What is DMLS: Process, Benefits & Applications
DMLS is the most commonly used process of Metal 3D Printing due to the multiple benefits it offers for a wide range of applications. Although Metal 3D Printing or Metal Additive Manufacturing produces parts by adding layers of metal on top of one another to achieve the desired shape, there are different techniques in Metal AM depending on how each layer is printed and how the bonding is achieved.
While Binder Jetting uses a binding agent to bond metal particles together and EBM uses an electron beam to melt the metal powder, Direct Metal Laser Sintering (DMLS) uses a laser to melt the metal powder resulting in instant solidification. The flexibility to manufacture complex shapes with better consistency makes DMLS the most preferred technique for Metal 3D Printing. DMLS is also known by other names such as Laser Powder Bed Fusion (LPBF), Direct Metal Printing (DMP) and Selective Laser Melting (SLM).
Let us now understand in detail how DMLS works, its advantages, limitations and applications.
DMLS Metal 3D Printing Process
It is a common misconception that 3D printing a part doesn’t take much effort and is accomplished by just switching on the 3D printer and pressing the start button. In reality, there is a lot that goes into getting a good printed part. This includes processing the input data (Pre-processing), printing the part (3D Printing) and finishing the part (Post Processing). Let us now look into each of these steps in detail.
First, the designed 3D CAD model is imported into a data preparation software. As the tolerances achieved in DMLS is around +/- 50 microns, if tighter tolerances are required, necessary stock for machining is added to the CAD model.
Support structures are generated and the model is oriented for favourable factors such as better surface finish, faster build time and easier post-processing. The thumb rule is that surfaces below 35 degrees to the XY-plane need support beneath them to get printed. Supports help in dissipating heat away from the part and also anchors the part to the base. Teeth are provided in the supports where the structures touch the part, so that the supports can be easily removed during post-processing.
The part along with supports is analysed to check the max deviation, stress build-up and possible recoater crashes. If the simulated results are not satisfactory, either the orientation is changed or the supports are reinforced.
Post successful simulation, The part and the supports are sliced into numerous layers, each 40 microns in thickness (could go up to 100 microns). Each slice contains specific details in the form of vectors which the laser follows while printing.
The slice file is then fed to the machine and the process parameters such as laser speed, wattage, hatch distance etc. are set respective to the material and layer thickness being used.
The build chamber inside the 3D printer contains 3 units next to one another. The Build platform unit on which the parts are printed, Dispenser unit which stores the powder and dispenses it onto the build plate and the Collector unit which stores the leftover powder after recoating.
A metal base plate, preferably same material as the metal powder, is mounted on the build platform. The base plate is preheated to a certain temperature and a thin layer of powder (size less than 50 microns in diameter) is coated on the base plate by a recoater blade.
Recoater blades could be made of either steel, ceramic, carbon fibre or rubber. Once the door is closed, inert gas is flooded in the chamber reducing the oxygen content to less than 0.01%. This is done to minimise oxidation at elevated temperatures during printing.
The laser beam diameter is around 100 microns and its movement is controlled by 2 deflecting scanner mirrors. The laser with a wattage of 400W/1000W hits the metal powder melting the powder, resulting in instant solidification. The laser moves as per the 2D vector data melting powder on its way, until it covers every contour on that particular layer.
Once a particular layer is printed, the build platform unit moves down by a layer thickness while the dispenser moves up and the recoater spreads a new layer of powder onto the build platform.
Laser again prints this new layer bonding the current layer to the previous layer. The build platform again moves down and the process keeps repeating until the final layer is printed.
Once the part is printed, the surrounding powder is removed and the part is unloaded from the machine. The part is then depowdered to remove the trapped powder inside the complex channels of the part. If the part is not depowdered, the trapped powder will solidify during heat treatment causing functionality issues.
Part is then subjected to heat treatment in an inert or vacuum atmosphere to relieve the internal stresses and improve the mechanical properties. Post heat treatment, the strength of additive parts is equivalent to that of wrought parts.
The part is then cut off from the build plate though Wire EDM or bandsaw. Supports are then removed either by manual chiselling or machining. If finer tolerances are required and region is accessible, machining is carried out. The part is then subjected to shot blasting or electropolishing to improve the surface finish.
Advantages of DMLS Metal 3D Printing
As there is no tooling or fixturing involved, even multiple parts with minor changes in design can be printed in one go. Also, all possible variations of a certain design can be printed at once to decide the best functional design among them.
Assembly of multiple components can be redesigned as a single component and then printed, which increases the life cycle of the part and reduces the final weight.
Improved Functional Performance
As Additive parts are designed for functionality rather than designed for manufacturability, the functional performance of a component is improved.
The process allows for printing parts with topology optimized designs and inclusion of lattice structures, thereby reducing the part weight while maintaining the required strength.
Improved freedom of design
Unlike subtractive manufacturing where parts need to follow stringent design rules, additive manufacturing allows almost any design to be printed.
The general build volumes are 250x250x300mm and 400mm cube. If parts are bigger than the above size, they need to be cut into sections, individually printed and then welded, which increases the final cost significantly.
Currently, only 8 to 10 different alloys are available for Direct Metal Laser Sintering. New material development could take anywhere between 6 months to a year.
Surface Finish and Geometric Tolerances
Since the size powder particles is around 50 microns in diameter, the tolerances achieved in DMLS parts is +/-50 microns for smaller parts and */-0.2% of the measured dimension for bigger parts. Further machining is to be carried out if finer tolerances are required. The surface finish of a printed DMLS part is around 8 microns Ra. Further surface finishing operations are required to improve this finish.
Some common applications of DMLS
Waveguides are used to transmit electromagnetic or sound waves with minimal loss of energy. Waveguides which were earlier machined and fastened together are now being printed as a single piece. This drastically reduces the manufacturing lead time.
Patient specific implants are manufactured using 3D printing in a biocompatible material such as titanium or cobalt chrome. Custom implants reduce the probability of a re-surgery and makes the surgery less complex.
Die and Mould Inserts
The inserts used in Dies of Metal Injection Moulding, Plastic Injection Moulding or Pressure Die Casting can employ conformal cooling channels which provide faster and better cooling thereby reducing the cycle time significantly.
The combustion chamber of rocket engines made using 3D printing have regenerative cooling channels which provide better efficiency. Manufacturing these channels through any other method is not possible.
In conclusion, Direct Metal Laser Sintering provides repeatability of quality metal 3d printed parts, as result of which, a lot of research is being carried out by both industry and academic organisations to improve the process.
New materials are being developed for different applications. Machines with twin and quad lasers are being introduced improving the productivity significantly. Bigger machines might also come out in the near future considering the rate at which people are embracing this technology.
At Rapid DMLS, we provide engineering solutions through Direct Metal Laser Sintering. Backed with the best infrastructure and minds, we handle the complete cycle involved in the development of metal additive parts. We support you right from the pre-processing stage until the part is ready to use. To get in touch for your Metal 3D Printing requirements, visit www.rapiddmls.com or contact us at firstname.lastname@example.org.