At external examination the ingots of alloy 1580 obtained at the SCCU were characterized by a high surface quality. Foundry equipment and general view of the ingot are shown in Fig. 4.
After cutting the bottom and gating parts of the ingots, as well as after milling all surfaces of the ingots, no casting defects in the form of cavities and inclusions were found, which indicates the correct selection of technology.
The microstructure of cast ingots, which was carried out before and after homogenization annealing is shown in Fig. 5.
On the microstructure of alloy No.1 with a Sc content of 0.055% (wt.) in the cast state, there is grains of an α-solid solution with chemical inhomogeneity over the cross section of dendritic cells (Fig. 5a). Along the boundaries of the cells, inclusions of the nonequilibrium phase β(Al8Mg5), the Mg2Si phase, and a large number of phases containing iron Al6(Fe, Mn) and Al15(Fe, Mn)3Si2 were also found. In this case, the structure of the ingots did not contain inclusions of primary intermetallic compounds Al3(Sc, Zr) [47]. Consequently, scandium and zirconium in the alloy were only in a supersaturated solid solution and the cooling mode during casting was chosen correctly.
In the two-stage annealing of ingots, according to [21], two processes occur: homogenization and heterogenization of the structure. For this alloy, upon homogenization, the dissolution of the nonequilibrium phase β(Al8Mg5), partly of the Mg2Si phase, and elimination of chemical inhomogeneity over the cross section of the dendritic cell and grain occurred. Heterogenization of the alloy led to the decomposition of the aluminum solid solution supersaturated with respect to Sc, Zr, Mn with the release of dispersed particles Al3(Sc, Zr) and Al6Mn and proceeded in two stages. At the first stage, at 350 °C, a homogeneous decomposition of a solid solution of scandium and zirconium occurred with the formation of particles of a stable coherent Al3(Sc, Zr) phase several nanometers in size and with a high distribution density. At the second stage of annealing at a temperature of 425 °C, the decomposition was completed with a small coarsening of dispersed particles of the hardening phase Al3(Sc, Zr). Also at this stage, the manganese solid solution decomposed with the formation of secondary precipitates.
The microstructure of alloy No.1 in the annealed state (Fig. 5b) practically did not differ from the cast structure, and insignificant differences in the microstructure can be explained by dissolution during annealing of the nonequilibrium β(Al8Mg5) phase and partial dissolution of the Mg2Si phase.
Analysis of the microstructure of alloys in polarized light showed (Fig. 5c, d) that the grain size of the alloys in the annealed state practically does not change in comparison with the cast state. Studies of the microstructure of the ingot of alloy No.5, containing 0.75% (wt.) Sc, did not reveal any noticeable differences from the microstructure of the ingot of alloy No.1.
The average values of the results of testing the mechanical properties of tensile samples cut from hot-rolled and annealed sheets are given in Table 3. Analysis of these data showed that alloys Nos. 1-3 with a scandium concentration of 0.055% (wt.) Are similar in their properties. Alloys Nos. 4-6 containing 0.075 have a higher level of strength properties, especially in terms of yield strength Rp, which can be explained by a higher content of scandium. At the same time, the ductility of all six alloys is at the same level.
Table 3 – Mechanical tensile properties of hot-rolled sheet semi-finished products 5 mm thick after annealing at 380 °C for 1 h
Alloy number
|
Sc, % (wt.)
|
Mechanical properties
|
Longitudinal direction
|
Cross direction
|
Rm, MPa
|
Rp, MPa
|
A, %
|
Rm, MPa
|
Rp, MPa
|
A, %
|
1
|
0.055
|
343
|
214
|
17
|
344
|
216
|
20
|
2
|
0.054
|
338
|
213
|
19
|
338
|
212
|
18
|
3
|
0.053
|
345
|
216
|
20
|
350
|
219
|
19
|
4
|
0.077
|
366
|
244
|
16
|
368
|
244
|
18
|
5
|
0.075
|
364
|
249
|
17
|
364
|
237
|
18
|
6
|
0.075
|
365
|
241
|
18
|
367
|
243
|
17
|
The average values of the results of testing the mechanical properties of tensile samples cut from cold-rolled and annealed sheets are given in Table 4.
Table 4 – Tensile properties of cold-rolled sheet semi-finished products 1 mm thick after annealing at 350 °C for 3 h
Alloy number
|
Sc, % (wt.)
|
Mechanical properties
|
Longitudinal direction
|
Cross direction
|
Rm, MPa
|
Rp, MPa
|
A, %
|
Rm, MPa
|
Rp, MPa
|
A, %
|
1
|
0.055
|
356
|
219
|
15
|
365
|
232
|
16
|
2
|
0.054
|
362
|
234
|
14
|
363
|
242
|
18
|
3
|
0.053
|
359
|
224
|
15
|
368
|
230
|
20
|
4
|
0.077
|
406
|
295
|
12
|
408
|
302
|
13
|
5
|
0.075
|
393
|
287
|
14
|
384
|
286
|
11
|
6
|
0.075
|
403
|
305
|
12
|
401
|
301
|
11
|
The analysis of these data showed that in alloys Nos. 4-6 the level of strength properties exceeds the level of alloys Nos. 1-3 by approximately 10-12%. But at the same time the plasticity of alloys Nos. 1-3 in the transverse direction significantly exceeds the plasticity of alloys Nos. 4-6.
The results of modeling the casting of ingots from alloy 1580 were tested in the industrial conditions of a Russian metallurgical enterprise when casting a large ingot with a cross section of 2100×500 mm. The aim was to obtain an ingot in which the scandium content, as in the experimental ingots, should be in the range from 0.05 to 0.075% (wt.). A general view of the industrial casting tooling and the large-sized ingot obtained on it is shown in Fig. 6.
The chemical composition of a large ingot made of alloy 1580 is presented in Table 5.
Table 5 – The chemical composition of a large-sized industrial ingot made of alloy 1580
Mass fraction of elements, %
|
Sc
|
Fe
|
Mn
|
Mg
|
The sum of other
|
Al
|
0.067
|
0.25
|
0.51
|
5.10
|
0.30
|
basis
|
From Table 6 it follows that the casting modes obtained during physical modeling at the SCCU, which were recommended for industrial testing, ensured the production of a large-sized ingot with a given scandium content. Upon external examination of the ingot and after its milling to a depth of 5-10 mm, defects of casting origin were not detected, and in the study of the microstructure, the precipitation of primary intermetallic compounds Al3(Sc, Zr) was not detected. From the ingot subjected to homogenization annealing under industrial conditions, a template was cut with a thickness of 40 mm and dimensions in plain view of 120×170 mm, which repeated the dimensions of a billet for rolling, obtained from an ingot cast at the SCCU. The ingot was subjected to hot and cold rolling on the equipment of the laboratory of the Department of Metal Forming of the Siberian federal university according to the modes used for rolling experimental ingots. At the same time, the modes of heat treatment of sheet semi-finished products were also repeated. The mechanical properties of 1580 alloy sheets rolled from an industrial ingot are presented in Table 6.
Table 6 – Mechanical tensile properties of semi-finished sheet products obtained by rolling a template cut from a large-sized ingot 1580, casted under industrial conditions
Billet characteristics
|
Mechanical properties
|
Longitudinal direction
|
Cross direction
|
Rm, MPa
|
Rp, MPa
|
A, %
|
Rm, MPa
|
Rp, MPa
|
A, %
|
Hot-rolled 5 mm thick, after annealing at 380 °C, 1 h
|
358
|
236
|
17
|
355
|
236
|
16
|
Cold-rolled 1 mm thick, after annealing at 350 °C, 3 h
|
400
|
285
|
12
|
398
|
295
|
13
|
Comparison of the data given in Table 3, 4 and 6, allows concluding that the tensile properties of experimental and industrial ingots are approximately between the level of properties of alloys Nos. 1-3 and alloys Nos. 4-6, which corresponds to the average content of scandium in the industrial ingot between these groups of experimental alloys.
Summary
The studies carried out allowed concluding the following. The use of physical modeling of the process of semi-continuous casting of aluminum alloys on the SCCU experimental installation makes it possible to develop casting modes for new alloys. Industrial testing of the casting modes of ingots from the new alloy 1580, obtained at the SCCU, showed that the structure, as well as the mechanical properties of sheet semi-finished products from experimental and industrial large-sized ingots, practically did not differ. This proves the reliability of the modes of casting ingots obtained at the SCCU and the validity of their application to the industrial conditions of semi-continuous casting of ingots from aluminum alloys.