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Prevention of galling in forming of deep hole with retreat and advance pulse ram motion on servo press
CIRP Annals - Manufacturing Technology 60 (2011) 315–318
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er. com/ci rp/ def a ult . asp
Prevention of galling in forming of deep hole with retreat and advance pulse ram motion on servo press R. Matsumoto a,*, S. Sawa a, H. Utsunomiya b, K. Osakada (1)c a
Division of Mechanical Engineering, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka, Japan Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka, Japan c Faculty of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka, Japan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Forming Lubrication Servo press
To prevent galling in backward extrusion of deep holes, an extrusion method utilizing a servo press and a punch having an internal channel for supplying liquid lubricant is proposed. On the servo press, the punch is pushed into the billet in a manner combining pulsed and stepwise modes. The lubricant is sucked into the formed hole through the internal channel during the retreat motion of the punch. Appropriate punch ram motions for preventing galling are determined from the surface observation of the formed hole. The maximum aspect ratio of the hole attained by the proposed method is discussed. ß 2011 CIRP.
1. Introduction
2. Extrusion with retreat and advance pulse ram motion
To reduce the energy consumption of vehicles and to improve the product quality, lightweight components are increasingly used in automobiles [1]. For reducing the weight of a component, application of the materials with high specific strength such as ultra-high-strength steel, aluminium, magnesium and titanium alloys is one of the solutions. Another important solution is to employ lightweight structures such as hollow components. Since long or complex shaped hollow components are not easily produced by metal forming, establishment of forming methods for the components with hollow shapes will be a crucial technical target. In backward extrusion of deep holes, the lubricant is difficult to supply to the deforming area at the bottom of the hole, although lubrication is critically important to avoid seizure in the component of a high strength and/or high friction material. In drilling of holes, the lubrication method by utilizing the tools having internal channels has been used for directly supplying liquid lubricant to the cutting part [2]. If a similar method is developed for backward extrusion, the lubrication problem may be solved. Servo press with flexible ram motion has led to new forming processes such as extrusion against counter tool for improving the geometrical defects [3], blanking with low noise [4], shearing with reduced burrs [5] and sheet forging with reduced friction [6]. In this study, a forming method for deep hole utilizing a servo press and a punch having an internal channel for liquid lubricant is proposed.
2.1. Backward extrusion method A newly proposed extrusion method for producing products by reducing the friction over the punch surface is given in Fig. 1. A punch having an internal channel for lubricant flow is pushed into the billet in a manner combining pulsed and stepwise modes for supplying liquid lubricant to the punch nose. The internal pressure in the cavity formed in the previous forming steps is depressurized by the retreat action of the punch, and the lubricant may be sucked into the cavity through the internal channel. After the retreat motion of the punch, the punch is advanced again to continue forming of the hole. If sufficient amount of the lubricant is supplied to the cavity during the retreat motion of the punch, forming of a hole can be carried out by keeping good lubrication in the next advance motion. To express the punch motion, these parameters are defined as following symbols: ntotal: total step number of forming sai: advance stroke at i-th forming step (i = from 1 to ntotal). sri: retreat stroke at i-th forming step (i = from 1 to ntotal). sfi: forming stroke at i-th forming step (= sai sri) (i = from 1 to ntotal). P ntotal stotal: total forming stroke of punch ¼ i¼1 sfi : In this study, sai, sri and sfi were kept to be constant at every forming step. Due to this, sai, sri and sfi are simply symbolized as sa, sr and sf, respectively. 2.2. Experimental set-up
* Corresponding author. 0007-8506/$ – see front matter ß 2011 CIRP. doi:10.1016/j.cirp.2011.03.147
Fig. 2 shows the tool arrangement for the proposed forming method. The punch with an internal channel for lubricant flow was
[()TD$FIG]
316
R. Matsumoto et al. / CIRP Annals - Manufacturing Technology 60 (2011) 315–318
Fig. 1. Retreat and advance pulse ram motion of punch with internal channel for lubricant in forming for a high aspect ratio hole (sai: advance stroke of punch, sri: retreat stroke of punch, sfi: forming stroke of punch, i = from 1 to ntotal).
[()TD$FIG]
[()TD$FIG]
Fig. 3. Mean punch speed–stroke diagram during advance and retreat ram motions on servo press (Komatsu Industrial Corp., H1F45).
tube is applied to the lubricant flow in the punch as follows. 4 4 DB ðP B P BI Þ DI ðPBI P IT Þ ¼p 8hLB 8hLI 2 2 4 DT ðP IT P T Þ ¼p 8hLT 2
Q Lub ¼ p Fig. 2. Schematic illustrations of tool arrangement and punch with internal channel for lubricant (LP: punch length, DP: punch diameter, DI: diameter of internal channel for lubricant).
connected to the lubricant tank with the tube. Any equipment such as pump and/or check valve for prevention of flow backward was not used for supplying the lubricant. The mineral oils with a kinematic viscosity of y = 10–280 mm2/s (at 40 8C) were used as lubricant. The punch diameter was DP = 6.0 mm and the diameter of the internal channel was DI = 1.5 mm. The punch and container surfaces were polished to be mirror-surfaces as 0.02–0.04 mmRa. The materials used for the punch and container were cemented tungsten carbides WC–10 mass%Co and WC–22 mass%Co, respectively, while the material tested for the specimen was an AA6061T6 aluminium alloy. A 450 kN servo press, Komatsu Industrial Corp., H1F45 driven by an AC servomotor through a mechanical link (0–70 spm) was used. The diagram of the mean advance and retreat punch speeds and the punch stroke is shown in Fig. 3. The number of the forming step was limited to less than five due to the press capacity. 3. Sucked amount of lubricant during retreat motion of punch To supply the lubricant to the deforming zone during the retreat motion of the punch, the internal channel should have an appropriate diameter depending on the lubricant viscosity. To examine the lubricant flow in the internal channel, the amount of the sucked lubricant in the formed hole cavity during the retreat motion of the punch was estimated. In the theoretical analysis, the Hagen–Poiseuille equation [7] on the assumption of Newtonian fluids in laminar flow in a circular
V Lub ¼
Z
t 0
Q Lub dt ¼
Z
sr 0
Q Lub
dt dz dz
(1)
(2)
where QLub is the flow volume of the lubricant per second, VLub is the flow volume of the lubricant, P is the lubricant pressure, h is the viscosity of the lubricant, D and L are the diameter and the length of the internal channel, respectively. Subscripts of B, BI, IT and T mean the bottom, bottom-internal, internal-top and top, respectively (see Fig. 2(b)). The duration of the retreat motion of the punch was determined from Fig. 3. At first, PT after a retreat motion of the punch was determined from atmospheric pressure (PAtm) and the assumed initial volume of the cavity (p(DT/2)2LT) on the assumptions of isothermal change and complete sealing between the billet and the cavity as follows. P T ¼ PAtm
DP DT
2
sr LT
(3)
Then PB, PBI, PIT and QLub were determined from Eq. (1). Finally, VLub was calculated by Eq. (2). In the experiment, the amount of the sucked lubricant was estimated from the weight difference of the billet including the lubricant before and after a retreat action of the punch on the assumption that the sucked lubricant adhered to the surface of the hole. The maximum scattering of the estimated volume of the sucked lubricant was approximately 20% when the lubricant with y = 280 mm2/s was used. Fig. 4 shows the calculated and the measured volumes of the sucked lubricant through the internal channel into the cavity. The
[()TD$FIG]
[()TD$FIG]
R. Matsumoto et al. / CIRP Annals - Manufacturing Technology 60 (2011) 315–318
317
Fig. 6. Relation between punch motion and occurrence of galling at hole surface in forming with pulse ram motion (y = 32 mm2/s). Fig. 4. Relation between retreat stroke of punch and volume of sucked lubricant in hole (y: kinematic viscosity of lubricant).
volume of the sucked lubricant increases with increasing the retreat stroke and decreasing the viscosity of the lubricant. The measured amount of the sucked lubricant in the experiment is lower than the calculated one possibly because the cavity is assumed to be completely sealed in the calculation. If the straight part of the punch contacting the sidewall of the formed hole is longer, the air seal will become tighter and the sucked amount of the lubricant may increase in the experiment. The nominal thickness of the sucked lubricant at whole surface area of the cavity (VLub/(p(DP/2)2 + pDPsr)) is thicker than 10 mm in the calculated and experimental results when the retreat stroke of the punch (sr/DP) is larger than 1.0. It is confirmed that the lubricant can be sucked through the internal channel to the forming zone by the retreat action of the punch. 4. Experimental results 4.1. Appropriate punch motion for preventing galling Fig. 5 shows the surface profiles and the roughness of the formed hole when the lubricant with y = 32 mm2/s is periodically supplied with the punch motion of sr/DP = 1.0. Galling of the hole surface is caused by the sliding of the punch during the advance or [()TD$FIG]
[()TD$FIG]
Fig. 7. Forming load-stroke diagram in forming of AA6061 aluminium billet with pulse ram motion (sr/DP = 1.0, stotal/DP = 6.0, y = 32 mm2/s).
retreat motion of the punch and is defined when the roughness exceeds 0.4 mmRa in this study. From Fig. 5, Galling occurs at the forming stroke of the punch (sf/DP) larger than 2.0, while the smooth surface (no galling) is observed under sf/DP 1.5. To determine the appropriate punch motion, the relation between the punch motion and the occurrence of galling is plotted into Fig. 6 in the range of sr/DP = 0.5–2.0. The maximum forming stroke of the punch for preventing galling at each forming step is found to be sf/DP = 2.0 (sliding distance of the punch at each forming step: (sa + sr)/DP = 3.0) in sr/DP = 0.5 from Fig. 6. 4.2. Forming load Fig. 7 shows the forming load–stroke diagram with the pulse ram motion. Although the lubricant is periodically supplied to the formed hole cavity before every forming step, the forming load is not sensitively affected by the lubricating condition because the contacting area between the sidewall of the formed hole and the straight part of the punch (1.0 mm in length as shown in Fig. 2(b)) [()TD$FIG] small. is
Fig. 5. Surface profiles of formed hole of AA6061 aluminium billet and influence of punch motion on surface roughness of formed hole of billet (sr/DP = 1.0, stotal/ DP = 6.0, y = 32 mm2/s).
Fig. 8. Influence of viscosity of lubricant on surface roughness of formed hole in forming with no pulse ram motion (stotal/DP = 2.5, ntotal = 1, supplied amount of lubricant: 20 mm3).
[()TD$FIG]
R. Matsumoto et al. / CIRP Annals - Manufacturing Technology 60 (2011) 315–318
318
Fig. 9. Relation between viscosity of lubricant and formed hole surface in forming with retreat and advance pulse ram motion (sr/DP = 1.0).
[()TD$FIG]
material (=560 GPa, WC–10 mass%Co), LP is the punch length, k is the radius of gyration of the punch (=1.55 mm). The marks plotted into Fig. 10 are experimentally obtained in forming of the billets of some metals using the WC punch, however, the buckling length of the WC punch in forming of Ti– 6 mass%Al–4 mass%V alloy is lower than the calculated line (Eq. (4)). Plastic deformation of the WC punch is caused before buckling in the forming of the steel and titanium alloys because 0.2% proof stress and compressive strength (sP) of the WC punch (WC–10 mass%Co) are 2.0 and 4.3 GPa, respectively. Due to this, Eq. (4) for elastic buckling is not appropriate in high forming pressure range. For high forming pressure range, the J.B. Johnson formula [8] for short beams with the following equation is applied.
P¼
s P s 2P ðLP =kÞ2 4C p2 EP
ðP 0:5s P Þ
(5)
The plotted results of Ti–6 mass%Al–4 mass%V alloy have good agreement with the calculated line (Eq. (5)) for buckling of the WC punch. Thus maximum depth of the hole is considered to be obtained as approximately 20 of the aspect ratio in the proposed pulse forming of AA6061 aluminium billet (P = 1.7 GPa) from Eqs. (4) and (5). To form deeper holes, sustaining methods of punch not to bend during extrusion should be developed. 6. Conclusions Fig. 10. Relation between buckling length of punch calculated by the Euler and J.B. Johnson equations and experimentally obtained forming pressure in forming of several billet materials using WC punch.
A backward extrusion method for holes utilizing a servo press and a punch having an internal channel was proposed. The following conclusions were obtained:
4.3. Influence of viscosity of lubricant Fig. 8 shows the influence of viscosity of the lubricant on the surface roughness of the formed hole in the forming with no pulse ram motion (ntotal = 1) with the lubricant amount of 20 mm3. Galling of billet tends to occur at an early stage of the forming when the lubricant with low viscosity is used. The relation between the forming stroke of the punch and the occurrence of galling in the pulse forming with the lubricant with different viscosities is plotted into Fig. 9 when the retreat stroke of the punch is kept to be constant as sr/DP = 1.0. The most effective viscosity of the lubricant for prevention of galling is found to be 32 mm2/s in the pulse forming.
(1) Sufficient amount of liquid lubricant for preventing galling of billet is periodically supplied to the deforming zone through the internal channel by the retreat action of the punch. (2) Appropriate punch motion for preventing galling of aluminium billet at each forming step is the advance and retreat strokes of 15 and 3 mm for the punch diameter of 6 mm. (3) Maximum aspect ratio of the hole in the pulse forming method is estimated to be approximately of 20 for aluminium alloy from viewpoints of buckling and strength of the punch.
References 5. Discussion on forming limit Because the punch with a high aspect ratio must be prepared in forming with deep holes, the forming limit is estimated from the viewpoints of buckling and strength of the punch. The buckling limit of the punch is calculated by the Euler formula [8] on the assumption of elastic deformation of long beams as following equation. P¼
C p2 EP ðLP =kÞ
2
(4)
where P is the forming pressure, C is the end condition of the punch (=2.05, fixed-pinned), EP is the elastic modulus of the punch
[1] Kleiner M, Geiger M, Klaus A (2003) Manufacturing of Lightweight Components by Metal Forming. Annals of the CIRP 52(2):521–542. [2] Weinert K, Inasaki I, Sutherland JW, Wakabayashi T (2004) Dry Machining and Minimum Quantity Lubrication. Annals of the CIRP 53(2):511–537. [3] Osakada K, Matsumoto R, Otsu M, Hanami S (2005) Precision Extrusion Methods with Double Axis Servo Press using Counter Pressure. Annals of the CIRP 54(1):245–248. [4] Otsu M, Yamagata C, Osakada K (2003) Reduction of Blanking Noise by Controlling Press Motion. Annals of the CIRP 52(1):245–248. [5] Junlapen K, Kubo M, Mizuochi K, Koga N, Keawtatip P (2008) Burr-Free Shearing using NC Servo Press Machine. Advanced Technology of Plasticity 2008:267–272. [6] Maeno T, Mori K, Osakada K (2009) Reduction in Friction in Forging of Plates by Load Oscillation. Journal of JSTP 50(585):951–955. (in Japanese). [7] Schlichting H (1968) Boundary-Layer Theory. 6th edition. McGraw-Hill Book Company. pp. 11–13. [8] Shigley JE (1956) Machine Design. McGraw-Hill Book Company. pp. 66–72.
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er. com/ci rp/ def a ult . asp
Prevention of galling in forming of deep hole with retreat and advance pulse ram motion on servo press R. Matsumoto a,*, S. Sawa a, H. Utsunomiya b, K. Osakada (1)c a
Division of Mechanical Engineering, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka, Japan Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka, Japan c Faculty of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka, Japan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Forming Lubrication Servo press
To prevent galling in backward extrusion of deep holes, an extrusion method utilizing a servo press and a punch having an internal channel for supplying liquid lubricant is proposed. On the servo press, the punch is pushed into the billet in a manner combining pulsed and stepwise modes. The lubricant is sucked into the formed hole through the internal channel during the retreat motion of the punch. Appropriate punch ram motions for preventing galling are determined from the surface observation of the formed hole. The maximum aspect ratio of the hole attained by the proposed method is discussed. ß 2011 CIRP.
1. Introduction
2. Extrusion with retreat and advance pulse ram motion
To reduce the energy consumption of vehicles and to improve the product quality, lightweight components are increasingly used in automobiles [1]. For reducing the weight of a component, application of the materials with high specific strength such as ultra-high-strength steel, aluminium, magnesium and titanium alloys is one of the solutions. Another important solution is to employ lightweight structures such as hollow components. Since long or complex shaped hollow components are not easily produced by metal forming, establishment of forming methods for the components with hollow shapes will be a crucial technical target. In backward extrusion of deep holes, the lubricant is difficult to supply to the deforming area at the bottom of the hole, although lubrication is critically important to avoid seizure in the component of a high strength and/or high friction material. In drilling of holes, the lubrication method by utilizing the tools having internal channels has been used for directly supplying liquid lubricant to the cutting part [2]. If a similar method is developed for backward extrusion, the lubrication problem may be solved. Servo press with flexible ram motion has led to new forming processes such as extrusion against counter tool for improving the geometrical defects [3], blanking with low noise [4], shearing with reduced burrs [5] and sheet forging with reduced friction [6]. In this study, a forming method for deep hole utilizing a servo press and a punch having an internal channel for liquid lubricant is proposed.
2.1. Backward extrusion method A newly proposed extrusion method for producing products by reducing the friction over the punch surface is given in Fig. 1. A punch having an internal channel for lubricant flow is pushed into the billet in a manner combining pulsed and stepwise modes for supplying liquid lubricant to the punch nose. The internal pressure in the cavity formed in the previous forming steps is depressurized by the retreat action of the punch, and the lubricant may be sucked into the cavity through the internal channel. After the retreat motion of the punch, the punch is advanced again to continue forming of the hole. If sufficient amount of the lubricant is supplied to the cavity during the retreat motion of the punch, forming of a hole can be carried out by keeping good lubrication in the next advance motion. To express the punch motion, these parameters are defined as following symbols: ntotal: total step number of forming sai: advance stroke at i-th forming step (i = from 1 to ntotal). sri: retreat stroke at i-th forming step (i = from 1 to ntotal). sfi: forming stroke at i-th forming step (= sai sri) (i = from 1 to ntotal). P ntotal stotal: total forming stroke of punch ¼ i¼1 sfi : In this study, sai, sri and sfi were kept to be constant at every forming step. Due to this, sai, sri and sfi are simply symbolized as sa, sr and sf, respectively. 2.2. Experimental set-up
* Corresponding author. 0007-8506/$ – see front matter ß 2011 CIRP. doi:10.1016/j.cirp.2011.03.147
Fig. 2 shows the tool arrangement for the proposed forming method. The punch with an internal channel for lubricant flow was
[()TD$FIG]
316
R. Matsumoto et al. / CIRP Annals - Manufacturing Technology 60 (2011) 315–318
Fig. 1. Retreat and advance pulse ram motion of punch with internal channel for lubricant in forming for a high aspect ratio hole (sai: advance stroke of punch, sri: retreat stroke of punch, sfi: forming stroke of punch, i = from 1 to ntotal).
[()TD$FIG]
[()TD$FIG]
Fig. 3. Mean punch speed–stroke diagram during advance and retreat ram motions on servo press (Komatsu Industrial Corp., H1F45).
tube is applied to the lubricant flow in the punch as follows. 4 4 DB ðP B P BI Þ DI ðPBI P IT Þ ¼p 8hLB 8hLI 2 2 4 DT ðP IT P T Þ ¼p 8hLT 2
Q Lub ¼ p Fig. 2. Schematic illustrations of tool arrangement and punch with internal channel for lubricant (LP: punch length, DP: punch diameter, DI: diameter of internal channel for lubricant).
connected to the lubricant tank with the tube. Any equipment such as pump and/or check valve for prevention of flow backward was not used for supplying the lubricant. The mineral oils with a kinematic viscosity of y = 10–280 mm2/s (at 40 8C) were used as lubricant. The punch diameter was DP = 6.0 mm and the diameter of the internal channel was DI = 1.5 mm. The punch and container surfaces were polished to be mirror-surfaces as 0.02–0.04 mmRa. The materials used for the punch and container were cemented tungsten carbides WC–10 mass%Co and WC–22 mass%Co, respectively, while the material tested for the specimen was an AA6061T6 aluminium alloy. A 450 kN servo press, Komatsu Industrial Corp., H1F45 driven by an AC servomotor through a mechanical link (0–70 spm) was used. The diagram of the mean advance and retreat punch speeds and the punch stroke is shown in Fig. 3. The number of the forming step was limited to less than five due to the press capacity. 3. Sucked amount of lubricant during retreat motion of punch To supply the lubricant to the deforming zone during the retreat motion of the punch, the internal channel should have an appropriate diameter depending on the lubricant viscosity. To examine the lubricant flow in the internal channel, the amount of the sucked lubricant in the formed hole cavity during the retreat motion of the punch was estimated. In the theoretical analysis, the Hagen–Poiseuille equation [7] on the assumption of Newtonian fluids in laminar flow in a circular
V Lub ¼
Z
t 0
Q Lub dt ¼
Z
sr 0
Q Lub
dt dz dz
(1)
(2)
where QLub is the flow volume of the lubricant per second, VLub is the flow volume of the lubricant, P is the lubricant pressure, h is the viscosity of the lubricant, D and L are the diameter and the length of the internal channel, respectively. Subscripts of B, BI, IT and T mean the bottom, bottom-internal, internal-top and top, respectively (see Fig. 2(b)). The duration of the retreat motion of the punch was determined from Fig. 3. At first, PT after a retreat motion of the punch was determined from atmospheric pressure (PAtm) and the assumed initial volume of the cavity (p(DT/2)2LT) on the assumptions of isothermal change and complete sealing between the billet and the cavity as follows. P T ¼ PAtm
DP DT
2
sr LT
(3)
Then PB, PBI, PIT and QLub were determined from Eq. (1). Finally, VLub was calculated by Eq. (2). In the experiment, the amount of the sucked lubricant was estimated from the weight difference of the billet including the lubricant before and after a retreat action of the punch on the assumption that the sucked lubricant adhered to the surface of the hole. The maximum scattering of the estimated volume of the sucked lubricant was approximately 20% when the lubricant with y = 280 mm2/s was used. Fig. 4 shows the calculated and the measured volumes of the sucked lubricant through the internal channel into the cavity. The
[()TD$FIG]
[()TD$FIG]
R. Matsumoto et al. / CIRP Annals - Manufacturing Technology 60 (2011) 315–318
317
Fig. 6. Relation between punch motion and occurrence of galling at hole surface in forming with pulse ram motion (y = 32 mm2/s). Fig. 4. Relation between retreat stroke of punch and volume of sucked lubricant in hole (y: kinematic viscosity of lubricant).
volume of the sucked lubricant increases with increasing the retreat stroke and decreasing the viscosity of the lubricant. The measured amount of the sucked lubricant in the experiment is lower than the calculated one possibly because the cavity is assumed to be completely sealed in the calculation. If the straight part of the punch contacting the sidewall of the formed hole is longer, the air seal will become tighter and the sucked amount of the lubricant may increase in the experiment. The nominal thickness of the sucked lubricant at whole surface area of the cavity (VLub/(p(DP/2)2 + pDPsr)) is thicker than 10 mm in the calculated and experimental results when the retreat stroke of the punch (sr/DP) is larger than 1.0. It is confirmed that the lubricant can be sucked through the internal channel to the forming zone by the retreat action of the punch. 4. Experimental results 4.1. Appropriate punch motion for preventing galling Fig. 5 shows the surface profiles and the roughness of the formed hole when the lubricant with y = 32 mm2/s is periodically supplied with the punch motion of sr/DP = 1.0. Galling of the hole surface is caused by the sliding of the punch during the advance or [()TD$FIG]
[()TD$FIG]
Fig. 7. Forming load-stroke diagram in forming of AA6061 aluminium billet with pulse ram motion (sr/DP = 1.0, stotal/DP = 6.0, y = 32 mm2/s).
retreat motion of the punch and is defined when the roughness exceeds 0.4 mmRa in this study. From Fig. 5, Galling occurs at the forming stroke of the punch (sf/DP) larger than 2.0, while the smooth surface (no galling) is observed under sf/DP 1.5. To determine the appropriate punch motion, the relation between the punch motion and the occurrence of galling is plotted into Fig. 6 in the range of sr/DP = 0.5–2.0. The maximum forming stroke of the punch for preventing galling at each forming step is found to be sf/DP = 2.0 (sliding distance of the punch at each forming step: (sa + sr)/DP = 3.0) in sr/DP = 0.5 from Fig. 6. 4.2. Forming load Fig. 7 shows the forming load–stroke diagram with the pulse ram motion. Although the lubricant is periodically supplied to the formed hole cavity before every forming step, the forming load is not sensitively affected by the lubricating condition because the contacting area between the sidewall of the formed hole and the straight part of the punch (1.0 mm in length as shown in Fig. 2(b)) [()TD$FIG] small. is
Fig. 5. Surface profiles of formed hole of AA6061 aluminium billet and influence of punch motion on surface roughness of formed hole of billet (sr/DP = 1.0, stotal/ DP = 6.0, y = 32 mm2/s).
Fig. 8. Influence of viscosity of lubricant on surface roughness of formed hole in forming with no pulse ram motion (stotal/DP = 2.5, ntotal = 1, supplied amount of lubricant: 20 mm3).
[()TD$FIG]
R. Matsumoto et al. / CIRP Annals - Manufacturing Technology 60 (2011) 315–318
318
Fig. 9. Relation between viscosity of lubricant and formed hole surface in forming with retreat and advance pulse ram motion (sr/DP = 1.0).
[()TD$FIG]
material (=560 GPa, WC–10 mass%Co), LP is the punch length, k is the radius of gyration of the punch (=1.55 mm). The marks plotted into Fig. 10 are experimentally obtained in forming of the billets of some metals using the WC punch, however, the buckling length of the WC punch in forming of Ti– 6 mass%Al–4 mass%V alloy is lower than the calculated line (Eq. (4)). Plastic deformation of the WC punch is caused before buckling in the forming of the steel and titanium alloys because 0.2% proof stress and compressive strength (sP) of the WC punch (WC–10 mass%Co) are 2.0 and 4.3 GPa, respectively. Due to this, Eq. (4) for elastic buckling is not appropriate in high forming pressure range. For high forming pressure range, the J.B. Johnson formula [8] for short beams with the following equation is applied.
P¼
s P s 2P ðLP =kÞ2 4C p2 EP
ðP 0:5s P Þ
(5)
The plotted results of Ti–6 mass%Al–4 mass%V alloy have good agreement with the calculated line (Eq. (5)) for buckling of the WC punch. Thus maximum depth of the hole is considered to be obtained as approximately 20 of the aspect ratio in the proposed pulse forming of AA6061 aluminium billet (P = 1.7 GPa) from Eqs. (4) and (5). To form deeper holes, sustaining methods of punch not to bend during extrusion should be developed. 6. Conclusions Fig. 10. Relation between buckling length of punch calculated by the Euler and J.B. Johnson equations and experimentally obtained forming pressure in forming of several billet materials using WC punch.
A backward extrusion method for holes utilizing a servo press and a punch having an internal channel was proposed. The following conclusions were obtained:
4.3. Influence of viscosity of lubricant Fig. 8 shows the influence of viscosity of the lubricant on the surface roughness of the formed hole in the forming with no pulse ram motion (ntotal = 1) with the lubricant amount of 20 mm3. Galling of billet tends to occur at an early stage of the forming when the lubricant with low viscosity is used. The relation between the forming stroke of the punch and the occurrence of galling in the pulse forming with the lubricant with different viscosities is plotted into Fig. 9 when the retreat stroke of the punch is kept to be constant as sr/DP = 1.0. The most effective viscosity of the lubricant for prevention of galling is found to be 32 mm2/s in the pulse forming.
(1) Sufficient amount of liquid lubricant for preventing galling of billet is periodically supplied to the deforming zone through the internal channel by the retreat action of the punch. (2) Appropriate punch motion for preventing galling of aluminium billet at each forming step is the advance and retreat strokes of 15 and 3 mm for the punch diameter of 6 mm. (3) Maximum aspect ratio of the hole in the pulse forming method is estimated to be approximately of 20 for aluminium alloy from viewpoints of buckling and strength of the punch.
References 5. Discussion on forming limit Because the punch with a high aspect ratio must be prepared in forming with deep holes, the forming limit is estimated from the viewpoints of buckling and strength of the punch. The buckling limit of the punch is calculated by the Euler formula [8] on the assumption of elastic deformation of long beams as following equation. P¼
C p2 EP ðLP =kÞ
2
(4)
where P is the forming pressure, C is the end condition of the punch (=2.05, fixed-pinned), EP is the elastic modulus of the punch
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