計畫成果自評

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2. 論值想當一致，且與實驗結果得到的抽拉

4. 少而降低。

本研究目前已發表兩篇國際期

1. Masahiro Hayashi and Jung-Chung Hung, “Simulation of Ultrasonic-Vibration Drawing Using the Finite Element Method (FEM)”, Journal of Materials Processing Technology, -vibration on hot

研討會論文

ference on Processing, Taipei, Taiwan.

洪景華, 2004, “超音波振動於鋁合金高溫壓縮之影響＂,中國工程師學會 由 FEM 分析所得到的臨界抽拉速度與理

速度相的符合。

3. AUD 和 RUD 之抽拉力波形變動的頻率平均值與實驗抽拉力量測結果非常一致。

由有限元素的定量分析驗證抽拉速度隨振幅減

刊及兩篇會議論文（如附錄）：

2003, Vol. 140, pp. 30-35

2. Jung-Chung Hung, Chinghua Hung “The influence of ultrasonic

upsetting of aluminum alloy”, Ultrasonics, 2005, Vol. 43, pp. 692-698

1.

Masahiro Hayashi and Jung-Chung Hung, 2003, “Simulation of Ultrasonic-Vibration Drawing Using the Finite Element Method (FEM)”, 6^th Asia Pacific Con

Materials

2.

洪榮崇,

第二十一屆全國學術研討會。

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附 錄

際期刊:

Masahiro Hayashi and Jung-Chung Hung, “Simulation of Ultrasonic-Vibration Drawing Using the Finite Element Method (FEM)”, Jour cessing Technology, 2003, Vol. 140, pp. 30-35

ng Hung, Chinghua Hung “The influence of ultrasonic-vibration on hot

研討

景華, 2004, “超音波振動於鋁合金高溫壓縮之影響＂,中國工程師學會第二

國

1.

nal of Materials Pro 2. Jung-Chu

upsetting of aluminum alloy”, Ultrasonics, 2005, Vol. 43, pp. 692-698

會論文:

1. Masahiro Hayashi and Jung-Chung Hung, 2003, “Simulation of Ultrasonic-Vibration Drawing Using the Finite Element Method (FEM)”, 6^th Asia Pacific Conference on Materials Processing, Taipei, Taiwan.

2. 洪榮崇, 洪

十一屆全國學術研討會。

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The influence of ultrasonic-vibration on hot upsetting of aluminum alloy

Jung-Chung Hung, Chinghua Hung

^*

Department of Mechanical Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 300 Taiwan, ROC Received 2 November 2004; received in revised form 2 March 2005; accepted 8 March 2005

Available online 23 March 2005

Abstract

The traditional ultrasonic apparatus cannot be operated at high temperature, explaining why the effect of ultrasonic-vibration on high temperature metal forming has seldom been addressed in literature. This study establishes an ultrasonic-vibration hot upsetting system. A cooling mechanism is used to solve the problem of high temperature. The effects of temperature and strain rate during ultrasonic-vibration on the upsetting of aluminum alloy were explored using this new system. Experimental results indicate that ultrasonic-vibration can considerably reduces the compressive forces during hot upsetting. The reducing effect on compressive forces decreases while the temperature increases. The strain rate does not significantly affect the reducing effect on compressive forces.

2005 Elsevier B.V. All rights reserved.

Keywords: Ultrasonic-vibration; Hot upsetting; Aluminum alloy

1. Introduction

Many new materials, such as titanium alloys, magne-sium alloys and inter-metallic compounds, are difficult to produce. Production depends on the development of new processes to overcome the difficulties that arise during the metal forming process. The technique of ultrasonic-vibration has been applied widely in metal forming. The difference between conventional metal forming and the ultrasonic-vibration metal forming is that the latter exploits ultrasonic energy to act on the die and then uses the die to deform the work-piece.

Some interesting effects arise in the application of ultrasonic-vibration for metal forming processes, such as the reduction of the friction between the die and the work-piece, the reduction of the forming forces, and de-creases of the spring-back angle during sheet metal forming. These effects increase the forming limit of

materials. Blaha and Langenecker were the first to inves-tigate the use of ultrasonic-vibration in relation to plas-ticity of metals[1,2]. They superimposed high-frequency vibrations onto the static load during the tensile testing of a zinc single crystal specimen. In the experiment, they observed a substantial reduction in the yield stress and the reduction of the flow stress. This phenomenon is the so-called Blaha effect. Kempe [3] proposed three mechanisms by which dislocations may absorb energy from vibrations; they are (1) a resonance mechanism, (2) a relaxation mechanism, and (3) a mechanism of sim-ple hysteresis. Nevill [4]attributed the reduction of the flow stress to the superposition of steady stress and the alternation of stress, and proposed the stress superposi-tion mechanism.

Lehfeldt and Pohlman[5]examined the feasibility of exciting a ball by vibration on a revolving plate in exper-iments on the influence of the ultrasonic-vibration on friction. The frictional forces are minimal at the contact surface when the direction of vibration is parallel to the direction of motion. Jimma et al.[6]applied ultrasonic-vibration to the deep drawing process and show that

0041-624X/$ - see front matter
2005 Elsevier B.V. All rights reserved.

* Corresponding author. Tel.: +886 3 5712121 55160; fax: +886 3 5720634.

E-mail address:[email protected](C. Hung).

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ultrasonic-vibration deep drawing is very effective in increasing the limiting drawing ratio (LDR) and sur-passed the theoretical value LDR of deep drawing by ideal tools without friction. Murakawa et al.[7,8] inves-tigated the effects of radial ultrasonic-vibration drawing (RVD) and axial ultrasonic-vibration drawing (AVD), and compared them to those of conventional wire draw-ing (CD). It was proven to be highly effective in increas-ing the critical drawincreas-ing speed by ultrasonic wire drawing, and the RVD operation appears to be more productive than the AVD operation.

Huang et al.[9]investigated the benefits of applying the axial ultrasonic-vibration of forming tools in an upsetting process; he used plasticine as a model material to simulate the hot metal. According to that study, applying an ultrasonic-vibration to the die reduces the mean forming force during upsetting. He concluded that the stress superposition effect and the reduction of inter-face friction contributed to the above phenomenon.

Conventional ultrasonic apparatus cannot be oper-ated at high temperatures, so relatively few investiga-tions have addressed the effect of ultrasonic-vibration metal forming at high temperature. This study, estab-lishes an ultrasonic-vibration hot upsetting apparatus to overcome this difficulty. The effects of temperature and strain rate during ultrasonic-vibration on the hot upsetting of aluminum alloy are investigated using this apparatus.

2. Ultrasonic-vibration hot upsetting apparatus 2.1. Hot upsetting machine

Hot upsetting is based on a process of unconfined uni-axial deformation of a cylindrical specimen between parallel rigid platens. In this study, an especially de-signed microcomputer server controls the hot upsetting machine. The machine has four conducting pillars and a server motor to control the velocity of the platens.

The maximum loading capacity is 2000 kg W; resolution of the force is 10N; the resolution of displacement is 0.005 mm and the range of velocity is 0.5–50 mm/min.

Fig. 1shows this machine.

2.2. Heating and cooling system

Materials are easily oxidized at high temperature, so the heating system is enclosed in a vacuum chamber, which can sustain a maximum temperature of 600C and vacuum pressure of 10
³Torr. The system was de-signed with a free moveable die and a vacuum furnace and so was ideal for performing experiments at a high temperature in a vacuum.

During the ultrasonic-vibration hot upsetting

experi-tus affecting the parts. The vacuum furnace and the both upper and lower dies also require a cooling system to prevent damage to the part. The heating of the dies pro-ceeds mainly by thermal radiation, so the rate of heating was lower than the rate of liquid cooling. An auxiliary heating system was designed on both the upper and lower dies to make compensate for the heat to increase the accuracy of the temperature control. Three independent PID controlled are used to control the temperature. This temperature compensation can maintains the tempera-ture between the dies and the inner furnace within

±1C.

2.3. Ultrasonic-vibration system

The ultrasonic-vibration system includes an ultra-sonic frequency generator, a piezoceramic vibration transducer, a resonator and an ultrasonic forming die.

The ultrasonic frequency generator has a maximum capacity of 2 kW and provides power for a piezoelectric transducer to generate ultrasonic-vibration. This gener-ator includes an automatic frequency-tracking control-ler, which is able to maintain the system resonant frequency at 20kHz ± 300Hz. A booster then amplifies the amplitude of vibration and transmits it to a horn.

The ultrasonic-vibration system was fixed by flange

Fig. 1. Ultrasonic hot upsetting experiment set up.

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which is resonant in a longitudinal mode, was uniquely designed for use in upsetting. In this investigation, finite element simulations software, ABAQUS, was used to determine the dimensions of this stainless steel horn.

The results of the tests showed that the simulated and experimental resonant frequencies were very close to each other.

3. Ultrasonic-vibration hot upsetting experiment 3.1. Experimental procedure

The ultrasonic-vibration hot upsetting experiment proceeded as follows. First, the ultrasonic-vibration sys-tem and vacuum furnace were set up on a hot bench controlled by a microcomputer server. Second, the spec-imen was placed between parallel dies. Then, a 20kg W preload was applied to the specimen. The heating con-troller was turned on. When the temperature reached the designated temperature, it was hold constant for 10min before the rest of the experiment was performed.

Whenever the loading reached 70kg W during an exper-iment, the ultrasonic-vibration was superimposed.

3.2. Experimental conditions

Table 1shows material properties and the hot upset-ting conditions used in the experiment. The specimens used in this study were aluminum alloy A6061 that was 6 mm high and 6 mm in diameter. The compression dis-placement was set to 4 mm (equivalent to a 66.7% com-pression ratio), and the true strain rate was controlled throughout the experiment. The tests were performed under dry conditions without lubricant. During ultra-sonic-vibration hot upsetting, the axial vibration fre-quency was 20kHz. The amplitude was set to 5.6 lm.

4. Experimental results and discussion

4.1. Effects of ultrasonic-vibration and temperature on upsetting

Fig. 2 shows the experimental results of load–dis-placement curves for the conventional upsetting (CU)

and axial ultrasonic-vibration upsetting (AUU). The temperature was set to 25C and the true strain rate was 0.03 1/s.Fig. 2(a) shows that for CU, the compres-sion ratio was 67% (with a displacement of 4 mm) under a compressive forces 1511 kg W for CU, but for AUU, a compressive forces was 1296 kg W was required to yield the same compression ratio. The compressive force was therefore 215 kg W lower for AUU. These results indi-cate that ultrasonic-vibration effectively reduces the material flow stress.Fig. 2(b) shows reduced compressive forces–displacement curves. Under ultrasonic-vibration, increasing the compression ratio increases the reduction in the compressive forces. Restated, ultrasonic-vibration strengthens the effect of the reduced compressive forces when the compressing ratio is increased.

Figs. 3–5 show the load–displacements curves of the CU and AUU, with a constant true strain rate of 0.03 1/s at temperatures set to 100C, 200 C and 250C, respectively. At all tested temperatures, the load decreased when the ultrasonic-vibration was applied.

Fig. 6(a) and (b) plot the relationship between

tem-Table 1

Material and hot upsetting conditions

Specimen material Aluminum alloy (A 6061) Tooling material Stainless steel (SUS304)

Size of specimen /6.0· 6.0mm

Lubricant N/A

Reduction (R) 66.7%

True strain rate (_e) 0.003 1/s, 0.03 1/s

Temperature of specimen 25C, 100 C, 200 C, 250 C

0

Compressive force, kgw

Strain rate=0.03, CU Strain rate=0.03, AUU

0

Reduced Compressive Force, kgw Strain rate=0.03

(a)

(b)

Fig. 2. Load–displacement curves for CU and AUU at 25C: (a) compressive forces–displacement curves for CU and AUU; (b) reduced compressive forces–displacement curves.

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displacement was 4 mm the compressive forces of AUU at 25C and 100 C were very close to those of CU at 100C and 200 C. Accordingly, the reduction in flow stress caused by increasing the temperature by 100C

vibration. However, when the compression displacement was less than 3 mm, the effect of ultrasonic-vibration exceeded than that of increasing the temperature by 100C.

Fig. 7(a) and (b) indicate that the increase in temper-ature reduced compressive forces for CU and AUU at a true strain rate of 0.03 1/s.Fig. 7(c) plots the load–dis-placement curve of the compressive forces reduced by ultrasonic-vibration at various temperatures. The reduc-tion in compressive forces caused by ultrasonic-vibra-tion was distinguished at 25C. The magnitude of the compressive forces reduction caused by ultrasonic-vibration decreases as the temperature is increased.

4.2. Effects of ultrasonic-vibration and strain rate on upsetting

In this part, the true strain rate was set to 0.03 1/s and 0.003 1/s in CU and AUU, respectively, at temperatures from 25C to 250 C.Fig. 8 shows that the strain rate 0

Compressive Force, kgw

Strain rate=0.03, CU Strain rate=0.03, AUU

Fig. 3. Load–displacement curves for CU and AUU at 100C.

0

Compressive Force, kgw

Strain rate=0.03, CU Strain rate=0.03, AUU

Fig. 4. Load–displacement curves for CU and AUU at 200C.

0

Compressive Force, kgw

Strain rate=0.03, CU Strain rate=0.03, AUU

Fig. 5. Load–displacement curves for CU and AUU at 250C.

0

Compressive Force, kgw

T=100, CU

Compressive Force, kgw

T=25, CU

Fig. 6. Relationship between temperature and ultrasonic-vibration in CU and AUU: (a) load–displacement curves of CU at 25C and 100C, AUU at 25 C; (b) load–displacement curves of CU at 100 C and 200C, AUU at 100 C.

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the temperature was increased to 250C, the loading force of CU and AUU decreased as the strain rate was lowered to 0.003 1/s. These results indicated that, at high temperature, the strain rate markedly affected the materialÕs flow stress, regardless of whether ultrasonic-vibration was applied.

Fig. 10shows the differences of compressive forces vs.

displacements of CU at true strain rates of 0.03 1/s and 0.003 1/s. The difference of compressive forces (F_d) is de-fined as

F_d¼ F_e¼0:03
F_e¼0:003 ð1Þ

where F_{_e¼0:03} is the loading associated with a strain rate of 0.03 1/s and F_{_e¼0:003} is the loading associated with a strain rate of 0.003 1/s.

Fig. 11plots the differences of compressive forces vs.

displacements of AUU at strain rates of 0.03 1/s and 0.003 1/s. At 250C for both CU and AUU, the strain rate significantly affects the compressive forces. Further-more, when the displacement exceeded 3 mm in 25C

comes negative because the ultrasonic-vibration opera-tion proceeds for a long time, increasing the systemÕs temperature, reducing the efficiency of the power

trans-0

Compressive Force, kgw

Strain rate=0.03, CU Strain rate=0.03, AUU Strain rate=0.003, CU Strain rate=0.003, AUU

Fig. 8. Load–displacement curves for CU and AUU at 25C; the strain rate was 0.03 1/s and 0.003 1/s.

0

Compressive Force, kgw ^{T=25, AUU}

T=100, AUU

Reduced Compressive Force, kgw

T=25

Compressive Force, kgw ^{T=25, CU}

T=100, CU T=200, CU T=250, CU

(a) (b)

(c)

Fig. 7. Experimental results for CU and AUU at 25C, 100 C, 200 C and 250 C: (a) load–displacement curves of CU at 25 C, 100 C, 200 C and 250C; (b) load–displacement curves of AUU at 25 C, 100 C, 200 C and 250 C; (c) reduced force-displacement curves for AUU at 25 C, 100 C, 200C and 250 C.

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4.3. Mechanisms of ultrasonic-vibration in upsetting Several investigations have demonstrated that the mechanisms of ultrasonic-vibration in metal forming are as follows: (1) reduction of flow stress; (2) reduction of the friction between die and work-piece, and (3) in-crease of the temperature between die and work-piece.

The most common causes of the reductions of flow stress are as follows: (1) dislocations might absorb en-ergy through resonance and overcome slip obstacles;

(2) dislocations are able to absorb energy from an ap-plied periodic stress and overcome the energy barrier;

(3) the internal friction effect; and (4) the superposition of steady stress and alternating stress. Overall, the effects of ultrasonic-vibration on metal forming are very com-plex. Apart from the reduction of flow stress, the friction between die and work-piece and the raised temperature

the friction effect is stronger when drawing or deep drawing is performed[6–8].

This study addresses the effect of ultrasonic-vibration on hot upsetting. During the experiments, the ultrasonic-vibration frequency was 20MHz, which is far from the ordinary natural frequency, 100 MHz, of a dislocation loop [10]; therefore, dislocations would absorb little energy of vibration due to the lack of res-onance. Accordingly, dislocations would not make much effect on flow stress reduction. However, as shown in Fig. 2(b), during ultrasonic-vibration, the ef-fect of compressive forces reduction was increased with the increase in the compression ratio. If the reduction of flow stress caused by ultrasonic-vibration was attrib-uted only to the superposition of steady stress and alternating stress, then the compressive forces reduc-tion must be a constant. Therefore, mechanisms other than the superposition of stresses should also be considered.

Fig. 7(c) shows that the magnitude of the reduction in force caused by ultrasonic-vibration decreases as the temperature is increased. The causes may be as fol-lows: (1) The materialÕs creep characteristic will gradu-ally come to dominate at high temperature. The deformation mechanism therefore differs from that at room temperature. The ultrasonic-vibration energy ab-sorbed by the material is reduced, weakening the effect of ultrasonic-vibration. (2) Ultrasonic-vibration in-creases the interface temperature between die and work-piece, and reduces the effect of the reduction in friction.

The experimental results indicate that the strain rate does not further reduce the material flow stress associ-ated with ultrasonic-vibration. Currently, the developed ultrasonic-vibration system is limited to a relatively short operation time; only two strain rates were used during experiments; therefore, the connection between 0

Compressive Force, kgw

Strain rate=0.03, CU Strain rate=0.03, AUU Strain rate=0.003, CU Strain rate=0.003, AUU

Displacement, mm

Fig. 9. Load–displacement curves for CU and AUU at 250C; the strain rate was 0.03 1/s and 0.003 1/s.

-150

Fig. 10. Differences of compressive forces vs. displacements of CU in strain rate of 0.03 1/s and 0.003 1/s.

-150

Fig. 11. Differences of compressive forces vs. displacements of AUU in strain rate of 0.03 1/s and 0.003 1/s.

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well explored. A detailed study will be required in the future.

5. Conclusions

This study established the ultrasonic-vibration hot upsetting system. By adopting a cooling mechanism, the system overcomes high temperature operation diffi-culty. The effects of temperature and strain rate during the ultrasonic-vibration upsetting of the aluminum alloy were investigated. Based on the results of this study, we conclude the following:

(1) During the hot upsetting process, the difference between the temperatures of the furnace and the work-piece was significant. Therefore, a device to heat the die was required to eliminate the differ-ence between the temperature of die and the work-piece.

(2) An axial ultrasonic-vibration can reduce the defor-mation resistance in hot upsetting.

(3) The effect of ultrasonic-vibration on hot upsetting cannot be explained by a simple mechanism, such as the effect of interface friction, or superposition of stress, or the absorption by dislocations of the ultrasonic-vibration energy.

(4) The magnitude of the reduction of forming stress in ultrasonic-vibration hot upsetting decreases as the temperature increases.

(5) The strain rate does not markedly affect the reduc-tion of the flow stress in ultrasonic-vibrareduc-tion hot upsetting.

Acknowledgments

The authors would like to thank the National Science Council of Taiwan, ROC for the grant NCS-92-2212-E-009-007, under which the investigation was undertaken.

The authors would like to thank the National Science Council of Taiwan, ROC for the grant NCS-92-2212-E-009-007, under which the investigation was undertaken.

在文檔中 超音波輔助複合擠製成形之研究(III) (頁 33-48)