Whitepaper of SLM (selective laser melting)

How does SLM work?

SLM (selective laser melting) is also called direct metal laser melting (DMLM). SLM is very similar to DMLS, and both processes are covered under the metal powder bed fusion umbrella. The laser melts the powder together, layer-by-layer, until the model is complete.

On a molecular level, the powder is melted together, resulting in a homogeneous part. Most commonly the printing materials are “pure” materials, for example, titanium. However, alloys are also used.
Although the powder bed lends support to the print while printing, often, because of the weight of the material, SLM requires support structures to be added to any overhanging features.

SLM materials and post treatment

The most useful metal 3D printing materials offer manufacturers the greatest value-add compared to traditional manufacturing methods. Oftentimes this stems from machinability. Hard-to-machine materials like tool steels and titanium are traditionally very tough to work with, but because machinability does not equate to printability, these metals can be made on a 3D printer with minimal labor at a fixed, low cost per part.

Let’s review some of the metal 3D printing materials available in IN3DTEC , and the pros and cons that each material brings to the manufacturing process.

Aluminum (AISi10Mg)
AISi10Mg is the most common aluminum alloy and features solid strength, hardness, and dynamic properties. Its light weight also supports good thermal properties and has strong buildability for use in challenge geometries. Uses include housings, ductwork, engine parts, and production tools.

Stainless steel (316L/17-4 PH)
Stainless steel is characterized by high strength and excellent corrosion resistance. This material is used across a vast range of industries and applications from manufacturing to assistive technology. Examples of 3D printed stainless steels include the extremely corrosion resistant 316L and the heat treatable 17-4 PH Stainless Steel.

Stainless steel 316L is a workhorse material used for manufacturing acid and corrosion resistant parts. Select 316L when stainless steel flexibility is needed; 316L is a more malleable material compared to 17-4 PH. Final parts built in 316L receive stress relief application.

Stainless Steel 17-4 PH is a precipitation hardened stainless steel that is known for its hardness and corrosion resistance. If needing a stainless steel option, select 17-4 PH for its significantly higher tensile strength and yield strength, but recognize that it has far less elongation at break than 316L. Final parts built 17-4 PH receive vacuum solution heat treatment as well as H900 aging.

Tool steel
As the name suggests, this class of steels is used for a variety of manufacturing tooling. Anything on a production line that cuts, stamps, molds, or forms is probably made out of tool steel. Tool steels can withstand such harsh conditions because of their high hardness, and excellent high heat and abrasion resistance. Because of these properties, tool steels are very difficult and expensive to machine, making them an ideal candidates to be 3D printed. Popular powders and filaments include 18Ni300

Titanium 64
This metal is strong, incredibly lightweight, and heat and chemical resistant. Normally, titanium is extremely challenging to machine (contributing to its high cost), making it a great candidate for 3D printing. The most common 3D printed titanium is Titanium 64 (Ti-6Al-4V) and is used in situations when a very high strength to weight ratio is beneficial, such as aircraft.

Nickel Alloy IN718
Parts show good tensile, fatigue, creep and rupture strength at temperatures up to 700°C making it ideal for many high temperature applications such as gas turbine parts, instrumentation parts, power and process industry parts, etc.

Post treatment

As-Printed Finish
Surface finish in the “as printed” state with no secondary ops.

Bead blasting
Finishing process to smooth surface without affecting tolerances.

Polishing
Mirror finish with a unique production polishing process. Ideal for aesthetic and functional requirements

Vibratory Finish
Machining process with an undefined cutting edge. The goal is to improve surface quality for small parts. This is done by rounding edges, smoothing processes, and grinding.

Design guideline

Support structures

Optimizing the build orientation of your part is critical to get the best build quality and pricing.

The correct build orientation helps minimize support structures, reduce the build time, improve surface finish and speed post build machining. Designing to reduce the need for support structures gives you better parts at a lower price.

Overhangs

Overhanging sections of your part may need support structures. Inclination angles of:
Less than 40˚ will need support structures.
Between 40˚ and 60˚ may need support.
Over 60˚ should not need supporting.
Overhangs, especially 90˚ should be avoided.

Support structures may be added to the base of the part to connect it to the build plate, geometry depending.

Managing Overhangs.

Support structures can be eliminated from overhanging features with the use of curves or chamfers:

Inner channels and holes

The ideal shape for inner channels is influenced by the need to avoid support structures. Tear-Drop shapes are ideal as much they minimize overhang and are self-supporting.

Holes. Post build drilling

Where holes are required in your design, design them undersized so they can be drilled as part of the post build processing. Drilling assures the accuracy and roundness of critical hole features.

Reduce volume

Lattice like structures used for strength while avoiding solid volume

Wall Thicknesses

Generally wall thicknesses of ≥ 0.5mm are possible but is dependent on material and part geometry. Horizontal walls should be ≥ 1mm. The thermal conductivity of your selected alloy influences the minimum wall thickness achievable.

Comparison between 3D printing and CNC machining

The only technology worth comparing with traditional CNC machining is metal 3D printing. The most popular techniques currently employed are SLS (Selective Laser Sintering) and SLM (Selective Laser Melting). These techniques use a form of metal powder that is placed layer by layer. Then a laser traces out the pattern of the part cross section at that layer height, fusing the powder together. See video above. Thereafter, another thin powder layer is placed and the process repeats.

Let’s compare some of the key factors of CNC machining and 3D printing:

1. Cost
Cost is usually the most important factor when deciding on how a part is to be manufactured. The costs are generally comparable when looking at prototypes. However, the per unit cost for 3D printing is always the same, so it quickly loses out to CNC when the quantity of parts increases. CNC parts become considerably cheaper as the number of units increase making this method ideal for larger production runs.

2. Speed
3D printing is significantly slower than CNC machining which gets faster with each new development. There is no way 3D printing can currently compete with the speed of a CNC machine. This then directly affects the cost for the customer.

3. Set-up Time
The set-up time for each of these machines is also significant. With CNC machining, the CAD part needs to be prepared for the machine. This requires the expertise of a CAM (Computer Aided Manufacturing) expert to set up the required tools and tool paths. With AM, the CAD part is imported into a slicer program which is like CAM, but requires a lot less expertise from the operator as the software does all the work. The operator just decides on the optimal orientation of the part on the build platform.

Once the gCode is sent to the 3D printer, the operator is no longer needed until the part is complete. Upon completion, the operator cleans all the unused powder from the part. In a normal CNC machine, the operator might have to turn the part over and restart the machine so that the underside can be machined.

4. Design Flexibility
When it comes to design flexibility, AM has a sizeable advantage as many features that CNC machines struggle with are trivial on an AM machine. Some examples are undercuts, multi axis features such as holes that are off the primary axis, and hollow parts with a lightweight honeycomb internal structure. AM may produce prototype parts that look amazing and futuristic, but the reality is that these prototypes cannot be cost effectively mass produced, and therefore can only serve industries that require complex and low production volume parts. This is often the case within the aerospace industry.

5. Tolerances
If accurate parts are needed, then CNC machining is the only option, as they can produce parts with extremely high tolerances. The general tolerance of a CNC machine is about 0.1mm for quick prototypes, and can be pushed to 0.005mm for high precision work. A 3D printer is typically capable of a 0.1mm tolerance.

Furthermore, if a polished surface finish is necessary then AM parts will not be suitable as they have a relatively rough finish due to the powder particles that are used to make up the parts. On less expensive machines the individual layers can be seen, further ruining the appearance. AM parts often need post-processing, and a small amount of CNC machining is typically required to bring near-netshape parts into tight tolerances for applications that require them. Whereas, CNC machines can produce high quality finishes with very low surface roughness values.

Conclusion
As with CNC machining when it was at its infancy, 3D printing will continue to become more relevant as a manufacturing technique. Advances in AM techniques are being made almost daily. Costs will continue to drop, and speeds will increase. There are currently technologies being developed that can print large batches of parts by using a metal infused resin that is then placed in a furnace that turns them into usable components.

However, CNC machines will continue to dominate the manufacturing scene for the foreseeable future, and despite its few disadvantages when compared to AM, it is a mature, widespread technology that you can bank on. When it comes to a cost effective, volume manufacture with tight tolerances and a good surface finish, then CNC is still the best solution hands down.