At the first stage of experimental studies in laboratory conditions, the rolling of billets with milled edges 50×130×330 mm in size (Fig. 1b) from the experimental alloy 1580 was simulated (Table 3). An assessment was made of the possibility of rolling plates from one heating under various modes of single reductions, a study of the structure, and determination of the mechanical properties of the resulting deformed semi-finished products.
Two workpieces were rolled (Fig. 2) with a change in the reduction value, selecting rational processing modes. The general scheme of deformation included hot rolling of a billet 50 mm thick, heated to a temperature of 430°C, with different values of a single reduction. As equipment for rolling, a two-roll laboratory mill DUO 330 was used, the technical characteristics of which are given in Table 5. In the course of rolling samples were taken for testing the mechanical properties and studying the structure of the metal.
Table 5
– Technical characteristics of the sheet rolling mill DUO 330
Parameter
|
Parameter value
|
Electric motor power, kW
|
90
|
Roll barrel length, mm
|
520
|
Roll diameter, mm
|
330
|
Maximum roll separation, mm
|
70
|
Roll rotation frequency, rpm
|
10
|
Maximum rolling force, MN
|
1,55
|
Maximum rolling moment, MN·m
|
0,82
|
The first workpiece heated to a temperature of 430±10°C was rolled to a thickness of 30 mm with a minimum single reduction of 2%, after which cracks began to appear on the side faces (Fig. 3). The total degree of deformation was 40%, and the number of passes was 21. The temperature of the metal after the last pass was 350°C. An analysis of the rolled workpiece showed that the metal flow for a given reduction scheme is uneven, while the peripheral layers are ahead of the central ones (Fig. 2a). This caused the appearance of significant tensile stresses and, as a result, with a decrease in the temperature of the metal by the end of rolling, the appearance of cracks on both side faces of the workpiece. It should also be noted that for given single reductions the energy-power parameters of rolling did not exceed the allowable ones (Table 5).
The second workpiece was rolled in a similar temperature regime, but with maximum reductions during rolling (Table 4) after which the billet opened at its end. The total degree of deformation was 37%. The temperature of the metal after the last pass was 370°C. It can be noted that a more uniform metal flow and the absence of cracks were observed during rolling (Fig. 2b), which indicates the development of both peripheral and central layers of the workpiece. Exceeding the permissible values of the energy-power parameters in this mode of compression was also not observed.
The mechanical properties of the obtained semi-finished products are given in Table 6.
Table 6
– Mechanical properties of samples from plates for an experimental alloy 1580
Workpiece number
|
Ultimate tensile strength Rm, MPa
|
Yield strength Rp, MPa
|
Elongation to failure A, %
|
1
|
405
|
343
|
9
|
2
|
410
|
379
|
8
|
For metallographic studies samples were cut from the fracture surface of the workpieces for fractographic studies (Fig. 4a, b). The analysis showed that the fracture surface of the workpieces has a matte, non-oxidized surface without visible defects of foundry origin (large pores and non-metallic inclusions). A significant amount of light inclusions is observed along the fracture cross section of the workpieces.
X-ray microanalysis (XRSA) of the second workpiece revealed the presence of intermetallic inclusions, as well as cracks along the grain boundaries (Fig. 5). Intermetallics are observed in lamellar form containing Al, Zr, Sc, Ti (Fig. 5, spectrum 1, 2) and in the form of polyhedra containing Al, Fe, Mn, Cr (Fig. 5, spectrum 3). Intermetallic compounds have sizes of 10-60 microns, single inclusions reach 150 microns.
The study of workpiece fracture on a scanning electron microscope showed that the fracture surface is mainly characterized by ductile fracture elements in the form of ridges, pits of various sizes and shapes (Fig. 6). The fracture also contains elements of brittle intergranular fracture in the form of flat facets.
Additional studies of the microstructure of the sample prepared from the transverse section of the destroyed workpiece showed that the propagation of cracks occurred along the accumulations of intermetallic inclusions located mainly along the grain boundaries (Fig. 7). The intermetallic compounds present in the structure of the workpieces are predominantly of crystallization origin.
The results of the experimental studies for test ingots made it possible to draw the following conclusions:
-
it is necessary to improve the technology of casting large-sized ingots, since the presence of heterogeneity of the grain structure and intermetallic compounds does not allow for the same reduction of its entire surface, while cracks in the metal during rolling begin to form in the area where large intermetallic compounds are located;
-
with reductions in the passages of about 9-11% and billet heating temperatures of 430±10°C, the deformation of the metal proceeds evenly without defects and the metal is worked out over the entire thickness;
-
at large individual degrees of deformation rolling with a minimum number of passes is possible and limited only by the allowable values of the rolling force and moment;
-
for the studied experimental ingots of the 1580 alloy the critical technological parameters that limit the possibility of deformation are: the heating temperature of the billets is 420-450°C; the minimum temperature of the metal after rolling is not lower than 350-370°C; the total degree of deformation is not more than 40%.
The results obtained during computer and physical modeling of the rolling process of 1580 alloy plates were used to test the reduction modes during metal deformation and to adjust the casting parameters of experimental ingots in order to obtain a high-quality metal structure necessary for rolling.
For experimental studies at the second stage, billets 300×1445×2200 mm in size with milled edges were made from a large-sized experimental ingot of 1580 alloy 445 mm thick (Fig. 1a). At the same time, the technology for manufacturing plates under industrial conditions included homogenization annealing, hot rolling, intermediate annealing, and cold rolling.
The workpieces were subjected to homogenization annealing according to the following regime: heating with a furnace at a rate of 1.16°C per minute to 350°C; exposure at this temperature for 11 hours; reheating to a temperature of 425°C at a rate of 1.25°C per minute; exposure at this temperature for 8 hours; air cooling [27].
The macrostructure of the 1580 alloy ingot was studied on templates cut from the peripheral and central zones along the thickness of the ingot after preliminary etching in a 15% NaOH solution for 30 min and subsequent clarification in nitric acid.
Metallographic studies have shown that the macrostructure over the cross section of the templates is uniform fine-grained with a grain size of up to 1 mm. Defects of metallurgical origin in the form of cracks, porosity, oxide films, non-metallic inclusions and intermetallic compounds were not found. The depth of the surface zone of liquates reached 3 mm.
The microstructure of the metal in the cast state (Fig. 8) is represented by an aluminum-based α-solid solution and inclusions of excess phases located along the boundaries of dendritic cells. In the central zone of the ingots, porosity was found with pore opening up to 0.02 mm (Fig. 8a) and intermetallic compounds up to 0.05×0.23 mm in size. In the process of homogenization the dissolution of the phase components along the boundaries of the dendritic cells and the decomposition of the solid solution with a uniform release of dispersed particles over the volume of the solid solution occurred (Fig. 8b). The study of the microstructure in polarized light (Fig. 9) showed that in the peripheral zone of the ingot there is an inhomogeneous structure with a grain size of 167 to 330 µm.
The central part of the ingot is characterized by a more uniform grain structure, the average grain size is 250 µm (Fig. 9a).
Hot rolling of ingots with dimensions of 45×2230×7600 mm was carried out on a Quarto 2800 mill [7, 32, 36]. The degree of deformation was 85%. The thermal deformation parameters of rolling were chosen on the basis of simulation results. The ingot heating temperature was 430±10°C.
The obtained values of the mechanical properties of the deformed and annealed semi-finished products are given in Table 7. Intermediate annealing of the slabs was carried out at a temperature of 320±10°C for 6 hours. From Table 7 it can be seen that annealing has practically no effect on the mechanical properties of hot-rolled semi-finished products.
The microstructure of hot-rolled plates was studied on microsections cut from the central part of the plates in the longitudinal and transverse directions (Fig. 10).
Table 7
– Mechanical properties of plates from alloy 1580
Sample cut direction
|
Ultimate tensile strength Rm, MPa
|
Yield strength Rp, MPa
|
Elongation to failure A, %
|
Hot rolled plates after rolling
|
longitudinal
|
389
|
247
|
18.0
|
transverse
|
358
|
234
|
12.0
|
Hot rolled plates after annealing
|
longitudinal
|
388
|
245
|
18.8
|
transverse
|
351
|
238
|
12.6
|
Cold rolled plates after rolling
|
longitudinal
|
414
|
386
|
6.6
|
transverse
|
413
|
372
|
5.2
|
Metallographic analysis showed that the microstructure is typical of an aluminum alloy in a hot-deformed state. Against the background of the α-solid solution, lines elongated in the direction of rolling and separate chains of fragmented fine phases are observed (Fig. 10a, b). An analysis of the grain structure in polarized light showed that the structure of all the studied samples was fibrous, non-recrystallized (Fig. 9b). It should also be noted that the microstructure of plate samples after hot rolling and annealing is similar to the structure of plates in the hot-rolled state.
Cold rolling of plates was carried out to a thickness of 31.5 mm while the degree of deformation reached 30%. It was noted that at the degree of deformation close to 30% cracks appeared on the side edges of the plates. The mechanical properties of cold-rolled plates with dimensions of 31.5×2000×7500 mm are also presented in Table 7.
An analysis of the metal microstructure of cold-rolled plates showed that lines and separate chains of crushed fine phases are observed in the rolling direction, which are located along the grains of the α-solid solution (Fig. 10c, d). The structure of plates in the cold-deformed state is non-recrystallized in the form of fibers elongated along the rolling axis (Fig. 9c).