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

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.

• 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)

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.

30.3.1.1 - • 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,