2014-The Ballistic Performance of SiC–AA7075 Functionally Graded Composite Produced by Powder Metallurgy

Materials and Design 56 (2014) 31–36

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Materials and Design

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The ballistic performance of SiC–AA7075 functionally graded compositeproduced by powder metallurgy

0261-3069/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.10.092

⇑ Corresponding author. Tel.: +90 328 8271000x3685.E-mail address: [email protected] (M. Übeyli).

Mustafa Übeyli a,⇑, Erhan Balci b, Bertan Sarikan c, M. Kemal Öztas� d, Necip Camus�cu c, R. Orhan Yildirim e,Ömer Keles� d

a Osmaniye Korkut Ata University, Engineering Faculty, Mechanical Engineering, Karacaoglan Yerles�kesi, 80000 Osmaniye, Turkeyb Kocaeli University, Engineering Faculty, Mechanical Engineering, Kocaeli, Turkeyc TOBB University of Economics and Technology, Mechanical Engineering, Sögütözü Cad. No: 43, 06560 Ankara, Turkeyd Gazi University, Engineering Faculty, Mechanical Engineering, 06570 Maltepe, Ankara, Turkeye Middle East Technical University, Engineering Faculty, Mechanical Engineering, Ankara, Turkey

a r t i c l e i n f o

Article history:Received 8 January 2013Accepted 24 October 2013Available online 9 November 2013

a b s t r a c t

The potential of silicon carbide reinforced Functionally Gradient Material (FGM) to be used as armormaterial was investigated under the impact of armor piercing projectile. For this purpose, the SiC–Aluminum Alloy (AA) 7075 functionally graded composite at different thicknesses was produced fromthe metallic and ceramic powders via powder metallurgy method. Before the ballistic testing, theprecipitation hardening behavior of the samples was determined. And also, the microstructural charac-terizations of the samples were done with the aid of microscopy techniques. Next, the FGM samples weretested using armor piercing projectile to analyze their impact behavior. In the produced samples, somepore formation was detected. The ballistic experiments showed that the investigated FGMs (up to athickness of 25 mm) did not withstand the impact of the projectile. At the tested samples, some majorcracks and plug formation were detected at macrolevel while there were some microcracks, deformedand elongated grains in the regions near to the deformation zone of the samples.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

In defense applications, the development of armor materialswith the possible lowest areal density is very crucial to enhancemaneuver capacity and saving of energy and structural material.Functionally Gradient Material (FGM) can be considered as apotential candidate armor material due to its distinctive features[1]. FGM, consisting of different layers chemically, has propertiesvarying from one layer to other [2–4]. For this reason, it mayappear to be a new material type, serving different actions at eachlayer. Various techniques have been used in the production ofFGMs [1–7]. Powder metallurgy is a very suitable method in pro-ducing the graded layers containing ceramic particle reinforcedmetal matrix composite.

In ballistic attacks, the most widely used threat type is theprojectile with various calibers, core hardness and velocity. There-fore, the ballistic resistance of armor against projectile is stronglyrequired to be determined. Utilization of a hard outer layer and atough inner layer in an FGM may lead to an effective protectionagainst ballistic threats [1,10]. It is expected that the outer hardlayer erodes and breaks up the projectile whereas the inner toughlayer stops the projectile via absorbing its kinetic energy [1,10].

The studies performed on the ballistic behavior of FGMs arevery rare and limited [8–10]. The ballistic response of Ti–TiB2

FGM against tungsten cored 7.62 mm projectile was investigatedby Pettersson et al. [8]. They obtained that the FGM showed supe-rior performance compared to the monolithic TiB2 [8]. On the otherhand, Jovicic et al. [9] made a modeling of the ballistic behavior ofgradient armor. And also, they used the finite element analysis tomake a simulation of the investigated structures. In a more recentstudy [10], the ballistic behavior of an FGM based on B4C–AA7075against 7.62 mm Armor Piercing (AP) projectile was examined. Itwas mentioned that the material did not provide a ballistic protec-tion up to a thickness of 25 mm [10].

In the current study, the production and characterization of theFGM samples based on silicon carbide reinforced aluminum alloy7075 were investigated by taking into account of two differentmaterial compositions. The main motivation point of the studywas to clarify the ballistic impact resistance of the silicon carbidereinforced FGMs.

2. Materials and methods

In this paper, the ballistic performance of SiC–AA7075 FGM,produced by powder metallurgy technique, was investigatedexperimentally. The starting raw materials used for the FGM sam-ples considered in this study were metallic and ceramic powders

32 M. Übeyli et al. / Materials and Design 56 (2014) 31–36

which were supplied from the market. The mean sizes of alumi-num and silicon carbide were measured to be 10 and 33 lm,respectively. Silicon carbide, one of the hardest ceramic types,was considered as a reinforcing component to enhance hardnessand abrasive efficiency of the FGM. It was added to the middleand top layers of the two FGM samples at different ratios (Table1). A heat treatable aluminum alloy, AA7075 was selected to bethe matrix material which can reach much higher hardness andstrength by aging compared to non-aged one [11–13]. Aluminumalloys due to their lower densities compared to steels can promisea lightweight armor production in some systems such as militaryvehicles.

The powders were initially mixed and cold-pressed to formthree green layers separately. And then, they were transferred tothe hot pressing mold at the Powder Metallurgy Lab of TOBBUniversity of Economics and Technology to combine them bysintering operation. The operation was performed in liquid phasesintering condition at 580 �C during 40 min to decrease orcompletely eliminate the formation of porosity. As well known,the porosity causes a significant reduction in the strength, loadcarrying capacity and hardness of materials. Moreover, it alsoreduces the ballistic resistance of potential armor materials. Theproduced and tested FGM samples in this work are given in Table1. The FGM samples, having a square size of 7 cm and a thickness of15, 20 and 25 mm, were produced to be tested for the sake ofcomparison with our previous study conducted on the boroncarbide reinforced FGMs [10]. After that, the micro-structural anal-ysis was done using a scanning electron microscope, whereas theaverage grain size and the porosity level were computed by animage processing program. Furthermore, the three points bendingtesting of the samples was carried out to determine the flexuralstrength values. Prior to the ballistic testing, the effect of temper-ature and operation time on the hardness of the FGM sampleswas examined during the artificial aging treatment at 100 and150 �C for comparison with our previous results gained at 120 �C[7]. The mechanical tests were performed for the FGM specimensusing the ASTM: E10-12 (http://www.astm.org/Standards/E10.htm) and the ASTM: B528-12 (http://www.astm.org/Stan-dards/B528.htm).

In the hardness testing, the load of 187.5 kg was applied to thesamples with a steel ball of 2.5 mm in diameter whereas, in thethree points bending testing, the sample thickness was consideredto be 15 mm and the force was applied through the compositelayer. The hardness measurements were made every 30 min duringthe aging period to monitor the hardness variation apparently. Thehardness measurement was repeated 5 times. In the three pointsbending tests, three unique samples were used for testing eachmaterial group. Finally, the mean values were recorded for eachcase.

Moreover, the ballistic tests were made using 7.62 mm APprojectile which is one of the most widely used bullet in armies.The velocity of the projectile changed between �775 m/s and800 m/s during the testing of the samples. Five samples for eachsample with the same composition and thickness were subjectedto ballistic shot. After finishing the ballistic tests, the macro and

Table 1The FGM types and compositions produced by powder metallurgy.

FGM type

1 2

Top layer 20% SiC–80% AA7075 40% SiC–60% AA7075Medium layer 10% SiC–90% AA7075 20% SiC–80% AA7075Bottom layer AA7075 AA7075

microobservations on the failed samples were conducted to seethe structural changes and impact damage.

3. Experimental results and discussion

3.1. Micro-structural observations

Fig. 1 illustrates the micro-view of the AA7075 layer for theFGM 1. There are some pores formed in the FGM samples afterthe production. The porosity level was computed to be 1.3%, 1.4%and 2.5% at the bottom, middle and top layers of the FGM 1, respec-tively. On the other hand, it was estimated to be 2.6%, 2.7% and3.2% at the same layers of the FGM 2, successively. It is clear thatthe porosity increased with increasing the ceramic reinforcementratio gradually. Although the porosity levels were quite low, thefull densification in the samples was not maintained, mostprobably due to high thicknesses of the samples and relativelylow operation pressure. On the other hand, the average grain sizewas determined to be between 5.4 and 6.3 lm depending on thelayers for the FGM samples. Moreover, the smooth transitionbetween the layers is shown in Fig. 2. The formation of a metallur-gical bond between the layers allowed for a structural integritybetween the layers with various chemical compositions. Theirregular shaped ceramic particulates can be seen apparently atdifferent layers of the FGM 2 (Fig. 3).

3.2. Precipitation aging properties

Figs. 4 and 5 represent the aging curves for the FGMs at 100 and150 �C, respectively. Generally, the peak hardness values werereached within 20 h for the layers of the two FGM types. Thehardness started to increase rapidly at the beginning of the agingtreatment. And then, there was a stabilization or very slow reduc-tion on the hardness observed in the graphs. The response of theunreinforced bottom layer was found to be very close for the twoFGMs at the same aging temperature. It resembled the properrepeatability of the FGM production. The highest hardness valueswere measured as HB 213, HB 177 and HB 161 for the top, middleand bottom layers of the FGM 2 at the aging of 120 �C, respectively[7]. The temporal variation in the hardness curves for the threelayers was not affected by the addition of the SiC ceramic particles.This means that the aging kinetics and formation of coherentprecipitates in the layers were not influenced by the compositeformation [6,7,10]. Rising the aging temperature to 150 �C led tofaster attainment to the peak points and overaging. It would besuitable to remind that the formation of g0 phase provides a stronghardening in AA7075 [11–13]. For this reason, by the application ofthe aging treatment, nano-sized precipitates enhance the hardness

Fig. 1. The microstructure of the AA7075 layer for the FGM 1.

(a) Between bottom and middle layer

(b) Between middle and top layer

Fig. 2. The transition regions between layers for the FGM 1.

Fig. 3. The middle (a) and top (b) composite layers of the FGM 2.

M. Übeyli et al. / Materials and Design 56 (2014) 31–36 33

and strength of the matrix alloy to a great extent [11–13]. Themain contribution to the hardness came from the matrix material,particularly in the FGM 1.

The three point bending results of the FGMs in two differentthermal treatment conditions are given in Table 2. It is clear thatthe aging treatment had a positive effect on the bending strengthof both FGMs since the resistance of the matrix material to plasticdeformation increases with hardness improved by the aging treat-ment. However, the increment was at a low rate. On the otherhand, an increase in the ceramic content caused a reduction inthe strength level of the FGM. This may be due to the brittle natureof ceramic particles causing crack initiation more easily in thematrix. Moreover, the reduction in the volume of matrix materialmade the FGM more brittle and sensitive to crack formation. Therewere no interfacial cracks found in the samples. This means that avery good bonding was occurred between the layers.

3.3. Ballistic results

The FGM samples in this work were tested in the peak hardnesscondition, derived from the aging curves. The ballistic testing wasperformed using the hard steel-cored 7.62 mm AP projectile. Theschematic ballistic setup is depicted in Fig. 6 which was also usedfor the testing of boron carbide reinforced FGMs [10]. The ballisticshots were carried out at the normal direction between the sampleand projectile. All the tests were repeated 5 times for one uniquesample. In addition every sample was subjected to one shot tosee the deformation clearly. The samples were fixed at the targetto cover the kinetic energy of projectile and to get a proper impactbetween them. The distance between the sample and projectileexit zone was 20 m.

All the investigated FGM samples were perforated by theprojectile. They did not withstand the projectile impact at all

thicknesses as in the case of boron carbide reinforced FGMs withthe similar thickness levels [10]. Although the propagation of theprojectile was obtained to be much harder in the samples with high-er ceramic content and thickness, the samples were unable to stopthe projectile. Fig. 7 shows a view of 25 mm thick FGM 2 sample afterthe ballistic testing. The hole created by the projectile motion is seenapparently. There are also some major crack formations on the sam-ple. Some part of the kinetic energy of the projectile was mainly con-sumed by the hole formation and radial crack formation. Theextensive crack formation and breaking in the samples are due tothe relatively lower toughness and ductility of the samples. The re-flected or generated tensile waves after the projectile hit were notcovered by the FGM samples successfully. Moreover, it is thoughtthat the pores act as crack initiation and easy propagation sites un-der the dynamic loading. Although the very hard ceramic particles insignificant quantity were in the layers of the FGM samples, theywere unable to erode the projectile effectively. This is because oftheir distribution as micron-sized particles in a much softer matrixmaterial acting a low obstacle to the projectile motion. Fig. 8 illus-trates the cross-sectional view of the same sample through the pro-jectile propagation. The deformation or shear of the layers occurredin the sample. However, there was no delamination or separationobserved in the samples. Furthermore, there was no pulverization

(a)

(b)Fig. 4. The aging curves at 100 �C for the SiC reinforced FGM 1 (a) and FGM 2 (b) atthe r top, s middle, t bottom layers.

(a)

(b)Fig. 5. The aging curves at 150 �C for the SiC reinforced FGM 1 (a) and FGM 2 (b)(the legend as in Fig. 4).

Table 2Bending strengths of the investigated FGMs.

Bending strength (MPa)

Thermal treatment FGM 1 FGM 2As solutionized 491 450As aged 510 475

Fig. 6. The setup for the ballistic impact testing of the FGMs [10].

34 M. Übeyli et al. / Materials and Design 56 (2014) 31–36

of the broken parts in the tested samples. All they mean that thesintering and so the bonding between the layers can be consideredto be well.

The ballistic impact can lead to microstructural changes inmetallic materials at a great extent [14–27]. These changes maycause crack formation and failure of materials [14–20]. There are

two different types of adiabatic band formation in the metallicmaterials under high strain rate deformation like ballistic impact[14–20]. Those are called deformed and transformed bands [14–16]. In deformed band, only change in the size and elongation ofgrains is observed but no crystal structure change takes place.However, in transformed band (white band), a phase change occursdue to the rapid heating and cooling in material [14–16]. In aprevious work [21], only the deformed bands containing highlydistorted grains were recorded for a dual phase steel tested by7.62 mm AP projectile. In earlier studies [22,23], the effect of heattreatment on the adiabatic band generation was examined in thesteels namely, AISI 4140, AISI 4340 and DIN100Cr6 which weretested by steel-cored 7.62 mm AP projectile. There were both thetransformed and deformed bands observed in the samples withthe hardness levels of 49 or 59 HRC [22,23]. In addition to thatthe hardness level of the transformed bands was found to be muchhigher than that of the deformed bands due to the fine grain andprecipitate formation in the transformed bands [22,23]. In a morerecent paper [24], only the deformed band formation was found atbacking layers of the alumina/steel (with dual phase microstruc-ture) laminated composites which were tested by 7.62 mm APprojectile. Mishra et al. [25] investigated the influence of temper-ing temperature on the formation of ASBs. They concluded thatincreasing the tempering temperature lowered the formation ofASBs. In a study related to the ballistic testing of tempered bainiticsteel [26], ASB formation near to the penetration zone and petal-ling type of deformation were detected. Jena et al. [27] alsoobserved the adiabatic shear band formations at Al-7017 andtempered low-alloy steel samples tested by 7.62 mm deformableprojectile. They recorded the shear plugging deformation on thesamples caused by ASBs [27].

The microstructural observations were made on the sectionsthrough the thickness of the samples. In the tested FGM specimens,only the deformed bands were detected. There were no trans-formed bands found. Fig. 9 shows the microstructure at the bottomlayer of the 20 mm-thick FGM 1 which is very close to hole createdby the projectile. It is apparent that the elongated and fine grainsformed along with the projectile direction. Upon the impactbetween sample and projectile, a significant amount of heat, caus-ing micro-structural changes, was produced. Rapid heating andcooling cycle as well as forced deformation caused to the elongatedand fine grain formation in the regions very near to hole. Moreover,there were some micro-cracks obtained in the tested specimens.These cracks were often found in connection with the pores. Thepores acted as crack initiation sites under dynamic loading. Themicrostructure at the middle layer of the same sample is repre-sented in Fig. 10. Again, there are some fine and directional grainsin the matrix. The deformed bands were detected in the samples

Fig. 7. The macrophotos of 25 mm thick FGM 2 sample after the ballistic testing: (a) front view, and (b) rear view.

Projectile affected zonePr

ojec

tile

dire

ctio

n

Fig. 8. The cross-sectional view of the FGM 2 sample after the ballistic testing. Theshear on the layers are seen clearly.

Deformed and elongated grains

Fig. 9. The microstructure at the bottom layer of the 20 mm-thick FGM 1 after theballistic testing.

Fig. 10. The microstructure at the middle layer of the 20 mm-thick FGM 1 after theballistic testing.

M. Übeyli et al. / Materials and Design 56 (2014) 31–36 35

after the ballistic testing while there was no transformed bandformation observed. Moreover, the investigated FGMs did notappear to be a good candidate for lightweight armor production

in comparison to the alumina/aluminum alloy [28] and alumina/steel [29] laminated composites.

4. Conclusions

The investigated FGMs did not show the successful ballisticprotection at all investigated thickness and compositions. In orderto get a full ballistic protection design, the FGMs with a thicknessgreater than 25 mm would be required. Nevertheless, in this casethe FGMs would be not a good solution to make a lightweightarmor production. The deformation type of the FGMs was foundto be brittle in general. There were some major cracks and plugformation at the tested samples at macrolevel. And also, there weresome microcracks, deformed and elongated grains especially in theregions near to the deformation or impact zone of the samples.

Acknowledgement

This work was supported by the Research Fund of TÜB_ITAK,Project # 110M034. The authors are thankful to TÜB_ITAK for itssupport. Moreover, they also thank to MKE Silahsan A.S�. (Kırıkkale)for its support on the ballistic tests.

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