J. Cent. South Univ. (2017) 24: 2231-2237

DOI: https://doi.org/10.1007/s11771-017-3632-4

Effect of zirconium on microstructures and mechanical properties of squeeze cast Al–5.0Cu–0.4Mn–0.1Ti–0.1RE alloy

MENG Fan-sheng(孟凡生), ZHANG Wei-wen(张卫文), HU Yuan(胡愿),

ZHANG Da-tong(张大童), YANG Chao(杨超)

National Engineering Research Center of Near-net-shape Forming for Metallic Materials,South China University of Technology, Guangzhou 510640, China

Central South University Press and Springer-Verlag GmbH Germany 2017

Abstract: Al–5.0Cu–0.4Mn alloys with different Zr additions have been prepared by direct squeeze casting. The effects of Zr on microstructures and mechanical properties of the as-cast and T6 heat-treated alloys were investigated by tensile test, optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the optimal tensile property of the as-cast alloy occurs at the Zr content of 0.15% (mass fraction) due to the “Zr poisoning” action and the appearance of bulky primary Al₃Zr, which decreases the grain refinement strengthening effect of the as-cast alloy. The peak values of ultimate tensile strength and yield strength of the T6 alloy occur at the Zr content of 0.25%, and that of the elongation occurs at Zr content of 0.05%. This is mainly attributed to the strengthening effect of dispersed precipitation of Al₃Zr and q θ'phases. The optimal mechanical properties of T6 heat-treated alloy are the tensile strength of 429 MPa, the yield strength of 327 MPa and the elongation of 18%, respectively when the squeeze pressure is 100 MPa and Zr content is 0.25%.

Key words: Zr; squeeze pressure; mechanical property; microstructure

1 Introduction

Al–Cu–based alloys are widely used in the fields of aviation, spaceflight, transportation and other industries owing to their high strength, good toughness and excellent machining performance. Squeeze casting is a kind of short process, high efficiency and precise forming technology combining the characteristics of casting and plastic processing. It is a common method for preparing high performance materials and components because of the reduction of the shrinkage and thermal crack defects in the alloy [1–3]. Micro-alloying is also an important way to improve the mechanical properties of the aluminum alloy. Much research work has demonstrated that Zr, V, Sc, Ti, Ce and other trace elements added in aluminum alloys could improve the properties of the alloy obviously [4–9]. Adding trace elements Zr in the aluminum alloys can refine the grain size. The fine and dispersed Al₃Zr strengthening phase generated by heat treatment processing could pin dislocations, inhibit recrystallization and then further improve the properties of the alloys [10–12].

Some studies have been carried out on the effect of Zr on Al–Cu alloys. JIA et al [13] studied the precipitation behavior of Al₃Zr in the cast Al–Cu alloy. It was found that the metastable Al₃Zr precipitated in the middle of the dendrite and helical-like and strip-like Al₃Zr existed in the interdendritic regions which are composed of many spherical tiny Al₃Zr precipitates. This was attributed to the Cu element accelerating the Al₃Zr precipitation phase from the L1₂ structure to the D0₂₃ structure. ZHANG et al [14] showed the effect of Al₃Zr precipitated phase on the microstructure, texture and properties of the Al–Zn–Mg–Cu alloy. It was found that the grading homogenization could promote the precipitation of Al₃Zr particles in the grain boundary region. The Al₃Zr particles which were coherent with matrix not only blocked the dislocation motion, but also hindered the thermal activation of the sub-grain boundaries and grain boundaries. Meanwhile the particles prevented the recovery and recrystallization in hot rolling process, which improved the mechanical properties of the alloy. SEPEHRBAND et al [15] regarded that the hardness of the alloy could be improved obviously through enough time of solid solution treatment when added 0.15% Zr (mass fraction) to cast A319 aluminum alloy. It was attributed to the lower kinetics of the Al₃Zr phase and a long time of precipitation. ZHENG et al [16] demonstrated that the mechanical properties of the alloy could be improved significantly when 0.5% Zr and 0.3% Ti were added in Al–Cu alloy simultaneously. The heterogeneous nucleation of Al₃ (Ti, Zr) leads to the a(Al) grain refinement. The mechanical properties of the alloy decreased sharply when 0.9% Zr and 0.5% Ti were added simultaneously in the alloy because of the coarse Al₃Ti phase formed in the solidification process. As reported by PENG et al [17], the superplasticity of the Al–Mg– Mn alloy can be improved by adding Zr and Sc. The elongation is much higher than that of the alloy without Zr and Sc. It is owing to the activation energy of superplasticity reduced by adding Zr and Sc, and the grain size of the alloy was refined at the same time. YIN et al [18] demonstrated that the grain size was refined significantly by addition of 0.2% Sc and 0.1% Zr in the Al–5Mg alloy because of the heterogeneous nucleation of Al₃Sc and Al₃Zr in the melt by Al, Zr and Sc.

It should be noted that the study on the effects of trace element Zr was mainly focused on the wrought aluminum alloys. So far, little work has been reported on the squeeze cast Al–Cu alloys by adding Zr element. In this work, the microstructure and mechanical properties of squeeze cast Al–5.0Cu–0.4Mn–0.1Ti–0.1RE alloy by adding different Zr contents were studied and the strengthening mechanism of Zr in the alloy was analyzed.

2 Experimental

Raw materials of 99.95% (mass fraction) high purity commercial aluminum, Al–50% Cu, Al–10% Mn, Al–10% Zr, Al–5% Ti–1% B, Al–10% RE master alloys were used in the charges of 10 kg in the resistance furnace of graphite crucible. The Al–Cu, Al–Mn, Al–RE, Al–Zr, and Al–Ti–B master alloys were added in turn after the pure aluminum was melted. The sodium salt was used as covering agent in the process of melting, and stirred after completely melting. The melt was degassed by nitrogen at 730 °C. The pouring temperature of the melt was about 730 °C. The direct squeeze casting was carried out on the 100 t vertical presser. The preheating mold temperature was 200 °C before pouring. The applied pressure was 0, 50 and 100 MPa. The holding time was about 30 s. The size of the sample was about d 80 mm×100 mm. The heat treatment for the sample was 540 °C solution for 12 h, water quenching at room temperature and then aging at 175 °C for 8 h. The chemical compositions of the alloys analyzed by optical emission spectroscopy are shown in Table 1.

The rod with diameter of 10 mm was cut from the casting center about 30 mm. The standard sample size was d 5 mm×25 mm. Tensile tests were performed on an SANS CMT5105 universal material testing machine. Tensile rate was 1 mm/min. At least three samples were tested at each condition. The sample for microstructure observation was etched with Keller reagent. The microstructure was examined using LEICA/DMI 5000M optical microscope (OM) and Quanta 2000 scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDS) analysis. TEM samples were prepared using twin-jet electropolisher with 3:7 volume ratio of HNO₃ in methanol solution and the temperature was –25 °C. The samples in the JEM-2200FS were observed when the acceleration voltage was 200 kV.

3 Results

3.1 Mechanical properties

Figures 1 and 2 show the mechanical properties of the as-cast and heat-treated alloys at different squeeze pressures and Zr contents, respectively. It is obvious that the effect of squeeze pressures is generally to improve the mechanical properties, especially the elongation. But as compared to the properties of 50 MPa and 100 MPa, the difference of this improvement is slight. Comparing the mechanical properties shown in Figs. 1 and 2, an increase of Zr content generally results in an increase in the mechanical properties first and a decrease later. The optimal mechanical properties occur at the Zr content of 0.15% for the as-cast alloys at different squeeze pressures, and the optimal ultimate tensile strength and yield strength occur at Zr content of 0.25% for the heat-treated alloys at different squeeze pressures, while the optimal elongation occurs at Zr content of 0.05%. The optimal tensile property of the as-cast alloy with the ultimate tensile strength of 210 MPa, yield strength of 113 MPa and elongation of 21.1%, respectively, is obtained at the squeeze pressure of 100 MPa and Zr content of 0.15%. The optimal ultimate tensile strength and yield strength of the heat-treated alloy are 429 MPa and 327 MPa at the squeeze pressure of 100 MPa and Zr content of 0.25%. The optimal elongation of the heat- treated alloy is 19.3% at the squeeze pressure of 100 MPa and Zr content is 0.05%.

Table 1 Chemical composition of alloys (mass fraction, % )

Fig. 1 Mechanical properties of as-cast alloys at different squeeze pressures and Zr contents:

3.2 Microstructures

Figure 3 shows the microstructure morphologies of the as-cast alloys under 100 MPa squeeze pressure and different Zr contents. Figure 4 shows the quantitative analysis for the average grain size of the as-cast alloy at different squeeze pressures and Zr contents. It is found that the grain sizes in the Zr-free alloys are very large while the grain is somewhat refined with 0.15% Zr addition. And then the average grain size coarsens when the Zr content exceeds 0.15%. However, the grain size reduces significantly with the increase of the squeeze pressure but there is no significant reduction when the squeeze pressure rising from 50 to 100 MPa. The minimum grain size of the alloy is about 79 μm when the Zr content is 0.15% and squeeze pressure is 100 MPa.

Fig. 2 Mechanical properties of heat-treated alloys at different squeeze pressures and Zr contents:

Fig. 3 Microstructures of as-cast alloys at 100 MPa squeeze pressure and different Zr contents:

Fig. 4 Grain sizes of as-cast alloys at different squeeze pressures and Zr contents

Figure 5 shows the image of the a(Al) matrix in the heat-treated alloys under 100 MPa squeeze pressure and different Zr contents. EDS is adopted to analyze the precipitated phases in α(Al) matrix. The results are demonstrated in Fig. 6. Only dispersed q θ'(Al₂Cu) phase exists in the Zr-free alloy (see Fig. 5(a)). A large number of spherical precipitated phases with the diameter of 40 to 60 nm appear when the content of Zr exceeds 0.15% (see Figs. 5(b)–(d)). The density of this phase increases with the increase of Zr content. From EDS, it can be seen that the particles are rich in Al and Zr, and the molar ratio of Al and Zr is about 74 : 26. Further referring to the results in Ref. [16, 17], it can be inferred that the spherical particles are Al₃Zr phase.

3.3 Fracture

Figure 7 shows the fracture morphologies of the as-cast and heat-treated alloys with 100 MPa squeeze pressure and different Zr contents. As for the as-cast alloys, there are lots of q(Al₂Cu) phases among the a(Al) dendrites (see Figs. 7(a) and (b)) and some bulk phases appear when the Zr content is 0.35% (see Fig. 7(b)). EDS analysis indicates that this phase is Al₃Zr according to the atomic ratio of Al:Zr of 74:26 and relevant Refs. [17, 18] .

In the heat-treated condition, the q phase almost dissolves completely into a(Al) matrix (see Figs. 7(c) and (d)). But the size and shape of the white Al₃Zr phase do not change significantly (see Figs. 7(b) and (d)).

4 Discussion

The optimal ultimate tensile strength, yield strength and elongation of the as-cast alloy occur at Zr content of 0.15% under different squeeze pressures. This slight increment of the mechanical properties is attributed to the grain refinement effect by Zr addition when the Zr content is below 0.15%. As the Zr content in the aluminide increases, the lattice matching of Al₃Zr with a(Al) improves because the interatomic distance of Al₃Zr is more close to a(Al) compared with the distance between bulky Al₃Ti and a(Al) (The interatomic distance of {114} Al₃Zr is 0.291 nm, more close to {111} Al of 0.286 nm, but {112} Al₃Ti is 0.275 nm) [19, 20]. When the Zr content is over 0.15%, the grain refinement becomes worse because of the “Zr poisoning” effect by the following reaction proposed by JONES et al [20].

Fig. 5 Precipitated phases in α(Al) matrix of T6 heat-treated alloys at squeeze pressure of 100 MPa and different Zr contents:

Fig. 6 EDS of Al₃Zr phase

                   (1)

This reaction occurs only when the concentration of Zr in solution is four times that of the residual Ti [20]. The reaction indicates a layer of ZrB₂ coated on the surface of the TiB₂ particles, which will decrease the nucleation potency for α(Al). Because of the large lattice parameters of ZrB₂ compared with TiB₂, ZrB₂ will be a bad match with a(Al) lattice (The interatomic distance of {0001} ZrB₂ is 0.317 nm, and that of {0001} TiB₂ is 0.303 nm). So this reaction will weaken the grain refinement effect of Ti on a(Al) when the Zr content exceeds 0.15%. Once the Zr content is high enough, such as 0.35%, the bulky primary Al₃Zr phase is prone to generate in the dendrite (see Fig. 7(b)), which will significantly deteriorate the mechanical properties of the alloy.

The previous results show that the ultimate tensile strength and yield strength of the T6 heat-treated alloy increase with the Zr content rising from 0 to 0.25% under different squeeze pressures. This is mainly attributed to the precipitation strengthening of the Al₃Zr particles and q θ' phase. The dispersive and tiny Al₃Zr particles tend to hinder the dislocation movement. According to the Orowan mechanism, the precipitation strengthening effect is determined by the size and volume fraction of the precipitated phase. Smaller size and higher volume fraction make the precipitation strengthening effect increase obviously [21]. With the increase of Zr content the volume fraction of dispersive Al₃Zr particles increases gradually (see Fig. 5). When the Zr content approaches 0.25%, the precipitation strengthening by dispersive nano-sized Al₃Zr particles results in the maximum ultimate tensile strength and the yield strength of the alloy. However, the mechanical properties decrease sharply at Zr content of 0.35% owing to the occurrence of the bulky Al₃Zr phase (see Fig. 7(d)).

Fig. 7 Tensile fracture morphologies of 100 MPa alloys at different Zr contents:

As for the plasticity of the heat-treated alloy, it should be noted that the best elongation occurs in the composition of 0.05% Zr under different squeeze pressures. Obviously, the refinement of the grain size by minor Zr addition is beneficial to the elongation of the alloy. However, the large amount of dispersive Al₃Zr phases increases in the a(Al) matrix with the Zr content more than 0.05%, and the bulky Al₃Zr particles form when the content of Zr reaches 0.35%, having a detrimental effect on the deformation of the alloy.

The mechanical properties of the as-cast and heat-treated alloys have been improved by the squeeze pressure. This is due to the squeeze pressure eliminating the shrinkage and porosity in the alloys. But the difference for improvement of mechanical property is subtle when the squeeze pressure is beyond 50 MPa. As for the alloys with different Zr contents, the effect of squeeze pressure on microstructure and mechanical property of the alloys is almost the same. The squeeze pressure only reduces the a(Al) grain size and porosity of the alloys with different Zr contents. However, it has little effect on the second phases, such as q θ'(Al₂Cu) and Al₃Zr precipitation as shown in Fig. 5. So, the optimal Zr content is the same for the mechanical property of the alloys no matter the squeeze pressure is applied or not.

5 Conclusions

Microstructure and mechanical properties of squeeze cast Al–5.0Cu–0.4Mn–0.1Ti–0.1RE alloy with different contents of Zr and squeeze pressures are studied. The conclusions are drawn as follows:

1) In the as-cast condition, the optimal tensile property achieves at the squeeze pressure of 100 MPa and Zr content of 0.15%.

2) The “Zr poisoning” effect weakens the grain refinement effect of Ti when the Zr content is over 0.15% under different squeeze pressures.

3) In the heat-treated condition, the optimal ultimate tensile strength, and yield strength achieve at the squeeze pressure of 100 MPa and Zr content of 0.25%, while the elongation achieves at the squeeze pressure of 100 MPa and Zr content of 0.05%. The optimal mechanical property is the tensile strength of 429 MPa, the yield strength of 327 MPa and the elongation of 18%.

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(Edited by FANG Jing-hua)

Cite this article as: MENG Fan-sheng, ZHANG Wei-wen, HU Yuan, ZHANG Da-tong, YANG Chao. Effect of zirconium on microstructures and mechanical properties of squeeze cast Al–5.0Cu–0.4Mn–0.1Ti–0.1RE alloy [J]. Journal of Central South University, 2017, 24(10): 2231–2237. DOI:https://doi.org/10.1007/s11771-017-3632-4.

Foundation item: Project(51374110) supported by the National Natural Science Foundation of China; Project(2015A030312003) supported by the Guangdong Natural Science Foundation for Research Team, China

Received date: 2016-04-12; Accepted date: 2016-07-11

Corresponding author: ZHANG Wei-wen, Professor, PhD; Tel: +86-20-87112272; E-mail: mewzhang@scut.edu.cn