1. Introduction
In current industrial practice, the extrusion with the use of porthole dies is commonly applied for aluminum as well as its low- and medium-strength alloys. However, there is a demand for hollow profiles with higher strength, and research efforts are aiming at an elaboration of similar technology for high-strength 2xxx, 5xxx, and Cu-containing 7xxx alloys [
1,
2]. Among parameters determining the quality of longitudinal welds, the temperature in the welding chamber is one of great importance [
1,
3,
4,
5,
6]. The hard-deformable Al alloys, in particular those of 7xxx series with Cu, are characterized by low solidus temperature [
7], which becomes a factor significantly limiting the permissible temperature of material in the welding chamber. Thus, the careful selection of alloys’ chemical composition as well as suitable billets preparation for extrusion are important steps in the elaboration of hard-deformable alloys extrusion technology.
The DC-cast billets of AlZnMgCu alloys are characterized by dendritic microstructure with numerous eutectics and particles. The commonly reported phases are the M-(Cu, Zn, Al)
2Mg, T-(Cu, Zn, Al)
49Mg
32, S-Al
2CuMg, and Fe-bearing ones [
8,
9,
10,
11,
12,
13]. The non-equilibrium solidification during DC casting results in low billets solidus temperature, most often values in the range 470–480 °C are noted [
9,
14,
15,
16], but lower can also be found in the literature [
17,
18]. The solidus temperature can be increased by homogenization annealing; however, obtained effects are strongly dependent on process parameters.
The initial soaking temperature is limited by unequilibrium solidus temperature, because incipient melting, causing irreversible microstructural damage, must be avoided [
9]. Thus, the first stage of soaking should be accomplished below 470 °C. The M and T phases can be effectively dissolved in these conditions. However, the S-phase, often observed in the as-cast billets microstructure or forming and growing during initial stages of annealing [
9,
13,
19,
20], is hard to redissolve, and it is frequently noted after homogenization at the mentioned temperature range [
13,
14,
15]. In that situation, the billets solidus temperature increases slightly, but it is still below 500 °C. The complete dissolution of the S-phase is in many cases obtained after multi-stage homogenization, when due to the rise of solidus temperature during the initial stage, the application of higher final soaking temperature is possible [
14,
21]. Although the microstructure evolutions taking place in AlZnMgCu billets during homogenization soaking are described in the literature for many alloy compositions, the values of solidus temperature, which can be obtained after homogenization with full dissolution of low melting structure components, are unfortunately reported rarely.
Another homogenization parameter influencing the solidus temperature of billets is a cooling rate after completed soaking. The billets of described alloys are usually slowly cooled after homogenization, which leads to the precipitation of phases showing limited solubility (e.g., M, T), depletion of solid solution from Zn, Mg, and Cu, and lowering of flow stress as well as extrusion pressure. However, if cooling is very slow, the mentioned phases can be precipitated in a form of large particles, which are incapable for dissolution during billets preheating and extrusion. Their presence in the billets microstructure can cause unequilibrium incipient melting [
9]. Similar effects are noted also for other Al alloys series [
22,
23]. Hence, taking advantage of obtaining a full dissolution of low melting microstructure components during homogenization heating and soaking is possible when the cooling rate is properly selected. This aspect of AlZnMgCu billets preparation for extrusion is also rarely described in the literature.
The mentioned lack of data regarding the attainable solidus temperature of AlZnMgCu alloys after homogenization results probably from the fact that the billets solidus temperature is not a crucial material property in the conventional solid products extrusion, where temperature in the deformation zone is usually lower. However, as it was already mentioned, the high solidus temperature is necessary for the selection of extrusion welding parameters. This work presents the results of investigations aimed at determining the homogenization conditions leading to obtaining the highest possible solidus temperature of AlZnMgCu alloys billets, which are intended for extrusion welding.
2. Materials and Methods
The billets, with the chemical composition presented in
Table 1 and diameter of 100 mm, were DC cast in semi-industrial conditions. Four alloys were investigated: three within EN AW-7075 and one within EN AW-7049 grade.
The specimens with dimensions of 10 × 20 × 20 mm3, intended for examination in the as-cast state as well as for laboratory homogenization experiments, were sectioned from obtained billets.
At the first stage of work, the materials in the as-cast state were subjected to DSC (Differential Scanning Calorimetry) analyses as well as microstructure observations. The DSC tests were performed using a Mettler Toledo 821e heat flux type calorimeter (Greifensee, Switzerland). The disc-shaped samples were inserted in ceramic pans into the cell with the temperature of 390 °C and heated 20 °C/min to the temperature of 700 °C in Ar atmosphere. The solidus temperature and heat of the incipient melting reactions were determined.
The specimens intended for microstructure examination were mounted in conductive resin and mechanically ground and polished using in sequence abrasive papers, diamond suspensions, and colloidal silica suspension. The billets microstructure was examined using LM (light microscopy) and SEM/EDS (Scanning Electron Microscopy/Energy-Dispersive Spectroscopy). The specimens intended for LM observations were etched with Keller reagent and examined using Olympus GX 51 microscope (Tokyo, Japan). The SEM/EDS analyses were performed on non-etched specimens using a Hitachi SU-70 scanning electron microscope (Tokyo, Japan) equipped with a Thermo Scientific EDS system (Thermo Fisher Scientific, Waltham, MA, USA). EDS analyses were applied to determine the chemical composition of the observed eutectic areas or particles and to measure main alloying elements content in the dendrites interiors.
At the second stage of work, soaking parameters were examined. The conditions of laboratory homogenization experiments were selected on the basis of literature data [
9,
14,
20,
21], earlier investigations, and the results of DSC tests of alloys in the as-cast state. The heat treatment experiments were accomplished using a Nabertherm forced convection chamber furnace. Three homogenization schemes were applied:
The standard homogenization with soaking at 465 °C
The high-temperature homogenization with two soaking stages at 465 and 475 °C
The high-temperature homogenization with two soaking stages at 465 and 485 °C.
In all cases, materials were heated from room temperature to 465 °C for 10 h. A similar heating rate, about 40 °C/h, was applied during heating between soaking stages. After completed soaking, specimens were quenched in water. The details of the homogenization experiments performed in the second stage of the work are presented in
Table 2.
At the third stage, the influence of cooling rate from the homogenization temperature, on the billets of alloys 1, 2, and 4 microstructure, was investigated. Specimens were subjected to homogenization with soaking conditions selected on the basis of stage 2 results and cooled to room temperature in three ways. The average cooling rates in the temperature range from 465 or 475 to 200 °C, estimated on the basis of specimens temperature measurements during cooling cycles, were about 500, 120, and 60 °C/h.
Materials after all homogenization experiments were subjected to DSC tests. In stage 2, they were used for the analysis of low-melting microstructure components dissolution. In stage 3, the DSC runs were applied in order to evaluate the precipitated particles dissolution ability during rapid heating. On the basis of the obtained DSC results, specimens for microstructure observations were selected. The DSC analyses as well as the microstructure observations of homogenized alloys were performed in the manner described above.
4. Discussion
In the case of all examined alloys, the low-melting microstructure components were dissolved during homogenization soaking in a degree sufficient in practice—no incipient melting peaks on the DSC curves are noted. As a result, the significant increase of solidus temperature was achieved, and the obtained values are within the range from 525 °C for alloy 4 to 548 °C for alloy 1 (
Table 5).
For alloys 1, 2, and 3 of 7075 grade, the low-melting components are in sequence: phase(s) containing Al, Zn, Mg, and Cu observed in the as-cast state, and after their dissolution in the early annealing stages, it is the phase S. In the case of alloys 1 and 3, the expected results are obtained after homogenization at a temperature of 465 °C with soaking time of 4 and 12 h, respectively. This is a rather low soaking temperature when compared to literature data [
9,
14,
20,
21]. It should be mentioned here that in many papers, e.g., [
12,
15,
25,
27,
28], only a significant decrease of low-melting phases content is described. The soaking time for alloy 1 can be assessed as very short, which results from low main alloying additions concentration. For alloy 3, with high Zn and medium Mg and Cu content (with respect to grade limits), it is three times longer, but it is still acceptable in the industrial practice. However, the application of higher temperature in the second soaking stage enables the shortening of total annealing time from 22 to about 16 h. In the case of alloy 2 with highest Cu content, the DSC test results after standard homogenization show the presence of phase S after soaking for 2 to 24 h. The incipient melting heat changes indicate that at this temperature, the dissolution of the S-phase is probably unattainable. Therefore, the application of high-temperature homogenization with soaking at 475 °C was necessary. As it was mentioned above, in all alloys of 7075 grade, the incipient melting in the course of homogenization (after initial solidus increase) is caused by the presence in the microstructure of the same phase S, and only for alloy 2 is the high-temperature homogenization required for its dissolution. Based on the phase diagram [
26], one may expect that this discrepancy results from the fact that for alloy 2, the S-phase solvus is above 465 °C.
The DSC curves of alloy 4, with composition of 7049 grade, indicate that during homogenization soaking at the temperature of 465 °C for at least 8 h, the incipient melting results from the presence of the same phase(s) as in the as-cast state i.e., containing Al, Zn, Mg, and Cu. The dissolution of low-melting microstructure components was noted after of 12 h soaking. The lack of solidus temperature change as a result of longer homogenization at 465 °C, as well as after homogenization with soaking at 485 °C (
Figure 5f,g), allows stating that for billets of this alloy, the mentioned standard homogenization is sufficient.
The obtained results clearly indicate that the cooling from the homogenization temperature may have an essential influence on the billets microstructure and solidus temperature. This is consistent with the literature data [
9,
14]. However, the present work shows that the effect of cooling rate is strongly dependent on alloy composition, and significant differences can be observed even within one grade. This observation, to the authors’ knowledge, is not described in the literature.
The billets of 7075 alloy 1 can be cooled very slowly without influencing the solidus temperature. In contrary, in the case of 7075 alloy 2, the lowering of solidus temperature is observed also in spite of fast cooling, at 500 °C/h. This results from the fact that during cooling from the homogenization temperature, the S-phase precipitates. As it was mentioned above, for this composition, the S-phase solvus temperature is high—the soaking investigations allow expecting that it is within the range of 465 to 475 °C. In addition, during rapid heating applied in this work, the time for particles dissolution is found to be too short, despite their small dimensions obtained as a result of fast cooling. In industrial practice, the billet preheating takes significantly longer. However, the billets are usually preheated to a temperature below the estimated above S-phase solvus [
7], and a further temperature increase during extrusion may be fast. The obtained result indicates that for alloy 2, the application of fast billets cooling, within limitations resulting for example from billets dimensions, may be insufficient for taking advantage of the solidus increase obtained during soaking. In this case, special attention should be paid to billets preheating in order to ensure that the S-phase will not cause the incipient melting during the extrusion, with a significant temperature increase in a welding chamber. For example, the slow billets preheating to the temperature slightly above the S-phase solvus but below 490 °C (where melting is noted) may be applied with short holding if necessary. Then, if the mentioned temperature is too high from the point of view of extrusion parameters, the billets (short, already cut to desired length) may be fast cooled to the needed temperature. A similar billets preheating manner is described for 6xxx alloys [
29].
In the case of high-Zn 7049 alloy 4, for which the precipitation of phase(s) containing Al, Zn, Mg, and Cu is noted, it is sufficient to cool the billets from homogenization temperature at moderate rate of 120 °C/h. The precipitated particles are able to dissolve during subsequent rapid heating, and the high solidus temperature of 525 °C is maintained.
Author Contributions
Conceptualization, A.W. and G.W. methodology, A.W.; investigation, J.M., B.L.-M., J.G. and G.W.; writing—original draft preparation, A.W.; writing—review and editing, G.W., B.L.-M., J.G., J.M. and D.L.; supervision, D.L. All authors have read and agreed to the published version of the manuscript.
Funding
This research was funded by THE NATIONAL CENTRE FOR RESAERCH AND DEVELOPMENT, grant number TECHMATSTRATEG2/406439/10/NCBR/2019 “Extrusion welding of high-strength shapes from aluminium alloys 7xxx series”.
Data Availability Statement
Not applicable.
Acknowledgments
The authors are grateful to Józef Zasadziński and Wojciech Libura for valuable discussions during investigations and results analysis.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
DSC curves of alloys in the as-cast state.
Figure 2.
Microstructure of 7075 alloy 3 in the as-cast state: (a) optical microscope photograph; (b) SEM image with marked analysis points and distribution of main additions across dendrite.
Figure 3.
SEM/EDS (Scanning Electron Microscopy/Energy-Dispersive Spectroscopy) microanalyses results of eutectics and particles in the as-cast billets: (a) 7075 alloy 1; (b) 7049 alloy 4.
Figure 4.
Exemplary DSC (Differential Scanning Calorimetry) curves of alloys subjected to homogenization: (a) 7075 alloy 1, standard homogenization; (b) 7075 alloy 2, high-temperature homogenization; (c) 7075 alloy 3, high-temperature homogenization; (d) 7049 alloy 4, standard homogenization.
Figure 5.
Changes of solidus temperature and incipient melting heat in the course of homogenization: (
a) 7075 alloy 1; (
b,
c) 7075 alloy 2; (
d,
e) 7075 alloy 3; (
f,
g) 7049 alloy 4.
Tables S1–S4 with determined values given in
Supplementary Materials.
Figure 6.
Exemplary microstructure of 7075 alloy 3 after homogenization: (a) soaking at 465 °C for 2 h; (b) soaking at 475 °C for 4 h (visible dispersoids reflecting former dendritic microstructure).
Figure 7.
Particles of phase S-Al2CuMg and Mg2Si, 7075 alloy 2 after soaking at 465 °C for 2 h.
Figure 8.
Examples of particles containing Al, Cu, and Fe as well as Mg2Si in observed in the microstructure of 7075 alloy 2 after soaking at 475 °C for 8 h.
Figure 9.
DSC curves of alloys subjected to various cooling rates from homogenization temperature: (a) 7075 alloy 1 after homogenization with soaking at 465 °C for 4 h; (b) 7075 alloy 2 after homogenization with final soaking at 475 °C for 8 h; (c) 7049 alloy 4 after homogenization with soaking at 465 °C for 12 h.
Figure 10.
Particles observed in the alloys microstructure after cooling from the homogenization temperature at 60 °C/h: (a) 7075 alloy 2; (b) 7049 alloy 4.
Table 1.
The chemical composition of investigated alloys, mass percentage.
Alloy Denotation | Si | Fe | Cu | Mg | Cr | Zn | Ti | Zr |
---|
7075 alloy 1 | 0.08 | 0.16 | 1.22 | 2.08 | 0.21 | 5.14 | 0.02 | 0.15 |
7075 alloy 2 | 0.08 | 0.17 | 2.02 | 2.50 | 0.20 | 5.94 | 0.02 | 0.15 |
7075 alloy 3 | 0.10 | 0.21 | 1.54 | 2.39 | 0.20 | 5.89 | 0.02 | 0.15 |
7049 alloy 4 | 0.11 | 0.23 | 1.57 | 2.36 | 0.20 | 8.02 | 0.02 | 0.16 |
Table 2.
Laboratory homogenization conditions applied in stage 2.
Scheme Denotation/Homogenized Alloys | Heating 1 | Soaking 1 | Heating 2 | Soaking 2 |
---|
1—standard homogenization/ all alloys | 10 h | 465 °C/ 0, 2, 4, 8, 12, 16, 20, 24 h 1 | - | - |
2—high-temperature homogenization/alloys 2 and 3 | 10 h | 465 °C/2 h | 15 min. | 475 °C/0, 2, 4, 8 h |
3—high-temperature homogenization/alloy 4 | 10 h | 465 °C/12 h | 30 min. | 485 °C/0, 2, 4, 8 h |
Table 3.
DSC test results of as-cast alloys.
Alloy | Solidus Temperature, °C | Incipient Melting Heat, J/g |
---|
7075 alloy 1 | 484.0 | 6.1 |
7075 alloy 2 | 482.6 | 11.9 |
7075 alloy 3 | 479.1 | 9.2 |
7049 alloy 4 | 479.6 | 11.8 |
Table 4.
DSC tests results of alloys 1, 2, and 4 after homogenization with differentiated cooling.
Alloy | Cooling Rate | Solidus Temperature 1, °C | Incipient Melting Heat 1, J/g |
---|
7075 alloy 1 soaking 465 °C/4 h | 500 °C/h | 545.7 | |
120 °C/h | 547.0 | |
60 °C/h | 547.7 | |
7075 alloy 2 soaking 475 °C/8 h | 500 °C/h | 491.3 | 0.3 |
120 °C/h | 495.0 | 0.5 |
60 °C/h | 478.0/490.9 | 0.3/0.8 |
7049 alloy 4 soaking 465 °C/12 h | 500 °C/h | 524.6 | |
120 °C/h | 523.4 | |
60 °C/h | 481.3 | 0.3 |
Table 5.
Homogenization soaking parameters enabling maximizing the solidus temperature.
Alloy | Soaking | Solidus Temperature, °C |
---|
7075 alloy 1 | 465 °C/4 h | 548.1 |
7075 alloy 2 | 465 °C/2 h + 475 °C/8 h | 531.2 |
7075 alloy 3 | 465 °C/12 h | 533.2 |
465 °C/2 h + 475 °C/4 h | 531.9 |
7049 alloy 4 | 465 °C/12 h | 525.5 |
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