30. Electrons

30.1 Electron Beam Machining (EBM)

• The basic physics is an electron beam is directed towards a work piece, the electron heat and vaporize the metal.

• Typical applications are,

annealing

welding

metal removal

• electrons accelerated with voltages of approx. 150,000V to create velocities over 200,000 km/sec.

• beam can be focused to 10 to 200 micro m and a density of 6500 GW/mm2

• good for narrow holes and slots.

e.g. a hole in a sheet 1.25 mm thick up to 125 micro m diameter can be cut almost instantly with a taper of 2 to 4 degrees

• the electron beam is aimed using magnets to deflect the stream of electrons

• a vacuum is used to minimize electron collision with air molecules.

• beam is focused using an electromagnetic lens.

 

• The process looks like,

 

• Some examples of cutting performance are given below,

 

 

• typical energy requirements for cutting are,

 

• e.g. to cut a 150 micro m wide slot in a 1mm thick tungsten sheet, using a 5KW power source, determine the cutting speed.

 

• Basic mechanics,

 

• e.g.

 

• the heat rise can be estimated using a one dimensional heat flow equation

 

 

• We can estimate the melting temperature with,

 

• e.g.

 

• Other effects of EBM

process done in vacuum, so it is best suited to small parts, but vacuum also reduces contamination

very high heat concentration reduces peripheral heating of surface less that 50 micro m from the cut the part is at room temperature.

• Summary of EBM characteristics,

mechanics of material removal: melting, vaporization

medium: vacuum

tool: beam of electrons moving at very high velocity

maximum mrr = 10 mm3/min

specific power consumption = 450W/mm3/min

critical parameters: accelerating voltage, beam current, beam diameter, work speed, melting temperature

materials application: all materials

shape application: drilling fine holes, cutting contours in sheets, cutting narrow slots

limitations: very high specific energy consumption, necessity of vacuum, expensive machine.

30.1.1 References

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30.1 Ghosh, A., Manufacturing Science, Ellis Horwood Ltd., Chichester, UK, 1986.

30.1.2 Problems

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Problem 30.1 a) If the typical energy requirement for an EBM cut slot (0.1mm wide by 0.5mm deep) is 6(W/(mm3/min)) for titanium, how fast can the hole be cut if a 1KW machine is used?

b) Explain why you think the result in #5.a) is accurate/inaccurate.

30.2 Ion Implantation

• original application was doping semiconductors with boron, arsenic, phosphorous, etc.

• Ions can be implanted with electron beams or lasers.

• Ion penetration is a result of high energy ions arriving at a surface. The penetration depth is a function of energy (10-200 KeV typically) and collisions with the target matrial lattice.

• Typical impacts, and impact depth are depicted in the figure below, [source, unknown]

 

• Energy is dissipated in two ways,

excitation of electrons

elastic deformation of the lattice

• The beam penetration basically has a Gaussian distribution about some central value.

• After the ion enters the surface it may collide with a number of atom, knocking them out of location and strain hardening the material. The ions will eventually come to rest at some depth in the material.

• The radiation effects causes a loss of energy as the incident ion beam strikes and displaces target atoms.

• Sputtering is the effect where incoming ions are reflected back in their direction of origin, and if energetic enough they will leave the surface.

• The cascade effect involves an ion with sufficient energy colliding with one atom, in turn causing more collisions.

• The implanted ion results in surface stress, and when concentrations are high enough, this can lead to partial deformations.

• The System,

 

• A collimator is in use to prevent ions from striking the surface at shallow angles and sputtering.

• The workpiece is rotated to ensure good surface coverage

• One high dose source can be replaced with a number of lower dose systems.

• Advantages,

low temperature processing reduces handling an stress problems.

no dimensional changes

good adhesion of treated surface

new alloys possible

can improve corrosion, oxidation, wear, hardness, friction, fatigue

• Disadvantages,

very shallow treatment (< 1 μm)

high cost

the surface can be weakened by radiation effects

• Applications,

Nitrogen implantation has been used to increase wear resistance and give longer life,

injection molding screws

high speed steel tools

a clutch housing tool

hip prosthesis

yttrium gives oxidation and wear resistance

titanium and carbon on iron gives lower friction and better wear.

chromium is used to maintain strength of holes.

• After layers of materials have been implanted the surface can be bombarded with another ion beam that causes collisions and better mixes the implanted ions more uniformly in the surface. This technique is known as ion beam mixing.

• Ion beam enhanced deposition (IBED) uses normal surface deposition techniques and an accompanying ion beam that increases the thickness of the deposited layer, and bonding between the new layer and the existing material.

30.2.1 Thin Layer Deposition

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• Thin layers can be mixed into surfaces using electron and laser beams,

1. A thin layer is deposited on a surface with other techniques,

a) electrostatic vapor deposition

b) a thin sheet is applied

c) the material is delivered as the beams work

2. A laser or electron beam melts the surface and mixes the two materials

• The rates can be greater than 1 m/s

• Advantages,

radiation penetration is shallow

no chemical contamination

no gas or jets to disturb molten surface

high precision for delivery

automation is possible

• Disadvantages,

high capitol cost

beams narrow, so multiple passes required (slow)

30.2.2 Problems

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Problem 30.2 TRUE / FALSE: Ion beam surface finishing is used to place material on a surface.

30.3 Electrostatic Spraying

• Used to coat an object to,

reduce corrosion

protect against scratching

stop contact with air/water/chemicals/etc

alter appearance

• Electrostatic painting methods

increases the production rate

reduces paint wastage

are well suited to automation

give good finishes

• The basic process is,

before/as the paint is sprayed, it is given a negative charge (extra electrons).

The workpiece is positively charged, thus attracting the paint particles

The paint is generally sprayed at the workpiece, the electrostatics even out the dispersion, and reduce wastage.

30.3.1 Electrostatic Atomization Methods

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30.3.1.1 - • The Blade Method 9Ransburg’s No. 1 process),

no spraygun used

a straight sharp edge is used charged to about 90 KV

the work is grounded

the paint is slowly pumped onto the blade, it spreads out, and by the time it reaches the tip it comes off as small charged droplets.

the shape of the workhead encourages a linear dispersion pattern

advantages,

high efficiency/low wastage
simple controls
suited to flat surfaces

disadvantages,

limited dispersion patterns

least cost effective and flexible

 

• The Bell Method (Ransburg’s No. 2 process)

uses a rotating bell or funnel to disperse the paint

the bell head is about 1 ft. from the workpiece

a potential of 90-100 KV is applied to the head while the work is grounded

the head rotates at 900 r.p.m. to cause good paint dispersion

advantages

good coverage for rounded surfaces
better efficiency than blade method
suited to spherical irregular shapes

disadvantages,

extra power/equipment to run motor

• ASIDE: the edge effect in electrostatic phenomenon leads to higher concentration of charges at corners and edges.

• The Disk Method (also Ransburg’s No. 2 process)

similar to Bell method except that a flat disk is used.

the geometry of the disk allows a much wider spray coverage

advantages (compared to bell),

wider coverage area
well suited to irregular and long parts

• Rotating Bell handgun

can be used for manual/movable spraying

is well insulated to prevent shocks

well suited to complex shape because of operator aiming

most efficient and cost effective method

• Consider the comparative tables [source unknown],

 

• Air/Hydraulic Electrostatic Spraying

Type I

large negatively charged (100KV) electrodes are placed at the side of a spray paint line
work on the central conveyor is rounded, and uncharged paint is sprayed in.
the sprayed paint enters the electric field, becomes negatively charged and is drawn to the work.
can be upgraded to a full electrostatic method easily
a higher paint wastage results.

Type II

a typical spray gun head is modified to deposit a high voltage charge on the paint particles
the paint is sprayed with a negative charge, and it is attracted to a grounded workpiece

Type III

the paint is charged before passing through the spray head

• Control systems (power supplies) for spraying a resistive/non-resistive. Resistive systems use resistors to limit the maximum current that can be drawn, thus protecting the users. Non-resistive sysems require added safety precautions and are best suited to automate systems.

• The edge phenomenon can result in uneven distribution of paint near edges and corners.

• The Faraday cage effect will result in recessed areas that get less paint coverage.

• Safety issues include electrical shocks, ventilation (most paints are solvent based), and arcs from poor grounding or unexpected metal contact.

• Some automation issues are,

part recognition on a conveyor

color changes

booths are costly, but provide safety and process advantages

30.3.2 Problems

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Problem 30.3 TRUE / FALSE: Parts can be electrostatically coated by dipping molds in a bath of thermo plastic powder.

Problem 30.4 TRUE / FALSE: Electrostatic painting uses electrical charge for better mixing of paint components.

30.4 Air-Plasma Cutting

• Basic process: uses an ionized gas jet (plasma) to cut material

• can be used on all materials that conduct electricity

• can be used to cut materials resistant to oxy-fuel cutting,

stainless steel

monel

super-alloy plates

• Plasma is generated by exposing a gas stream to the electrons from an electric arc. High velocity electrons generated by the arc impact gas molecules, and ionize them.

 

• The gas is forced through the nozzle, and the jet heats the metal, and blasts the molten metal away.

• Advantages

3 to 5 times faster than conventional gas cutting

can deal with any conducting material, including those not suited to normal gas cutting.

stainless steels
chromium-nickel alloys
aluminum
copper
etc

works best on ranges from .03” to 1”

More efficient than other types of gas plasma

can cut up to .15 m/sec continuously.

 

• Summary of Air-Plasma characteristics,

mechanics of material removal: melting

medium: plasma

tool: plasma jet

maximum temperature = 16,000C

maximum velocity of plasma jet = 500 m/sec

maximum mrr = 150 cm3/min

specific energy = 1000 W/cm3/min

power range = 2 to 200 KW

maximum plate thickness = 200 mm (depends on material)

cutting speed = 0.1 to 7.5 m/min

voltage 30 to 250 V

current <= 600 A

critical parameters: voltage, current, electrode gap, gas flow rate, nozzle dimensions, melting temperature

materials applications: all conducting materials

shape application: cutting plates

limitation: low accuracy

30.4.1 References

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30.2 Ghosh, A., Manufacturing Science, Ellis Horwood Ltd., Chichester, UK, 1986.

30.3 Flexible Manufacturing Systems

 

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