INTRODUCTION

The aluminum Al–Cu-base alloys are characterized by a sufficiently high strength at both room and high temperatures and very low manufacturability upon casting [1–6]. The high manufacturability upon casting, in particular, the low solidification cracking susceptibility is of importance for both shaped castings and semicontinuous casting ingots [2–7]. The alloying with eutectic-forming elements, such as Fe, Si, Mn, Ni, and Ca, favors an increase in the manufacturability upon casting [3–9]. However, often, in reaching a low solidification cracking susceptibility, the alloys have a highly heterogeneous structure and low plasticity [6], which, in turn, makes the preparation of wrought half-finished products difficult. An alternative variant is the search for new alloying systems, for which the structure with fine phases of solidification origin and the narrow solidification range might be combined. The Al–Cu–Ce- [10, 11], Al–Cu–Y- [12, 13], Al–Cu–Er- [13, 14], and Al–Ca-based [8, 9, 15] alloys are among such systems. The distinctive feature of the yttrium- and erbium-containing alloys containing also zirconium and/or scandium is the possibility of precipitation strengthening in the course of annealing of ingots [16–36]. Low yttrium and erbium additions to the aluminum-based alloy [16–23] and to magnalium [24–28] increase the strengthening effect during annealing of ingots and restrain the softening during annealing after rolling at the expense of the increase in the density of dispersoid precipitates formed during the first heat treatment. Erbium efficiently modifies the grain structure [24–26, 30] and decreases the hot-brittleness of the Al–5Cu alloy [31]. The ternary Al–Cu–Y and Al–Cu–Er alloys [12–14] are characterized by narrow solidification range, and the phases of solidification origin are characterized by small sizes and high thermal stability. The alloying with zirconium [32, 33] and jointly zirconium and manganese [34, 35] leads to a substantial increase in the strength characteristics of alloys after deformation. The manganese addition leads to the formation of Al25Cu4Mn2Er [34] and Al25Cu4Mn2Y [35] phases of solidification origin. Iron impurity is dissolved in the phases of solidification origin and does not change their morphology, whereas silicon leads to the formation of sufficiently compact Al3Er2Si2 and Al11Cu2Y2Si2 [36, 37] phases. The complexly alloyed Al–Cu–Y- and Al–Cu–Er-based alloys with magnesium, manganese, zirconium, and titanium additions exhibit the adequate manufacturability upon casting, high strength, and mechanical properties at elevated temperatures [38]. The main disadvantage of these alloys is not high plasticity [38].

This study is aimed at the investigation of the structure and properties of new wrought aluminum Al–Cu–Y- and Al–Cu–Er-based alloys with the low contents of main alloying elements, such as copper, yttrium, erbium, zirconium, manganese, and titanium, which also contain magnesium and iron and silicon impurities.

EXPERIMENTAL

The Al–4.5Cu–1.6Y–0.9Mg–0.6Mn–0.2Zr–0.1Ti– 0.15Fe–0.15Si (AlCuYMg) and Al–4.0Cu–2.7Er–0.8Mg–0.8Mn–0.2Zr–0.1Ti–0.15Fe–0.15Si (AlCuErMg) (wt %) alloys were melted at 780°С using a resistance furnace, A7 aluminum, Mg90 magnesium, and Al–51.7Cu, Al–10Y, Al–8Er, Al–10Mn, Al–5Zr, and Al–5Ti–1B master alloys and were cast into a copper water-cooled mold with the internal hollow 20 mm × 40 mm × 120 mm in size. The compositions studied in this work differ from analogous casting alloys [38] in the lower copper, yttrium, erbium, zirconium, manganese, and titanium contents. The density of alloys was determined by hydrostatic weighing method. A Labsys Setaram differential scanning calorimeter (DSC) was used to determine the solidus and liquidus temperatures. The homogenizing annealing was performed at 575°С for 3 h. After homogenizing annealing, the alloys were rolled at 500°С and room temperature to thicknesses of 10 and 1 mm, respectively. After deformation, the alloys were annealed at 100–550°С for different times. The aging at 150–210°С for 0.5–6 h was performed after deformation and subsequent quenching from 575°С; the holding time at this temperature was 15 min. Metallographic studies and identification of phases were performed by light microscopy (LM) using a Zeiss optical microscope and scanning electron microscopy (SEM) using a TESCAN VEGA 3LMH scanning electron microscope. The Vickers hardness (HV) was measured in accordance with the standard procedure using the 5‑kg load. Tensile tests were carried out using a Zwick/Roll Z250 universal testing machine. Total corrosion tests were performed using artificial sea water.

RESULTS AND DISCUSSION

The liquidus temperature of the alloys under study is 635–637°С (Fig. 1) and is higher by 3–5°С than that of analogous casting alloys [38]. The liquidus temperature is determined, in accordance with the ternary phase diagrams [39, 40], by the contents of the main additions, such as copper, yttrium, and erbium. The solidus temperature is almost unchanged and is 588°С (see Fig. 1). The alloys are characterized by the narrow solidification range, namely, 47–49°С, which determines the high manufacturability upon casting. The density of the studied AlCuYMg and AlCuErMg alloys is 2.81 and 2.86 g/cm3, respectively; these values are by 0.2–0.3 g/cm3 lower than that of casting alloys containing a great amount of alloying additions [38].

Fig. 1.
figure 1

DSC curves for the (a) AlCuYMg and (b) AlCuErMg alloys.

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Figure 2 shows the microstructure of the alloys in the cast state. The decrease in the concentration of the main modifier, namely, titanium, to 0.1% leads to the formation of grains 80–100 µm in size in both alloys (Figs. 2a, 2b). For comparison, the grain size of the Ti-free Al–Cu–Y–Zr alloy is ~190 µm, whereas the grain size of the AlCuErMg alloy with 0.15Ti is 25 µm. The decrease in the concentration of the main alloying elements does not affect the phase composition of the alloys (Figs. 2b, 2d and data of [38]). The microstructure consists of the aluminum solid solution, fine eutectic, and intermetallics formed by manganese, magnesium, silicon, and copper additions (Figs. 2b, 2d). Iron does not form phases typical of aluminum alloys.

Fig. 2.
figure 2

(a,c) Grain structure (LM) and (b, d) microstructure (SEM) of the (a, b) AlCuYMg and (c, d) AlCuErMg alloys in the cast state.

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Before quenching, the ingots of the alloys were annealed at 575°С for 3 h in accordance with the regime used for casting alloys [38]. The dissolution of nonequilibrium excess of phases of solidification origin leads to the fact that the copper and magnesium concentrations in the solid solution increase to 2.1–2.2 and 0.9–1.0%, respectively. In the course of annealing, the fragmentation and spheroidization of the phases of solidification origin take place (Figs. 3f, 3b), which partially are crushed and elongate along the deformation direction in the course of rolling (Figs. 3c, 3d). As a result, their size is 1–5 µm. Together with the homogenization processes, Al3(Zr,Er), Al3(Zr,Y), and Al20Cu2Mn3 phase dispersoids precipitate from the solid solution supersaturated with zirconium, yttrium, erbium, and manganese [34, 35]. SEM images of the microstructure (Fig. 3) show fine bright inclusions in the aluminum solid solution, which correspond to the described phases.

Fig. 3.
figure 3

Microstructure of the (a, c) AlCuYMg and (b, d) AlCuErMg alloys subjected to (a, b) annealing at 575°С for 3 h and (c, d) subsequent rolling.

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Deformed sheets were annealed at 100–550°С in order to determine the recrystallization temperature range and to analyze changes in the hardness (Fig. 4). For both alloys, the annealing at temperatures below 350°С retains the nonrecrystallized structure. In this case, the hardness first (at temperatures below 150°С) slightly increases and, then, decreases. The increase in the hardness is likely to be related to occurred aging; the analogous effect was observed for the magnesium-free alloys of the same systems [34, 35]. The softening occurs at the expense of polygonization processes, and recrystallized grains are found after annealing at 400°С (inset in Fig. 4). In this case, the recrystallized grain size in both alloys is 6–8 µm. The increase in the annealing temperature to 550°С leads to the grain growth to 10–12 µm. In this case, the hardness is unchanged and is 65–68 HV.

Fig. 4.
figure 4

Dependences of the hardness HV on the temperature of 1 h annealing of the rolled (a) AlCuYMg and (b) AlCuErMg alloys.

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Figure 5 shows the dependences of the hardness of wrought alloys on the annealing time at low temperatures. As was noted above, at temperatures close to 150°С, the slight strengthening occurs during first hours of annealing. As the temperature increases to 180°С, the softening is observed after 1-h holding, which is related to the occurrence of polygonization. In the course of annealing at 210°С, the hardness decreases after 3-h holding and remains unchanged at the holding time increases to 6 h. In this case, two opposite processes occur; these are the strengthening related to aging and softening determined by recovery and polygonization. Table 1 gives results of tensile tests of the alloys in the strained and annealed state. The yield strength of the both alloys in the strained state is 380–390 MPa and the relative elongation is 1.8–2.4%. In this case, the increase in the yield strength during annealing at 150°С is found only for the AlCuYMg alloy. After 6-h annealing, the yield strength increases from 380 to 405 MPa; the plasticity also increases from 1.8 to 4.5%. During tensile tests, the hardness and yield strength demonstrate different sensitivity to structural changes. The analogous effect was noted for the magnesium-free alloys having the close composition [34, 35]. The increase in the annealing temperature to 210°С leads to the decrease in the yield strength to ~300 MPa, whereas, in this case, the elongation remains not high.

Fig. 5.
figure 5

Dependences of the hardness HV of the (a) AlCuYMg and (b) AlCuErMg deformed alloys on the annealing time.

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Table 1.   Mechanical characteristics (tensile tests) of the alloys in strained and annealed states
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After rolling, the alloys were quenched from 575°С after 15-min holding at this temperature and aged at 150, 180, and 210°С. Figure 6 shows the dependences of the hardness (HV) on the time of aging of sheets after quenching. The observed dependences are qualitatively analogous in the fact that they are obtained upon aging of analogous casting alloys [38]. The hardness increases during aging from 64–66 to 105–115 HV. The recrystallized structure formed in the alloys (inset in Fig. 6) allows the plasticity of the alloys to be substantially increased. According to tensile test data, the relative elongation after aging at 210°С for 3 h is 11.3–14.5% (Table 2). In this case, the yield strength is 264–266 MPa, whereas the ultimate tensile strength is 356‒365 MPa. For comparison, the yield strength of the wrought D16 alloy in the form of sheets in the hard-down and annealed states is 230–360 MPa; its ultimate tensile strength is 365–475 MPa and the relative elongation is 8–13%; the alloy in the form of rods [42] exhibits a yield strength of 325–345 MPa, an ultimate tensile strength of 450–470 MPa, and a relative elongation of 8–10%. The yield strength of recrystallized rods is 265 MPa [42] and their ultimate tensile strength is 410 MPa at a relative elongation of 12%. In this case, the manufacturability upon casting of the D16 alloy is substantially lower than that for the compositions under study. The wrought АК4-1 alloy characterized by the high heat resistance [42], which is prepared in the form of rods, exhibits a yield strength equal to 335 MPa and the ultimate tensile strength equal to 390 MPa at a relative elongation of 6%. Thus, the new wrought Al–Cu–Y- and Al–Cu–Er-based alloys can provide competition for the available industrial alloys.

Fig. 6.
figure 6

Dependences of the hardness HV of the (a) AlCuYMg and (b) AlCuErMg sheet alloys after quenching from 575°C after 15-min holding on the aging time.

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Table 2.   Mechanical characteristics (tensile tests) of alloys subjected to rolling and subsequent quenching from 575°С after 15-min holding and aging at 210°С for 3 h
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It is known that the aluminum—copper alloys corrode. In this study, we performed a simplified estimation of the corrosion resistance in determining the total corrosion resistance in artificial sea water. After the tests, the yield strength of the AlCuYMg alloy decreases from 405 to 374 MPa, whereas the relative elongation decreases from 4.5 to 3% (Table 3). The decrease in the yield strength of the AlCuErMg alloy is less pronounced.

Table 3.   Mechanical characteristics of the alloys subjected to rolling and subsequent annealing at 150°С for 6 h determined by tensile tests before and after total corrosion tests
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CONCLUSIONS

The structure and properties of new wrought aluminum Al–Cu–Y- and Al–Cu–Er-based compositions alloyed with zirconium, manganese, magnesium, and titanium and containing iron and silicon impurities were studied. The alloys are characterized by the narrow solidification range equal to 47–49°С, which ensures the high manufacturability upon casting. The density of the AlCuYMg and AlCuErMg alloys under study is 2.81 and 2.86 g/cm3, respectively. After homogenizing annealing and rolling, the structure, which consists of the aluminum solid solution strengthened with fine Al3(Zr,Er), Al3(Zr,Y), and Al20Cu2Mn3 phase particles and the compact thermally stable phases of solidification origin 1–5 µm in size, forms. The softening in the course of annealing performed after rolling occurs at the expense of polygonization processes at temperatures below 350°С, whereas the recrystallization occurs at the higher temperatures. After annealing at 400°С, the grain size is 6–8 µm and increases to 10–12 µm after 1-h annealing at 550°С. At temperatures of 150–180°С, the hardness slightly increases; this is related to the aging. The analogous effect was found for the casting alloys of these systems. The yield strength of the AlCuYMg alloy subjected to rolling and subsequent 6-h annealing at 150°С is 405 MPa at a relative elongation of 4.5%. The increase in the annealing temperature to 210°С leads to the decrease in the yield strength to 300 MPa, whereas the relative elongation remains unchanged. The plasticity of the alloy subjected to rolling and subsequent quenching and aging at 210°С substantially increases to 11.3–14.5%; the yield strength and ultimate tensile strength are 264–266 and 356–365 MPa, respectively. As a result, the new wrought Al–Cu–Y- and Al–Cu–Er-based alloys can provide competition for the available industrial alloys.