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Development of Tungsten Heavy Alloy

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Development of Tungsten Heavy Alloy with Hybrid Structure for Kinetic Energy Penetrator Woon-Hyung Baek 1, a , Eun-Pyo Kim 1, b , Heung-Sub Song 1, c , Moon-Hee Hong 2, d , Seong Lee 1, e , Youngmoo Kim 1, f , Sung Ho Lee 1, g , J oon-Woong Noh 1, h and J oo Ha Ryu 3, i 1 Technology R&D Center, Agency for Defense Development, Daejeon, Korea 2 Defense Agency for Technology and Quality Assurance, Seoul, Korea 3 Angang plant, Poongsan Corp. Angang, Kyungju, Korea a whbaek@add.re.kr
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  Development of Tungsten Heavy Alloy with Hybrid Structure for Kinetic Energy Penetrator Woon-Hyung Baek 1, a , Eun-Pyo Kim 1, b , Heung-Sub Song 1, c , Moon-Hee Hong 2, d , Seong Lee 1, e , Youngmoo Kim 1, f  , Sung Ho Lee 1, g , Joon-Woong Noh 1, h  and Joo Ha Ryu  3, i   1  Technology R&D Center, Agency for Defense Development, Daejeon, Korea 2 Defense Agency for Technology and Quality Assurance, Seoul, Korea 3  Angang plant, Poongsan Corp. Angang, Kyungju, Korea a whbaek@add.re.kr, b dharmakim@add.re.kr, c heungs@add.re.kr, d moonheehong@hanmail.net, e seongl@add.re.kr, f  ymkim78@add.re.kr, g shlee62@add.re.kr, h  jwnoh@add.re.kr, i  joo-ha.ryu@poongsan.co.kr. Keywords:  tungsten heavy alloy, penetrator, hybrid structure Abstract.  A new tungsten heavy alloy with hybrid structure was manufactured for the kinetic energy penetrator. The tungsten heavy alloy is composed of two parts: core region is molybdenum added heavy alloy to promote the self-sharpening; outer part encompassing the core is conventional heavy alloy to sustain severe load in a muzzle during firing. The fracture surfaces of the specimen is observed after ballistic tests. The core region revealed brittle behavior with W/W inter-granular fracture which activates self-sharpening. On the other hand, outer part exhibited conventional ductile fracture mode. From ballistic test, it was found that the penetration performance of the hybrid structure tungsten heavy alloy is higher than that of conventional heavy alloy. This heavy alloy is thought to be very useful for the penetrator in the near future. Introduction A kinetic energy round defeats a target by penetrating an armor with the kinetic energy of  penetrator. Tungsten heavy alloy(WHA) or depleted uranium(DU) is used as penetrators in all cannon-fired kinetic energy projectiles. The DU penetrator is manufactured by casting and forging[1]. The density, typical ultimate tensile strength and tensile elongation of this alloy are 18.5 g/cm 3 , 1600 MPa, and 12%, respectively. The WHA projectile is fabricated through powder metallurgy techniques. Elemental powders are blended, pressed to shape and sintered. The sintered  bar is heat-treated and cold swaged. WHA is two-phase mixture of body centered cubic(BCC) tungsten particles surrounded by face centered cubic(FCC) Ni-based matrix[2]. Despite the superior performance of the DU munitions[3], there are many environmental concerns,  political controversies and additional costs associated with their manufacture, their use in the  battlefield, the subsequent battlefield clean up which may be required, and the final disposal of these rounds. These concerns have prompted counting efforts to develop a less hazardous, environmentally more benign, tungsten based penetrator material which is capable of equalling or surpassing the performance of DU[4,5]. In the present investigation, we proposed a tungsten heavy alloy penetrator with hybrid structure which exhibits excellent penetration performance and can be mass produced. The hybrid structure  penetrator is composed of a core and a housing encompassing the core. The core region is molybdenum added heavy alloy to promote the self-sharpening; the housing part is conventional heavy alloy to sustain the severe load in a muzzle during firing. In this study, the procedures to fabricate the hybrid structure WHA has been presented. Material  properties and microstructure were also investigated. The penetration performances of the conventional WHA penetrator and the hybrid one were compared by a ballistic test to evaluate the applicability of this new material to a kinetic energy projectile.  Materials Science Forum Vols. 534-536 (2007) pp. 1249-1252online at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland  All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without thewritten permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 129.137.163.99-09/07/07,19:52:45)  Experimental Procedure Two types of specimens, a conventional and a hybrid structure heavy alloys, were prepared from W, Mo, Ni and Fe elemental powders with 2.5, 2.5, 3.0 and 3.5 μ m in average size, respectively. The conventional heavy alloy was used as a reference specimen. Its composition was 93W-4.9Ni-2.1Fe and was fabricated by the usual powder metallurgy processes. A schematic drawing of a hybrid structure WHA penetrator is shown in Fig.1. The composition of the core material was 92.8W-4.6Mo-1.1Ni-1.5Fe. The proportioned powder was mixed without  binder in a tubular mixer for 8 hours. The mixed powder was compacted under 200 MPa into a round bar and then sintered under hydrogen atmosphere at 1425°C for 2 hours. Sintered core material was positioned at the center of a rubber bag for cold isostatic pressing with a fixture. Mixed powder for the housing material, 90W-7Ni-3Fe in composition, was packed around the core and compacted. The compact was sintered under hydrogen atmosphere at 1475°C for 2 hours. Fig. 1. Schematic drawing of a hybrid structure WHA penetrator. Brittle core induces self- sharpening and housing material sustains structural integrity. The sintered specimen was heat-treated repeatedly under nitrogen atmosphere at 1150°C for 8 hours followed by water quenching. Cold working of 18% area reduction was carried out to increase the strength of the material. The density of the specimen was measured and the microstructure was observed by a scanning electron microscope. Ballistic test was conducted with a solid propellant gun. Its caliber was 30 mm. Projectiles were made of WHAs with a conventional and a hybrid structures. The striking velocity measured by MVRS(Muzzle velocity Radar System) was about 1140 m/sec. The target was rolled homogeneous armor(RHA) plate of 100 mm wide and 150 mm thick. Penetration depth was measured from the cross-section of the target after the test. Results and Discussion The microstructures of the core and the housing materials after they were sintered together are shown in Fig.2. Tungsten particles are round in both cases. The tungsten particle size of the housing is larger than that of the core. It is noted that tungsten particle growth of the core also occurred. This seems to be the result of fully developed liquid phase sintering at the final sintering step which was carried out at higher temperature than that of the first one. The chemical analysis found about 10.5% molybdenum in weight in the core material. However, no trace of molybdenum was detected in the housing. So it can be thought that the difference in tungsten particle size in the core and the housing was resulted from the presence of molybdenum which altered the chemical composition of the matrix. The sintered density of the conventional and the hybrid structure WHAs were 17.74 g/cm 3  and 17.29 g/cm 3 , respectively. For the hybrid WHA, the theoretical density of the core material is 17.94 g/cm 3  and that of the housing 17.15 g/cm 3 . Care should be taken not to allow the increase in the sintering temperature or sintering time because the density difference between the core and the housing results in displacement of the core out of the center of the penetrator. Progress in Powder Metallurgy 1250    (a) (b) Fig. 2. Final microstructure of core and housing materials. (a) Core and (b) Housing The fracture surfaces of the hybrid structure WHA were shown in Fig.3. The brittle intergranular fracture was revealed in the core material, which showed no ductile failure of the matrix. The brittle fracture mode is expected to lead to self-sharpening during penetration. On the contrary, the housing material showed ductile fracture behavior. The observation found precipitates at the tungsten/tungsten interfaces and ductile failure of the matrix[8]. The housing material is thought to  be able to maintain the structural integrity of the projectile inside a gun bore and at the impact. (a) (b) Fig. 3. Scanning electron micrographs of fracture surface of hybrid structure WHA. (a) Core - Brittle fracture and (b) Housing - Ductile fracture The hybrid structure penetrators were found to have improved penetration performance over the conventional ones showing 10% increase in penetration depth. Irregularly shaped tungsten particles found in tungsten heavy alloy made by another processing method are shown in Fig. 4[6]. In this heavy alloy, the failure mode was observed to be brittle transgranular fracture while ductile intergranular failure is typical in the conventional heavy alloy with round tungsten particles[7]. The change from ductile fracture to brittle one is caused by protrusions or intrusions of the particles since they act as stress-concentration points like notches. The brittle fracture in WHA was known to  be beneficial to penetration performance because brittle fracture results in self-sharpening of the  penetrator[8]. Hence, it can be inferred that the fracture behavior of the core material induced self-sharpening and contributed to the increased penetration of the hybrid structure WHA in spite of decrease in the density. Materials Science Forum Vols. 534-536 1251    (a) (b) Fig. 4. Microstructure of WHA(93W-4.9Ni-2.1Fe) re-sintered after cyclic heat-treatment to form irregularly shaped tungsten particles. (a) Optical micrograph and (b) Scanning electron micrograph of fracture surface Conclusions According to a new concept of combining brittle core material to ductile housing, a hybrid structure WHA penetrator was fabricated by powder metallurgy techniques. The core material showed brittle intergranular fracture while the housing material exhibited ductile one, typical in conventional tungsten heavy alloy. The penetration depth of the hybrid structure WHA penetrator was found to  be higher than that of the conventional one by 10%. The self-sharpening of the penetrator resulted from brittle fracture of the core contributed to the increased penetration performance of the hybrid structure projectile. References [1] P.K. Johnson: Inter. Defense Review, Vol. 5 (1983), p.643. [2] E.C. Green, D.J. Jones and W.R. Pitkin: 28th Sympo. on Powder Metallurgy, special report No. 58 (1954), p.253. [3] L.S. Magness: Proc. the 1st Inter. Conf. on Tungsten and Tungsten Alloys, edited by A. Bose and R.J. Dowding (1992), p.15. [4] M.H. Hong, J.W. Noh, W.H. Baek, E.P. Kim, H.S. Song and S.Lee: Metall. Trans. B, Vol. 28B (1997), p.835. [5] E.P. Kim, M.H. Hong, W.H. Baek, and I.H. Moon: Metall. Trans. A, Vol. 30A (1999), p.627. [6] K.J. Park, S.H. Lee, W.H. Baek, J.W. Noh, H.S. Song, M.H. Hong, S. Lee, E.P. Kim, J.W. Suh: Korean Patent 257,463 (2000) [7] D.K. Kim, S.H. Lee and H.S .Song: Metals and Materials, Vol.5 (1999), p.211. [8] E.P. Kim, H.S. Song, I.H. Moon: J. of Korean Institute of Metals and Materials, Vol. 36 (1998),  p.901. Progress in Powder Metallurgy 1252
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