Modeling and developing processes of integrated aluminum and aluminum alloys processing аbased on the methods of continuous casting, rolling and extrusion
Sidelnikov S.B., Dovzhenko N.N., Dovzhenko I.N.
(State University of Non-ferrous Metals and Gold, Krasnoyarsk, Russia)
Abstract
The results of theoretical and experimental studies of integrated aluminum and aluminum alloys processing operations based on the methods of continuous casting, rolling and extrusion are presented. The efficiency of applied method for obtaining small cross-section extruded articles from non-ferrous metals and their alloys is demonstrated. The analysis of continuous casting and extrusion of aluminum and aluminum alloys enabled to formulate specific targets and methods to achieve these targets. The original technical solutions implementing different schemes of continuous metal processing methods are offered. The results of forming tests and power consumption parameters for integrated rolling and pressing processes, obtained by method of mathematical modeling for different roller passes are presented. Modeling of thermal processes involved in integrated casting, rolling and extrusion process was performed. The algorithm and soft wear to control the procedure of the tool design and technological process of continuous aluminum alloys processing was developed. The advanced technology based on the methods of integrated casting, rolling and extrusion was developed and applied for industrial production.
INTRODUCTION
Currently the metal processing industry employs high energy and metal consuming equipment to produce small cross-section profiles from aluminum alloys: casting-rolling facilities and drawing mills, horizontal and vertical hydraulic and mechanical presses, heating equipment. All these types of equipment are used for big production volumes and are not flexible when it is necessary to convert rapidly from profiles of one size to another in case of small orders. The technology of finish products, particularly long size ones, includes many cycles and a number of technological operations. These factors cause high cost and as a consequence low competitiveness of products. The efforts of the world metal processing industry are aimed at designing integrated mini production lines (MP) which employ technologies enabling to obtain articles from non-ferrous metals and alloys based on processing in one continuous line liquid metal-casting-processing. Practically all schemes use the processes of lengthwise rolling [1], as one of the most widely spread methods of metal plastic working. Solving the problem of continuous metal processing and developing new technology is very important for domestic industry with a big number of metallurgical companies as it can allow both reducing power consumption and improving competitiveness. The western companies employ technologies of new generation called technologies of continuous extrusion (Conform, Extrolling, Linex) and continuous casting, rolling, extrusion (Castex, Caster) [2, 3] аwhen shapes with small cross-section from aluminum and copper alloys are produced in small volumes. Modular equipment designed for this purpose distinguishes by high mobility, flexibility to switch from one standard size to another and comparatively high productivity. It can be used as basic equipment for mini production.
Analysis of integrated processes of casting and extrusion application in terms of energy savings [4] demonstrated that the main barrier on the way is different organization of these processes because at many plants the extrusion operation remains discrete and casting operation is continuous. In case billets are continuously supplied to the deformation center, which is specific for continuous extrusion, this problem can be solved. According to data presented [4] energy savings will account to 600 kJ/kg.а
The objectives of this research was to develop a complex of technological and technical solutions to design flexible easy readjusted mini production lines based on integrated process of casting, rolling and extrusion in order to raise competitiveness of extruded articles with small cross-section from aluminum and aluminum alloys.
MATERIALS AND RESEARCH METHODS
Aluminum and aluminum alloys were selected for this study. The major final consumers of products from aluminum and aluminum alloys are machine building and civil construction industries and cans and packing materials producers [5]. In 2001 the share of aluminum consumed by domestic market was 240,000 t and export 190,000 t. At present the main consumers of aluminum semi finished products are three regions: the North America, Europe and Asia. Consumption in these regions amounted to correspondingly: the USA - 25 kg per capita, in Europe Ц 18.3 kg (including in Germany - 24 kg, in Italy - 19 kg, in Japan Ц 17.5 kg, in Russia Ц 2.5 kg).
Industrial assortment of extruded sections from aluminum alloys, varies and at present includes tens of thousands different types and sizes. However despite such variety all sections based on their geometrical shape can be divided into the following groups: sections with solid cross-section; sections with variable cross-section; hollow sections; wire; panels. The most widely used products are rods, solid cross-section sections and wire (Table 1).
Table 1.
|
Product description |
Size, mm |
Alloys |
|
Rods |
diameter 6 Ц 20 |
1060, 1350,1100, 2014, 2618, 6061, etc. |
|
Angle sections |
wall height 12 - 100, wall thickness 1 - 100 |
|
|
T-bars and I-bars |
wall height 15 - 70, wall thickness 1 - 100 |
|
|
Z-profiles |
wall height 20 - 50, wall thickness 1 - 50 |
|
|
Channelа sections |
wall height 25 - 80, wall thickness 1 - 50 |
|
|
Rolled wire |
diameter 9 - 14 |
|
|
Wire |
diameter 0 wall height 1 - 6,5 |
Review of scientific and technical literature shows that currently to produce small cross-section sections and rolled wire from aluminum alloys the following groups of wrought aluminum alloys are used: technically pure aluminum of grades 1060, etc.; aluminum alloys Al-Mg-Si ; aluminum alloys Al-Si, Al-Si-Cu-Mg; alloys Al-Ti-B, applied to modify cast ingots.
The problems discussed in this paper were solved applying the method of mathematical modeling of plastic deformation with account of boundary conditions and friction in the contact between metal and tool using the variation method of minimum capacity. The nature of the deformed material and its plastic characteristics effect on the deformation mechanism and metal properties by continuous extrusion was studied. The strained metal state was investigated by method of simultaneous solution of differential equilibrium equations and conditions of plasticity and method of coordinate scale. The power characteristics were determined using tensometry.
RESULTS AND DISCUSSION
аа The main stages of the study are:
1. Theoretical and experimental study of deformation center, determination of its structure and calculation of casting-extrusion process stability.
2. Mathematical modeling of the integrated casting-extrusion process in order to obtain analytical dependences and calculate parameters of forming and energy-power characteristics within the definite limits of non-dimensional parameters changes, unambiguously characterizing the deformation center.
3. Modeling thermal processes of integrated casting, rolling and extrusion and predicting thermal fields in the crystallization and deformation zones.
4. Elaborate technical solutions to implement new types of modular equipment for integrated processing, design pilot facilities and select technological modes for better properties of the extruded articles.
5. Develop algorithm and soft wear for tool design and new technologies.
а Based on the proposed technical solutions (certificates of authorship and patents of the Russian Federation ╣1667979, ╣1692739, ╣1701040, ╣1785459, ╣2100113Ц Figure 1) the metal forming characteristics in the process of integrated casting-extrusion were investigated [6].
 |
For this purpose on the laboratory equipment by rolls stop at the process of rod extrusion the unfinished sections were obtained. They were used to observe the changes of the process main geometric parameters along the deformation center and metal flow stages. It was found that when the billet enters the deformation center it bends in the direction of roll with blind pass, due to velocity difference of metal flow in different billet parts. The spread is constrained by rolling and at the moment of maximum pressing out the side walls of roller pass are in full contact with the both sides of the billet. Before matrix the metal is forced back and is pressed out along the roller pass height to the size equal to the matrix height. Metal flow from the matrix face is radial, which corresponds to the well known extrusion regularity. In the place where the pressed out part of billet turns into the rod a nick can be observed, which evidences matrix elastic deformation. The metal flow in the pressing out zone of deformation center corresponds to characteristics of deformation with active friction which is confirmed by metal flow practically along matrix face. The analysis of unfinished sections shows that the metal flow significantly differs from that in the process of normal direct extrusion and corresponds to pressing mechanism when the active friction forces are involved. The main indications of this as in the case of extrusion method with active friction forces are: outstrip metal flow on periphery to matrix; elimination of standstill zone near the matrix plane. When rolling-extrusion process is used an additional deformation type appears Ц regular deformation, i.e. each elementary metal volume first is subjected to vertical pressing deformation and horizontal elongation and after passing minimum gap between the rolls it is subjected to opposite deformation. This deformation mechanism enhances plasticity particularly of cast metal, extends extrusion speed limits. From the analysis of metal forming it is clear that geometrical deformation center includes three zones: 1 Ц zone of billet grip and rolling; 2 Ц zone of pressing out;ааа 3 Ц extrusion zone. Some features of metal deforming condition in zones indicate to active friction forces from the roller pass walls that favor the process and improves mechanical properties of finished extruded product. Study of process kinematics and strained state applying the method of coordinate scale [7] showed that at the first stage of deformation the changes in point relative position of coordinate scale completely coincide with the rolling stage. Thus, displacement of coordinate scale ordinate points begins before geometrical deformation center, the billet cross-sections bend in the direction opposite to the rolling direction, and metal particles contacting rolls pass ahead of intermediate metal layers, which are axial. However in the pressing out zone a gradual leveling of cross-sections curvature occurs and the before matrix the axial metal layers passing ahead the contact layers can be observed. This completely corresponds to extrusion stage. Changes in velocity component Vx for curve piece, corresponding to a part of deformation center after metal pressing out with rolls by rolling, are characterized by changes similar to changes of metal flow in contact, intermediate and axial layers. Thus, velocity Vx at metal entrance into deformation center is equal to initial metal velocity (V0). It grows in rolling zone and the maximum values are achieved in the plain passing through common rolls axis at the moment of maximum pressing by rolling. Later it gradually goes down to cross-section, passing through matrix face, and sharply goes up in the extrusion zone. The dependence nature reiterates both for metal contact, intermediate and axial layers. The values of vertical velocity component are not big and typical feature for diagram vertical component Vє is insignificant changes in velocity for axial metal layers. For intermediate and contact metal layers velocity Vє diminishes on the diagram piece from metal entrance in the deformation center up to typical plateau passing through common rolls axis. Then in the zone of metal pressing out the velocity component increases and achieves its maximum value before matrix, then in the pressing out zone velocity sharply goes down. It should be noticed, that the described above mechanism of velocity horizontal component changes of metal flow along the deformation center corresponds to theoretical calculations based on the designed mathematical model of the integrated rolling-extrusion process. Evaluation of metal strained state was performed by calculation and drawing dependences of logarithmic deformations lne11 and lne33 along flow lines. Analysis of the obtained dependences demonstrates that their typical feature is maximum and minimum points in planes passing through the common rolls axis and roller pass overlap by matrix. The dependences for lne33 component have different nature. Deformations in vertical direction have minimum value in the rolling zone, and then continuously grow, achieving their maximum values at the extrusion. Such sharp growth of deformation components in the extrusion zone is typical for horizontal components, and it was found that in absolute values they are significantly higher than maximum deformations occurring in the rolling zone. In this case it is due to the fact that deformation value by extrusion is by 2.7 times higher than by rolling. It should be also noticed that near contact metal layers are subjected to higher deformation than intermediate and axial layers. In order to solve the problem of possibility to implement the process of integrated rolling-extrusion it was taken as the basic statement that the process is possible in case the force due to active friction forces ╨1, appearing on the contact surface of roller pass and billet is higher than force P2, needed to perform deformation by extrusion and overcome friction forces on the surface of calibrating element. To evaluate the process stability criterion kє = (╨1 Ц P2) 100 / ╨1 was selected. The methods were developed and the stability of the rolling-extrusion process by extruding billet with square cross-section 14´14 mm using different values of absolute reduction and distance of matrix face from plane passing through rolls rotation axes at fixed matrix channel were tested. Calculations based on proposed program showed that by preset process parametersа (extruded rod diameter 7.2 mm, rolls radius 100 mm and fixed absolute reduction by rolling 2 mm) for alloys with resistance to deformation 50 MPa the rolling-extrusion process within the entire range of the deformation center parameters is impossible. Calculations of absolute reduction value for billet 4 mm and 7 mm indicated that the optimal reduction value is 7 mm (relative reduction by rolling is 50%), in this case practically within the entire parameters range of deformation center the process should be stable. Analyzing calculated data, it can be concluded, that developing rollingЦextrusion process in each individual case, to provide process good stability, it is necessary to calculate criterion k = (╨1 Ц ╨2) /╨1, i.e. to perform parametric correction of design parameters.
Mathematical modeling of integrated rolling-extrusion process was performed using method of power balance and variation principle of full power minimum [8]. The values of pressure created by rolling on rolls with different circular velocity were derived from the process energy balance, formulated to determine forces needed to extrude metal through matrix. It is presented as NE1 + NE0 Ц NF Ц NC1 Ц NC0 Ц NP = 0,а (where NE1 and NE0 are power on the roll side with open and blind roller passes respectively; NF is power needed to change shape; NC1 and NC0 are sliding power in the contact of rolls with metal with open and blind roller passes respectively; NP is power to accomplish extrusion process). As a result of power components calculation, the dependence for calculation of created extrusion pressure upon rolls diameter, roller pass size, metal reduction value and geometrical sizes of deformation center was obtained. Analysis of the calculation results applying the model demonstrated, that rolls radius increase results in linear growth of produced pressure, and in case of smallа drawing out it is possible to extrude billets of significantly bigger cross-sections than using a well-known method лConform╗. In order to find the matrix distance from the common rolls axis a variation problem applying mathematical model was solved. This model is based on the equation of powers balance given above and variation equation of full power minimum principle, presented as: dN=d(NEH + NCP Ц NCK Ц NB) = 0 (where NEH is power of internal forces, NCP is shearа strength on the velocities rupture surfaces, NCK is friction power, NB is power developed by rolls). Simultaneously the geometrical model describing plastic deformation center with accuracy up to varying parameters was designed, cinematically possible field of velocities was plotted and boundary conditions at contact metal Ц tool formulated.а The model was used to obtain dependences to calculate matrix distance from common rolls axis for deformation process in blind box type and bar type roller passes. It was established that deformations of metal having low friction coefficient or deformation of sections or pipes having complex configuration, it is necessary to use blind bar type roller passes, as the active friction forces in this case are higher than in box type roller passes.
Using A.TselikovТs method [1] the theoretical analysis of contact strains by rolling-extrusion, acting on the boundary of rolls contact and processed metal was conducted. Analysis of the obtained results proved that distribution of contact strains sx along deformation center corresponds to data obtained in the experiment tests of deformation mechanism. Thus, in the first zone stress sx growth up to maximum value in the plane passing through common rolls axis was observed. However in the pressing out zone the obtained results differed from results presented in [3], at first strains go down and then go up. It is probably due to the fact that in the pressing out zone both active and reactive friction forces are accounted. In the extrusion zone the strains achieve their maximum value, and then drop to sx=0. As a result of analytical evaluation the formulas to calculate metal temperature and roll size for each zone of deformation center were obtained.
In order to verify the results of mathematical modeling the tests of rolling-extrusion process for different aluminum alloys were carried out [6] using pilot equipment on the basis of rolling mill DUO 400. Rolling mill specification is given in Table 2.
Table 2
|
Parameter |
Unit |
Value |
Roll diameter |
mm |
400 |
|
Roll neck diameter |
mm |
140 |
Roll side length |
mm |
240 |
|
Roll circular velocity |
m/min |
6,3 |
|
Electric motor capacity |
kW |
75 |
Analysis of the results showed that deformation strength on rolls and matrix increase with growth of drawing out coefficient. In addition, with increase of drawing out coefficient the pressure in the pressing out zone grows and necessary capacity supplied by rolls also goes up. Due to temperature increase the forces on matrix and rolls decrease, which corresponds to generally accepted principles of theory of working metals under pressure and is a result of reduced metal resistance to deformation. When the tests were conducted it was found, that forces on matrix and rolls diminish if the deformation speed increases. This is probably due to the fact that the processes of softening, occurring by preheating temperature increase for studied alloys are predominant as compared to rapid deformation strengthening. Comparative analysis of quantitative dependences of forces acting on rolls and matrix for aluminum 1060 and alloy 6063 proved that they are lower for 1060, than for alloy 6063. This can be explained by lower resistance to deformation of 1060as compared with 6063 under other equal conditions. Test diagrams of energy power parameters for studied alloys Al-Ti-B are also specified by temperature increase at one and the same drawing out and the deformation strength decrease both on rolls and matrix. Increases drawing out coefficient causes the forces growth on matrix and rolls, however growth of forces on rolls differs depending on master alloy composition. For master alloy Al-3.2%Ti-0.4%B the sharp growth of power parameters is typical. This can be probably explained by peculiar behavior of alloys Al-Ti-B studied by physical metallurgy, specifically by relaxation of internal strains, acting on the boundaries of strips being deformed along grain boundaries and in the interphase boundaries. Characteristic feature of these dependences is that by small reduction values the difference in deforming forces is also small; when reduction increases the difference in deforming forces grows due to specific behavior of Al-Ti-B system at high reduction and due to the presence of intermetallic compounds in alloy. Study of macro- and microstructure of alloy 1060 showed the existence of optimal parameters combination which provides the best structure characteristics in terms of homogeneity and particles dispersion in distinguished phase. The combined parameters are: billet preheating temperature 580 0╤, drawing out Ц 3.5 and deformation rate Ц 0.54 s-1. Comparison of test and calculated data showed good agreement and the error of power parameters calculation is less 5-7% on average.
In order to improve the developed technologies the technical solutions related to integration of continuous casting and rolling-extrusion processes were offered (patents Russia ╣2100136, ╣2200644). As the basic process remained the process of integrated rolling- extrusion studied before, the main objective was modeling thermal processes and prediction of thermal fields in zones of crystallization and deformation. Modeling thermal processes in the casting-rolling-extrusion unit was performed based on solution of deferential equations of internal and external heat transfer using method of coordinate scale and difference ratios [9]. To solve this problem a program was developed in object oriented environment DELFI. It enabled to make calculations for wide range of technological modes of integrated casting, rolling and extrusion in various conditions of external heat transfer and for different thermal and technical specifications of crystallized and deformed material and produced article sizes both for cooled and uncooled rolls. Modeling enabled to find optimal equipment parameters providing temperature in the central point in matrix entrance to reduction zone necessary for metal or alloy heat treatment with all design specifications for individual unit components. After analyzing the existing designs of roll type units for direct rolling, in order to implement the process in the laboratory conditions, the variant of unit with metal supply from the top was selected. In this case the melt comes into the gap between rolls not only due to metallic static pressure but also under the pressure of liquid metal column, which direction coincides with rolling and extrusion line. When crystallized metal appear in the process using rolls permits to avoid metal spread, because they form closed roller pass. Experiments aimed at obtaining rods from aluminum └5, └7, alloys 6063 and 6082, arranged using laboratory unit for integrated casting and rolling-extrusion based on rolling mill DUO 200, allowed to test the results of theoretical studies and obtain pilot lots of extruded articles applying new technology. The analysis of structure and properties of obtained rods proved that they can be used to modify aluminum alloys [10].
To control the processes of tool design and technology of integrated rolling-extrusion the algorithm and soft wear as CAD subsystem, implemented in DELFI environment, were developed. Subsystem allows making calculations of process energy power parameters, determining process stability and obtaining matrix and rolls drawings for designed variant based on derived formulas geometrical characteristics of deformation center [11]. The performed calculations enabled to make tool working drawings (shape accuracy control unit, instrumental unit, etc) and fabricate them for pilot industrial units based on rolling mills DUO 260 (Figure 2) and DUO 400. An industrial test [12] of integrated metal processing technology allowed to produce several lots of rods (each about 80 kg) from aluminum 1060 and alloy Al-Mg-Mn. The mechanical properties of extruded items were measured.
ANALYSIS OF RESULTS
аа As a result of conducted studies the optimal sizes of billets were found and optimal technological modes for processing aluminum 1060 and alloy Al-Mg-Mn applying the method of integrated rolling-extrusion for industrial conditions were
Figure 2. Diagrams showing rolling-extrusion process based on rolling mills DUO 260
proposed. The design of different units of modular equipment improved, deformation process energy power parameters were measured, major mechanical characteristics determined, the structure and properties of obtained extruded products studied and the pilot industrial unit was installed at JSC лVerkhne-Saldinsk Metallurgical Production Complex ╗ and лSEGAL, Ltd╗ (Krasnoyarsk). Compared to western similar equipment Ц continuous extrusion equipment of "Holton Machinery", operated based on лConform╗ method, the designed experimental industrial units applying method of integrated rolling-extrusion are specified by reduced energy consumption (capacity of electric motor drive is 2-3 times lower) at the same time the properties of articles produced by this method are better Ц they posses better dimensional uniformity. When the processes of rolling-extrusion are integrated with continuous casting of ingots in rotating molds the process of obtaining small cross-section extruded articles with small diameters becomes more profitable. However, we believe the most advantageous process for non-ferrous metals processing is the method of direct rolling-extrusion with metal crystallization on water cooled rolls with further deformation by billet extrusion through matrix having opening of the preset shape and size. The cost-effectiveness due to introduction of new equipment compared to traditional technologies (discrete extrusion using hydraulic presses), presently applied for production in Russia, can be 30-80% higher. The productivity can be increased 2-3 times, and properties of extruded articles meet Russian GOST requirements.
CONCLUSIONS
ааааааааааааааа As a result of performed theoretical and experimental studies it was stated that conversion from discrete technologies of producing different sections from non-ferrous metals to continuous technologies has many advantages. The modular equipment has been designed and proposed to implement different modes of continuous casting, rolling and extrusion based on patented technical solutions. The designed modular equipment and developed technology are recommended for production of small cross-section extruded articles from aluminum and aluminum alloys.
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