Effect of Ce on microstructure, mechanical properties and corrosion behavior of high-pressure die-cast Mg–4Al-based alloy
Research highlights
▶ High-pressure die-casting was carried out on Mg–4Al–xCe–0.3Mn alloys. ▶ Fine microstructure was obtained due to extremely high cooling rate and constitutional supercooling effect of Ce and Al. ▶ Al–Ce strengthening phases were characterized in detail. ▶ Optimized alloy composition and improved mechanical properties of HPDC Mg–4Al–xCe–0.3Mn alloys were obtained.
Introduction
Over the last decade, the requirement to reduce the weight of car components as a result of legislation limiting CO2 emission has created a renewed interest in magnesium (Mg) alloys [1]. Auto manufacturing companies have reduced vehicle weight by the use of magnesium die castings, which account for about 30% of the total reported consumption of Mg alloys [2]. The high-pressure die-cast (HPDC) Mg–Al-based alloys like AZ91D and AM60B are widely used in non-critical parts such as valve covers and instrument panels due to their excellent combination of die castability, room-temperature mechanical properties and corrosion resistance [3], [4]. However, the applications of these alloys are still limited because of their poor creep resistance at temperatures above 125 °C [5], [6].
In recent years, improving the elevated temperature properties has become a critical issue for the potential applications of Mg alloys in major powertrain applications [7], [8]. It has been reported that the elevated temperature properties of Mg–Al alloys can be improved by the addition of elements such as RE [9], [10], [11], [12], [13], [14], [15], Ca [2], [6], [16], Sr [2], [17], Si [4], [5], [18], Sn [5] and Sb [4], [18]. Among them Mg–Al–RE system is a major development in heat-resistant Mg–Al-based alloys. AE42 (Mg–4Al–2RE, wt.%), a benchmark heat-resistant HPDC Mg alloy, exhibits major improvement in creep resistance, which is commonly thought to arise from the suppression of Mg17Al12 phase due to the preferential reaction of Al and RE to form the highly thermally stable Al11RE3 and Al2RE intermetallic phases [19]. However, the deterioration in creep resistance of AE42 at temperature above 150 °C was also reported, and the related mechanism was still in dispute. Powell et al. [20] mainly attributed the poor creep resistance of AE42 at 175 °C to the instability and partial decomposition of Al11RE3 phase. While the recent work by Zhu et al. [21] showed that both Al2RE and Al11RE3 phases were stable at temperature up to 200 °C, with no decomposition observed even after 2 weeks. According to their work, continuous precipitation of Mg17Al12 due to the supersaturation of Al solute in Mg matrix was responsible for the loss of creep resistance. Recently, a new HPDC alloy, AE44 (Mg–4Al–4RE, wt.%), was developed by Hydro Magnesium. The heat-resistance of AE44 alloy is further improved due to the higher RE content [21].
The typical composition of Ce-rich mischmetal added in AE42 and AE44 is 52–55 wt.% Ce, 23–25 wt.% La, 16–20 wt.% Nd, and 5–6 wt.% Pr [20], [22]. The microstructures and performances of these HPDC alloys containing Ce-rich mischmetal are determined by the combined action of above all RE elements, thus it is difficult to confirm the effect of single RE element in these alloys. Wang et al. [12] reported that addition of Ce could improve the microstructure and enhance the mechanical properties of as-cast and hot-rolled Mg–5Al–0.3Mn alloy. The influences of single La, Pr and Nd on the microstructure and mechanical properties of HPDC Mg–4Al-based alloy have been studied, and the results indicate that the behavior of these alloys could be affected by the kind of RE elements to a certain extent [13], [14], [15]. In order to further confirm the effects of different RE elements in HPDC Mg–4Al-based alloy, in this work, the microstructure, mechanical properties and corrosion behavior of HPDC Mg–4Al–xCe–0.3Mn (x = 0, 1, 2, 4, 6 wt.%) alloys were investigated in detail.
Section snippets
Experimental procedures
The nominal compositions (wt.%) of investigated alloys were Mg–4Al–0.3Mn, Mg–4Al–1Ce–0.3Mn, Mg–4Al–2Ce–0.3Mn, Mg–4Al–4Ce–0.3Mn and Mg–4Al–6Ce–0.3Mn, while the reference alloy was commercial AE44 containing Ce-rich mischmetal. Commercial pure Mg and Al were used, Mn and Ce were added in the form of Al–10 wt.% Mn and Mg–20 wt.% Ce master alloys, respectively. Specimens were produced using a 280 tonnes clamping force cold chamber die-cast machine. The metal was hand-ladled into the die casting
Microstructure
Fig. 1 shows the representative tensile and creep test bars obtained from the HPDC AlCe44 alloy castings. It can be observed that the die-cast samples do not present any macroscopic defect on the surface, a result that partly confirms the good die-castability of these Mg–Al–Ce–Mn alloys. The metallographic samples were cut from the middle portion of the tensile test bars.
Fig. 2(a) shows the typical cross-section microstructure of the HPDC AlCe44 alloy. A narrow band that follows a contour
Conclusions
The microstructure, strengthening phases, mechanical properties and corrosion behavior of high-pressure die-cast Mg–4Al–xCe–0.3Mn (x = 0, 1, 2, 4 and 6 wt.%) alloys have been investigated. The main conclusions can be drawn as follows:
- (1)
The Mg–4Al–xCe–0.3Mn alloys exhibit an excellent die-castability. A narrow band with a fine structure divides the cross-section of test bar into the fine skin region and the relatively coarse interior region.
- (2)
With addition of Ce, the grains of Mg–4Al–0.3Mn alloy are
Acknowledgements
This work was supported by the National Natural Science Foundation of China (no. 50871033), the Key Project of Science and Technology of Harbin City (2008AA4CH044, 2009AA1AG065, 2010AA4BE031), the Fundamental Research funds for the Central Universities (HEUCF101001), the Heilongjiang Postdoctorial Fund (LBH-Z09217) and China Postdoctoral Science Foundation (20100471015).
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