Altan T. Metal Forming Handbook

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High local plastic deformations occur during friction in sheet forming,

i.e. with the alteration of the surface of the workpiece a transformation

in the topography takes place. Surface transformation takes place

depending upon the state of stress, strength of the sheet metal material

and of the grain size and alignment. This surface transformation which

can alter considerably the amount of lubricant that is retained at the

die/material interface. These mechanisms receive minimum considera-

tion in the laws of friction, as does the influence of frictional tempera-

ture and pressure on lubricant viscosity during the sliding operation.

The coefficient of friction can be considered neither as a material

variable nor as a constant for the die/workpiece pair. However, if the

coefficient of friction is selected for making predictions, its dependence

on various process parameters must be considered. In sheet metal form-

ing, the following such parameters are of major importance:

–the material pair and the metallic surfaces which are in contact with

each other,

–the die geometry,

–the temperature-dependent properties of the lubricant as well as its

volume and the location of lubricant application,

–the actual normal contact stress, which is a function of the load bear-

ing surface portions of both surfaces,

–the magnitude of the sliding velocity and the frictional path of the

contacting surfaces and

–the surface characteristics of the workpiece material as well as the

changes of these characteristics that take place during the process.

Wear

In general, the assessment of the wear behavior of dies is difficult

because of the presence of several superimposed wear mechanisms. In

order to describe the various forms of damage, wear mechanisms

should be divided into five basic fundamental phenomena (Fig. 4.2.8):

Deformation

Under external loads, a micro-geometrical adaptation of the surface pair

takes place, initiated by the local deformation at the contacting peaks of

the roughness of the workpiece surface.

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Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

Wear at surface layer

During the application of load chemical reactions between the die and

the workpiece may take place. These are due to the influences of fric-

tional heating, lubricant and the surrounding medium which lead to

the built-up of an interface layer at the die/material interface. The shear

strength of the interface layers is less than that of the metals, so that

they are removed under frictional shear stress. Consequently, the coef-

ficients of friction occurring between the die/workpiece surfaces are low

when an interface layer is present. Thus, it is possible to maintain the

functional capability of the contact surfaces even during short term

overloading through wear of the interface layers.

Adhesion

Adhesion – also-called cold welding – occurs as a result of nuclear adhe-

sive forces between the frictional partners. The emergence of adhesive

forces pre-supposes the following:

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Sheet metal forming and blanking

Fig. 4.2.8 Depth effect of wear mechanisms on metallic surfaces

adsorption layer

outer (non-

charac-

teristic)

boundary

layer

inner

(charac-

teristic)

boundary

layer

3-5 Å

10-100 Å

> 50.000 Å

1 324

1 wear at

surface layer

2 adhesive

wear

3 abrasive

wear

4 fatigue

wear

metal in chemically

bonded form

(oxide or reaction

layer)

layer damaged by

forming, inhomogenous

in terms of hardness,

strength, degree of

deformation, intrinsic

tension and texture

substrate material

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

– plastic deformation at the peaks of the roughness of contacting sur-

faces,

– surface expansion brought about by the forming process, which

removes the adhesion presenting boundary layers and brings bright

metal surfaces in contact with one another,

– similar material structures of the frictional partners.

The greater is the difference between the metals and alloys of the fric-

tional partners, the lower is the tendency to develop cold welding. This

tendency is at its lowest between metallic and non-metallic materials of

corresponding hardness.

Abrasion

Abrasive wear includes all parting or separation processes that take place

at the interface of frictional partners, where material is released in sub-

microscopic particles through a material removal process. The difference

in the hardness between the two surfaces of frictional partners is an

important factor for assessing abrasive friction. The wear resistance of a

die increases with the hardness of its surface. Due to the relative move-

ment of the two surfaces of the frictional partners, a part of the energy

used is converted into heat. With most materials, the hardness drops as

the temperature increases and the resistance to wear is reduced. Abrasive

wear represents a generally inevitable long-term wear effect on the die.

Surface fatigue

Fatigue wear is considered to be the separation of micro- and macro-

scopic material particles. This separation is caused through fatigue

cracking, progressive cracking and residual fracture due to mechanical,

thermal or chemical stress-related conditions on the loaded surfaces. As

a rule, surface fatigue plays a minor role in sheet forming dies. However,

fatigue is common in cutting tools, as an increasing normal tension

stress is applied onto the front surface in addition to a variable friction-

al shear stress that exists over the whole external surface area.

Lubrication

Lubricants are subject to different temperatures and compressive stress-

es during their use. Important variables influencing the correct choice

of suitable lubricants are viscosity, density and compressibility, where-

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Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

by viscosity demonstrates the greatest dependence on the two parame-

ters: pressure and temperature. Particularly at contact points with high

surface pressure, attention must be paid not only to the temperature,

but also to the influence of pressure. Lubricants can be classified as fol-

lows:

– water soluble,

– non water soluble,

– solid lubricants,

– foils and varnishes.

Liquid lubricants are predominantly oil based. These are generally blends

without a consistent composition, as the portions of paraffin, aromatic

and naphtha content varies according to the origin of the oils. Animal-

based, vegetable and synthetic oils also belong in this category. Paste-

type lubricants are stabilized blends of mineral or synthetic oils, greas-

es and waxes as well as soap. Powdered or needle shaped hard waxes

and hard soaps are used as solid lubricants. The properties of lubricants

can be altered by additives and adjusted to suit the respective applica-

tion. Additives serve to improve load bearing capacities, bonding

strength, and viscosity-temperature-pressure characteristics, and also

assist in preventing corrosion. The additives create either physical

adsorption layers or chemical reaction layers.

By means of so called “friction modifiers” (for example animal and

vegetable fats, fat soap etc. also belong in this category), the lubricant

bonds itself to the metal surface without causing any chemical reaction.

This type of physically-acting additives are, however, inherently tem-

perature-dependent. The reduction in cohesive and adhesive strength

under increasing temperatures leads to an increase in the coefficient of

friction. The effect of these additives ranges from a total absence of

physical adsorption up to a stable chemical bond (formation of metal-

lic soap). At high levels of interface pressures, anti-wear additives are

used. They form a wear-reducing protective layer. A sub-division of this

group are the extreme pressure (EP) additives, which form reaction lay-

ers under higher temperatures. With a combination of selective addi-

tives, in this range lubricants can be optimized to suit just about any

particular application.

Based on the discussion given above, the following is a summary of

the performance requirements of lubricants:

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Sheet metal forming and blanking

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–development of a cohesive pressure and temperature-resistant lubri-

cation film, which separates the surfaces of the workpiece and the

die,

–high adhesive and shearing strength as well as good wetting capabil-

ity,

–no involuntary physical or chemical reactions on the surface of the

frictional partners,

–easy and complete removal of the lubricant from the finished part,

–no unhealthy or environmentally harmful substances.

4.2.4Hydro-mechanical deep drawing

Hydro-mechanical deep drawing involves the use of a pressure medi-

um, generally oil/water emulsion, to perform the forming process (cf.

Fig. 2.1.13). Hydro-mechanical deep drawing is carried out using dou-

ble-action hydraulic presses. In addition, a bed cushion is utilised for

hydro-mechanical reverse drawing.

Die layout

The lower die is a so-called pressure medium reservoir or water con-

tainer. It is designed as a pressure chamber and serves as a holder for the

part-specific female die (Fig. 4.2.9). The water container is connected to

the pressure regulator in the press. In order to fasten the female die on

the water container a clamp or a shrink ring is used. This ring has

grooves, which carry away emerging leakage fluid through the over-

flow. The female die has a circular groove at the edge of the drawing

radius in order to locate a string like polyurethane seal. This is over-

lapped at both edges by an angle of approx. 45°when inserted in the

sealing groove. This sealing groove can be eliminated in the case of

rotationally-symmetrical hollow components, provided that the female

die is designed accordingly.

The upper part of the die consists of the blank holder and the draw

punch. The blank holder is normally a casting into which the part-

specific blank holder insert is attached. This contains a circular splash

ring designed to contain leaking fluid. This portion of the die serves

also to bridge a larger blank holder opening in the press. The draw

punch is located inside the blank holder. This is, in general, provided

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Deep drawing and stretch drawing

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

with an extension piece. The punch extension is guided in the blank

holder. In production dies, an internal mechanical stop should be pro-

vided to limit the draw depth.

Functional sequence

The press is opened and the water container is filled in the home posi-

tion (Fig. 4.2.9). After the insertion of the blank, the press closes and the

blank holder grips the blank. The blank holder pressure, set at the press,

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Sheet metal forming and blanking

Fig. 4.2.9 Layout and function of a hydro-mechanical deep drawing die

slide

blank holder

slide

draw punch

blank holder

splash ring

pressure medium

tank

female die

shrink collar

press bed

pressure control device

seal

overflow

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

seals the pressure chamber and the actual forming process is initiated.

The medium pressure builds up as a result of penetration of the draw

punch into the water container. During deformation, the sheet metal is

pressed against the draw punch. During the forming phase, the control

system which is linked to the pressure chamber controls the application

of the hydraulic pressure in function of the draw depth.

After reaching the mechanically limited draw depth, the pressure in

the chamber is released and the press travels back to its home position.

Special features of the Hydro-Mec forming process

The reaction pressure activated within the water container by the pene-

tration of the draw punch has a multi-lateral effect during the forming

process and forces the metal to be shaped against the punch (Fig. 4.2.10).

The dry friction between the punch and the sheet metal is thereby sub-

stantially increased. As a result, the drawing forces increase to levels con-

siderably higher than those reached in conventional deep drawing. At

the same time, the pressure of the fluid in the exposed part of the blank

between the female die and the punch causes the material to bulge

upwards. This deformation creates radial tensile stresses and tangential

compressive stresses.

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Deep drawing and stretch drawing

b = R+2 mm

s: sheet metal thickness, w: water gap,

R: drawing radius, b: web width (at least 7 mm)

Fig. 4.2.10

Formation of the drawing gap during

hydro-mechanical deep drawing

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

Hydro-mechanical deep drawing allows to achieve a significantly

larger draw ratio than that achievable in conventional drawing opera-

tions. Whilst the limiting drawing ratio using conventional technology

is b< 2.0, deep drawing ratio limits of up to 2.7 can be achieved in

hydro-mechanical deep drawing. Since no intermediate drawing and

annealing operations are required, very cost-effective forming is possi-

ble. The tool costs can also be reduced because of the reduction in the

number of necessary forming operations. Another notable benefit is the

quality of the surfaces of the drawn parts, as the sheet metal has not

been drawn over a rigid drawing edge but over a fluid bead. Costs for

finish processing operations such as polishing or grinding are substan-

tially reduced or often completely eliminated.

Due to the pressing of the blank against the punch which reduces the

amount of spring back, dimensionally accurate part production is pos-

sible. This is particularly important in reflector production, as not only

measurement tolerances but also optical qualities are tested. The thick-

ness of the sheet metal for hydro-mechanically deep drawn parts remains

consistent within narrow limits. This applies particularly to the small

reduction in thickness on the base radii, so that thinner blank can fre-

quently be used for forming the desired part.

Press force is higher in hydro-mechanical drawing than in other

forming methods using rigid tools, due to the reaction pressure in the

water container. Here, the slide force F

[kN] is the sum of the conven-

tional forming force F

[kN] and the reaction force F

[kN], which acts

on the punch surface through the pressure medium (Fig. 4.2.11).

Depending upon the particular blank materials being processed, the

following pressures occur in the water container:

–aluminium: 50 – 200 bar

–steel: 200 – 600 bar

–stainless steel:300 – 1,000 bar

4.2.5Active hydro-mechanical drawing

Depending on their design, large-panel components for the automobile

industry such as hoods, roofs and doors possess only minimal buckling

strength in the center region of the parts. The reason for this is the low

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level of actual deformation in this area. The small amount of deforma-

tion means that insufficient strain hardening takes place and the pan-

els are liable to give way under only a low application of pressure,

resulting in low dent resistance. This low component stability has a

negative effect on crash resistance of vehicles and resistance to impact

from hailstones. In addition, undesirable vibrations can occur at certain

driving speeds.

As a result, higher strength materials or reinforcing elements and

beadings are used. In some cases, this can considerably raise the unit

cost of the part and increase the component weight. “Active hydro-

mechanical drawing” (Active Hydro-Mec) is a deformation process that

allows to overcome these problems and reduce the tool costs.

Range of parts

The Active Hydro-Mec method can be used to produce a wide range of

parts, including practically all large panels and, particularly, flat exter-

nal body parts of a vehicle such as hoods, trunklids, roofs, doors and

fenders.

The smaller the degree of deformation required in the first operation

when producing an outer panel (conventionally: deep drawing of the

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Deep drawing and stretch drawing

Fig. 4.2.11 Force versus displacement during hydro-mechanical deep drawing

slideforce

F=F+F

ST U Re

reaction

forceF

formingforce

drawingpaths

force

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

contoured blank), the more suited is the operation for using the Active

Hydro-Mec method.

Operating principle

The Active Hydro-Mec method involves forming with an operating

medium (oil-water emulsion), in which a large-size blank is clamped all

around with a watertight seal between the female die and the blank

holder – in the form of a water container (Fig. 4.2.12/2). At this stage, a

defined, part-specific gap exists between the clamped blank and the

punch. As soon as the blankholding force has built up, the emulsion is

actively introduced into the water container by means of a pressure

intensifier and a defined pressure is built up (depending on the part

between 20 and 30 bar). This pressure causes a controlled bulging of the

blank which is pre-stretched over what is to become the entire surface

of the component until it comes to rest against the punch in the center

of the punch surface. This controlled expansion of the sheet metal

results in work hardening of the workpiece which leads to a substantial

improvement in buckling strength of the formed part.

After the pre-stretching process, the whole system – blank holder,

blank and female die – is pressed upwards against the stationary punch.

This process corresponds to the counter-drawing principle (Fig.4.2.4)

in which the emulsion is displaced. During the process, the pressure

level can be kept constant or can be regulated to increase or decrease

progressively or degressively. After completion of the drawing process,

the emulsion pressure is actively built up (to a maximum of 600 bar) in

order to ensure optimum application and formation of the blank

against the contour of the punch.

Press

The press developed for the Active Hydro-Mec-process is a double-

action straight-sided press design with Hydro-Mec pressure intensifier

and a quick die changing system. Figure 4.2.13provides a schematic

view of the press.

The press force of max. 50,000 kN is applied from below by eight

cylinders located in the press bed. Six blank holder cylinders in the

press crown apply a blankholding pressure of max. 16,000 kN during

the pre-stretching process. Six powered mechanical stops can be adjust-

ed for each individual part to determine the limit of the draw depth.

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Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998