information technologies in the investigation of the oxide coatingsformationmechanisms
Chigrinova N., Chigrinov V.
State Scientific Institution "Powder Metallurgy Institute, Minsk, Belarus
Obtaining a predicable complex of the characteristics of the coatings by the anode microarc oxidizing method (AMAO) in the problems of formalization (calculated and technological grounding of the methods selected) needs an appropriate scientific and algorithmic description of the named process with the adaptive control elements which essentially deepens the traditional concepts of the strengthening technologies this method is related to.
A drawback of the existing AMAO models is the excessive abstraction from the phenomenal data at modeling the processes taking place in the charges. Remaining strong is the tendency to give a detailed consideration on the micro level to the phenomena only on 1 of the borders of the separation (normal electrolyte – a film or electrolytic gas) and inside the charge with abstraction from a real complex of conditions in the maternal macroscopic system, where the complex discharge subsystems (i.e. sole charges or their specific multitudes) rise and pass all phases of their development. In the end the link between macro and microcharacteristics of the present system is lost.
A complex system of interlinked physical and chemical phenomena on the surfaces of contacting bodies (on micro and macro scales), causing alteration of the physical-mechanical properties of materials including in the spots of the actual contact, influence of the temp gradients and stochastic nature of decomposition of the bacteria complicate obtaining a full mathematic description of basic processes during the microplasma-spark treatment of the material.
In the sameness with many processes of metallization to describe the specifics of microarc oxidation it is possible to apply traditional calculated schemes and generalized characteristics. To realize the regulatory function of an object behavior during its transfer to the final state the system of adaptive control must include a subsystem of determination of the existing state of the object, the subsystem of comparison of the existing state of the object with its preplanned state, a subsystem of selecting parameters affecting the object on the basis of the comparison results, a subsystem of formation of management for the selected acting parameters.
With regard for a division of the construction into separate functional blocks at the development of the automatic control means the adaptive-control system construction may be formally divided into the measurement block, the executive block and the Intel core (fig. 1).
При этом подсистемы сравнения текущего состояния и выбора
At that the subsystems of comparison of the existing state and selection of the parameters affecting the object based on the results of comparison comprise the core of the ACS. The subsystem of determination of the object state is realized on the ground of the measurement block that usually includes converters of the original physical figures (detectors) and electronic measurement schemes that help gain the detectors readings and the primary processing of the obtained data. The subsystem of forming management actions for the chosen affecting parameters is realized on the basis of the executive block that as a rule includes the electronic schemes and mechanisms facilitating generation of the control signals for the regulator. Fig.2 shows the algorithm of the control system operation by the microplasma-spark action during materials treatment used for the investigation of the AMAO specific features.
И - the measurement block; Р – regulator; ИВ- source of action; ОУ- object of control
Fig. 2. Algorithm of operation of the microplasma-spark action control system during treatment of materials.
From the point of view of the control theory (control object or CO) the electric outburst causing the formation of coatings in the process of microarc oxidation can be presented as a reaction to the external effect of the electric field created between the electrodes (regulator Р). At this the control parameters are voltage, current and intensity of the electric action in the outburst region. Based on this, microarc oxidation process can be generally presented as follows:
- to consider the physical figures characterizing the outside action on the CO as the input parameters (хi) that form the vector of outside action , t – time;
- Outside action vector is settled with Р by means of its executive mechanisms;
- to consider the physical figures (controlling voltage), that influence Outside action vector the control parameters (uk), that form the control vector ;
- the vector affects Р, thus inflicting the alteration;
- to consider the physical figures characterizing the state ОУ as the output parameters (yj), that form the vector of the state , at that taken out from yi should be the visible (yi1) in the process of formation of coatings and the invisible (yi2) parameters, i.e. yi = yi1+ yi2.
In view of the papers [1,2] to describe the mechanism of microarc oxidation from the point of the adaptive control it is admitted:
- Outside action vector is settled with the regulator;
- to consider the physical values (controlling voltage), influencing Outside action vector the parameters (uk) of formation of the control vector ;
- to consider the visible physical figures (temp, electric current, etc..) characterizing the state of the CO as the input parameters (yj1) that form the state vector ;
- the invisible physical values (concentration of the charges in the electric field rang, their mobility, etc.) form the vector of hidden control , connected with the vector of state be line dependence:
- to consider the duration (Δt) of the outside factors a criterion of effectiveness of the technological process.
Introduction of the named allowances allows formalization of the microplasma-spark processes through various transferring media especially AMAO and control models presenting it by the analytical equation
,(1)
,(2)
Where , – the functionals describing the technological mode and the state of the regulator.
It is shown that if during AMAO is affected by specific parameters of rj, (for example, intensity of the energetic action, electric current), relating to yj, the aim of the adaptive control will be minimization of the quality criteria:
,(3)
And the algorithm of the quality criterion of the work of the ACS will be presented as a task directed towards solution of the system from the obtained equations (1) – (3).
Based on the above allowances the control of the microplasma-spark processes on the open cycle occurs with the help of the program determining their technological modes starting from the preliminary data about CO. this principle of control sets high requirements to the accuracy of the mathematical model, executive mechanisms of Рand their performance. The high accuracy of calculations can be reached if using the expressions (2) and (3) clearly established is a unique connection between the duration of AMAO, the affecting parameters and the state of CO. however, as the analysis of the papers showed [3–9], due to the synergetic nature of the microarc oxidation process this dependency cannot be attained. Practically, this means that during the development of tech process firstly calculated are the bottom and top borders of the technological modes and on their basis determined is the time interval Δt. This parameter influences the spread in end values yik, characterizing the state ОУ. The greater is the spread yf the lower is the quality of the obtained products. Thus, Δt can be selected by the efficiency criteria of the microplasma-spark processes.
The condition of putting out the stages of the process of formation of a coating at the sparkle discharge allows more precise calculation the intermediate parameters of the modes of the microplasma-spark processes and determination of in the unclear form of the unique connection between the outside effect duration, the affecting parameters Р and the state ОУ. Practical application of such a connection is possible with the help of the closed control cycle this is realized because of the return connection between ОУ and Р. The type of connection where adaptive control can be used allows analyzing the current state ОУ and if necessary modifying the modes of oxidation. The adaptive control in this case is aimed at optimization by the disturbance parameters of the duration of effect on ОУ from Р, which is practically realized by means of the ACS.
The developed block diagram (fig.3) allows realizing the control and operation in the real-time mode of the base parameters of AMAO – electrical determining the dynamics of on-growing the oxide coating, the state of its structure and properties.
At the analysis of the dynamics of on-growing coatings at the modifying technological parameters of AMAO determined was the role of each of them in the formation of coatings of various thicknesses. According to the developed identification model of control and the created in the ACS in real-time mode by means of the metering-controlling instrument based on PC with the analogue signal converters registered were the effective (medial-integral) and average values of current and voltage and their impulse properties. AMAO was realized in conditions of the altered intensity of the energetic impact in the electrolytes subjected to chemical (E 1), electrical (E 2) and machine (E 3) activation.
At studying the features of AMAO of the AL alloy А 99 it was noticed that the sparkling phenomenon indicating the start of a coating forming in electrolytes appeared at various electrical parameters of the process (fig. 4).
Current, A0 3 6 9 12 15 18 21 24 27 30
AMAO time, min / Voltage, V
0 3 6 9 12 15 18 21 24 27 30
AMAO time, min
Fig . 4. MAO electrical parameters in real-time mode at oxidising the AL alloy in variously activated electrolytes. Electrolytes:E 1- chemically activated; E 2- electrically activated; E 3- mechanically activated.
In the initial period of carrying out AMAO in all the electrolytes under study due to the gate properties of the all-time superficial AL oxide film of several Angstroms the applied alternating stress is growing with the resultant emergence around the dipped-in anode of a cloud of micro discharges inciting the acceleration of the oxide film growth. The most active growth of the current of AMAO is present in the electrolyte ЭЩ 1.this effect is probably connected with the more intensive than in the E 2 and E 3, behavior of chemical reactions with the formation of new connections for which oxidizing in the microarc media requires an increase of the initial current. AMAO at higher currents also causes the 1.5 times increase of the speed of the coating growth compared to AMAO in standard solution of E with the 1.3 times lower energetic consumption on formation of 1 µm of the coating (table 1).
Thickness of the coating, mkmThickness of the coating after AMAO of the alloy А99 in the electrolytes under study. / Table 1. Energy consumption for obtaining
Coatings on the surface of the alloy А99 in the electrolytes under study
Electro-
lyte / Speed of the coating growth
µm/min / Energy consumption on oxide formation, J / Energy consumption ratio, times E/E n
E / 1.4 / 20299 / -
E 1 / 2.2 / 15750 / 1.28
E 2 / 2.9 / 11819 / 1.46
E 3 / 3.9 / 9123 / 2.22
Influence of the electrical activation on the electrolyte with increasing duration of AMAO is evident in the increase of frequency of current and voltage impulses (fig. 5.).
Current impulse,A/s Voltage impulse, V/s
0 3 6 9 12 15 18 21 24 27 30
AMAO time, min /
0 3 6 9 12 15 18 21 24 27 30
AMAO time, min
Fig. 5. Frequency of current (а) and voltage (б) impulses during AMAO in the standard (curve 1) and activated (curve 2) electrolytes.
It means that distinct from AMAO in a standard solution where impulse voltage and current characteristics are altered in time insignificantly (curve 1on the diagrams) and obtaining a constant additional growth of the layer width is impossible, the effectiveness of AMAO process in E 2 is not decreased.
The smallest values and stable alterations of volt-ampere characteristics were registered at AMAO in mechanically activated ultrasonic solution E 3 [10], which is conditioned by cavitational effects [11], that includes agitation of the electrolyte with leveling the concentration of eons, degassing of electrolyte, increase of the cathode’s active surface. More active growth of the coating at practically the same values of current and voltage with a treatment in the solution E 2 is also connected with cavitations that are conditional of microexplosions of gas bubbles round the cathode which leads to the pressure increase in the area of disruption. Alongside with obtaining high values of the energy of oscillations created are the fields with a high energy level [11] that determine the ultrasound radiation in the form of impulses. This enables realization in the media a precise time selection of the propagating impulses and thus controlling the processes of solution and chemical interaction between the anodic material and products of the electrolyte hydrolysis meaning the thickness, composition and structure of the formed coatings. The emerging shock waves during the microexplosions of the gas bubbles also provide for intensifying all the occurring interactions thus keeping up the high temp in the reaction area and providing a prolongation of the thermolysis stage in the electrolyte. This incites extra reactions of interaction of not just formed in the pre-sparking mode oxides on the anode but also oxides on the basis of the electrolyte components, metal oxides from the composition of original salts in form of cations and oxides of the chemical elements of the anion complex ensuring the intensified growth of the width of oxide layers. These shock waves activate the treated surface leading to a more intensified nominal growth of the coatings at lower volt-ampere characteristics of AMAO (table 1). The sonic-capillary effect provides for the percolation of the electrolyte into the tiniest pores and cracks of the coating creating the additional centers of origination of extra oxide phases that causes dispersing of a solid body – anode, deaeration of the solution thus preventing formation of the hollow cavities (pores) in the coating, accelerates the crystallization of the forming phases and, most importantly, intensifies electrochemical and diffusive processes [11]. Happening at УЗ shack waves provide treatment of the anodic surface from the existing not quality, loose patches of oxidation thus uncovering the metal regions that interact with oxygen with the formation of new centers of origination of the oxide phase. All the above provides for obtaining thicker, denser and equally thick coatings (fig.5).
With the aim to determine which of AMAO parameters has a priority influence on the dynamics of the coatings growth including activation of the electrolyte with ultra sound and also to find out the mechanism of influence of the percolated through the electrolyte ultrasound on the kinetics of interaction of the anode and cathode carried out were experiments with fixing the value of the ultrasonic signal by means of the high-frequency digital oscillograph В-421. At that, oscillograms were read out of:
- at autonomous operation of the US-generator;
- at autonomous operation of the AMAO plant;
- at operation of AMAO plant with putting through the ultrasound but with no anode dipped in the bath;
- at similar work but with the dipped-in anode.
Current is altered by means of the based on Hall Effect sensor DTPX-50. For the experiments used were a serial plant for AMAO, the ultrasonic generator US-1,5 with frequency 22 kHz. As anode applied was AL alloy А 99.
Fig. 6 shows the oscillograms of the periodical dampening signal at the autonomous work of the ultrasonic generator with the signal frequency 2 MHz (fig. 6 а), as at the autonomous work of the serial AMAO plant (fig. 6 b), but the range of signal modification is much narrower – about 150 mV which corresponds to the 2 А current against 350 mV current of 5 А in the AMAO plant.
a / bFig. 6. Oscillograms of the periodical dampening signal; а – autonomous operation of US- generator; b – autonomous operation of AMAO plant.
During the signal analysis by the ultrasound pass through the electrolyte at the running plant AMAO (fig.7), it was stated that its frequency was 2 MHz, its alteration range – 700 mV, which is equal to the current of 10 А.
Fig. 7. Oscillogram of the signal passing through ultrasound during the AMAO plant work.
It is the electrolyte that has a certain influence on the change of impulse characteristics of the current during ultrasonic action.
Thanks to the developed software and the AMAO control system it is established that at passing through the electrolyte of ultrasonic fluctuations the average height of the stress impulses is increased by 68%, and current impulses — by 87%, the frequency of current and voltage impulses grows up by 90%, causing the additional energetic impulse during the microplasma-spark impact on the treated material and thus conditioning intensification of the AMAO process meaning a more efficient growth of the formed oxide layers.
Experimental conclusions were confirmed during the immersion of an oxidized part into the mechanically activated electrolyte. At that, observed is the approx. 4-times growth of the frequency of the signals impulses being the evidence of the reduction of their amplitude and the 1.3 times increased range of alteration of the impulses compared to the same parameter at the read-out of the oscillogram in the activated solution but without the anode. This effect connected with the emergence of a greater quantity of high frequency dying oscillations of the irregular form with a high current amplitude (from 5 А above) is explained by the appearing synergetic effect which causes optimization of the AMAO parameters and a more intensive growth of the coating.
Calculation of the average speed of the coating growth showed that during oxidation without ultrasound for all alloys this parameter does not exceed 0, 7 -0, 75 µm/min and at AMAO with ultrasound increases 2,5 times reaching up to 1,6 -1,9 µm/min. the said above is illustrated on the bar chart of average width of the coating after oxidizing various gate metals in the electrolyte with and without ultrasound (fig. 8).
Average coating width
1, 2- titanium alloys ВТ 6, ВТ1-0; 3 - Zr 635; 4, 5 - aluminum alloys - A 99; D16 Т