Module 2: Open Challenges Flashcards
What are the Process Parameters of SLM?
- Beam energy P
- Layer thickness t
- Scanning speed v
- Hatch spacing h
Laser Power Density = P/hv
Volume Power Density VPD = P/hvt
What are the scanning strategies for SLM?
- Depending on the applied scanning strategy, the condition for solidification changes -> formation of texture
- Lower risk of unmolten material in adjacent tracks
- In-plane anisotropy can be minimized by using an adapted laser strategy
- Standard scanning strategy includes:
1) Island scanning (checker board)
2) Alternating bi-directional scanning
3) 67deg hatch vector rotation between consecutive layers
Discuss about the thermal gradients in SLM.
- During AM, only a localized portion of the part is molten followed by rapid solidification
- High thermal gradients in the direction towards the build platform and the sides of the melt pool
- Grains are usually elongated along the build-up direction and expand over more than one layer causing non-equilibrial columnar microsteructures
- Part thickness variability, scan patterns, overhangs can result in different microstructure
Discuss defects in SLM (porosity and optimal process window)
A. At high laser power, a low scanning speed (high density energy) leads to increased porosity (keyhole effect)
B. For lower laser power, the decrease in energy density at high scanning speeds leads to incomplete melting and fusion of the material
What is the Key Hole Effect?
- High laser energy input, plasma and evaporation of material
- Instability in liquid metal can entral gas -> formation of pores
What is lack of fusion/porosity?
- Poor bonding, due to insufficient molten metal or overlap between successive layers or adjacent melted tracks
- Defects can cause a surface roughness that leads to interlayer defects. Interlayer defects may extend to multi-layer defects
- Not sufficient energy introduced. Defects with un-melted metal powder, caused by low laser energy input
- Metal alloys that oxidize easility may form an oxide layer at the surface, this leads to a poor bonding between layers
- Often small residual porosities, in particular below the surface in the transition zone to the contour
What is balling/swelling?
- Ball formation is the solidification of melted material into spheres instead of solid layers due to surface tension effects
- Balling is observed in case the metal powder layer melted by the layer does not wet the substrate lying under it
- Surface tension is the physical phenomenon that drives melt balling
- Oxide contamination layer on the substrate and the surface of the melt might play a role
- High speed and high power, may trigget melt balling
- Swelling: can occur due to surface tension effects of the melt pool geometry (preliminary stage of balling)
Discuss cracking in SLM.
- Cooling rates are extremly high, it leads to a great temperature gradient and correspondingly large residual thermal stresses
- High temperature gradients combined with great residual stresses often cause crack intiation and propagation
- High temperature gradients combined with great residual stresses often cause deformation or warping of parts
- The following materials are regarded to be susceptible to thermal cracking:
1) stainless steels
2) Nickel-based alloys
3) Titanium
What does roughness depend on and how can it be reduced?
Roughness depends on building direction and can be reduced by jet blasting or plasma polishing
What are the steps for an AM project?
1) Design
2) Material Supply
3) Processing
4) Post processing
5) Qualification
Discuss about design
Design:
- Is the first basic step for a successful project
- Shall take into account design limitations and manufacturing constraints
- Software covering orientation, design, simulation and supporting structure is needed
- Strength of supporting structure
Why is the need for design rules?
Processes to take into account:
- Powder Bed based processes
- Blown powder based processes
- Wire based processes
- Lamination based processes
- Electrochemical based processes
Materials to take into account:
- Metals
- Polymers
- Ceramics
- The exotics
What are the general design rules for powder bed processes?
Geometrical functional requirements:
- Circular, good finish, functional (eg optical)
- Orientation vs gas-flow and recoating direction
Manufacturing requirements
- Support structure - mechanical + thermal + removal
- Overhangs
- Mix of thin and think
- Build surface density
- Change of build surface density
- Layer thickness
Post manufacturing requirements
- Powder removal and cleanliness
- Tool access - liquid access
- Handling for finishing or testing
- Verification visual inspection or X-ray CT
Geometrical features, quality assurance, cleanliness requirements and instrumentation requirements.
Discuss about supports.
- Due to induces thermal stresses resulting from reduction in tempearture from the melting point: metal would ben
- Supporting structures keep part in its position
- Thermal stresses
- WIll be partially relieved when cutting the part off the base plate and when the supporting structure is removed
- Different (elastic) deformation for different materialsW
What are some design rules for supports?
1) Flat, down-facing areas should be designed as archs. Otherwise the desig nwould most likely result in a building error and the surface quality is poor and the layer would be welded into loose powder bed leading to recoater crash
In the arch design only very small areas are welded into the loose powder bed.
2) Anisotropic surface roughness when building circular shapes in horizontal direction. Whenever possible, circular shapes should be built vertically
3) Flat, long parts buolt horizontally will bend after being removed from the base plate. This depends on the moduluts of elasticity (important for titanium or aluminium). Stress relief annealing of the whole base plate with parts.
4) Flat down-facing surfaces shall show an angle of a>45 degrees. The steeper is the angle, the better
5) Threads can readily be built. Dependent on achievable surface roughness of the certain material. Threads (circular shapes) shall be built vertically.
6) Any welded area must be connected to the base plate at any time, otherwise building error.
7) Specimens that were connected directly to the building platform had fractures occuring in the building platform.
A radius shall be created at the interface to the building platform to prevent stress concentration.
What are some PB processes not in the design rules?
Laser Path/Parameters:
1) Width of the stripes (or dimensions of the islands)
2) Strategy for the stripes
3) Strategy for the stack
4) Strategy for the contour
5) Variation in laser power density
Support structure and strategy:
1) Choice of orientation and support
- Mass production
- Fastest job
- Easiest removal
2) Verticality and stiffness of the support
3) Connection between part and platform
- Line support
- Cone support
- Tree support
- Open cell block support
- Ghost support
- Thin wall support
What are the general design rules for other processes?
1) Blown powder and wire fed processes rules
- Surface finish is substractive -> geometrical design rules same as substractive to allow final machining
2) Binder jetting process rules
- Need to remove entrapped powder -> same as powder bed for cleaning
- Need to debind -> competitive kinetic processes between “off gassing” and “sintering”, difficult to mix thin and thick sections
3) Slurry based processes
- Need to debind
- Shrinkage
What is not in the design rules for blown powder and wire fed processes?
1) Where to start, where to stop, tool path, which line first - from which direction comes the wire
2) How to accomodate (L and T) crossing
3) How to change wall thickness
What is not in the design rules for jet binding?
1) Raw material selection
2) Binder/powder ratio
Why is there the need for optimization?
- Manual iterations are time consuming and the concept is driven by the engineer working on the project. No guarantee that the best solution was found
- Optimization runs 100+ iterations within short time, converging to an optimal solution according to the defined constraints
- Trend towards simulation driven design
- FE model anyways needed, adding an optimizaiton formulation on top is simple. Of course a prerequisite in an FE model with a correct representation of the physical behaviour
- Optimization in combination with AM.
- Very limited manufacturing constraints, complex part can be realised
- Mass reduction of component directly linked to cost reduction
What ar esome optimization aspects?
There are several aspects that constitute to an optimized product:
- Choice of material
- Performance (Mechanical, Thermal, Electrical, Flow,…)
- Manufacturability (optimized for manufacturing)
- Accuracy
- Integration concept
- Number of parts in assembly
- AM Specific: building direction, support structures, finishing process.
Not all aspects can be considered in an FE based optimization. Often the main focus is on the performance.
How to define the optimization problem?
An optimization problem consists of the following three main definistions and is added on top of the mechanical problems (loads, BC, …):
1) Objective: What is the goal of the optimization?
2) Design Variables (What can be changed to achiee the objective)
3) Constraints: What needs to be considered (eg static BC, dynamic analysis)
An optimization needs 1 objective and at least 1 design variable, but doesnt guarantee that the part fullfills its functions -> constraints needed. Evaluate relevant constraints for your part and define the complete problem formulation. Only defined constraints are considered by the optimization algorithm.
Describe the topology optimization process.
- Within a defined volume (design space), the solver determines which elemends need to be present and which are not requiired by controlling the density of each element (design variable).
- Tries to converge to a discrete solution: material/nomaterial -> main load path
- Typically also intermediate density elements are density. Manufacturing of a 50% density element, not really possible.
Describe the design and non design spaces.
1) Interfaces (IF)
- Where the part is mounted to the structure, or a unit is mounted to the part
2) Stay-in volume (I)
- Envelope in which the part needs to be to avoid collisions with neighboring parts and allow enough clearance for integration
3) Stay-out volume (O)
- Locations within the stay-in volume where no material is allowed (eg due to bolt accessbility, cable pass through holes). In case of an inteface change, the stay-out volume moves along with the interface
4) Non-design space (N)
- To define volume where material has to be allocated although may not needed for a structural point of view. The goal is to minimize the non design space to the essential
5) Design space (O)
Volume inside ), outside ) and N in which the topology optimization algorithm decides where material is needed to fullfill the structural requirements of the part.
Discuss about the material supply.
1) Material supply: Essential for mechanical/physlcal properties
2) Typical processes include:
- Gas/water atomisation (non ferrous/ferrous metals)
- Tungstent rotating electrode process
- Electrolysis process -> costs, ecological footprint
3) Space quality? Chemical composition, shape, size, flowability, humidity -> Certification of Comformance (CoC)
4) Handlning/reuse specifications
Discuss about material supply challenges.
- Material powder is essential for mechanical properties and final product’s performances
- Often powder suppliers do not provide sufficient information on the powder quality
- Poor mechanical properties discovered on the final samples may be linked to powder contamination
Discuss about CT -scan
- The CT-scan is used for obtaining information on pores/voids/volumetric defects/inclusions
- Non-destructive
- No contact
- 3D imaging of sample
- Little or no sample preparation required
- Obtainable internal and external information
- When using CT scan to measure defects, we should always correlate with physical measurements (miscrosectioning)
Describe the powder procurement process.
Slide 76.
-First comes the procurement specification that is the responsibility of the customer. –From that, the procurement specification, there is a powder purchase order, which includes the material and the quantity.
- Passing to the supplier, there is an incoming order, the order is processed and passed to the powder provision.
- With the data of manufacture, blending, post-processing and order from other suppliers, the powder is tested.
- If the powder properties are as expected, a CoC is issued and the product is dispatched.
- Passing back to the customer, the product is delivered and the powder is tested and the results are compared with the CoC.
-Ultimately, the batch is accepted or refused.
What are AM raw material types?
1) Powder
- Powder bed fusion (laset, electron beam)
- Blown powder, cold spray
2) Wire
- Wire Arc AM (WAAM)
- Direct Energy Deposition (DED)
- FDM
3) Slurries
- Lithography
4) Liquids
- VAT process (polymer)
Discuss metal powders
Metal powders consists of solids (powder particles), liquids (moisture) and gases (interstitial air).
- Powders are materials with components in different states
- Powder properties are very difficult to predict
- Have a crucialimpact on AM process and final part quality
What are the particle and powder properties?
Particle properties:
1) Particle size
2) Particle shape
3) Chemistry
4) Surface state
All these are influenced by the environment (humidity, air, gas)
Powder properties:
1) Flow
2) Density
3) Physical properties
Powder characterisation techniques: Chemistry
1) ICP- OES (inductively couples plasma - optimical emission spectrometry) - commonly used
2) SEM-EDS (scanning electro microscope - energy dispersive scepctroscopy)
3) GD-AES (glow discharge - atomic emission spectroscopy) - rarely used, wont detect hydrogen
4) Inert gas fusion - commonly used
Powder characterisation techniques: Particle Size and Morphology
1) Laser light diffraction - commonly used
2) Dynamic image analysis
3) Static image analysis
4) Automatic SEM - rarely used
5) Sieve analysis - commonly used
6) XCT (X-ray computer tomography) - rarely used
7) BET surface area - rarely used
