SLM or Selective Laser Melting Technology : Explained Briefly

Selective Laser Melting or Direct Laser Metal Melting is a rapid additive manufacturing process which uses high power laser beam as power source to melt and bind metallic powder or powdered alloy layer-by-layer to form a 3-Dimensional object.

SLM is similar to SLS 3D printing and both the processes are grouped under Powder Bed Fusion technology. The major difference between the two is SLS 3D printer uses Nylon polymer materials and SLM 3D printer uses metals and alloy materials.

SLM additive manufacturing was first introduced in 1995 at the Fraunhofer Institute ILT in Aachen, Germany as a part of German research project. The ASTM International F42 standards committee has grouped SLM technology under “Laser Sintering” umbrella which is inappropriate because the SLM technology completely melts the metal powder to form 3D object.

The first SLM 3D printer was commercialized in Karlsruhe, Germany in 1999 by the SLM Solutions. In developing this technology many companies were involved and the SLM Solutions is a by-product of these companies.

People also misunderstand SLM technology with the DLMS (Direct Laser Metal Sintering) technology which is a name trademarked under EOS brand. Both the SLM and DLMS technology uses metal and alloy powders as material to make 3D model. 

But the major difference between the two is SLM technology melts the metal powders and binds them whereas in DLMS technology is similar to SLS technology which heats the powder metal to the point that they fuse together on a molecular level.

SLM 3D Printing Process

Pre-Printing Process

Every 3D Printing process starts by developing the 3D model in the 3D design software called the CAD (Computer Aided Design) file. This file is then converted into .STL file also known as Stereolithography or Surface Tessellation Language file.

This STL file is then repaired using software like Materialise Magics or Autodesk Netfabb which is recommended in every 3D Printing processes. For example, in some cases the file is damaged or have empty shells than its recommended to repair or fill the empty part of the file else that part will not get printed in the 3D printer and there are 90% chance of printing failure.

Unlike SLS technology, here it is mandatory to add supports to any overhang structure in SLM technology due to the material weight and minimize the distortion caused by the high temperatures required to fuse the metal particles. This is one of the major things that differs SLS and SLM, where the unsintered powder material is used as support and hold the overhang structure.

The STL file is then uploaded into the printer program where it is cut into slices as per the micron (usually between 20 to 100 micrometers) we set. For example, if we set 50 microns in printing parameter than the program will divide 1 mm height into 20 slices, which means the thickness of each slice will be 0.05 mm.

SLM 3D Printing Process

Schematic Diagram of SLM 3D Printing

The SLM 3D printing is a Powder Bed Fusion technology which uses high powered Ytterbium Fiber Laser of 200 watts to melt small granules of raw materials like copper, aluminium, stainless steel, tool steel, cobalt chrome, titanium and tungsten into a 3D model. The Yb-fiber laser beam selectively melts the raw material layer by layer according to the geometry of 2D slices.

SLM 3D printing starts by filling the powder delivery chamber with raw material or material in powder form. The chamber is then filled with inert gases like argon or nitrogen at oxygen levels below 500 part per million making it a controlled atmosphere suitable for melting metal powder materials.

The roller spreads the powder material on the built platform. The laser is focused on the scanner which contains scanning mirrors fixed with galvanometer which guides the laser to trace the 2D geometry of the slice onto the powder bed and perform the melting process. The small granules of material on the platform fuses with each other as the temperature is raised by the laser.

The scanned part gets solidify and gains mechanical strength. Based on the micron configuration set by the operator the built platform moves one layer downward and the roller disperse the powder onto the built platform for laser to solidify the next cross section of the layer.

The whole process is repeated for each layer until the whole 3D part is printed. Once the printing process is completed the operator removes platform and transfer it to the post processing area.

Post Processing SLM Parts

After the printing process is done the 3d model needs to cool down before transferring built plate to the cleaning station for post processing the built part. As we know that parts made by SLM 3D printing needs supports due to the material weight so, first the extra powder material is removed with the help of compressed air or hand brush.

Powder Removal Process. Image source: 3DHUBS

After the powder removal process the supports are removed with the help of band-saw, files and other mechanical instruments. The support material and part material are same so removing supports will be difficult for the operator as he has to be very careful to not damage the 3D model.

After removing supports different post processing like thermal annealing or heat treatment is done to improve mechanical properties, CNC machining for achieving dimensional accuracy, media blasting, metal plating, and polishing are used to improve surface quality and fatigue strength of the printed metal part.

Common SLM Materials

SLM technology uses various metals as their material for making 3D model. Some of the materials are copper, aluminium, stainless steel, tool steel, cobalt chrome, titanium and tungsten. Metals with high melting point and high ductile and brittle transition temperature can be used in SLM technology.

These materials are used in almost all the industries for manufacturing various parts including aerospace, defense and medical industry. SLM technology also uses precious metals like gold, silver, platinum, and palladium, but the cost of powder material is very high and mostly these metals are used in jewelry manufacturing.

Materials	Characteristics
Aluminium Alloys	+ Good mechanical and thermal properties + Good electrical conductivity + Low density
Stainless Steel	+ High wear resistance + Great hardness + Good ductility and weldability
Titanium Alloys	+ Corrosion resistance + Excellent strength-to-weight ratio + Low thermal expansion + Biocompatible
Cobalt-Chrome  Super Alloys	+ Excellent wear and corrosion resistance + High hardness + Biocompatible + Good properties at high temperature
Nickel Super Alloys	+ Good mechanical properties + High corrosion resistance + Temperature resistance upto 1200℃ + Used in extreme environment
Precious Metals	+ Used in jewellery making

Some More Information

The Selective Laser Melting is very useful for those who want rapid manufacturing of small parts in metal or metal alloys. Additive manufacturing can build complicated designs with high accuracy and smooth surface and with the SLM technology this can also be possible in metal parts.

With SLM technology one can make objects with intricate geometries having thin walls which will make the object lightweight. SLM uses almost all types of metal powders including copper, aluminium, stainless steel, tool steel, cobalt chrome, titanium and tungsten which gives good mechanical properties so that it can handle heavy stress and loads.

Complex Geometry by SLM Process. Image source: Forecast3D

SLM technology is only useful for small batch production companies as the melting process takes time and after printing the 3d model take time for cooling.

The ability to 3D print metal, SLM technology is widely used in motor parts like rotors, impellers, cooling channels. Benefits of metal printing is encouraging the economical industries like aerospace, automotive, research, dental and medical to innovate technologies.

SpaceX CEO Elon Musk shared an image of SpaceX's regeneratively-cooled SuperDraco rocket engine chamber manufactured in EOS 3D metal printer on September 5, 2013. The rocket engine was made with 'Inconel Superalloy' which is an alloy of nickel and iron, later in May 2014 SpaceX announced the first qualified fully 3D metal printed engine rocket named SuperDraco. 

French automobile company Bugatti was the first to test titanium 3D printed brake caliper in its most fastest and most powerful super sports car Bugatti Chiron which is the successor to the Bugatti Veyron.

Limitations

SLM 3D printing is an expensive process due to the use of metal powders and the power source to melt the powder. So, it is preferable to use this technology only if the parts are difficult to manufacture by traditional method and has complex geometry.

Operator should be skilled and has good knowledge of designing and manufacturing skills. As the SLM process requires support structure, one should know how to cut the supports so that it do not affect the main part.

Parts made by SLM process requires lots of post processing to achieve mechanical properties which degrades overtime. Lots of polishing and finishing process is needed for smooth or mirror surface finish.

Limited size of built platform because of the need of controlled manufacturing environment which would be difficult to achieve in case of bigger built volume. Dimensional error in Z axis can be found in some cases.