Effect of Si Addition on the Mechanical Properties and Material Structure of Al-Zn-Mg Alloys

Abstract

Among Al-Zn-Mg alloys, A7003 alloy is considered to be an excellent alloy from the viewpoint of weldability because it is an alloy with relatively low Zn and low Mg composition. In this paper, changes in mechanical properties and microstructure during aging treatment are investigated by the addition of Si in the Al-5.6 mass%Zn-0.75 mass%Mg alloy containing Cu, Mn, Zr and Fe. Cast billets of alloys with different Si contents (Si: 0.05 mass%, 0.15 mass%, and 0.30 mass%) were prepared, and these cast billets were homogenized and extruded. After extrusion, four aging treatments were performed: one step aging at 423 K for 8 hours, and two step aging at 373 K for 3 hours, 6 hours, and 9 hours, followed by 423 K for 8 hours. The higher the amount of Si, the smaller the thickness of recrystallization layer near the inner surface and near the outer surface, and the smaller the existence rate of recrystallized grains in the cross section. After one step aging at 423 K for 8 hours, Si: 0.05 mass% showed lower strength than Si: 0.15 mass% and Si: 0.30 mass%. On the other hand, two-step aging resulted in Si: 0.05 mass%, Si: 0.15 mass%, and Si: 0.30 mass% with similar strength. The low strength of Si: 0.05 mass% after one step aging at 423 K for 8 hours is thought to be due to the coarse η phase, which was precipitated in the grain boundaries and grains. In Si: 0.15 mass% and Si: 0.30 mass%, Al(Mn,Fe)Si which formed during homogenization process is present in the grain boundaries and grains after extrusion. It is thought to suppress the generation of the precipitate of the coarse η phase in the aging at 423 K for 8 hours, which is a condition that the coarse η phase is easy to generate.

 

This Paper was Originally Published in Japanese in J. JILM 74 (2024) 167–172.

1. Introduction

In recent years, with the background of increasing social demands for lightweight automobile bodies in consideration of the global environment, aluminium alloy materials have been applied to automobile body parts such as panels (outer and inner panels of hood, door, roof, etc.) and reinforcement materials such as bumper reinforcement and door beams, in place of conventional steel materials such as steel sheet. Al-Zn-Mg alloys of the A7000 series, which are manufactured by hot extrusion, are widely used as aluminum alloy materials because of their excellent age hardenability. Al-Zn-Mg alloys can be roughly classified into two series: high-strength series containing Cu and having the highest strength among aluminum alloys, and medium-strength series containing no Cu and low Zn and Mg content, thereby improving weldability, extrusion workability, and corrosion resistance. For automotive structural members, Al-Zn-Mg alloys with low Zn and Mg compositions and without Cu are used because they are required to have excellent weldability, extrusion workability, and corrosion resistance, and A7003 is attracting attention.

When Cr, Mn, or Zn is added to an Al-Zn-Mg alloy, a dispersed phase is obtained by homogenization treatment. The dispersed phase becomes denser as the temperature rises slower to reach the homogenization treatment temperature, and the denser the dispersed phase, the more effective is the suppression of recrystallization [1–3].

The changes in mechanical properties of extruded shapes after aging treatment when Si, Cu, Fe, Mn, and Cr are added to Al-Zn-Mg alloys alone have already been investigated [4].

However, the effects of Si addition on the mechanical properties and material structure of extruded shape after aging treatment in the case of combined addition of elements other than Zn and Mg have not yet been clarified.

In the manufacturing process of Al-Zn-Mg alloys, it is not desirable to keep the amount of Si addition as low as possible from the viewpoint of the cost of ingot to be procured, and it is important to determine a certain amount of Si addition that is acceptable.

Therefore, the purpose of this paper is to investigate the effect of Si addition on the mechanical properties and microstructure of Al-Zn-Mg alloys by aging in the presence of Cu, Mn, Zr, and Fe in A7003, an Al-5.6 mass%Zn-0.75 mass%Mg alloy. In A7003, Cu is added to increase the strength, Mn and Zr are added to suppress the formation of recrystallization layers on the surface of shape, and a certain amount of Fe is unavoidable as an unavoidable impurity. Therefore, in this paper, the effects of Si addition on the mechanical properties and material structure of Al-Zn-Mg with Cu, Mn, Zr, and Fe as more versatile alloys were investigated.

2. Experimental Methods

Three different alloy compositions with Si contents of 0.05 mass%, 0.15 mass%, and 0.30 mass% (hereafter referred to as Si: 0.05 mass%, Si: 0.15 mass%, and Si: 0.30 mass%, respectively) as shown in Table 1 were used as the specimens. Cast billets of 89 mm in diameter with these three different alloy compositions were cut to 380 mm in length and homogenized in an atmospheric furnace at 743 K for 1 hour. The billets were then removed from the furnace and fan cooled at a cooling rate from 743 K to 473 K for 30 minutes.

Table 1 Alloy compositions of samples with different Si contents.

Then, a 530 ton extruder was used to extrude the shape shown in Fig. 1 with an extrusion ratio of 47.7. The extrusion conditions were billet temperature of 743 K, container temperature of 723 K, ram speed of 5.0 mm/s, and shape speed of 9.4 m/min.

Fig. 1

Shape of extruded material.

After extrusion, the extruded material was air-cooled using a fan under the cooling table and stretch straightened with a stretch amount of 100 mm. The extruded material after stretch straightening was cut into 20 mm long samples, and then aged in a heat treatment furnace for 8 hours with 423 K in air (referred to as 423 K×8 h), 373 K for 3 hours, 6 hours, and 9 hours, followed by two-step aging with 423 K for 8 hours (referred to as 373 K×3 h-423 K×8 h, 373 K×6 h-423 K×8 h, and 373 K×9 h-423 K×8 h). After stretch straightening, the steel was left at room temperature for 2 hours before aging treatment was applied.

For each sample before aging treatment, a sample was cut to observe the cross-section parallel to the extrusion direction shown in Fig. 2(b). After mirror polishing and etching of the resin-filled sample, optical microscopic observation within the cross-section was performed to observe the recrystallization state within the cross-section in the extrusion direction. For Si: 0.15 mass%, microstructures were observed by transmission electron microscopy (TEM) (JEOL JEM3010) in the center of the billet after homogenization at 723 K for 1 hour and cooling by fan, and in the extruded shapes after solution annealing after extrusion. Solution treatment was performed under the condition of holding at 723 K for 1 hour followed by water cooling to remove the effect of aging precipitation that occurred between the extrusion and TEM observation. TEM observations were also made on the extruded shapes after aging treatments of 423 K×8 h for Si: 0.05 mass% and Si: 0.15 mass%.

Fig. 2

Comparison of the macrostructure in the material by the difference in quantity of Si. (a) Optical microscopy observation results, (b) Sampling position for optical microscopy observation.

Thin film specimens for TEM observation of the extruded material after extrusion and solution annealing at Si: 0.15 mass% and the aged material at Si: 0.05 mass% and Si: 0.15 mass% for 423 K×8 h were prepared by cutting out the shaded position in Fig. 3, grinding the center of the specimen in the thickness direction, and then performing the twin-jet method.

Fig. 3

TEM observation sampling position.

Furthermore, the specimens after heat treatment under each aging condition were processed into JIS No. 5 specimens in a tensile test shape, and then tensile tests were conducted using an Instron-type universal testing machine (Instron 5567 (load cell capacity: 30 kN)) at a test speed of 1 mm/min and test speed: room temperature.

The aging behavior of each sample was investigated by differential scanning calorimetry (DSC) (DSC8230D manufactured by Rigaku Corporation) before aging treatment and after heat treatment under each aging condition. The samples before aging were solution annealed at 723 K for 1 hour and then water cooled to remove the effect of aging precipitation that occurred between the extrusion and the DSC measurement. After solution annealing, the specimens were left at room temperature for one day, and then DSC measurements were performed.

In the DSC measurements, the temperature increase rate was 10 K/min, and a pure Al (99.9999%) sample was used as the standard sample for the measurement sample.

3. Results and Discussion

3.1 Macrostructure in cross section of shapes

Macrostructural observation using an optical microscope was performed on the cross sections of 89 mm cast billets extruded in three alloy compositions with different Si contents (Si: 0.05 mass%, Si: 0.15 mass%, and Si: 0.30 mass%) in the direction parallel to the extrusion direction. The results are shown in Fig. 2(a). Figure 2(a) shows that the surface of the cross section in contact with the molding section when extruded from the extrusion mold has recrystallized grains for all conditions, while the inside of the cross section has a fibrous structure except for the surface area.

Macrostructural observations were made on the cross sections of shapes with Si: 0.05 mass%, Si: 0.15 mass%, and Si: 0.30 mass%, respectively, and the thickness of the recrystallized layer was measured at three positions per view in three different fields of view to obtain a total of nine positions, with the average of the nine positions being the average recrystallized layer thickness.

Here, it has been confirmed that the addition of Zr precipitates Al₃Zr during the homogenization process, and that the precipitates of Al₃Zr suppress the generation of recrystallized grains and contribute to the formation of a fibrous structure [5–7]. Therefore, it is considered that the fibrous structure in the center of the alloy at Si: 0.05 mass%, Si: 0.15 mass%, and Si: 0.30 mass% is influenced by the precipitates of Al₃Zr.

As shown in Fig. 4, the thickness of the layer with recrystallized grains near the inner and outer surfaces of the shapes became smaller as the Si content increased. As shown in Fig. 5, the presence of recrystallized grains in the cross section became smaller. The percentage of recrystallized grains in the cross section was calculated by (thickness of the layer containing recrystallized grains on the outside of the profile + thickness of the layer containing recrystallized grains on the inside of the profile) / 2 mm of the profile wall thickness.

Fig. 4

Comparison of the recrystallization layer thickness of materials with different Si contents.

Fig. 5

Ratio of recrystallized layer to plate thickness.

3.2 Comparison of microstructures of materials added with Si: 0.05 mass% and Si: 0.15 mass%

The center of the billet after homogenization treatment with Si: 0.15 mass% was cut out for TEM observation and EDX analysis. As shown in Fig. 6(a), Fig. 6(b) and Fig. 6(c), the comparison of EDX analysis between compound (A) and matrix (B) revealed that a number of Al(Mn,Fe)Si was found in the matrix phase. The Al(Mn,Fe)Si is a crystalline product formed during the homogenization process, according to previous literature [8].

Fig. 6

TEM photograph and results of EDX analysis of the center of the billet after the homogenization process of Si: 0.15 mass%. (a) Microstructure, (b) EDX analysis results for (A), (c) EDX analysis results for (B).

As shown in Fig. 7, TEM observation of the extrudate at Si: 0.15 mass% after solution treatment confirmed the presence of Al(Mn,Fe)Si on the grain boundary.

Fig. 7

TEM photograph of extruded material at Si: 0.15 mass% after solution treatment.

The microstructures of the extrudate after aging treatment at 423 K×8 h with Si: 0.05 mass% and Si: 0.15 mass% were investigated by TEM observation. Figure 8(a) shows the microstructure within the grain and Fig. 8(b) shows the microstructure near the grain boundary. Figure 9 shows the microstructure of the extrudate after aging treatment at 423 K × 8 h for Si: 0.15 mass%. Figure 9(a) shows the microstructure within the grain and Fig. 9(b) shows the microstructure near the grain boundary. The composition of the coarse compounds in Fig. 8(a), Fig. 8(b), Fig. 9(a), and Fig. 9(b) were investigated by EDX analysis, and the results of EDX analysis of Fig. 8(c), Fig. 8(d), and Fig. 8(e) indicate that the coarse compounds in Fig. 8(a) and Fig. 8(b) are MgZn₂ (η-phase), and that there are many coarse precipitates of the η-phase both in the grain and on the grain boundary for Si: 0.05 mass%. In contrast, as shown in Fig. 9(c), Fig. 9(d) and Fig. 9(e), for Si: 0.15 mass%, the coarse compound was judged to be Al(Mn,Fe)Si, and many coarse Al(Mn,Fe)Si compounds were found in the grains and on grain boundaries. However, no coarse precipitates of the η-phase were observed for Si: 0.15 mass%.

Fig. 8

TEM photographs and results of EDX analysis for Si: 0.05 mass% after aging treatment of 423 K×8 h. (a) Microstructure in the grain, (b) Microstructure near the grain boundary, (c) EDX analysis result for (C), (d) EDX analysis result for (D), (e) EDX analysis result for (E).

Fig. 9

TEM photographs and results of EDX analysis for Si: 0.15 mass% after aging treatment of 423 K×8 h. (a) Microstructure in the grain, (b) Microstructure near the grain boundary, (c) EDX analysis result for (C), (d) EDX analysis result for (D), (e) EDX analysis result for (E).

3.3 Mechanical properties of extruded materials after aging treatment

From Fig. 10 and Fig. 11, the strength of aging at 423 K×8 h in Si: 0.05 mass% was about 260 MPa in tensile strength and about 220 MPa in 0.2% proof stress, and it was lower than 340∼360 MPa in tensile strength and 300∼320 MPa in 0.2% proof stress obtained by aging at 373 K×3 h-423 K×8 h, 373 K×6 h-423 K×8 h, and 373 K×9 h-423 K×8 h in Si: 0.05 mass%. However, for Si contents of 0.15 mass% and 0.30 mass%, the strength obtained by aging at 423 K×8 h was about 330 MPa for both tensile strength and about 300 MPa for both 0.2% proof stress, which was higher than that of Si: 0.05 mass%, and slightly lower than that of 350∼360 MPa for tensile strength and 310∼320 MPa for 0.2% proof stress obtained by aging at 373 K×3 h-423 K×8 h, 373 K×6 h-423 K×8 h, and 373 K×9 h-423 K×8 h. This is different from the previous study that the strength of Al-4 mass%Zn-1 mass%Mg alloy decreases with Si addition [4]. This difference in strength results may be due to the fact that Al (Mn, Fe) Si affected the precipitation state of the phase of MgZn₂ in the materials of this study because more Al (Mn, Fe) Si was produced during homogenization than in the previous study. That is, the effect of Si addition in the state without Mn addition was investigated in the previous study, while the effect of Si addition in the state with Mn addition was investigated in this study. Therefore, it seems to be different that Al (Mn, Fe) Si is not easily formed in the previous study, while Al (Mn, Fe) Si is easily formed in this study.

Fig. 10

Effects of amounts of Si addition on the tensile strength after each aging treatment.

Fig. 11

Effects of amounts of Si addition on 0.2% yield stress after each aging treatment.

3.4 Investigation of MgZn₂ precipitates in microstructure by DSC thermal analysis

As shown in Fig. 12, a mountainous exothermic peak at around 510 K was confirmed in the solution-treated materials at Si: 0.05 mass%, Si: 0.15 mass%, and Si: 0.30 mass%. This is considered to be an exothermic peak when η′ and η phase precipitates precipitate [9–12]. As shown in Fig. 13, a mountainous exothermic peak was observed around 500 K for Si: 0.05 mass% in 423 K×8 h aging. However, in the case of Si: 0.15 mass% and Si: 0.30 mass% at 423 K×8 h, no mountainous exothermic peak was observed around 500 K, unlike the case of Si: 0.05 mass%. From this fact, it is considered that the precipitation of η′ and η phase is little in Si: 0.15 mass% and Si: 0.30 mass% during the temperature rise in the DSC measurement, and it is considered that the precipitation of η′ and η phase is much generated in the matrix during the aging treatment of 423 K×8 h. On the other hand, in Si: 0.05 mass%, the precipitation of η phase was confirmed by aging at 423 K×8 h from the TEM structure observation shown in Fig. 8, and in addition, η′, η phase seemed to precipitate during the temperature rise during the DSC measurement.

Fig. 12

DSC curve of each Si amount after solution treatment.

Fig. 13

DSC curve of each Si amount of aging treatment 423 K×8 h.

As shown in Fig. 14, in the 373 K×6 h-423 K×8 h material, a valley endothermic peak was observed around 485 K under any condition of Si: 0.05 mass%, Si: 0.15 mass% and Si: 0.30 mass%. This endothermic peak is considered to be due to the solid solution of the η′ phase. A mountainous exothermic peak at the peak around 525 K is considered to be caused by the precipitation of η phase. In the case of Si: 0.30 mass%, it is considered that the precipitation of η phase is small because a mountainous exothermic peak is small. The peak exothermic reaction was slightly smaller in Si: 0.15 mass% than in Si: 0.05 mass%, suggesting that the precipitation of η phase was less in Si: 0.15 mass% than in Si: 0.05 mass%. In addition, the valley endothermic peak is observed at around 570 K for Si: 0.05 mass% and Si: 0.15 mass%, which is considered to be the solid solution of η phase. In Si: 0.30 mass%, the valley endothermic peak was not confirmed, so the solid solution of η phase was considered to be small.

Fig. 14

DSC curve of each Si amount of aging treatment 373 K×6 h-423 K×8 h.

Therefore, from the DSC results, it was found that in the case of 423 K×8 h aging, the amount of precipitation of η′ and η phase during 423 K×8 h aging tended to increase as the amount of Si added decreased. Considering the relationship between material structure and strength, it is considered that the precipitation amount of η′ and η phase was large and the coarsening of η phase was advanced at 0.05 mass% with small Si addition, and the precipitation amount of fine η′ which contributed to the strength decreased, resulting in the lower strength after aging than Si: 0.15 mass% and Si: 0.30 mass%. In addition, the strength of Si: 0.05 mass% after aging at 423 K×8 h was lower than that of Si: 0.15 mass% and Si: 0.30 mass%, because the percentage of recrystallized grains in the cross section of Si: 0.05 mass% after extrusion was larger than that of Si: 0.15 mass% and Si: 0.30 mass%. [13]

In addition, the strength of Si: 0.05 mass% after 373 K×6 h-423 K×8 h aging was higher than that of Si: 0.05 mass% in 423 K×8 h one step aging, and in 373 K×6 h-423 K×8 h two step aging, the strength was similar in the Si addition range of this study. In the Si addition range of this study, it is estimated that the precipitation density of η′ phase becomes similar by the two step aging of 373 K×6 h-423 K×8 h, but the elucidation of the detailed mechanism is a future problem.

4. Conclusion

In this paper, the effect of Si addition in Al-5.6 mass%Zn-0.75 mass%Mg alloy containing Cu, Mn, Zr and Fe on aging mechanical properties and material structure of Al-Zn-Mg alloy was investigated. The results are shown below.

1. (1)    The higher the Si content, the smaller the thickness of the recrystallized layer near the inner and outer surfaces of the mold, and the smaller the percentage of recrystallized grains in the cross section.
2. (2)    At 423 K×8 h, the strength of Si: 0.05 mass% was lower than that of Si: 0.15 mass% and Si: 0.30 mass%. This is considered to be caused by the precipitation of coarse η phase at grain boundaries and within grains in Si: 0.05 mass%. In the case of Si: 0.15 mass% and Si: 0.30 mass%, it is considered that Al (Mn, Fe) Si, which is a crystallized product formed in the process of homogenization, existed in the grain boundary and in the grain in the mold after extrusion, and contributed to the suppression of the formation of coarse η phase precipitates in the 423 K×8 h aging.
3. (3)    Si: 0.05 mass%, Si: 0.15 mass%, and Si: 0.30 mass% had similar strength in a two step aging treatment of 373 K×6 h-423 K×8 h.

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