SLM & DMLS: what is the difference?
Selective Laser Melting (SLM) and Direct Metal Laser Sintering (DMLS) are two metal additive manufacturing processes that belong to the powder bed fusion 3D printing family. The two technologies have a lot of similarities: both use a laser to scan and selectively fuse (or melt) the metal powder particles, bonding them together and building a part layer-by-layer. Also, the materials used in both processes are metals that come in a granular form.
The differences between SLM and DMLS come down to the fundamentals of the particle bonding process (and also patents): SLM uses metal powders with a single melting temperature and fully melts the particles, while in DMLS the powder is composed of materials with variable melting points that fuse on a molecular level at elevated temperatures.
SLM produces parts from a single metal, while DMLS produces parts from metal alloys.
Both SLM and DMLS are used in industrial applications to create end-use engineering products. In this article, we use the term metal 3D Printing to refer to both processes in general and we describe the basic mechanisms of the fabrication process that are necessary for engineers and designers to understand the benefits and limitations of the technology.
There are other additive manufacturing processes that can be used to produce dense metal parts, such as Electron Beam Melting (EBM) and Ultrasonic Additive Manufacturing (UAM). Their availability and applications are limited though, so they won’t be presented here.
How does Metal 3D Printing work?
The basic fabrication process for SLM and DMLS are very similar. Here is how it works:
- The build chamber is first filled with inert gas (for example argon) to minimize the oxidation of the metal powder and then it is heated to the optimal build temperature.
- A thin layer of metal powder is spread over the build platform and a high power laser scans the cross-section of the component, melting (or fusing) the metal particles together and creating the next layer. The entire area of the model is scanned, so the part is built fully solid.
- When the scanning process is complete, the build platform moves downwards by one layer thickness and the recoater spreads another thin layer of metal powder. The process is repeated until the whole part is complete.
When the build process is finished, the parts are fully encapsulated in the metal powder. Unlike polymer powder bed fusion process (such as SLS), the parts are attached to the build platform through support structures. Support in metal 3D printing is built using the same material as the part and is always required to mitigate the warping and distortion that may occur due to the high processing temperatures.
When the bin cools to room temperature, the excess powder is manually removed and the parts are typically heat treated while still attached to the build platform to relieve any residual stresses. Then the components are detached from the build plate via cutting, machining or wire EDM and are ready for use or further post-processing.
Characteristics of SLM & DMLS
In SLM and DMLS almost all process parameters are set by the machine manufacturer. The layer height used in metal 3D printing varies between 20 to 50 microns and depends on the properties of the metal powder (flowability, particle size distribution, shape etc).
The typical build size of a metal 3D printing system is 250 x 150 x 150 mm, but larger machines are also available (up to 500 x 280 x 360 mm). The dimensionally accuracy that a metal 3D printer can achieve is approximately ± 0.1 mm.
Metal printers can be used of small batch manufacturing, but the capabilities of metal 3D printing systems resemble more the batch manufacturing capabilities of FDM or SLA machines than that of SLS printers: they are restricted by the available print area (XY-direction), as the parts have to be attached to the build platform.
The metal powder in SLM and DMLS is highly recyclable: typically less than 5% is wasted. After each print, the unused powder is collected, sieved and then topped up with fresh material to the level required for the next built.
Waste in metal printing though comes in the form of support structure, which are crucial for the successful completion of a build but can increase the amount of the required material (and the cost) drastically.
Various post-processing techniques are used to improve the mechanical properties, accuracy, and appearance of the metal printed parts.
Compulsory post-processing steps include the removal of the loose powder and the support structures, while heat treatment (thermal annealing) is commonly used to relieve the residual stresses and improve the mechanical properties of the part.
CNC machining can be employed for dimensionally crucial features (such as holes or threads). Media blasting, metal plating, polishing, and micro-machining can improve the surface quality and fatigue strength of a metal printed part.
Benefits & Limitations of Metal 3D Printing
Here are the key advantages and disadvantages of metal 3D printing processes:
- Metal 3D printing processes can be used to manufacture complex, bespoke parts with geometries that traditional manufacturing methods are unable to produce.
- Metal 3D printed parts can be topologically optimized to maximize their performance while minimizing their weight and the total number of components in an assembly.
- Metal 3D printed parts have excellent physical properties and the available material range includes difficult to process otherwise materials, such as metal superalloys.
- The material and manufacturing costs connected with metal 3D printing is high, so these technologies are not suitable for parts that can be easily manufactured with traditional methods.
- The build size of the metal 3D printing systems is limited, as precise manufacturing conditions and process control are required.
- Already existing designs may not be suitable for metal 3D printing and may need to be altered.