In the past 20 years, application of magnesium alloy castings in automotive industry has been growing rapidly, and they are mainly produced by die casting. Traditional die-cast magnesium alloys are mainly AM60B and AZ91D alloys. Mg-Al alloy has excellent die-casting properties, mechanical properties and corrosion resistance. However, due to its low yield strength and poor creep resistance, it is difficult to use it at higher temperatures, especially at temperatures above 150℃. This is because, firstly, intermetallic compound Mg17Al12 softens at high temperatures, making sliding of grain boundaries easier; secondly, discontinuous precipitation of Mg17Al12 at grain boundaries makes it more difficult to improve mechanical properties through heat treatment.
Compared with Mg-Al alloys, Mg-Zn alloys exhibit obvious age hardening effects, which makes it possible to improve their mechanical properties through heat treatment. Strengthening precipitation phases in Mg-Zn alloys are mainly β1′ and β2′ phases. Both β1′ and β2′ are Laves phases, composition is approximately MgZn2, and they have a hexagonal structure (a=0.520nm, c=0.857nm); rod-shaped β1′ phase is considered to be transition phase of MgZn′, and its length direction is parallel to direction of α-Mg matrix, while disc-shaped β2′ phase is parallel to basal surface of α-Mg matrix.
Mg-Zn alloys exhibit high yield strength under T6 conditions. However, hot cracking is prone to occur during casting process, especially under die-casting conditions. Although numerous studies have proposed a large number of mechanisms related to thermal cracking, thermal cracking is still a complex solidification phenomenon that is not fully understood. Hot cracking tendency of Mg-Zn binary alloy initially increases with increase of Zn content, but when Zn content reaches 1.5%, hot cracking tendency decreases again. Increasing preheating temperature of mold can reduce tendency of hot cracking because the higher preheating temperature of mold, the smaller cooling rate. This study mainly studies effect of Al content on hot cracking behavior and microstructure of Mg-10Zn alloy, aiming to provide a reference for development of high-strength, low-cost die-cast magnesium alloys.

Mg-10Zn-yAl (y=0, 2, 3.5, 5, mass fraction) alloy was prepared using commercially available high-purity Mg (>99.9%), Al (>99.9%) and Zn (>99.9%). Melt 40kg of Mg raw material in a low carbon steel crucible and pass it through a protective gas (volume fraction of 95% N2 + 0.5% SF6). A BD-350V5 3500kN cold chamber die-casting machine is used. Mold preheating temperature is 200℃, low speed is 0.25m/s, high speed is 4m/s, pressure is 70MPa, and pouring temperature is 690℃. Schematic diagram of a die-casting part with a pouring system is shown in Figure 1. It consists of 4 different test rods (diameter 5~17mm) and a flat plate (thickness 2.5mm), see Figure 2. It can be seen from shape of mold that test bar A is the most difficult to fill during die-casting process and has the greatest possibility of hot cracking. From A to E, tendency of hot cracking gradually decreases. In order to make difference in hot cracking of different castings more obvious, a hot cracking factor (Frod) is set for each test bar, where FA=2, FB=4, FC=8, FD=16, FE=32; A has the smallest hot cracking factor because it is more likely to crack.HTS=∑ (Qtear×Frod) is used to evaluate hot cracking tendency of die-casting alloys, where Qtear refers to number of cracks on each test bar.

 

Figure 1 Schematic diagram of die-cast formed parts used for HTS testing

 

Figure 2 Test rod size

 

Figure 3 Die-cast Mg-10Zn-yAl alloy test rod
(a)Mg-10Zn (b)Mg-10Zn-2Al
(c)Mg-10Zn-3.5Al (d)Mg-10Zn-5Al

 

Figure 4 HTS value of Mg-10Zn-yAl alloy
Figure 4 is hot cracking susceptibility factor of alloy. It can be seen that Mg-10Zn alloy has the largest hot cracking tendency value, and Mg-10Zn-5Al alloy has the smallest hot cracking tendency value. As Al content increases, hot cracking tendency value of Mg-10Zn-yAl alloy becomes smaller and smaller. This trend also appears in other aluminum-containing magnesium alloys. It can be seen that Al can effectively improve hot cracking tendency of Mg-Zn alloy.

 

Figure 5 T-Fs curve of Mg-10Zn-yAl alloy (Pandat8.1Scheil model)

Table 1 Calculated values of Fl and FTR

 

Figure 6 Relationship between HTS, Fl and FTR
Figure 7 is an SEM image of Mg-10Zn-yAl alloy. Table 2 shows second phase volume fraction and stoichiometric ratio of Mg-10Zn-yAl alloy. It can be seen from Table 2 that volume fraction of second phase continues to increase as Al content increases due to elimination of feeding and thermal cracking. In addition, it can be seen from Figure 7 that second phase transforms into an arc shape and forms a network structure. In ZA105 (Mg-10Zn-5Al) alloy (see Figure 7d), maximum size of second phase exceeds 10 μm, which may form a crack source during deformation process. However, it has been confirmed that arcuate second phase in magnesium alloys is beneficial to eliminate hot cracking.

 

Figure 7 SEM image of cast Mg-10Zn-yAl alloy

Table 2 Second phase volume fraction and stoichiometric ratio of Mg-10Zn-yAl alloy

 

Figure 8 XRD pattern of cast Mg-10Zn-yAl alloy

 

Figure 9 EDS results of the ZA series alloy marked in Figure 6

(1) Mg-10Zn alloy has the largest hot tearing tendency (HTS) value, and Mg-10Zn-5Al alloy has the smallest HTS value. HTS value decreases with increasing aluminum content. Thermodynamic calculations show that Fl value increases with increase of Al content, HTS and Fl are negatively correlated.
(2) Volume fraction of second phase in Mg-10Zn-yAl alloy increases with increase of Al content. Second phase in Mg-10Zn is Mg5Zn2. A quasi-crystalline phase was found in cast Mg-10Zn-yAl(y=2, 3.5, 5) alloy, and its composition is Mg62~69Zn22~25Al6~16.
(3) Eutectic reaction in Mg-10Zn-yAl (y=2, 3.5, 5) alloy is L→α-Mg+I-Mg62～69Zn22～25Al6～16, and eutectic reaction in Mg-10Zn is L→α-Mg+Mg5Zn2.