Metallic AM: State of the Art Review & Prospects
Abstract
AM is used for more than 25 years
No longer confined to prototyping applications
Mostly used in niche markets (medical applications, aerospace)
Provides improvements
over traditional processes
Time-to-market
ecological impact
design
Current metallic AM processes
studied in this paper
SLS
Direct Metal LS (DMLS)
SLM
Electron Beam Melting (EBM)
Direct Metal Deposition (DMD)
1. Intro
Traditional Processes are multi-staged:
rough-part creation & material removing
AM processes can build fully functional
parts in a single operation
2. Definition
Manufacture metallic parts which meet
the designer's specification in terms of:
shape (geometry)
material
mechanical behaviour
3. Direct Metallic
AM processes
Classification
Type of Material
Polymer
Stereolithography
Polyjet
Fused Deposition Material (FDM)
paper
Laminated Object
Manufacturing (LOM)
wood
Stratoconception
metal
Layer
Direct Deposition
State of Raw Material
discrete particle #
Type
3.1. Layer based
metallic AM
Types
SLM
More powerful laser than both SLS and DMLS
Fully melys the powder
The parts manufactured have no or few porosities (if the gap between the scanning paths is small enough).
The optimization of the parameters is cruciual to obtain good surface quality
Higher temperatures involved
Shrinkage
Thermal Distortion
SLS
1st processed invented (1979)
Initially the only available powder was polymer powder
Since 1990, it widened the reange of available materials to ceramics and metalic alloys
After the fabrication is completed, the part is placed in an oven to vaporize the binding polymer, sinter the part and infiltrate it witha molten metal with a lower fusion temperature (such as bronze) to improve the mechanical behaviour and fill in porosities
High Building speed
Long curing phase
EBM
Similar to SLM, using elctron beam instead of laser
High building speed
The lack of moving parts to guide the building spot makes high scanning speed possible (up to several km/s)
The increase of energy density at the building spºot allows the use of a large variety of metal alloys
Does not require curing phase
shrinkage occurs
Scanning strategy is important
minimize heat diffusion inside the powder bed
improve the part's quality
DMLS
Variant of SLS able to build metallic parts without using polymer to bind the particles (no curing phase)
The laser melts the peripheral region of the particle while its core remains solid
The molten metal acts as a binder,
creating gates between the particles
The powder can include several metals, in that case, the metal with the lowest fusion temperature acts as the binder
The parts are porous with
reasonable mechanical props
Useful to manufacture filters or gas storage systems for example.
To obtain fully dense or gas proof parts, an infiltration is required
Characteristics
Operation
Start from a 3D model of the part which is sliced
into 20-150 microns-thick cross sections
Sections are built sequentially
An energy source (laser or electron beam) is used
to scan each of powder to bind the material
After the section has been scanned , the piston of the building chamber is moved down and a roller deposits and presses down a new layer of powder
This processed is repeated until
the part is completed
Once built, the part (or parts) is separated from the unbound powder and cleaned. The remaining powder is filtered and stored to be used later
Constraints
Using Supports
Purpose: prevent the collapse of molten (or sintered) metal inside when manufacturing large overhanging surfaces and dissipate heat
Generated during pre-processing stage and amde from the same material athan the part (contrary to photopolymer based procs)
Removed after completion of the aprt
Building rate
The building chamber is usulaly heated to minimize the quantity of energy to be brought at the focal poiint
Binding Mechanism
Particles are fully melted (SLM, EBM, DMD)
Partially melted (SLS, DMLS)
Energy Source
Laser
Electron Beam
3.2. Direct Metal Deposition
Consists in spraying the metallic powder direclty onto a laser beam (usually a kilo-watt CO2 laser)
The molten drops are then used to build the parts
Various metallic alloys are available and it is possible to gradually and continuosuly change from a material to another while manufacturing
Possible to manufacture multimaterial parts
The nozzle is usually mounted onto a 5 axis
CNC structure to produce complex parts
In that case, a preprocessing phase is required to generate the nozzle trjectories
This generation is complex due to its high influence on the final result
Contrary to layer based processes, the thickness
of the DMD joint is not constant. Depends on:
Speed of the nozzle
Rate of material deposition
Powder flow
laser power
gas flow, etc
Any difference between the manufactured and expected thickness can cause the failure of the construction since the distance between the nozzle and the surface can slowly grow, and consequently the moltewn particles solidify before reaching the part.
To prevent this phenomen, an optical system can be used to monitor the distance between the nozzle and the part.
4. Metallic AM
processes Evaluation
Main criteria: time-cost-quality triangle
- Can become a square by addition
of the environmental impact
Quality
Surface Quality
Granular aspect due to the binding of
unmolten particles on the exterior of the parts
Arithmetic rugosity of the surfaces is below
15 microns for the powder bed based processes
The surfaces built with SLS (and with DMLS + infiltration) have a better quality than the ones made through SLM and unilfiltered DMLS (the infiltration smoothens the surfaces)
Rugosity of EBM is 25-35 microns (according to Arcam)
DMD produces surfaces with Ra between 10-25um (according to ROM)
Materials and mechanical props
Nowadays, it's possible to buiold parts with CNC like amterial [13] and some processes can manufacture multi-material parts [14]
The invrease iin power of the Laser sources used in SLS, DMLS and SLM allow the use of high melting point metallic alloys
The mechanical props of the sintered and molten material tend to be similar or even better tnhan the machined one, the microstructrue being more and more doncotrolled
Dimensional Quality
SLS, DMLS and SLM produce parts with dimensional erros of less than 0.1mm for a 100mm length
EBM is half as good
DMD is 3x worse
On a general note, when good surface or dimensional quality is needed, finishing ioperations are necessary
Time
Few studies focus on the manufacturing speed of different RM processes since it's difficult to build a part under the same conditions on different manufacturing processes
AM Processes based on sintering (SLS and DMLS) are fairly faster than SLM
DMD and EB; are able to produce non-porous parts, as SLM does, with a higher building speed
This data is usualçly measured with max. layer thickness (except for DMD, not layer based)
SLS and DMLS, though having similar building sppeed than EBM and DMD, require an infiltration to obtain nearly fully dense parts
EBM and DMD are tghhe fastest processes to manufacture parts without ffinishing operations
EBM allows the user to change the diamter of the building spot diameter from 200um to 1mm. With this process, it's possible to build small entities as well as fill in quickly large vols. (compared to SLM, for example, where the focal spot has a fixed diameter of 70um
To have the same flexibility on laser based processes, machines with multiple laser are experimented to have multiple scanning spot at the same time [12]
Cost
Depend on
Machine operating cost
raw material cost
consumables cost
manufacturing time
Generally, for a medium building chamber volume, sintering based processes are least-expensive, whereas EBM and DMD are most expensives
The high price of these machines is balanced by the very short pre-production phase for small series
The price of metallic powders is greatly impacted by the atomization process which reduces the price between different alloys
Environmental Impact
About 95% (according to Arcam) of the unused powder can be filtered and used right away
Thr environmental impact of SLS and SLM machines manufacturing a test part was qauntified [18] and shows that the fabrication imopact can't be disregarded compared to extraction and creation phases
5. Conclusions
Using these processes should now be considered from the early desgning phases to take advantage of the freedom of shape which can lead to building less massive or more fucntional parts (fig. 7)
Current CAD tools and, more generally, the numerical
chain should change to take into account the
new features of rapid manufacturing built parts:
mukltimaterial parts
Inner structures
Coloured surfaces, etc. [20]