What is DMLS
DMLS is abbreviation of Direct Metal Laser Sintering, which is a metal additive manufacturing process belong to powder bed fusion 3D printing. In DMLS process, a laser system will scan and selectively fuse metal powder particles, bond them together to build parts layer-by-layer. The materials used in this process are granular form metal. These materials have variable melting points, which can fuse on molecular level at elevated temperatures. DMLS technology is applied for metal alloys production. We will describe basic mechanism of DMLS process, and its benefits and limitations in this article.
How DMLS Work
DMLS manufacturing process:
- The build chamber is filled with insert gas like argon, in order to minimize the metal powder oxidation at optimal building temperature.
- A thin layer of metal powder is spread over the build platform, then the laser system will scan the component cross-section, melting or fusing the metal powder to create a new layer. As entire model area is scanned, the final parts are fully solid.
- Once a scanning process is complete, the build platform will move downwards in one layer thickness, then re-coating another thin layer of metal powder. This process is repeated until whole part is complete.
Once this manufacturing process is finished, the parts are encapsulated in metal powder. The support structure is created with same material as parts, it is required to avoid warping and distortion in high processing temperature.
Then the build platform will cool to room temperature, the remaining metal power will be removed manually. Printed parts one build platform will need heat treatment to relieve residual stresses. Finally cutting or machining method are used to detach the final components.
DMLS printer parameters
In DMLS, almost all process parameters are set by the machine manufacturer. The layer height in DMLS varies between 20 to 50 microns, which is depended on metal powder properties like flowability, power size distribution, shapes.
The typical size of DMLS system is 250×150×150 mm, the dimension accuracy is approximately ±0.1 mm. DMLS is widely sued for small batch production, as parts are always attached to build platforms, it is restricted by X and Y direction print area.
The metal powder in DMLS is high recyclable, only less than 5% is wasted. After printing process, unsintered powder will be collected, sieved and then filled with fresh materials for next printing. The only waste is the support structure, which is crucial for successful completion of final parts. This will increase the amount of materials and cost drastically.
DMLS parts have nearly isotropic mechanical and thermal properties, there are very little internal porosity on solid surface, the porosity is less than 0.2 to 0.5% in as-sintered state. After thermal processing, this will reduce to nearly zero. DMLS parts have superior mechanical properties and higher hardness, but they are more prone to fatigue. Due to metal powder have granular form, DMLS parts surface roughness is approximate 6 to 10 microns. This also determines the lower fatigue strength of final parts.
Support structure & Part orientation
Support structure is required in DMLS process, in reason of high processing temperature.
DMLS support structure functions:
- Provide suitable platform for the next layer.
- Anchor parts to build plate, in order to prevent warping.
- Draw away heat from printed parts like heat sinks, control cooling rate at suitable speed.
DMLS parts should be oriented with angle to minimize warping likelihood and maximize strength in critical directions. There is no doubt that, this will increase printing time, material waste and total cost. Warping also can be minimized by randomized scan patterns, this will prevent residual stress in any particular directions, and also add a characteristic surface texture.
As high cost of DMLS, simulations are used to predict printing behavior. Topology optimization algorithms can be used to maximize the mechanical performance and create lightweight parts, minimize support structure need and warping likelihood.
Hollow sections & Lightweight structure
DMLS cannot use large hollow sections as difficult removal of support structure. For internal channels lager than diameter of 8 mm, we recommend to apply diamond or tear-drop cross sections instead of circular, reason of these features don’t need any support structures.
Skin and cores design in DMLS is an alternative to hollow sections. This design can be processed with different laser powder and scan speed, in order to create different material properties. This process is very useful for large solid section manufacturing, it will reduce printing time and warping likelihood, and produce parts with high stability and excellent surface quality.
Lattice structure is a common strategy in DMLS to reduce parts weight, topology optimization algorithms can assist the design of organic light-weight form.
Common DMLS Materials
DMLS can produce various parts form a wide range of metal alloys, including aluminum, stainless steel, titanium, cobalt chrome. These materials will satisfy most industrial application requirement. The cost of metal powder is very high, so we need to minimize parts volume and support structure.
The key strength of DMLS is its compatibility with high strength metal alloys, like nickel or cobalt-chrome supperalloys, this alloys are difficult to manufacture with traditional methods. DMLS technology will save cost and time significantly, and can be post-processed to high surface finishes.
|Aluminium alloys||Good mechanical & thermal propertiesLow densityGood electrical conductivity||Low hardness|
|Stainless steel||High wear resistanceGreat hardnessGood ductility and weld-ability|
|Titanium alloys||Corrosion resistanceExcellent strength-to-weight ratioBio-compatible|
|Cobalt-chrome supperalloys||Excellent wear & corrosion resistanceGreat propertiesHigh hardnessBio-compatible|
|Nickel superalloys||Excellent mechanical propertiesHigh corrosion resistanceTemperature resistance at 1200℃|
|Precious metals||Jewellery application||Not widely available|
Various post processing technologies are applied to improve DMLS parts properties, accuracy and appearance. Compulsory post-processing steps include loose powder removal, support structure removal, while heat treatment like thermal annealing is used to relieve residual stress and improve mechanical properties. CNC machining can be applied for crucial features like holes or threads. Media blasting, metal plating, polishing and micro-machining will improve parts surface quality and fatigue strength.
DMLS Benefits & Limitations
- DMLS can produce custom parts with complex geometries, which traditional manufacturing method are unable to produce.
- DMLS parts can be topologically optimized to maximize performance and minimize total weight.
- DMLS parts have excellent physical properties, the available materials include superalloy metal, which is difficult to process.
- DMLS material and manufacturing cost is very high, it is not suitable for easy manufacturing with traditional methods.
- DMLS build size is limited, as precise production conditions and process control.
- Design adjustment is required to suit DMLS process.