Indian Journal of Engineering & Materials Sciences Vol. 12, June 2005, pp. 221-226

Ballistic impact testing of AA2219 aluminium alloy welded plates and their metallurgical characterisation

Abhay K Jha,* Naga Shiresha, S V S N Murty, V Diwakar & K SreeKumar Material Chracterisation Division, Vikram Sarabhai Space Centre,

Indian Space Research Organisation (ISRO), Trivandrum 695 022, India

Received 24 September 2004; accepted 2 February 2005

Ballistic impact experiments were conducted on 6.4 mm and 7.8 mm thick aluminium alloy AA 2219 sheets using cylindrical ballistic projectiles made of hard steel. The impact velocities varied between 380 ms-1 and 890 ms-1 and flash infra red technique was used to determine projectile velocity and to assure the normality of impact. The microstructural damage associated with impact was analyzed by optical and scanning electron microscopy (SEM). The microstructure consisted of adiabatic shear bands formed in an extremely short time by the combined effects of the highly localized shear deformation and the high temperature rise that occurred within the shear bands. Scanning electron microscopy revealed features of typical melting within the band.

IPC Code: C22C 21/00

The AA 2219 aluminium alloy is being used to fabricate liquid propellant tanks for satellite launch vehicles. These tanks have to be subjected to various functional tests. During the functional tests, one such tank failed catastrophically at the test stand. Evidences collected during analysis of failed tank confirmed failure occurred due to high strain rate. However, the source for high strain rate could not be established. Even though the phenomenon of high strain rate deformation of materials is extensively studied, literature on the behaviour, especially on aluminium alloy AA2219 material under these conditions is scarce. This has prompted us to conduct detailed investigations on the high strain rate behaviour of AA2219 parent material as well as welded joints. The aim of the present investigation was to conduct very high strain rate deformation of AA2219 (of the order of >104 s-1) and to carry out detailed metallographic investigation of the tested coupons. This paper highlights the details of experiment carried out and the salient observations at microscopic level on the tested coupons. Experimental Procedure Materials AA 2219 Al. alloy with a nominal composition (wt%) of Al 6.3Cu 0.3Mn 0.1Zr 0.06Ti welded sheets

of sizes 200 × 200 × 6.4 mm (two nos) and 200 × 200 × 7.8 mm (two nos.) in T87 condition were subjected to ballistic impact test. The welding process used was automatic alternating current tungsten inert gas (Auto ac -TIG). Welding was carried out across the rolling direction of the plate in single pass with welding parameters (current 340-350 A and voltage 15 V). Welding filler used was AA2319 of 1.6 mm dia with nominal composition (wt%) Al 6.1Cu 0.3Mn 0.15Zr 0.15Ti. The mechanical properties were evaluated as per ASTM B557 and the values obtained were, UTS 260 MPa, YS- 160 MPa, %El (50 mm GL) 6 for welded joints, while that of parent material were UTS 450 MPa, YS 360 MPa, %El (50 mm GL) 14.

Method The ballistic impact experiments were carried out at room temperature. Experiments were conducted such that the root side of the weldment always faced the projectile. This was necessary to simulate the condition of tank during its failure. Four different types of guns, hereafter referred as source ‘A’, source ‘B’, source ‘C’ and source ‘D’ were used for testing. The velocities of the projectiles measured were in the range 380-800 ms-1. The principle of measurement involved allowing the projectile to interrupt two infra red (IR) signals at 2 m distance apart. The schematic of test range arrangement is shown in Fig. 1. The diameter of the projectiles were 9 mm (source 'A'),

__________ *For correspondence

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7.02 mm (source 'B'), 5.56 mm (source 'C') and 7.62 mm (source 'D'). Each welded plate was pierced by two different types of projectiles, both on the weldment and the parent material. The distance between the projectile and target was maintained at 10 m. Photographs of ballistic impact tested plates are shown in Fig. 2. Experimental parameters are given in Table 1. Results and Discussion Visual observations revealed that in case of source 'A' , the projectile in speed range of 390-420 ms-1, could penetrate only partially. This has resulted in

absorption of energy within the material and caused the material to flow in the vicinity of impact. Material flow lines resulted due to localized strain by the projectile impact (partial penetration) on the parent material are shown in Fig. 3. However, for the same partially penetrated projectiles, no cracks were observed on the other side of the sheet (Fig. 4). Source 'A' projectile, while making partial penetration

Table 1—Details of experimental parameters

Plate no.

Plate thickness Shot no. Gun used Velocity ms-1

Hole dia. (mm)

entry side

Strain rate (v/d)

× 105 S-1

Remarks

5 6.4 1

2 3 8* 9*

B

A

785 793 791 389 391

7.93 7.97 7.56 8.99 12.23

0.99 0.94 1.04 0.43 0.31

Weld Parent Parent Weld Parent

6 6.4 10 11 12 18 19 20 21

C

D

891 882

- 825 830 818 833

7.05 6.50 6.63 8.46 7.33 8.18 7.84

1.26 1.36

- 0.97 1.13 1.00 1.06

Parent Weld Weld Parent Parent HAZ HAZ

7 7.8 4 5 6* 7*

B

A

789 793 418 394

7.78 7.79 9.92 11.0

1.01 1.02 0.42 0.36

Weld Parent Parent Weld

8 7,8 13 14 15 16 17

C

D

873 880 824 824 830

6.32 7.69 7.74 7.24 8.24

1.38 1.14 1.06 1.14 1.01

Weld Parent Parent Weld Weld

* Projectile has not pierced the plate.

Fig. 1⎯Schematic of test range arrangement

Fig. 2—Photographs showing AA2219 welded plates impacted by different projectiles

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on the weld root has caused a crack (Fig. 5) on the fusion line (shot no. 7) and on the reinforcement side of weld pool (shot no. 9). Projectiles fired from the three sources, i.e., 'B', 'C' and 'D' with velocity in the range of 785-900 ms-1 pierced the plate and ejected through them. However, projectiles with velocities less than 400 ms-1 could pierce the plate partially. Metallographic examination was carried out on the locations of impact. Specimens were taken out from such locations across the thickness and prepared by conventional metallographic technique. Micro-structural observations on all the specimens subjected to ballistic impact revealed the presence of adiabatic shear bands. These bands were due to complete

absorption of the energy of the impact by the plate resulted in localized yielding. Typical photomicro-graphs of AA2219 aluminium alloy plate before and after the ballistic impact study are shown in Figs 6 and 7 respectively. Adiabatic shear bands as noticed were at an angle of about 45º to the rolling direction

Fig. 3—Photographs showing metal flow lines on the entry side of partially penetrated projectile (parent)

Fig. 4—Photographs showing absence of crack on the exit side of partially penetrated projectile on the parent material.

Fig. 5—Photographs showing cracks on the fusion line and in the weld pool on the exit side of partially penetrated projectile

Fig. 6—Optical microphotographs showing adiabatic shear bands due to high strain rate.

Fig. 7—Hardness profile of adiabatic shear band

Fig. 8—SEM fractographs showing globular features on the fracture surface.

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of the plate, i.e., the direction of stress experienced by the plate under impact. It is interesting to note that similar adiabatic shear bands were also noticed during analysis of the propellant tank of a satellite launch vehicle failed due to high strain rate of loading. The micro indentation hardness measurements were carried out across the adiabatic shear band. Within the band, the hardness values were high compared to the surrounding matrix. A typical hardness profile is shown in Fig. 8. Fractograph observations were carried out on the petals formed on both entry and exit side due to the high velocity projectiles. Observations under scanning electron microscope revealed the presence of globular features, molten and solidified debris on the fracture surface. These features were seen on all the specimens. Typical fractographs indicating globular features, molten and solidified debri for each plate (plate nos 5,6,7,8) were shown in Figs 9 and 10 respectively In the present investigation, the aim was to study the microstructural manifestations of AA2219 aluminium alloy under high strain rate through

ballistic impact testing using different guns and projectiles (in order to give different strain rates more than 104 s-1 by varying the velocity of the projectiles). Observations of the affected areas under metallographic and electron microscopy revealed adiabatic shear bands, very fine dimples, signatures of melting and solidification (globular features) of debris in all the coupons. Mechanism for the formation of such microstructural features is well understood and is in total conformity with the literature information1-6. At high strain rates, there is not enough time for the material to dissipate the energy input to the material and the conditions are adiabatic in nature. In general, strain rates of the order 10 s-1 and above are considered to be adiabatic. Under this condition, there will be a local rise in temperature, which in turn reduces the local flow stress of the material. On continued deformation, the region at higher temperature, having comparatively lower flow stresses, will deform prior to the surrounding material. This gives rise to a local and narrow region of deformed zone commonly referred to as 'adiabatic

Fig. 9—SEM fractographs showing signatures of molten and solidified debris.

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shear band'1. Under severe conditions these bands may get forked, and under extreme conditions may be associated with cracks also. The higher hardness of the adiabatic shear band region is associated with the inter-relation of the two mechanisms of the shear band formation2, i.e., the hardening effect of grain refinement by the dynamic re-crystallization as well as high dislocation density and softening effect by temperature rise. Because the temperature rise is the highest at the center of the shear band, it can be expected that the thermal softening effect is the highest there, but the hardening effect is also in operation due to grain refinement caused by the increased dislocation density and the dynamic re-crystallization. When deformation stops, the softening effect quickly disappears because the heat is rapidly emitted into the matrix and thus, the hardening effect predominates over the softening effect; yielding the increased hardness at the shear band. The presence of adiabatic shear bands, as noticed in the present study confirmed the attainment of high temperature within narrow bands. These bands form at an angle of 45° to the stress direction. Similar

observations were made in the present investigation (Fig. 7). The specimen polished across the thickness revealed adiabatic shear bands at about 45° to the rolling direction of the sheet, the plane which experiences the maximum stress. The presence of globular features and molten and solidified debris on the fracture surface confirmed "generation of excessive heat during fracture". A more probable explanation of the separation process is that due to the heat of plastic working during penetration, the material within the shear band becomes molten. Immediately after penetration by the projectiles, the molten material in the shear band begins to solidify. During solidification, fracture of the band occurs by the release of stored elastic energy in the target. The fracture follows the path that is still molten to give a fracture surface consisting of the globular asperities. The heat generated within the narrow adiabatic shear bands causes the separation of surfaces resulting in globular features, melting and solidified debris on the fracture surface. Stock and Thomson3 explained the phenomena of separation process in aluminum alloy AA2014, due to heat of plastic working resulting in melting of material within shear bands.

Fig. 10⎯SEM fractographs showing signatures of molten and solidified debris. a) plate no. 5, b) plate no. 6, c) plate no. 7, d) plate no. 8

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During solidification, fracture of bands occurs by release of stored elastic energy in the material. The investigators developed a model for calculation of temperature rise within the bands. Assuming all the energy spent to form the band, gets converted into heat, they computed a temperature rise of the order of 1040°C. However, the band would melt before this temperature is reached, causing separation of surfaces. It is to be noted from present experiment that projectiles with velocities less than 400 ms-1 and target at a distance of 10 m could pierce the plate partially, whereas projectiles with velocities in the range of 785-900 ms-1 could penetrate the plate fully. This indicated that momentum transfer and ability of material to accommodate the energy also play a major role in piercing the plate. Conclusions The penetration of aluminium alloy by projectile results in (i) The formation of adiabatic shear bands at an angle of about 45° to the rolling plane, the plane which experienced maximum stress. (ii) The higher hardness value within the band than the matrix was attributed to the hardening effect which predominates the softening effect. (iii) Formation of these bands, the heat generated by plastic flow within a narrow band and insufficient time for the heat to dissipate was sufficient to raise the temperature locally. (iv) The presence of globular features, molten and

solidified debris were the result of local temperature rise. Acknowledgements The authors would like to express their profound sense of gratitude to Shri M.C. Mittal, Group Director, MMG, VSSC and Shri K.S. Sastri, Deputy Director, VSSC for their technical guidance in carrying out this work. They would like to thank Dr B.N. Suresh, Director, VSSC for his encouragement. References 1 Shockey D A , in Metallurgical application of shock–wave

and high–strain rate phenomena, edited by Lawrence E Murr et al., (Marcel Dekker, Inc, New York), 1986, 633.

2 Chang Gil Lee, Woo Jin Park, Sungbak Lee & Kwang Seon Shin, Metall Mater Trans A, 29A (1998) 477.

3 Stock T A C & Thompson K R L, Metall Trans, 1 (1970) 219.

4 Grebe H A & Meyers Marc A, Metalll Trans A, 16A (1985) 761.

5 Holt W H, Mock W, Soper W G, Coffey C S, Ramachandran V & Armstrong R W, J Appl Phys, 1973 (8) (1993) 3753.

6 Backman E Marvin, Finnegan Stephen A & Schulz Tan C, in Proc Metallurgical application of shock wave & high strain rate phenomena, edited by Murr L E, Staudhammer Karl P & Mayers Marck A, (Marcel Dekker, Inc, New York), 1986, 675.

7 Metals handbook, Fractography and Atlas of Fractographs, vol. 9, 8th ed (ASM, Metals Park, Ohio), 1974, 242.