Deformation characterization of cartridge brass

Indian Journal of Engineering & Materials Sciences Vol. 20, August 2013, pp. 283-288 Deformation characterization of cartridge brass Arun Kumar Verma a, A Shingweker b, M Nihichlani b, V Singh b & Prantik Mukhopadhyay a * a Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad 500 058, India b Department of Metallurgical Engineering, National Institute of Technology, Raipur 492 010, India Received 2 July 2012; accepted 23 April 2013 Cu-30 Zn (wt%) alloy is widely used for cartridge case. As received hot rolled (HR) cartridge brass is rolled about 15%, 30%, 40%, and 50% by plane strain rolling at normal room temperature in laboratory. The quantitative microstructure details such as mean grain size and grain size distribution are characterized by optical microscope equipped with microstructure analysis software. The developments of deformation texture are measured by X-ray texture Goniometer and analysis of texture details is done by LaboTex-Edu texture analysis software. The mean grain size is increased with rolling in rolling direction. The grain size distribution shows higher frequency for larger grains with deformation by rolling. The initial stages of rolling produce bi-modal grain size distribution. Later it shifts to multi-modality. The deformation strengthens Bs, L, R, Goss, Taylor, Cu and S texture components. Though the Cu texture orientation shows an increasing trend, the strength of Cu orientation is much less than that of Bs and S components. The transition of texture for Cu, S and Bs texture components is not found in this study. Keywords: Rolling, Cartridge brass, Microstructure, Texture Copper alloyed with 30% Zn (wt. %) is used for cartridge case manufacturing, which incorporates rolling and cold drawing in several stages with intermediate annealing, final taper and stress relieving annealing 1,2. The cold drawing capabilities of this alloy are directly associated with formability characteristic 3,4, which are governed by the structural development in micro-scale during deformation and annealing. The properties are liable to vary with the processing for same alloy with certain composition 5. Generally the formability issue if addressed by high plastic strain ratio (R) value, low yield point, low strain aging, high work hardening exponent, high uniform and total ductility 6. Definite microstructural control and proper texture engineering by the induction of {111} texture are the crucial factors to enhanced formability 7. Advanced microstructure design, incorporating multi-modality in grain size and associated change in grain orientation might be beneficial for formability 8. The forming character of low SFE brass is not similar to the forming character of high SFE metals and alloys 9. Despite elaborate research work on brass, the formation of rolling texture Bs {110} 112 and Cu {112} 111 texture components are not properly understood 10-14. Cold drawing of cartridge brass enhances the hardness in *Corresponding author (E-mail: prantikmukherje@yahoo.com) the actual practice but the same level of hardness can also be obtained by 50% cold reduction of this alloy 2. Hence, the deformation scenario by cold drawing might be approximated by cold rolling. The cold rolled texture and microstructure is addressed in this study. Materials and Methods Cold rolling As received hot rolled (HR) sheet of cartridge brass was rolled at normal room temperature in laboratory. About 15%, 30%, 40% and 50% cold rolling (CR) were carried out in successive stages from the hot rolled sheet. Initial thickness of hot rolled sheet was 3.5 mm, which was reduced to 3.01 mm, 2.45 mm, 2.10 mm and 1.75 mm by about 15%, 30%, 40% and 50% cold reduction respectively (Table 1). The deformations were only plane strain deformations without significant plastic strain along transverse direction (TD). Microstructure characterization The HR sheet was rolled at normal room temperature to produce desirable microstructure for quantitative analysis and advanced characterization. Table 1 Sample data name, thickness and plastic strain Sample As received 15% CR 30% CR 40% CR 50% CR Thickness 3.5 3.01 2.45 2.10 1.75 (mm) True strain (ε) Hot rolled 0.15 0.35 0.51 0.70

284 INDIAN J. ENG. MATER. SCI., AUGUST 2013 Standard metallographic sample preparation first by belt grinding followed by grinding with emery paper was done. Polishing of ground samples was carried out with advanced and automated polisher using alumina solution. The final etching of samples was performed by 15 g ferric chloride in 300 cc distilled water with 60 cc HCL to reveal the microstructure. The structural constituents such as grain size of the samples was measured by optical upright microscope, equipped with quantitative microstructure analysis software. The grain size was measured as per ASTM line intercept method. The digitized micrograph was captured by connected camera with length scale assigned by analyzer. The Sigma-Pro 5 software was used to measure grain size L (line intercept length) with proper calibration for measuring distance between pixels, converting pixel distance to length scale. Bulk texture characterization The bulk texture of deformed samples was measured by an advanced X-ray texture Goniometer. The texture pole figures of {111}, {200}, {220} and {113} diffractions were measured. The prominent ideal orientations were selected from pole figure derivative orientation distribution function (ODF) and then the texture strength of those ideal orientations was derived from Gauss distribution surrounding the ideal orientation with Gaussian half width angle 5. The sophisticated texture analysis software Labo Tex-Edu was used for advanced qualitative and quantitative texture study. mean value of line intercept length increases with deformation. The grain size distribution shows the number frequency versus L (Fig. 2), which compares the character of grain size distributions quantitatively of HR and cold rolled samples. The grain size distribution is log normal in character for HR condition but 15% and 30% cold reductions change the distributions to bi-modality, which are further changed by 40% and 50% cold reduction towards multi-modalilty trends. The frequency of larger grains increases with rolling as observed in all grain size distributions. Bulk texture Figure 3 (a-d) shows the {111} pole figures of the rolled samples and Fig. 4 (c-d) shows the respective Fig. 2 Grain size distribution of hot rolled and cold rolled samples. Results and Discussion Microstructure The mean values of the line intercept length (grain size) along the rolling direction of HR and cold rolled samples are compared (Fig. 1). Deformation by rolling in successive stages from 15% to 50% increases the grain size in rolling direction and the Fig. 1 Trend of mean grain size with cold reduction Fig. 3 {111} pole figure of deformation texture (a) 15% rolled, (b) 30% rolled, (c) 40% rolled and (d) 50% rolled.

VERMA et al.: DEFORMATION CHARACTERIZATION OF CARTRIDGE BRASS 285 Fig. 4 (a-b) Developments of texture with rolling (ODF representation) orientation distribution functions for 15% to 50% deformation. The locations of ideal texture orientations of cartridge brass are shown in Fig. 4(a) while the orientation distribution function of HR sample is shown in Fig. 4(b). The observed salient features of the deformation are (i) developments of high strength of α-fiber and β-fiber with stronger Bs, L, R, Goss, Cu and S components. Higher strength of Bs and S components was found than that of Cu texture orientation. (ii) Cube texture orientation disappears from ODF (below 1 random intensity) after only 15% cold reduction. (iii) very low strength of H texture orientation (H highest = 1.45 in HR) and normal direction Fig. 4 (c-f) Developments of texture with rolling(odf representation)

286 INDIAN J. ENG. MATER. SCI., AUGUST 2013 rotated cube CH texture orientation (CH highest = 2.06 in HR) which are reduced with the advancement of cold rolling. The high strength developments of fiber texture such as α-fiber and β-fiber show that those fiber textures strengthen during deformation of cartridge brass. Highest intensities along the α-fiber and β-fiber are compared in Fig. 5(a-b), which says that these fibers strengthen with deformations from 15% to 50%. The rotation and/or fragmentation of hot rolled (HR) Cube grains with deformations are shown from the angle of rolling direction rotation (Fig. 6). Cold rolling from 15% to 50% considerably alters the intensity of Cube orientation towards rolling direction rotated orientations, e.g., the Goss orientation. The strength of the ideal texture orientations are given in Table 2. The strength of the Bs, L, R, Goss, Taylor Cu, and S texture components increases with deformation. Though the increasing trend of Cu orientation could be found, the strengthening of Cu is Table 2 Texture strengths of rolled ideal texture orientations Orientation {hkl} uvw HR 15% CR 30% CR 40% CR 50% CR Cube {001} 100 2.37 1.53 1.06 1.03 0.77 Bs {110} 112 3.45 4.23 6.33 7.26 7.75 Cu {112} 111 1.83 2.17 2.61 3.11 4.04 R {123} 412 4.32 5.45 7.27 7.55 9.05 S {123} 643 4.74 5.75 8.14 8.50 10.75 Fig. 5a Highest intensity lines of α-fiber Goss {110} 001> 2.45 2.34 3.37 3.47 3.50 Taylor 1.44 1.88 2.46 2.57 2.63 {4411} 11118 H{001} 110 1.45 1.17 0.90 0.75 0.71 Q 1{021} 100 4.93 4.62 5.18 5.02 4.04 CH{001} 120 2.06 1.73 1.30 1.22 1.05 L {011} 522 3.75 4.05 6.57 7.03 7.75 Q 2 {113} 332 1.34 1.75 2.26 2.00 2.16 Q 3 {236} 385 3.56 4.22 4.30 3.86 3.56 Fig. 5b Highest intensity lines of β-fiber Fig. 6 Rolling direction rotated Cube orientation

VERMA et al.: DEFORMATION CHARACTERIZATION OF CARTRIDGE BRASS 287 much less than the strength of Bs, L, R and S components. The reducing texture strength of Cube, H and CH with deformations shows the indication of plane strain rolling, while the unsteady texture strength of Q 1, Q 2 and Q 3 reveal that those are transient texture orientations during deformation. The laboratory rolling at normal room temperature used in the study was plane strain rolling, which elongated grains only along rolling direction without much strain in the transverse direction (TD) of the samples. The mean grain size measured by line intercept methods revealed the increase in grain size along rolling directions with rolling. The HR microstructure shows the log normal grain size distribution. Deformation shifts the log normal distribution due to preferential elongation of grain structure along rolling direction. During 15% and 30% deformation, the soft grains elongate while the hard grains do not change the size significantly, based on the orientation dependent Taylor factors of grains and produce bimodal grain size distributions. Distribution of deformation to all grains at higher deformations such as 40% and 50% rolling develops high frequency for large grains. The SFE of cartridge brass (with 30% Zn) is 0.025 Jm -2 compared to SFE of aluminium (0.170 Jm -2 ) and pure copper (0.08 Jm -2 ). The deformation mechanisms of metals and alloys by slip and twinning vary with their SFE 3. The deformation texture developed by rolling of pure copper and cartridge brass should not be expected to be similar. An elaborate pole figure and orientation distribution function analysis of pure copper and brass has been given elsewhere 11. The development of Bs component has been reported 11 and that has been supported by other researchers 3. The salient features of prior research 11 on this issue are (i) no significant increase of Cu and S texture components rather decrease or transition of texture beyond 60% cold reduction by rolling and (ii) strengthening of α-fiber with stronger Goss texture component. An earlier research work in contrary to the this work 11-14 said that (i) cold reduction (0-50)% developed Cu component in both pure copper and cartridge brass and (ii) deformation twinning occurred in brass after (40-50)% cold reduction preferably near {112}<111> texture component and deformation heterogeneities such as shear bands started forming beyond 60% cold reduction contributing to shear texture orientation. Though the transition of texture was tried to be explained by SFE one recent research work said that transition of texture could be still present without the variation of SFE of pure copper and its alloys 14. The present study on rolling texture development of cartridge brass deals with the deformations up to 50% in successive stages of 15%, 30%, 40% and 50% from HR sheet to clarify the deformation texture component whether Bs component, S component or Cu and also to check the texture transition if any (such as the reduction of Cu with deformation) because all these issues regarding texture directly associated with formability of cartridge brass during deep drawing. The results of present study show that the strength of the texture components such as Bs, L, R, Goss, Taylor, Cu and S increases with deformations. Though the Cu orientation shows an increasing trend the strength of Cu is much less than the strength of Bs and S. The texture transition for Cu, S and Bs is not found in this study. The strengthening of α-fiber and β-fiber is found with deformation. The texture strengths of Cube, H and CH reduce with rolling while the texture strengths of Q 1, Q 2 and Q 3 show transient trend. The formability of low SFE annealed brass is reported to be benefitted by Bs orientation supplemented by grain growth components. Generally, random texture crucially induces degree of isotropy. Dissimilar orientations of grains associated with random texture tends to result in the isotropic formability 15 whereas the rolling texture in high SFE fcc metals and alloys are the cause of earing problem during deep drawing 16. Conclusions The mean grain size measured by line intercept methods shows the increase in grain size in rolling direction with rolling at normal room temperature. The initial stages of deformation such as 15% and 30% reductions produce bi-modal grain size distribution while the 40% and 50% rolling induce the high frequencies for larger size grains with multi-modalilty in distribution. The deformation strengthens Bs, L, R, Goss, Taylor, Cu and S texture components. Though Cu shows an increasing trend, the strength of Cu is much less than the strength of Bs and S components. The transition in texture for Cu, S and Brass is not found in this study. The strengths of α-fiber and β-fiber increase with deformation. The texture strengths of

288 INDIAN J. ENG. MATER. SCI., AUGUST 2013 Cube, H and CH reduce which indicates plane strain deformations while transient texture strengths are observed for Q 1, Q 2 and Q 3 orientations. Acknowledgement The research grant of Defence Institute of Advanced Technology, Pune, for the project Comparison of deformation and recrystallization character of α-brass suitable for cartridge case manufacturing is acknowledged. References 1 Avner S H, Introduction to physical metallurgy, (TATA Mc-Graw Hill, New Delhi), 2001. 2 Doig A, Military metallurgy, (Maney, London), 1998. 3 Humphreys F J & Hatherly M, Recrystallization and related annealing phenomena, (Elsevier, Oxford, U K), 2004. 4 Dieter G E, Mechanical metallurgy, (Mc-Graw Hill, New York), 1986. 5 Gottstein G, Physical foundation of material science, (Springer, Berlin), 2004. 6 Pickering F B, Physical metallurgy and the design of steels, (Applied Science Publisher Limited, London), 1978. 7 Salari M & Akbarzadeh A, J Mater Process Technol, 182 (2007) 440-444. 8 Suresh K S, Sinha S, Chaudhary A & Suwas S, Mater Charact, 70 (2012), 74-82. 9 Oeztuerk T & Davies G J, Mater Sci Technol, 5 (1989) 186-193. 10 Leffers T & Ray R K, Progr Mater Sci, 54 (2009) 351-396. 11 Hirsch J & Luecke K, Acta Metall, 36 (1988) 2863-2882. 12 Leffers T & Juul Jensen D, Texture Microstruct, 8-9 (1988) 467-480. 13 Hutchinson W B, Duggan B J & Hatherly M, Met Tech, 6 (1979) 398-402. 14 Engler O, Acta Mater, 48 (2000), 4827-4840. 15 Hills R & Abbaschian R, Physical metallurgy principles, (PWS-Kent Publishing, Boston), 1992. 16 Liu J, Banovic S W, Fields R J & Morris J G, Metall Mater Trans A, 37A (2006), 1887-1891.