effect of heat treatment process on hydroforming of

European Journal of Scientific Research ISSN 1450-216X Vol.68 No.3 (2012), pp. 377-388 © EuroJournals Publishing, Inc. 2012 http://www.europeanjournalofscientificresearch.com

Effect of Heat Treatment Process on Hydroforming of

Tubular Materials

A. S. Selvakumar Associate Professor, Department of Mechanical Engineering

B.S. Abdur Rahman University, Vandalur, Chennai, Tamilnadu, India

K. Kalaichelvan Associate Professor, Department of Production Technology M.I.T Campus, Anna University, Chennai, Tamilnadu, India

S. Venkataswamy

Former Professor, Department of Production Technology M.I.T Campus, Anna University, Chennai, Tamilnadu, India

E-mail: [email protected]

Abstract

Hydroforming uses the force of water or hydraulic fluids to shape a single part. A pressurized fluid is introduced into ends of the tube. The applications of hydroforming are in automobile, aerospace and sanitary use. Tube hydroforming is an advanced plastic forming technology used in making complicated shaped hollow parts with thin wall. In tube hydroforming process an internal fluid medium is used as a punch and the tube is bulged into the desired shape through simultaneous application of axial feed and internal pressure.

This paper aims to evaluate the effect of deformation characteristics on tubular materials before and after heat treatment in hydroforming process. The parameters considered for experimental work are axial feed, fluid pressure, and fluid medium. Due to effect of axial feed, the pressure during forming and expansion of tube were analyzed. When pressure exceeds the limit, failures of wrinkle and bursting was occurred. It was found that annealing would result in good formability of tubular materials. Keywords: Formability, Annealing, Tube Hydroforming, Expansion Zone

1. Introduction Hydroforming is one of the sheet-metal-forming technologies for making components with sheet metal and tubular materials. In hydroforming method, the friction at the contact interface between the work piece and die can be reduced and better formability of the product can be achieved (Yeong and Wen, 2005).Components produced by this process have been widely used in automotive and aerospace applications (Lang et al, 2004). Exhaust system, chassis parts, power train components and seat frame are among many components produced by hydroforming process (Muammer and Taylan, 2001).

Hydroforming can be used to make complex shaped sheet metal parts and tubular components. In this process, sheet metal and tubular materials are deformed using pressurized fluid medium in closed dies. In tube hydroforming, the fluid medium inside the tube serves as forming medium, and the

Effect of Heat Treatment Process on Hydroforming of Tubular Materials 378

tube is bulged into the desired shape through simultaneous application of axial feed and internal pressure (Fillice et al, 2001). The combination of process parameters will produce the best quality part in the minimum amount of time (Johnson et al, 2004). In order to achieve the economic product with best performance, it is important to choose proper material and process selection (Carleer et al, 2000).The amount of axial feeding in coordination with internal pressure is a critical factor to improve the formability of tubes(Ahmetoglu et al, 2000).

Buckling, wrinkling and bursting are the three failure modes that limit the expansion process in hydroforming (Dohmann and Hart, 1996). Wrinkling occurs in tube hydroforming due to incompatible relationship between internal pressure and axial feeding (Shijian et al, 2006). Bursting occurs when the internal pressure is very high (Dohmann and Hart, 1997). Formability of the tube material was evaluated by varying the process parameters. In our experimental work, a tubular specimen was placed between the two halves of a split die and filled with oil or water as the forming medium. This tooling was mounted on a hydraulic press as shown in Fig. 1. The punch was advanced into the tube to increase fluid pressure and simultaneously for providing axial feed of the tube. The optimal expansion zone geometry was identified by various experimental trials so as to achieve a flaw-free component.

Hydroforming offers many advantages in terms of improved structural integrity for the product, reduction in production cost, material saving and reduction in the number of joining processes. Other advantages of hydroforming include improved structural strength and stiffness, lesser tooling parts, uniform thickness of formed component, lesser secondary operations, reduced scrap, reduced dimensional variations, and weight reduction through more efficient section design (Ken-ichi and Masaaki 2002).

The chemical analysis test for aluminum material (A9 6063) was carried out. Its chemical composition is shown in Table 1. Table 1: Chemical composition of aluminum A9 6063

Si Fe Cu Mn Mg Zn Ni Cr Pb Sn Ti Al 0.42811 0.35 0.0184 0.0249 0.4506 <0.0020 <0.0010 <0.0010 <0.0010 .0051 0.0056 98.77

2. Experimental Work 2.1. Hydroforming without Annealing Treatment The experimental work was performed without annealing process on tubular materials of aluminum, brass and mild steel. The die and punch were fabricated using low carbon steel as per dimensions shown in Fig. 3. In the bottom surface of die cavity, a hole was made to enable measurement and control fluid pressure inside the deformation zone. The test was carried out by placing the tube into the die and filling the tube completely with SAE 90 fluid. The movement of punch provides both axial feeding of the tube and increasing pressure inside the tube (Ahmetoglu and Altan, 2000). A Teflon sealing was mounted on the front face of the punch which will be first to come in contact with the tube to ensure necessary sealing to the system even when the pressure is quite high.

The die setup was mounted on a 100-ton hydraulic press for conducting experiments as shown in Fig. 2. The parameters such as axial feed, pressure and fluid medium were considered to obtain expansion zone geometry. The movement of punch increases the fluid pressure. The pressure can be built up by compressing the fluid when the punch forces the blank downwards (Klaus et al, 2000). Due to the development of high pressure, the desired shape of component may be obtained (Sokolowski et al, 2000; Fuh-Kuo et al, 2007). The punch and die sketch with dimensions are shown in fig. 3.

In trial I, the experimental work was performed on aluminum, brass and mild steel tubular materials of thickness 1.5 mm, length 106 mm and diameter 38 mm. The required bulging diameter of tube was 54 mm. The punch axial feed for brass, aluminum and mild steel considered for experimental work were 10 mm, 10 mm and 5 mm. The different pressures that developed during the forming

379 A. S. Selvakumar, K. Kalaichelvan and S. Venkataswamy

process were recorded and shown in Table 2. The failures were due to leakage of fluid through pipe joints. The shape of formed component is shown in Fig. 5.

In trial II, different values of punch axial feed applied for aluminum were 10 mm, 13 mm and 15 mm. The values of pressure developed during the forming process are given in Table 3. The modes of failure observed are wrinkling and bursting, which may be due to leakage of oil through pipe joints and side wall of the tube. The formed components are shown in Fig. 6.

In trial III, after proper sealing by means of Teflon tape, leakage was arrested and experiments were continued. The punch attachment of height 25 mm shown in Fig. 4 was fabricated and punch axial feed of 25 mm was considered for experimental work. Due to increase in axial feed, the pressure inside the tube increased, which initiated partial deformation. The results are shown in Table 4. The formed components are shown in Fig 7.

Figure 1: Experimental setup in hydraulic press

Figure 2: Die setup view


Figure 3: Punch and die sketch for bulging in hydroforming

Figure 4: Punch attachments of height 33 mm, 30 mm and 25 mm.

Trial I Table 2: Effect of axial feed on brass, aluminum and mild steel tubes without annealing

Material of specimen: brass, aluminum and mild steel; length: 106 mm; thickness of specimen: 1.5 mm; medium: SAE 90 Oil

S. No Material Thickness

(mm) Length (mm)

Working medium

Axial feed (mm)

Pressure (N/mm2) Observation

1 Brass 1.5 106 Water 10 22.06 Wrinkle failure 2 Aluminum 1.5 106 SAE 90 Oil 10 15.85 Wrinkle failure 3 Mild steel 1.5 106 SAE 90 Oil 5 15.85 Wrinkle failure


Figure 5: Components with defects in wrinkling in trail I

Trial II Table 3: Effect of axial feed on aluminum tube without annealing

Material of specimen: Al; length: 101 mm, 96 mm, 94 mm; thickness of specimen: 1.5 mm, 1 mm and 0.7 mm; medium: SAE 90 Oil

S. No Material Thickness

(mm) Length mm

Working medium

Axial feed (mm)

Pressure (N/mm2) Observation

1 Aluminum 1 101 SAE 90 Oil 10 20.68 Wrinkling failure 2 Aluminum 1 96 SAE 90 Oil 13 19.30 Initiation of forming 3 Aluminum 1.5 94 SAE 90 Oil 15 20.30 Initiation of forming 4 Aluminum 0.7 96 SAE 90 Oil 13 18.2 Tear and buckling

Figure 6: Components with defects in trail II

Trial III Table 4: Effect of axial feed on aluminum tube using punch attachment

Material of specimen: Al; length: 106 mm; thickness of specimen: 1 mm; medium: SAE 90 Oil

S. No

Material Thickness (mm)

Length (mm)

Working medium

Axial feed (mm)

Pressure (N/mm2)

Observation

1 Aluminum 1 106 SAE 90 Oil 25 14.47 Partial deformation without defects.


Figure 7: Components with partial deformation from trail III

2.2. Hydroforming after Annealing Treatment The tubular materials of aluminum, brass and copper were annealed by heavy-duty electric furnace as shown in Fig. 8. The annealing temperatures considered for aluminum, brass and copper materials were 413C, 540C and 520C. The holding time in the furnace was 2 hours and then cooled. The specifications of furnace are maximum temperature 1000, 6.5 kW and 230 V.

Figure 8: Annealing process in electric furnace

Trial IV: Effect of Axial Feed and Pressure After ensuring no leakage in the set-up, experiments were carried out. The experiments were performed by the movement of punch attachment into the tubular specimen of diameter 38 mm, length 106 mm and thickness 1.5 mm. The axial feed of the punch was 30 mm. After the formability test, the tube was removed from the die and the diameter of expanded zone was measured to be 53.45 mm using coordinate measuring machine. The formed component is shown in Fig. 9 and results are shown in Table 5. Table 5: Effect of axial feed on annealed aluminum tube using punch attachment

Material of specimen: Al; length: 106 mm; thickness of specimen: 1.5 mm; medium: SAE 90 Oil

S. No Pressure (N/ mm2) Axial feed (mm) Observation 1 10.29 30 Flaw-free deformation obtained 2 9.80 30 Flaw-free deformation obtained


Figure 9: (a) and (b) Components with uneven deformation

Trial V: Effect of Axial Feed and Pressure In trial V, at an axial feed of 30 mm, pressure developed during forming was measured to be 10.78 N/mm2 and better deformation was obtained. However, at an axial feed of 32 mm, a high pressure of 12.25 N/mm2 was generated and thereby resulted in wrinkle defect of the component. The component is shown in Fig. 10 and the results are shown in Table 6. Table 6: Observations of trial V


S. No Pressure (N/mm2) Axial feed (mm) Observation

1 10.78 30 Better deformation in the expansion zone but non-uniform thickness

2 12.25 32 Wrinkling failure

Figure 10: (a) and (b) Components with uneven bulging and wrinkle defects

Trial VI: Effect of Axial Feed and Pressure In trial VI, when axial feed of 33 mm was applied, pressure developed was 19.60 N/mm2 and wrinkle defect occurred in the tubular component. The formed components are shown in Fig. 11. When axial feed of 30 mm was applied, pressure developed during forming process was 9.6 N/mm2 and the required expansion zone geometry was obtained. The results are shown in Table 7.


Table 7: Effect of axial feed on annealed aluminum tube using punch attachment


S. No Pressure (N/mm2) Axial feed (mm) Observation 1 19.6 33 Wrinkle defect 2 9.6 30 Required deformation obtained

Figure 11: (a) and (b) Components with defects of wrinkle and deformation

Trial VII: Effect of Axial Feed and Pressure In trial VII, when axial feed of 30 mm was applied on brass specimen, a high pressure of 22.06 N/mm2 was developed and bursting type of failure occurred. When axial feed of 30 mm was applied in copper material, pressure developed was 16.54 N/mm2 and better formability of bulging was obtained. The formed components are shown in Fig. 12 and the results are shown in Table 8. Table 8: Effect of axial feed on annealed brass and copper tubes using punch attachment

Material of specimen: brass and copper; length: 106 mm; thickness of specimen: 1.5 mm; medium: SAE 90 Oil

S. No Pressure (N/mm2) Axial feed (mm) Observation

1 22.06 30 Bursting failure occurred 2 16.54 30 Good deformation obtained

Figure 12: Component with burst failure and better deformation


Figure 13: Measurement of bulging diameter using coordinate measuring machine – aluminum material

Figure 14: Measurement of bulging diameter using coordinate measuring machine – brass and copper materials

3. Theoretical Pressure Calculations The theoretical pressure required to deform the tubular work piece is calculated using the formula 3.1.1. Pressure Calculation

2

2 2 0ln 0

03 3

nr r

p k tr r

Where k- Strength coefficient obtained from power law r- Bulged tube radius = 27 mm ro- Original tube radius = 19 mm


t0- Original tube thickness = 1.5 mm n- Strain hardening coefficient obtained from stress strain curve. σ-True stress ε- True strain

3.1.2. Power Law σ= K εn

220 = K (0.15)0.2292

220 = 0.647 K K= 339.887 N/mm2

3.2. Strain Hardening Exponent (n) The strain hardening exponent noted as n, is a material constant which is used in calculations for stress-strain behavior in work hardening.

Figure 3.1: Stress-Strain Curve of A9 6063

The values obtained from Stress-Strain curve are (0.15, 220) and (0.2, 235). By plotting these values in log chart we obtain the value of ‘n’ n = (log 235 – log 220) / (log0.2 – log 0.15)

= 0.292. The values of k ,r,ro, n,to are substituted in the theoretical pressure formula. P =2/√3 * 339.837 (2/√3 ln (27/19)0.2292 *1.5 * (19/272))

= 12.4 N/mm2 4. Results and Discussion The experimental work was performed using a split die to obtain expansion zone diameter of 54 mm from 38 mm diameter aluminum tube (A9 6063) and brass, copper and mild steel materials. The process parameters considered were axial feed, pressure and fluid medium.


The coordinate measuring machine (TESA Micro-hite 3D) shown in figure 13 and 14 was used to measure bulging diameter of aluminum, brass and copper tubes. The specifications of the machine were measuring range 460510420 mm, repeatability limit 0.3 μm and air pressure 4.8–8.3 bar.

The experimental work was performed in two phases. In phase I, without heat treatment process, three trials of experiments were carried out on brass, aluminum and mild steel tubes. Wrinkle and bursting failures were observed on components in trial I and II. This was due to leakage of oil through pipe joints and side wall of the tube.

For trials III to VII, in order to increase fluid pressure, punch attachments of height 33 mm, 30 mm and 25 mm were fitted on the punch face. This helped in arresting the leakage.

After arresting leakages, trial III was conducted using punch attachment of height 25 mm. A bulging diameter of 42.904 mm was achieved.

In phase II, after annealing treatment to tubular specimens, four trials were performed using punch attachments of heights 33 mm and 30 mm. Wrinkle failure and uneven bulging diameter were observed initially due to high speed of the ram in trials IV and V. After reducing the speed of ram, the desired diameter of 53.736 mm was obtained in trial VI.

In trial VII on annealed brass and copper tubes, the experimental work was performed to evaluate the formability of tubular materials. A bulging diameter of 40.144 mm with bursting type of failure was observed in brass tubular specimen due to improper ram speed. In copper material, a diameter of 48.66 mm of deformed component was obtained.

By proper annealing treatment, leakage arresting and suitable punch movement, the desired shape of the component without failure was obtained. 5. Conclusion Hydroforming is an innovative forming process. It is becoming more popular in automobile and aerospace Industries.

The experimental work was performed in two phases. In phase I, without heat treatment process, three experimental trials were conducted, and wrinkling, bursting and initiation of bulging formation were observed. The maximum bulging diameter obtained in phase I was 42.904 mm.

In phase II, after annealing treatment, four trials of experiments were performed. The required bulging diameter was formed from trial VI in aluminum material at optimum parameters of axial feed 30 mm and pressure 9.6 N/mm2. The bulging diameter obtained was 53.735 mm.

Appreciable deformation was obtained for copper material from trial VII. The annealed specimens showed good formability than those without heat treatment. Aluminum A9 6063 has good formability and is a good candidate for tube hydroforming.

The pressure values obtained from experimental work are ranges from 9.6 N/mm2 to 12.25 N/mm2 and the theoretical pressure value obtained is 12.4 N/mm2. Acknowledgements The experimental work using hydraulic press was carried out at Department of Production Technology, Anna University, MIT Campus, Chennai,India.The tubular specimen preparations, annealing treatment process, tensile test by U.T.M and Measurements of bulging diameter by C.M.M f was carried out in the Department of Mechanical Engineering, B.S.Abdur Rahman University, Chennai, India. The authors thank the Departments of Production Technology,Anna university,M.I.T Campus and Department of Mechanical Engineering, B.S.Abdur Rahman University for permitting to use all the equipment facilities and providing all the technical support during the research work.


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effect of heat treatment process on hydroforming of

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