evaluation of the significance of the model coefficients is carried out on the basis of Student's t-test: where t cr -the critical value of the Student's distribution for the confidence probability of 95 % and the number of degrees of freedom φ=3, s -the standard deviation determined on the basis of formula (10).

Investigation of parameters of aluminum-based DSCMs of SAP-ISML type
The input and output parameters of research are presented in Table 4.

Table 4
Input and output parameters of research
Given the lack of experimental data for the value av 2 x , the value of this parameter is determined by plotting the dependence on existing data using MS Excel 2010, which is acceptable, given the nature of the experimental studies (t=260 o C), The visual analysis of the response surface also confirms the fact that both the Al 2 O 3 content and the test temperature have a significant effect on the yield strength.Analyzing the obtained response surface, it can be concluded that the yield strength increases with increasing Al 2 O 3 content and decreases with increasing test temperature of the sample of aluminum-based DSCMs of SAP-ISML type.

Discussion of research results of the parameters of aluminum-based DSCMs of SAP-ISML type
Analysis of the response surface describing the values of the ultimate strength (US) confirms that the Al 2 O 3 content has no significant effect on the ultimate strength and the only significant factor is the temperature of the test samples.Analyzing the resulting response surface, it can be concluded that the strength of the material decreases with increasing the test temperature of aluminum-based DSCMs of SAP-ISML type.
Analysis of the response surface describing the yield strength (YS) confirms that both the Al 2 O 3 content and the test temperature have a significant effect on the yield strength.Analyzing the obtained response surface, it can be concluded that the yield strength increases with increasing Al 2 O 3 content and decreases with increasing test temperature of the sample of aluminum-based DSCMs of SAP-ISML type.
Based on the experimental and obtained data, it can be concluded that the nature of the dependence of the yield strength and ultimate strength on temperature and Al 2 O 3 content is the same.Increasing the volume fraction of Al 2 O 3 can slightly increase the yield strength and ultimate strength, but this increase has a limitation related to the temperature of operation of these materials and the fact that aluminum-based DSCMs of SAP-ISML type are produced with a maximum Al 2 O 3 fraction of 15 %.

Conclusions
As a research result, mathematical models have been constructed to determine the relationship between the chemical composition of aluminum-based disperse-strengthened composite materials of SAP-ISML type and the test temperature with the ultimate strength and yield strength values for these materials.The obtained models are regression equations, the estimated parameters of which are obtained by realizing the second order complete central orthogonal central design of the factor experiment.Analysis of these models and the resulting response surfaces shows that the ultimate strength for aluminum-based DSCMs of SAP-ISML type is most dependent on the test temperature (operation), and the Al 2 O 3 content in the alloy has no significant effect.
The yield strength is almost equally dependent on both the test temperature (operation) and the Al 2 O 3 content.It is established that an increase in the Al 2 O 3 content and test temperature of the samples will lead to a decrease in the yield strength, and an increase in the Al 2 O 3 content will increase it.
Research results can be used to select the necessary materials in the manufacture of various equipment at known temperatures of operation, as well as to make compromise optimization of the investigated parameters for controlling the quality of aluminum-based composite alloys such as SAP-ISML.

( 2 )
where x 1 and x 2 -the normalized values of the input variables, the natural values of the input variables, 1

Table 1
Mechanical properties of aluminum-based DSCMs of the SAP-ISML type