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Extrusion behavior of RQ Al composite powder manufactured by a stone mill type crusher
Materials Science and Engineering A304–306 (2001) 559–563
Extrusion behavior of RQ Al composite powder manufactured by a stone mill type crusher H.T. Son∗ , T.S. Kim, J.H. Lee, D.Y. Maeng, S.J. Hong, C.W. Won, S.S. Cho, B.S. Chun Rapidly Solidified Materials Research Center, Chungnam National University, Taejon 305-764, South Korea
Abstract The aim of the present investigation is to predict surface cracking and reinforcement distribution during hot extrusion in Al 6061 and 5083 composite powder reinforced by hybrid TiC–Al2 O3 particles. The composite powders were manufactured by crushing in the newly developed stone mill crusher using twin rolled flakes. With increasing initial billet temperature, surface cracking occurred during extrusion due to a decrease in damage criterion. It was enlightened to obtain an optimal distribution of ceramic particles in Al alloy matrix as a function of milling cycles. Optimal distribution of the reinforcement in the matrix was taken into consolidation with milling cycles. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Extrusion; Damage criterion; Surface cracking; Twin roll
1. Introduction Aluminum alloy composite materials reinforced by ceramic particles such as Al2 O3 , SiC and TiC are known to have good mechanical properties and generally applied to the aerospace and automotive industries where higher wear property and low thermal expansion coefficient are required [1–4]. Since various MMC manufacturing processes were developed, rapid solidification (RS) is regarded as a promising route for further improved MMC in the mechanical properties due to a recent development of technique for the working processes such as extrusion, forging, and rolling. In terms of powder metallurgy for manufacturing MMC, it finds many advantages in the extrusion process as follows [5]: (1) the break up of agglomerated reinforcement, (2) a more homogeneous distribution of reinforcement in the matrix, (3) the improvement of the mechanical properties and microstructures, and (4) high productivity and low product cost. An important issue during metal-forming process is how the desired deformation can be accomplished without fracture of the work-piece. In industrial practice, however, the experience of the designer is decisive for fracture-free quality of the products, but often requiring very costly trial and error. Thus, in order to obtain an MMC product with quality, it strongly needs to predict the formation of the fracture during extrusion.
∗
Corresponding author.
In this study, TiC–Al2 O3 reinforced Al 6061 and 5083 alloy composite bars were manufactured by twin rolling, stone mill type crushing and hot extruding [6]. The objective of the present research is to estimate the cracking at the surface and distribution of TiC–Al2 O3 particles in the matrix during extrusion. Numerical analysis using a commercial finite-element (FE) software was also carried out to examine the agreement with the experimental results.
2. Experimental procedure 2.1. Materials Commercial Al 6061 and 5083 alloys were used to prepare the MMC composites, where 6061 and 5083 alloys consisted of 0.9 wt.% Mg–0.48 wt.% Si–0.17 wt.% Cu–0.17 wt.% Fe–0.08 wt.% Cr–0.04 wt.% Zn–0.02 wt.% Ti and 4.35 wt.% Mg–0.68 wt.% Mn–0.12 wt.% Cr–0.11 wt.% Si–0.15 wt.% Fe–0.06 wt.% Zn, respectively. The TiC–Al2 O3 powders for reinforcement were synthesized by self-propagating high temperature synthesis (SHS) at sizes of 1–3 m in diameter. 2.2. Twin rolling, crushing and composing Each alloy was heated to 800◦ C in an electric furnace and poured into a tundish preheated to 800◦ C, then the melt was delivered to water cooled revolving Cu twin rolls through a Si3 N4 melt delivery nozzle of 2 mm in diameter. Flakes of about 60 mm long and 5 mm wide were produced at a rate
0921-5093/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 5 3 4 - 3
560
H.T. Son et al. / Materials Science and Engineering A304–306 (2001) 559–563
of 0.5 kg/min. They were subsequently chopped into 5 mm long jaw crusher before feeding into stone mill type crusher. TiC–Al2 O3 particles were mixed with the chopped flakes in the stone mill type crusher rotating at a rate of 1500 rpm, having 10 vol.% in the matrix.
Table 1 The critical damage values of composites measured by tensile test (expressed in N/mm2 ) Initial billet temperature (◦ C)
2.3. Consolidation The hybrid ceramic particle reinforced 5083 and 6061 composite powders were cold compacted to 85% of theoretical density by 350 t press, and then degassed for 1 h at 400◦ C down to 10−2 Torr. The consolidated and degassed bulk materials were hot extruded using at 800 t press after 1 h holding at different preheated temperatures (450, 500, and 550◦ C) with a reduction ratio of 23:1 into a bar of 15 mm diameter. In order to prevent the effect of the equipment on the extrusion, container and die were consistently preheated at 450◦ C. 2.4. Numerical analysis In the present study, on the basis of the simulation results using DEFORM, a rigid-thermoviscoplastic FE program is developed. The FE formulation is only outlined briefly here, because the basic mathematical description of the methods, as well as the solution techniques, are given in several books [7,8]. From the variational principle, the functional Π for rigid–viscoplastic material can be written as follows: Z Z (1) t¯i vi dS Π = E(˙εij ) dV − F
SF
where E(˙εij ) is the viscoplastic potential, t¯i the traction specified on the boundary SF and vi the velocity component. The incompressibility constraint on the admissible velocity fields may be removed by introducing a penalty constant K and modifying the functional Π. Then, the solution of the original boundary-value problem is obtained from the solution of a dual-variational problem, where the first-order variation of the functional vanishes; Z Z Z t¯i ␦vi dS = 0 ␦Π = σ¯ ␦ε˙¯ dV + K ε˙ F ␦ε˙¯ F dV − F
F
SF
(2) where ␦vi is the arbitrary variation and ␦ε˙¯ and ε˙¯ F are the variation in the strain rate. Eq. (2) can be converted to non-linear algebraic equations by utilizing the finite-element discretization procedure. The solution of the non-linear simultaneous equation is obtained iteratively using the Newton–Raphson method. Modeling of surface cracking in the extrusion was undertaken to determine optimum processing conditions with preheating temperatures of the billets. Basic idea of the fracture criterion is that the fracture occurs when the value of a damage parameter reaches a critical value, generally called as damage criterion. In the present study, the
450 500
Composites 6061/10 vol.% TiC–Al2 O3
5083/10 vol.% TiC–Al2 O3
7.6 3.5
7.9 6.5
Table 2 Process conditions for FE simulation of extrusion Material conditions
Flow stress, σ Friction coefficient Damage criterion Ram speed (mm/s)
Extrusion temperature, 6061/10 vol.% TiC–Al2 O3 (◦ C) 450
500
64.77ε 0.0359 0.15 7.6 10
45.55ε 0.0893 0.15 3.5 10
Crockcroft–Latham [9] criterion is used to determine the critical damage value if and where surface cracking occur during the extrusion process. The formula is expressed as follows: Z εf σ ∗ dε (3) C= 0
where σ ∗ is the maximum tensile strength, εf the fracture strain and C the Crockcroft–Latham constant. The mechanical properties and critical damage value (criterion) of the material were measured from uniaxial tensile test. The damage criterion for composites at various temperatures are presented in Table 1. The computational conditions in the FE simulation of the extrusion process are summarized in Table 2.
3. Results and discussion Fig. 1 shows the effect of the initial billet temperature on the peak ram pressure during extrusion of the 6061 or 5083/TiC–Al2 O3 alloy composites. With increasing initial billet temperature, the peak extrusion pressure was decreased due to the reduction in flow stress. It is shown that the maximum extrusion pressure of 6061/TiC–Al2 O3 composite is lower than that of 5083/TiC–Al2 O3 composite. The difficulty for extrusion in 5083 Al matrix composite may due to the higher strength than that of 6061. Fig. 2 shows the surface of the extruded bars of both the alloys for given initial billet temperatures after extrusion. Surface cracks were observed on the surface of 6061 Al matrix and 5083 Al matrix composite bars above 450 and 500◦ C, respectively. Severe tearing is shown in the 6061 composite bar extruded at 550◦ C (Fig. 2a), decreasing with
H.T. Son et al. / Materials Science and Engineering A304–306 (2001) 559–563
561
Fig. 1. The maximum extrusion pressure of 5083/10 vol.% Al2 O3 –TiC and 6061/10 vol.% Al2 O3 –TiC in extrusion at various initial billet temperatures.
decreasing temperature to 500◦ C. The surface cracking was only observed at the initial billet temperature of 550◦ C in the 5083/10 vol.% TiC–Al2 O3 composites. Comparing the surface cracks and damage criterion (Table 1), damage criterion was decreased and surface crack was increased with increasing initial billet temperature. With consideration to the relationship, the surface cracking formed during extrusion can be anticipated by calculating the damage criterion at the composite bars. It is well known that the calculation of the damage criterion using Crockcroft–Lantham equation (3). Fig. 3(a) and (b) shows the surface cracking
Fig. 3. Prediction of surface cracking and distribution of damage value for extrusion at initial billet temperature of (a) 500◦ C and (b) 450◦ C.
and distribution of the damage values calculated by DEFORM, a commercial FEM-simulation software, during extrusion of 6061/10 vol.% TiC–Al2 O3 composites as a function of the initial billet temperatures at 450 and 500◦ C,
Fig. 2. Surface quality of extruded bars showing extent with initial billet temperatures: (a) 6061Al/10 vol.% Al2 O3 –TiC and (b) 5083Al/10 vol.% Al2 O3 –TiC.
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H.T. Son et al. / Materials Science and Engineering A304–306 (2001) 559–563
respectively. The calculated maximum damage values, which is the value over the criterion, are 6.89 N/mm2 (Fig. 3(b)) and 4.60 N/mm2 (Fig. 3(a)) at given temperatures of 450 and 500◦ C, respectively. The calculation results show the highest damage values distributed along the surface of the extruded rods, possibly due to the friction stress formed between the extruded body and the die wall, resulting in a restriction of the metal flow. However, it is already shown that the measured critical damage values (=criterion) in 6061 composite bar at the same temperatures with the calculation were 7.6 and 3.5 N/mm2 , respectively (Table 1). Comparison of the measured and calculated results at 500◦ C shows that the latter exceeds the measured damage criterion of C = 3.5 N/mm2 , indicting a crack formation. In addition, we can understand the crack initiation at the surface of the bar corresponding to the maximum distribution of the values at the surface. On the contrary, the calculated damage of 6.89 N/mm2 does not exceed the damage criterion of C = 7.6 N/mm2 at 450◦ C, resulting in no surface crack. Fig. 4(a)–(d) shows the microstructure of the as-milled powder and the extruded bar with the milling cycles. The reinforced ceramic particles were distributed along the edge of the milled powder after a cycle, thus particle rich and depleted zones were formed after extrusion (Fig. 4(a) and (b)), where Fig. 4(b) is the enlarged form of Fig. 4(a). On the other hand, much longer milling cycles result in a homogeneous distribution of the particles through the milled
powders (Fig. 4(c)), thus no lamella type reinforcement distribution appeared as shown in Fig. 4(d). The mechanism of the lamella type distribution is possibly enlightened in the next part of this paper. Fig. 5 shows procedures for the formation of the lamella structure, as shown in Fig. 4(a), during extrusion from the composed powder which have the reinforcement distribution along the rim side by milling. Here, the schematic on the left bottom in Fig. 5 represents the die geometry at extruding machine, where (a) is dead metal zone, (b) die entry and (c) part extruded bar in the die. In part (a) in the die, it is clearly seen that the initial composite powders were present without any deformation, which usually called to dead metal zone of billet (Fig. 5(a)). That is an intimate contact between composite powders has been attained, but the shape and the reinforcement distribution of the composed powder remain almost unchanged. Large deformation starts only as the composite powder approached the die entry (see Fig. 5(b)). In this zone, the composite powders were elongated along the metal flow toward the die entry. The matrix and the reinforcement forms a lamella structure distributed in the extrusion direction out of the die entry (Fig. 5(c)). To sum up, this lamella type distribution of the reinforcement in composite material may affect the final property with deterioration compared to the composite with homogeneous distribution. Thus, in terms of the deformation behavior during extrusion, it is important to make an initially homogeneous composite powder.
Fig. 4. Microstructure of the milled powder and the extruded bars with milling cycles: (a) the milled powder and (b) the extruded bar of one cycle; (c) the milled powder and (d) the extruded bar of four cycles.
H.T. Son et al. / Materials Science and Engineering A304–306 (2001) 559–563
563
Fig. 5. Procedures forming lamella type distribution of the reinforcement in Al composite during extrusion.
4. Conclusion
References
The Crockcroft–Latham damage criteria of 5083/10 vol.% TiC–Al2 O3 and 6061/TiC–Al2 O3 composites were experimentally obtained. It was possible to explain the formation of surface cracking during extrusion by the damage criterion of the composites. The composite bar extruded from the composite powders, which is prepared by twin rolls and the newly designed stone mill crusher, have lamella type microstructure of reinforcement rich and depleted zones. The distribution of reinforcement was dependent on the initial composed powder structure and the deformation mechanism of the composite powders during the extrusion was examined.
[1] F.P. Kehoe, G.A. Chadwick, Mater. Sci. Eng. A 135 (1991) 209. [2] S.V. Nair, J.K. Tien, R.C. Bates, Int. Metall. Rev. 30 (1985) 275. [3] N. Tsangaraskis, J. Nunes, J.M. Slepetz, Eng. Fract. Mech. 30 (1988) 565. [4] D. Zhao, F.R. Tuler, D.J. Lyoyd, Scripta Metall. Mater. 27 (1992) 41. [5] C.H.J. Davies, W.C. Chen, E.B. Hawbolt, I.V. Samarasekera, J.K. Brimacomb, Scripta Metall. Mater. 32 (3) (1995) 309. [6] J.H. Lee, T.S. Kim, S.J. Hong, D.Y. Maeng, H.T. Son, C.W. Won, S.S. Cho, B.S. Chun, Mater. Sci. Eng. A, this volume. [7] O.C. Zienkiewicz, The Finite Element Method, 3rd Edition, McGraw-Hill, UK, 1977. [8] S. Kobayashi, S.I. Oh, T. Altan, Metal Forming and Finite Element Method, Oxford University Press, London, 1988. [9] M.G. Crockcroft, D.J. Latham, J. Inst. Met. 96 (1968) 33.
Extrusion behavior of RQ Al composite powder manufactured by a stone mill type crusher H.T. Son∗ , T.S. Kim, J.H. Lee, D.Y. Maeng, S.J. Hong, C.W. Won, S.S. Cho, B.S. Chun Rapidly Solidified Materials Research Center, Chungnam National University, Taejon 305-764, South Korea
Abstract The aim of the present investigation is to predict surface cracking and reinforcement distribution during hot extrusion in Al 6061 and 5083 composite powder reinforced by hybrid TiC–Al2 O3 particles. The composite powders were manufactured by crushing in the newly developed stone mill crusher using twin rolled flakes. With increasing initial billet temperature, surface cracking occurred during extrusion due to a decrease in damage criterion. It was enlightened to obtain an optimal distribution of ceramic particles in Al alloy matrix as a function of milling cycles. Optimal distribution of the reinforcement in the matrix was taken into consolidation with milling cycles. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Extrusion; Damage criterion; Surface cracking; Twin roll
1. Introduction Aluminum alloy composite materials reinforced by ceramic particles such as Al2 O3 , SiC and TiC are known to have good mechanical properties and generally applied to the aerospace and automotive industries where higher wear property and low thermal expansion coefficient are required [1–4]. Since various MMC manufacturing processes were developed, rapid solidification (RS) is regarded as a promising route for further improved MMC in the mechanical properties due to a recent development of technique for the working processes such as extrusion, forging, and rolling. In terms of powder metallurgy for manufacturing MMC, it finds many advantages in the extrusion process as follows [5]: (1) the break up of agglomerated reinforcement, (2) a more homogeneous distribution of reinforcement in the matrix, (3) the improvement of the mechanical properties and microstructures, and (4) high productivity and low product cost. An important issue during metal-forming process is how the desired deformation can be accomplished without fracture of the work-piece. In industrial practice, however, the experience of the designer is decisive for fracture-free quality of the products, but often requiring very costly trial and error. Thus, in order to obtain an MMC product with quality, it strongly needs to predict the formation of the fracture during extrusion.
∗
Corresponding author.
In this study, TiC–Al2 O3 reinforced Al 6061 and 5083 alloy composite bars were manufactured by twin rolling, stone mill type crushing and hot extruding [6]. The objective of the present research is to estimate the cracking at the surface and distribution of TiC–Al2 O3 particles in the matrix during extrusion. Numerical analysis using a commercial finite-element (FE) software was also carried out to examine the agreement with the experimental results.
2. Experimental procedure 2.1. Materials Commercial Al 6061 and 5083 alloys were used to prepare the MMC composites, where 6061 and 5083 alloys consisted of 0.9 wt.% Mg–0.48 wt.% Si–0.17 wt.% Cu–0.17 wt.% Fe–0.08 wt.% Cr–0.04 wt.% Zn–0.02 wt.% Ti and 4.35 wt.% Mg–0.68 wt.% Mn–0.12 wt.% Cr–0.11 wt.% Si–0.15 wt.% Fe–0.06 wt.% Zn, respectively. The TiC–Al2 O3 powders for reinforcement were synthesized by self-propagating high temperature synthesis (SHS) at sizes of 1–3 m in diameter. 2.2. Twin rolling, crushing and composing Each alloy was heated to 800◦ C in an electric furnace and poured into a tundish preheated to 800◦ C, then the melt was delivered to water cooled revolving Cu twin rolls through a Si3 N4 melt delivery nozzle of 2 mm in diameter. Flakes of about 60 mm long and 5 mm wide were produced at a rate
0921-5093/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 5 3 4 - 3
560
H.T. Son et al. / Materials Science and Engineering A304–306 (2001) 559–563
of 0.5 kg/min. They were subsequently chopped into 5 mm long jaw crusher before feeding into stone mill type crusher. TiC–Al2 O3 particles were mixed with the chopped flakes in the stone mill type crusher rotating at a rate of 1500 rpm, having 10 vol.% in the matrix.
Table 1 The critical damage values of composites measured by tensile test (expressed in N/mm2 ) Initial billet temperature (◦ C)
2.3. Consolidation The hybrid ceramic particle reinforced 5083 and 6061 composite powders were cold compacted to 85% of theoretical density by 350 t press, and then degassed for 1 h at 400◦ C down to 10−2 Torr. The consolidated and degassed bulk materials were hot extruded using at 800 t press after 1 h holding at different preheated temperatures (450, 500, and 550◦ C) with a reduction ratio of 23:1 into a bar of 15 mm diameter. In order to prevent the effect of the equipment on the extrusion, container and die were consistently preheated at 450◦ C. 2.4. Numerical analysis In the present study, on the basis of the simulation results using DEFORM, a rigid-thermoviscoplastic FE program is developed. The FE formulation is only outlined briefly here, because the basic mathematical description of the methods, as well as the solution techniques, are given in several books [7,8]. From the variational principle, the functional Π for rigid–viscoplastic material can be written as follows: Z Z (1) t¯i vi dS Π = E(˙εij ) dV − F
SF
where E(˙εij ) is the viscoplastic potential, t¯i the traction specified on the boundary SF and vi the velocity component. The incompressibility constraint on the admissible velocity fields may be removed by introducing a penalty constant K and modifying the functional Π. Then, the solution of the original boundary-value problem is obtained from the solution of a dual-variational problem, where the first-order variation of the functional vanishes; Z Z Z t¯i ␦vi dS = 0 ␦Π = σ¯ ␦ε˙¯ dV + K ε˙ F ␦ε˙¯ F dV − F
F
SF
(2) where ␦vi is the arbitrary variation and ␦ε˙¯ and ε˙¯ F are the variation in the strain rate. Eq. (2) can be converted to non-linear algebraic equations by utilizing the finite-element discretization procedure. The solution of the non-linear simultaneous equation is obtained iteratively using the Newton–Raphson method. Modeling of surface cracking in the extrusion was undertaken to determine optimum processing conditions with preheating temperatures of the billets. Basic idea of the fracture criterion is that the fracture occurs when the value of a damage parameter reaches a critical value, generally called as damage criterion. In the present study, the
450 500
Composites 6061/10 vol.% TiC–Al2 O3
5083/10 vol.% TiC–Al2 O3
7.6 3.5
7.9 6.5
Table 2 Process conditions for FE simulation of extrusion Material conditions
Flow stress, σ Friction coefficient Damage criterion Ram speed (mm/s)
Extrusion temperature, 6061/10 vol.% TiC–Al2 O3 (◦ C) 450
500
64.77ε 0.0359 0.15 7.6 10
45.55ε 0.0893 0.15 3.5 10
Crockcroft–Latham [9] criterion is used to determine the critical damage value if and where surface cracking occur during the extrusion process. The formula is expressed as follows: Z εf σ ∗ dε (3) C= 0
where σ ∗ is the maximum tensile strength, εf the fracture strain and C the Crockcroft–Latham constant. The mechanical properties and critical damage value (criterion) of the material were measured from uniaxial tensile test. The damage criterion for composites at various temperatures are presented in Table 1. The computational conditions in the FE simulation of the extrusion process are summarized in Table 2.
3. Results and discussion Fig. 1 shows the effect of the initial billet temperature on the peak ram pressure during extrusion of the 6061 or 5083/TiC–Al2 O3 alloy composites. With increasing initial billet temperature, the peak extrusion pressure was decreased due to the reduction in flow stress. It is shown that the maximum extrusion pressure of 6061/TiC–Al2 O3 composite is lower than that of 5083/TiC–Al2 O3 composite. The difficulty for extrusion in 5083 Al matrix composite may due to the higher strength than that of 6061. Fig. 2 shows the surface of the extruded bars of both the alloys for given initial billet temperatures after extrusion. Surface cracks were observed on the surface of 6061 Al matrix and 5083 Al matrix composite bars above 450 and 500◦ C, respectively. Severe tearing is shown in the 6061 composite bar extruded at 550◦ C (Fig. 2a), decreasing with
H.T. Son et al. / Materials Science and Engineering A304–306 (2001) 559–563
561
Fig. 1. The maximum extrusion pressure of 5083/10 vol.% Al2 O3 –TiC and 6061/10 vol.% Al2 O3 –TiC in extrusion at various initial billet temperatures.
decreasing temperature to 500◦ C. The surface cracking was only observed at the initial billet temperature of 550◦ C in the 5083/10 vol.% TiC–Al2 O3 composites. Comparing the surface cracks and damage criterion (Table 1), damage criterion was decreased and surface crack was increased with increasing initial billet temperature. With consideration to the relationship, the surface cracking formed during extrusion can be anticipated by calculating the damage criterion at the composite bars. It is well known that the calculation of the damage criterion using Crockcroft–Lantham equation (3). Fig. 3(a) and (b) shows the surface cracking
Fig. 3. Prediction of surface cracking and distribution of damage value for extrusion at initial billet temperature of (a) 500◦ C and (b) 450◦ C.
and distribution of the damage values calculated by DEFORM, a commercial FEM-simulation software, during extrusion of 6061/10 vol.% TiC–Al2 O3 composites as a function of the initial billet temperatures at 450 and 500◦ C,
Fig. 2. Surface quality of extruded bars showing extent with initial billet temperatures: (a) 6061Al/10 vol.% Al2 O3 –TiC and (b) 5083Al/10 vol.% Al2 O3 –TiC.
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H.T. Son et al. / Materials Science and Engineering A304–306 (2001) 559–563
respectively. The calculated maximum damage values, which is the value over the criterion, are 6.89 N/mm2 (Fig. 3(b)) and 4.60 N/mm2 (Fig. 3(a)) at given temperatures of 450 and 500◦ C, respectively. The calculation results show the highest damage values distributed along the surface of the extruded rods, possibly due to the friction stress formed between the extruded body and the die wall, resulting in a restriction of the metal flow. However, it is already shown that the measured critical damage values (=criterion) in 6061 composite bar at the same temperatures with the calculation were 7.6 and 3.5 N/mm2 , respectively (Table 1). Comparison of the measured and calculated results at 500◦ C shows that the latter exceeds the measured damage criterion of C = 3.5 N/mm2 , indicting a crack formation. In addition, we can understand the crack initiation at the surface of the bar corresponding to the maximum distribution of the values at the surface. On the contrary, the calculated damage of 6.89 N/mm2 does not exceed the damage criterion of C = 7.6 N/mm2 at 450◦ C, resulting in no surface crack. Fig. 4(a)–(d) shows the microstructure of the as-milled powder and the extruded bar with the milling cycles. The reinforced ceramic particles were distributed along the edge of the milled powder after a cycle, thus particle rich and depleted zones were formed after extrusion (Fig. 4(a) and (b)), where Fig. 4(b) is the enlarged form of Fig. 4(a). On the other hand, much longer milling cycles result in a homogeneous distribution of the particles through the milled
powders (Fig. 4(c)), thus no lamella type reinforcement distribution appeared as shown in Fig. 4(d). The mechanism of the lamella type distribution is possibly enlightened in the next part of this paper. Fig. 5 shows procedures for the formation of the lamella structure, as shown in Fig. 4(a), during extrusion from the composed powder which have the reinforcement distribution along the rim side by milling. Here, the schematic on the left bottom in Fig. 5 represents the die geometry at extruding machine, where (a) is dead metal zone, (b) die entry and (c) part extruded bar in the die. In part (a) in the die, it is clearly seen that the initial composite powders were present without any deformation, which usually called to dead metal zone of billet (Fig. 5(a)). That is an intimate contact between composite powders has been attained, but the shape and the reinforcement distribution of the composed powder remain almost unchanged. Large deformation starts only as the composite powder approached the die entry (see Fig. 5(b)). In this zone, the composite powders were elongated along the metal flow toward the die entry. The matrix and the reinforcement forms a lamella structure distributed in the extrusion direction out of the die entry (Fig. 5(c)). To sum up, this lamella type distribution of the reinforcement in composite material may affect the final property with deterioration compared to the composite with homogeneous distribution. Thus, in terms of the deformation behavior during extrusion, it is important to make an initially homogeneous composite powder.
Fig. 4. Microstructure of the milled powder and the extruded bars with milling cycles: (a) the milled powder and (b) the extruded bar of one cycle; (c) the milled powder and (d) the extruded bar of four cycles.
H.T. Son et al. / Materials Science and Engineering A304–306 (2001) 559–563
563
Fig. 5. Procedures forming lamella type distribution of the reinforcement in Al composite during extrusion.
4. Conclusion
References
The Crockcroft–Latham damage criteria of 5083/10 vol.% TiC–Al2 O3 and 6061/TiC–Al2 O3 composites were experimentally obtained. It was possible to explain the formation of surface cracking during extrusion by the damage criterion of the composites. The composite bar extruded from the composite powders, which is prepared by twin rolls and the newly designed stone mill crusher, have lamella type microstructure of reinforcement rich and depleted zones. The distribution of reinforcement was dependent on the initial composed powder structure and the deformation mechanism of the composite powders during the extrusion was examined.
[1] F.P. Kehoe, G.A. Chadwick, Mater. Sci. Eng. A 135 (1991) 209. [2] S.V. Nair, J.K. Tien, R.C. Bates, Int. Metall. Rev. 30 (1985) 275. [3] N. Tsangaraskis, J. Nunes, J.M. Slepetz, Eng. Fract. Mech. 30 (1988) 565. [4] D. Zhao, F.R. Tuler, D.J. Lyoyd, Scripta Metall. Mater. 27 (1992) 41. [5] C.H.J. Davies, W.C. Chen, E.B. Hawbolt, I.V. Samarasekera, J.K. Brimacomb, Scripta Metall. Mater. 32 (3) (1995) 309. [6] J.H. Lee, T.S. Kim, S.J. Hong, D.Y. Maeng, H.T. Son, C.W. Won, S.S. Cho, B.S. Chun, Mater. Sci. Eng. A, this volume. [7] O.C. Zienkiewicz, The Finite Element Method, 3rd Edition, McGraw-Hill, UK, 1977. [8] S. Kobayashi, S.I. Oh, T. Altan, Metal Forming and Finite Element Method, Oxford University Press, London, 1988. [9] M.G. Crockcroft, D.J. Latham, J. Inst. Met. 96 (1968) 33.