What is Selective Laser Melting? How it works?
Selective Laser Melting (SLM) is an additive manufacturing process that is used to produce metal parts. It was first started in 1995 at the Fraunhofer Institute ILT in Germany. This 3D printing method is very similar to Selective Laser Sintering (SLS) as both methods create parts layer-by-layer and fuse them, with the most significant difference being the material used. SLS require polymer powder where as SLM use metal powder. Understanding any one of these methods will make understanding the other fairly simple. Many consider SLM to be a sub-category of SLS, which is quite reasonable.
Another 3D printing method, the Direct Metal Laser Sintering (DMLS), works on the same principle. DMLS uses alloy powder to form layers, which is the only significant difference between DMLS and SLM. Essentially SLS, SLM and DMLS work on the same principle and process with the only significant difference between them being the material/powder used. Let us explore these methods and understand the processes used to make a finished metal part.
Note: The terms SLM and DMLS can be used interchangeably in this article unless stated otherwise.
SLM is a type of Power Bed Fusion in which layers of powder material are fused together using a laser or other type of concentrated energy source. This process largely differs from other additive manufacturing methods such as FDM, SLA or PolyJet. SLM uses a high-power laser to melt and fuse the metal layers together and create the required part.
The first step in any 3D printing method is to create a CAD model of the desired part. Once the CAD model is completed, it is entered in the 3D printing software which further process the CAD file accordingly. In SLM, the CAD model is divided into multiple layers which instruct the laser how each and every layer is going to be printed. The thickness of these layers will also determine the resolution and accuracy of the part, which is discussed below.
SLM printers generally consists of a Laser Scan System, Re-coater, Build Platform and Powder Bin. The part is printed from bottom to top on the build platform. The printing process starts with the build chamber, inside which the printing takes place, being filled with chemically inert gas such as Argon. This is done to reduce oxidation of metals as metals tend to get oxidized easily at higher temperatures. Then the build platform, on which the part will be sintered, is heated to the required temperature before the laser starts printing. As the optimal temperature is achieved, the re-coater moves across the powder bin and spreads a thin, uniform layer of powder on the build platform, usually 20-100 microns.
As the thin layer of powder is spread across the build platform, the laser selectively melts the powder according to the required design. When printing of the layer is completed, the build platform moves down by exactly one-layer thickness and the re-coater moves across the powder bin, spreading another layer of powder on the build platform. The laser sinters the layer accordingly and the build platform moves down again. This process is repeated till the entire part is printed.
After the printing is completed, the part is encapsulated in the unfused powder. The powder bin and build platform are left to cool down before removing the part.
In SLM, some post processing is a required as the part is encapsulated in unfused powder. The parts are also printed with support structures, unlike SLS which doesn’t require any support structures. Once the build platform has cooled down to room temperature, the excess powder is removed using brushes or compressed air. Then the support structures are removed using metal cutters etc. After this, the part is ready for use or other post processing, as per the requirement. The unfused metal powder is highly recyclable with wastage of less than 10%, unlike polymer powders used in SLS which aren’t highly recyclable.
As mentioned before, the resolution of SLM printed vary from 20 to 100 microns. SLM parts have almost isotropic thermal mechanical properties throughout. The printed parts have high strength and quite hard as they are metal.
The need of support structures in SLM is due to the fact that metal lose their shape very easily when melted which may lead to deformation of the part. Metal 3D printing require considerably high temperatures, which get very close to melting point of the metal powder used which makes support structures necessary. They also act as heat sinks, drawing away heat from the part allowing it to cool down faster.
Metal 3D printing is also compatible with high strength materials such as Aluminium Alloys, Stainless Steel, Titanium Alloys, Nickel Superalloys, Precious Metals, etc.
SLM and DMLS can be used to manufacture complex parts with geometries with high accuracy, which is not possible with the traditional printing methods. The parts created also have good physical properties which make them both reliable and durable.
One of the most significant drawback of metal 3D printing the need for support structures. They increase the cost of printing considerably as support structures are of no use once removed from the printed part and is considered as waste product. Another drawback is its limited build size as printing larger parts tend to have a negative effect on the accuracy and precision of the parts.
SLM and DMLS are allows users to print complex designs and geometries using various metals and alloys. This provides high strength and durability which is always desirable. Metal 3D printing opens up opportunities for people to manufacture parts which would not be possible with the traditional methods. Although the cost of printing can get high, it remains the most used metal 3D printing in the market.
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