Appunti sulle lezioni generali di Advanced manufacturing processes

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1| Water jet

Waterjet Considerations Suffer Delamination in the first part of WJ drilling as water

Composite Materials:

enter between layers Because of this starting hole is done by mechanicall drilling. in this

case WJ cutting heads host a spindle to mount drills.

Requires to be drill at low preassure (i.e 70Mpa) to reduce WJ thriust force

Glass:

which is high on the first instants whjen the hole is blind. After drilling, preassure can

be increased to standar values 250MPa and more.

no particular problems but when drilling is long, diameter increase is expected

Metals:

due to water reflection in the blind hole condition.

intricate profiles since is not as fast as other ones in straight cuts.

Natural work field:

Marble, multilayyer glass, kevlar, stainless steel, and wood.

Pumps

Intensifiers

Increassing the number of pumps is iseful to increase water flow rate when one single

pump feeds multiple cutting heads.

Ultra High Pressure pumps exists on the market even if the relability is lower than the

"usual" 400MPa ones. Moreover they cost 3 times more. An interesting case can be the

600MPa that can be cost effective when drilling tens of hundreds of holes as the case

of tube sheets. The high pressure value cuts the drilling time dramatically making the

productivity of this operation economically interesting.

Autofretage 2

1| Water jet

Pure Water jet Cutting Head

More Coherents jets are obtained at lower diamiters, sharp orifices and lower pressures.

Small orifices produce longer coherent as they assure a lower reynolds number.

Abrasive Granulometry

Catcher

It is basically a pool under the waterjet table is used to catch debris and also since the

power jet force has to be calculated according to the surfaces roughness wanted (so more

than the power needed to just cut the piece) the pool of water actas as a damper for this

excess energy.

Pure Jet velocity of compressible fluid

The velocity coefficient is difficult to determine experimentally. Among the available

c v

techniques, Laser Doppler Velocimetry (LDV) is one of the most suitable methods because

it is non-intrusive and does not disturb the water jet. The natural irregularities of the

jet scatter the laser light produced by two intersecting laser beams, allowing accurate

velocity measurements.

The contraction coefficient cannot be measured directly. It is obtained indirectly from

c c

experimental measurements of the volumetric flow rate and the jet force.

The discharge coefficient is defined as the product of the velocity and contraction

c d

coefficients: c = c c

d v c

It relates the actual volumetric flow rate to the theoretical flow rate, which is given

Q

w

by the product of the nominal orifice cross-sectional area and the theoretical jet velocity.

The kinetic power of a waterjet can be calculated from the water mass flow rate and

m w

the jet velocity. The mass flow rate is obtained from the volumetric flow rate

m Q

w w

using the orifice coefficients.

The kinetic power of a pure waterjet can be expressed as a function of the nominal orifice

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1| Water jet

diameter and the water pressure, taking into account the orifice coefficients, which depend

on both the orifice and the fluid. The nominal orifice diameter has a stronger influence

on the jet power than the pressure, since its exponent is whereas the pressure exponent

2,

is 1.5.

The derivative of the jet power with respect to pressure defines the hydraulic power

density. Water pressure is the only relevant jet parameter affecting the power density,

while the orifice cross-sectional area has a negligible influence, provided that the Bernoulli

assumptions hold (high upstream pressure and small nozzle diameter).

In waterjet cutting, the jet is an unconventional tool whose cutting capability depends on

power, force, and velocity. After leaving the orifice, the jet is no longer pressurized and its

pressure energy is converted into kinetic energy. Therefore, waterjet cutting effectiveness

is governed by jet velocity rather than pressure. The physical quantity that best describes

the cutting capability is the power density, i.e. the ability to concentrate power over a

small area.

Videos recorded using a transparent cutting head show the influence of air on waterjet

behavior. The focusing tube is mounted even without abrasive. When the abrasive inlet is

plugged, no air enters the mixing chamber and the jet remains more coherent, maintaining

its diameter over a longer distance. When the inlet is open, air is sucked into the chamber,

disturbing the jet surface and generating droplets, which may also cause abrasive clogging.

Although air strongly affects the jet periphery, it does not significantly alter the coherent

core. Therefore, the jet cutting capability is not strongly affected by air, but air reduces

jet stability in time and space, which is critical in high-precision cutting. For this reason,

air intake must be controlled.

The pressure difference governs air suction into the mixing chamber. If the abrasive

−p

p

a v

inlet is fully plugged, the air flow rate is zero and the vacuum inside the chamber

Q

air

is maximum. If the inlet is fully open, atmospheric pressure is established inside the

chamber and becomes zero. Intermediate conditions correspond to partial plugging

−p

p a v

of the inlet. Increasing water pressure increases air suction, shifting the operating curves

accordingly.

Air flow rate is proportional to the square root of air pressure, analogously to water

flow rate being proportional to the square root of water pressure. The hose diameter at

the abrasive inlet strongly affects . Larger hose diameters reduce flow restrictions,

Q air

increasing air flow, but excessive air reduces the Venturi effect and lowers abrasive particle

velocity, impairing abrasive transport. 4

1| Water jet

When abrasive is added, the jet behavior is analyzed using conservation of momentum.

Initially, air and abrasive have negligible velocity, while water flows at velocity Down-

v.

stream of the mixing process, water, air, and abrasive ideally reach a common velocity

, forming a multiphase abrasive jet. Air remains within the jet, representing up to

v abr

95% of its volume, and induces jet divergence after some distance, without significantly

reducing cutting capability.

The abrasive waterjet kinetic power is defined as the kinetic power of the mixed jet. Using

the loading ratio, the mixed jet velocity is found to be proportional to the theoretical

v

abr

water velocity and pressure. The abrasive waterjet kinetic power is proportional to

2 3/2

∝

P d p

kin n

which is the same dependence as for a pure waterjet. However, increasing the abrasive

mass flow rate reduces the kinetic power of the jet.

Material Interaction

The waterjet cutting process develops through three temporal stages, defining three cut-

ting zones. the jet initially interacts with the material and different cutting

Stage 1 (entry zone):

mechanisms develop until the maximum penetration depth is reached. A traverse distance

produces a penetration depth , dominated by erosion at small angles of impact

X h

1 1

(abrasion). Further penetration from to during traverse to is mainly due to

h h X X

1 2 1 2

erosion at large angles of impact. the cutting process becomes cyclic and continues until

Stage 2 (steady cutting zone):

the jet reaches the end of the plate. Penetration is fully developed when the total traverse

reaches , and further depth increase from to is controlled by erosion at large

X h h

3 2 3

angles of impact associated with jet upward deflection. This zone is responsible for the

roughness at the bottom of the kerf.

the cutting process ends and is characterized by sideways jet

Stage 3 (exit zone):

deflection, which produces an uncut tailing.

In steady cutting, the jet impacts the material surface at shallow angles, removing material

from the upper surface. As depth increases, particle deflection and velocity reduction

reduce the local removal rate, increasing interface curvature. Small steps are formed due

to jet traversal; these steps are removed by subsequent perpendicular impacts. Successive

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1| Water jet

steps become larger with depth, and the jet deflection angle increases up to , after

◦

90

which a new cutting cycle begins.

For brittle materials, cutting mechanisms at large angles of attack (zones 2 and 3) domi-

nate penetration. For ductile materials, erosion at small angles of attack (zone 1) is the

primary mechanism governing depth of cut.

Quality parameters

AWJ cutting quality is commonly classified using the so-called which provide a

Q levels,

qualitative evaluation of the kerf surface finish. Most AWJ machine builders base their

process models on these quality levels.

corresponds to a separation cut, where the objective is only to separate parts, regard-

Q1

less of surface quality. represents the highest cutting quality, characterized by the

Q5

absence of striations and a smooth kerf surface, as described by the deformation wear

zone in Hashish’s model.

Although the Q levels provide a qualitative description, they are effective in industrial

practice. High quality levels (e.g. are also associated with good kerf quality parame-

Q5)

ters such as reduced edge rounding and absence of burrs, even though these aspects are

not explicitly included in the Q-level definition.

Machine builders use the required Q level as an input from the user and automatically

select the appropriate process parameters for a given material and thickness. 6

2| Introduction to Advance

flexible sheet forming process

Trends in sheet and tube metals for automotive

The automotive industry is driven by demographic, regulatory and competitive pressure

leading to:

• Electrification and co2 regulations

• Weight reduction and anergy efficiency

• Global platforms needed fot material and process flexibility

• Competition for new technologies (Gigacasting, composites,etc.)

OEMs are converging towards multi-material strategies where specific stiffness is more

relevant than absolute weight. in material selection we must consider: Structural in-

tegration, manufacturability, Recycling and energy consumption, cost vs per-

formance trade offs

Conventional Deep Drawing and stamping

Conventional metal tools are wideley used and they are the only possible selection in

most applications, Metals tools are very stiff hence provide a very good precision and

repeatability of the sheets and tube metal parts other thing is they are durable and in

some applications wear is so small that doesn’t represent a concern.

The drawbacks are related to the high prices of the machine itself and the long production

time needed. Conventional rigid tools are meant for medium to large production batches

cause they have long durability. of course is difficult to make minor changes to the part

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2| Introduction to Advance flexible sheet forming process

design and difficult to repair.

• Expensive for small sized batches of complex geometry

• Induce defects such as scratches, striation or local stress

• difficult to use on low ductility materials (High strength steel, hs Al alloys, Ti alloys

and Mg Alloys) if complex geometry required (Small corner radius, large drawing

ratios)

• Difficult on materials with high spring-back if large radius and thickness required.

Formability and Forming Limit Diagram

Formability can be defined as the capability of a sheet metal forming solution with large

deformation and without defects (Thinning, necking, fracture, thickening,buckling,wrinkling)

Biaxial Stretching→ Fracture occurs earlier.

Plane Strain→ Lowest Forming limit.

Deep drawing with draw in→ Higher formability.

FLDs are obtained experimentally (Etched grids) or via FEM.

Spring-back problem

Elastic recovery after unloading, common to almost all flexible processes. Characteristic

problem is an increase in the radius with an angle decrease after unloading. Strongly

affected by Young modulus (-E = +SB) Hardening coefficient K ratio / thickness

ρ

Equation of spring back angle is not very aqua rate since pure bending does not occurs

in real processes but it explains well the factors that influence the spring-back.

Rationale For advance and flexible processes

To overcome the difficult of rigid tools in case of small batch productions of complex

geometry there is the option for flexible forming processes.

• Tools made out of hard plastic (Thermosetting if ma-

Semi rigid Rapid tools:

chined or thermoplastic if are 3D) Can be rapidly produce and do not require thermal

or surface treatment nor grinding operation. 8

2| Introduction to Advance flexible sheet forming process

• The tool is compliant as in rubber pad forming or fluid forming

Soft flex tool:

(Hydro-forming). in this case 1 tool is replaced by flexible medium like rubber or

water and the shaping is provided by a regular rigid tool

Classification of processes

semi rigid Rapid Tools

Basically Rapid Tools replace conventional tools almost with the same category.

PA, PC, PEI (Ultem) im thermoplastics category or thermoset Pu OR 3D

Materials:

printed ABS or GFPC

Reduce cost and lead time, nos scratching and suitable for small batches.

Advantages: Lower stiffness that lead to tool deflection, limited durability and FEM

Limitations:

based compensation algorithms related to predict their deflection and compensate their

shape.

Indirect Rapid Tool is preferable, obtained by Additive manufacturing or machining be-

cause od their shorter and simpler production cycle SLA,SLS/SLM, EAM are AM and

SLS sintering reinforced plastics ex. copper reinforced polyamide where copper provides

heat transfer properties.

Compliant Tools

Is used in sheet metal forming. The key aspect is that rubber tools for bending acts as a

flexible confirming medium that distributes pressure more evenly over the materials.

Adapt to a range of geometries without need for custom hard dies for every

Benefits:

shape. The holes in the dies increase the adaptability to different geometries so we

can introduce stiffens bars to control the required stiffness. Surface protection and cost

effective. poor control over small radius and higher spring-back.

Limitations: Shore Hardness A to D and nearly incompressible v = 0.5

Key rubber properties:

Fluid forming Processes

Fluid can act as Punch (higher pressure) or Die (Lower pressure) 9

2| Introduction to Advance flexible sheet forming process

Role of fluid, in or dependent blank-holding, presence of mem-

Classification Criteria:

brane, temperature (Cold, Warm or Hot) and blank type(Sheet or tube)

Fluid replace punch, sealing and blank-holding are coupled,

Sheet hydro-forming:

leads to biaxial stretching and needs very high pres force (Several Thousands of kN).

Uses a rubber membrane for sealing, blank-holding independent, alloys

Flex-forming:

undercuts and limited formability and large radius only. Use for low deformation and

big radius are required Fluid Die + Solid punch, allows pre bulging (used

Hydro-mechanical Deep Drawing

only for low deep drawing or non complex parts), reduces spring-back but still limited by

sealing and pressure control

Super Plastic Gas Forming

Widely used in automotive and aerospace engineering to shape complex and lightweight

aluminum alloy parts particularly body panels. 450-550 degrees, gas pres-

Conditions:

sure 0.5 - 5 Mpa and very low strain rates.

Complex shapes, no spring back, excellent surface quality, thin and lightweight

Benefits:

parts. long cycle times, high cost, large scrap, low volume high end vehicles.

Limitations:

CNC Dieless forming - Air Bending

Air bending is the best example of nearly dieless forming.

Simple shape that allows to produce different shapes by increasing or decreasing the punch

stroke. the angle vs stoke is the key to success while machines have a software to estimate

by hand final adjustments are needed to obtain the desire bending angle.

This process is less precise, less repeatable but much more flexible and lower bending

forces are required than bottom bending.

Good on small bending radius.

Hot Stamping

Used for ultra high strength steels heat to 850 (Austenite), Stamping in cooled

Process:

dies, Quenching (Martensite) Ys up to 1200, almost zero spring back and very

Effects:

high dimensional accuracy. 10

2| Introduction to Advance flexible sheet forming process

Direct (Al-Si coating) and Indirect(Preforming before heating)

Variants:

Pros: high strength,reduce spring back, lighter structural parts Cons:Complex

dies with cooling, slow press speed, limited post forming operations, laser cutting often

required 11

3| CNC Tube bending

Tubular components are widely use in automotive applications because closed cross section

tubes provide a very high stiffness to weight ratio.

• Structural parts (Engine cradles, chassis, frames, torsion bars)

• Exhaust systems

• Fluid Carrying lines

• Emerging lightweight structures

Pre-forming before hydro-forming

Tube hydro-forming is very effective for expansion and calibration but not for generating

sever bends. thus pre forming is required when:

• Tight radius are needed

• Geometry include multiple bends

most common pre-forming i