Failure observation of the AZ31B magnesium alloy and the effect of lead addition content under ballistic impact

Abstract

The association of the failure behavior of the AZ31B magnesium alloy with absorption energy under ballistic impact condition was investigated in this study, as well as the effect of varying percentages of added lead (Pb) content. The compositions of Pb in AZ31B were 1%, 5%, and 10% by percentage weight of Pb. Two types of projectile were used to compare the experimental and simulation results. Ballistic testing on a magnesium alloy plate was performed at the velocity of 435 m/s for the 9 mm×19 mm Parabellum projectile and 976 m/s for the 5.56 mm×45 mm NATO projectile. Simulation was performed using the Johnson–Cook model to determine the maximum stress that the material can withstand. Deformation and failure occurred in the ballistic simulation when the different projectiles were used. In the 9 mm×19 mm Parabellum projectile, no penetration was observed in the magnesium alloy, whereas in the 5.56 mm×45 mm NATO projectile, complete penetration was observed throughout the thickness of the alloy. The addition of Pb on AZ31B affected the absorption energy, and the addition of 1% Pb produced the most suitable absorption energy for further analysis.

Keywords

Ballistic failure Johnson–Cook model lead magnesium alloy

Introduction

Many technologies require new materials that are lightweight but have high strength.¹ Magnesium-based alloys are of current interest to the US military industry because these alloys are the lightest among all structural metal alloys.² The density of magnesium is approximately 35% lower than that of aluminum and approximately 77% lower than that of steel.³ Magnesium alloy is the lightest metallic material that has high potential for weight reduction, thereby decreasing the amount of fuel used in automobile and aerospace applications. However, compared with several conventional materials, such as steel, fewer studies have been performed on the relationship between magnesium alloys and impact loading,⁴ particularly under ballistic condition.

Mukai et al.⁵ reported the dynamic behavior of ZK60 and AZ91 magnesium alloys and determined the dynamic absorption energy in magnesium alloy. Staroselsky and Anand⁶ developed an equation considering both the sliding and twinning deformation mechanisms of dynamic magnesium alloy. The high energy absorption efficiency of magnesium alloy confers excellent anti-penetration performance to the material.⁴ However, the most efficient energy absorption is achieved by adding high-density elements, such as lead (Pb), gold, rare-earth elements, and others.⁷ Magnesium alloy offers a unique combination of high tensile strength (up to 410 MPa), low density (up to 1.8 g/cm³), and superior shock absorbency 100 times greater than aluminum alloys.⁸ Magnesium also has the highest specific damping capacity among the metals used in armor applications. Thus, magnesium is an excellent choice for ballistic applications because of its enhanced energy absorption and shock mitigation. Previous studies on magnesium alloy characterization^4–6 indicate that the determined properties are appropriate for ballistic applications. Hence, this alloy is suitable for application in an armor plate to reduce armor weight, simultaneously increase the fuel consumption of armor vehicles, and provide ballistic resistance to these vehicles. Currently, the armor plate used in rolled homogeneous armor (RHA) is steel-based. Furthermore, replacing RHA with magnesium alloy offers a new alternative that solves problems in armor weight and fuel consumption while providing the same penetration resistance.

The AZ31B series from the magnesium alloy family was chosen in this study because AZ31B is normally used in aerospace and automotive applications.^5–7 AZ31B has good room temperature strength, ductility, corrosion resistance, and excellent weldability.⁷ This alloy is nonmagnetic and has high electrical and thermal conductivities; it is also used for radio-frequency interference and electromagnetic interference shielding in computer and electronic applications. AZ31B plate and sheet are used in applications that require medium strength at temperatures of <150 °C. The superplastic formation of AZ31B sheet at high temperatures can be used in complicated automotive parts. The “AZ” in AZ31B stands for aluminum and zinc, which are the two main alloying elements of the material. The numbers 3 and 1 indicate the percentage of aluminum and zinc (3% and 1%, respectively) rounded-off to the nearest whole number. The “B” in AZ31B is the second composition, which is registered with the American Society for Testing and Materials (ASTM).

AZ31B is a material that can potentially replace RHA in armor vehicles because of its impact behavior. However, the composition of AZ31B is not strong enough to support energy absorption. Pb is a material that exhibits high energy absorption. Therefore, Pb could be added to AZ31B to obtain higher energy absorption. The objective of this study is to investigate the failure behavior of magnesium alloy under ballistic impact and the effect of Pb composition on the absorption energy of magnesium alloys. The results demonstrated the deformation of magnesium alloy under ballistic and stress distribution. The addition of Pb provided the magnesium alloy material with good absorption energy.

Methodology

The methodology used for this study is shown in the flow diagram in Figure 1. The experiments were first conducted to characterize the material in terms of its properties and energy absorption under ballistic impact. The relevant properties of the material were used as input for the simulation model to determine its similarities to the experimental data. Based on the simulation procedure, the analysis results of material deformation and stress distribution were used to demonstrate the failure mode of AZ31B. The stress constriction theory was used for the failure analysis. The comparison of the experimental and the simulation results is discussed based on the deformation effect on AZ31B in ballistic conditions.

Figure 1.

Flow diagram of the methodology.

Experimental procedure

AZ31B was the magnesium-based alloy used in the experiment because this alloy series exhibits high energy absorption, especially for ballistic impact.⁸ Based on a previous study, Pb can enhance the energy absorption of magnesium alloy⁷ because the properties of Pb, such as high density and high ductility, enhance energy absorption. The composition percentage of AZ31B is shown in Table 1 and follows the standard ASTM B90/B90M-12:2012.⁹

Table 1.

Composition percentage element of AZ31B.⁹

	Composition %
	Al	Mn	Zn	Ca	Cu	Fe	Ni	Si	Each	Mg
AZ31B	2.5–3.5	0.2–1.0	0.6–1.4	0.04	0.05	0.005	0.005	0.10	0.30	Balance

Based on the original AZ31B composition, certain quantities of Pb were added to AZ31B with the following percentages: 1%, 5%, and 10% Pb. Several compositions were chosen based on a previous study.⁷ The effect of Pb addition on hardness and surface roughness was investigated. Pb was added to the sample in the given proportions via the disintegrated melt deposition (DMD) technique.¹⁰ The DMD technique employs higher superheat temperatures and lower impinging gas jet velocities, and the end product is bulk alloy only. Upon reaching the superheat temperature of 750 °C, the molten alloy melt was mechanically stirred using a mild steel impeller blade to facilitate the incorporation and uniform distribution of the reinforcement materials in the metallic matrix. The magnesium alloy sample that was used for the ballistic test is shown in Figure 2. The sample was 100 mm in length (L), 100 mm in width (W), and 25 mm in thickness (t). The important factor in the sample size is its thickness because the selected thickness of 25 mm is the standard thickness² of steel RHA used in armor plates.

Figure 2.

AZ31B sample used for the experimental ballistic impact.

Two experimental procedures were performed, that is, the ballistic test and material characterization. For the ballistic test, two types of projectile were used, namely, a 9 mm×19 mm Parabellum (used on magnum-type firearms) and a 5.56 mm×45 mm NATO (used on M16 rifles). The projectile consists of a jacket made of copper material and Pb-base filler core. The projectiles were chosen based on the National Institute of Justice (NIJ) standard¹¹ level IIIA. The level IIIA standard requires a maximum penetration of 44.0 mm into the sample. Figure 3 shows the two types of projectile that were chosen for the ballistic impact experiment.

Figure 3.

Types of projectile: (a) 9 mm×19 mm Parabellum and (b) 5.56 mm×45 mm NATO.

The distance between the test barrel and the plate was 5 m, which was chosen because of the NIJ standard requirement based on an experiment using handgun rounds. The angle of attack was normal or at 90 ° to the target. This angle will produce the maximum energy absorption of penetration.¹¹Figure 4 shows the schematic diagram and image of the experimental setup. The velocity of the projectile was detected using a high-speed velocity sensor. The actual velocities obtained during the tests were used as the input values for the simulation procedure.

Figure 4.

(a) Schematic diagram of the experimental setup according to the relevant guideline.¹¹ (b) Arrangement of the ballistic equipment during the test.

Upon completion of the ballistic test, the tested specimens were cut into several small sections for material characterization using the hardness test. The Rockwell hardness test was performed using a steel ball with a diameter of 6.35 m and at a total test force of 588.4 N, which was consistent with the standard of ASTM E18-12:2012.¹²Figure 5 shows the hardness tester machine used in the experiment.

Figure 5.

Rockwell tester used for the hardness characterization.

Simulation process

A specific simulation software package was used to develop a three-dimensional (3D) model for the ballistic tests. The 3D model shows the effect of ballistic impact on the AZ31B plate. Finite element analysis was used to accommodate the Johnson–Cook (JC) material model. The JC material model was used because it considers the damage evolution in the fracture and the thermal sorting effect in the material behavior. This model is commonly used for impact simulation.^13,14 The JC model is used to determine the strain rate and temperature dependence of visco-plastic material models. The JC model is represented by the following equation¹⁵

σ_{Y} = (A + B ε^{n}) (1 + Cln ε^{• *}) (1 - T^{* m})

(1)

where $σ_{Y}$ is the effective stress; $ε$ is the effective plastic strain; $ε^{• *}$ is the normalized effective plastic strain rate (typically normalized to 1.0 s⁻¹); n is the work hardening exponent; and A, B, C, and m are the material constants. Where A = 224 MPa, B = 380 MPa, n = 0.761, C = 0.012, and m = 1.554 followed previous study of AZ31B.¹⁶

The value of T* is defined by the following equation

T^{*} = \frac{T - T_{room}}{T_{melt} - T_{room}}

(2)

where T_room is the room temperature and T_melt is the melting temperature, which is also typically considered as the solidus temperature of the material being tested.

Table 2 shows the types of samples and the average velocity of projectiles recorded in the ballistic experiment. Figure 6 shows the simulation model obtained for both types of projectiles using the material composition values from Table 1. The velocities of the 9 mm×19 mm Parabellum and the 5.56 mm×45 mm NATO projectiles were 435 and 976 m/s, respectively. The simulation model of the 25-mm-thick AZ31B, AZ31B + 1% Pb, AZ31B + 5% Pb, and AZ31B + 10% Pb geometrical plate used the actual size and scale of the types of projectile. The aim of the authors is to develop a simulation model that can provide a good indication to determine the effect of different projectiles on the materials.

Table 2.

Specimen types and the recorded velocities of the projectiles.

Projectiles	Velocity (m/s)
Projectiles	Run 1	Run 2	Run 2	Average
9 mm×19 mm Parabellum	433	437	435	935
5.56 mm×45 mm NATO	970	979	973	976

Figure 6.

Geometric model used for the simulation procedure with different projectiles: (a) 9 mm×19 mm Parabellum and (b) 5.56 mm×45 mm NATO.

Results and discussion

Figure 7 shows the effects of ballistic impact on the specified magnesium alloy on experiment and simulation when the 9 mm×19 mm Parabellum projectile was used. However, no complete penetration was observed on the sample, although the area of projectile penetration varied among the types of magnesium alloy. As shown in Table 3, a smaller diameter of 13 mm was observed for AZ31B + 1% Pb, but the depth of penetration was greater in AZ31B. This result is attributed to the optimum composition of AZ31B + 1% Pb. Therefore, AZ31B + 1% Pb can absorb and effectively transfer the energy to other parts in the material. This process will affect the diameter size of penetration, which was not too large compared with the other compositions. However, the hardness of AZ31B + 1% Pb was less than that of AZ31B, which resulted in greater depth compared with AZ31B alone. In contrast, the 5.56 mm×45 mm NATO projectile completely penetrated the entire AZ31B as well as the respective Pb composition panels. The diameter size of the penetration was similar for all types of alloys at 7 mm for experiment and 6.5 mm for simulation. Figure 8 shows the front and rear views of the projectile penetration between experiment and simulation.

Figure 7.

Front view of the target plate after projectile penetration: 9 mm×19 mm Parabellum: (a) experiment and (b) simulation.

Table 3.

Projectile penetration results on experiment and simulation: (a) 9 mm×19 mm Parabellum and (b) 5.56 mm×45 mm NATO.

Specimen	Diameter (mm)				Depth (mm)
	Experiment		Simulation		Experiment		Simulation
	(a)	(b)	(a)	(b)	(a)	(b)	(a)	(b)
AZ31B	17	7	15	6.5	8	25	7.5	25
AZ31B + 1% Pb	13	7	10	6.5	10	25	9	25
AZ31B + 5% Pb	18	7	17	6.5	12	25	11	25
AZ31B + 10% Pb	17	7	16	6.5	11	25	9.5	25

Figure 8.

Front and rear views of the target plate after projectile penetration: 5.56 mm×45 mm NATO for experiment and simulation.

Figure 9 shows the schematic diagram of an indicator that was used to measure the effect of the ballistic impact, which is important to determine the shape of the penetration on the plate. The diameter and depth were measured because these parameters reflect the effect of absorption energy on the behavior of the material. Table 3 shows the results of the penetration using the indicator measurement shown in Figure 7. All types of magnesium alloy exhibited no penetration for the 9 mm×19 mm Parabellum projectile, whereas the plate exhibited complete penetration for the 5.56 mm×45 mm NATO projectile.

Figure 9.

Schematic diagram of (a) diameter and (b) depth of projectile penetration.

Table 3 shows the projectile penetration results on experiment and simulation for 9 mm×19 mm Parabellum and 5.56 mm×45 mm NATO. The correlation graph between experiment and simulation is shown in Figure 10. It was shown that the error was within the margin error which is 20% error. AZ31B + 1% Pb exhibited a smaller diameter size with the 9 mm×19 mm Parabellum projectile compared with the other compositions. However, the depth of penetration of AZ31B is smaller. The addition of Pb to AZ31B can be further explored because the optimized value for the added Pb is still not accurate. However, from this experiment alone, the addition of 1% Pb results in the optimal performance among the other compositions.

Figure 10.

Correlation graph simulation versus experiment.

Figure 11 shows the energy (J) respond versus time (μs) on magnesium alloys using 9 mm×19 mm Parabellum projectile. It can be seen that the resulting graph of response shows that the energy absorption of the magnesium alloys is uniform at 22 μs. Figure 12 shows the velocity (m/s) respond versus time (μs) on magnesium alloys using 9 mm×19 mm Parabellum projectile. It can be seen in the graph, the starting velocity at 435 m/s, a reduction of velocity up to 0 m/s, and the momentum side back cause negative velocity.

Figure 11.

Energy (J) respond versus time (μs) on magnesium alloys using 9 mm×19 mm Parabellum projectile.

Figure 12.

Velocity (m/s) respond versus time (μs) on magnesium alloys using 9 mm×19 mm Parabellum projectile.

Figure 13 shows the energy (J) respond versus time (μs) on magnesium alloys using 5.56 mm×45 mm NATO projectile. It can be seen that the resulting graph of response shows that the energy absorption of the magnesium alloys is uniform at 22 μs. Figure 14 shows the velocity (m/s) respond versus time (μs) on magnesium alloys using 5.56 mm×45 mm NATO projectile. It can be seen in the graph, the starting velocity at 976 m/s, a reduction of velocity up difference velocity depending on types of materials. However, it can been seen that the velocity was flat at 26 μs; it means the projectile was through the plate sample.

Figure 13.

Energy (J) respond versus time (μs) on magnesium alloys using 5.56 mm×45 mm NATO projectile.

Figure 14.

Velocity (m/s) respond versus time (μs) on magnesium alloys using 5.56 mm×45 mm NATO projectile.

Hardness and absorption energy analysis

The quality of defect of a projectile penetration depends on the hardness of the material,^17,18 the values of which were recorded using the Rockwell hardness test. The results of the hardness test are shown in Figure 15. AZ31B and AZ31B + 5% Pb exhibited high and low hardness, respectively. The percentage of Pb added in AZ31B decreased the hardness of AZ31B. In particular, 7.7% and 14.1% decrement of the original hardness of AZ31B was observed when 1% and 5% Pb were added, respectively. When the amount of Pb added was increased to 10%, the hardness decreased by 8.9%. The results of the hardness test show that Pb addition must not exceed 5% because the change in composition reduces the maximum hardness.

Figure 15.

Graph of the hardness of the different types of magnesium alloy.

Magnesium alloy has superior shock absorbency that is 100 times greater than aluminum alloy. However, the magnesium alloy is also less ductile. The addition of Pb to the magnesium alloy improves the ductility of the magnesium alloy. The ductility of the material is important to achieve maximum energy absorption.^19,20 The material properties of Pb improve energy absorption. Pb has a high density at 11.34 g/cm³. Thus, the addition of Pb to AZ31B, which has a lower density at 1.77 g/cm³, increases the energy absorption capacity of the material, especially for ballistic impact. The energy absorption theory indicates the following

w = \int_{0}^{ε} \frac{1}{2} \frac{σ^{2}}{E}

(3)

where $ε$ is the strain, $σ$ is the stress, and E is the elastic modulus of the material used. Equation (3) describes the energy absorption in the linear-elastic stage. Figure 16 shows the bar graph energy (J) of each material on different projectiles. Energy of AZ31B + 1% Pb gives the minimum energy which is 874 J for 9 mm×19 mm Parabellum projectile. But on the 5.56 mm×45 mm NATO, energy of AZ31B + 1% Pb at second higher which is 1584 J. Figure 17 shows the bar graph velocity (m/s) of end penetration for each material on different projectiles. It shows that the lowest velocity is AZ31B + 1% Pb which is 233 m/s compared to other materials using 5.56 mm×45 mm NATO projectiles. Figure 18 shows that the correlation graph between maximum energy (J) and time (μs) was projectile stop or through the plate samples.

Figure 16.

Bar graph energy (J) of each material on different projectiles.

Figure 17.

Bar graph velocity (m/s) of end penetration for each material on different projectiles.

Figure 18.

Correlation graph between maximum energy (J) and time (μs) was projectile stop or through plate samples.

Figure 19 shows the microstructure of materials’ research, 1% Pb AZ31B, 5% Pb AZ31B, 10% Pb AZ31B, and AZ31B. It can be seen from the graph analysis from Figures 16 to 18 may be described the effective absorption of energy from projectiles by 1% Pb AZ31B is because the Pb content in the structure. It can also be seen in the structure of Pb spots AZ31B + 5% Pb and AZ31B + 10% Pb which resulted in inconsistencies in the energy transfer. The addition of optimum Pb can increase the strength of the dislocation between the molecules of the material which is less on AZ31B. Pb content can enhance the absorption of molecular materials.

Figure 19.

Microstructure of sample in magnification of 1000 μm: (a) AZ31B + 1% Pb, (b) AZ31B + 5% Pb, (c) AZ31B + 10% Pb, and (d) AZ31B.

Based on the energy absorption and material deformation behavior, AZ31B is a potential material that can be used in ballistic applications. Furthermore, the properties of AZ31B can be improved with the addition of Pb to the original composition. Pb content could improve the ductility of the magnesium alloy and can confer protection against corrosion.⁷ Nonetheless, the addition of other elements that can improve the strength and ductility of the material should also be studied.

Conclusion

The effect of the amount of Pb added to the behavior of AZ31B alloy was investigated. The percentage of Pb added to AZ31B caused a difference in absorption energy. The optimum percentage of Pb that should be added to AZ31B is 1% Pb. The simulation results demonstrate that the energy that can be applied to AZ31B + 1% Pb with the 9 mm×19 mm Parabellum and 5.56 mm×45 mm NATO projectiles are 874 and 1584 J, respectively. Failure occurred in one-third of the material thickness when the 9 mm×19 mm Parabellum projectile was used. Energy absorption and hardness are important factors in controlling the diameter and depth of penetration, respectively. However, failure occurred because the energy applied was beyond the energy absorption. Thus, the use of magnesium alloy in ballistic applications is feasible, but several of its properties need to be improved. The percentage error between the experimental and simulation results is in error margin of 20%. Our results show that the magnesium alloy is a suitable material for ballistic and military applications. Furthermore, addition of an element in the original alloy composition enhances the durability characteristics and results in a difference in energy absorption and hardness.

Footnotes

Academic Editor: R Xu

Declaration of conflicting interests

The authors declare that there is no conflict of interest.

Funding

The authors would like to express their gratitude to Ministry of Higher Education Malaysia via Universiti Kebangsaan Malaysia and Universiti Pertahanan National Malaysia (Research funding: FRGS/1/2012/TK04/UPNM/0304 and LRGS/2013/UPNM-UKM/DS/04) for supporting this research.

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