Talat Lecture 3803: Process Technologies

TALAT Lecture 3803

Process Technologies 15 pages, 21 figures Advanced Level prepared by K. Siegert and T. Werle, Institut für Umformtechnik, Universität Stuttgart

Objectives: − to describe fabrication processes for superplastic forming, i.e. female and male die forming, and the criteria for selecting the correct process

Prerequisites: − General background in production engineering and material science

Date of Issue: 1994 © EAA – European Aluminium Association

TALAT 3803

1

3803

Process Technologies

Table of Contents 3803

Process Technologies ...................................................................................2

3803.01 Superplastic Fabrication Methods ............................................................ 2 3803.02 Female Die Superplastic Forming ............................................................. 4 3803.03 Superplastic Forming with Male Dies....................................................... 7 3803.04 Criteria for the Choice between Male Die and Female Die Forming... 11 3803.05 List of Figures............................................................................................ 15

3803.01 Superplastic Fabrication Methods Figure 3803.01.01 defines in general terms the application of superplastic forming of sheets metals.

Superplastic Forming of Sheets

Manufacturing complex shaped components in small and medium series using simple tools and alloys having a special fine-grained microstructure

alu

Superplastic Forming of Sheets

3803.01.01

Training in Aluminium Application Technologies

In principle, superplastic forming of metals can be subdivided into bulk metal and sheet metal forming, see Figure 3803.02.02. Hot die forging, precision forging and isothermal forging are examples for bulk superplastic forming. Bulk forming, however, shall not be treated in this lecture.

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Forming Superplastic Materials

Superplastic sheet forming

Superplastic bulk forming

Air injection process

Precision forging process

-Female die forming -Male die forming

- Isothermal forging - Hot die forging

alu

Forming Superplastic Materials

3803.01.02

Training in Aluminium Application Technologies

Processes for superplastic forming of sheet metals are subdivided further into three main pneumatic forming processes: die forming, patrizen forming and membrane forming, see Figure 3803.01.03. The economy of female die forming and male die forming can be increased by using a number of tool inserts assembled in one tool so as to produce a number of parts simultaneously.

Superplastic Forming Processes for Sheet

Male Die Forming

Female Die Forming Membrane Forming

Female Die Forming with Inserts

alu

Male Die Forming with Inserts

Superplastic Forming Processes for Sheet

3803.01.03

Training in Aluminium Application Technologies

Fields of application for the different manufacturing processes are shown in Figure 3803.01.04. Superplastic forming can best be applied for producing medium series of parts and for complicated shaped components. The field of application of superplastic forming is limited by conventional process technologies. Small series of complicated parts generally have to be produced as single-parts. Classical sheet forming technologies are suitable for parts which are not so complicated. The relatively expensive pressure die casting is most suitable for producing large series of complicated parts.

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Fields of Application for the Different Manufacturing Processes

3

1 Single-piece production Complexity

4

100

2 Deep drawing and stretch forming 3 Pressure die casting 4 Superplastic forming

1

2

101

102

103

104

105

Series size alu Training in Aluminium Application Technologies

Fields of Application for the Different Manufacturing Processes

3803.01.04

3803.02 Female Die Superplastic Forming Female die forming utilises a concave form, see Figure 3803.02.01. During the whole forming process, the flange of the sheet blank is clamped tight between the top and lower pressure chambers. A flow-in of the material into the tool interior is thus not possible. The forming zone, therefore, consists of the blank membrane area within the tool. The pressure difference applied in the first phase, causes free forming (pneumatic sinking) of the blank membrane in the direction of the lower pressure. In the second phase, the gradually increasing doming leads to contact between tool and blank. The gas membrane then divides into sub membranes, leading to varying flow rate conditions and consequently to non-homogeneous material flow. Furthermore, the locally varying frictional conditions result in an unfavourable wall thickness distribution.

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Female Die Forming po po 1st process step

Process start Tightly clamped blank

Free forming of a dome

pu

pu

2nd process step End of process Hindering of material flow due to tool contact starts

alu Training in Aluminium Application Technologies

po Formed part pu

Female Die Forming

3803.02.01

Figure 3803.02.02 illustrates the forming steps during female die forming, described in Figure 3803.02.01, using a photographic technique. The first phase of undisturbed free forming can be seen clearly. In the second phase, two sub membranes exist.

Forming Steps During Female Die Forming

Source: Photographic Art, R. Häselich alu Training in Aluminium Application Technologies

Forming Steps During Female Die Forming

3803.02.02

Figure 3803.02.07 shows a tool insert for a Landrover front part. The complicated internal design with very narrow webs is clearly visible. The use of die inserts reduces the cost significantly.

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Tool for a Landrover Front Part

Source: Superform Metal Ltd. alu

Tool for a Landrover Front Part

3803.02.03

Training in Aluminium Application Technologies

Figure 3803.02.04 shows a complete equipment for female die forming. One sees a rather simple machine construction which is, however, equipped with a complicated control and regulation system for gas and heat flow.

Equipment for Female Die Forming

Source: Superform Metal Ltd. alu

Equipment for Female Die Forming

3803.02.04

Training in Aluminium Application Technologies

The rear wheel housing of an ASTON MARTIN LAGONDA is a typical component produced with female die forming, see Figure 3803.02.05

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Superplastically Formed Wheel Housing with the Female Die Forming Process

Rear Wheel Housing - ASTON MARTIN - LAGONDA Source: Superform Metals Ltd. alu

Superplastically Formed Wheel Housing

3803.02.05

Training in Aluminium Application Technologies

.

3803.03 Superplastic Forming with Male Dies The male forming process works with a convex form, see Figure 3803.03.01. During the whole forming process, the flange of the sheet blank is clamped tight between the top and lower pressure chambers. A flow-in of the material into the tool interior is thus not possible. The forming zone, therefore, consists of the blank membrane area within the tool. The pressure difference applied in the first phase, causes free forming (pneumatic sinking) of the blank membrane in the direction of the lower pressure. In the second phase, when the doming is large enough, the punch with the positive form is moved in and the material pressed on to the form by changing the pressure direction. Frictional contact and local thinning of the membrane occurs much later than in female die forming. As a result, the wall thickness distribution is more favourable than in female die forming.

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Superplastic Male Die Forming Process Start

1st Process Step

2nd Process Step pO

pO

End of Process pO

pU pU

Tightly clamped blank

alu

Free forming

Form punch moved in and air pressure direction reversed

Superplastic Male Die Forming

Formed part

3803.03.01

Training in Aluminium Application Technologies

Figure 3803.02.11 shows the complete equipment used for superplastic male die forming: preheating furnace, machine and regulation and control systems for gas and heat flow.

Equipment for Superplastic Male Die Forming

Source: Superform Metal Ltd. alu Training in Aluminium Application Technologies

Equipment for Superplastic Male Die Forming

3803.03.02

Figure 3803.03.03 shows an opened malew die forming machine where the material flanges are visible. The punch is at the top-end of the stroke. TALAT 3803

8

Machine for Male Die Forming

Source: Superform Metal Ltd. alu

Machine for Male Die Forming

3803.03.03

Training in Aluminium Application Technologies

Figure 3803.03.04 shows the tool for male die forming. One can clearly see the crimped seam, running around the periphery, used to seal the gas pressure cavities as well as the connections for heating the tool. The punch has moved to the top-end of the stroke.

Tool for Male Die Forming

Source: Superform Metal Ltd. alu

Tool for Male Die Forming

3803.03.04

Training in Aluminium Application Technologies

Figure 3803.03.05 illustrates the different stages in the manufacturing of a component using the male die forming process. The three intermediate stages of the left show the

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initial phase of free forming. The last three stages show the steps of forming after pressure reversal.

Stages in the Manufacturing of a Component Using the Male Die Forming Process

Source: Superform Metals Ltd. alu Training in Aluminium Application Technologies

Stages in the Manufacturing of a Component Using the Male Die Forming Process

3803.03.05

Figure 3803.03.06 shows a tool set, consisting of a punch, punch holder plate and blank holder, for male die forming of an external rear view mirror housing for a car together with an untrimmed and a trimmed component.

Tool Set for Manufacturing a Car External Rear View Mirror Housing Tool for Manufacturing a Car External Rear View Mirror Housing

Process: Male Die Forming

Source: Superform Metal Ltd. alu Training in Aluminium Application Technologies

TALAT 3803

Tool Set for Manufacturing a Car External Rear View Mirror Housing

10

3803.03.06

Figure 3803.03.07 shows a head rest for a pilot seat. The characteristically high form of the part makes it most suitable for male die forming. A sectional view is shown on the left. A good wall thickness distribution exists above the trimming line.

Wall Thickness Distribution in a Head Rest - Male Die Process Starting sheet thickness s = 3 mm Section A - A

Sectional line A-A

1,13mm 1,20mm 1,45mm 37,7% 1,45mm 48,3%

40,0% 48,3%

1,38mm

46,0% 48,0%

1,79mm 59,7%

1,44mm

Cutting line

3,00mm

3,00mm

Wall thickness along part Part: Head rest of a pilot seat Source: Schneider, K.E. alu

Wall Thickness Distribution in a Head Rest

3803.03.07

Training in Aluminium Application Technologies

3803.04 Criteria for the Choice between Male Die and Female Die Forming Figure 3803.04.01 shows the criteria required in principle for choosing the male die forming process. The male die forming process allows an accurate replication of the interior contours. Due to local differences in material flow, the external contour do not exactly conform to the required geometry. Since the male die forming process exploits the material volume better than die forming, it is possible to work with thinner starting sheets in the former case. Secondary form elements protruding out of the basic form in the blowing direction have a positive effect on the male die forming. The maximum ratio for the surface increase is 0.6.

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Criteria for Choosing Male Die Forming b

l

The ratio of surface area increase A0/A1 is greater than 0.4.

h

A0 = l ⋅ b

A0 = l ⋅ b + 2 ⋅ h(b + l ) z

Exact replication of contours on inside of part. External surface of part free from manufacturing blemishes (scratches etc.). Wavy external surface of part due to wall thickness variations.

z Actual contour Required contour

Weight of part and sheet thickness required to be as low as possible.

s

Secondary form element alu

Secondary form elements make female die forming very difficult.

Criteria for Choosing Male Die Forming

3803.04.01

Training in Aluminium Application Technologies

Principle criteria for choosing the female die forming process are discussed in Figure 3803.04.02. The maximum ratio for the surface increase is 0.4.

Criteria for Choosing Female Die Forming An exact form replication on the outside surface of the part is essential.

Part interior surface free from manufacturing blemishes (scratches etc.). Wavy interior surface of part due to variations in wall thickness.

z Required contour Actual contour

Ribs divide the part.

Secondary form elements make male die forming difficult.

The blank size necessary is not conducive to male die forming.

alu

Criteria for Choosing Female Die Forming

3803.04.02

Training in Aluminium Application Technologies

Figure 3803.04.03 compares the component requirements, blank dimensions, geometrical and dimensional requirements, economy and the controlling parameters as a help for selecting the right process for superplastic forming processes.

TALAT 3803

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Choosing a Process for Superplastic Forming Dimensional accuracy

Forming ratio

Control of log. strain rate

+

--

---

+++

--

Female die process with insets

--

+

+

--

---

++

--

Male die process

+

-

-

+

+

+

+

Male die process with insets

+

-

-

+

+

+

+

fk < 0,6

+

Tool costs

--

External surface

Female die process

> 900

Constant

Economic aspects

Internal surfacce

Part wall thickness

External surface

Defect-free surface

Control possibilities

Internal surfacce

Process

< 900 x 1300

< 2300 x 1300

Part size (possible)

Criterion

Economy

fk < 0,4

Dimensional and geometrical requirements

Multiple forming

Part requirements

ϕ

Source: Superform Metals Ltd. alu

3803.04.03

Choosing a Process for Superplastic Forming

Training in Aluminium Application Technologies

Figure 3803.04.04 gives a method for estimating the required gas pressure for forming which can be calculated using the formula p = 2 ⋅ kf ⋅ s/r. This formula is based on the equilibrium principle of the strength theory: p ⋅ Aproj. = kf ⋅ Astressed

Estimating the Required Gas Pressure dA cosα Hemisphere

dA

dFn

σl σu

α

p

σl

α F

σu σr

s

r = average sphere radius

p = internal pressure

kf = flow stress

s = sheet thickness

Equilibrium conditions

p Aproj. = kf Astressed

p = kf 2s r alu

Estimating the Required Gas Pressure

3803.04.04

Training in Aluminium Application Technologies

In order to obtain kf from the flow curves it is necessary to estimate the logarithmic ,as outlined in Figure 3803.04.05, where vwzg is the tool velocity (see also strain rate, ϕ! TALAT 3300, Figure 3303.02.04).

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Estimating the Log. Strain Rate (or True Strain Rate)

Elongation of a tensile specimen element

ϕ = ln dl

l0

"

ϕ=

alu Training in Aluminium Application Technologies

TALAT 3803

l l0

vwzg dl dϕ = = dt l ldt

Estimating the Log. Strain Rate (or True Strain Rate)

14

3803.04.05

3803.05 List of Figures

Figure No. Figure Title (Overhead) 3803.01.01 3803.01.02 3803.01.03 3803.01.04

Superplastic Forming of Sheets Forming Superplastic Materials Superplastic Forming Processes for Sheet Fields of Application for the Different Manufacturing Processes

3803.02.01 3803.02.02 3803.02.03 3803.02.04 3803.02.05

Female Die Forming Forming Steps during Female Die Forming Tool for a Landrover Front Part Equipment for Female Die Forming Superplastically Formed Wheel Housing

3803.03.01 3803.03.02 3803.03.03 3803.03.04 3803.03.05 3803.03.06 3803.03.07

Superlastic Male Die Forming Equipment for Superplastic Male Die Forming Machine for Male Die Forming Tool for Male Die Forming Stages in the Manufacturing of a Component Using the Male Die Forming Process Tool Set for Manufacturing a Car External Rear View Mirror Housing Wall Thickness Distribution in a Head Rest

3803.04.01 3803.04.02 3803.04.03 3803.04.04 3803.04.05

Criteria for Choosing Male Die Forming Criteria for Choosing Female Die Forming Choosing a Process for Superplastic Forming Estimating the Required Gas Pressure Estimating the Log. Strain Rate (or True Strain Rate)

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