What is metal 3D printing and how does it work?

What is metal 3D printing? The differences between SLM and DMLS

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.

Essentially, 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 is similar for both SLM and DMLS. Here's 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 the polymer powder bed fusion process (such as SLS printing or MJF), 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.

What are the characteristics of metal 3D printing? Get to know SLM & DMLS

SLM & DMLS printer parameters

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 and more).

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 dimensional accuracy that a metal 3D printer can achieve is approximately ± 0.1 mm.

Metal printers can be used for 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 build.

Waste in metal printing comes in the form of support structures, which are crucial for the successful completion of a build but can increase the amount of the required material (and the cost) drastically.

Layer adhesion

Metal SLM and DMLS parts have almost isotropic mechanical and thermal properties. They are solid with very little internal porosity (less than 0.2 - 0.5% in the as-printed state and close to none after thermal processing).

Metal printed parts have higher strength and hardness and are often more flexible than parts that are manufactured using a traditional method. However, they are more prone to fatigue.

For example, take a look at the mechanical properties of the AlSi10Mg EOS metal 3D printing alloy and the A360 die-cast alloy. These two materials have a very similar chemical composition, high in silicon and magnesium. The printed parts have superior mechanical properties and higher hardness compared to the wrought material.

Due to the granular form of the unprocessed material, the as-built surface roughness (Ra) of a metal 3D printed part is approximately 6 - 10 μm. This relatively high surface roughness can partially explain the lower fatigue strength.

Support structures & part orientation

Support structures are always required in metal printing, due to the very high processing temperature and they are usually built using a lattice pattern.

Support structures in metal 3D printing serve three different functions:

• They offer a suitable platform for the next layer to be built upon.

• They anchor the part to the build plate and they prevent warping.

• They act as heat sinks, drawing heat away from the part and allowing it to cool at a more controlled rate.

Parts are often oriented at an angle to minimize the likelihood of warping and maximize part strength in critical directions. However, this will increase the amount of required support, the build time, the material waste and (ultimately) the total cost.

Warping can also be minimized using randomized scan patterns. This scanning strategy prevents the buildup of residual stresses in any particular direction and will add a characteristic surface texture to the part.

Since the cost of metal printing is very high, simulations are often used to predict the behavior of the part during processing. Topology optimization algorithms are also used not only to maximize mechanical performance and create lightweight parts but also to minimize the need for support structure and the likelihood of warping.

Hollow sections & lightweight structures

Unlike polymer powder bed fusion processes like SLS, large hollow sections are not commonly used in metal printing as support structures cannot be easily removed.

For internal channels larger than Ø 8 mm, we recommend using diamond or tear-drop cross sections instead of circular ones, as they require no support structures. More design guidelines on SLM & DMLS can be found in this article.

As an alternative to hollow sections, parts can be designed with skin and cores. Skin and cores are processed using different laser power and scan speed, resulting in different material properties. Using skin and cores is very useful when manufacturing parts with large solid sections, as they significantly reduce the print time and the likelihood of warping and produce parts with high stability and excellent surface quality.

Using a lattice structure is also a common strategy in metal 3D printing for reducing the weight of a part. Topology optimization algorithms can also aid in the design of organic, lightweight forms.

What are the common materials for 3D metal printing?

SLM and DMLS can produce parts from a large range of metals and metal alloys including aluminum, stainless steel, titanium, cobalt chrome and Inconel. These materials cover the needs of most industrial applications, from aerospace to medical. Precious metals, such as gold, platinum, palladium and silver can also be processed, but their applications are mainly limited to jewelry making.

Powder for metal 3D printing is, as you’d imagine, very expensive. For example, a kilogram of stainless steel 316L powder cost approximately $350 - $450. For this reason, minimizing the part volume and the need for support is key to keeping the cost as low as possible.

A key strength of metal 3D printing is its compatibility with high-strength materials, such as nickel or cobalt-chrome superalloys, that are very difficult to process with traditional manufacturing methods. Significant cost and time savings can be made, by using metal 3D printing to create a near-net-shape part that can is later post-processed to a very high surface finish.

Post-processing methods for metal 3D printing

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.

What are the 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 are 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.

The main characteristics of SLM and DMLS systems are summarized in the table below.