Microstructures and mechanical properties of AZ91D magnesium alloy processed by low pressure die casting

JIANG Hai-yan(蒋海燕), FU Peng-huai(付彭怀), YU Yan-dong(于彦东), ZHAI Chun-quan(翟春泉)

National Engineering Research Center of Light Alloys Net Forming,

Shanghai Jiao Tong University, Shanghai 200030, China

Received 28 July 2006; accepted 15 September 2006

Abstract: AZ91D alloy components were cast by low pressure die casting (LPDC) process. The mechanical properties of cast components with different microstructural features (shrinkage and distribution of Mg₁₇Al₁₂ second phase) were investigated under as-cast states. Compared with gravity casting, AZ91D with LPDC has much coarser grain size and second phases(Mg₁₇Al₁₂ and Al₈Mn₅). The different size and distribution of Mg₁₇Al₁₂ phase and shrinkage correspond to different mechanical properties. The ultimate tensile strengths and elongations are mainly decided by the content and distribution of shrinkage porosity, while the yield strengths are determined by the percentage and distribution of Mg₁₇Al₁₂ phase. The more and finer Mg₁₇Al₁₂ phase in the alloy, the relatively higher the yield strengths are. In the alloy without shrinkage, the mechanical properties are mainly determined by the size and distribution of Mg₁₇Al₁₂ phase. The finer Mg₁₇Al₁₂ phase, the better the mechanical properties are. Under optimal process, the density and mechanical properties of LPDC AZ91D are improved with fine microstructures.

Key words: AZ91D; magnesium alloys; low pressure die cast; microstructure; porosity; mechanical property

1 Introduction

Magnesium alloys represent unique structural materials combining high specific strength with the capability to absorb shock and vibration energy. Cast magnesium alloy AZ91D is most widely used in aircraft and engine building industries due to its high castability, corrosion resistance and mechanical properties[1-2]. Properties of castings depend on the microstructure of the material; however, fundamental factors of microstructural formation and evolution during different casting methods are still not fully understood. Magnesium alloys are mainly used as die casting components, which scatter in fatigue and monotonic properties for relative defects[3-6], such as gas porosity and oxide film. Low pressure die casting (LPDC) is one process that can obtain compact structural part because of feeding well during solidification under low pressure, with little porosity and without knit lines for stable filling process[7]. It is believed that the cost of low-pressure casting is lower than that of die-casting and the process provides better quality than gravity casting. In spite of so many advantages, the LPDC process has not yet been fully appreciated and used widely. The main problem is the lack of detailed understanding of the process. There are plenty of literatures on the microstructures and mechanical properties about AZ91D magnesium alloy die casting[3-6], but few documents about low pressure die casting of magnesium alloys[8-9], which did not mention the mechanical properties and relationship between microstructure and corresponding properties. It is necessary to evaluate the magnesium alloy’s mechanical behavior under LPDC process for future use.

In the present study, the effects of microstructural features (the distribution of porosity and Mg₁₇Al₁₂ phase) on the mechanical properties of LPDC cast AZ91D Mg alloy are examined. The microstructure and mechanical properties under optimal LPDC process are also investigated.

2 Experimental

Commercial magnesium alloy AZ91D was used in the experiments. The chemical compositions of the experimental alloy are listed in Table 1. Alloys were melted in an electrical furnace using a mild steel crucible under a protection of mixture gas, SF₆, CO₂ and air, and were refined at about 750 ℃ using magnesium alloy refine flux. After holding for 30 min, when the temperature of alloy liquid dropped to the designed value, it is cast into the parts using LPDC process. The optimal process parameters are listed in Table 2. During the casting process, mixture gas of SF₆, CO₂ and air was used as pressurized gas to force the liquid metal to rise in the tube. Gravity cast AZ91D alloy with a smaller block-shaped mould[10] under the condition of pouring temperature of 710 ℃ and die temperature of 150 ℃ was also produced for comparison.

Table 1 Chemical compositions of AZ91D alloy (mass fraction, %)

Table 2 Optimal process parameters of LPDC AZ91D alloy

Tensile samples were taken from the casting parts under different processes with different microstructures. Tensile tests were performed in a Zwick/Roell material test machine at ambient temperature. Microstructure analysis was performed using optical microscope and samples were taken from the middle of castings. The densities of the alloy were measured using Achimedes principle. Density samples were taken from the middle position on the wall of cast parts.

3 Results and discussion

3.1 Microstructural features and corresponding mechanical properties

The alloy’s microstructure is mainly determined by fundamental factors. The different processes would produce different microstructures and different mechanical properties. The microstructural features under different LPDC process are: the distribution of shrinkage porosity, and the size and distribution of Mg₁₇Al₁₂ phases (grain size). The coarse grains are always concomitant with rough Mg₁₇Al₁₂ phases, so here only Mg₁₇Al₁₂ phase is considered.

Fig.1 and Table 3 show different distributions of porosity and the corresponding mechanical properties of LPDC as-cast AZ91D alloy. From the microstructures in Fig.1, two kinds of shrinkage can be found. One is larger single shrinkage void(Figs.1(a) and (b)), and the other is smaller scale shrinkage (interdendritic)(Figs.1(c) and (f)). Clearly, both of the two kind shrinkages will reduce the mechanical properties. As for the single shrinkage void, its size determines its harmfulness to the mechanical properties. The larger the single shrinkage voids are, the more harmful to mechanical properties (samples a, b in Table 3). Here mechanical properties refer to ultimate tensile strength (UTS) and elongation. Yield tensile strength (YTS) is little affected by the distribution of both kinds of shrinkage (Table 3). Larger shrinkage (Fig.1(a)) may correspond to higher YTS (sample a). When comparing the microstructure and YTS in Figs.3(a), (b) and samples a, b in Table 3, it appears that the higher YTS corresponds to finer and higher concentration of Mg₁₇Al₁₂ phases under similar density. As for the smaller scale shrinkage, its scale will determine its detriment. The more serious small scale shrinkage (Figs.1(c)-(f)) is, the larger the reduction of mechanical properties (samples c, d, e and f in Table 3). When comparing the microstructure and YTS in Figs.1(e), (f) and samples e and f, it appears that the higher YTS does not correspond to finer and high concentration of Mg₁₇Al₁₂ phases if the difference of density is large. Higher density can also lead to higher YTS.

Table 3 Mechanical properties and density corresponding to microstructures in Fig.1

Fig.2 and Table 4 show different sizes and distributions of Mg₁₇Al₁₂ phases and the corresponding mechanical properties of LPDC as-cast AZ91D. There is no shrinkage porosity in the microstructure and the effect of distribution of Mg₁₇Al₁₂ phases is more clear. With homogeneous distribution and fine Mg₁₇Al₁₂ phase (Fig.2(a)), the alloys have better YTS, UTS and elongation (sample a in Table 4). On the other hand, with heterogeneous distribution and coarse Mg₁₇Al₁₂ phase (Fig.4(b)), the alloys have worse YTS, UTS and elongation (sample b in Table 4). Finer Mg₁₇Al₁₂ phase leads to higher mechanical properties. As Mg₁₇Al₁₂ phases are barriers to the dislocations motions[3], finer Mg₁₇Al₁₂ phase would be more likely to generate more serious concentration of dislocation, which leads to higher strength.

Table 4 Mechanical properties and density corresponding to microstructures in Fig.2

Fig.1 Optical micrographs of different sizes and distributions of shrinkage porosity of LPDC AZ91D alloy: (a) and (b) Larger single shrinkage void; (c)-(f) Smaller scale shrinkage

Fig.2 Optical micrographs of different size and distribution of Mg₁₇Al₁₂ phases of LPDC AZ91D alloy without shrinkage

3.2 Microstructure and mechanical properties of as-cast AZ91D by optimal LPDC process

Under optimal LPDC process, the microstructure of AZ91D compared with that of gravity casting alloy is shown in Fig.3. The white phases are Mg₁₇Al₁₂ and the black ones are Al₈Mn₅[9] as indicated. Phase content of Mg₁₇Al₁₂ in LPDC process reduces, but the size of single particle is not decreased. The average grain size under LPDC process is 150-200 μm, similar to that of gravity casting(Fig.3(c)). The eutectic structure is large and like that of sand casting. The Mg₁₇Al₁₂ phase and grain size depend on process parameters in LPDC process which is not discussed here. The mechanical properties under the optimal process are: σ_0.2=92.15 MPa, σ_b=180.4 MPa, and δ=3.42%. Compared with these of gravity casting σ_0.2=82.73 MPa, σ_b=178.35 MPa, δ= 3.59%，YTS in LPDC increases by about 10 MPa, with comparable UTS and elongation. The density of AZ91D alloy under LPDC process is ρ=1.815 5 g/cm³, with increase of 0.302% based on gravity casting, and is much more increased than that of high pressure die casting[4].

Fig.3 Optical micrographs of AZ91D alloy: (a) and (b) Under optimal LPDC process: (c) Under gravity casting process

4 Conclusions

1) Under LPDC process, shrinkage is the key factor to determine the UTS and elongation, while the YTS is mainly decided by the size and distribution of Mg₁₇Al₁₂ second phases. In the alloy without shrinkage, the mechanical properties are mainly determined by the distribution of Mg₁₇Al₁₂ second phase.

2) Compact AZ91D castings are produced under optimal LPDC process. Compared with gravity casting, YTS is increased by about 10 MPa, with comparable UTS and elongation.

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(Edited by YANG Bing)

Corresponding author: FU Peng-huai; Tel: +86-21-28527966; E-mail: fph112sjtu@sjtu.edu.cn