How Direct Metal Laser Melting (DMLM) Works
What Is Direct Metal Laser Melting?
Direct metal laser melting (DMLM) is an additive manufacturing process that uses lasers to melt ultra-thin layers of metal powder to build a three-dimensional object. Objects are built directly from an .stl file generated from CAD (computer-aided design) data. The use of a laser to selectively melt thin layers of tiny particles yields objects exhibiting fine, dense and homogeneous characteristics.
The DMLM process begins with a roller spreading a thin layer of metal powder on the print bed. Next, an .stl file directs a laser to create a cross-section of the object by completely melting metal particles. The print bed is then lowered so the process can be repeated to create the next object layer. After all layers are printed, the excess unmelted powder is brushed, blown or blasted away. The object typically requires little, if any, finishing.
DMLM Vs. DMLS
The direct metal laser sintering (DMLS) process uses lasers to partially melt particles so they adhere to one another. The DMLM process is very similar, except that the material is completely melted to create ultra-thin liquid pools, which solidify as they cool.
The term “DMLS” is often used to refer to both processes, although the term “DMLM” is gradually emerging as the preferred way to reference the process when complete melting occurs.
The power and precision of lasers used in the DMLM process make it possible to use extremely durable metals delivered as extremely fine powders. Machines using direct metal laser melting craft elaborate, yet super-tough parts used in demanding applications in aerospace, auto racing and petrochemicals.
Titanium is one of the most popular materials used in the direct metal laser melting process. Titanium parts endure high pressures and temperature extremes. Parts fabricated by direct metal laser melting machines are valuable where a quick turnaround of limited product runs is a strategic advantage.
Since stainless steel is known for its strength, toughness and ductility, it is frequently used to print functional prototypes and production parts. When low-carbon content is required, 316L stainless steel is an option. It is a tough, ductile, weldable compound that is highly resistant to pitting and corrosion. Maraging steel is used to create internal conformal cooling channels important in injection molding. This tooling steel is readily machined and easily polished in post-processing.
Inconel 718 is a superalloy with properties required in rocket and jet engines. Its heat and corrosion resistance also makes it ideal for use in various chemical industry applications. Cobalt chrome is another superalloy that offers high-temperature resistance and toughness. It is also used in the printing of turbine and engine parts.
Aluminum alloys possess excellent fusion characteristics that are important in additive manufacturing. DMLM is used to create hard aluminum objects capable of handling significant loads. Highly machinable aluminum components are used in automotive, racing and thermal applications.
Selective Laser Melting (SLM) & Selective Laser Sintering (SLS)
SLM and SLS are two AM processes differentiated by the degree to which materials are melted. SLM involves the full melting of material, while SLS involves sintering (partially melting) material. In both instances, the term “selective” refers to the precise melting of ultra-thin layers of build material.
SLS is a lower-temperature process than SLM, although SLS still produces parts with dimensional accuracy and complex geometries. Support structures are not required during printing. SLS uses either single- or dual-component powders. When using the latter substance, lasers melt away the outer layer, and the inner material fuses with adjacent particles.
With SLS, it is possible to reduce shrinking and warping by heating the build chamber to a temperature just below that required for sintering powdered metal alloys, plastics, glass and ceramics. Surface porosity commonly associated with sintering is addressed with the application of a sealant.
Since selective laser melting (SLM) requires complete melting at very high temperatures, object distortions and stresses are more of an issue. However, full melting minimizes porosity.
Stresses introduced by the high-temperature SLM process make it vital to keep the object firmly secured to the print bed during printing. A heated build chamber combined with proper support structures helps to minimize distortion. Post-processing heat treatment while the object is still on the print bed also reduces internal stresses. The SLM process uses atomized metallic powders, including titanium, tungsten, maraging steel, cobalt chrome, stainless steel, aluminum and copper.
Designers take advantage of the fact that direct metal laser melting yields intricate parts that reduce weight while retaining requisite strength and durability. Parts are often used in applications where weight reduction is vital, as in satellites, rocket thrusters and jet engines. Robotics and injection molding also benefit from the low-run, highly durable, precision components produced by direct metal laser melting.
The temperature and pressure extremes in a rocket engine make it the perfect laboratory for demonstrating the unique capabilities of direct metal laser melting. The propellant of choice in rocket engines is typically liquid hydrogen — a lightweight fuel with a high exhaust velocity and high reaction rate. However, liquid hydrogen must be stored at minus 423 degrees F. At combustion, it generates temperatures exceeding 5,500 degrees F.
High-precision DMLM parts possess exceptional surface characteristics along with mechanical properties equivalent to those found in traditional wrought materials.
Surface quality and minimal porosity are two key advantages of the direct metal laser melting process. Since it is possible to move the print bed in as little as 20-micron increments, objects exhibit a smooth surface quality that minimizes the need for post-production finishing. To put a thickness of 20 microns in perspective, consider that the diameter of a red blood cell is about five microns, and a human hair is about 75 microns thick.
The direct metal laser melting process minimizes the porosity common with sintering. In fact, it is possible to achieve close to 100 percent density. Enterprises can reuse the valuable unmelted metal powders.
Direct metal laser melting offers short lead times ideal in situations where repeated testing of functional metal prototypes is necessary. Where traditional production times are often measured weeks, the direct metal laser melting process only requires hours or days.
The DMLM process gives designers the freedom to create objects with intricate structures and significant undercuts that are usually impossible to create using conventional methods. Quicker design cycles are vitally important in the highly competitive environments common in many industries. DMLM makes possible a design-driven process with significant benefits.
For More Information
Find more information about DMLM machines from Concept Laser.