Mustafa ÜBEYLİ

EFFECT OF AGING TREATMENT ON THE BALLISTIC BEHAVIOR OF ALUMINUM ALLOY 7075 AGAINST 7.62 mm ARMOR PIERCING PROJECTILES Teyfik DEMİR 1 , Mustafa ÜBEYLİ 1 and R. Orhan YILDIRIM 2 1 TOBB ETU Mechanical Engineering, Söğütözü Cad. No:43, Ankara-TURKEY E-mail: [email_address] 2 Middle East Technical University, Mechanical Eng., Ankara-TURKEY

INTRODUCTION EXPERIMENTAL PROCEDURE RESULTS AND DISCUSSION CONCLUSIONS

INTRODUCTION Selection of suitable armor materials for defense applications is very crucial with respect to increasing mobility of the systems as well as maintaining safety. Therefore, determining the material with the lowest possible areal density that resists the predefined threat successfully is required in armor design studies.

Steel is the most widely used armor material due to : its superior mechanical properties, availability, large technological database and relatively cheapness.

After steel, high strength aluminum alloys have also some potential to be used as armor due to relatively higher specific strength. Another potential candidate material is titanium alloys but their relatively high cost restricts their use significantly.

This study presents the ballistic behavior of aluminum alloys of 7075 and 5083 and HSLA steel, AISI 4140 against 7.62 mm armor piercing projectile. In addition to the variety of the alloys, effect of mechanical properties and areal density on the ballistic behavior of these alloys was investigated.

EXPERIMENTAL PROCEDURE Five different areal densities, namely 55, 70, 85, 100 and 115 kg/m 2 were selected to be used for each alloy to see the change in the ballistic resistance with thickness of the material and compare these materials.

Nomenclature for the specimens was used as shown below: Areal Density: 1) 55, 2) 70, 3) 85, 4) 100, 5) 115 kg/m 2 Heat Treatment: A) 38 HRC (for the AISI 4140) H111 (for the AA5083) TO: annealed (for the AA7075) B) 50 HRC (for the AISI 4140) and T651 (for the AA7075) C) 53 HRC (for the AISI 4140) and T7351 (for the AA7075) D) 60 HRC (for the AISI 4140) Material type: 1) AISI 4140, 2) AA5083, 3) AA7075 1A2

After that, various heat treatments were applied to the alloys, AISI 4140 and 7075 to get different mechanical properties. Table 1 gives the heat treatment procedure for these materials. On the other hand, the alloy 5083 was used as in received condition (H111).

A heat treatable aluminum alloy 7075 was received in two different heat treatment conditions, T651 (aged) and T7351 (overaged). Furthermore, a third heat treatment condition, namely annealing at 420  C was also applied to the 7075 to see the effect of mechanical properties on the ballistic behavior of this alloy more clearly

Table 1. Heat treatment procedures applied for the AISI 4140 and 7075 180 420 60 7075 Annealing Time (minute) Annealing Temperature (  C) Hardness (HB) 250 60 400 53 450 50 120 580 90 860 38 AISI 4140 Tempering Time (minute) Tempering Temp . (  C) Austenitizing Time (minute) Austenitizing Temperature (  C) Hardness Level (HRC) Material

Table 2 gives the thicknesses of the investigated materials corresponding to the areal densities mentioned above. After finishing the heat treatment processes, the standard mechanical tests, tension and hardness, were applied to determine important mechanical properties .

Table 2. Thicknesses of the materials corresponding to areal densities 3C 3B 3A 40 35 30 25 20 Thickness (mm) 2A 1D 1C 1B 14.4 12.7 10.8 9 7.2 Thickness (mm) 1A 115 100 85 70 55 Areal Density (kg/m 2 ) Specimen Group

Next, b allistic experiments were performed with A P projectiles of 7.62 x 51 mm in a ballistic laboratory. Five separate specimens for every specimen group were tested as each specimen group had five different areal densities. Targets were subjected to a single shot for every specimen and all tests were repeated five times.

The impact velocity of projectile was recorded to be 7 9 0  4 . 5 m/s in the experiments. Finally, macro observations were made on the samples to clarify their failure modes.

Fig. 1. A schematic view of the experimental setup

RESULTS AND DISCUSSION Any ballistic threat has to be stopped without perforation at the backing layer of the composite for safety. Therefore, t he samples, of which backing plates resisted to the projectile without perforation, were accepted as satisfactory.

Table 3 gives some important mechanical properites of the investigated alloys. There is a significant change in the strength and hardness level with respect to heat treatment conditions.

Table 3 . Some important mechanical properties of the investigated materials 13.0 505 435 1.2 150 3C 11.0 570 500 1.3 170 3B 17.0 230 105 1.5 60 3A 16.0 300 190 1.4 85 2A Ductility (% Elongation) Tensile Strength (MPa) Yield Strength (MPa) Standard Dev . in Hardness Hardness (HB) 8 .0 1800 1500 1.3 60 1D 12.7 1640 1400 1.1 53.4 1C 13.8 1570 1250 0.9 49.9 1B 17.0 1450 1150 0.9 37.8 1A Ductility (% Elongation) Tensile Strength (MPa) Yield Strength (MPa) Standard Dev . in Hardness Hardness (HRC) Specimen Group

Ballistic performance of the investigated alloys was evaluated by considering the probability of non- perforation of these alloys out of 5 shots which is given in Table 4 . The specimen groups; 2A and 3A, were fully perforated at all areal densities since they did not resist the impact of the projectiles efficiently due to their lowest hardness and strength among the investigated specimen groups.

Table 4 . Probability of the non-perforation of the specimen groups a Specimen was not penetrated but broken into several pieces. b Specimen was successful but rear surface had some cracks. 100 b 100 b 0 0 0 3C 100 100 100 0 0 3B 0 0 0 0 0 3A 0 0 0 0 0 2A 80 a 0 a 0 a 0 a 0 a 1D 100 100 0 a 0 a 0 1C 0 a 0 a 0 0 0 1B 0 0 0 0 0 Performance (%) 1A 115 100 85 70 55 Areal Density (kg/m 2 ) Specimen Group

Figure 2. Macro views of the front (at top of the Figure) and rear (at bottom of the Figure) of the 3A having the areal density of a) 55 kg/m 2 , b) 70 kg/m 2 , c) 85 kg/m 2 , d) 100 kg/m 2 and e) 115 kg/m 2 after the ballistic impact.

Figure 3 . Macro cross-sectional view of the specimen 3A5 through the hole created by the projectile after the ballistic testing. Steel core of the projectile perforated the sample easily by leaving its brass jacket at nearly 10 mm away from the impact point. Brass jacket of the projectile

On the other hand, the specimen group 3B was satisfactory at an areal density  85 kg/m 2 , whereas 3C samples provided the full ballistic protection for an areal density  100 kg/m 2 . Figure 4 represents the cross sectional view of the specimens 3B3, 3B4 and 3B5 through the centerline of the impact point after the ballistic testing.

Figure 4. Macro view of the cross-section of the specimens a) 3B3, b) 3B4 and c) 3B5 through the centerline of the impact point after the ballistic test. It can be seen that the penetration depth of the projectile was almost the same for the 3B samples having different thicknesses. (Scale was given in mm).

For the steel AISI 4140, the samples having a hardness of ~ 38 HRC were perforated by the projectiles via forming a ductile hole as seen in the failed aluminum alloy specimens (Figures 5 and 6 ). When its hardness was increased to ~ 50 HRC, the brittle fracture was observed for the specimens at all investigated areal densities (Figure 7 ).

Figure 5. Photographs of the specimen group 1A with a) 55 kg/m2, b) 70 kg/m 2 , c) 85 kg/m 2 , d) 100 kg/m 2 and e) 115 kg/m 2 after the ballistic impact. All samples failed under the impact of AP projectiles. a) b) c) d) e)

Figure 6. Ductile hole formation in the specimen of 1A5. The projectile inlet and outlet regions were shown as in the circles of A and B, respectively. Petalling was observed only at the rear side of the specimen.

Figure 7. Photographs of the 1B specimens having an areal density of a) 55 kg/m 2 , b) 70 kg/m 2 , c) 85 kg/m 2 , d) 100 kg/m 2 and e) 115 kg/m 2 after the ballistic impact. All samples fractured into several pieces in a brittle manner. Radial fractures on the samples occurred. a) b) c) d) e)

Among the steel specimen groups, the best performance was found for the 1C having a hardness of ~ 53 HRC. As shown in Figure 8 , the samples 1C4 and 1C5 withstood the impact of the AP projectile successfully and maintained the full ballistic performance. Moreover, there were no cracks observed on these specimens. When the hardness was increased to 60 HRC, the specimens were fractured into several pieces depending on the areal density (Figure 9 ).

Figure 8. Front views of the 1C specimens with a) 55 kg/m 2 , b) 70 kg/m 2 , c) 85 kg/m 2 , d) 100 kg/m 2 and e) 115 kg/m 2 after the impact of the projectile. The samples 1C4 and 1C5 achieved to stop the AP projectiles whereas the others failed. a) b) c) d) e)

Figure 9. View of the specimen 1D failed by fracturing into several pieces under the impact of the AP projectiles. The number of fractured pieces decreased when its areal density was increased from a) 55 kg/m 2 , b) 70 kg/m 2 , c) 85 kg/m 2 , d) 100 kg/m 2 and e) 115 kg/m 2 .

CONCLUSIONS Among the investigated materials, the best performance was attained with the aluminum alloy 7075-T651. Increase in the hardness of the aluminum alloys led to increase in the resistance to the projectiles effectively. On the other hand, the best performance in the steel, AISI 4140 was achieved for a hardness level of ~ 53 HRC. Although the increase in the hardness of the steel improved its ballistic behavior, the steel specimens having either 50 or 60 HRC were broken in a brittle manner rather than perforation by the projectiles. The aluminum alloy 7075-T651 could provide at least a reduction in the weight of armor by ~ 25 % in comparison to RHA of 380 HB

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