Effect of Cu addition on microstructures and tensile properties of high-pressure die-casting Al-5.5Mg-0.7Mn alloy

1. Introduction

The lightweight car is an effective means to solve the problem of energy consumption and environmental pollution [1], [2], [3], [4]. For new energy electric vehicles, it is also necessary to maximize the weight reduction of the body structure to offset the problem of weight gain caused by the battery system, so as to achieve long-lasting mileage [5], [6], [7], [8]. As a lightweight structural material commonly used in automobiles, aluminum alloy has high strength and good shock absorption performance, and can also bring higher safety performance and more comfortable ride experience [4,9]. Compared with other casting methods, high-pressure die-casting is more suitable for mass production in automotive industry, which is due to its high production efficiency, capacity of producing complex thin-walled parts and good comprehensive properties, and die-casting Al alloys are increasingly used in automotive parts [10], [11], [12]. What’s more, high-quality die-casting Al alloys with high strength and high ductility are demanded in the automobile structural parts to meet the safety standard [13,14]. The Al-Mg-(Si) alloys with excellent ductility and high strength to weight ratio are getting an increasing attention to be used for the automobile structure components produced by high-pressure die-casting [13,[15], [16], [17].

At present, the research of die casting Al-Mg-(Si) based alloys have been mainly focused on the improvement of the mechanical properties based on rigorous control of chemical compositions and special casting technologies [13,16,18,19]. It has been reported that the low-iron Magsimal^®-59 alloy exhibits the excellent tensile properties. The yield strength (YS), ultimate tensile strength (UTS) and elongation of the alloy are about 160 MPa, 300 MPa, 15%, respectively. In addition, T5 heat treatment can significantly improve the YS of the alloy (increased by about 30%) [18]. Zhang et al. developed a high strength and ductility Al-5Mg-0.6 Mn alloy (YS > 160 MPa, UTS > 300 MPa and elongation>10%) by high-pressure die-casting [19], which is very attractive to automotive powertrain and structure applications due to its high strength, ductility and corrosion resistance etc. However, it should be noted that the strength of Al-Mg alloys mostly results from the solid solution strengthening and have a relatively low strength [20], [21], [22]. In addition, classical Al-Mg alloys lose part of the strength during the baking treatment [23,24] (equivalent to aging at 160-180 °C for 20-30 min). These features would limit their extensive application in the structural components. So, it is meaningful to develop a new alloy to improve the strength of the material on the basis of guaranteeing excellent ductility of Al-Mg alloy.

Generally, microalloying is the main method to develop the new die-casting Al alloys with the excellent mechanical properties. Cu is a strengthening element commonly used in Al alloys [25], [26], [27], [28], either dissolved into the matrix or forming intermetallics such as Al₂Cu and Al₂CuMg etc. For example, the addition of Cu in the die-casting Al-Si alloys (such as AlSi9Cu3) can improve the strength of the castings [29,30]. Recently, Zhang et al. [31] developed a new die-casting Al-Si-Cu alloy with high strength (YS of 206 MPa and UTS of 331 MPa) and ductility (elongation of 10%). What’s more, Cu was added to 5××× Al-Mg based alloys to improve the strength by introducing precipitation hardening [32], [33], [34], [35], [36]. Ji et al. [13] pointed out that the addition of Cu can increase the yield strength under an as-cast condition or heat treated condition with scarifying the ductility in the die-casting Al-Mg-Si alloy. But he didn’t report the effect of Cu on the microstructures of the alloy and strengthening reason also didn’t mention. What’s more, age hardening may also come from the precipitate of Mg₂Si not only from Cu-containing precipitates. It can suppose that the addition of Cu can improve the strength of die-casting Al-Mg-(Si) alloys. But, it is necessary to systemically study the effects of Cu on the microstructures and mechanical properties of the Al-Mg-(Si) alloys prepared by high-pressure die-casting.

In this study, the addition of Cu in the die-casting Al-Mg-Mn alloy is to improve the tensile properties of the alloys. The effects of Cu on the microstructures, tensile properties and fracture behaviors of the die-cast Al-Mg-Mn alloy were systemically investigated. In addition, an aging process at 175 °C was also carried out to further improve the tensile strength of the Al-Mg-Mn-xCu alloys. The microstructures and tensile properties of the alloys after T5 treatment were also studied in detail.

2. Experimental

The castings with nominal chemical compositions of Al-5.5Mg-0.7 Mn-xCu (x = 0, 0.5, 0.8 and 1.5, wt%) were prepared from high purity Al, pure Mg and the master alloys of Al-10 Mn, Al-5Ti-B, Al-50Cu in an electrical resistance furnace. The activated element Mg was over added by 8 wt% to compensate its burning loss during the melting. When adding pure magnesium, the protective solvent is sprinkled on the melt to prevent Mg burning. Fe content in the alloys was strictly controlled at a low level of 0.1-0.15 wt%. The pouring temperature and the mold temperature were controlled at about 700 °C and 200 °C, respectively. Before pouring, the melt was kept in the electrical resistance furnace for 15-20 min to ensure homogenization of the chemical composition and the dissolution of the intermetallic particles.

The sample castings with different Cu content (shown in Fig. 1) were produced using a TOYO-BD350V5 cold chamber die-casting machine with the same casting parameters (shown in Fig. 2). The castings were directly quenched into warm water (about 60 °C). Tensile specimens (6.4 mm × 50 mm gauge length) were cut from these castings. Some tensile specimens were aged at 175 °C for 0-12 h in an oil bath furnace (designed as T5). The actual chemical compositions of the castings were determined by an Optima 7300DV inductively coupled plasma analysis (ICP) (shown in Table 1).

Room temperature tensile tests were carried out on a Zwick/Roell-Z100 tensile machine using the specimens with a gauge length of 50 mm and gauge dimension of 6.4 mm (at a tensile rate of 1 mm/min). The values of YS, UTS, and elongation of the alloys were calculated from the tensile stress-strain curves. For each alloy, at least five tensile specimens were tested and the average value of these tests was considered as the achieving properties of the alloys.

The metallographic samples for microstructural observation were taken from the grip areas of the tensile samples. After mechanical grinding and polishing, the samples were etched in a solution composed of 0.5 ml HF and 100 ml H₂O. The identification of the phases was performed using an Ultima IV X-ray diffractometer and a JEOL JSM-6460LA scanning electron microscope (SEM) with an energy-dispersive spectroscopy (EDS), respectively. The fracture behaviors of the alloys were observed using the SEM.

To investigate the content and the size of phases in the Al-5.5Mg-0.7 Mn-xCu alloys, an image analysis software (Image-pro plus) was used to count the information of microstructure observation by SEM. The area (μm²) was used to consider as the size of each phase. The area proportion (%) was defined as the content of each phase. Every value was statistics by five pictures of the microstructure.

The samples for transmission electron microscope (TEM) analysis were taken from the gauge areas of untested tensile samples. Thin foil samples for transmission electron microscope (TEM) analyses were mechanically grounded to about 160 μm in thickness and then etching polished using a solution of 25% nitric oxide and 75% methanol at a temperature of -30 °C and a voltage of 8 V. Finally, the ion beam thinner was applied to expand the thin area of the foil. Conventional TEM observations were performed using a PHILIPS CM 300 at 200 kV.

3. Results and discussion

3.1. Microstructures

3.1.1. As-cast alloys

Fig. 3 shows the typical microstructures of the as-cast Al-5.5Mg-0.7 Mn-xCu alloys produced by high-pressure die-casting. Fig. 4 and Table 2 illustrate the XRD analyses results and the quantitative EDS analyses of chemical compositions of various phases in the as-cast alloys. Combined with SEM-EDS and XRD analyses, the microstructure of the as-cast Al-5.5Mg-0.7 Mn alloy mainly consists of α-Al matrix and a few dark gray Al₁₂Mg₁₇ eutectic compounds (shown as dark arrows in Fig. 3). In addition, the as-cast Al-5.5Mg-0.7 Mn alloy also contains some light-white Fe-containing intermetallic phases (marked by red arrows in Fig. 3) distributed along grain boundaries. Because of the low Fe content and the appropriate casting technology, the shape and size of the iron phase were well controlled.

Compared with the as-cast Al-5.5Mg-0.7 Mn alloy, except for the eutectic compounds and the (Fe, Mn)-riched phases, irregular-shaped gray-white intermetallic compounds (marked by a blue arrows in Fig. 3(b)) were also observed in the as-cast Al-5.5Mg-0.7 Mn-xCu alloys. Quantitative EDS analysis indicates that the irregular-shaped phase consists of Al, Cu, and Mg (as shown in Table 2). The results of XRD analyses in Fig. 4 confirm that the compound is similar to Al₂CuMg phase. Quantitative EDS analysis (Table 2) also show that a part of Cu dissolved into the matrix and Cu content in the matrix increases with the increase of Cu in the Al-5.5Mg-0.7 Mn alloy.

Fig. 5(a) shows the evolution of the Al₂CuMg phase with respect to Cu content in the as-cast Al-5.5Mg-0.7 Mn alloy. Fig. 5(b)-(d) shows the relationship between Al₂CuMg size and proportion for the as-cast Al-5.5Mg-0.7 Mn-xCu alloys containing different contents of Cu. Fig. 5(a) indicates that the area fraction of Al₂CuMg phase increases from about 3.8% to 8.4% when Cu content in the alloys is improved from 0.5 to 1.5 wt%. Therefore, it’s believed that the amount of the Al₂CuMg phase mainly depends on the Cu content in the as-cast Al-5.5Mg-0.7 Mn alloys.

Obviously, different Cu content leads to the change in the size and content of the Al₂CuMg phase in the as-cast Al-5.5Mg-0.7 Mn-xCu alloy. It can be seen that the amount of the Al₂CuMg phase with bigger size apparently increases with the increase of Cu content. In contrast, increasing Cu content significantly reduces the number of finer particles. When Cu content increases from 0.5 to 1.5 wt%, the proportion of Al₂CuMg phase with a smaller size (<1 μm²) decrease from 65% to 20%. The proportion of the coarser particles (>10 μm²) is improved from 7% (0.5Cu) to 49% (1.5Cu).

3.1.2. T5-treated alloys

In order to preliminarily determine the change of microstructures of Al-5.5Mg-0.7 Mn-xCu alloys after the aging process. The aging hardness was first measured and the hardness curve aging at 175 °C was obtained. Fig. 6 shows that the hardness of Al-5.5Mg-0.7 Mn alloy hardly not change during the aging process. With the addition of Cu, the hardness of the alloy increases with aging time and maximum hardness was achieved at 8 h. The hardness results indicate that Al-5.5Mg-0.7 Mn containing Cu alloys own the precipitate strengthening and some precipitates precipitate during the aging process.

Fig. 7 shows the backscattered SEM micrographs of the Al-5.5Mg-0.7 Mn-0.8Cu alloys aged at 175 °C for 8 h. Compared with the as-cast counterparts, the microstructures of the T5-treated alloy do not change significantly on the basis of SEM observation. Fig. 7(b)-(d) indicates that the content of the Al₁₂Mg₁₇ eutectic particle, Fe-containing phase and Al₂CuMg phase in the as-cast and T5-treated Al-5.5Mg-0.7 Mn-0.8Cu alloy were almost same. In addition, the influence of Cu addition on the area fraction of the Al₁₂Mg₁₇ phase in the Al-5.5Mg-0.7 Mn-0.8Cu alloys is marginal (as-cast: 1.53%-1.69%, T5 treated condition: 1.63%-1.80%). Similar results were obtained in the Al-5.5Mg-0.7 Mn-0.5Cu and Al-5.5Mg-0.7 Mn-1.5Cu alloys under as-cast and T5-treated conditions.

Fig. 8 shows the results of TEM analysis of the precipitates in the matrix of the Al-5.5Mg-0.7Mg-0.8Cu alloy aged at 175 °C for 8 h. Bright field images observed from [001]_Al zone, [011]_Al and [$\bar{1}$ 12]_Al are shown in Fig. 8(a)-(c), respectively. As shown in Fig. 8(a), the precipitates grow along two directions including [0 $\bar{1}$ 0]_Al and [$\bar{1}$ 00]_Al when the observation direction is parallel to [001]_Al. On [$\bar{1}$ 12]_Al zone, the angle between precipitate D and E is about 36.87°, which is similar to the angle between (210)_Al and (201)_Al plane. The results observed from [110]_Al indicate that all precipitates in the Al-5.5Mg-0.7 Mn-0.8Cu alloy are the same precipitate. The chemical compositions of the precipitates in the T5-treated Al-5.5Mg-0.7 Mn-0.8Cu alloy in Fig. 8 indicate that the element content is close to the Al₂CuMg precipitate. These lath-shaped precipitates have a width of 2-4 nm and length of 15-20 nm. In addition, the particles grow along the direction of [001]_Al and parallel to (021)_Al plane, which is conformed to the Al₂CuMg precipitates growing as laths in the <100>_Al directions with {210}_Al habit planes [32], [33], [34], [35], [36]. Therefore, the precipitate in the T5-treated Al-5.5Mg-0.7 Mn-xCu alloys is considered as Al₂CuMg precipitate.

3.2. Mechanical behavior

Tensile properties of the as-cast Al-5.5Mg-0.7 Mn-xCu alloys are shown in Fig. 9. The YS, UTS, and elongation of the as-cast Al-5.5Mg-0.7 Mn alloy are about 160 MPa, 300 MPa, and 11%, respectively. The YS of the as-cast Al-5.5Mg-0.7 Mn-xCu alloys increases with the increase of Cu content. The as-cast Al-5.5Mg-0.7 Mn-1.5Cu alloy shows the highest YS (∼182 MPa), improved by 22 MPa in comparison with the as-cast Al-5.5-0.7 Mn alloy. However, no apparent variation in the UTS was observed in the as-cast Al-5.5Mg-0.7 Mn-xCu alloys. The elongation of the as-cast alloys (from 11% to 7.5%) decreases with the increase of Cu content (from 0 to 1.5 wt%). In this work, the as-cast Al-5.5Mg-0.7 Mn-0.8Cu alloy exhibits the excellent comprehensive properties (i.e. YS of 179 MPa, UTS of 303 MPa and elongation of 8.7%).

Fig. 10(a)-(c) shows the variations of YS, UTS, and elongation of the Al-5.5Mg-0.7 Mn-xCu alloys with the aging treatment time. Fig. 11(d) compares the tensile properties of the Al-5.5Mg-0.7 Mn-0.8Cu alloy aged at 175 °C for 8 h and the as-cast counterpart. It can be seen that the effect of aging treatment on the tensile properties of the Al-5.5Mg-0.7 Mn alloy is marginal. In contrast, the YS of the Al-5.5Mg-0.7 Mn alloy containing Cu element increases with the increase of aging treatment time (Fig. 10(a)). It is also seen that increasing aging treatment time slightly decreases the ductility of the Al-5.5Mg-0.7 Mn-xCu alloys (Fig. 10(b)). As shown in Fig. 10(c), the UTS of the Al-5.5Mg-0.7 Mn-xCu alloys show no significant variation during aging treatment. In addition, these alloys aged at 175 °C for 8 h exhibit the highest YS, increased by about 8.8% (0.5Cu), 10.6% (0.8Cu), 10.4% (1.5Cu), respectively, in comparison with the as-cast counterparts. Besides, Al-5.5Mg-0.7 Mn-0.8Cu alloy obtained the optimum combination properties. Compared to the base alloy, the yield strength of Al-5.5Mg-0.7 Mn-0.8Cu alloy increased by 23.7% and the elongation decreased by 27.3% after 8 h aging at 175 °C.

3.3. Existence form of Cu in die-casting Al-5.5Mg-0.7Mn alloy

3.3.1. Existence as Al₂CuMg phase distributed along grain boundary

According to the microstructure analyses mentioned above, the addition of Cu mainly promotes the formation of Al₂CuMg phase distributed along the grain boundary. The amount and size of Al₂CuMg particles remarkably increase with the increase of Cu content. What’s more, the elongations of Al-5.5Mg-0.7 Mn-xCu alloys decrease with the addition of Cu.

Fig. 11 shows the fracture surface of the as-cast Al-5.5Mg-0.7 Mn-xCu alloys. The cracked Al₂CuMg phases appeared in the fracture after addition of Cu. In order to explore the effect of the phase on the fracture behavior of the alloys, the microstructure near the fracture surface of the as-cast Al-5.5Mg-0.7 Mn-xCu alloys specimens was observed using SEM (as shown in Fig. 12). The area fraction of the cracked phases near the fracture of the as-cast alloys was calculated and the results are shown in Fig. 13.

With the increase of Cu content, more cracked Al₂CuMg phases can be observed in the fracture surface of the specimen (Fig. 12(b)-(d)). This result is also consistent with the decrease of the ductility of the alloys with the increase of Cu content. When Cu content increase from 0.5 to 1.5 wt%, the ratio of the cracked Al₂MgCu phases near the fracture surface of the as-cast Al-5.5Mg-0.7 Mn-xCu alloy specimens improved from about 54.4% to 69% (as shown in Fig. 13). In contrast, the ratio of the cracked (Fe, Mn)-riched phase decreases from 45.5% (0.5Cu) to 31% (1.5Cu). Therefore, it is believed that the Al₂CuMg phase results in the reduction of elongation in the die-casting Al-5.5Mg-0.7 Mn-xCu alloys. Similar results can be obtained in the aged alloys.

3.3.2. Existence as solid solution atom or precipitate

Due to the rapid cooling speed of die-casting, a part of the Cu supersaturated into the matrix. With the increase of Cu addition, the content of Cu dissolved into the matrix also improved. However, it should be noted that the content of Cu dissolved in the matrix hardly not changes between the Al-5.5Mg-0.7 Mn-0.8Cu and Al-5.5Mg-0.7 Mn-1.5Cu alloys. It indicates that the addition of Cu mainly leads to the formation of Al₂CuMg phase distributed along the grain boundary and the solid solution in the Al matrix has certain limitation. Besides, the yield strength of as-cast Al-5.5Mg-0.7 Mn-0.8Cu and Al-5.5Mg-0.7 Mn-1.5Cu alloys are almost the same which imply the strengthening effect are mainly come from the solid solution atom rather than Al₂CuMg distributed along the grain boundary.

Fig. 14 illustrates that the content of Cu in the matrix (designated as C, wt%), the content of Al₂CuMg phase (S, %) and the increment of YS (designated as ΔY=YSxCu-YS0Cu, MPa) of the as-cast Al-5.5Mg-0.7 Mn-xCu alloys. The numerical fitting find that the ΔY of the as-cast Al-5.5Mg-0.7 Mn-xCu alloys can be expressed as follows:

ΔY=42×C+0.86×S (1)

The expression can be considered to consist of two parts: the strengthening from the solid solution 42×C; the strengthening from the Al₂CuMg phase 0.86×S. It also shows that the strength enhancement from Cu solid atoms is stronger than the Al₂CuMg phase distributed along the grain boundaries. This may be the reason that the yield strength of Al-5.5Mg-0.7 Mn-0.8Cu and Al-5.5Mg-0.7 Mn-1.5Cu alloys is almost the same.

The yield strength of Al-5.5Mg-0.7 Mn alloys containing Cu element was improved during the aging. TEM observation shows that the precipitate of Al₂CuMg phase in Al-5.5Mg-0.7 Mn-0.8Cu alloy aged at 175 °C for 8 h. It is reasonable that the precipitate of Al₂CuMg phase improves the yield strength of Al-5.5Mg-0.7 Mn-xCu alloys during the aging process. Because the content of solid solution Cu has little difference, the aging strengthening effect is almost the same for Al-5.5Mg-0.7 Mn-0.8Cu and Al-5.5Mg-0.7 Mn-1.5Cu alloys. The area number density of Al₂CuMg phase in the Al-5.5Mg-0.7 Mn-0.8Cu and Al-5.5Mg-0.7 Mn-1.5Cu alloys also prove this situation. The relation between area number density of the Al₂CuMg precipitate and Cu content in the Al-5.5Mg-0.7 Mn-xCu alloys is shown in Fig. 15. It can be seen that the area number density of Al₂CuMg precipitate increases with the increase of Cu content. However, the area number density of the precipitate between Al-5.5Mg-0.7 Mn-0.8Cu (1.72 × 10¹⁴ m^-2) and Al-5.5Mg-0.7 Mn-1.5Cu (1.78 × 10¹⁴ m^-2) is similar.

3.3.3. Further work to make full use of Cu in die-casting Al-5.5Mg-0.7Mn alloy

In this work, the addition of Cu increases the strength and decrease the ductility of alloys. Al₂CuMg phase distributed along the grain boundary acts as major crack sources during the tensile test and the elongation of castings have a significant reduction. The strength enhancement from the Al₂CuMg phase is weaker than Cu solid atoms and the precipitate of Al₂CuMg phase. Therefore, the content of Al₂CuMg phase distributed along the grain boundaries should be controlled in a low level and the content of solid solution Cu or Al₂CuMg precipitate should be improved. It is a promising way to enhance the yield strength of Al-5.5Mg-0.7 Mn alloy, meanwhile, it obtains an excellent elongation.

The vacuum die-casting maybe a hopeful choice for Al-5.5Mg-0.7 Mn-xCu alloys, and T6 treatment can be used to reduce the content of Al₂CuMg phase distributed along the grain boundary and increase the content of solid solution Cu which will improve the strengthening effect or promote the precipitate of Al₂CuMg phase during the latter aging treatment. T6 might further improve both the strength and elongation of the alloy. Besides, the SEM-EDS analysis results show that the total quantity of the solid solution of atoms (Mg + Cu) is almost the same (variation between 4.87 wt% and 4.93 wt%) with the addition of Cu. Maybe it is possible to reduce the addition of Mg and increase the addition of Cu to promote the content of solid solution Cu, then both of the strength and the ductility of the alloy would be improved.

In general, the addition of Cu is an effective way to improve the properties of the die-casting Al-5.5Mg-0.7 Mn alloy, especially, conducting an aging treatment after die casting process. Certainly, some other work, like how to improve the content of solid solution Cu dissolved in the matrix, will continue to conduct to further improve the properties of die-casting Al-5.5Mg-0.7 Mn-xCu alloys.

4. Conclusions

In this work, the effects of Cu addition on the microstructures, tensile properties and fracture behaviors of the die-casting Al-5.5Mg-0.7 Mn alloy under as-cast and aging states were investigated. The conclusions are as follows:

(1) Additions of Cu lead to the formation of Al₂CuMg particles along grain boundaries in Al-5.5Mg-0.7 Mn alloy. The amount and size of Al₂CuMg particles remarkably increase with the increase of Cu content.

(2) Fine lath-shaped Al₂CuMg particles with a width of 2-4 nm and length of 15-20 nm precipitate in the Cu-containing Al-5.5Mg-0.7 Mn alloys aged at 175 °C for 8 h. The number density of the Al₂CuMg precipitates increases from 1.03 × 10¹⁴ to 1.78 × 10¹⁴ m^-2 with the increase of the addition of Cu from 0.5 to 1.5%.

(3) The additions of Cu cause the increase of the yield strength of the Al-5.5Mg-0.7 Mn alloy under the as-cast condition. Among them, the as-cast Al-5.5Mg-0.7 Mn-0.8Cu alloy exhibits comprehensive properties with a yield strength of 179 MPa, ultimate tensile strength of 303 MPa and elongation of 8.7%.

(4) Al-5.5Mg-0.7 Mn-xCu (x = 0.5, 0.8, 1.5) alloys aged at 175 °C for 8 h exhibited the YS, increased by about 8.8% (0.5Cu), 10.6% (0.8Cu), 10.4% (1.5Cu), respectively, in comparison with the as-cast counterparts. The Al-5.5Mg-0.7 Mn-0.8Cu aged at 175 °C for 8 h possesses the optimal comprehensive performance: the yield strength 198 MPa, tensile strength 305 MPa and elongation of 8%.

Acknowledgments

This work is supported financially by the National Key Research and Development Program of China (No. 2016YFB0301001). The authors are also grateful to Instrument Analysis Center in SJTU for experimental support.

The authors have declared that no competing interests exist.

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