Advanced manufactoring routes Flashcards
Functional materials
- They are considered for thir properties of magnetism, porosity, shape, interaction& reaction with enviroments.
Processing issues of functional materials
- Theese materials are not simple metallic alloys, but kind of more difficult to be processed by conentional working routes
- Ceramics are plastically deformable, but they can be casted in order to achive complex shapes.
- Powder based routes are a way to realize small pieces with tight tolerances
The atomization of powder
This process is based on a high-pressure fluid that breaks up a molten metal stream into fine droplets
- Several types of atomization exist as water, plasma, gas,vacuum, centrifugal,rotating disk, ultrasonic
- The more accurate is produced by gasses
Powders by mechanical metods
Mechanical methods are the cheapest way for powder production, an example is milling with bowls containing hard spheres.
Powder properties
A large set of materials is commercially available as powders.
Powders must posses distinct key properties:
- Chemical composition
- Particle size
- Morphology
- Apparent density
- process releted properties
- powders recycling
- safety for enviroments
Particle size distributio PSD
- Frequency distribution: indicate the percentage of particles existing in respective size particles
- Cumulative distribution: express the percentage of particle below a certain value
- Size percentile values: indicate the size below which percentage of all particles are found.
PSD
Depending on the manufacturing technology exist an optimal size and shape.
- For press and sintering irregular shapes are preferred.
- For additive manufacturing spherical particles are the best choice regarding to size particles must be not too finer (over 20 microns).
Rheological properties
To evaluate rheological properties of metal powders in AM (how they can be packed and how can easily flow) are used to main methods:
- Apparent tap density= mass of loose powder/ the bulk powder volume;
- Flow rate and angle of repose: the last one is determined statically with an orifice and measuring the cone formed by the powder, dynamically with a rotating disk with powder inside.
Press and sintering work flow
- Preparation of the powder blend by mixing the material powder with lubricant
- forming the green part by filling a die with a defined amount of powder and applying a pressure
- Sintering the green part in a protective/reducing atmosphere at high temperature (generally below melting temp). liquid sintering if a second phase melts during sint.
Application of sintering
Tools for machining hard component
Ceramics items
Brake pads
Bronze filter cartridges
Powder mixing and die filling in sintering
- the aim is to form an homogeneous blend also with the right amount of lubricant to reduce the friction with die walls.
- The lubricant usually consists in stearic acid added for about 0.2% of total amount.
- mixed lubrication when lubricant is added to the powder
- Die-wall lubrication: when graphite is applied to the die walls.
- the powder then is compressed with pressure up to 1000 MPa.
Press and sintering stage
Despite the action of lubricant a gradient of pressure exists, in fact the most stresses region is the upper one where the plunge presses, but to limit this effect exist double action machines.
Powder compaction
- the cold pression phase consists in the green part generation
- Filling the die
- Press to fill voids and to elastically deform the particles
- Plastic deformation and mechanicsl interlocking
- Bulk plastic deformation
- Pressure up to 1400 MPa.
Sintering
- is the core stage of the process
- Mechanicla links and atomic diffusion
- Porosity decrease, grain growth
- Sintering is carried out in a protective enviroment (reducing enviroments for metals).
- A slight shrinkage is expected
Post sintering operations
- Filling: Generally is done by a filler metal (copper with ferrous parts).
- Impregnation: same of filling but with organic material
- Steam treatment: Used for PM steel-based, to involve the formation of an iron oxide to improve corrosion resistance.
- Heat treatments
- Machining
Hot pressing
For ceramics and metal powders hot pressing can be alternative route and adds some advantages:
- Hot metal powder is softer and therefore it can be more efficiently pressed in the die.
- The density and strength after hot pressing are suitable for service.
cons are longer time, higher costs.
Hot isostatic pressing
The process is carried out in a furnace at 0.8 of melt. temp. with a pressure constant around the pieces of 100-200 MPa.
This promotes the isotropic properties and almost full densification.
Can be used also for: densification of castings, sintered parts, reduction of defecto of AM parts.
Metal injection moulding
The process:
- Mixing binder + powder
- Injection moulding
- Debinding with cleaning and thermally
- The green body obtained is sintered in a controlled atmosphere.
The advantages are the strict tolerances, high efficiency of material use, high production volume, complex geometries at relative low price.
Process based on powder-bed fusion
- A thin layer of powder is distribuited on a surface
- A focused heat source selectively melts the powder
- The platform is then lowered and another layer of powder is deposited.
Advantages: Thanks to the inert gas contamination are avoided, the powder surro
Binder jetting
- Generation of a green part using a binder to stick the powder together. The binder is selectively spreaded to shape the part.
- Curing the binder, meaning that the part is stable and the particles and bonded strongly (thermally,UV,chemically).
- Debinding
- Sintering
Laser metal deposition
- A torch gun delivers powder to melt and the heat source, generally is a well automated process using robotic arms.
- The heat sources are many laser, plasma, electon beam.
