Effect of Cooling Rate after Solution Heat Treatment on Aging Properties of Al–Zn–Mg Alloy

Abstract

This paper investigated the effect of cooling rate after solution heat treatment and pre-aging at 303 K after cooling on aging behavior of Al–6 mass%Zn–0.75 mass%Mg alloy to elucidate the aging behaviors and strengthening mechanism throughout aging heat treatment in the case of low cooling rate, such as furnace cooling, followed by solution heat treatment. Regardless the absence of pre-aging, the maximum hardness obtained in the materials with furnace cooling after solution treatment was almost the same as that of water-cooled and pre-aged materials during the same aging time. The result of microstructure observations using transmission electron microscope show that fine precipitates consisting of η phase have been formed in all peak-aged materials.

(a) Aging hardening curves and (b) electrical conductivity changing curves of specimens isothermally aged at 433 K after solution treatment at 723 K for 3.6 ks followed by cooling at each cooling rate and pre-aging.

1. Introduction

Owing to their specific strength, Al–Zn–Mg alloys are mainly used as structural materials for transportation, such as aircraft. These alloys are precipitation-hardening materials that are strengthened by aging treatment to finely disperse precipitates. The widely known precipitation sequence of Al–Zn–Mg alloy is as follows [1–10].

  

\begin{align*} \text{Supersaturated solid solution}&\to \text{GP zones}\\ &\to \eta'\to \eta\ (\text{MgZn$_{2}$}) \end{align*}

The transformation of η′ into η, which contributes to the strengthening of this alloy, is well established [5–7, 11–13], and 11 types of η have been reported [6, 11, 12].

Throughout the heat treatment process of this alloy, rapid cooling (quenching) must be generally performed following with the solution heat treatment to obtain a supersaturated solid solute state. However, the quenching process generates large material distortion, causing problems such as degeneration of the dimensions and precision of the products. Omitting the water quenching step could also be desirable to simplify the manufacturing process.

In contrast, this alloy exhibits low quench sensitivity. Baba [14] reported that approximately 95% of the maximum hardness of water-quenched alloy was obtained in an Al–6 mass%Zn–1.8 mass%Mg alloy without impurity elements after solution heat treatment at 738 K and furnace cooling at the rate of 0.025 K/s. Yoshida et al. [15] reported that even in the cases of air cooling (cooling rate: 3 K/s) and furnace cooling (cooling rate: 0.006 K/s) after solution heat treatment, an Al–6 mass%Zn–0.75 mass%Mg alloy showed significant natural age-hardening and achieved hardness similar to that of water cooling following artificial aging heat treatment. Rowolt et al. [16] analyzed in situ quench-induced precipitation of Al-6.1 mass%Zn-0.77 mass%Mg alloy in a wide range of cooling rates varying between 0.0003 and 3 K/s and reported a precipitation reaction at around 373 K was observed during cooling. However, the precipitation behavior and age-hardening mechanism at low cooling rates such as furnace cooling have not been clarified.

This study investigates the age-hardening behavior of an Al–6 mass%Zn–0.75 mass%Mg alloy to elucidate the effect of cooling rate after solution heat treatment and the strengthening mechanism during aging is discussed.

2. Experimental Procedure

Al–6 mass%Zn–0.75 mass%Mg alloy cold rolled sheets with 1.0 mm thick were used. Figure 1 shows a schematic illustration of the heat treatment process. Solution heat treatment was conducted at 723 K for 3.6 ks, followed by cooling at different cooling rates, water quenching (hereafter referred to as “WQ”), and furnace cooling (hereafter referred to as “FC”) at a cooling rate of 0.02 K/s using the temperature control system of an electric furnace. To investigate the effect of pre-aging (PA) because this alloy exhibits “positive effect” [15] in terms of higher hardness when artificially aged after PA than without PA, the specimens with pre-aged for 604.8 ks at 303 K after quenching were also prepared (hereafter WQ with PA and FC with PA referred to as “WQPA” and “FCPA”, respectively). Artificial aging at 433 K up to 172.8 ks (2 days) was carried out immediately after cooling and PA. After the aging treatments, Vickers hardness and electrical conductivity measurements were conducted. The Vickers hardness test was performed at a load of 4.9 N at room temperature, and the electrical conductivity was measured with eddy-current at room temperature. Microstructure observation by transmission electron microscopy (TEM) was performed at an acceleration voltage of 200 kV using JEOL JEM-7001F. To prepare TEM specimens, mechanical polishing was performed to prepare 100-µm-thick and 3-mm-diameter disk were punched from them. Subsequently, electrical polishing with the twin-jet method was performed using a solution of nitric acid and methanol (3:1 volume ratio) at approximately 263 K.

Fig. 1

Schematic illustration of heat treatment process of this study.

3. Results and Discussion

3.1 Aging properties

Figure 2(a) and (b) shows the age-hardening curves and electrical conductivity change curves of the materials during aging at 433 K. The hardness of WQ and FC immediately after cooling (A.C.) was approximately 40 HV and 70 HV, respectively, and the hardness of both materials increased to approximately 90 HV after PA. From these results, significant natural age hardening was confirmed, even in the case of furnace cooling after solution treatment. In contrast, the electrical conductivity of both WQ and FC decreased after PA, and the decrease was larger for FC than for WQ.

Fig. 2

(a) Aging hardening curves and (b) electrical conductivity changing curves of specimens isothermally aged at 433 K after solution treatment at 723 K for 3.6 ks followed by cooling at each cooling rate and pre-aging.

In the initial stage of artificial aging (approximately 0.1 ks), the hardness of all materials decreased from that of the pre-aged materials. Although WQPA and FCPA had almost had the same hardness before artificial aging, the amount of hardness reduction was different. Focusing on the peak aging, while the hardness of WQ was approximately 70 HV at an aging time of approximately 86.4 ks, the hardness of FC, WQPA, and FCPA was about 100 HV at about 14.4 ks, indicating that the specimens of FC, WQPA, and FCPA materials reached similar hardness at almost the same aging time. The above results indicated that peak hardness equivalent to that of WQ with PA can be obtained in the same aging period in the FC sample regardless of the presence or absence of PA.

3.2 Microstructure observations

Figure 3 shows TEM bright-field images and selected area electron diffraction patterns of each sample aged to peak hardness at 433 K. The incident electron beam direction is parallel to [100] and [110] of the aluminum matrix. In all materials, disc-like precipitates were observed in the [100]_Al and rod-like precipitates edge-on at the {111}_Al plane were observed in the [110]_Al. Figure 4 shows a schematic illustration of the precipitate at peak hardness [17]. The morphologies show that all precipitates were estimated as η₂ and/or η₃. The crystal orientation relationship between them and the aluminum matrix is as follows [6, 7, 11, 13, 18–21];

  

\begin{equation*} (0001)_{\eta_{2}}\parallel (1\bar{1}\bar{1})_{\text{Al}},[10\bar{1}0]_{\eta_{2}}\parallel [110]_{\text{Al}} \end{equation*}

  

\begin{equation*} (0001)_{\eta_{3}}\parallel (1\bar{1}\bar{1})_{\text{Al}},[11\bar{2}0]_{\eta_{3}}\parallel [110]_{\text{Al}} \end{equation*}

Fig. 3

Transmission electron microscope bright-field images and selected-area electron diffraction patterns of WQ ((a), (e)), WQPA ((b), (f)), FC ((c), (g)), and FCPA ((d), (h)) specimens isothermally aged at 433 K to peak hardness. Incident beam direction of the upper images (a)–(d) was parallel to [100] of the aluminum matrix, and one of the lower images (e)–(h) was parallel to [110] of the aluminum matrix.

Fig. 4

Model of precipitate we observed at peak hardness [20].

The FC, WQPA, and FCPA precipitates were finer and denser than the WQ precipitate. This result may explain why their hardness was higher than that of WQ. The mechanisms of precipitating in each process will be clarified using TEM microstructure observations.

4. Conclusion

This study investigated the effect of cooling rate after solution heat treatment and PA at 303 K on the aging properties of an Al–6 mass%Zn–0.75 mass%Mg alloy using Vickers hardness, electrical conductivity, and microstructure observations by TEM. The main conclusions are summarized below.

1. (1)    The peak hardness in FC samples was confirmed to be equivalent to that of WQPA, regardless of the presence or absence of PA.
2. (2)    The result of microstructure observation using TEM indicated that FC, WQPA, and FCPA precipitates were finer and denser than that of WQ. The mechanisms of precipitation in each process will be clarified using TEM microstructure observations.

Acknowledgments

The specimens used in this study were supported by “Age-hardening behavior of 7000 series aluminum alloys research subcommittee” of The Japan Institute of Light Metals. The authors also thank to Dr. W. Inagaki in NIPPON STEEL TECHNOLOGY Co., Ltd. (Futtsu, Chiba, Japan), who supported the microstructure observation of this alloy.

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