Recitation: Topology Opt and Design for AM Flashcards
Discuss about Metal PBF.
Energy beam:
- Laser beam (LPBF or SLM)
- Electron beam (EB-PBF or EBM)
Max part size:
- Up to 400x400x500
Advantages
- High resolution (30-100μm)
- Complex features (lattices, internal channels)
Disadvantages
- Low build rate
- Required suppot
What is the process chain of AM.
1) Design
- CAD file
- Topology optimization
2) Process setup
Choose:
- material
- scan strategy
- process parameters
- building strategy
3) Printing
- Build the component
4) Post-processing
- Part removal
- Supports removal
- Heat treatment
- Surface finish
5) Quality testing
Measure:
- Distortion
- Residual stress
- Porosity
What is topology optimization?
A mathematical approach that optimises material layout within a given design space, for a given set of loads and boundary conditions such that the resulting layout meets a prescribed set of performance features.
How to approach an optiization problem?
1) Design variables
A function or vector that describes the design, and which can be changed during optimization. It may represent geometry or choice of material.
2) Objective functions
A metric used to classify designs: mass, stiffness or even cost of production
3) Constraint functions
Design requirements that have to be satisfied: maximum stress, minimum natural frequency, instability
Why AM?
1) Complexity for free
2) Near net shape
3) Undercuts, internal channels
Additive manufacturing allows to produce anything.
What are the product quality and process parameters for PBF?
1) Product quality assessment
- Dimensional accuracy
- Geometrical accuracy
- Surface finishing
- Mechanical properties (yield, UTS, elastic modulus, elongation at break)
- Physical properties (density)
Process parameters:
1) Laser related
- Power
- Spot size
- Pulse duration
- Pulse frequency
2) Scan-related
- Scan speed
- Scan pattern
- hatching parameters
3) Powder-related
- Particle size
- Shape and distribution
- Layer thickness
- Materal properties
4) Temperature related
- Chamber temperature
- Pre-heating
5) Part-related
- Shape
- Orientation
- Support geometry
The first 4 are predefined for each material-machine combination
Discuss the orientation parameters.
Orientation:
It’s a tradeoff between buildability, quality and cost.
Orientation
1) Buildability
- Aspect ratio
- Angle to recoater
- change of cross-section
2) Quality
- Support removability
- Surface roughness
- Dimensional accuracy
- Mechanical properties
3) Cost
- Build time
- Nesting suitability
- Amount of supports
Discuss buildability:
Buildability
1) Aspect ratio
A minimm wall thickness of 400μm is manufacturable. Rz = 120-150μm and tw>800 recommended
2) Overhangs
90>δ>60 good manufacturability
60<δ<45 manufacturable
45>d manufacturing only with supporting structure
3) Surface angle
small surface angle lead to reduction of the surface quality and warping of the structure
What to avoid?
1) large cross sectional areas
2) Part edges inclined towards the wiper
3) Thin wall sections parallel to the recoater motion (possible damage to rubber recoaters)
Discuss quality.
The layer-by-layer manufacturing scheme results in anisotropic mechanical properties of the final part. It is possible to orient the part according to the maximum stress direction. Supports removability and costs must be carefully evaluated.
Buildability and final product quality often overlap, as optimizing the buildability results often in the optimization of the product quality. Solid metal below the melt pool:
- strong weld
- efficient heat dissipation
- good surface finish
Powder below the melt pool(overhang)
- heat accumulation -> stress accumulation
- sintering of surounding powder -> low dimensional and geometricla accuracy, poor surface finish
What is the purpose of supports?
1) Avoid direct building on building plate and simplify part removal
2) Support overhang surfaces
3) Anchor points to mitigate part distortion due to residual stresses
4) Heat sink to avboid burning of thermally insulated feaatures (eg acute cornersor for bulk parts)
Disadvantages:
1) Waste of material
2) Post processing time and cost
3) Difficult fixturing
Discuss suports for overhangs and holes/channels
Overhangs:
Redesign large overhangs to become self supporting. Chamfer must be 45 degrees or above to become self supporting. Fillet must be no greater than 10mm radis. Avoid creating tall supports. When building tall supports, there is a change the supports may fail due to the supports shifting.
A bridge can go unsupported up to 3-4mm. Soem materials allow more than this it depends on the geometry. One sided unsupported 0degree overhangs shall not extend more than 1mm.
Holes/channels
Diameters as small as 0.4-0.6mm can be produces but this depends on material, build orientation and layer thickness. A diameter up to 10mm can be built without support. Holes larger than 10mm in diameter should be considered for redesign,.
