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DMLS design guide V4
Transcript of DMLS design guide V4
The DMLS (Direct Metal Laser Sintering) process
1. A layer of powder (approx 0.02mm to 0.08mm thick) is deposited on the build platform by a recoating blade. The bottom layer of the part is then created by the laser, which locally melts the powder.
2. The roller then deposits another layer of powder as the build platform lowers one layer and the powder reservoir is raised by the same amount.
3. The laser then fuses the second layer to the first.. and so on...
4. As the layers build, the level in the build chamber goes down and the powder reservoir base rises.
5. When the final layer of the part has been built, the powder is removed.....
...revealing the part attached to the build platform.
The build platform
Benefits of DMLS
DMLS and the environment
EOS Aluminium AlSi10Mg is a typical casting alloy with good casting properties and is typically used for cast parts with thin walls and complex geometry. It offers good strength, hardness and dynamic properties and is therefore also used for parts that are subject to high loads. Parts in EOS Aluminium AlSi10Mg are ideal for applications which require a combination of good thermal properties and low weight. They can be machined, spark-eroded, welded, micro shot-peened, polished and coated if required.
CC MP1 is a fine powder mixture which produces parts in a cobalt-chrome-molybdenum based super-alloy. This class of super-alloy is characterized by having excellent mechanical properties (strength, hardness etc.), corrosion and temperature resistance. Such alloys are commonly used in biomedical applications such as dental and medical implants and also for high-temperature engineering applications such as in aero engines.
Cobalt Chrome Alloy (CC MP1)
718 Alloy is a nickel based heat resistant alloy in fine powder form. Its composition corresponds to UNS N07718, AMS 5662, AMS 5664, W.Nr 2.4668, DIN NiCr19Fe19NbMo3. This kind of precipitation-hardening nickel-chromium alloy is characterized by having good tensile, fatigue, creep and rupture strength at temperatures up to 700oC. In718 Alloy also has outstanding corrosion resistance in various corrosive environments. This material is ideal for many high temperature applications such as gas turbine parts, instrumentation parts, power and process industry parts etc.
In718 (718 Alloy)
MS1 is a pre-alloyed ultra high strength steel in fine powder form. Its composition corresponds to US classification 18% Ni Maraging 300, European 1.2709 and German X3NiCoMoTi 18-9-5. This kind of steel is characterised by having very good mechanical properties, and being easily heat-treatable using a simple thermal age-hardening process to obtain excellent hardness and strength. Ideal for many tooling applications such as tools for injection moulding, die casting of light metal alloys, punching, extrusion, it is also good for high performance industrial and engineering parts, for example aerospace and motor racing applications.
Maraging Steel 1.2709 (MS1)
316L Stainless Steel is a pre-alloyed austenitic stainless steel in fine powder form. This powder meets the chemical requirements of AISI 316L, DIN 17006 X2CrNiMo17-12-2, W.Nr1.4404. This kind of steel is characterised by having higher corrosion resistance and mechanical properties than the more common 304 alloy, and can be used over a wide temperature range down to cryogenic temperatures. This type of steel is widely used in a variety of food processing, medical, aerospace, oil and gas, and other engineering applications requiring high strength and corrosion resistance.
316L Stainless Steel
StainlessSteel PH1 is a pre-alloyed stainless steel in fine powder form. Its composition corresponds to US classification 15-5PH and European 1.4540 and fulfils the requirements of AMS 5659 for Mn, Mo, Ni, Si, C, Cr and Cu. This kind of steel is characterised by having very good corrosion resistance and mechanical properties, especially in the precipitation hardened state. This type of steel is widely used in metal prototypes and a variety of medical, aerospace and other engineering applications requiring high hardness, strength and corrosion resistance.
Stainless Steel 15-5PH (SS PH1)
Titanium Ti64 is a pre-alloyed Ti6AlV4 alloy in fine powder form. This well-known light alloy is characterised by having excellent mechanical properties and corrosion resistance combined with low specific weight and biocompatibility. This material is ideal for many high-performance engineering applications, for example in aerospace and motor racing, and also for the production of biomedical implants. Parts built in Titanium Ti64 fulfill the requirements of ASTM F1472 regarding maximum concentration of impurities. Standard processing parameters use full melting of the entire geometry. Parts built from Titanium Ti64 can be machined, spark-eroded, welded, micro shot-peened, polished and coated if required. Unexposed powder can be re-used.
Titanium Ti64 Alloy
Build platform is made of steel plate. One standard size for the base is 250mm x 250mm.
Parts are built vertically up from the base. Multiple layers of parts are only possible if they can be stacked on top of each other and cut apart after the build.
Multiple parts stacked
Before the parts are removed from the base, the whole platform is placed in a furnace for several hours to relieve mechanical stresses. The parts are then removed from the base by EDM (Electrical Discharge Machining) - also known as 'wire cutting'. The path can help define the geometry of the finished part.
Material left on the base (usually only a few millimetres) is removed by machining....
....and the machined base is now ready for another build.
The primary benefits of DMLS are:
Internal as well as external complexity
The potential to combine a number of parts into one part
The absence of any tooling, allowing complex shapes and geometry to be produced
The ability to vary wall sections to achieve optimum strength
Metal parts can be supplied 'as-built' or finished using various post processing and finishing techniques. Depending on the material used, there are various forms of finishing available:-
Shot peening entails impacting a surface with shot (round metallic or ceramic particles) with force sufficient to create small indentations or dimples. It is similar to sandblasting, except that it operates by the mechanism of plasticity rather than abrasion. This means that less material is removed by the process, and less dust created. Shot peening may be used for cosmetic effect.
Polishing is a metal-finishing operation where articles are polished using abrasives or mops, in a multistage process. Firstly, coarse grit abrasives are applied at high speed to remove surface defects.Then fine grit abrasives are used to remove the residue and smooth the surface. Finally, cotton mops are used to give a mirror-like finish to the articles.
Build platforms are typically 250mm x 250mm at the base and either 215mm or 315mm high.
Building layers in DMLS
The most straightforward geometry to build in DMLS is a vertical 'extruded' form from the build platform, where each layer builds on the geometry directly below it.
Angled surfaces and holes
The powder in the build chamber does not provide any support to the part as it builds, so any angled surfaces will ideally be self-supporting.
If the angle is too acute, the surface will need a supporting structure built in as part of the model. This supporting structure will then need to be removed by machining or wire cutting, increasing energy use.
The minimum angles that will be self supporting are approximately:
- Stainless steels: 30 degrees
- Inconels: 45 degrees
- Titanium: 20-30 degrees
- Aluminium: 45 degrees
- Cobalt Chrome: 30 degrees
If the angle is near the point where it needs supports, the downward facing surface will become rough an may require considerable post-finishing.
Small holes can be accomodated easily. Holes of less than 6mm diameter are ideal.
Larger circular holes will result in a roughened surface at the top which may need post-machining.
Large holes will require support structures to be added in the centre to prevent the part collapsing or becoming distorted during the build process. These supports will need to be removed by wire cutting or machining.
If the hole has an angled or arched upper area it will probably not require any supports. This is one of the features of DMLS that can have a significant impact on the design process.
Downward facing surfaces
Direction of build and cross sections
Any downward facing surface will require support. Support structures will need to be removed by wire cutting or machining, which will increase the energy and waste involved in the process.
The most simple support structure will fill the hole that creates the downward facing surface. This can be removed by wire cutting or machining.
An offset support structure can be used that will be easier to remove. In this case, the base of the support will be cut when the part is removed from the base by wire cutting, leaving one edge to be cut in order to remove the rest of the support.
If the top surface of the hole can be made of a series of angles (which are self supporting) the supports can be minimised to the base of each angled surface.
If the hole is simply for weight reduction or cooling, for example, it can be modified as a series of semi-circular topped slots which will not require supports. However, the 'pillars' between the holes need to be self-supporting (see part strength - below).
An alternative to this approach will be to turn the part through 45 degrees to make all the surfaces angled and remove the need for supports. Orientation is a major issue in finding the most efficient build method - please see item 3 in Other Issues (below) for more details on the limits and possible pitfalls of using angled edges like the ones shown above...
As the recoater blade passes over the part, depositing another layer of powder, it can touch the layer below, sometimes with some force. The orientation of the part is, therefore, important.
The ideal geometry is a circular profile which provides a smooth lead in for the blade, and a stable cross section as it builds.
An open 'U' or similar shape is also ideal, as the lead in for the blade is again rounded, and the basic profile will be strong as it builds, resisting the force of the recoating blade.
The 'worst case' geometry would be a thin section parallel to the recoater blade. The blade will tend to 'bounce' off the parallel wall, and the section itself will not resist the force of the blade as it builds.
Any flat surfaces need to be at least 5 degrees from parallel with the blade to allow the blade to touch the part at a point, not a face.
In addition to touching the part at an angle, it helps if the geometry is inherently stiff, which will resist bending forces as the recoater blade passes over the part.
Long, thinner parts with rounded ends will build well, as they also provide a smooth lead in for the blade and are inherently stiff. However, all these issues need to be considered in parallel with the other limits (build angles, etc) mentioned elsewhere in this section.
Partially built part
Part strength during the build process
As the recoater blade passes over the part, more force will be applied to the geometry as it gets taller. As a rule of thumb, the ratio between the section and the height should be no more than 8:1.
The exact proportions will always depend on the specific geometry, but if the section gets too high, there is a danger that the recoater blade will bend the part, and possibly damage itself in the process, terminating the build sequence.
To prevent these problems, vertical sections need to be bridged at certain points. The best method of achieving this will be to use 'arches' to avoid the creation of downward facing flat surfaces.
Even a part that will be strong when it is finished may need some support during the build process. This triangular section will be very weak as the build gets close to the apex.
This kind of structure may need a simple support structure up the middle to provide some rigidity before the part is completed.
If the reason for the open structure is simply weight reduction, it may be easier to perforate it with holes (ideally less than 6mm in dia) that will reduce weight, but not require any supports.
Part will be very
weak at this point in the build process
1. Avoid sharp edges
Very sharp edges cannot be built in DMLS, and it is better to design parts with minimum rads of approximately 0.5mm.
2. Avoid thick sections
The heat build up when creating very large horizontal sections can affect the build geometry, particularly when using titanium. A better approach is to angle the part to minimise the horizontal section at any one time.
3. Avoid angles facing into the recoater blade
Angled parts that lean into the path of the recoater blade may cause the blade to collide with the part and terminate the build.
4. Avoid sharp edges
Sharp corners can act as 'stress raisers' in DMLS in the same way as they can in most processes. Always try to use rads on corners instead of sharp edges.
5. Use the wire cut removal path
The path used to wire cut the part from the base can be used as an integral part of the component design, rather than simply as a straight cut.
6. Build multiple parts
The nature of the DMLS process allows for multiple parts to be built 'in situ'. This can save considerable time and assembly cost for appropriate geometry.
What do supports do?
Supports are a 'necessary evil' in the DMLS process. Good design practice will minimise them, as they use a lot of energy - both in their construction and removal - but they also fulfill a number of vital functions within the process:
1. They support the newly melted surface, particularly on downward facing surfaces and shallow angles.
2. They can prevent the new geometry from deforming.
3. They dissipate heat away from the newly formed geometry, and
4. They provide temporary support for geometry that will be strong when complete, but that is weak during the build process. (see 'part strength during the build process').
The ideal situation is to design a part that requires no supports at all (see images on the right for theoretical worst and best case scenarios).
The reality is that it is rarely possible to design parts that require no supports at all, but minimising them will save time, energy, and money...
Large amount of support structure that needs to be built (and then removed) to support downward facing surfaces during the build process
Geometry changed to simple curve that can be built without supports
Types of support
1. Simple fill in
The most simple form of support is to fill in the area that needs support, and then cut this out when the build is complete by wire cutting or machining. If the support area is to be removed with wire cutting, a small hole needs to be placed in the support area to allow the wire to be located.
Simple fill in with hole for cutting wire
Support area removed
2. Offset supports
Offset supports require less machining. They rise vertically and then angle in to support specific surfaces. The base of the support is usually removed with the wire cut removal of the part, requiring only the supported surface to be machined.
All support structures are formed from fine lattices, to minimise energy consumption and build time
3. Overhanging surfaces
Horizontal overhanging surfaces can be supported from the base, although this will require a considerable amount of material and energy. A better solution is to 'buttress' the surface from the main geometry at an angle. Better still, design the support into the geometry and remove the need for any additional work!
Support from base
'Buttress' support from main geometry
4. Supports for curved surfaces
Sometimes, it is necessary to support a downward facing curved surface to prevent the geometry failing or a very rough surface being formed.
In this case, a support structure is formed under the part which is then removed by wire cutting or machining when the part is removed from the base.
The impact of part stresses
Forming threads in DMLS parts
DMLS material specifications
A conventional 'rat trap' bicycle pedal (left) has a large number of surfaces. If it is built in the horizontal plane, the large number of downward facing surfaces will require a significant amount of support (right). A large number of these can be offset, which will reduce the removal time, but building the part would require a considerable amount of energy.
If the geometry is modified to reduce the number of downward facing surfaces (mainly by putting in a number of 45degree angled surfaces) the amount of supports needed is reduced significantly (right).
However, by changing the orientation of the part to vertical, the number of supports needed is dramatically reduced.
This vertical orientation, combined with design changes to the pedal, would allow designs to be be produced that require no supports at all.
Airline seat buckle
The airline buckle is one of the SAVING projects aimed at reducing energy use by creating new, lighter parts for use in mass transit aircraft.
One of the main features of an airline seat buckle is the opening in the base which allows the belt to be clamped in place in an emergency.
One benefit of the DMLS process is that multiple parts can be built as an assembly. In this case. the belt tensioning bar can be built into the body of the buckle (a small gap needs to be built in to the assembly to prevent the parts fusing during the build process).
The first version was strong enough when completed, but the open geometry meant that the part often failed in mid-build, as one side was pushed over by the movement of the recoater blade.
This meant that there appeared to be no choice but to fill in the triangular section and wire cut it to achieve the desired geometry.
This side was pushed over half way through the build process
An alternative approach was tried to minimise the supports needed. However, the angled geometry collided with the recoater blade during the build process.
Support would only by needed in the centre
The next approach was to fill in the whole area during the build process, with an offset support to allow the tensioning bar to be built as part of the assembly. A hole was built in to allow for simple wire cutting of the triangular hole.
Conventional airline seat belt buckles weigh between 155g (Steel) and 120g (Aluminium). When made in Titanium with DMLS, the weight is reduced to 68g without compromising strength. This is a maximum potential weight saving of 87g.
An Airbus A380 when configured for economy passengers has 853 seats. A DMLS titanium buckle would lead to a total weight saving of 74kg. This could equate to a saving of 3,300,000 litres of fuel over the life of the plane.
3D Lattice supports
1. 3D lattices are made up of repeated structures that occupy a low fraction of the volume that makes up the part. This means that they use less material and take less time to build than conventional lattice supports.
2. The open nature of the lattice allows the easy removal of surplus powder, making it easier to remove the part and platform from the build chamber.
These types of support lattice require the use of specialised CAD tools that are still under development. For more information, contact Simpleware on +44 (0)1392 428 750 or visit www.simpleware.com.
Whereas standard supports (see previous section) are based on extruded lattices inside a closed 'skin', 3D lattices use an open structure that offers a number of benefits:
Heat treatment, support removal and finishing
The build platform needs to be heat treated before the parts and supports are removed. There are two reasons for this:
Age hardening of the parts, and
Stress relieving prior to removing the parts
There are three methods commonly used:
Electrical furnace (above)
Vacuum furnace, and
Hot Isostatic Pressing (HIPing).
In addition to removing the supports, DMLS parts usually require some post finishing. This can include any of the following:
CNC machining (to achieve the required tolerances)
Blasting (to improve surface finish and relieve stresses):
Shot peening: Steel & ZrO2 media
Ti: Wet blasting with Al2O3 media
Polishing (to achieve a good cosmetic finish):
Automatic (Microtek-MMP Process) To find out more about this process, visit www.firstsurface.co.uk.
Post finishing operations
Removal of supports
Supports can be very light tubular forms, like the ones shown above supporting dental copings, or heavy sections supporting large downward facing surfaces.
They are usually removed either by wire cutting - where there is clear path for the wire - or machined off, either by CNC equipment or manually.
The best supports are those that can be removed easily by the same wire cutting process that is used for removing the parts themselves.
One of the most exciting aspects of DMLS is its ability to produce geometry that is not limited by normal engineering manufacturing processes.
This means that it can be used to 'grow' lattice structures that can support the build process and/or make a part far stiffer for a very low increase in weight.
Lattice structures can be used effectively to support complex geometry during the build process. Because it is an open structure, lattice supports allow easy removal of the waste powder, and use less material than conventional supports.
If lattice support structures are placed inside complex geometrical parts, the part weight can be reduced whilst the strength and stiffness of the part is increased. However, these benefits need to be weighed against an increase in build time due to the complex structure formed by the lattice.
The very high temperatures involved in the DMLS process can cause significant stresses to build up in parts as they build.
These stresses can result in:
Delamination of the layers
Cracks in the part
Warpage during post finishing
These problems can be minimised or avoided by shot peening the parts to relieve stresses and/or heat treating prior to removing the parts from the build platform.
Strong support structures can also minimise the build up of stresses in the part.
Threads can be formed directly into parts, depending on the size of the thread and the orientation.
Threaded areas should always be vertical, and ideally have sufficient clearance around the thread to allow a tap or die to be used to ensure that it is clean.
Smaller threaded areas should be left off the CAD file, and post-machined (Drilled and tapped or thread milled).
Wall thicknesses are somewhat material dependent, but as a rule of thumb, wall sections should not fall below 1mm.
Very thin wall sections - or placing a thin section against a thick section - may result in significant distortion due to the very high temperatures involved in the process.
Fine detail is possible, however, particularly in the vertical plane. The illustration on the right shows a section of a pipe with a wall section of 0.9mm with a hole running through it of 0.4mm diameter.
0.9mm wall thickness
Energy efficient DMLS parts:
Funded by the:
A practical guide for designers
This guide is part of the SAVING project, aimed at finding ways of reducing energy consumption through the application of additive layer manufacturing techniques.
Opening for belt to be tensioned
The final design uses round holes to reduce weight and a single support in the centre of the hole in the base that allows the belt tensioner to be be built as part of the assembly.
The single support is removed by wire cutting as the same time as the part is removed from the build platform to minimise energy use.
Conventional belt buckle
The build orientation of the parts made it possible to almost eliminate supports. The wire cutting path is shown - note how it is used to create some of the geometry.
The efficiency of heat sinks is affected by material, surface area and air flow. The experimental heat sinks shown above explore different methods of increasing surface area within the design rules of DMLS - avoiding supports and post-machining.
This 'tree' form of heat sink builds well from the base and maintains the 8:1 ration of thickness to height.
This heat sink failed during the build process, probably because the individual fingers get thinner, and meant that they did not meet the 8:1 rule.
The downward facing surfaces of this design were resolved by curving the surfaces to arches, which are self supporting. This created a series of 'chimneys' in the heat sink.
This heat sink is a series of concentric cylinders with perforations to increase surface and air flow. The arched tops of the holes allow them to be built without supports.
This heat sink is similar to the other 'chimney' version, with archways to allow the part to be built without supports.
This example illustrates the realities of modifying an existing product to make it suitable for production using DMLS.
The original product was machined from a billet of aluminium.
There are a number of obvious features in the original parts that make them unsuitable for cost-effective production using DMLS.
Downward facing horizontal surfaces
When starting to re-design the part, the first thing to decide is the best orientation for the build and layout of the cut plane.
Direction of build
The next step is to chamfer, where possible, all downward facing horizontal surfaces to allow them to build without supports.
In some cases, these surfaces will still need some supports or post-machining to achieve the original design intent.
These chamfers will need to be machined out to achieve the intended design
This surface will still need supports, or it will collapse during the build process
Significant design changes may be necessary to make the parts suitable for production using DMLS. In this case the rear half of the casing needed the lids combining on one side, and the open positions reversed, to create the right orientation for the main body.
The DMLS process requires a considerable amount of energy, particularly if the costs of primary material extraction and processing are taken into consideration.
However, considerable energy savings are possible if the process is used to create parts that reduce CO2 emissions as a result of their long term use. Most of these applications involve reduced weight. Any part that accelerates, for example, will save energy if its weight is reduced. This is particularly true of any part that is used in Aerospace applications, especially if the part is used many times. Another area is improved thermal efficiency, particularly of parts like heat sinks, etc.
The way DMLS parts are designed can also have a significant effect on their energy use, particularly in terms of the way the part is built within the process, the amount of post-machining that is required and how much waste is generated.
These issues will all be examined in the following sections.
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