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High-velocity compaction of titanium powder and process characterization
Powder Technology 208 (2011) 596–599
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
High-velocity compaction of titanium powder and process characterization Zhiqiao Yan a,⁎, Feng Chen b, Yixiang Cai a a b
Guangzhou Research Institute of Nonferrous Metals, Guangzhou 510650, PR China Department of Applied Physics, Hong Kong Polytechnic University, Kowloon, Hong Kong, PR China
a r t i c l e
i n f o
Article history: Received 2 August 2010 Received in revised form 1 December 2010 Accepted 18 December 2010 Available online 25 December 2010 Keywords: High-velocity compaction (HVC) Titanium powder Impact energy per unit mass Green density Huang Pei-yun equation
a b s t r a c t Pure titanium powder was pressed into two kinds of samples through high-velocity compaction technology. The first was rings with 60 mm outer and 30 mm inner diameters and a mass of 57 g, and the second was cylinders with 20 mm diameter and a mass of 10 g. For the rings, the maximum shapable relative density reached 76.2% at impact energy of 2283 J. For the cylinders, however, it reached 96.0% at much lower impact energy of 1217 J. The reasons for the contradictory effects were analyzed, and a new quantity, impact energy per unit mass, was put forth to well characterize the difference. In addition, the relations between peak pressure and green density of the two kinds of samples were found to comply with Huang Pei-yun equation, and the densification mechanism was discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Powder metallurgy (PM) is a near net shape method to prepare titanium alloys and composites with low cost. Unfortunately, titanium powder is very difficult to press into dense compacts through conventional compaction (CC) methods due to its high work hardening rate. Although some advanced forming methods, such as powder forging and hot isostatic pressing, etc., are effective to increase green density, they are extremely expensive. The dilemma between properties and costs in forming titanium powder now is hopeful to be solved by the high-velocity compaction (HVC) technology emerged in the past decade. HVC is a mass production technique to prepare high-density PM parts with high efficiency, whose densification process is achieved by intensive shock waves, created by a hydraulically operated hammer, that transfer the compaction energy through the compaction tool to the powder [1]. This technique has several advantages over other existing forming technologies, including high and uniform green density, costeffective, low springback, and low ejection force. It realizes an excellent balance between properties and costs and is extremely competitive to manufacture PM parts with high density, high strength, high precision, and low cost [2,3]. Many powder materials can be pressed by HVC, such as iron-based and stainless steel powders [4–9], ceramics [10,11] and polymers [12,13]. However, research on forming powders with intrinsic high work hardening rate, such as titanium [14] and molybdenum is
⁎ Corresponding author. Tel.: + 86 20 61086627; fax: + 86 20 37238669. E-mail address: [email protected] (Z. Yan). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.12.026
relatively rare. In addition, the forming ability of HVC is generally characterized by either peak pressure or impact energy. For the former, Sethi et al. [4] found that HVC yielded a lower density than CC when the peak pressure in HVC was equal to the constant pressure in CC for Astaloy Mo. For compacting a similar Astaloy CrM, however, Torive et al. [9] found that the HVC density is in average 0.5% higher than CC density. Comparison of HVC and CC based on pressure leads to contradictory results. Moreover, peak pressure is unable to characterize the effect of multiple high-velocity impacts. It seems that HVC cannot be appropriately characterized by the peak pressure in it, although CC is generally characterized by its pressure. As for impact energy, we will demonstrate in the paper that it is not quite appropriate, either. In this research, titanium powder, with high work hardening rate, was pressed into two types of green compacts by HVC and the forming effects were compared. A new quantity, impact energy per unit mass, was found to characterize HVC process more appropriately. 2. Experiments A hydride/dehydride titanium powder was pressed using Hydropulsor HYP35-7 high-velocity compaction machine. Characteristics and image of the powder were shown in Table 1 and Fig. 1, respectively. Details of the machine were introduced in Ref. [5]. As titanium is very active and easy to be polluted, no lubricant or binder was added into the powder. But before powder filling, the die wall was lubricated with zinc stearate dissolved in acetone to facilitate the ejection of the samples. Two types of green compacts were pressed. Their detailed information was shown in Table 2. The specimen density was measured through Archimedes method, and
Z. Yan et al. / Powder Technology 208 (2011) 596–599 Table 1 Characteristics of titanium powder. w(Ti) w(O) (%) (%) 99.7
w(N) (%)
w(H) (%)
Table 2 Detailed information of the two green compacts.
Apparent density Median diameter Specific area (g/cm3) (μm) (m2/g)
0.180 0.0131 0.0169 1.46
597
91.58
Sample type
Cross-section
Powder filling amount (g)
Powder filling height (mm)
The rings
60 mm outer and 30 mm inner diameters
57
18
The cylinders
20 mm diameter
10
20
0.018
relative density was calculated based on theoretical density of 4.506 g/cm3 for titanium. The fracture surface was observed by scanning electron microscope (SEM). During high-velocity compaction, impact energy can be adjusted by varying stroke length of the hammer and calculated based on Eq. (1):
The relations between peak pressure and green density are fitted by Huang Pei-yun equation as follows: m lg ln
ð1Þ
E = Fh
where E is the impact energy, F is the force applied to the hammer by the hydraulic system, h is the stroke length, which is the distance between starting position and impact position of the hammer. For the HYP35-7 high-velocity compaction machine, F is 76.087 kN and the hammer weight (m) is 135 kg. The geopotential energy of the hammer can be ignored [5]. As can be seen from Eq. (1), impact energy is proportional to impact stroke. Similarly, impact velocity can be calculated based on impact stroke [15]. For the least stroke length of 6 mm and the largest stroke length of 88 mm of the HYP35-7 highvelocity compaction machine, the corresponding impact velocity is about 2.61 and 9.98 m/s, respectively.
ðρm −ρ0 Þρ = lgP− lg M ðρm −ρÞρ0
ð2Þ
where ρm is the theoretical density of full dense material; ρ0 is the apparent density of powder; ρ is the density of green compact; P is the pressure, referring to the peak pressure of the shock wave here; m is index of work hardening, equal to the reciprocal of the slope of the compacting equation; and M is the compacting modulus. Plot of ðρm −ρ Þρ
lg lnðρ −ρ0Þρ versus lg Pis shown in Fig. 4(a). m 0 According to the slope of the fitted line, it can be calculated that the value of m is 1.00074, indicating that no work hardening happens during the whole HVC process. The densification is achieved mainly in
3. Results and discussion 3.1. Forming effect of the rings In the experiment, stroke length began with 20 mm and increased with 5 mm interval. The bulk density of the rings is presented in Fig. 2(a). It shows that the maximum shapable green density reaches 76.2% at a stroke length of 30 mm, which corresponds to impact energy of 2283 J. The green compacts were completely delaminated when the stroke length increased to 35 mm (Fig. 3). The delamination might be caused by the tensile stresses aroused by the intense shock wave during HVC process when it reflects on the free upper surface. Azhdar et al. [13] found that relaxation assisting devices, located above lower punch and beneath upper punch, could reduce compaction defects by giving a more homogeneous opposite velocity and a better locking of the powder bed in the compacted form.
Fig. 1. SEM image of titanium powder.
Fig. 2. Green density as function of impact energy and stroke length, (a) the rings; (b) the cylinders.
598
Z. Yan et al. / Powder Technology 208 (2011) 596–599
Fig. 3. Delmination of green compact formed at stroke length of 35 mm.
the form of sliding and rearranging of powder particles. The particles are loosely packed, many large pores exist, and the relative density is very low (Fig. 5(a)). 3.2. Forming effect of the cylinders As cross-section of the cylinders was much smaller than that of the rings, stroke length began with 6 mm, and increased with 2 mm interval. The bulk density of the samples is presented in Fig. 2(b). When the stroke length is smaller than 10 mm, the density increases linearly with increasing stroke length. Once the stroke length is bigger than 10 mm, the density increases with increasing stroke length according to a parabola law. The maximum shapable green density reaches 96.0% at a
Fig. 5. Fracture surface SEM images of green compacts, (a) the ring sample; (b) the cylinder sample.
stroke length of 16 mm, which corresponds to impact energy of 1216 J. With further increasing stroke length, the green compacts were still perfectly pressed, but the ejection force significantly increased. For the advanced PM cold and hot isostatic pressing (CHIP) process, a titaniumbased compact with approximately 85% is produced after CIP. After vacuum sintering, the relative density is 95% [16]. Obviously, relative density of green compacts formed by HVC in this research is almost equal to that in CHIP process after vacuum sintering. The relations between peak pressure and green density are still consistent with Huang Pei-yun equation, as shown in Fig. 4(b). Index of work hardening, m, is 1.69690 according to the slope of the fitted line. This value is larger than 1, which indicates that certain work hardening happens during the compaction process. Generally speaking, work hardening is mainly caused by plastic deformation. So besides sliding and rearranging of powder particles, densification is also attributed to plastic deformation. Therefore, the compact is much denser than the ring sample, which is verified by comparing fracture surface SEM images in Fig. 5. 3.3. Comparison of the forming effects of the two types of samples
Fig. 4. Huang Pei-yun equation of high-velocity compacting titanium powder, (a) the rings; (b) the cylinders.
Impact energy is regularly used to characterize the effect of HVC. A relative compact density of 76.2% is obtained at impact energy of 2283 J in Section 3.1. However, that of 96.0% is obtained at impact energy of 1217 J in Section 3.2. Obviously, the latter reaches a higher density at a smaller impact energy, which is only about half of the former. Thus, the two forming results cannot be compared based on impact energy. In addition, in Ref. [14], relative green density of pure titanium powder reached 93.5% after one strike at impact energy of 335 J, where the samples were cylinders with 14 mm diameter and a mass of 3.0 g. It is difficult to evaluate which forming effect is better in Section 3.2 and in Ref. [14] just according to impact energy. It seems that the forming ability of HVC is not only determined by impact
Z. Yan et al. / Powder Technology 208 (2011) 596–599
599
100
suitable to characterize the pressing effect of different samples. For forming pure titanium powder, the density of 76.2% and 96.0% was reached at impact energy per unit mass of 40.1 and 121.7 J/g for the rings and the cylinders, respectively. In addition, the relations between green compact density and peak pressure of HVC process agree with Huang Pei-yun equation. The densification of the rings is mainly in the form of sliding and rearranging of powder particles, while certain plastic deformation happens in the cylinders.
Relative density (%)
95 90 The rings
85
The cylinders 80
Result in Ref. [14]
75
Acknowledgments 0
100
200
300
400
500
This work was financially supported by the National Natural Science Foundation of China (51004040). The authors are grateful to the Hydropulsor-East Precision Machinery of Nanjing in China for the HYP35-7 machine used for this investigation.
600
Impact energy per mass (J/g) Fig. 6. Relations between green density and impact energy per unit mass.
energy but also much related with powder filling amount. Thus, a new quantity, impact energy per unit mass, defined as dividing the impact energy by the powder filling amount, shown in Eq. (3), is put forward to compare the three results. The comparing result is shown in Fig. 6. I=
E M
ð3Þ
where I is impact energy per unit mass, E is impact energy, and M is powder filling amount. Fig. 6 shows that for the rings in Section 3.1, the maximum shapable impact energy per unit mass is 40.1 J/g, and the corresponding green density is 76.2%. For the cylinders in Section 3.2, that is 121.7 J/g, and the corresponding green density is 96.0%. In Ref. [14], 93.5% green density was obtained at impact energy per unit mass of 111.7 J/g. Impact energy per unit mass in Section 3.2 and in Ref. [14] is close, so the comparable density is obtained. While for the rings, impact energy is larger, but impact energy per unit mass is much smaller, which leads to a lower green density. Therefore, the three different results become comparable through utilization of impact energy per unit mass, which is more suitable to characterize the features of HVC technology than conventional impact energy. Doremus et al. [17] also noted that to achieve a given density, the required energy is proportional to the mass of the powder. It suggests that the same density would be attained if the powder was pressed at the same impact energy per unit mass. Therefore, the relations among impact energy, green density, and powder filling amount can be designed if impact energy per unit mass of a certain powder is confirmed. It is meaningful for designing parameters in HVC process and forecasting the maximum density for a type-confirmed HVC machine and certain powder. 4. Conclusions The forming ability of HVC technique is related with impact energy and powder filling amount. Impact energy per unit mass is quite
References [1] P. Skoglund, High density PM parts by high velocity compaction, Powder Metall 44 (2001) 199–201. [2] P. Skoglund, M. Kejzelman, I. Hauer, HVC punches PM to new mass production limits, Met Powder Rep 57 (2002) 26–30. [3] C.E. Grant, Höganäs promotes potential of high velocity compaction, Met Powder Rep 56 (2001) 6. [4] G. Sethi, E. Hauck, R.M. German, High velocity compaction compared with conventional compaction, Mater Sci Technol 22 (2006) 955–959. [5] J.Z. Wang, H.Q. Yin, X.H. Qu, J.L. Johnson, Impact of multiple impacts on high velocity pressed iron powder, Powder Technol 195 (2009) 184–189. [6] B. Barendvanden, F. Christer, L. Tomas, Industrial implementation of high velocity compaction for improved properties, Powder Metall 49 (2006) 107–109. [7] P. Jonsén, H.Å. Häggblad, L. Troive, J. Furuberg, S. Allroth, P. Skoulund, Green body behaviour of high velocity pressed metal powder, Mater Sci Forum 534–536 (2007) 289–292. [8] T. Ericsson, P. Luukkonen, Residual stresses in green bodies of steel powder after conventional and high speed compaction, Mater Sci Forum 404–407 (2002) 77–82. [9] L. Troive, J. Furuberg, P. Skoglund, S. Allroth, High-density PM components by high velocity compaction a comparison with conventional compaction, Euro PM 2005, Prague, Czech Republic, October 2005. [10] D. Souriou, P. Goeuriot, O. Bonnefoy, G. Thomas, F. Doré, Influence of the formulation of an alumina powder on compaction, Powder Metall 1–2 (2009) 152–159. [11] F. Dore, L. Lazzarotto, S. Bourdin, High velocity compaction: overview of materials, applications and potential, Mater Sci Forum 534–536 (2007) 293–296. [12] D. Jauffrès, O. Lame, G. Vigier, F. Doré, Microstructural origin of physical and mechanical properties of ultra high molecular weight polyethylene processed by high velocity compaction, Polymer 48 (2007) 6374–6383. [13] B. Azhdar, B. Stenberg, L. Kari, Determination of dynamic and sliding friction, and observation of stick–slip phenomenon on compacted polymer powders during high-velocity compaction, Polym Test 25 (2006) 114–123. [14] M. Eriksson, M. Andersson, E. Adolfsson, E. Carlström, Titanium-hydroxyapatite composite biomaterial for dental implants, Powder Metall 49 (2006) 70–77. [15] J.Z. Wang, X.H. Qu, H.Q. Yin, M.J. Yi, X.J. Yuan, High velocity compaction of ferrous powder, Powder Technol 192 (2009) 131–136. [16] S. Abkowitz, S.M. Abkowitz, H. Fisher, P.J. Schwartzl, CermeTi® discontinuously reinforced Ti-matrix composites: manufacturing, properties, and applications, JOM 56 (2004) 37–41. [17] P. Doremus, Y.L. Guennec, D. Imbault, G. Puente, High-velocity compaction and conventional compaction of metallic powders; comparison of process parameters and green compact properties, Proc. ImechE, Part E: J. Process Mechanical Engineering 224 (2010) 177–185.
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
High-velocity compaction of titanium powder and process characterization Zhiqiao Yan a,⁎, Feng Chen b, Yixiang Cai a a b
Guangzhou Research Institute of Nonferrous Metals, Guangzhou 510650, PR China Department of Applied Physics, Hong Kong Polytechnic University, Kowloon, Hong Kong, PR China
a r t i c l e
i n f o
Article history: Received 2 August 2010 Received in revised form 1 December 2010 Accepted 18 December 2010 Available online 25 December 2010 Keywords: High-velocity compaction (HVC) Titanium powder Impact energy per unit mass Green density Huang Pei-yun equation
a b s t r a c t Pure titanium powder was pressed into two kinds of samples through high-velocity compaction technology. The first was rings with 60 mm outer and 30 mm inner diameters and a mass of 57 g, and the second was cylinders with 20 mm diameter and a mass of 10 g. For the rings, the maximum shapable relative density reached 76.2% at impact energy of 2283 J. For the cylinders, however, it reached 96.0% at much lower impact energy of 1217 J. The reasons for the contradictory effects were analyzed, and a new quantity, impact energy per unit mass, was put forth to well characterize the difference. In addition, the relations between peak pressure and green density of the two kinds of samples were found to comply with Huang Pei-yun equation, and the densification mechanism was discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Powder metallurgy (PM) is a near net shape method to prepare titanium alloys and composites with low cost. Unfortunately, titanium powder is very difficult to press into dense compacts through conventional compaction (CC) methods due to its high work hardening rate. Although some advanced forming methods, such as powder forging and hot isostatic pressing, etc., are effective to increase green density, they are extremely expensive. The dilemma between properties and costs in forming titanium powder now is hopeful to be solved by the high-velocity compaction (HVC) technology emerged in the past decade. HVC is a mass production technique to prepare high-density PM parts with high efficiency, whose densification process is achieved by intensive shock waves, created by a hydraulically operated hammer, that transfer the compaction energy through the compaction tool to the powder [1]. This technique has several advantages over other existing forming technologies, including high and uniform green density, costeffective, low springback, and low ejection force. It realizes an excellent balance between properties and costs and is extremely competitive to manufacture PM parts with high density, high strength, high precision, and low cost [2,3]. Many powder materials can be pressed by HVC, such as iron-based and stainless steel powders [4–9], ceramics [10,11] and polymers [12,13]. However, research on forming powders with intrinsic high work hardening rate, such as titanium [14] and molybdenum is
⁎ Corresponding author. Tel.: + 86 20 61086627; fax: + 86 20 37238669. E-mail address: [email protected] (Z. Yan). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.12.026
relatively rare. In addition, the forming ability of HVC is generally characterized by either peak pressure or impact energy. For the former, Sethi et al. [4] found that HVC yielded a lower density than CC when the peak pressure in HVC was equal to the constant pressure in CC for Astaloy Mo. For compacting a similar Astaloy CrM, however, Torive et al. [9] found that the HVC density is in average 0.5% higher than CC density. Comparison of HVC and CC based on pressure leads to contradictory results. Moreover, peak pressure is unable to characterize the effect of multiple high-velocity impacts. It seems that HVC cannot be appropriately characterized by the peak pressure in it, although CC is generally characterized by its pressure. As for impact energy, we will demonstrate in the paper that it is not quite appropriate, either. In this research, titanium powder, with high work hardening rate, was pressed into two types of green compacts by HVC and the forming effects were compared. A new quantity, impact energy per unit mass, was found to characterize HVC process more appropriately. 2. Experiments A hydride/dehydride titanium powder was pressed using Hydropulsor HYP35-7 high-velocity compaction machine. Characteristics and image of the powder were shown in Table 1 and Fig. 1, respectively. Details of the machine were introduced in Ref. [5]. As titanium is very active and easy to be polluted, no lubricant or binder was added into the powder. But before powder filling, the die wall was lubricated with zinc stearate dissolved in acetone to facilitate the ejection of the samples. Two types of green compacts were pressed. Their detailed information was shown in Table 2. The specimen density was measured through Archimedes method, and
Z. Yan et al. / Powder Technology 208 (2011) 596–599 Table 1 Characteristics of titanium powder. w(Ti) w(O) (%) (%) 99.7
w(N) (%)
w(H) (%)
Table 2 Detailed information of the two green compacts.
Apparent density Median diameter Specific area (g/cm3) (μm) (m2/g)
0.180 0.0131 0.0169 1.46
597
91.58
Sample type
Cross-section
Powder filling amount (g)
Powder filling height (mm)
The rings
60 mm outer and 30 mm inner diameters
57
18
The cylinders
20 mm diameter
10
20
0.018
relative density was calculated based on theoretical density of 4.506 g/cm3 for titanium. The fracture surface was observed by scanning electron microscope (SEM). During high-velocity compaction, impact energy can be adjusted by varying stroke length of the hammer and calculated based on Eq. (1):
The relations between peak pressure and green density are fitted by Huang Pei-yun equation as follows: m lg ln
ð1Þ
E = Fh
where E is the impact energy, F is the force applied to the hammer by the hydraulic system, h is the stroke length, which is the distance between starting position and impact position of the hammer. For the HYP35-7 high-velocity compaction machine, F is 76.087 kN and the hammer weight (m) is 135 kg. The geopotential energy of the hammer can be ignored [5]. As can be seen from Eq. (1), impact energy is proportional to impact stroke. Similarly, impact velocity can be calculated based on impact stroke [15]. For the least stroke length of 6 mm and the largest stroke length of 88 mm of the HYP35-7 highvelocity compaction machine, the corresponding impact velocity is about 2.61 and 9.98 m/s, respectively.
ðρm −ρ0 Þρ = lgP− lg M ðρm −ρÞρ0
ð2Þ
where ρm is the theoretical density of full dense material; ρ0 is the apparent density of powder; ρ is the density of green compact; P is the pressure, referring to the peak pressure of the shock wave here; m is index of work hardening, equal to the reciprocal of the slope of the compacting equation; and M is the compacting modulus. Plot of ðρm −ρ Þρ
lg lnðρ −ρ0Þρ versus lg Pis shown in Fig. 4(a). m 0 According to the slope of the fitted line, it can be calculated that the value of m is 1.00074, indicating that no work hardening happens during the whole HVC process. The densification is achieved mainly in
3. Results and discussion 3.1. Forming effect of the rings In the experiment, stroke length began with 20 mm and increased with 5 mm interval. The bulk density of the rings is presented in Fig. 2(a). It shows that the maximum shapable green density reaches 76.2% at a stroke length of 30 mm, which corresponds to impact energy of 2283 J. The green compacts were completely delaminated when the stroke length increased to 35 mm (Fig. 3). The delamination might be caused by the tensile stresses aroused by the intense shock wave during HVC process when it reflects on the free upper surface. Azhdar et al. [13] found that relaxation assisting devices, located above lower punch and beneath upper punch, could reduce compaction defects by giving a more homogeneous opposite velocity and a better locking of the powder bed in the compacted form.
Fig. 1. SEM image of titanium powder.
Fig. 2. Green density as function of impact energy and stroke length, (a) the rings; (b) the cylinders.
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Z. Yan et al. / Powder Technology 208 (2011) 596–599
Fig. 3. Delmination of green compact formed at stroke length of 35 mm.
the form of sliding and rearranging of powder particles. The particles are loosely packed, many large pores exist, and the relative density is very low (Fig. 5(a)). 3.2. Forming effect of the cylinders As cross-section of the cylinders was much smaller than that of the rings, stroke length began with 6 mm, and increased with 2 mm interval. The bulk density of the samples is presented in Fig. 2(b). When the stroke length is smaller than 10 mm, the density increases linearly with increasing stroke length. Once the stroke length is bigger than 10 mm, the density increases with increasing stroke length according to a parabola law. The maximum shapable green density reaches 96.0% at a
Fig. 5. Fracture surface SEM images of green compacts, (a) the ring sample; (b) the cylinder sample.
stroke length of 16 mm, which corresponds to impact energy of 1216 J. With further increasing stroke length, the green compacts were still perfectly pressed, but the ejection force significantly increased. For the advanced PM cold and hot isostatic pressing (CHIP) process, a titaniumbased compact with approximately 85% is produced after CIP. After vacuum sintering, the relative density is 95% [16]. Obviously, relative density of green compacts formed by HVC in this research is almost equal to that in CHIP process after vacuum sintering. The relations between peak pressure and green density are still consistent with Huang Pei-yun equation, as shown in Fig. 4(b). Index of work hardening, m, is 1.69690 according to the slope of the fitted line. This value is larger than 1, which indicates that certain work hardening happens during the compaction process. Generally speaking, work hardening is mainly caused by plastic deformation. So besides sliding and rearranging of powder particles, densification is also attributed to plastic deformation. Therefore, the compact is much denser than the ring sample, which is verified by comparing fracture surface SEM images in Fig. 5. 3.3. Comparison of the forming effects of the two types of samples
Fig. 4. Huang Pei-yun equation of high-velocity compacting titanium powder, (a) the rings; (b) the cylinders.
Impact energy is regularly used to characterize the effect of HVC. A relative compact density of 76.2% is obtained at impact energy of 2283 J in Section 3.1. However, that of 96.0% is obtained at impact energy of 1217 J in Section 3.2. Obviously, the latter reaches a higher density at a smaller impact energy, which is only about half of the former. Thus, the two forming results cannot be compared based on impact energy. In addition, in Ref. [14], relative green density of pure titanium powder reached 93.5% after one strike at impact energy of 335 J, where the samples were cylinders with 14 mm diameter and a mass of 3.0 g. It is difficult to evaluate which forming effect is better in Section 3.2 and in Ref. [14] just according to impact energy. It seems that the forming ability of HVC is not only determined by impact
Z. Yan et al. / Powder Technology 208 (2011) 596–599
599
100
suitable to characterize the pressing effect of different samples. For forming pure titanium powder, the density of 76.2% and 96.0% was reached at impact energy per unit mass of 40.1 and 121.7 J/g for the rings and the cylinders, respectively. In addition, the relations between green compact density and peak pressure of HVC process agree with Huang Pei-yun equation. The densification of the rings is mainly in the form of sliding and rearranging of powder particles, while certain plastic deformation happens in the cylinders.
Relative density (%)
95 90 The rings
85
The cylinders 80
Result in Ref. [14]
75
Acknowledgments 0
100
200
300
400
500
This work was financially supported by the National Natural Science Foundation of China (51004040). The authors are grateful to the Hydropulsor-East Precision Machinery of Nanjing in China for the HYP35-7 machine used for this investigation.
600
Impact energy per mass (J/g) Fig. 6. Relations between green density and impact energy per unit mass.
energy but also much related with powder filling amount. Thus, a new quantity, impact energy per unit mass, defined as dividing the impact energy by the powder filling amount, shown in Eq. (3), is put forward to compare the three results. The comparing result is shown in Fig. 6. I=
E M
ð3Þ
where I is impact energy per unit mass, E is impact energy, and M is powder filling amount. Fig. 6 shows that for the rings in Section 3.1, the maximum shapable impact energy per unit mass is 40.1 J/g, and the corresponding green density is 76.2%. For the cylinders in Section 3.2, that is 121.7 J/g, and the corresponding green density is 96.0%. In Ref. [14], 93.5% green density was obtained at impact energy per unit mass of 111.7 J/g. Impact energy per unit mass in Section 3.2 and in Ref. [14] is close, so the comparable density is obtained. While for the rings, impact energy is larger, but impact energy per unit mass is much smaller, which leads to a lower green density. Therefore, the three different results become comparable through utilization of impact energy per unit mass, which is more suitable to characterize the features of HVC technology than conventional impact energy. Doremus et al. [17] also noted that to achieve a given density, the required energy is proportional to the mass of the powder. It suggests that the same density would be attained if the powder was pressed at the same impact energy per unit mass. Therefore, the relations among impact energy, green density, and powder filling amount can be designed if impact energy per unit mass of a certain powder is confirmed. It is meaningful for designing parameters in HVC process and forecasting the maximum density for a type-confirmed HVC machine and certain powder. 4. Conclusions The forming ability of HVC technique is related with impact energy and powder filling amount. Impact energy per unit mass is quite
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