Metal 3D printing

What’s the Difference Between Metal 3D Printing Processes?

How do you know what type of metal 3D printing to use for an application? Find out some of the key differences and some basic selection criteria.

Materials and processes is often a freshman level course in college. With the expansion of new technology, some engineers might find a refresher course that includes additive manufacturing, also called 3D printing, valuable. It can be difficult when looking for new solutions and become inundated with new technology.

This article will mention some of the difference between metal 3D printing processes and selection criteria to help understand this technology better. This does not cover all the processes. There is a lot of great research happening in this area, but the following introduces some of the more common processes.  

New technology is reducing the cost of metal 3D printing. This lower barrier to entry is expanding the technology to more industries and offering a competitive edge to companies that understand the innovative process.

First, I always suggest checking with a professional, and specifically one that works with multiple processes. For example, if a service bureau only has electron beam melting (EBM), it might say that it can processes soft metals like magnesium (Mg)or zinc (Zn) alloys. While it is possible, the high temperatures of EBM might not be the best for your specific application. Mg/Zn may even vaporize at EBM processing temperatures. Basically, a company that only offers EBM will find a way to make the process work, but it might not be the best solution.  

Processes

When it comes to metal 3D printing, the most used technology is powder bed process. There are multiple powder bed processes such as binder jetting (BDJ), selective laser melting (SLM), and EBM. Other processes might say selective laser sintering, but they may not be the same as selective laser melting. Sintering normally uses a lower power laser. The energy is enough to compact, but not melt the powder. This might also be a term that is lost in hype and marketing, so it is important to understand whether a company offers sintering, melting, or both.

SLS diagram: While some people might think support structures aren’t needed, supports can reduce low spots that form from materials sintered on top of unsupported powder. By Materialgeeza - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4032088

Metal sintering, also called direct metal laser sintering (DMLS) can reduce internal stresses cause by excessive heat. Without the thermal stresses caused by some of the other processes, including 3D printing and traditional, DMLS parts are capable of operating in the aerospace and automotive industry for applications that may see high stresses. DMLS can also eliminate post-thermal processing such as annealing. The strength, elongation, and hardness charts show an idea of common materials and material properties associated with DMLS.

DMLS Material Strength (Credit: Protolabs)

DMLS Material Elongation (Credit: Protolabs)

DMLS Material Rockwell Hardness (Credit: Protolabs)

Selective laser melting (SLM) melts selected sections of material together, then adds another layer of powder to repeat the process. Performing this process in a closed inert environment that is often pressurized eliminates oxygen contamination. The inert gas might change based on what will react with the material being processed. In addition, the build plate can be heated to minimize cooling rates. According to an article published by the National Center for Biotechnology, SLM’s important process parameters are:

  • Laser power
  • Laser scan speed
  • Hatch distance
  • Hatch overlaps
  • Scanning pattern


Schematic diagram of SLM process parameters (Credit: Ohiolink.edu)

According to the NCBI paper, SLM is regarded as the most versatile AM process because it can process a wide spectrum of materials, including aluminum, titanium, iron, nickel, cobalt, copper-based alloys, and their composites.

The laser power, scan speed, and layer height will probably be determined by the manufacturer of the machine. However, this will tell you what type of diameters and scan speed a machine is capable of. The linear laser energy density (LLED) is a unit of joules per millimeter (J/mm). LLED estimates the laser energy input to melt the powder layer.

LLED=P/v

Where P = laser power (W), and v = scan speed (mm/s)

This, along with the hatching space and layer height, will influence the densification of a part. A paper from Ohio Link says a hatch overlap is necessary for a solid sample, and in most SLM processes an overlap of at least 20% produced better quality samples. The paper continues saying the scanning patterns is the design or orientation of the hatches between layers, and can also affect bonding between layers.  

Electron Beam Melting (EBM) is similar to SLM, but an electron beam is used instead of a laser. Bed temperatures are higher (~870 K), and the EBM environment is performed in a vacuum rather than the Inert gas used in SLM. One drawback to EBM is that is involves more process parameters that means the process will take more expertise and time to optimize. The NCBI paper outline the following parameters involved in the EBM process:

  • Beam power
  • Beam scanning velocity
  • Beam focus
  • Beam diameter
  • Beam line spacing
  • Plate temperature
  • Pre-heat temperature (including the repetitions, speed, and power of the beam)
  • Contour strategies
  • Scan strategy

The complexity of the process limits EBM materials. EBM has been known to use Ti grade 2, Ti6Al4V, Inconel 718, and CoCrMo. SLM cannot produce many brittle materials such as intermetallics because its high cooling rate will cause internal stresses. Longer cooling times can be both a pro and a con for EBM.

Binder jetting (BDJ) is a powder bed process but instead of lasers or electron beams, a binder agent holds the powder together. A (green) part is produced faster than any of the other powder bed processes, but this is only the first step. Post-processing—such as curing, de-powdering, sintering, infiltration, annealing, and finishing the part—will often take more time than creating the first green part.

6-axis robots are increasingly being used in many industries. These types of arms have printed extruded plastic. However, by taking advantage of binder jetting with advance robotics, Viridis prints casting molds. While an indirect way of using 3D printing for metal parts, it saves time and cost of traditional molds.

6-axis robots are increasingly being used in many industries. These types of arms have printed extruded plastic. However, by taking advantage of binder jetting with advance robotics, Viridis prints casting molds. While an indirect way of using 3D printing to generate metal parts, it saves time and cost of traditional molds.

It can handle metals and alloys, including aluminum, copper, iron, nickel, and cobalt-based alloys. In addition, BDJ can process ceramics, including glass, sand, and graphite. BDJ is said to work with any powder that allows for color printing. For applications BDJ is often not suitable for structural applications due to porosity that may occur from the conventional sintering process. Sintering also means green parts that are not near net tolerance. There is software the will compensate for shrinkage during processing, and scale green parts accordingly. However, it is imperative that a designer understand whether software they are using will scale for shrinkage or not. Below are some further pros and cons for BDJ.

Materials

Soft materials. There are design requirements that may lean towards a type of material. From the previous example, binder jetting (BDJ), selective laser melting (SLM), and EBM are all competitive when considering metals/composites. However, EBM temperatures can exceed 870 K and are a concern if a material has a low boiling point, such as Al/Mg/Zn.

The ProX DMP 320 claim to offer high throughput and high-quality parts from the most challenging alloys. While the name indicates DMP meaning direct metal printing, it doesn’t necessarily indicate which process it uses. The information on the website does say loose metal particles are melted together indicating a selective laser melting powder bed process.

The ProX DMP 320 claims to offer high throughput and high-quality parts from the most challenging alloys. While the name indicates DMP meaning direct metal printing, it doesn’t necessarily indicate which process it uses. The information on the website does say loose metal particles are melted together, indicating a selective laser melting powder bed process.

Harder materials. While EBM might not be great for aluminum, if you add some titanium it might change which process you want to use. For TiAl or hard intermetallic, all three processes will work. However, SLM has a high cooling rate. Al-based alloys that processes between 473 K and 673 K might work well, but for TiAl, intermetallic, or brittle metals, a high cooling rate can create internal stresses and cracks. The processing and cooling temperature can be controlled in SLM, but TiAl and intermetallic materials make BDJ and EBM a better choice depending on available technology capabilities and application. 

High entropy alloys (HEA). Generally considered to be brittle and to contain more than three of four elements in equi-atomic configuration with a range of melting points, it can difficult to processes these materials with any fusion method. Therefore, HEA materials are an obvious choice for the BDJ process. It isn’t impossible to process HEAs with EBM or SLM, but it is not widely used.

Crystalline materials. There are some basics of materials that easily carry over into 3D printing. Crystalline materials crystalize based on their exposure to heat, so they can’t work in an EBM process because the temperatures are too high. While BDJ processes at low temperatures, and the de-binding and sintering processes are lower than EBM, the thermal processes takes long enough to crystalize materials. SLM involves heat, but it has a high cooling rate that can prevent materials from crystallizing.    

The process selected might also vary the material properties. SLM has shown that it can tune Al-12Si yield strength between 235 and 290 MPa, ultimate strength between 220 and 460 MPa, and ductility between 2.8% and 9.5% in tension.

This table shows the mechanical property difference of Ti6Al4V when processed with EBM and SLM. SLM’s high cooling creates a better martensitic structure that increases the strength and brittleness of the material.

When considering cooling temperature, it should be noted the powder bed will need to cool before parts can be removed. Higher temperatures will increase the time between printing and removal. In the case of EBM, a powder bed may need to sit overnight. This time slows production and can increase part cost. 

Selection Criteria

There are several things to consider when selecting the right process and material. The NCBI and Stratasys both have seven selection criteria, but they are not entirely the same. Stratasys includes polymer 3D printing into its consideration in Identifying the Best 3D-Printing Process for Your Applications, from which this list was compiled.

It can be hard to find good information on 3D printing, as internet searches tend to be crowded with the hype surrounding the processes. In general, Protolabs, Stratasys, 3D Hubs, and Xometry (and of course, Machine Design) are good sources for ungated content.

I’ve also found great technical papers. You can use Google Scholar to search exclusively for technical journals and scholarly articles. They often want you to pay to access them, but there is a way to get around this. You can often read the abstract, and maybe a page or two to determine if it’s a paper you want to read, or the one that might answer your question. Then look for the authors and do an internet search for their contact information.

I’ve been surprised by the authors I was able to find, and if I can find them they normally respond with a full copy of their paper. Most scholarly journals that charge keep 100% of the money, so the professor isn’t losing money sending a copy to you. They might also have some additional insight to any questions you have.

If you have any comments or questions, feel free to reach out to me at [email protected].

 

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