Effect of Aluminium Content on the Tensile Properties of Mg-Al-Zn Alloys

The aim of this study is to investigate the effect of aluminium content on microstructure, tensile properties and work hardenability of Mg-Al-Zn alloys. Two types of magnesium alloys were investigated i.e. AZ31 and AZ61, where the aluminium contents were 3% and 6%, respectively. Microstructure observation revealed that higher aluminium content decreases the grain size and increases the volume of Mg17Al12 precipitations. From the tensile test, AZ61 demonstrated higher yield stress and tensile strength while maintaining the elongation as compared to AZ31. The work hardening rate for AZ61 was also greater as compared to that of AZ31.


INTRODUCTION
In recent years, magnesium alloys are widely used as structural materials in automotive industry due to the combination of low density, high strength properties and good machinability (Blawert et al. 2004;Gaines et al. 1995).Excellent mechanical properties of magnesium alloys have attracted engineers' attention to choose these alloys for engineering structures but so far the application in major load bearing components in which reliability, durability and safety are of major concern is still very limited.One of the most popular magnesium alloys used in automotive and industry are the Mg-Al-Zn alloy series i.e.AZ31, AZ61 and AZ90.These alloys are now becoming the premier choices for light weight application due to their improved mechanical properties with addition of certain percentage of aluminium and zinc (Marya et al. 2006).
Addition of aluminium as an alloying element in magnesium alloys is very important for high precipitation density of Mg 17 Al 12 phase.This β-Mg 17 Al 12 will act as strengthening phases that increase the tensile strength of magnesium alloys.Beside aluminium, the addition of zinc will also affect the strengthening of magnesium alloy (Cheng et al. 2009;Kainer 2003;Marya et al. 2006).However, the different composition of aluminium and zinc elements in Mg-Al-Zn alloy series could results in different mechanical properties especially the work hardenability and the tensile properties.In present study, effect of aluminium content on microstructure, tensile properties and work hardenability of constant Zn content in Mg-Al-Zn alloys were investigated.

EXPERIMENTAL PROCEDURE
The materials used in this study were extruded AZ31 and AZ61 of the Mg-Al-Zn alloy series.The two letters followed by two numbers designate the name of the magnesium alloys.The two letters tell the main alloying elements (A is for aluminium and Z is for zinc).The numbers tell the nominal compositions of main alloying elements respectively.Detail chemical compositions of AZ31 and AZ61 being used in this study are listed in Table 1.Both alloys were polished and etched to reveal their microstructure.They were then observed under the optical microscope.To obtain the mechanical properties of the magnesium alloys, tensile tests were performed on dumb-bell shaped specimens with a gage length and gage diameter of 10 mm and 3 mm, respectively.Tensile test was conducted using the Universal Testing Machine with a capacity of 100 kN at strain rate of 1 × 10 -3 s -1 in room temperature.Detail stress-strain responses were recorded using a data acquisition system.Further analyses of work hardening behavior for both alloys were performed and identifi ed by using the following equations ( 1) to (3).
where, σ is the true stress, F is the force and A T the area of cross section at the force, F and the true strain ε is defi ned as: where dL is the elongation and L O the original gage length.
The work hardening rate is given as: where dσ is the increment of true stress and dε the increment of true strain.
RESULT AND DISCUSSION

MICROSTRUCTURE
Figure 1 shows the microstructures of AZ31 and AZ61 observed under the optical microscope.From the fi gure, it shows that both alloys have an equiaxed grain structure.The average grain size of AZ31 and AZ61 were 24 μm and 15 μm as shown in Figure 2.  From Figure 1, it is clearly seen that the AZ31 has a larger grain size compared to that of the AZ61.Microstructure of AZ31 as shown in Figure 1(a) indicated more twinning, as indicated by the arrow, which probably due to the extrusion process while, less twinning for AZ61 as shown in Figure 1(b).Higher density of twinning was observed in AZ31 which had a larger grain size compared to that of the AZ61.Twinning formation is assumed to be resulted from the extrusion process as stated by Barnett (2007a) (Barnett 2007b).For AZ61 as in Figure 1(b), it is assumed that more precipitates in the alloy pinned the twinning formation so that less twinning was observed compared to that in AZ31.
The smaller grain size and higher density of Mg 17 Al 12 precipitation in AZ61 are believed to be caused by higher aluminium content that formed the solid solution in the alloy (Kainer 2003, St John et al. 2005).The Mg 17 Al 12 precipitation observed in AZ61 is shown in Figure 3. Sajuri (2005) mentioned that during the extrusion process of magnesium alloys, the casting defects will almost disappear when as-cast alloy heated to 350-400°C and squeezed through an extrusion die.SEM observation showed that after the extrusion process, the β-Mg 17 Al 12 phase in AZ61 alloy was almost completely dissolved and there was no casting defects presented in the matrix (Sajuri 2005).On the other hand, the existence of twinning in a material would also contribute to higher fl ow stress and better work hardening (Yablinsky et al. 2006).However, in most magnesium alloy series, fi ner grain size and Mg 17 Al 12 precipitation have more infl uenced to improve the strength, work hardening and ductility despite of the smaller grain size would be suppression the twinning (Barnett 2007;Khan et al. 2006).
precipitation (Khan et al. 2006, Ryu et al. 2000).The tensile properties of AZ31 and AZ61 were summarized in Table 2. Yield stress and ultimate tensile strength of AZ61 were higher than that of AZ31 with 14.7% and 36.3%, respectively.More work hardening of AZ61 could be recognised by the large increment of differences between yield stress and ultimate tensile strength in nominal stress-strain curve.On the contrary, AZ31 clearly possessed less work hardening compare to AZ61 (Latif et al. 2014).However, the elongation for both AZ31 and AZ61 were found identical.Figure 4 shows nominal stress-strain curves of AZ31 and AZ61 magnesium alloys.The result shows that AZ61 exhibited higher tensile strength and better work hardening as compared to that of the AZ31, which caused by the grain size reduction and Mg 17 Al 12 precipitation.Khan et al. (2006) and Ryu et al. (2000) have reported that the higher tensile strength was due to the effects of smaller grain size and higher density of Work hardening rate for AZ31 and AZ61 were clearly shows in Figure 5.It is noted that the work hardening rate for AZ61 was greater compared to that of AZ31 due to the existence of precipitation and fi ner grain size of alloy.These phase.The grain size refi nement and precipitation increases tensile strength and allow for more work hardening of the alloy due to more dislocation piles up and blocked at the grain boundaries before further deformation of the material.AZ61 exhibited better tensile properties than that of AZ31 with the yield stress and ultimate tensile strength of 14.7% and 36.3%, respectively.However, both AZ31 and AZ61 fractured in ductile manner with more dimples and microvoids in fracture surfaces.
factors have infl uenced to the dislocation piles up against the microstructure for delayed fracture.Work hardening rate for AZ61 was considered after yielding up to the ultimate tensile strength, while the hardening process for AZ31 was diminished after yielding.The diffi culty of deformation would be induced by the presence of dislocation density to allow more hardening of the alloy (Latif et al. 2014).

FRACTURE SURFACE
Figure 6 shows the fracture surface of AZ31 and AZ61.
Both alloys clearly show ductile fracture behaviour where many dimples observed on the fracture surface.Similar observation was reported by Marya et al. (2006).However, fracture surface of AZ61 as shows in Figure 6

FIGURE 2 .
FIGURE 2. Average grain sizes of AZ31 and AZ61

FIGURE 4 .
FIGURE 4. Nominal stress-strain curves of AZ31 and AZ61 Figure6shows the fracture surface of AZ31 and AZ61.Both alloys clearly show ductile fracture behaviour where many dimples observed on the fracture surface.Similar observation was reported byMarya et al. (2006).However, fracture surface of AZ61 as shows in Figure6(b) indicated more ductile fracture behaviour where more dimples and micro voids present in the fracture surface than that of AZ31 at Figure 6(a).

TABLE 1 .
Chemical composition of AZ31 and AZ61

TABLE 2 .
Tensile properties of AZ31 and AZ61 CONCLUSIONAluminium content in Mg-Al-Zn alloy series infl uenced the microstructure, grain size and precipitation of Mg 17 Al 12