To determine the influence of the cold briquetting pressure on the density of cylindrical briquettes made of 6063 alloy, depending on the mass of the fill, experimental studies were carried out the results of which are shown in Fig. 6.
It has been found that the minimum required level of the relative density of briquettes for subsequent deformation should be at least 60-70%, for which a briquetting pressure of at least 80-100 MPa must be applied. In this case, the greater the mass of the fill, and, consequently, the higher the height of the resulting briquette, the lower its density and the more significant the inhomogeneity of its distribution. A decrease in porosity and, consequently, an increase in density at the same values of briquetting pressure can be achieved if the chips are compacted in a form heated to a certain temperature. The heating temperature, corresponding in this case to the conditions of hot briquetting, is selected on the basis that the resistance to deformation of the chip material at this temperature should be lower than the level of applied briquetting pressures.
To confirm this fact, experiments were carried out on hot briquetting of the same 6063 alloy chips in a press-form, in one case heated to a temperature of 300±20°C, and in the other - up to 400±20°C. The integral density of the obtained compacts was: at 300°C 2.3-2.4 g/cm3, which corresponded to the relative density of about 85%, and at 400°C 2.5-2.6 g/cm3, which corresponded to the relative density of about 90%.
The temperature and deformation rate at which discrete extrusion on a vertical press with a force of 1000 kN took place were identical each time and corresponded to the parameters recommended for extrusion cast billets from 6063 alloy, i.e. when the billet temperature was in the range T = 430-450°C, and the extruding speed Vextr = 50-150 mm/s. Compression was used to obtain rods with a diameter of 6 mm (the elongation coefficient µ was ≈ 56), 8 mm (µ ≈ 32), and 12 mm (µ ≈ 14). It was revealed that regardless of the extrusion ratio with which extrusion is carried out, the outflow of the rods from the die according to a given temperature and speed regime is quite stable. The maximum extrusion force for the 6063 alloy is about 500 kN, which is in satisfactory agreement with the calculated data obtained using the formula of I.L. Perlin [61].
In the course of practical testing, it was found that preliminary hot or cold briquetting of chips does not have a significant effect on the formation of a given level of mechanical characteristics of wire-rod products. In any case, it is important to provide only the specified temperature and rate conditions of deformation at the moment of the beginning of the outflow of metal from the working hole of the die. In this case, the mechanical properties of the rods after hot extrusion were: ultimate tensile strength Rm = 130-140 MPa, elongation to failure A = 15-17%.
To obtain briquettes from AlSi12 alloy chips, two methods were also used: cold and hot briquetting.
The results of experiments on cold briquetting showed that the maximum relative density, which can be achieved by cold briquetting of chips with a pressure of about 180 MPa, is about 70-75%. At the same time, the use of briquetting pressures below 100 MPa did not allow achieving a stable bound state at all. When removing the briquettes compacted at this pressure from the press-form, they crumbled into separate fragments.
Experiments on hot briquetting of the same chips at different briquetting temperatures were carried out with the following parameters: the weight of the sample of the chips was about 100 g, the diameter of the container was 42 mm, the maximum applied briquetting pressure was 180 MPa, and the holding at this pressure was 5 min.
In the course of briquetting and at the end of it, the value of the current and final integral density of the briquettes was determined, the values of which, depending on the temperature and pressure of briquetting, are shown in Fig. 7. The maximum integral density of the obtained briquettes at a temperature of 400°C was 2.35-2.45 g/cm3, which corresponded to a relative density of about 90%.
The temperature and speed mode of performing the discrete extrusion operation in this case corresponded to the parameters recommended for extruding rods from hard-to-deform aluminum alloys: billet heating temperature T = 450-470°C, extruding speed Vextr = 50-100 mm/s [61]. By extruding, rods with a diameter of 6 mm (µ ≈ 56) and 8 mm (µ ≈ 32) were obtained. A comparative analysis of the achieved mechanical characteristics showed that, regardless of the diameter of the hot-extruded rod, the average values of the considered indicators of the strength and plastic properties of the material differ little from each other. The range of variation of the values of the ultimate tensile strength Rm is from 160 to 170 MPa, the elongation to failure A is from 15 to 17%, the area reduction Z is from 30 to 40%. For example, Fig. 8 shows the typical structures of a rod with a diameter of 6 mm, obtained from chips of silumin AlSi12 by the method of discrete extrusion.
Metallographic analysis showed that the structure in the longitudinal section is row-like, with the light sections of the aluminum phase elongated in the direction of outflow from the die interspersed with darker areas saturated with silicon (Fig. 8). In this case, the concentration, size of silicon particles, and the order of their distribution against the background of the aluminum phase are relatively inhomogeneous.
Computer modeling of the combined rolling-extruding process was carried out in the QForm V8 program in a 3D setting with one plane of symmetry (Fig. 9). The data on mechanical properties required for modeling were taken from works [62–70]. Aluminum alloy 6063 was chosen as the workpiece material. The rolls had different rolling diameters, which made it possible to increase the feasibility of the CRE process [58–60].
Figure 9a shows the distribution of metal temperature along the deformation zone. When in contact with a colder tool, the temperature of the billet decreases, which is explained by its small transverse dimensions and a significant difference between the heating temperatures of the billet and the rolls. The only positive effect of this effect can be that the metal of the billet does not recrystallize and, therefore, does not lose the strength achieved as a result of hardening. The distribution of the degree of deformation shown in Fig. 9b shows a gradual increase in this parameter, starting from the entry of the billet into the rolls with a sharp increase near the surface of the die. In this case, at the entrance to the deformation zone, the distribution of the degree of deformation is asymmetric: the maximum degree of deformation is obtained by the layers adjacent to the roll having a larger diameter (roll with a protrusion).
Figure 10 shows the results of studies of changes in the power parameters of the CRE process and the relative density of the metal. From the graphs of the dependence of the value of the rotation moment of the rolls on the displacement (Figure 10a), it can be seen that the rotation moment of the roll with a protrusion is lower than that of a roll with a small diameter (a roll with a groove). This is explained by the fact that a roll with a groove has a larger contact area with the metal being processed than a roll with a protrusion; therefore, the amount of contact friction is greater.
From the picture of the distribution of the relative density of the billet material along the deformation zone, it can be seen (Fig. 10b) that the compaction process begins immediately at the entrance to the deformation zone, and this process is also asymmetric: on a roll with a protrusion, the process begins earlier. However, as the metal advances in the course of rolling-extruding, the metal layers adjacent to the groove roll are already more compacted. When the metal approaches the die, the highest density is observed closer to the center of the billet. In the plane connecting the axes of the rolls, the compaction process practically ends, and an almost compact material enters the extrusion zone, the relative density of which is close to 100%. With further advancement of the metal, this density no longer changes.
To check the simulation results, experimental studies were carried out on the production of briquettes from the investigated alloys and their deformation by the CRE method. The cross section of the briquette had dimensions of 15×15 mm, the length was 200 mm, and the mass of the fill was taken equal to 130, 150, and 180 g. A special split form was used for briquetting the chips [33]. The general view of the briquettes, as well as the dependence of their density on the pressure of cold briquetting, is shown in Fig. 11. The maximum relative density at pbr = 100 MPa was 80%.
The resulting briquettes (3 pieces at the same time) before rolling-extruding were heated to a temperature of 480±20°C in a resistance furnace; the total heating time was about 60 minutes. In parallel, the rolls of the CRE-200 unit were heated to a temperature of Tr = 80-100°C.
Using the CRE method, rods with a diameter of 7 and 9 mm were obtained, the level of strength and plastic characteristics of which turned out to be approximately the same and amounted to: ultimate tensile strength Rm = 180-190 MPa, elongation to failure A = 12-16%, area reduction Z = 38-42%.
Analysis of the results of metallographic studies of rods obtained from 6063 alloy chips (Fig. 12) showed that it is necessary to increase the degree of deformation during extrusion, since it is insufficient to ensure high-quality seizure of the chips particles in the process of their joint deformation. The microstructures show clearly defined interfaces between individual chips, which are surface oxide films, and rather rare discontinuities. There is no fundamental difference between the structures of the samples cut from extruded rods with a diameter of 7 and 9 mm. Bridges of seizure between the chips are not observed, i.e. the formation of physical contact occurs mainly on the microroughnesses of the chips with partial spreading (but not destruction) of the oxide film over the entire contact surface.
Billets made of chips from the AlSi12 alloy for CRE were formed by hot briquetting at a temperature Tbr = 350 °С and a briquetting pressure pbr = 100 MPa, the relative density being 85-87%. The temperature of heating the briquettes before the CRE was chosen equal to 500-520 °С, the total heating time was 60 minutes. As a result, rods with a diameter of 7 and 9 mm were manufactured, and, as before, the level of strength and plastic properties of the resulting rods was practically the same value: ultimate tensile strength Rm = 220 - 230 MPa, elongation to failure A = 5-7%, area reduction Z = 8-9%. The characteristic structures of rods also do not have fundamental features, as in the case of rods made of 6063 alloy chips.
Comparison of the mechanical properties of rods manufactured using discrete extrusion and the CRE method from the same types of chips showed that due to the difference in temperature conditions for the implementation of the processes, it is advisable to pre-anneal the rods obtained by the CRE method before they are cold drawn by drawing according to the following modes:
- for alloy 6063: Tanneal = 350 °С; tanneal = 60 min;
- for alloy AlSi12: Tanneal = 400 °С; tanneal = 60 min.
After the annealing of the rods, the strength and plastic characteristics of the metal were:
- for alloy 6063: ultimate tensile strength Rm = 125-135 MPa, elongation to failure A = 17-18%, area reduction Z = 50-55%;
- for alloy AlSi12: ultimate tensile strength Rm = 105-115 MPa, area reduction Z = 10-11%.
At the final stage of research, computer modeling of the process of drawing bars obtained by discrete and continuous extrusion from chips of 6063 and AlSi12 alloys, as well as experiments on obtaining wire from them, were carried out [33].
Modeling of the process of drawing a porous rod was carried out in the ABAQUS program. Workpiece material - aluminum alloy 6063, the initial relative density of the material was 90%. The coefficient of friction was chosen as a variable parameter, which varied at four levels: 0.05; 0.1; 0.2 and 0.3.
The effect of the friction coefficient on the compaction process is shown in Fig. 13 (for example, the distribution of the relative density for the friction coefficients of 0.05 and 0.3 is shown). It can be seen that with a small friction coefficient in the direction of the radial coordinate, two zones of compaction can be distinguished, and in the peripheral zone the compaction is greater than in the central zone. With an increase in the friction coefficient, a third zone appears in the center with a density reduced to the initial value, which expands with an increase in the friction coefficient to 0.3.
The results of calculating the distribution of the Pressure value (the value opposite in sign with respect to the hydrostatic pressure) are shown in Fig. 14. It can be seen that at a low coefficient of friction, zones of intense compression are located in the peripheral regions of the billet, adjacent to the surface of the die. At the level of the calibrating band, tensile zones appear which are caused by the action of the pulling force. With an increase in the friction coefficient, the zones of action of compression stresses are increasingly localized near the contact surfaces, without penetrating into the central region. This picture is in qualitative agreement with the density distribution shown in Fig. 13.
The distribution pattern of the degree of deformation PEEQ (Fig. 15) reflects an increase in this value from the entrance to the deformation zone to exit from it and its characteristic non-uniform distribution along the radius.
The maximum value of this indicator is typical for the periphery of the wire, and the smallest for the central area. Thus, qualitatively, the distribution of the degree of deformation also corresponds to the distribution of density.
Additional shear stresses also contribute to the compaction of the deformable material. In the case of axisymmetric deformation, they can be estimated by the stress tensor component σrz. This component, designated as S12, is depicted in Fig. 16 areas and lines of equal level.
This value is characterized by the presence of two signs - "plus" and "minus", which corresponds to a different direction of shear deformation. At the entrance to the deformation zone, a stress extremum with a minus sign is observed, and at the exit - with a plus sign. With an increase in the friction coefficient, the zone of increased shear stresses at the entrance to the deformation zone increases, and at the exit, it decreases. The values of the maximum shear stresses are located closer to the contact surface, i.e., in the regions where the highest density is observed. Therefore, the contribution of increased shear stresses to the effect of increasing density cannot be rejected.
Thus, summarizing the simulation results presented above, it is possible to predict a scenario of deformation development in which the influence of the discontinuities initially embedded in the metal structure will be minimal. This approach can become the next step in the development of ideas about the deformation of porous materials, including according to the technological scheme described above.
Further, in the work, rods from the investigated alloys were drawn on a chain mill with a force of 50 kN, obtained using various versions of the extruding process. Preliminarily, using data on the mechanical characteristics of the metal [70], the parameters of the wire drawing process were determined. Table 2 shows the results of calculating these parameters for the 6063 alloy.
Table 2
– Technological and energy-power parameters of the process of drawing wire obtained from 6063 alloy chips
Drawing process parameters
|
Pass number
|
1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
Initial diameter Di, mm
|
6
|
5.5
|
5.1
|
4.7
|
4.35
|
4.1
|
3.75
|
3.4
|
Final diameter Di − 1, mm
|
5.5
|
5.1
|
4.7
|
4.35
|
4.1
|
3.75
|
3.4
|
3
|
Initial cross-sectional area Fi, mm2
|
28.3
|
23.7
|
20.4
|
17.3
|
14.9
|
13.2
|
11.0
|
9.1
|
Final cross-sectional area Fi − 1, mm2
|
23.7
|
20.4
|
17.3
|
14.9
|
13.2
|
11.0
|
9.1
|
7.1
|
Elongation coefficient µ
|
1.19
|
1.16
|
1.18
|
1.17
|
1.13
|
1.20
|
1.22
|
1.28
|
Total relative reduction εΣ, %
|
16.0
|
27.8
|
38.6
|
47.4
|
53.3
|
60.9
|
67.9
|
75.0
|
Average resistance to deformation σs(av), MPa
|
150
|
175
|
193
|
206
|
215
|
221
|
227
|
232
|
Drawing force Рd, kN
|
1.21
|
1.00
|
1.02
|
0.88
|
0.61
|
0.82
|
0.76
|
0.80
|
Drawing tension Kd, MPa
|
51
|
49
|
59
|
59
|
46
|
74
|
84
|
113
|
Safety factor γs
|
3.0
|
3.6
|
3.3
|
3.5
|
4.6
|
3.0
|
2.7
|
2.1
|
The drawing route was taken as a basis: 6.0 mm → 5.5 → 5.1 → 4.7 → 4.35 → 4.1 → 3.75 → 3.4 → 3.0 mm. In this case, preliminary and intermediate annealing was not provided. In the course of drawing, in each case, samples were taken at separate diameters (3 pieces for each diameter) and the mechanical characteristics were determined on a universal testing machine LFM10.
Comparison of the achieved mechanical characteristics was carried out on a cold-deformed wire with a diameter of 4.7 mm (the total relative reduction ε at that time was 39%), a diameter of 4.1 mm (ε = 53%) and a diameter of 3.0 mm (ε = 75%). The results of the mechanical tests carried out in the form of the corresponding diagrams are presented in Fig. 17. Their comparative analysis and comparison with the calculated data obtained from the results of the regression analysis (the formulas (1-4) are given below), indicates a high convergence of the results.
Figure 18 shows the microstructures of a wire made of 6063 alloy of various diameters, obtained from a rod with a diameter of 6 mm. Their analysis showed that with an increase in the relative reduction, the structure is refined with a gradually increasing fragmentation in the near-surface layers of the wire.
The drawing of the bars obtained from the chips of the AlSi12 alloy by the method of discrete extrusion, as before, was carried out on a chain drawing mill with a force of 50 kN. The average unit reduction during drawing was 15-20%, and during intermediate annealing the following regime was used: annealing temperature Tanneal = 400 °С, holding time annealing = 1 hour. Taking into account the pre-calculated values of the safety factor, the following drawing route was used: 6.0 mm → 5.5 → 5.0 → 4.35 → 3.85 → 3.3 → 2.8 → 2.4 → 2.1 → 1.7 → 1.5 → 1.2 mm.
The results of mechanical tests of wires of various diameters made of chips of the AlSi12 alloy in comparison with the calculated data are presented in Fig. 19.
The study of the microstructure (Fig. 20) using the example of a wire with diameters of 6.6 and 5.0 mm, each of which was drawn from the corresponding rod with a total relative reduction of 30%, shows that as the diameter of the wire decreases, though not quite uniform length, but a noticeable crushing of silicon. The boundaries between the individual shavings practically do not appear, that is, we have obtained an almost homogeneous solid material, which is a uniformly distributed siliceous phase over the body of a α-solid solution of aluminum.
To assess the results obtained and automate the calculation of the parameters of the drawing process using the methods of full factorial experiment and least squares, regression equations were derived to determine the ultimate tensile strength Rm and elongation to failure A during cold drawing of wire obtained from chips of 6063 and AlSi12 alloys, depending on total relative reduction εΣ, expressed as a percentage.
For the 6063 alloy, these equations are:
R m = 132.9 + 2.177 ⋅ ε – 0.011 ⋅ ε2; (1)
A = 15.8 – 0.226 ⋅ ε + 0.001 ⋅ ε2. (2)
For alloy AlSi12:
R m = 167.2 + 2.747 ⋅ ε – 0.016 ⋅ ε2; (3)
A = 14.4 – 0.272 ⋅ ε + 0.002 ⋅ ε2. (4)
Possible areas of practical application of wire-rod products made from waste chips of the investigated alloys are their use as modifiers or deoxidizers, as well as welding wire made of AlSi12 alloy for soldering special-purpose structures made of aluminum alloys.