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manufacturing routes Flashcards

1
Q

Processing issues of functional/smart materials

A

these materials are not exactly simple metallic alloys; they cannot easily be processed by conventional metalworking routes
- Ceramics and other materials cannot be plastically deformed or cast in complex shapes successfully
- Powder-based routes are often the only feasible route for small devices with complex shape and tight control of tolerances

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2
Q

Why do we use metal/ceramic powders

A
  • low waste
  • High melting point materials and difficult to cast alloys can be easily processed
  • Constituents that would otherwise react at the liquid stage can be combined at the solid phase
  • We can combine several constituents, types of materials, that otherwise couldn’t have been combined
  • Consolidation can be partially performed to obtain porous materials
  • usefull as feedstock for additive manufacturing
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3
Q

How do we produce powder?

A
  • Powder atomization from liquid phase
  • Powders by mechanical methods
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4
Q

describe Powder atomization from liquid phase

A
  • A high pressure fluid jets break up a molten metal stream into fine spherical droplets which then solidify
  • there exist several variants of this project
  • gas atomization produce high quality powders with almost spherical shape
  • with water atomization the rapid cooling doesn’t give the surface tension enough time to make the powder spherical
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5
Q

Different types of powder morphology

A

Spherical, rounded, cylindrical, spongey, acicular, flakey, cubic, aggregated

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6
Q

Powders by mechanical method

A
  • cheapest way to product powder
  • example is milling within bowls containing hard spheres, where several actions are exerted on the particles:
  • impact of powder particle against another one
  • attrition = production of wear debris due to the rubbing among particles
  • Shear = cutting of particles resulting in fracture and fragmentation
  • particles broken in fine particles by squeezing action under compression stress
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7
Q

powder properties

A
  • chemical composition (special care to contaminants on particle surfaces)
  • particle size distribution (PSD)
  • morphology
  • physical properties such as apparent density, tap density,
  • process-related properties, such as flowability and powder bed density for powder bed fusion, green density for press & sintering, …

additionally:
- storage ad aging of powders
- reusability of powder after manufactuing
- health, safety and environmental issues

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8
Q

1)particle size distribution.

2)preferred PSD depending on application

A

is an index indicating what sizes of particles are present in what proportions.

cumulative distribution expressed as the percentage of the amounts of particles below a certain size.

For press and sintering, irregularly shaped particle are preferred since the mechanical locking upon pressing is more
favoured

For additive-manufacturing processes based on powder bed routes, spherical powders are the best choice and finer powder particles (below 10 or 20 microns) shall be avoided, as they are detrimental to the powder flowability. In
addition a broader PSD can help producing a more compact and homogeneous layer

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9
Q

Rheological properties of powder

A

they are very important for metal powders used in AM, both for powder handling and in the case of powder bed systems to form uniform layers of powders, to evaluate how much they can be packed and how easily they can flow.

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10
Q

methods to test Rheological properties of powder

A
  • Apparent and tap(ped) density
  • Flow rate and angle of response
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11
Q

what are:
- apparent density and volume
- tap density

A
  • The apparent density refers to the mass of the loose powder divided by the bulk powder volume
    the apparent volume includes the volumes of the solid particles, interparticle voids, and pores in the particles.
  • tap density value can be determined by mechanically tapping or vibrating under specified conditions a container, producing a rearrangement of the powder particles, thus reducing the volume of the interparticle voids
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12
Q

describe the test based on Flow rate and angle of repose

A

The angle of repose is related to interparticle friction and flowability of powders.
methods for measuring it:
- statically, through a funnel with a standard orifice, by measuring the angle of the cone formed by the deposited powder
- dynamically, by measuring the angle formed by a volume of powder inside a rotating drum

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13
Q

describe Press and sintering

A

it’s a Widely used process to deal with powder, both ceramic or metallic (Widely used for tool steels or for producing tools for machining)

it consists of 3 steps:
- Preparation of the powder blend (mix powder and lubricant)
- forming the green part
- sintering

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14
Q

press and sintering: powder mixing and die filling

A
  • we want to form a homogeneous blend of powder with right amount of lubricant (to reduce friction between powder particles and between powder and die walls)
  • lubricant=stearic acid, zinc stearate or other organic waxy compounds, added to
    amounts of 0.5 to 2 wt.% of the charge.

2 types of lubrication:
- mixed lubrication when the lubricant is blended to the powder to reduce interparticle friction and aid packing, but the lubricant has to be removed before sintering to avoid contamination
- die-wall lubrication to be applied on die and punch surfaces to ease sliding of the powder along the die walls

  • mixed powder is then compacted, into a rigid closed die made of hard steel or of carbides, under pressure
  • pressure applied to the powder inside the die does not spread uniformly through the volume due to friction
  • tools can exert a single action or a double action to improve powder compaction (The cost of a punch double ended is much higher)
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15
Q

press and sintering: forming the green part

A
  • filling a die with a defined amount of powder and applying a pressure. The
    resulting compact will possess just enough cohesion to be handled and moved safely to next stages

steps are:
a) filling of the die with defined amount of powder
b) filling interparticle voids by relative movement of powder particles
and elastic deformation at contact areas
c) mechanical interlocking and plastic deformation of powder particle surfaces
d) bulk plastic deformation and extended interlocking at particle surfaces, to achieve the required strength of the green part.

  • maximum permissible compaction pressure is controlled by the need to avoid tool failures and the max size of the part is a direct function of the capacity of the press
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16
Q

describe sintering

A

the compact held at high temperature acquires its strength by promoting atomic diffusion

  • porosity:
    porosity decreases, the pores become rounded and smaller, recrystallization and grain growth take place, eventually generating a fully recrystallized grain structure with limited residual porosity.
  • environment:
    carried out in a protective environment to avoid reaction with ambient gases. For metals we use reducing gases to prevent oxidation and even to reduce the surface
    oxide (Ceramics doesn’t have big problems with oxidation so we don’t need a particularity protected environment)
  • shrinkage is expected due to welding and reduction of porosity
    (We need to compensate for the shrinkage, we need to design the final shape using and oversizing at the beginning)
17
Q

Post-sintering operations

A

with sintering Near net shape is already achieved but we could have some other treatments as:

  • infiltration = Filling the surface-connected pores with a liquid metal
  • impregnation = pores are filled with an organic material (an example are oil-impregnated bearings)
  • steam-treatment
  • heat treatments (e.g. quench and tempering)
  • Machining
18
Q

Hot pressing

A

we try to do pressing and sintering at the same time = pressing at high temperature

advantages:
- Hot powder is softer so can be more efficiently pressed with lower load
- density and strength achieved are in some cases enough for a direct use of the part
- sometimes a shorter sintering stage is applied to achievethe highest performance

drawbacks:
- production cycles are longer due to additional heating and cooling times.
- cost and productivity concerns. (Heatresistant dies and heating systems have to be designed, also implementing controlled atmospheres to prevent oxidation even during the pressing stage)

19
Q

Hot Isostatic Pressing - HIP

A
  • pressure to the powder not just in vertical direction but in all the directions ( not unidirectional but isostatic pressure)
  • promotes isotropic properties and almost full densification

steps:
- design and construction of the container
- filling with powder and sealing
- hot isostatic pressing
- container removal
- post-processing operations

commonly used also for the other purposes, even starting from already consolidated workpieces:
✓ Densification of castings
✓ Densification of already sintered parts
✓ Reduction of defects in heavy duty AM parts
✓ Diffusion bonding between parts

20
Q

Metal Injection Moulding - MIM

A

consists in incorporating a high volume fraction of metal (or even ceramic) powder into a binder, allowing the injection of the composite mix into a mould

  • After consolidation and extraction of the green part, a debinding stage follows, aimed at removing most (but not all) of the binder.
  • debinded metallic shape is then carefully transferred to a sintering stage
21
Q

Additive manufacturing / 3D printing

A

build layer by layer the material, generating a 3D part
- No moulds or other tools are required just a digital model
- Cost of part does not depend on
complexity of the shape
- no Geometrical constraints that exist for
conventional processing
- New shapes become feasible
- For simple shape machining is convenient
instead for complex shape additive
manufacturing (AM) is more convenient

22
Q

Processes based on powder-bed fusion

A
  • most diffused additive manufacturing technique
  • powder bed is distributed over a flat surface, the building platform
  • A focused heat source (laser, electron beam) selectively scans the surface and melts the powder
  • The platform is then lowered and a new layer of powder is deposited and selectively melted together with a fraction of the previously deposited layers so as to progressively
    build a solid 3D object.
  • carried out in a protected chamber filled with inert gas to avoid contamination and reaction of powder with ambient gases
  • After all layers have been deposited, the building plate with the 3D solid parts is cleared from the surrounding loose powder and extracted from the chamber.
  • The parts are separated from the platform and from their supports
    (unmelted powder removed from the build platform can be re-used)
23
Q

Binder jetting

A
  • Generation green part, starting from a powder bed on which a binder is selectively spread layer by layer to allow the part to be safely separated from the platform and moved to post-processing furnaces
  • consolidation of the green part by curing the binder
  • removing the excess of binder (debinding stage)
  • sintering the part at high temperature in a controlled environment in order to achieve the desired density and properties.
24
Q

Laser metal deposition and hybrid processes

A

Also called Directed Energy Deposition (DED), it is based on the use of a torch mounted on an arm. The torch delivers heat and the molten feedstock materia coaxially (powder or a wire) onto a substrate where it solidifies to progressively build the 3D object by overlapped passe

smart materials (15 decks)

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