An investigation on the workability of sintered copper-silicon carbide preforms during cold axial upsetting

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 19, April 1, pp An investigation on the workability of sintered copper-silicon carbide preforms during cold axial upsetting M Sumathi a * & N Selvakumar b a Department of Mechanical Engineering, National Engineering College, Kovilpatti 68 53, India b Department of Mechanical Engineering, Mepco Schlenk Engineering College, Sivakasi 66 5, India Received June 11; accepted 19 March 1 An investigation has been carried out for the determination of the workability behaviour of Cu-SiC composites under two different stress conditions namely plane and tri-axial state. Cylindrical preforms with 95% initial preform density possessing aspect ratio of.6 are prepared from copper with various percent content of silicon carbide (up to %) metal matrix composites using a die and punch assembly with a 1. MN capacity Universal Testing Machine. The sintering operation is carried out in an electric furnace at 85 ± 1 C for a period of one hour in argon inert atmosphere and subsequently they are furnace cooled. Each sintered preform is subjected to an incremental compressive loading under inc stearate as lubricant. Based on the experimental results, the phenomenon like axial strain (ε ), various stress ratio parameters (namely (σ m /σ eff ) and (σ /σ eff )) and the formability stress index (β) under two different stress state conditions are systematically analysed considering the barreling effects. Keywords: Metal matrix composite, Copper, Silicon carbide, Upsetting, Workability, Formability stress index The first metal matrix composite (MMC) was developed in 197s for high-performance applications using continuous fibers and whiskers for reinforcement. The recent recognition that additions of ceramic reinforcement enable tailoring of physical as well as mechanical properties of MMCs has led to increasingly widespread use of these materials in electronic packaging and thermal-management applications. Recent market forecasts for MMCs use, suggest the prospect for accelerating growth as the materials are more widely understood and costs decrease, suggesting a bright future for this class of materials 1. Workability refers to the relative ease with which a material can be shaped through plastic deformation and it is a function of the material as well as the process. A number of researchers have reported the experimental and analytical research works on workability of cylindrical preforms. Abdel- Rahman and El-Sheikh 3 studied the workability and the effect of the relative density on the forming limit of P/M compacts by conducting upsetting test. A workability factor was presented for the case of uniaxial stress state condition which describes the effect of the mean and the effective stresses. Ahmed and Hasmi 4 have presented the results from the *Corresponding author ( nselva@mepcoeng.ac.in) simulation of bulge forming with pressure and inplane compressive load using the ANSYS finiteelement code. Park et al. 5 used upper bound analysis for the rotational upset forming and predicted that the forming load and the relationships between the rotational velocity of the die and the forming parameters. Selvakumar and Narayanasamy 6 have proposed lower aspect ratio preforms give uniform density and this is due to rapid load transfer resulting in extensive work hardening. However, it is not true for the higher aspect ratio preforms. Inigoraj et al. 7 have evaluated the strain-hardening phenomenon and experienced that in sintered aluminium-3.5% alumina composite preforms during axial compression tests. The attention on the hoop strain and axial strain behavior of any powder metallurgy (PM) compacts is much required for any cold forging processes. Sowerby et al. 8 made a research work on the effect of the hoop strain and the axial strain at the free surface of an upsetting specimen and from which they have obtained the associated stresses. Finite element analysis (FEA) were used to predict the surface strains and compared with experimental works. Kuhn and Downey 9 studied the basic deformation behaviour of sintered iron powder by conducting a simple uniaxial compression tests. Lee and Altan 1 investigated the influence of flow stress and friction

2 1 INDIAN J. ENG. MATER. SCI., APRIL 1 on metal flow in upset forging of ring and cylinders considering the work hardening of material. Narayanasamy et al. 11 studied the workability behaviour of Al-Fe composites during cold upsetting under triaxial stress state conditions and concluded that there is a drastic change in the formability behavior of compacts because of different content of iron and its particle sies and aspect ratios. A number of research works by Narayanasamy et al have been carried out previously on the workability behaviour of various composites under uniaxial, plane and triaxial stress state conditions. Their results have shown that the workability behaviour of metals/alloys/composites of P/M components depend on the aspect ratio, the preform geometry, the particle sie and the percentage of reinforcement for the composites, the die geometry, the lubricants and the compacting load. Ramesh et al. 16 studied the effect of second phase particle sie on the workability behaviour of composite. The experimental results were analysed under tri-axial stress state condition. The formability stress index, various stress ratio parameters were obtained for each particle sie additions and aspect ratios. The relationship between various stresses to the axial strain was discussed. Kumar et al. 17 discussed the effect of glass and SiC in aluminium matrix on workability and strain hardening behaviour of PM hybrid composites. Copper matrix composites reinforced with SiC particles, which combine the high electrical conductivity of copper and the high modulus of SiC, can be applied into many fields. The importance of the composites could be attributed to its high stiffness, superior room and elevated temperature strengths, improved wear resistance and low coefficient of thermal expansion 18. Novel copper matrix composites reinforced with silicon carbide fibers are being considered as a new generation of heat sink materials for the diverter of future fusion reactors. Hence, the present work is a complete investigation on the working behavior of Cu-SiC composite with different SiC percentage additions, together with the unreinforced matrix material at room temperature. In addition, an attempt is made to establish the relationship between the various stress ratio parameter namely (σ m /σ eff ) and (σ /σ eff ) and formability stress index (β) at triaxial and plane stress state conditions. Experimental Procedure In this study, electrolytic copper powder procured from M/s The Metal Powder Company (P) Ltd, Madurai, India and purity was found as 99.7%. The particle sie of the copper powder was 15 µm and for silicon carbide powder the particle sie was 8.54 µm were taken for this study. To investigate the particle sie distribution, metallographic studies were carried out under scanning electron microscope (SEM) and the respective image is shown in Fig. 1. The required amount of copper and silicon carbide powders of different silicon carbide contents (-%) were measured and blended on weight percent basis in a high energy ball mill (planetary mono mill- Fritsch Pulverisette-6) in order to obtain homogeneous mix. And also inc stearate of quantity, 1% of the mass of Cu-SiC powder was mixed to act as a lubricant 17 during compacting. Once again the metallographic study was carried out to make sure about the homogeneous mixing of powders and the SEM image of the same is as shown in Fig.. The powders were compacted in a 1. MN UTM by closed die set assembly technique. Mass constancy principle has been adopted for the preparation of Cu-SiC composites. Fig. 1 SEM image of (a) copper powder and (b) silicon carbide powder

3 SUMATHI & SELVAKUMAR: COPPER-SILICON CARBIDE PREFORMS 13 and workability parameters (β) were determined using the expressions specified below for plane and triaxial stress state conditions, as described elsewhere 11-14,16. Theoretical Analysis The present investigation is based on analytical determination of the deformed density, stresses, strains and formability stress index (β). In the present investigation, the material under consideration is porous. As explained elsewhere,1 the expression for the bulging of a cylindrical preform of a fully dense material can be written as follows, provided by the bulging contour assumes the form of a circular arc. Fig. SEM image of Cu-15SiC composite The compacts were prepared with the aspect ratio of.6. The compacting load range was considered as 1.9±1 MPa in order to obtain the initial preform density of 95% of the theoretical density. The inc stearate dust was also applied over the inner surfaces of the die and outer surfaces of the punch. Then the preforms were sintered in a muffle furnace at a temperature of 85 ± 1 C for a period of one hour. To avoid oxidation the preforms were heated in argon inert atmosphere. After the sintering schedule, the compacts were cooled in the furnace itself. The sintered preforms were cleaned and the dimensional measurement made before the deformation. The upsetting tests were conducted as described elsewhere 11-15,19 on UTM. Fine inc stearate powder was used as lubricant during cold upsetting. In order to evaluate the compressibility, the sintered preforms were deformed at room temperature between flat dies at an incremental loading step of kn, increased until the appearance of first visible cracks on the free surface of the preform. Immediately after each incremental loading, the contact diameter at the top (D ct ), the contact diameter at the bottom (D cb ), the bulged diameter (D b ), the height of the preforms (h f ) and the density ( f ) were recorded. Before upsetting, the initial diameter (D ), the initial height (h ) and the initial preform density ( ) of the specimens were measured. Moreover, the density measurements of the preforms were carried out using the Archimedes principle 6,11. Using the load, dimensional parameters and others (namely density), different true stresses (namelyσ, σ, σ m and σ eff ) and different true strains, (namely ε and ε ) ( D b + D c ) h f π D h = π 4 1 (1) Equation (1) is based on the volume constancy principle. However, the same is not true for porous preform upsetting; instead, the mass constancy principle is employed. Applying the mass constancy principle before and after deformation, Eq. (1) converts to π D h = 4 th 1 f ( ) D + b Dc hf th π Equation can be rearranged as: h = th th hf 3D D + D b c () (3) Taking natural logarithms on both sides of Eq. (3), the following expression is obtained. f ln th + h = ln ln th h f Db + D ln 3D However, Eq. (4) can be further simplified to: e th h = ε ln and ( D b + Dc ε = h ) f 3D f ε ε = th where, c (4) (5)

4 14 INDIAN J. ENG. MATER. SCI., APRIL 1 Since the ratio is taken to be constant, th Eq. (5) shows an exponential relationship between the f density ratio and between the difference in the th two strains ε and ε. According to Narayanasamy and Pandey the state of stress in plane stress condition is as follows: σ eff.5 (.5 + α )[ 3( 1+ α + α )] σ =... (6) where σ eff is the effective stress. α is the Poisson s ratio and σ is the axial stress. Since the radial stress, σ r is ero at the free surface and the hoop stress σ is σ = ( 1+ α ) σ ( + α ) and the hydrostatic stress σ m is: (7) As an evidence of experimental investigation implying the importance of the spherical component of the stress state on fracture called a formability stress index (β) 3,11,4 β 3σ m = (1) σ eff This index explains the fracture limit as explained elsewhere 4. Results and Discussion Figure 3 has been drawn between the stress ratio parameter (σ m /σ eff ) and the fractional theoretical 1 σ = + 3 [ σ ] m σ (8) According to Narayanasamy and Ponalagusamy 3 the state of stress in a triaxial stress condition is given as follows: The hoop stress can be determined by α + σ α + th th th σ = (9) Since σ r = σ in case of triaxial stress condition, the hydrostatic stress is given by, σ + σ σ = m (1) 3 and the effective stress is σ eff σ + σ th = 1 th ( σ + σ σ ).5 (11) Fig. 3 Stress ratio parameter (σ m /σ eff ) vs. fractional theoretical density (a) plane stress state condition and (b) triaxial stress th state condition

5 SUMATHI & SELVAKUMAR: COPPER-SILICON CARBIDE PREFORMS 15 density for Cu-SiC composite with varying th SiC contents for aspect ratio of.6. This plot shows that the stress ratio parameter (σ m /σ eff ) with respect to the fractional theoretical density increases th with increasing SiC content in the Cu-SiC composites irrespective of the stress state conditions. Cu-5SiC composite shows a very high increasing value of the stress ratio parameter with respect to fractional theoretical density. Figure 4 has been plotted between the stress ratio parameter (σ /σ eff ) and the fractional theoretical density for Cu-SiC composite with varying SiC th contents. It is noted that as the SiC content in the Cu-SiC composite increases, the stress ratio parameter (σ /σ eff ) also increases with fractional theoretical density irrespective of stress state conditions. th Figure 5 shows the plot between the formability stress index (β) and the fractional theoretical density for the upsetting of Cu and Cu-SiC powder th Fig. 4 Stress ratio parameter (σ /σ eff ) vs. fractional theoretical density (a) plane stress state condition and (b) triaxial stress th state condition Fig. 5 Formability stress index (β) vs. fractional theoretical density (a) plane stress state condition and (b) triaxial stress th state condition

6 16 INDIAN J. ENG. MATER. SCI., APRIL 1 compacts under plane and triaxial stress state conditions. It is observed that, the formability stress index (β) varies exponentially with increasing amount of the fractional theoretical density for triaxial th stress state condition. The level of enhancement of formability stress index is very high for Cu-5SiC composite irrespective of the stress state conditions. The reason may be associated with stacking fault energy. In the case of copper the stacking fault energy is low and this improves the work hardening effect. Figure 6 shows the relationship between the axial strain (ε ) and the stress ratio parameter (σ m /σ eff ) for various SiC content. It is observed that as the increasing of SiC content decreases the higher axial strain irrespective of the stress state condition because of large number of pores associated with high hydrostatic stress. Figure 7 has been drawn between the axial strain (ε ) and the stress ratio parameter (σ /σ eff ) for the upsetting of Cu, Cu-SiC powder compacts for the various percentage addition of SiC under plane and triaxial stress state conditions. It observed that the axial strain of the compacts purely depends on the percentage additions. From the Fig. 7, it is further noted that for the compacts with higher addition of the SiC, the initiation of crack appeared at a low axial strain. Figure 8 has been plotted between the slope value of the axial stain versus stress ratio parameter and the percentage of SiC content in the Cu-SiC composites. It is noted from the plot the slope value increases, reaches the maximum value and then decreases with increasing SiC content irrespective of the stress ratio parameters and stress state conditions. Fig. 6 Axial strain ( ε ) vs. Stress ratio parameter (σ m / σ eff ) (a) plane stress state condition and (b) triaxial stress state condition Fig. 7 Axial strain ( ε ) vs Stress ratio parameter (σ /σ eff ) (a) plane stress state condition and (b) triaxial stress state condition

7 SUMATHI & SELVAKUMAR: COPPER-SILICON CARBIDE PREFORMS 17 (vi) 1.5 for the case of plane and triaxial stress state conditions respectively) is obtained. For the compacts with higher value of SiC additions, the initiation of crack appeared at a low axial strain. Fig. 8 The relationship between slope of axial strain vs. stress ratio parameter and percentage of SiC content (a) plane stress state condition and (b) triaxial stress state condition Conclusions The following conclusions can be drawn from the present investigation for the Cu-SiC composites: (i) The enhancement of strength property is very high for the case of 5% of SiC with copper compared with pure copper and 1, 15, % SiC composites. (ii) For lower SiC content (5%), the increase in axial strain is obtained. (iii) For higher SiC content in the Cu-SiC composite, the larger number of pores is developed and hence the higher hydrostatic stress is developed for closing the pores during plastic deformation. (iv) The slope value of plots between the stress ratio parameter and the axial strain, increases, reaches a peak value and then decreases with increasing SiC content. (v) For the case of compacts with Cu-5SiC, a very high formability stress index (β) (1.9 and Nomenclature D = initial diameter of the preform (mm) D = bulged diameter of the preform after deformation (mm) b D c = contact diameter of the preform after deformation (mm) h = initial height of the cylindrical preform (mm) h = height of the preform after deformation (mm) f D cb = contact bottom diameter of the preform after deformation (mm) D ct = contact top diameter of the preform after deformation (mm) = initial preform density of the cylinder (g/cc) f = density of the preform after deformation (g/cc) th = theoretical density of the fully dense material (g/cc) ε = true strain in the axial direction ε = true strain in the hoop direction β = formability stress index σ eff = true effective stress (MPa) σ m = true hydrostatic or mean stress (MPa) σ = true stress in the hoop direction (MPa) f R = = relative density th UTM = universal testing machine SEM = scanning electron microscope References 1 Introduction to application Composites -1 ASTM handbook. 3, 1. George Dieter, Hand book on workability analysis and applications, 1 st ed, (ASM publications, USA),. 3 Abdel-Rahman M & El-Sheikh M N, J Mater Process Technol, 54 (1995) Ahmed M & Hashmi M S J, J Mater Process Technol, 77 (1998) Park J H, Kim Y H & Jin Y E, J Mater Process Technol, 111 (1) Selvakumar N & Narayanasamy R, J Mater Process Technol, 14 (3) Inigoraj A J R, Narayanasamy R & Pandey K S, J Mater Process Technol, 84 (1998) Sowerby R, O Reilly I, Chandrasekaran N & Dung N L, J Eng Mater Technol Trans ASME, 16 (1984) Kuhn H A & Downey C L, Int J Powder Metall, 7 (1) (1971) Lee C H & Altan T, J Eng Ind, 94 (197) Narayanasamy R, Ramesh T & Pandey K S, J Mater Sci Eng A, 391 (5) Narayanasamy R, Ramesh T & Pandey K S, Mater Des, 7 (6)

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