Effects of Heating Temperatures on Hot Spinning of TAlS Alloy Thin-walled Shell

Mei Zhan, Tian Li, He Yang, Qiaoling Wang, Hu Li

State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechni­cal University ,P. 0. Box 520 ,Xi' an 710072 ,China

Titanium alloy thin-walled shells, formed by hot spinning, have been attracting more and more applications in aerospace, aviation, military and

other technical industries, due to its highly forming precision and satisfyingly increasing needs for thin-walled and high strength/weight ratio

products. The hot spinning of titanium alloy thin-walled shells is a complex multi-physics problem with multi-factor coupling interactive effects,

and the heating temperatures have significant influences on the spinning qualities of workpieces. Therefore, in this study, using a 3D elastic-plas­

tic finite element (FE) model for the hot spinning of TAI 5 alloy thin-walled shell. the influence laws of preheating temperature of mandrel and

heating temperature of blank on forming qualities are revealed. The results show that: preheating temperature of mandrel and heating tempera­

ture of blank have significant influence on maximum tangential tensile stress. With the increasing of preheating temperature of mandrel and heat­

ing temperature of blank, the tangential tensile stress first decreases then increases The reasonable preheating temperature and the heating tem­

perature are about 400"C and 750"C , respectively. Finally, the satisfied workpieces are obtained with the above parameters. The results may pro­

vide a theoretical guide for the determination and optimization of forming temperature of relative spinning process.

Keywords: Titanium alloy thin-walled shell .hot spinning ,coupled thennal-mecha11ical ,heating temperature, preheating temperature

1. Introduction

Due to the inherent advantages and flexibility of the process such as simple tooling and low forming loads, plus the rapid emerging trend in modern indus­

tries towards near net shape manufacturing of thin sec­tioned lightweight parts, metal spinning has been at­tracting more attentions)). With the development of

aerospace, aviation, weapon and other high technology industries, metals with light weight and high strength under high temperature are being used widely. As one kind of lightweight alloys in the wide applications, TA15 alloy, for its fine thermal stability, weldability, high creep resistance and strength, is used to be vital candidate for components of aero-engine, which works at temperature above 500°C 2>. In order to reduce its de­

form resistance and improve its plasticity, the TA15 al­loy workpiece should be spun at elevated temperature.

According to the research on TA15 alloy, the flow stress decrease gradually with increasing temperature at given strain rate3>. The investigation by Hong Quan et al1>

indicated that when heated below 600°C, the plasticity of

TA15 is poor due to work-hardening, so a little larger de­formation would lead to cracks. When heated above 600°C ,due to the obviously dynamic recovery and dynamic

recrystallization, the plasticity can be improved. Hence, the

heating temperature is a vital factor for hot spinning. The investigation by Xu Wenchen et al5> indicated that the op­timum temperature of hot spinning of TA15 alloy is in the range of 600-700°C . The spinning temperature should

keep higher relatively to avoid crack when tube wall is a little thicker, and the temperature should be decreased to

control pileup of metal before the rollers when tube wall

becomes thinner.

The hot spinning of T Al 5 alloy thin-walled shells

is a complex multi-physics problem with multi-factor

coupling interactive effects. Due to the limitation of ex-

perimental study and theoretical analysis, FE method is becoming a very important tool for the research on the effects of parameters and forming quality of structural component6

"1 ll. Based on the FE software ABAQUS/

Explicit, Li Hu et aJ7>, Sun Linlin et al8> studied temper­

ature, stress and strain distribution of the power spin­ning for titanium alloy thin-walled shell and Ni-Cr-W­Mo superalloy workpiece with curvilinear shape, respec­tively. External thermal sources were transformed to in­ternal thermal sources by dividing the blank into several

annulations according to the feed rate of rollers and heating area. However, neither one analyzed the effect of

temperatures on hot spinning. By means of FE software Ansys/Ls-dyna, Li Qijun et al9> analyzed the stress­

strain status of different areas at different step and cau­ses of drawback occurring. But the temperature distribu­tion was not analyzed. Shan Debin et al1°> used FEM to analyze the effect of roller path, flanged length and feed ratio on spinning. The temperature was assumed to be constant during spinning process. Chen Yu et al1 ll estab­lished a 3D rigid-plastic FEM simulation coupled ther­

mal model for TC4 titanium alloy by warm shear spin­

ning. The model can not be used to analyze springback due to the assumption of rigid-plasticity model.

In this paper, using FEM, temperature field distri­

butions under different paths are obtained. And the in­

fluencing laws of different heating temperatures of blank and preheating temperature of mandrel on the tangential tensile and compressive stress, and shear stress (along circumferential direction in tangential plane) are investigated. Then, optimum heating tem­

perature and preheating temperature are obtained. With

the simulation results, satisfied workpiece is obtained

by experiment.

2. Finite Element Model and Computational Conditions

The FE model used in the study is the same as the

• 1784 • Proceedings of the 12'h World Conference on Titanium

model in the published paper by Li Hu et aln , as shown in Figure 1. Blank and mandrel are defined as 3D de­formable solid bodies, rollers are defined as analytical rigid bodies, and spindle is defined as a discrete rigid body. Blank is partitioned into several annuluses in re­sponse to the heating area and feed rate of rollers. Body heat flux is applied to each annulus following the movement of roller. The model has been proved to be stable in reference7>.

The thermo-physical parameters of workpiece and mandrel are taken from reference12

•13>. The true stress­

strain curve is taken from reference15>. The process pa­

rameters of hot spinning are shown in Table 1.

Figure 1. FE model of hot spinning Cfor meshing) n

Table 1. Process parameters of hot spinning

Initial diameter of blank/ mm

Initial thickness of blank/ mm

Diameter o f roller/ mm

Nose radius of roller/ mm

Setting angle of roller/ degree

Larger diameter of mandrel/mm

Smaller diameter of mandrel/mm

H alf cone angle/ degree

Radius of corner of mandrel/ mm

Rotational speed of mandrel/ rad s- 1

Distance between roller and mandrel/ mm

Feed ratio of rolle r/ mm r- 1

Gap conductance (blank-mandrel) / W ( m2 "C) - i

Heat transfer coeffi cient

( blank-envi ronment) / W(m2 °C) - 1

Heat transfer coeffi cient

(mandrel-environment) / W(m2 "C ) - 1

Radiation coefficient of blank

Radiation coeffi cient of mandrel

Preheating temperatu re/ "C

H eating temperature/ "C

3. Results and Discussion

260

4

170

4

30

280

80

45

10

20.944

2.828

1450

16. 5

20

o. 6

0. 8

350,400,450

700 ,750 , 800

Two paths are established to analyze the tempera­ture distribution in workpiece during spinning process, as shown in Figure 2. Path-1 is along the radial direc­tion, while path-2 is along the circumferential direction.

Figure 3 shows the temperature distributions along different paths during spinning process. As seen from

Figure 3 a) , the temperature is about 750°C at the de-

Figure 2. Definitions of two paths

forming area. After being deformed, the temperature in the current area decreases sharply, and large tempera­ture difference of deformed area can be seen from Figure 3(a) ,about 320°C . This is because of the long contact between mandrel and deformed area and low tempera­ture of mandrel. There are much longer time for thermal conductance between mandrel and top of blank, which lead to the lowest temperature of blank on the top area, about 420°C. According to Figure 3 ( b), the tempera­

ture distribution is nearly even along path-2.

800

700

~ 600 ~ 500

g 400 0.

~ 300 r 200 ....... _fo_n_n-in-g-ra--..tio_=_,2-5°,...Yo_,1

+ forming ratio=50% 1 OO ..... formi ng ratio=75%

0 '*forming ratio= I 00% .t::::c0=5:c::O::i':::ll O=O=l=r=5=0 =20:c:.0_2__,5_0~3_.._00~3 5._0_4__,0_0~4......J50 ( a) Path- I

True distance along path-I /mm

7QQ e • e 8 I •I I I I I I I I I I I I I I e I I

600 p ~ 500

~ 400 g_ E 300 ~

200

100

........ forming ratio=25% --+- forming ratio=50%

_..._forming ratio=75% _.... forming ratio= 100%

Lll::: ___ t:tt:! ___ !::lt::j1::t1::-::lt:!l~LJ ( b ) Path-I 0 I 00 200 300 400 500

True distance along path-2/mm

Figure 3. Temperature distributions at different time along

different paths

3. I Effects of Preheating Temperature of Mandrel As estimated above, heating temperatures have

the important effects on the forming quality and preci­sion of deformed workpiece, so the corresponding influ­encing laws should be investigated3>. According to ref. is-m , cracks, bulge and warping defects are easy to

occur during hot spinning process. Tangential tensile stress can predict the tendency of cracks, while tangen­tial compressive stress can predict the tendency of bulge, and shear stress can predict the degree of war­

ping. Therefore, in this paper, these three factors are

taken into considerations.

7. ear Net Shape Processing • 1785 •

Figure 4 shows the influencing laws of preheating temperature of mandrel on maximum tangential com­pressive stress, tangential tensile stress and shear

stress. As seen from this Figure , with the increasing of preheating temperature, the maximum tangential com­pressive stress increases , and the tangential tensile stress first decreases then increases , but the shear stress is in the opposite tendency. When the preheating tem­perature increases , the temperature difference in thick­ness directions decreases , and the deformation of blank becomes more even and easier, thus the required tangen­tial tensile stresses decreases , but tangential compressive

stress increases, and bulging is likely to occu r. At the same time , the shear stress increases, which means war­ping would be easy to occur. However, if the preheating temperature increases continually, above 400°C , the

maximum distance between inner surface of workpiece and mandrel would increases due to the expanded diam­eter, and the required deformation stress increases , thus the tensile stress increases. At the same time , the com­

pressive stress increases a little due to the decreases of deform resistance with the increasing of temperature. Furthermore , the shear stress decreases because of the homogeneous temperature dist ribution.

800 -0- Shear stress 216

790 -+-Tangential 212 "' 780 tens il e stress "' 0...

208 ~ ~ 770 ~

~ 760 204 "' 750 ~

~ 200 ~

740 "' 5

OJ

Oll 730 196 65 = ~ 720 192

710 700 188

350 400 450

Preheating temperature/ ·c

Figure 4. Influencing laws of preh ating temperature of mandrel

According to the above analysis, prehea ting tem­perature has a great influence on the tangential tensi le

stress , lower or higher preheating temperature would

cause large tensile stress, and the optimum preheating

temperature of mandrel is about 400°C .

3. 2 Effects of Heating Temperature of Workpiece Figure 5 shows the influencing laws of heating

temperature of blank on maximum tangential tensile and compressive stress and shea r stress. According to

this Figure , with the increasing of hea ting tempera­

ture , the compressive stress first increases then decrea­ses , but the opposite tendency are with the tangential

tensile stress and shear stress. The reason is when

temperature increases , metal becomes soft and easy to

flow, and the tensile stress and shear stress decrease, at

the same time, bulge would arise in front of rollers, and

more compressive stress is needed for rollers to pass

through the workpiece, thus the compressive stress increa­ses. With the temperature increases continually, due to the

dynamic crystallization and dynamic recovery, the plasticity of workpiece increases sharply, which makes it easier to be deformed, and the required compressive stress decrea­ses. However, tangential tensile and shear stress in­creases due to the low hardness. The higher the tem­perature in a reasonable range, the easier metal can be deformed by hot spinning.

900 Shear stress 270

--+-- Tangentia l compressive 260

"' 850 -+- Tangential tensile stress 250 0...

240 cl:: ~ stress ~ 800 230 ~ ~ "' ;;; 220 ~ ~ 750 "' 210 ~

~ "' <I.I 00 200 ei ~ 700

190

650 180 750 800

Heating temperature/ ·c

Figure 5. Influencing laws of hea ting tempera ture of blank

According to the above analysis, heating temperature of workpiece has great influence on tangential tensile stress. The temperature should be appropriate, neither too

low nor too high, to avoid the larger tensile stress , and the optimum heating temperature is about 750°C

4. Experiment

With the heating temperature of the blank of nea r­ly 400°C and the prehea ting tempera ture of the man­drel of about 750°C, the experiments are carri ed out by PT30501 spinning machine (shown in Figure 6) , and

the satisfi ed workpiece is gained , as shown in Figure 7.

.. -

Figure 6. Power spinning machine CNC PT30501

Figure 7. Workpiece obtained by expe riment

5. Conclusions

In thi s study, using FEM and experimental meth­ods, the influencing laws of heating tempera tures on

spinning are obtained, the results are as fo llows :

(1) The temperature distribution along the tangen­

tial direction isn' t even , and the highest temperature

distributes nea r the contact areas of workpiece and roll-

• 1786 • Proceedings of the 12'h World Conference on Titanium

er. The temperature distribution along the circumferen­tial direction is nearly even during spinning process.

( 2) The preheating temperature of mandrel and heating temperature of blank have significant influ­ences on maximum tangential tensile stress. With the increasing of preheating temperature of mandrel and heating temperature of workpiece, the maximum tan­gential tensile stress first decreases then increases, and the reasonable preheating temperature and heating temperature are about 400°C and 750°C, respectively.

Acknowledgements This research 1s supported by the National High.

Technology Research and Development Program of China (No. 2008AA04Zl22), the National Natural Sci­ence Foundation of China (No. 50405039), and the au­thors wish to express their gratitude.

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