Tube Bending and Hydroforming

Question 1

What are the risks of wrinkling and thinning in tube bending, and how are they mitigated?

Answer 1

Tube Hydroforming (THF) is a process where pressure from a hydraulic fluid is applied to a tube in order to shape it to a desired shape. The tube is held by dies which move axially. This method differs from traditional tube fabrication as welding is not necessary

Question 2

Compare shaft, spoon, and ball mandrels in terms of applications and complexity.

Answer 2

Exhaust pipes, chassis frames and torsion bars

Question 3

When is crushing required before hydroforming, and how is it performed?

Answer 3

The T-shape is made causing a bulge in the tube. The pressure is used to shape the tube in the desired T-shape

Question 4

Define the three principal stresses ((\sigma_1, \sigma_2, \sigma_3)) in THF and their typical signs (tensile/compressive).

Answer 4

1) Insert the tube in place and close the press
2) Fill in the water while closing the axial punches
3) Move in the horizontal cylinders and increase the water pressure. Retract the transverse punch to control the shape of the bulge
4) Open press and eject the part

Question 5

What is the stress ratio ((\alpha = \sigma_2/\sigma_1)) under plane strain conditions?

Answer 5

Its role is to keep the 3 way flat, reducing thinning and the risk of fracture and allowing for a longer branch

Question 6

Why does maximum thinning occur at the dome of a T-shape protrusion without a counterpunch?

Answer 6

Because of water properties. At high pressures, the water could change phase. If the material is too thick, it might require higher pressures

Question 7

Explain the relationship between hoop strain ((\varepsilon_1)), axial strain ((\varepsilon_2)), and thickness strain ((\varepsilon_3)) via volume constancy.

Answer 7

The axial pistons advance axially in order to favour the deformation of the tube

Question 8

What is a Forming Limit Diagram (FLD), and how is it used in THF?

Answer 8

In order to achieve shapes that are not possible to get with hydroforming like bendings for example

Question 9

List the three main defects in THF and their root causes (buckling, wrinkling, bursting).

Answer 9

The tube is feed into the machine and place in position. A bending die together with a clamping die rotate making the tube to bend while a mandrel inside the tube is introduced to avoid wrinkling in the operation as its diameter is really close to the one of the tube. There is always a extrados an intrados section, the first one is stretched and the second one compressed.

Question 10

How does excessive axial compression with low pressure lead to buckling?

Answer 10

These ratios give an idea of how much the material would be stretched and the tools needed to be used in order to avoid fracture

Question 11

What process parameters control the risk of bursting?

Answer 11

Wrinkling happens when the intrados material is subjected to high compression stress up to the point that the material yields and form wrinkles. Prevented by the presence of the wiper die and the mandrel
‘Thinning happens when the extrados material is subjected to high tension stress up to the point that plastic deformation occurrs and the thickness of the material diminishes in that section. This is mittigated using a booster which function is to compress the material at the other side in to compensate for the high tension stress

Question 12

Describe the “safe window” for THF in terms of axial force and internal pressure.

Answer 12

Ball mandrels are the most expensive ones as they are made out of multiple parts and are needed for bending operations where the bend is high. Shaft and spoon mandrels are more simple and required for not too complicated operations

Question 13

What are the three stages of hydroforming (filling, forming, calibration), and how do pressure and axial feed vary in each?

Answer 13

It is needed when the outer diameter of the initial tube is larger than the die allowance or when the minimum distance from the bottom to the top of the die in the cavity is smaller than the initial tube outer diameter OD.

Question 14

How is calibration pressure ((P_{cal})) estimated, and why is it higher than bulging pressure?

Answer 14

sigma 1.- Hoop stress or circumferencial stress / Almost always tensile
sigma 2.- Axial stress (in the direction where the water would flow in a pipe) / often tensile (in the area of maximum expansion of the bulge) but can be into compression thanks to the axial trust)
sigma 3.- Thickness or normal stress / compressive

Question 15

What is the minimum axial force ((F_{ax})) required to prevent leakage?

Answer 15

It is 0.5 when sigma 3 is equal to 0.

Question 16

How does friction affect axial force in long tubes?

Answer 16

As there is no counterpunch, the material is deformed in plastic zone and the “easiest” direction” to flow is where no force reaction is generated

Question 17

What factors determine the die closing force ((F_c))?

Answer 17

The sum of the three strains is equal to 0: epsilon 1 + epsilon 2 + epsilon 3 = 0

Question 18

Why is annealing performed after bending tubes before hydroforming?

Answer 18

It is a diagram that represents the principal strains Epsilon 1 and Epsilon 2 against eachother in a plane. It represents the major and minor deformation. Major deformation always in the vertical axis.

Question 19

What is the significance of the hardening exponent ((n)) in material formability?

Answer 19

Buckling: caused due to an excess of axial compression with very low normal stress
Wrinkling: caused due to an excess of axial compression with some normal stress
Bursting: caused due to an excess of hoop tension with some axial tension

Question 20

Give examples of automotive structural parts made via THF and their material choices.

Answer 20

As the pressure is too low during the beggining of the forming process, there is the risk of creating big stress while feeding axially which leads to the deformation of the part in a kind of “banana” shape

Question 21

How does hydroforming improve stiffness-to-weight ratios in vehicle components?

Answer 21

Axial force and internal pressure. Bursting will ocurr when high internal pressure is applied but not enough axial force is present

Question 22

Why are tilted parting surfaces used in some hydroforming dies, and what challenges do they introduce?

Answer 22

The “safe zone” is obtained at mid or lower internal pressures and mid axial forces, before wrinkling

Question 23

What are the benefits of forming multiple parts simultaneously in THF?

Answer 23

1.- Filling: a rapid filling of the tube is wanted. Axial feeding is small or inexistent but low pressure and high flow rate is present
2.- Forming: Main and longest phase. Both pressure and feed increase
3.- Calibration: During calibration feed stops (as it would generate wrinkles otherwise). Pressure is increased significantly for a short time in order to coin the smallest internal radii that require high pressure

Question 24

How is hydroforming applied in bicycle frame manufacturing?

Answer 24

Because of the strain hardening. In order to achieve the final form, a bigger pressure is required

Question 25

How are THF processes classified into Groups A–D based on axial feed (\(d_{ax}/L_i\))?

Answer 25

The minimum axial force is simply the cross section area of the tube multiplied by the internal pressure (just by calculating the equilibrium)

Question 26

What distinguishes Group A (no feeding) from Group D (large feeding) processes?

Answer 26

As the guided zone is long, the applied axial force can be completely eroded by friction and makes it useless to move the axial punches

Question 27

What are typical cycle times for THF parts like exhaust manifolds vs. large structural components?

Answer 27

It is determined by the crossing section on the separation plane multiplied by the pressure of calibration

Question 28

31. Why is annealing performed after bending tubes before hydroforming?

Answer 28

Annealing is often performed after bending in order to increase the material formability (which was reduced by roll forming, by welding and by bending)

Question 29

33. What is the significance of the hardening exponent (\(n\)) in material formability?

Answer 29

It is an indicator of formability of the material. The higher this exponent, the better

Question 30

34. Give examples of automotive structural parts made via THF and their material choices.

Answer 30

Low to medium carbon steels and aluminum as materials. Mainly used for chassins and body, steering and suspension and safety parts

Question 31

35. How does hydroforming improve stiffness-to-weight ratios in vehicle components?

Answer 31

Because the tubes are lightweight parts with closed cross sections

Question 32

36. Why are tilted parting surfaces used in some hydroforming dies, and what challenges do they introduce?

Answer 32

They are tilted in order to save space but it creates a non vertical load on the press structure which creates problems on the press

Question 33

37. What are the benefits of forming multiple parts simultaneously in THF?

Answer 33

As THF is a relative slow process, forming multiple parts allows for an increased production rate

Question 34

38. How is hydroforming applied in bicycle frame manufacturing?

Answer 34

THF allows to produce complicated 3D engineered shapes which account for strategically located facets and curves which alter how the frame behaves under a rider's load

Question 35

39. How are THF processes classified into Groups A–D based on axial feed (\(d_{ax}/L_i\))?

Answer 35

Group A: THF processes that do not need feeding (a small bulging ratio) or can be fed very little (a part with too many bendings, protrusions, etc, along the tube). Example: engine craddle. No feeding or litle feeding so little risk of wrinkling in this group. Risk of leaking could be high specially in zero feeding. Group B: Expansion ration larger than those in group A, more axial feeding is essential for the successful bulging of the parts Group C: This process involves parts with characteristics of a relatively small locally bulged region, with low to medium expansion ratio Group D: These are the parts with strong requirements of axial feed (typical example are T-shape, Y shape and X-shape conjuctions) They have 1 or two side protrusions

Question 36

40. What distinguishes Group A (no feeding) from Group D (large feeding) processes?

Answer 36

The requirement of axial feed needing. A needs no feeding or very little in contrast to D parts which require high feeding

Question 37

42. What are typical cycle times for THF parts like exhaust manifolds vs. large structural components?

Answer 37

Exhaust manifolds require between 15 to 20 seconds in contrast to large parts that require less than a minute.