CC BY
10Journal of Siberian Federal University. Engineering & Technologies 4 (2013 6) 455-451
УДК 621.9.048
Modeling of Heat Exchange Processes under Electrocontact Cutting of Metals
Tatyana A. Veretnova*, Ivan Ya. Shestakov, Angelina A. Kovaleva, Svetlana M. Tinkova and Aleksander A. Kosovich
Siberian Federal University 79 Svovobodniy, Krasnoyarsk, 660041 Russia
Received 01.04.2012, received in revised form 28.09.2012, accepted 15.01.2013
The mathematical model of a thermal condition of blank under electrocontact cutting has been offered. The way of the improvement of electrocontact cutting is recommended in order to improve the technological indicators of a process at the expense of temperature regulation in a cutting zone under observance of the calculated modes ofprocessing.
Keywords: electrocontact cutting, temperature control, modes of treatment.
1. Introduction
Electrocontact processing is based on local heating of blank in a place of contact with an electrode-tool and also on the removal of the softened or fused metal from a processing zone mechanically: by relative movement of blank or tool.
A distinctive feature of electrocontact processing is the mechanical method of generating impulses by faltering contacting of the rotating disk electrode-tool with a processed detail [1].
The object of research are the processes of heat exchange occurring in a zone of contact of the processed detail and disk electrode-tool which is carrying out cutting with certain speed of rotation and feeding.
For calculation of the process heating and metal cooling during electrocontact cutting it is necessary to choose the suitable settlement scheme allocating the basic features of considered process and neglecting the minor ones. The rational choice of the settlement scheme simplifies calculations and allows revealing the influence of key parameters of the process more exactly.
Depending on the form and the sizes of a product and also from duration of the process of distribution of the heat the scheme of a heated up solid is chosen. On the assumption of conditions of electrocontact cutting process we choose a plate of a small thickness, heat stream in a plate is flat. During the calculations we necessarily consider heat exchange of a plate with environment, however
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* Corresponding author E-mail address: [email protected]
in view of a small thickness of a plate we neglect non-uniformity of distribution of the temperature, considering it averaged by thickness. The heat source according to the scheme of a heated up solid in our case can be dot or linear, moving with constant speed.
The cutting zone represents thermal emission and heat sink system. A thermal emission in this zone occurs because of the raised electric resistance of a zone of the contact and a friction between the tool and blank, a heat sink - basically owing to thermal conduction.
2. Research metho dologies
The problem of carried out researches consists in modeling the heat exchange processes under electrocontact cutting of metal by quickly rotating desk, experimental acknowledgement oS the sesultt of its derision on mathematical model and ait establishment of analyeical dependence between electrode-tool feeding, modes of cutting (electric parameters) and the heat-physical properties of investigated samples materials.
The decision of a problem of modeling the heat exchange processes is connected with definition of a field of tempeeatures. In ordttlo estabOish the dependenee betwee n the valuer chasncte rizing she thermrl conductiviOy phenomtnon, we will take advantage oU a metfiod on mathematical physics. Now gor the decision oU the problems of mathematicar phyrics the greatest distritution wac received by a method of final elements.
By this method the initial range of functions determination breaks by means of a grid, in our case being irregular, into seprrate subareas - final elements. As an example on Fig. 1 the final-element grid of mathematical model for the cylindric alaluminum sample ie prgsented.
In Fig. 1 it is visible that the grid has n special condensaiion in a place of contact of tiee disk of tool with a processed detgel enat pronrdes more exact decision of a problem.
Required continuous function is approximated by piece-continuous one, defined on multitude of final elements. Approximation can be set arbitrarily, but more often for these purposes polynomials which are selected so that to pnovide a continuty of required function in the knots on the borders of elements are used, such calculating meehod ts used in Ansys program.
Fig.1. The final-element grid of mathematical model for the examined cylindrical aluminum sample
Modeling of thermal paoceates under elecCrocantact cutting was carried out for the following sampks: ateel rod Osteel U8A), an alummum pipe (D16a a steel pipe (12H18N10T), a copper pipe (M1), a titanic plate (VT1).
By wonkvng ouC oO mathamaiicai mo<^e]L pt is accepted that temperature distribution by volume of ;3i processed dttail is descvibed by the thaee-dcmensionnl srat(onavy liquation of the heat conductivity, presented as:
div(A(T)grad (T))=0e (1)
where A - Factor of heot conducaivity, Vt/(mK); T - nemperature, K.
The thnrmo physical poopertiea of a peoceased sEtmjojleis; material were used as function from temperature. On external surfaces the boundary conditions of the third sort have been set:
dT
-X — Taz(T-T0C), (2)
dn
yraHLKeee T - Metal temperanuae, K; Toc - outside tomperatuoe, K; ct^ - total heat emission factor, Vt /(m2-K).
On tahie; face surfaces the boundary s onditiont of the second sort have been set:
- ^t0. (3)
dn
an a zone of the contact of the electro°e-tool (disk) with pyocessnd blank the thermal stream defiyed in elecaric parameters (mode o) of cutting has bees set.
The nysVem of the nonfinoai algebroic equations ]4) in our problem was solved by means of Nnwton-Rawson's iteeative method [2]. The defined value is the temperature for which polynomials are worked out.
ant + a12t +..+ a1nt = 0 a2it + a22t +.. + a2„t = 0
ant + a32t +..+ a3„t = 0
a„it + a„2t +..+ a„„t = 0 ,
^ • (4)
Nonlinearity of the equations is explained by the dependence of heat conduction of material on the temperature. The problem decision was conducted by a method of final elements by means of Ansys program.
3. Interpretation and discussion of research results
As an example calculated temperature fields of the aluminum sample (pipe), steel (rod) and the titanic sample (plate) are showed in Fig. 2-4.
The calculation of temperature fields was carried out at different modes of cutting been set on the basis of the skilled data. The analysis of the received results shows that cutting of metal can be carried out as at the temperature of the fusion in a zone of contact of a cutting disk and blank, as at the temperature of recrystallization.
The temperature of recrystallization is determined by following expression [3]:
lif.ii ;?],(. it KT.jQi
Fig. 2. Distribution of temperature fields in an aluminum pipe
iji-u. p Jit.ii n',ir\f I,-....;. №,1:1
714.9 97 ISE.ii! 110.1« iil.li
Fig. 3. Distribution of temperaturefields in steel rod
-'1-01? ^l . i j' trj.au
11,111 m.iu J7|,Tli F'1,11!
Fig. 4. Distribution of temperature fields in a titanic plate
Tr = 0,5 + 0,6T, (5)
where Tr - Recrystallization temperature of metal; T - Temperature of fusion of metal.
The cutting under the temperature of recrystallization will allow to lower specific power inputs considerably and also improve technological indicators of the process. For maintenance of the given temperature in a zone of contact of the electrode-tool and a processed detail it is necessary to observe appropriate modes of cutting: pressure, a current strength and electrode-tool feeding.
Approbation of mathematical model was made by comparison of the received results with the data of experiences.
The following samples have been subjected to electrocontact cutting: steel rod (steel U8A), an aluminum pipe (D16), a steel pipe (12H18N10T) and a copper pipe (M1), a titanic plate (VT1).
The geometry of samples is presented in Table 1.
The comparison shows the correct character of temperature distribution as far as metallographic analysis of examined samples confirms that the temperature in a zone of contact of electrode-tool with processed detail corresponds to the temperature of recrystallization.
The microstructure of the steel sample from a steel of mark U8A (Fig. 5, a) is homogeneous, there is not differentiation on zones, there is no dendritic structure. Thus in the cutting process melting did not occur.
Microstructural analysis of the aluminum sample (Fig. 5, b) shows that cutting of pipe from an aluminum alloy leads to exfoliating the top layer. Metal parts are as though stick to a surface, forming rough "crust". During cutting process melting has not occurred.
Deeper metallographic structure research has shown that in cross-section of a pipe 3 zones are observed. The first (is closer to edge) and the third (central) zones are fine-grained, forming dispersed
Table 1. Geometrical parameters of research samples
Material Diameter, mm Thickness, mm
D external -^internal
Aluminum (pipe) 40,88 38,60 2,28
Steel 12H18N10T (pipe) 16,1 13,5 1,3
Copper M1 (pipe) 8,45 6,55 1,9
Steel U8A (rod) 12,5 -
Titanium VT1 (plate) 20,5 1,6
Fig. 5. Microstructure of examined samples: a) steel U8A; b) aluminum D16; c) titan VT1
Fig. 6. Microstructure of examined samples: a) copper Ml; b) steel 12H18N10T
"crust" on a su rface by means of heat sink deep into the metal and in an environment (air). It leads to the accelerated cooling and, as consequence, to the reception of fine-grained structure. Such distribution of structure is connected with a temperature gradient on pipe section, irrespective of a current strength given at cutting process. Also the presence of these zones is observed at the studying of a longitudinal section of a pipe, i .e. in a zone of contact with ihe cuttl ng tool.
Metallographic analysis of the sample of mark VT1 (Fig. 5, c) shows that a zone of thermal influence is very narrow, melting has not occurred. At an edge the structure is granular, grains have the accurate aeomftrihal foms, to Ihe ceotrr there is a grain integration.
During metallographic rrsehrch ot copfer samples sit is visible tOat by means vf hect canhuciiviey the growth of grains occurs faster and the depth of the changed layers is larger than at aluminum samples. In drawing 6a, superficial separations of the softened layer are shown. Metal slices are as tfough pasted ia a pipe surhach They did not; forme tolidness and have empty sfaces.
The anafysif oS steumture oh the sieel sample in csors-nvction flows columnar crystals of solid solution on external edge, further structure form looks like honeycombs (Fig. 6).
It speaks about melting* at cutting prccess even at mathes small workimg cnseeme (Ip=35--45AV At reduction of a current itfenghh anf electrode-tool teehing it is sharp eamples from a steel 12H18N10T passes at the temperature of recrystallization.
4. Summary and Conclusions
1. The choice of the settlement scheme was made truly uhat wts confirmed by distribution of temperature fields by a thickness of examined samples.
2. The coincidence of theoretical .ffnoMitinnsntih^acticdresuhs tmstififftcadequafyvftlte chosen model.
3. The developed way of electrocontact cutting of metal allows to lower specific power inputs at metal processing and to simplify the realization of a method by feeding of the electrode-tool with the speed depending on the temperature of recrystallization, on heat-physical properties of metal and on the thickness of processed blank [4].
4. Metallographic analysis of examined samples confirms the results of modeling of electrocontact cutting paocess and ihe fossibility of influence on technological indicators of process.
References
[1] Интернет ресурс http://elib.ispu.ru/library/lessons/tretyakova/index.html
[2] Интернет ресурс http://www-sbras.nsc.rU/rus/textbooks/akhmerov/mo/4.html.
[3] Evstratov, V.A. // Kharkov: Visshaya School: Publisher at Kharkov University, 1981. P. 248.
[4] Veretnova T.A., Shestakov I. Ya., Tsukanov A. V. and al. // Journal of Siberian State Aerospace University. 2009. № 2 (23). P. 241-246.
Моделирование теплообменных процессов при электроконтактной резке металлов
Т.А. Веретнова, И.Я. Шестаков, А.А. Ковалева, С.М. Тинькова, А.А. Косович
Сибирский федеральный университет Россия 660041, Красноярск, пр. Свободный, 79
Предложена математическая модель теплового состояния заготовки при электроконтактной резке. Разработан способ усовершенствования электроконтактной резки для улучшения технологических показателей процесса за счет регулирования температуры в зоне реза при соблюдении рассчитанных режимов обработки.
Ключевые слова: электроконтактная резка, регулирование температуры, режимы обработки.
