Modern manufacturing Techniques NUST

Non Traditional Machining (NTM) Processes

Comparison of Conventional and

Non- Conventional Machining Processes

CONVENTIONAL NON-CONVENTIONAL

Generally macroscopic chip formation by shear deformation

chips are of generally microscopic size

There may be a physical tool present There may not be a physical tool present

Energy domain can be classified as mechanical

Mostly NTM processes do not necessarily use mechanical energy to provide material removal

Conventional machining involves the direct contact of tool and workpiece

Non-conventional machining does not require the direct contact of tool and workpiece

Comparison of Conventional and

Non- Conventional Machining Processes

CONVENTIONAL NON-CONVENTIONAL

Lower accuracy and surface finish Higher accuracy and surface finish

Suitable for every type of material economically

Not Suitable for every type of material economically

Higher waste of material due to high wear

Lower waste of material due to low or no wear.

Noisy operation mostly cause sound pollutions

Quieter operation mostly no sound pollutions are produced

Lower capital cost Higher capital cost

Skilled or un-skilled operator may required

Skilled operator required

Non Traditional Machining (NTM)

Processes

The term non traditional machining refers to the group of

processes, that remove excess material by various

techniques involving;

Mechanical energy

Thermal energy

Electrical energy

Chemical energy

Non Traditional Machining (NTM) Processes

The NTM processes have been developed largely in response to the

new and unusual machining requirements that could not be satisfied

by conventional methods.

These requirements include:

The need to machine newly developed materials (metals, nonmetals, composites etc.). These new materials often have

special properties (e.g., high strength, high hardness, high

toughness) that make them difficult or impossible to machine by

conventional methods.

The need for unusual or complex part geometries that cannotbe easily accomplished and in some cases are impossible to

achieve by conventional machining.

The need to avoid surface damage that often accompanies thestresses created by conventional machining.

Types of NTM Processes

The NTM processes are often classified according to principal

form of energy used to effect material removal. By this

classification, there are four types:

1. Mechanical Erosion of the work material by a high velocity

stream of abrasives or fluid is the typical form of mechanical

action in these processes.

2. Electrical These non traditional processes use

electrochemical energy to remove material.

Types of NTM Processes

3. Thermal These processes use thermal energy to cut or

shape the workpart. The thermal energy is generally applied

to a very small portion of the work surface, causing that

portion to be removed by fusion and/or vaporization of the

material.

4. Chemical Most materials (metals) are susceptible to

chemical attack by certain acids or other corrosive

chemicals. In chemical machining, chemicals selectively

remove material from portions of the workpart, while other

portions of the surface are protected by a mask.

Mechanical Energy Processes

Some of the NTM processes that utilizes mechanical

energy for material processing are:

(1) Ultrasonic Processes

(2) Water jet cutting

(2a) Abrasive water jet cutting

(2b) Abrasive air jet machining

Ultrasonic Processes

Ultrasonic processing of solids utilizes the effect of the high

frequency mechanical vibration producing friction and abrasion

actions.

Common ultrasonic processes are:

Ultrasonic machining

Ultrasonic welding

Ultrasonic homogenizing

Ultrasonic cleaning

Ultrasonic degassing

Ultrasonic Machining (USM)

Ultrasonic machining (USM) is a non traditional machiningprocess in which abrasives contained in a slurry are driven at highvelocity against the work by a tool vibrating at low amplitude(around 0.075mm) and high frequency (ultrasonic) approximately20 kHz.

The tool oscillates in a direction perpendicular to the worksurface, and is fed slowly into the work, so that shape of the toolis formed in the part.

It is the action of the abrasive, impinging against the worksurface, that performs the cutting.


Common tool materials used in USM include soft steel and stainlesssteel.

Abrasive materials in USM include boron nitride, boron carbide,aluminum oxide, silicon carbide and diamond.

Grit sizes ranges between 100 and 2000 microns

The vibration amplitude should be set approximately equal to gritsize, and the gap size should be maintained at about two times grit

size. To a significant degree grit size determines the surface finish on

the new work surface.

The slurry in USM consists of a mixture of water and abrasiveparticles. Concentration of abrasives in water ranges from 20% to

60%.

The cutting action in USM operates on the tool as well as the work.As the abrasive particles erode the work surface, they also erode the

tool, thus effecting its shape.


Applications:

Tight-tolerance round thru-holesfor semiconductor processing

equipment components

Micro-machined and micro-structured glass wafers for

micro-electromechanical

systems (MEMS) applications

Ultrasonic machining can be used to form intricate, finely detailed graphite electrodes


Applications:

Ultrasonic machining of a carbide die for

the aerospace industry.

Ultrasonic machining centers can

perform both conventional and

ultrasonic machining operations.

By combining these technologies in

one machine, the user has the

capability to machine across theentire material spectrum.

Ultrasonic Welding (USW)


Sonotrode: A tool that creates ultrasonic vibrations and applies this

vibrational energy to a gas, liquid, solid or tissue. A sonotrode usually

consists of a stack of piezoelectric transducers attached to a tapering

metal rod.

Piezoelectric transducer is a device thattransforms one type of energy to another bytaking advantage of the piezoelectric properties ofcertain crystals or other materials. When apiezoelectric material is subjected to stress orforce, it generates an electrical potential orvoltage proportional to the magnitude of theforce. This makes this type of transducer ideal as aconverter of mechanical energy or force intoelectric potential.


Advantages:

Welding occurs at low temperatures relative to other methods

The process occurs in fractions of a second to seconds.

It's a safer process as it does not require flammable fuels and open

flames.

Ultrasonic welds are as strong and durable as conventional welds of

the same materials

Disadvantages:

Workers' hearing may be damaged by exposure to high-frequency

sound

The depths of the welds are less than a millimeter

Ultrasonically welding dissimilar materials requires an additional

material


Applications:

The materials in the upper portion of

this New Balance athletic shoe were

assembled by ultrasonic weldingrather than traditional sewing.

An ultrasonic welder compressing onto the contact.

Two materials are welded together

Water Jet Cutting (WJC)

WJC uses a fine, high pressure, high-velocity stream of waterdirected at the work surface to cause cutting of the work. (alsocalled hydrodynamic machining)

To obtain the fine stream of water a small nozzle opening ofdiameter 0.1 to 0.4 mm is used.

To provide the stream with sufficient energy for cutting, pressuresup to 400 MPa are used and the jet reaches a velocity up to 900m/s. The fluid is pressurized to the desired level by a hydraulicpump.


Important process parameters includes

standoff distance

nozzle opening diameter

water pressure

cutting feed rate

The standoff distance is the separation between the nozzleopening and the work surface. It is generally desirable for

this distance to be small to minimize dispersion of the fluid

stream before it strikes the surface.


(m3/s)


Size of the nozzle orifice affects the precision of the cut; smalleropening is used for finer cuts on thinner materials.

The cutting feed rate refers to the velocity at which WJC nozzle istraversed along the cutting path.

The WJC process is usually automated using computer numericalcontrol or industrial robots to manipulate the nozzle unit along thedesired trajectory.

Water jet cutting can be used effectively to cut narrow slits in flatstock such as plastic, textiles, composites, floor tile, carpet, leatherand cardboard.


Advantages:

(1) No crushing or burning of the surface (2) Minimal material loss (3)No environmental pollution (4) Ease of automating the process usingNC or industrial robots.

Disadvantages:

(1) Cuts slower than plasma cutting process, reducing materialprocessing productivity (2) Not suitable for cutting brittle materialsbecause of their tendency to crack during cutting (3) higher entrycost than the plasma cutting machines (4) abrasive material used forcutting harder materials tends to be quite expensive.

The WaterJet Orifice (Jewel Orifice)

The waterjet orifice is the singlemost overlooked component in a

waterjet cutting machine; without it

the entire system would fail to

function.

Waterjet orifice design utilizesmaterials like diamond, corundum,

ruby, and sapphire mainly due to

their high hardness property.

Orifice Failure Modes

1. FAILURE DUE TO IMPACTS

Debris and Garnet particles pulled back through the jewels orifice via thevacuum created when the stream is cycled off.

Metal or plastic particles from High Pressure tubing and filters.

Other line born contaminants.


2. FAILURE DUE TO OVAL SHAPED THROUGH HOLES

Faulty manufacturing of orifices may lead to oval shaped or out-of round through

holes within the orifice if proper quality control processes are not followed.

If this type of failure mode has occurred, typically you will see a fluctuating jet

stream and may also hear spitting and sputtering sounds while this occurs.


3. FAILURE TO SEAL

When a manufacturer machines an orifice mount, it is very important that it is

machined in a way that leaves concentric machine lines. If inconcentric machine

lines result, the jewel is unable to seat properly in the mount and therefore will

not seal.

If unable to seal, water as well as foreign debris is able to pass around the jewel

and cause immediate orifice failure.


4. ORIFICE RETAINER FAILURE

The key objective of the orifice retainer is to hold the jewel in place without

affecting the coherency of the jet stream.

If the optimal retaining method and materials are not used for each varying

application, this can cause improper jewel retention leading to blown out orifices,

angled jet streams and fractured jewels.

The Effects of Cutting with a

Defective Orifice

Damaged work piece material

Premature failure of mixing tube

Slower cut speeds

Poor edge cut quality

Decreased pressures

Over stroking

Increased flow rate

Inefficiently focused garnet

Abrasive Water Jet Cutting (AWJC)

When WJC is used on metallic parts, abrasive particles mustusually be added to the jet stream to facilitate cutting. Thisprocess is called abrasive water jet cutting (AWJC).

Introduction of abrasive particles into the stream increases thenumber of parameters that must be controlled. Additionalparameters (other than previously seen) are abrasive type, gritsize, and flow rate.

Aluminum Oxide, silicon dioxide and garnet (a silicate material) aretypical abrasive materials used, grit sizes ranging between 60 and120 microns.

One possible configuration is when abrasive particles are added tothe water stream after it has exited the WJC nozzle.

Abrasive Water Jet Cutting (AWJC)

Difference b/w WJC and AWJC

Abrasive waterjet cutting differs from pure waterjet cutting:

In pure waterjet cutting, the supersonic stream erodes thematerial. In the abrasive waterjet, the waterjet streamaccelerates abrasive particles and those particles, not thewater, erode the material.

The abrasive waterjet is much more powerful than a purewaterjet and is capable of cutting hard materials such asmetals, glass, stone, and composites.

Abrasive Waterjet Attributes

No Heat Affected Zones No mechanical stresses Easy to program 10 inch thick cutting Stack cutting Little material loss due to cutting Simple to fixture Low cutting forces One jet setup for nearly all abrasive jet jobs Quickly switch from pure waterjet to abrasive waterjet Reduced secondary operations Little or no burr

Metal

Composites

Fish

Metal

Abrasive Air Jet Machining (AJM)

Abrasive jet machining (AJM) is a material removal processthat results from the action of a high velocity stream of gascontaining small abrasive particles.

The gas is dry and pressure of 0.2-1.4 MPa is used to propelthe gas through the nozzle orifices.

Gases include dry air, nitrogen, carbon dioxide and helium.

The process is usually performed by directing the nozzle atthe work. The work station must be arranged to provideproper ventilation for the operator.

Abrasive Jet Machining (AJM)

AJM is normally used as a finishing operation rather than aproduction cutting process. Applications include deburring,trimming and deflashing.

Cutting is accomplished successfully on hard, brittlematerials (glass, ceramics) that are in the form of thin flatstock.

Abrasive Jet Machining (AJM)

Electrochemical Machining Processes

An important group of nontraditional processes use electricalenergy to remove material. This group is identified by theterm Electrochemical processes, because electrical energy isused in combination with chemical reactions to accomplishmaterial removal.

Electrochemical Machining

Electrochemical machining (ECM) removes metal from anelectrically conductive workpiece by anodic dissolution (theprocess of dissolving a solid substance into a solvent to makea solution)

The shape of the workpiece is obtained by a formedelectrode tool in close proximity but separated from theworkpiece by a rapidly flowing electrolyte.

ECM is basically a deplating operation (reverse ofelectroplating)

The workpiece is the anode (+) and the tool is the cathode (-).


The workpiece material is deplated from the anode anddeposited onto the cathode in the presence of anelectrolyte bath.

The electrolyte bath flows rapidly between the two poles tocarry off the deplated material, so that it does not becomeplated on the tool.

The electrode tool, usually made of copper, brass orstainless steel is designed to possess approximately theinverse of the desired shape of the part.

An allowance in the tool size must be provided for the gapthat exists between the tool and the work.


-

+


In addition to carrying off the material that has beenremoved from the workpiece, the flowing electrolyte alsoserves the function of removing heat and hydrogen bubblescreated in the chemical reaction.

Electrochemical machining is generally used in applicationswhere the work metal is very hard or difficult to machine,or where the workpart geometry is difficult to accomplishby conventional methods.


During ECM, there will be reactions occurring at the electrodesi.e. at the anode or workpiece and at the cathode or the toolalong with within the electrolyte.

Let us take an example of machining of low carbon steel which isprimarily a ferrous alloy mainly containing iron.

For electrochemical machining of steel, generally a neutral saltsolution of sodium chloride (NaCl) is taken as the electrolyte.

The electrolyte and water undergoes ionic dissociation aspotential difference is applied


As the potential difference is applied between the work piece(anode) and the tool (cathode), the positive ions move towardsthe tool and negative ions move towards the workpiece.

Thus the hydrogen ions will take away electrons from thecathode (tool) and from hydrogen gas as:

Similarly, the iron atoms will come out of the anode (work piece) as:


Within the electrolyte, iron ions would combine with chloride ions toform iron chloride and similarly sodium ions would combine withhydroxyl ions to form sodium hydroxide

In this manner, the work piece gets gradually machined and getsprecipitated as the sludge

As the material removal takes place due to atomic leveldissociation(separation), the machined surface is of excellent surfacefinish and stress free.


Process Parameters

(Output)

(Output)


Advantages: (1) Little surface damage to the workpart

(2) No burrs (3) low tool wear (4) Relatively high metal

removal rates for hard materials

Disadvantages: (1) significant cost of electrical power

(2) problems of disposing of the electrolyte sludge.

Applications: (1) Machining of irregular shapes and

contours into forging dies, plastic molds and other

shaping tools (2) Multiple hole drilling, especially holes

that are not round (3) Deburring


Applications:

Thermal Energy Processes

Material removal processes based on thermal energy arecharacterized by very high local temperatures; hot enough toremove material by fusion or vaporization.

Because of the high temperatures, these processes cause physicaland metallurgical damage to the new work surface.

Processes of commercial importance are:

(1) Electrical discharge machining (EDM)

(2) Electron beam machining

(3) Laser beam machining

Electric Discharge Machining (EDM)

EDM is one of the most widely used non-traditional processes.

The shape of the finished work surface is produced by a formedelectrode tool.

The sparks occur across a small gap between tool and worksurface.

The EDM process must take place in the presence of a dielectricfluid, which creates a path for each discharge as the fluidbecomes ionized in the gap.

The discharges are generated by a pulsating direct current powersupply connected to the work and the tool.


Ram EDMs have four sub-systems: a DC power supply to provide the

electrical discharges, with controls forvoltage, current, duration, duty cycle,frequency, and polarity a dielectric system to introduce fluid intothe voltage area/discharge zone and flushaway work and electrode debris, this fluid isusually a hydrocarbon or silicone based oil a consumable electrode, usually of copperor graphite a servo system to control feed of theelectrode and provide gap maintenance


Once the power supply is turned on, thousands of direct current,or DC, impulses per second cross the gap, beginning the erosionprocess.

The spark temperatures generated can range from 7760 to11,649 Celsius so that small portion of the workpiece is suddenlymelted and removed. The flowing dielectric flushes away thesmall particle.

Two important process parameters in EDM are discharge currentand frequency of discharges. As either of these parameters isincreased, metal removal rate increases.

Surface roughness is also affected by current and frequency. Thebest surface finish is obtained in EDM at high frequencies and lowdischarge currents.


Electrodes are made of graphite, copper, brass, copper tungsten,silver tungsten and other materials.

The selection of the electrodes depends on the type of powersupply circuit available, the type of work material, and whetherroughing or finishing is to be done.

Graphite is generally preferred for many applications because ofits melting characteristics. In fact it does not melt and vaporizes atvery high temperatures.

Dielectric fluids used in EDM include hydrocarbon oils, keroseneand distilled water. The dielectric serves as an insulator in the gapexcept when ionization occurs in the presence of a spark. Its otherfunctions are to flush debris out of the gap and to remove heatfrom tool and workpart.


Advantages:

EDM is a non-contact process that generates no cutting forces,permitting the production of small and fragile pieces

Burr-free edges are produced

Intricate details and superior finishes are possible

Disadvantages:

Low metal removal rates compared to chip machining

Lead time is needed to produce specific consumable electrodeshapes

The high spark temperatures that melt the work also erodes/meltsthe tool

Chance of flash fire in the dielectric fluid if the level falls too low


Applications of electric discharge machining includeboth tool fabrication and parts production.

Most of the tooling equipments are made by EDM,including molds for plastic injection molding,

extrusion dies, wire drawing dies, forging dies and

sheet metal dies.

EDM is also used for certain parts production.Examples include delicate parts that are not rigid to

withstand conventional cutting forces, hole drilling

where the axis of the hole is at an angle to the surface

and production machining of hard metals.

Comparison between EDM and ECM

The tool and workpiece are separated by a very small gap,i.e. no contact in between them is made.

The tool and material must both be conductors of electricity.

Needs high capital investment.

Systems consume lots of power.

A fluid is used as a medium between the tool and the workpiece (conductive for ECM and dielectric for EDM).

The tool is fed continuously towards the workpiece tomaintain a constant gap between them.

High metal removal rates are possible with ECM, with nothermal or mechanical stresses being transferred to the part,

and mirror surface finishes can be achieved.

Electric Discharge Wire Cutting (EDWC)

or Wire EDM

Electric discharge wire cutting commonly called wire EDM is aspecial form of electric discharge machining that uses smalldiameter wire as the electrode to cut a narrow kerf in the work.

The cutting action in wire EDM is achieved by thermal energy fromelectric discharges between the electrode wire and the workpiece.

The workpiece is fed continuously and slowly past the wire in orderto achieve the desired cutting path. NC is used to control theworkpart motion during cutting.


or Wire EDM

As it cuts, the wire is continuously advanced between a supply pooland a take up pool to present a fresh electrode of constantdiameter to the work.

As in EDM, wire EDM must also be carried out in the presence ofdielectric.


or Wire EDM

The four basic wire EDM subsystems include:

the DC power supply the dielectric system the wire feeding system the work positioning system


or Wire EDM

Since the wire electrode is very thin, power used is limited andremoval rates are slow.

A two-axis wire EDM can only make cuts at right-angles to the worktable.

Independent four-axis machines can cut tapered angles and makecuts that result in different top and bottom profiles. This capabilityis needed in making extrusion dies and flow valves.


or Wire EDM

Electron Beam Machining (EBM)

Electron beam machining (EBM) is one of several industrial processesthat use electron beams.

Electron beam machining uses a high velocity stream of electronsfocused on the workpiece surface to remove material by melting andvaporization.

An electron beam gun generates a continuous stream of electronsthat is accelerated to approximately 75% of the speed of light andfocused through an electromagnetic lens on the work surface.

The lens is capable of reducing the area of the beam to a diameter assmall as 0.025mm.

On impinging the surface, the kinetic energy of the electrons isconverted into thermal energy of extremely high density that meltsor vaporizes the material in a very localized area.

Electron Beam Machining


The EBM beam is operated in pulse mode.

Beam current is directly related to the number of electronsemitted by the cathode.

Increasing the beam current or pulse duration directlyincreases the energy per pulse.

High-energy pulses (in excess of 100 J/pulse) can machinelarger holes on thicker plates.

The energy density (J/mm3) and power density (W/mm3) isgoverned by energy per pulse duration and spot size.


Electron beam machining is used for a variety of high precisioncutting applications on any known material.

EBM must be carried out in a vacuum chamber to eliminatecollision of electrons with gas molecules and to avoid oxidizingof tungsten filament.


Advantages:

There is no mechanical contact between tool and work piece,hence no tool wear.

Very small holes can be machined in every type of materialwith high accuracy

Drilling of extremely small diameter holes down to 0.002 in.

Drilling holes with high depth/diameter ratios, greater than100:1.


Disadvantages:

Cost of equipment is high.

Rate of material removal is low.

It can used for small cuts only.

Vacuum requirements limits the size of work piece.

The interaction of the electron beam with the work piece can produce hazardousx-rays, and only highly trained personnel should use EBM equipment.


Applications:

Drilling of holes in pressure differential devices used in nuclearreactors, air craft engine

Machining of wire drawing dies having small cross sectionalarea.


Comparative advantages and disadvantages of the EBW and LBW processes

Laser Beam Machining (LBM)

Separate module

Chemical Machining

Chemical machining (CHM) is a nontraditional process in whichmaterial removal occurs through contact with a strongchemical etchant (acid).

The use of chemicals to remove unwanted material from aworkpart can be applied in several ways, and several differentterms have been developed to distinguish the applications.

Material is removed by microscopic electrochemical cell action,as occurs in chemical dissolution of a metal.

This controlled chemical dissolution will simultaneously etch allexposed surfaces even though the penetration rates of thematerial removal may be only 0.00250.1 mm/min

The chemical machining process consists of several steps. Differencesin applications and the ways in which the steps are implementedaccount for different forms of CHM.

The Steps are:

(1) Cleaning: The first step is a cleaning operation to ensure thatmaterial will be removed uniformly from the surfaces to be etched(engraved or cut).

(2) Masking: A protective coating called a maskant is applied to certainportion of the part surface. This maskant is made of a material that ischemically resistant to the etchant. It is therefore applied to thoseportions of the work surface that are not to be etched.

Chemical Machining

(3) Etching: This is the material removal step. The part is immersedin an etchant(chemical/acid) that attacks those portions of the partsurface that are not masked.

The usual method of attack is to convert the work material into a saltthat dissolves in the etchant and is thereby removed from thesurface.

When the desired amount of material has been removed, the part iswithdrawn from the etchant and washed to stop the process.

(4) Demasking: The maskant is removed from the part.

Chemical Machining

Maskant materials include neoprene, polyvinylchloride, polyethyleneand other chemicals.

Masking can be accomplished by different methods, two majorprocesses are:

(1) cut and peel

(2) photographic resist

Chemical Machining

Cut and peel method:

The cut and peel method involves application of the maskant over theentire part by dipping, painting or spraying.

After the maskant has hardened, it is cut using a scribing knife andpeeled away in the areas of the work surface that are to be etched.

The maskant cutting operation is performed by hand, usually guidingthe knife with a template. The cut and peel method is generally usedfor large workparts, low production quantities and where accuracy isnot a critical factor.

Chemical Machining

Photographic resist method (photoresist method):

It uses photographic techniques to perform the masking step.

The masking material contain photosensitive chemicals.

The mask (photoresist) is applied to the entire work surface andexposed to light through a negative image of the desired areas to beetched.

The mask removes from the areas where UV light is exposed.

The naked areas can then be removed using etchant spray.

This procedure leaves the desired surfaces of the part protected bythe maskant and the remaining areas unprotected, exposed tochemical etching.

Chemical Machining

Selection of the etchant(chemical/acid) depends on work materialto be etched, desired depth, rate of material removal and surfacefinish requirements.

The etchant must also be matched with the type of maskant that isused to ensure that the maskant material is not chemically attackedby the etchant.

Chemical Machining

Common work materials and etchants in CHM, with typical penetrationrates and etch factors.

Chemical Machining

Penetration rate:

Material removal rates in CHM are generally indicated as penetrationrates, mm/min (in/min), because rate of chemical attack of the workmaterial by the etchant is directed into the surface.

Under cut:

Along with the penetration into the work, etching also occurs sideways under the maskant, this

effect is referred to as the undercut. It must be accounted for in the design of the mask for the

resulting cut to have the specified dimensions.

The undercut is directly related to the depth of cut.

Chemical Machining

The constant of proportionality for the material is called the etch factor, definedas

where Fe = etch factor; d = depth of cut(mm); and u = undercut(mm)

Chemical Machining

The etch factor can be used to determine the dimensions of the cutaway(inner) areas in

the maskant, so that the specified dimensions of the etched areas on the part can beachieved.

Principle chemical machining processes are;

(1) chemical milling

(2) chemical blanking

(3) chemical engraving

(4) photochemical machining

Chemical Machining Processes

Chemical milling


Photochemical machining


Advantages:

1. No effect of workpiece materials properties such as hardness

2. Simultaneous material removal operation

3. No burr formation

4. No stress introduction to the workpiece

5. Low capital cost of equipment

6. Easy and quick design changes

7. Requirement of less skilled worker

8. Low tooling costs

9. Low scrap rates (3%).


Disadvantages:

1. Most of the chemicals such as cleaning solutions are veryhazardous liquids. Therefore handling and disposal of them iscostly.

2. Industries struggle to select more environmentally acceptedetchants for chemical machining process


Applications:


Missile skin-panel section contoured by chemical

milling to improve the stiffness- to- weight ratio of the part (Kalpakjain & Schmid)

Applications:


Laser Cutting

There are six ways that a laser can be used to cut materials,

Vaporisation cutting

Fusion cutting (melt & blow)

Reactive fusion cutting (melt, burn & blow)

Scribing

Controlled fracture

91

Laser Cutting Methods

Vaporisation cutting:

In vaporization cutting the focused beam heats the surface of thematerial to boiling point and generates a keyhole. The keyholeleads to a sudden increase in absorptivity quickly deepening thehole. As the hole deepens and the material boils, vapor generatederodes the molten walls blowing ejecta out and further enlargingthe hole.

Non melting material such as wood, carbon and thermoset plasticsare usually cut by this method.

92


Fusion cutting:

Fusion or melt and blow cutting uses high pressure gas to blowmolten material from the cutting area, greatly decreasing thepower requirement. First the material is heated to melting pointthen a gas jet blows the molten material out of the kerf avoidingthe need to raise the temperature of the material any further.Materials cut with this process are usually metals.

93


Reactive fusion cutting:

Reactive Fusion or melt, burn and blow cutting uses O2 gas to blowmolten material from the cutting area as well as to interact withmolten material to give extra heat (more than doubled). Thesuitable materials to cut are Fe and Ti and main disadvantage ofthis process is high heat-affected zone (HAZ).

94


Scribing:

Laser scribing is a two stage process and is quite similar totraditional mechanical scribe and break method. In the first stage,partially penetrating holes, a groove or deep vents at depths ofone third to one half of the material thickness are produced usinga laser. In the second stage, breaking by applying a mechanicalforce takes place.

95


Controlled fracture:

In this technique, thermal stresses are used to induce the crackand the material is separated along the cutting path by extendingthe crack. No melting is produced in this process and the suitablematerials to cut are glass, ceramics or semiconductors.

96

Key Factors in Laser Processing

of Materials

Laser power (W)

Cutting speed (mm/s)

Beam spot size (m)

Beam geometry

Gas type (Ar, He, CO2, N2) & its pressure (Pa)

Laser type (consider all parameters of laser beams)

Type of workpiece material and its optical properties

97

Key Responses for Quality

Assessment in Laser Cutting

Striations - number of parallel lines or scratches on the surface of a material

Dross - The residue of resolidified melt on the lower edge of the laser cut

Kerf - Width of cut opening

Heat Affected Zone (HAZ) - Region of material where the properties changes after the processing

Surface Roughness - Measure of the texture of the surface. It is quantified by the vertical deviations

of a real surface from its ideal form.

98

Striation Pattern

Typical striation patterns formed during the laser cutting of 1.25 mm mild steel using 300 W CW laser using cutting speed of 1.8 m/min and oxygen pressure of 2.0 bar

99

Dross Illustration

100

Example: Kerf

Kerf width

101

Example: Heat Affected Zone

(HAZ)

102

Conventional vs.


Contact process

Not a clean process

Chipping on the cut edges

Tool wear

Poor surface finish

Inexpensive

Non-contact process

Clean process

Chip-free cut edges

No tool wear

High quality surface finish

Relatively expensive

Conventional cutting Laser cutting

103

Example: Laser Cutting

Laser cut Blanks in Aluminum, Stainless Steel and Alloy Steel 104

Pulsed Laser

105

Mean power=peak power duty cycle

Duty cycle=pulse length / period

Wave definition for pulsed lasers

Laser Drilling

There are three modes/approaches of laser drilling processes,

Single Pulse Drilling

Trepanning

Percussion Drilling

106

Laser Drilling

Single Pulse Drilling:

Single pulse drilling is used for drillingnarrow (less than 1 mm dia) holesthrough thin (less than 1 mm thick)plates. High pulse energies are suppliedin drilling with single pulse because theirradiated energy levels must besufficient to vaporize the material insingle pulse.

107

Laser Drilling

Trepanning:

In trepanning, wider holes (less than3 mm dia) in thicker plates (less than 10mm thick) are produced by drilling aseries of overlapping holes around acircumference of a circle so as to cut acontour out of the plate. Trepanningcan be performed by translating eitherthe workpiece or the focusing optic.

108

Laser Drilling

Percussion Drilling:

In percussion drilling, a series of shortpulses (10-12 to 10-3 s) separated bylonger time periods (10-2 s) aredirected on the same spot to form athrough hole. Percussion drilling is usedto produce narrow holes (less than 1.3mm dia) through relatively thicker (up to25 mm) metal plates. High speed of thepercussion drilling makes it the mostcost-effective method.

109

Physical Effects in Laser

Drilling

Schematic of the physical effects taking place during laser drilling110

Geometrical Features of

Drilled Hole

Typical geometrical features of laser drilled hole111


Assessment in Laser Drilling

Hole Size

Circularity & Taper

Heat Affected Zone

Spatter & Dross

Recast (Re-solidified) Layer

Micro-cracking

112

Example: Laser Drilling

Aircraft engine turbine blade

Laser drilling of cooling holes in aerospace gas turbine parts such as turbineblades, nozzle guide vanes, combustion chambers, and afterburners is anestablished technology.

113

Laser Welding

There are two modes/approaches of laser welding processes,

Conduction Limited Welding

Deep Penetration (keyhole) Welding

Laser Welding

Conduction Limited Welding:

In this approach, the laserprocessing conditions are such thatthe surface of the weld pool remainsunbroken. In this approach theenergy transfer into the depth of thematerial takes place by conduction.

Laser Welding

Deep Penetration (keyhole) Welding:

In this approach, the keyhole is created in theweld pool. Higher power density andirradiation time compared to first approachresults in surface vaporisation at the moltenweld pool. Recoil Pressure of the vapour formsthe depression in the molten material whichsubsequently develops into a keyhole. It acts asa blackbody trapping the laser beam andincreases the absorption due to multiplereflection.

Laser Welding


Assessment in Laser Welding

Porosity (It is measure of void/spaces in a material)

Cracking

Heat Affected Zone

Hardness (Resistance of a material to localized deformation)

Tensile strength of the Joint

Schematic of Porosity

Schematic showing the formation of large void due to incomplete filling of keyhole during laser welding

Joint Configurations in

Laser Welding

Commonly used joint configurations in laser welding

Joining Efficiencies of

Different Welding Processes

Relative joining efficiencies of different welding processes

Comparison of Welding

Processes

Advances in Laser Welding

Significant progress has been made to improve the quality of the laser welds,welding efficiency, and productivity. Some of the major areas are,

Arc-Augmented (Hybrid) Laser Welding

Multibeam/Dual-Beam Laser Welding

Laser Welding of Tailor-Welded Blanks


Arc-Augmented (Hybrid) Laser Welding:

This process involves setting of an electric arc into the keyhole or the lasermaterial interaction zone produced during laser welding. Arc augmentationgives a welding speed which is almost four times the speed obtained withlaser welding alone. It was reported that maximum welding speedincreases with increasing arc current.

Schematic Illustration of Arc-

Augmented Laser Welding:

Configuration of laser beam and arc-welding electrode in laser-arc hybrid welding


Multibeam/Dual-Beam Laser Welding:

The dual-beam laser welding offers flexible processing since the power andother laser parameters can be independently altered in two laser beams.One of the beams can be used for melting of the workpiece duringwelding, while, the other beam can be used either for preheating ofworkpiece ahead of welding beam or post-heating (heat treatment) of thewelds past the welding beam.

Schematic Illustration of Dual

Laser Beam

Configuration of dual laser beam


Laser Welding of Tailor-Welded Blanks (welded vehicle body components):

Tailor Welded Blanks are made from individual steel sheets ofdifferent thickness, strength and coating which are joined togetherby laser welding. Laser welded automotive blank technology allowsfor the placement of various steel grades and thicknesses within aspecific part, placing steels attributes where they are most neededand removing weight that does not contribute to performance.Significant reduction in the weight of the vehicle can be achievedby welding the blanks (of various materials, thicknesses, properties,surface treatments, etc.) into a single tailor-welded blank (TWB).

Example: Tailor welded Blanks

Examples of typical tailor welded blanks


Advantages of Laser welding of TWBs:

Cost benefits due to reduction in the number of dies,minimization of scrap, reduction in the number of processsteps, etc.

TWB offers significant improvements in the dimensionalaccuracy and properties of the formed components.

Laser welding of the TWBs is challenging due to differingproperties of the blanks. However, laser processing parameterscan be effectively optimized to achieve the desired results.

Example: Laser Welding

The photograph shows a car door panel being subjected to laser welding (right) and laser hybrid welding (left) processes.

Video Clip: Laser Welding

Video Clip: Laser Cutting & Welding

References

Dahotre, N. B., and Harimkar, S. P. (2008) LaserFabrication and Machining of Materials, SpringerScience + Business Media, New York.

Ion, J. C. (2005) Laser Processing of Engineering Materials, Elsevier, London

Steen, W. M. (2003) Laser Materials Processing,Springer-Verlag, London.

Ready, J. F. (2001) LIA Handbook of LaserMaterials Processing, Laser Institute of America,Magnolia Publishing, Inc.

Beesley, M. J. (1971) Lasers and TheirApplications, Taylor & Francis, London.

Additive Manufacturing

Top 10 Emerging Technologies of 2015

https://agenda.weforum.org/2015/03/top-10-emerging-technologies-of-2015-2/

The World Economic Forums Meta-Council on Emerging Technologies, a panel of 18 experts, draws on the collective expertise of the Forums communities to identify the most important recent technological trends.

1. Fuel cell vehicles

2. Next-generation robotics

3. Recyclable thermoset plastics

4. Precise genetic engineering techniques

5. Additive manufacturing6. Emergent artificial intelligence

7. Distributed manufacturing

8. Sense and avoid drones

9. Neuromorphic technology

10. Digital genome

What is Additive Manufacturing?

Additive Manufacturing (AM) refers to a process by which digital 3D design data isused to build up a component in layers by depositing material. (from theInternational Committee F42 for Additive Manufacturing Technologies, ASTM).

The term "3D printing" is increasingly used as a synonym for AM. However, the latteris more accurate in that it describes a professional production technique which isclearly distinguished from conventional methods of material removal.

Subtractive VS. Additive Manufacturing

Material subtraction (removal) top-down

Material addition bottom-up

Could this be machined?

Features that represent problems using CNC machining

An early adopter Panasonic bread maker: heating element clip broke during move, model had been discontinued

Used sketchup (free CAD) to copy design, shapeways to print ($14)

Had to widen part to fit minimum shapeways size, and drill out the hole because glaze thickened thewall.

What Industry Sees

GE Leap fuel nozzle

Assembly directly made by laser melting of CoCr powder (also used inmedical/dental parts).

Previously 20 parts, now 1 part; 20 injectors per engine.

Estimate 25% lighter and 5X more durable than current design; new engine withexpected 15% fuel savings.

Expected production volume ~10s of thousands per year

May eventually need more metal AM machines (SLM) than current annualworldwide demand.

GE announced $50 million 3DP investment in Auburn, Alabama plant(7/16/14).

GE Leap fuel nozzle

from GE press releases and various news sources.

How do you use the parts made on your AM machines?

The first engineered AM parts ..

The cornerstones are >20 years old

so, why is AM the big thing now? (what has enabled all the interest and attention?)

Software (especially wide adoption of CAD/CAE) Improved (and lower cost) electromechanical components; thus, improved machine

performance Availability of printable materials Intellectual property (including key expirations) Proven results, momentum and customer/public mindest

Why AM for industry?

Rapid prototyping; short-run production.

Complex geometries, enabling new designs and improvedperformance (e.g., weight, strength, heat etc.).

Part consolidation (reduction in number of parts, printing assemblies)

Reduced lead time and inventory.

Decentralized (or, more efficient centralized) manufacturing supply chain efficiency.

Computational Steps in Rapid Prototyping

Figure A: The computational steps in producing a stereolithography file. (a) Three-dimensional description of eachpart. (b) The part is divided into slices (only one in 10 is shown); STL convert. (c) Support material is planned. (d) Aset of tool directions is determined to manufacture each slice.

STL (file format)

STL is a file format native to the Stereolithography CAD software created by 3DSystems. STL is also known as standard Tessellation Language.

This file format is supported by many other software packages; it is widely usedfor rapid prototyping and computer-aided manufacturing.

STL files describe only the surface geometry of a three-dimensional object without anyrepresentation of color, texture or other common CAD model attributes.

The common file format for geometry interchange, IGES, is not robust enough becauseof tolerancing issues.

STL (file format)

Users almost never have to worry about the file because the programs they use tocreate their geometry automatically generate STL files in the proper format.

If you do need to write your own routine to output an STL file, it is fairly simple.

The file can be a text file (ASCII) or a binary file.

Faceting

The program that creates the STL file goes through the topology(geometry) of the modeland meshes it:

1: First it puts points on all of the shared edges of all the surfaces

2: Then it creates triangles on each surface

Faceting

There are two things to note about faceting.

The first is that each corner(vertice) must be coincident with atleast one other corner. No corners can touch the edge ofanother triangle.

The second is that a triangle is flat and your surface can becurved. To make your curved surface look curved you needenough triangles(refined meshing) to make it appear like acontinuous surface.

Problem - Leaked geometry

The 7 AM Technologies (ASTM)

1. Vat photopolymerization or Stereolithography (SLA)

2. Material jetting or Ballistic particle manufacturing

3. Material extrusion or Fused deposition modeling (FDM)

4. Binder jetting or Three-dimensional printing

5. Powder bed fusion or selective laser sintering/melting (SLS/SLM)

6. Directed energy deposition

7. Sheet lamination or Laminated object manufacturing (LOM)

1. Stereolithography (SLA)

Stereolithography is an additive manufacturing process which employs avat of liquid curable photopolymer "resin" and an ultraviolet laser to buildparts' layers one at a time.

For each layer, the laser beam traces a cross-section of the part pattern onthe surface of the liquid resin. Exposure to the ultraviolet laser light curesand solidifies the pattern traced on the resin and joins it to the layerbelow.

After the pattern has been traced, the SLA's elevator platform descendsby a distance equal to the thickness of a single layer, typically 0.05 mm to0.15 mm. Then, a resin-filled blade sweeps across the cross section of thepart, re-coating it with fresh material. On this new liquid surface, thesubsequent layer pattern is traced, joining the previous layer. Acomplete 3-D part is formed by this process.

After being built, parts are immersed in a chemical bath in order to becleaned of excess resin and are subsequently cured in an ultraviolet oven.

Stereolithography requires the use of supporting structures which serveto attach the part to the elevator platform, prevent deflection due togravity and hold the cross sections in place so that they resist lateralpressure from the re-coater blade. Supports must be removed from thefinished product manually.



Schematic layout of a Stereolithography process


Schematic layout of a Stereolithography machine

Advantages:

High level of accuracy and good finishRelatively quick processMax model weight: 200-400 kg

Disadvantages:

Relatively expensiveLengthy post processing time and removal from resinLimited photopolymers (resins) availableOften requires support structures and post curing for parts to be strong enough for structural use


Material jetting is similar to inkjet document printing, but instead ofjetting drops of ink onto paper, PolyJet 3D printers jet drops of liquidphotopolymer onto the build tray.

Multiple print heads jet material simultaneously to create each layer,and UV light is then used to cure the layers. These layers build up one at atime in an additive process to create a 3D model.

Fully cured models can be handled and used immediately withoutadditional post-curing.

Along with the selected model materials, a gel-like support materialfacilitates successful printing of complicated geometries. Support materialcan be removed by hand or by a high-powered water jet station.

2. Ballistic Particle Injection

Material jetting is the only additive manufacturing technology that cancombine different print materials within the same 3D printed model in thesame print job.

Additionally, the multi-material printing process is capable of constructingfunctional assemblies, which reduces the need for multiple builds.



Schematic layout of a ballistic particle injection machine

Advantages:

The process benefits from a high accuracy of deposition of droplets and therefore low wasteThe process allows for multiple material parts and colours under one processMaterial can be changed during the build stage

Disadvantages:

Support material is often requiredA high accuracy can be achieved but materials are limited and only polymers and waxes can be usedHigh level of droplet control and positioningAccuracy in detail features lags compared to stereolithography


In fuse deposition modelling (FDM), material is drawn through a nozzle,where it is heated (keeping the temperature just above the its meltingpoint) so that it flows easily through the nozzle and is then depositedlayer by layer. The plastic hardens immediately after flowing from thenozzle and bonds to the layer below.

The nozzle can move horizontally and a platform moves up and downvertically after each new layer is deposited.

FDM is similar to all other 3D printing processes, as it builds layer by layer,it varies in the fact that material is added through a nozzle under constantpressure and in a continuous stream. This pressure must be kept steadyand at a constant speed to enable accurate results.

3. Fuse Deposition Modelling

A range of materials are available including ABS, polyamide,polycarbonate, polyethylene, polypropylene, and investment casting wax.

Material layers can be bonded by temperature control or through the useof chemical agents. Material is often added to the machine in spool formas shown in the diagram.



Schematic layout of a fuse deposition modelling machine

Advantages:

Widespread and inexpensive processABS plastic can be used, which has good structural properties and is easily accessible

Disadvantages:

Accuracy and speed are low when compared to other processesAccuracy of the final model is limited to material nozzle thickness/radiusConstant pressure of material is required in order to increase quality of finish


Three Dimensional Printing (3DP) technology was developed at theMassachusetts Institute of Technology.

The process is similar to the Selective Laser Sintering (SLS) process, butinstead of using a laser to sinter the material, an ink-jet printing headdeposits a liquid adhesive that binds the material.

3D Printing offers the advantage of fast build speeds, typically 2-4 layersper minute. However, the accuracy, surface finish, and part strength arenot quite as good as some other additive processes.

3D Printing is typically used for the rapid prototyping of conceptualmodels (limited functional testing is possible).

4. Three-Dimensional Printing

The 3D printing process begins with the powder supply being raised by apiston and a leveling roller distributing a thin layer of powder to the top ofthe build chamber.

A multi-channel ink-jet print head then deposits a liquid adhesive totargeted regions of the powder bed. These regions of powder are bondedtogether by the adhesive and form one layer of the part.

The remaining free standing powder supports the part during the build.After a layer is built, the build platform is lowered and a new layer ofpowder added, leveled, and the printing repeated.

After the part is completed, the loose supporting powder can be brushedaway and the part removed. 3D printed parts are typically infiltrated witha sealant to improve strength and surface finish.



Schematic layout of a three-dimensional printing machine

Advantages:

Parts can be made with a range of different coloursUses a range of materials: metal, polymers and ceramicsThe process is generally faster than othersIt allows for a large number of different binder-powder combinations

Disadvantages:

Not always suitable for structural parts, due to the use of binder materialAdditional post processing can add significant time to the overall process


Powder bed fusion includes following commonly used printingtechniques: Direct metal laser sintering (DMLS), Electron beam melting(EBM), Selective heat sintering (SHS), Selective laser melting (SLM) andSelective laser sintering (SLS).

Powder bed fusion (PBF) methods use either a laser or electron beam tomelt/sinter and fuse material powder together. Electron beam melting(EBM), methods require a vacuum but can be used with metals and alloysin the creation of functional parts.

All PBF processes involve the spreading of the powder material overprevious layers. There are different mechanisms to enable this, including aroller or a blade.

5. Powder Bed Fusion

A hopper or a reservoir below the fusion bed (platform) provides freshmaterial supply.

Direct metal laser sintering (DMLS) is the same as SLS, but with the use ofmetals and not plastics.

Selective Heat Sintering differs from SLS by using a heated thermal printhead to fuse powder material together.


Sintering is the process of compacting and forming a solid mass of material by heat and/or pressure without melting it to the point of liquefaction


Schematic layout of a powder bed fusion machine

Advantages:

Relatively inexpensiveSuitable for visual models and prototypes(SHS) Ability to integrate technology into small scale, office sized machinePowder acts as an integrated support structureLarge range of material options

Disadvantages:

Relatively slow speed (SHS)Lack of structural properties in materialsSize limitationsHigh power usageFinishing is dependent on powder grain size


Selective Laser Sintering (SLS) was developed at the University of Texas in Austin,by Carl Deckard and colleagues. The technology was patented in 1989 and wasoriginally sold by DTM Corporation. The basic concept of SLS is similar to that ofSLA.

It uses a moving laser beam to trace and selectively sinter powdered polymerand/or metal composite materials into successive cross-sections of a three-dimensional part.

As in all rapid prototyping processes, the parts are built upon a platform thatadjusts in height equal to the thickness of the layer being built. Additionalpowder is deposited on top of each solidified layer and sintered. This powder isrolled onto the platform from a bin before building the layer.

The powder is maintained at an elevated temperature so that it fuses easily uponexposure to the laser. Unlike SLA, special support structures are not requiredbecause the excess powder in each layer acts as a support to the part being built.

Selective Laser Sintering (SLS)

With the metal composite material, the SLS process solidifies a polymer bindermaterial around steel powder (100 micron diameter) one slice at a time, formingthe part.

The part is then placed in a furnace, at temperatures in excess of 900 C, wherethe polymer binder is burned off and the part is infiltrated with bronze toimprove its density.

The burn-off and infiltration procedures typically take about one day, after whichsecondary machining and finishing is performed.

Recent improvements in accuracy and resolution, and reduction in stair-stepping,have minimized the need for secondary machining and finishing.

SLS allows for a wide range of materials, including nylon, glass-filled nylon,SOMOS (rubber-like), Truform (investment casting), and the previously discussedmetal composite.


Direct Metal Laser Sintering (DMLS) was developed in 1994 jointly by RapidProduct Innovations (RPI) and EOS GmbH to produce metal.

With DMLS, metal powder (20 micron diameter), free of binder or fluxing agent,is completely melted by the scanning of a high power laser beam to build thepart with properties of the original material.

Eliminating the polymer binder avoids the burn-off and infiltration steps, andproduces a 95% dense steel part compared to roughly 70% density with SelectiveLaser Sintering (SLS).

An additional benefit of the DMLS process compared to SLS is higher detailresolution due to the use of thinner layers, enabled by a smaller powderdiameter. This capability allows for more intricate part shapes.

Direct Metal Laser Sintering (DMLS)

Material options that are currently offered include alloy steel, stainless steel, toolsteel, aluminum, bronze, cobalt-chrome, and titanium.

DMLS is often used to produce rapid tooling, medical implants, and aerospaceparts for high heat applications.

The DMLS process can be performed by two different methods, powderdeposition and powder bed, which differ in the way each layer of powder isapplied. In the powder deposition method, the metal powder is contained in ahopper that melts the powder and deposits a thin layer onto the build platform.In the powder bed method, the powder dispenser piston raises the powdersupply and then a recoater arm distributes a layer of powder onto the powderbed. A laser then sinters the layer of powder metal.

The powder deposition method offers the advantage of using more than onematerial, each in its own hopper. The powder bed method is limited to only onematerial but offers faster build speeds.


Directed Energy Deposition (DED) covers range of technologies: Laser engineerednet shaping (LENS), directed light fabrication (DLF), direct metal deposition(DMD), laser cladding, laser free-form fabrication (LFF) and many more.

Although the general approach is same, difference between these technologiescommonly includes laser power, spot size, laser type, powder delivery method,inert gas delivery method, feedback and motion control etc.

As all of the above mentioned technologies/processes involves deposition,melting and solidification of powder material using a traveling melt pool, theresulting part attains high density during the build process.

A typical DED machine consists of a nozzle mounted either fixed or on a multi axisarm to deposit the material from any angle using to 4 and 5 axis machines.

6. Directed Energy Deposition

DED processes are NOT used to melt a material that is pre-laid in a powder bedbut are used to melt materials as they are being deposited.

The process can be used with polymers, ceramics but is typically used withmetals, in the form of either powder or wire.

DED processes uses focused heat source (typically a laser or electron beam) tomelt the feedstock material and build up three dimensional objects in a mannersimilar to the extrusion based processes.



Schematic layout of a directed energy deposition machine

Advantages:

Ability to control the grain structure to a high degree, which lends the process to repair work of high quality, functional partsA balance is needed between surface quality and speed, although with repair applications, speed can often be sacrificed for a high accuracy and a pre-determined microstructure

Disadvantages:

Finishes can vary depending on paper or plastic material but may require post processing to achieve desired effectLimited material useFusion processes require more research to further advance the process into a more mainstream positioning


7. Sheet Lamination

Sheet lamination processes include ultrasonic additive manufacturing(UAM) and laminated object manufacturing (LOM).

The Ultrasonic Additive Manufacturing (UAM) process uses sheets orribbons of metal, which are bound together using ultrasonic welding. Theprocess does require additional cnc machining and removal of theunbound metal, during the welding process.

UAM uses metals and includes aluminium, copper, stainless steel andtitanium (Ultrasonic Additive Manufacturing Overview, 2014).

The process is low temperature and allows for internal geometries to becreated. The process can bond different materials and requires relativelylittle energy, as the metal is not melted.

7. Sheet Lamination

Laminated object manufacturing (LOM) uses a similar layer by layerapproach but uses paper as material and adhesive instead of welding.

A laser or any mechanical device is designed to cut to a depth of onelayer thickness; cuts the cross-sectional outline.

The unused material is left in place for support and diced using cross-hatch pattern into small rectangular cubes.

During post-processing the cross-hatched pieces of excess material areseparated from the part using typical wood carving tools (calleddecubing).

Laminated objects are often used for aesthetic and visual models and arenot suitable for structural use.

7. Sheet Lamination

Schematic layout of a laminated object manufacturing machine

7. Sheet Lamination

(a) Schematic illustration of the laminated-object-manufacturing process. (b) Crankshaft-part made by LOM

7. Sheet Lamination

Advantages:

Benefits include speed, low cost, ease of material handling, but the strength and integrity of models is reliant on the adhesive usedCutting can be very fast due to the cutting route of the shape outline but not the entire cross sectional areaSimple and inexpensive setup

Disadvantages:

Finishes can vary depending on paper or plastic material but may require post processing to achieve desired effectStructural quality of parts is limited

Characteristics of AM Rapid Prototyping Technologies

Powder Metallurgy

Powder Metallurgy

Powder metallurgy (PM) is a process forfabricating metal parts from finely compactedmetal powders.

Advantages of Powder Metallurgy1. PM parts can be mass produced to net shape or near net shape,

eliminating or reducing the need for subsequent processing.

2. The PM process itself involves very little waste of material; about 97%of the starting powders are converted to product.

3. Owing to the nature of the starting material in PM, parts having aspecified level of porosity can be made. This feature lends itself to theproduction of porous metal parts such as filters and oil-impregnatedbearings and gears.

4. Certain metals that are difficult to fabricate by other methods can beshaped by powder metallurgy.

5. PM compares favorably with most casting processes in terms ofdimensional control of the product.

6. PM production methods can be automated for economical production.

Disadvantages of Powder MetallurgyThese include the following:

1. Tooling and equipment costs are high.

2. Metallic powders are expensive.

3. Difficulties with storing and handling metal powders (suchas degradation of the metal over time, and fire hazardswith particular metals).

4. Metal powders do not readily flow laterally in the dieduring pressing, and allowances must be provided forejection of the part from the die after pressing.

5. Maintaining uniform material density throughout the partis difficult, especially for complex part geometries.

Characterization of Engineering Powders

A powder can be defined as a finely divided particulate solid.

The geometry of the individual powders can be defined by thefollowing attributes: (1) particle size (2) particle shape andinternal structure, and (3) surface area.

Screen Mesh for sorting particle sizeSeveral of the possible (ideal) particleshapes in powder metallurgy

Basic Steps in Powder Metallurgy

Powder Production

Blending and Mixing

Powder Compaction

Sintering

Sizing

Impregnation & Infiltration

Finishing etc.

Primary operations

Secondary operations

FLOW CHART

Outline of processes and operations involved in making powder-metallurgy parts.

Powder Production

Almost any metal can be made into powder form.

There are three principal methods by which metallic powdersare commercially produced.

The methods are:

(1) Atomization (Gas, water, centrifugal)

(2) Chemical

(3) Electrolytic

Atomization

Atomization involves the conversion of molten metalinto a spray of droplets that solidify into powders.

It is the most versatile and popular method forproducing metal powders today.

There are multiple ways of creating the molten metalspray, several of which are illustrated in Figure 1 (nextslide).

Figure 1: Atomization Methods (a) & (b) gas atomization (c) water atomization (d) centrifugal atomization

Gas Atomization Two of the methods shown in Figure 1(a) and 1(b) are based on

gas atomization, in which a high velocity gas stream (air orinert gas) is utilized to atomize the liquid metal.

In Figure 1(a), the gas flows through an expansion nozzle,siphoning molten metal from the melt below and spraying itinto a container. The droplets solidify into powder form.

In a closely related method shown in Figure 1(b), molten metalflows by gravity through a nozzle and is immediately atomizedby air jets. The resulting metal powders are collected in achamber below.

* Siphon: a tube/pipe that uses atmospheric pressure to draw liquid from one container,place, or level to another.

Water Atomization The approach shown in Figure 1(c) is similar to 1(b), except that

a high-velocity water stream is used instead of air. This isknown as water atomization.

This method is particularly suited to metals that melt below1600C (2900F).

The disadvantage of using water is oxidation on the particlesurface. A recent innovation involves the use of synthetic oilrather than water to reduce oxidation.

In both air and water atomization processes, particle size iscontrolled largely by the velocity of the fluid stream; particlesize is inversely related to velocity.

Centrifugal Atomization

Several methods are based on centrifugal atomization.

In one approach, the rotating disk method shown in Figure 1d,the liquid metal stream pours onto a rapidly rotating disk thatsprays the metal in all directions to produce powders.

Blending and Mixing

Blending: when powders of the same chemicalcomposition but possibly different particle sizes areintermingled. Different particle sizes are often blendedto reduce porosity.

Mixing: when powders of different chemistry beingcombined.

Blending and Mixing

Blending and mixing are accomplished by mechanical means.Four alternatives are illustrated in Figure (a) rotation in a drum(b) rotation in a double-cone container (c) agitation in a screwmixer and (d) stirring in a blade mixer.

Blending and MixingOther ingredients are usually added to the metallic powdersduring the blending and/or mixing step.

These additives include:

(1) Lubricants: to reduce friction between particles and at thedie wall during compaction

(2) Binders: to achieve adequate strength in the pressed butunsintered parts

(3) Deflocculants: an agent for thinning suspensions or slurries.It is used to reduce viscosity and improve flow characteristicsduring subsequent processing

Compaction

In compaction, high pressure isapplied to the powders to form theminto the required shape.

The conventional compaction methodis pressing, in which opposingpunches squeeze the powderscontained in a die. The steps in thepressing cycle are shown in Figure A.

(1) is the filling the die cavity withpowder, done by automatic feed inproduction, (2) initial position, (3)final positions of upper and lowerpunches during compaction, and (4)ejection of part.

Compaction The applied pressure in compaction results initially in

repacking of the powders into a more efficient arrangement,eliminating bridges formed during filling, reducing porespace within the particle, and increasing the number ofcontacting points between particles.

As pressure increases, the particles are plastically deformed,causing interparticle contact area to increase and additionalparticles to make contact. This is accompanied by a furtherreduction in pore volume.

Isostatic Pressing Isostatic pressing is a powder processing process.

There are 2 types of isostatic pressing processes.

Cold isostatic pressing (CIP), where powder is sealed in aflexible mould and is then subjected to a uniform hydrostaticpressure without heating.

Hot isostatic pressing (HIP), where components are loadedinto a furnace and then placed in a pressure vessel so thatheat and pressure can be applied simultaneously.

Isostatic pressure is a pressure that is applied from all directionssimultaneously.

Cold Isostatic Pressing (CIP)

Cold isostatic pressing (CIP) involves compactionperformed at room temperature.

The mold, made of rubber or other elastomermaterial, is oversized to compensate for shrinkage.

Water or oil is used to provide the hydrostaticpressure against the mold inside the chamber.

Cold Isostatic Pressing (CIP)Figure B illustrates the processing sequence in cold isostaticpressing.

Cold isostatic pressing: (1) powders are placed in the flexible mold; (2) hydrostaticpressure is applied against the mold to compact the powders; and (3) pressure isreduced and the part is removed

Cold Isostatic Pressing (CIP)Advantages of CIP:

more uniform density,

less expensive tooling,

greater applicability to shorter production runs

Limitations of CIP:

Good dimensional accuracy is difficult to achieve in isostaticpressing because of the flexible mold.

Subsequent finish shaping operations are often required toobtain the required dimensions, either before or aftersintering.

Hot isostatic pressing (HIP) is used, as indicated, topress and sinter simultaneously in an inertatmosphere (usually using Argon gas).

The powder in the hot isostatic pressing process hasto be protected from the atmosphere so that oxidefilms can be avoided.

Hot pressing needs the powder to be heated,pressurised and cooled in the protectiveatmosphere.

Hot Isostatic Pressing (HIP)

The mold in which the powders are contained ismade of sheet metal to withstand the hightemperatures.

It is a relatively expensive process and itsapplications seem to be concentrated in theaerospace industry.

PM parts made by HIP are characterized by highdensity (porosity near zero).


After pressing, the green compact formed after CIP lacksstrength and hardness and can easily crumbled under lowstresses.

Sintering is a heat treatment operation performed on thecompact to bond its metallic particles, thereby increasingstrength and hardness.

Most metals are sintered at 70% to 80% of the meltingtemperature.

Shrinkage occurs during sintering as a result of pore sizereduction

Sintering

The heat treatment consists of three steps, accomplished inthree chambers in these continuous furnaces: (1) preheat, inwhich lubricants and binders are burned off; (2) sinter; and (3)cool down. The treatment is illustrated in Figure C

Sintering

An alternative approach that combines pressing and sintering but

overcomes some of the problems in hot pressing is spark

sintering.

The process consists of two basic steps (1) powder or a green

compacted preform is placed in a die and (2) upper and lower

punches, which also serve as electrodes, compress the part and

simultaneously apply a high-energy electrical current that burns

off surface contaminants and sinters the powders, forming a

dense, solid part in about 15 seconds. The process has been

applied to a variety of metals.

Spark Sintering

Secondary Operations of PM

1. Impregnation

2. Sizing

3. Coining

4. Infiltration

5. Plating / Coating

Secondary Operations Impregnation(soaking)

Utilizes inherent porosity of PM components byimpregnating them with a fluid (oil) to reduce itdegradation properties and improve machinability.

Sizing

Sizing is the pressing of a sintered part to improvedimensional accuracy.

Coining

Coining is a press working operation on a sintered partto press details into its surface

Secondary Operations

Infiltration

An operation in which the pores of the PM part are filledwith a molten metal. The melting point of the fillermetal must be below that of the PM part. The processinvolves heating the filler metal in contact with thesintered component so that capillary action draws thefiller into the pores.

Plating / Coating

Plating and coating operations are applied to sintered parts to improve appearance and corrosion resistance.

METAL POWDER PRODUCTS

1. METALLIC FILTERS

2. CEMENTED CARBIDES

3. GEARS & PUMP ROTORS

4. MOTOR BUSHES

5. POROUS BEARINGS

6. MAGNETS

7. CONTACT PARTS

8. AUTOMOTIVE COMPONENTS

APPLICATIONS

ADVANTAGES LIMITATIONS

ELIMINATION OF MACHINING

HIGH PROD RATES

COMPLEX SHAPES CAN BE PRODUCED

WIDE VARIATIONS IN COMPOSITIONS

SCRAP IS ELIMINATED

INTERIOR STRENGTH PROPERTIES

RELATIVELY HIGH DIE COST

HIGH MATERIAL COST

DESIGN LIMITATIONS

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Modern manufacturing Techniques NUST