Installing a Metal 3D Printer: Part 5 (Housekeeping)

Download all 5 parts of this series as a single PDF here.

This is my final post in our 5 part series discussing things we learned installing a metal 3D printer (specifically, a laser powder bed fusion machine). If you haven’t already done so, please read the previous posts using the links below.

If you prefer, you can register for a webinar to be held on July 26, 2017 @ 2pm EDT (US) where I will be summarizing all 5 parts of this blog series. Register by clicking on the image below:

Housekeeping may seem too minor a thing to dedicate a post to, but when it comes to metal 3D printing, this is arguably the single most important thing to do on a regular basis once the equipment, facilities, safety and environmental considerations are addressed up front. In this post, I list some of the activities specific to our Concept Laser MLab Cusing R machine that we do on a routine basis as indicative of the kinds of things that one needs to set aside time to do, in order to maintain a safe working environment. In this post, I break down the housekeeping into the 3D printer, the wet separator and the filter change.

1. The 3D Printer

All 3D printers need to be routinely cleaned, but for powder based metal 3D printers, this needs to be done after every build. Three steps need to be performed during cleaning of the printer:

  • Powder Retrieval: After the build, the powder is either still in the dose/feed chamber or not. All powder that is not in the dose chamber needs to be brushed to the overflow chamber for recycling. While it is possible to vacuum this powder, that is not recommended since it results in greater loss of powder and also increases the burden on cleaning the vacuum and creating wet waste.
  • Process Chamber Cleaning: The process chamber after a build gets covered with fine combustion particles (soot) that need to be wiped away, as shown in Figure 1. The recommendation is to do this cleaning using lint-free or clean room wipes moistened with an ammonia based cleaner like Windex Original.
  • Lens Cleaning: Special lens cleaning wipes are to be used to clean the protective lens that separates the chamber from the laser. Standard lens cleaning wipes can be used for this, in a gentle single-pass movement.

It is important to wear appropriate PPE and also NOT contaminate the lens. Improper or irregular cleaning will result in soot particles interfering in subsequent builds. Soot particles can occasionally seen in subsequent builds especially when the inert gas and the ventilator (circulating fan) are turned on – this is more likely to happen if the chamber is not routinely and properly cleaned.

Figure 1. Post-build cleaning of the 3D printer and required materials

2. Wet Separator

The wet separator (vacuum) sucks up stray powder and suspends it in a water column. The metal particles will descend to the bottom of the water column (as shown in Figure 2) and need to be routinely cleaned out. This cleaning procedure is recommended daily for reactive metals – failing this, the metal particles will weld themselves to the metal container and prove to be very difficult to scrape out. For non-reactive metals, a daily flush may be excessive (since this will add to the cost in terms of labor and disposal) and a weekly routine may be preferable for a wet separator that serves 1-2 machines.

To reduce the water needed to flush out the powder sludge at the bottom, a standard pump sprayer is very effective. Further reduction in water usage and disposal can be achieved by a filtration device such as the one developed by the folks at Kinetic Filtration.

Figure 2. Cleaning a wet separator

 

3. Filter Change

Filters need to be changed periodically as shown in Figure 3. A video below (set to start at the 2:58 mark) shows how the filter change is performed for our MLab, for a non-reactive metal, so I shall not describe the procedure further. A reactive metal alloy filter needs to be stored in water to passivate it at all times, even through disposal. Other OEMs recommend sand and other materials, so it is important to follow the specific instructions provided by your supplier for passivation.

Figure 3. Removing, passivating and disposing the filter

 

Summary

Good housekeeping for metal 3D printing is vital and more than just aesthetic – there is a modest chance that failing to follow your supplier’s instructions on one or more of the items above will result in a safety incident. This is especially true for reactive alloys, where filter changes are recommended after each build and wet separator clean on a daily basis.

Disclaimers

  • This is intended to supplement the supplier training you must receive before using the equipment and not meant to replace it – in case of conflicting information, your supplier’s training and equipment requirements override any discussion here. PADT and the author assume no legal responsibilities for any decisions or actions taken by the readers of this document.
  • My personal experience derives specifically from the use of Laser-based metal 3D printing tools, specifically Concept Laser’s MLab Cusing R equipment. I expect majority of this information to be of use to users of other laser based powder bed fusion metal systems and to a lesser extent to Electron Beam systems, but have no personal experience to vouch for this.

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Final Thoughts

This concludes my 5-part post on what we learned installing a metal 3D printer. If you have any thoughts on the content or would like to discuss this subject further, please let me know by messaging me on LinkedIn or by sending an email to info@padtinc.com, citing this blog post. I will be happy to include any suggestions in my posts with due credit.

Thank you for reading – I hope this has added value to the discussion on safely and effectively advancing metal 3D printing technology.

Installing a Metal 3D Printer: Part 3B (Safety Risks – Prevention & Mitigation)

Download all 5 parts of this series as a single PDF here.

How can you minimize safety risks in powder-based metal 3D printing?

This is the second half of my third post in a 5-part series discussing things we learned installing a metal 3D printer (specifically, a laser powder bed fusion machine). If you haven’t already done so, please read the previous posts using the links below, in particular, part 3A which is a prequel to this post. I also recommend reading my post on the difference between reactive and non-reactive alloys in the context of this process.

In the previous post, I identified four main risks associated with operating a laser-based powder bed fusion metal 3D printer such as the one we use at PADT, a Concept Laser MLab Cusing R. In this post, I address three of these risks in turn and first discuss how the risk can be prevented from manifesting as a hazard (prevention) and then address how it can be mitigated in case it does result in an incident (mitigation). I will deal with the fourth risk (environmental damage) in the next post. As with the previous post, my intent is to inform someone who is considering getting a metal 3D printer and not be comprehensive in addressing all safety aspects – the full list of disclaimers is at the end of this post.

If you prefer, you can register for a webinar to be held on July 26 @ 2pm EDT (US) where I will be summarizing all 5 parts of this blog series. Register by clicking on the image below:

Risk 1: Fire and Explosion

1.1 Prevention:

Fig 1. ESD wrist-strap

The key to preventing a fire is to remember that it needs three things (“the fire triangle”): fuel (metal powder or soot), an ignition source (laser or spark) and oxygen. While certified equipment is designed to operate in a safe manner when bringing the laser and the metal powder in contact by doing so in an inert gas environment, you as the operator, are responsible for avoiding any ignition sources when handling powder or soot outside of the inert environment. This is because two of the three aspects have been met: fuel (powder or soot) and oxygen (in the ambient). As long as basic risks are eliminated (sparking equipment, smoking etc.), the primary risk that remains is Electro Static Discharge (ESD) and thus the main piece of preventive equipment is an ESD wrist-strap, as shown in Figure 1, or equivalent ESD management methods.

It helps to appreciate the life cycle of the powder, as it goes from purchased jar to ending up returned as recycled powder (the majority of the powder), or in the wet or dry waste streams. This is shown in Figure 2. While this looks quite complex, coming out of the machine, the powder and soot only have 4 streams that you have to follow: the powder trapped in the part, the powder that you will recycle, the soot and powder trapped in the filter and finally, what will be cleaned with wipes and accumulate on gloves. While this is not comprehensive (internal hoses and shafts can also accumulate powder), these are the ones operators will deal with on a regular basis.

Fig 2. Metal powder life-cycle

1.2 Mitigation:

In addition to doing everything we can to prevent fire, we also need to be prepared in case it does happen. There are (at least) four aspects that need to be considered, dealt with in turn below:

1.2.1 Personal Protective Equipment (PPE)

Fig 3. Extended PPE

PPE is your self-defense in case of a fire and it is thus a critical element of the safety procedures you need to pay attention to and remember. Tasks are of varying risks, and our supplier recommends PPE for this process in three categories:

  • Protective Clothing: A lab-coat that covers your arms, protective gloves, ESD strap if working with reactive metals
  • Standard PPE: Respirator, nitrile gloves, face mask (if not integrated with respirator), ESD strap
  • Extended PPE: Standard PPE PLUS
    fire-rated bunny suit, fire-rated gloves (see Figure 3)

Below is a list of all activities that involve some risk of ignition (or inhalation, to be discussed in the next section) and the associated level of PPE recommended.

Table 1. PPE recommendations for different tasks (Courtesy: Concept Laser, Inc.)

PPE can be tricky to implement consistently since as seen above, there are several tasks of varying risk levels that require different PPE. The conservative approach is to prepare for the worst case and wear Extended PPE at all times, but this can make you uncomfortable for long periods of time, reduce your mobility for some tasks, and introduce human error. Instead, here is the 3-step logic I use for remembering what to wear:

  • Always wear gloves, goggles and protective clothing (lab-coat) when you work with the machine – make this a rule even for the simplest of tasks like using the keyboard and mouse
  • If you are directly handling (i.e. not through a glove box) virgin or recycled non-reactive metal alloy powder (i.e. no reactive powders or combustion products), standard PPE is adequate
  • For everything else, you need extended PPE

1.2.2 Fire Extinguishing

Fig 4. Class D Fire Extinguisher (must be mounted or on a trolley, NOT as shown here on the floor)

There are several recommendations for how to manage fire extinguishing. This is an area where you need to get your fire marshal to weigh in. What is clear is that water and CO2 are not safe choices for metal fires [NFPA 484 6.3.3.5(1)]. For extinguishing fires, the consensus is to use Class D fire extinguishers, such as the one shown in Figure 4. The fire extinguisher needs to be a Class D since this is the one rated for metal fires. The main training aspect is to ensure it is pointed down at the base of the fire rather than at it, followed by sweeping.

What to do with Water Sprinklers?: Water can be dangerous for metal fires, but the risk of not having any sprinklers may outweigh the risk of water exacerbating the fire. This is a function of how much risk you are introducing (amount of powder, proximity to other flammable sources, area surrounding the printer etc.) and is a decision best made together with your fire marshal.

1.2.3 Powder Storage

Fig 5. Flammables Cabinet

Powder storage will involve powder in unopened jars, opened jars as well as in the overflow collector which is on the machine. It is best to store opened and unopened jars in a flammable cabinet as shown in the adjacent figure. This is not essential for non-reactive alloys, but necessary for reactive metal alloys. For large quantities of reactive alloys, blast proof walls may be necessary – this is again something your city officials and fire marshal can guide you on, but do not neglect the importance of getting their buy-in early. Finally, most cities will require you to fill in some paperwork and show on a plan (map) where you are storing your powders, and what their composition is. This is to help inform the fire-fighters that there are metal powders onsite, and where they are located, in case of a building fire. If you do plan on working with reactive alloys in particular, you must involve your fire marshal sooner rather than later.

 

Risk 2: Powder Inhalation and Contact

2.1 Prevention

The main method of minimizing risk of powder inhalation is through the use of a respirator. These come in many forms, but the two most recommended ones for this process are respirators with built-in face-masks (as shown in Figure 3), and more preferable, the PAPR respirator, which delivers a positive pressure of air (for more information, read OSHA’s guide on respirators). N95 and higher respirator filters are recommended, though N100 are ideal.

Contact with powder is avoided by wearing gloves at all times when handling the machine. It is also useful to minimize risk of carrying powder outside the metal 3D printer area:

  • Before starting work, put away watches, wrist jewelry and cell phones.
  • Once done with the work, take off your protective coat and wash your hands and arms up to elbows before handling anything else.
  • Consider installing an adhesive floor mat for you to step on as you walk out of the room.

2.2 Mitigation

Fig 7. SDS binder

What to do in case of exposure is typically documented in the SDS (Safety Data Sheets), which is specific to the material in question, as shown in Figure 6 below. Ensure you have an SDS from your powder supplier for all powders you order, and collect them in a folder that is stored close to the entrance for easy retrieval, as shown in Figure 7.

Fig 6. Example of SDS information on responding to exposure

 

Risk 3: Inert Gas Asphyxiation

3.1 Prevention

Fig 7. O2 Sensor

Inert gas (Nitrogen or Argon) is used for every build and is either stored in cylinders (argon) or piped from a generator (Nitrogen). Proper, leak-free facilities setup and equipment performance is essential, as is following recommended supplier maintenance on the equipment itself. An inability to drop to required oxygen PPM levels in the build chamber, or large fluctuations in maintaining them may be associated with a leak and should be addressed with the supplier before proceeding. Users of the equipment must know where the shut-off valves for the gases are, in case they need to turn it off for any reason.

3.2 Mitigation

The main mitigation device is an Oxygen sensor such as the one in Figure 7. This is an important sensor to have especially in confined spaces around any equipment that relies on inert gases, including the 3D printer and furnace. If oxygen levels fall below safe values, an alarm is triggered and immediate evacuation is required.

4. References

  1. National Fire Protection Association’s standard for combustible metals, NFPA 484
  2. OSHA on Oxygen Deficiency
  3. OSHA’s Guidance on Dust Explosions
  4. OSHA Respirator guide
  5. J.M. Benson, “Safety considerations when handling metal powders,” Southern African Institute of Mining and Metallurgy, 2012
  6. R. G. Goldich, “Fundamentals of Particle Technology,” Chapter 15, Midland IT and Publishing, UK, 2002

Disclaimers

  • This is intended to supplement the supplier training you must receive before using the equipment and not meant to replace it – in case of conflicting information, your supplier’s training and equipment requirements override any discussion here. PADT and the author assume no legal responsibilities for any decisions or actions taken by the readers of this document.
  • My personal experience derives specifically from the use of Laser-based metal 3D printing tools, specifically Concept Laser’s MLab Cusing R equipment. I expect majority of this information to be of use to users of other laser based powder bed fusion metal systems and to a lesser extent to Electron Beam systems, but have no personal experience to vouch for this.
  • Local, state and federal regulations vary, and are important – partner with your local fire marshal (or equivalent authority) as a starting point and take them along with you every step of the way. If in the US, familiarize yourself in particular with OSHA’s guidance on dust explosions and NFPA 484, the National Fire Protection Association’s standard for combustible metals (links above).

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Any other tips or ideas I have not covered, please let me know by messaging me on LinkedIn or by sending an email to info@padtinc.com, citing this blog post. I will be happy to include them in this post with due credit. My aim is only to add to the discussion, not be the last word on it – and I look forward to suggestions that can make operating this technology safer for all of us and the ones that rely on us coming home every day.

Please find the 4th part of the series here.

PADT Open House 2017, image courtesy James Barker

Installing a Metal 3D Printer: Part 3A (Safety: Risks)

Download all 5 parts of this series as a single PDF here.

What are the safety risks in laser powder bed fusion metal 3D printing?

This is the first half of the 3rd post in a series of 5 on things we learned installing a metal 3D printer (laser powder bed fusion). Links to previous posts are below:

The most important discussions around installing and operating a metal 3D printer involve safety. The requirements can be difficult to pin down since they depend on several things: whether you are using reactive or non-reactive alloys (read a previous post on this subject here), the risk perception of your local building safety director and fire marshal, local regulations (and exceptions) and the volume of material you are dealing with. As with all things safety and more so because of how nascent metal 3D printing technology is, I list a few disclaimers at the bottom of this page.

There is so much to say about safety in this process that half-way through writing this post, it became clear it would not fit in one post. Therefore, I have split the content into two: in this post (3A) I talk about the risks: where they come from and why they matter. In the next post (3B), I will discuss how these risks can be mitigated.

1. Sources of Risk

Broadly speaking, I like to think of two sources of risk in this process since as an operator of these machines you have to think differently about how you interact with these sources.

1.1 Metal Powder

Metal 3D printing involves fusing together powder in a bed. Typical metal powders used for laser based 3D printing are spherical in shape and range from 10-70 microns in diameter, as shown in Figure 1. At this size, a metal can be prone to fire and explosion (under the right circumstances) and there is also the physiological concern of long-term inhalation of, and contact with, these powders. The powder also has a long life cycle and requires human interaction at many steps – from arriving in a container (as shown in Figure 2), through multiple recycling steps through final disposal. These risks come into play just when handling the powder (independent of its use in the process) – an additional risk comes from the melting process itself.

Fig 1. Typical powder particle size distribution is in the range of 10-70 microns for the laser powder bed fusion process.
Fig 2. Metal powders are delivered and stored in plastic containers such as the one shown above

1.2 The Laser Fusion Process

The powder in the bed described before is fused together into a solid using a laser that locally melts the powder one layer at a time. This is conducted in an inert atmosphere (Argon or Nitrogen) and is the second source of risk since these gases can displace Oxygen from a closed environment. Additionally, the process of laser melting of metals creates vaporized soot (see video below), some of which deposits on the process chamber and in the extraction module and filter. The smoke particles can be even finer than the powder itself, and need to be cleaned out with care on a regular basis.

2. Risks

There are 4 main risks arising from the laser powder bed fusion process: fire and explosion, powder inhalation and contact, inert gas asphyxiation and the environmental impact of the wastes generated.

2.1 Fire and Explosion

In May 2014, OSHA cited a 3D printing company for 10 violations deriving from the workplace safety standards surrounding the operation of a metal 3D printer (including not having the proper Class D fire extinguisher). The disregard of multiple safety measures during a routine build setup procedure resulted in a fire which caused life-threatening burns to the operator of the printer. While this incident was the result of gross negligence, it is nonetheless a cautionary tale that should drive us to understand the fundamental reasons why a metal 3D printer can cause fires and to appreciate the underlying reasons for why suppliers recommend the safety measures they do.

Fire and explosion require a combination of conditions as shown in the commonly cited image below used by OSHA and other agencies to communicate risks of powder handling.
As shown in Fig. 3, when handling powder in ambient atmospheres (with oxygen), all that is needed is a suitable ignition source to initiate a fire. Further, if this occurs in the presence of a dust cloud with many particles dispersed in a contained area (such as a small room or an air duct), this could lead to a more damaging explosion.

Fig 3. The oft cited fire triangle and explosion pentagon. Users of powder based metal 3D printing are often working with 2 of the 3 elements in the triangle and the key is to avoid the third.
  • Fire: When handling metal powder, the user needs to be aware that she/he already has 2 of the 3 requirements of a fire met and the main aim must be to ensure protection against any ignition source. There are several sources that could cause an ignition, the most likely one for a user of a metal 3D printer is static electricity. Additionally, it is possible that a fire can be initiated by hot surfaces, flames, hot gases and particles, mechanically generated sparks and strayelectrical currents.
  • Explosion: With regard to explosions, in addition to the 3 requirements above, dust clouds in contained areas can exacerbate any ignition to a much larger impact within milliseconds. Therefore, the prevention of the formation of metal dust clouds (as unlikely as that may seem), is of paramount importance.

In addition to the requirements above, there are levels associated with each requirement that need to be met together for an actual fire or explosion to occur. The risk of ignition increases with reducing particle diameter and given a certain particle size, the most significant factor governing risk is the nature of the metal. Reactive metals (Aluminum alloys, Titanium and its alloys, as well as smoke particles from both reactives and non-reactives) pose much higher risk than non-reactive metals (steels, Inconels, bronze, Cobalt Chrome alloys) – this is a subject I wrote about in more detail in a previous post.

2.2 Powder Inhalation & Contact

As discussed before, most metal 3D powder particles range in size from 10-70um. This is at the very edge of what is considered respirable and damaging to our lungs. While contact physically is to be avoided since it may initiate irritation and potential dermatitis, there is greater concern about the long-term inhalation exposure risks of these powders. Particles of the size range in this process can get deposited in the tracheo-bronchial region per Jenson [1] and Goldich [2]. Ultimately, these particles are discharged from the body or swallowed, but effects of long term exposure for the wide range of metals and alloys is not fully studied – which is why suppliers insist on respirators (more on that in the next post). It is worth pointing out though, from the work published by Jenson and Goldich, that it appears that while metal 3D printing powders are small enough to travel past the nasal cavity if inhaled, their sizes are large enough that respiratory damage in the lungs is highly unlikely – only particles under 2 microns are at risk of making it all the way to the alveoli and causing lung disease [2].

Fig 4. The human respiratory system. Particles in the 20-100 micron range, if inhaled, can make it to the trachea and bronchial regions [1, 2]  (Image Credit – public domain: National Institute of Health: National Heart, Lung and Blood Institute)

2.3 Inert Gas Asphyxiation

Inert gases are used in laser metal 3D printers to reduce the reactivity of the metal for processing purposes. Most metal 3D printers either use Nitrogen or Argon. Inert gas asphyxiation is the main risk due to oxygen being displaced by either of these gases that have leaked for some reason. Since both gases are not detectable by humans, victims do not realize that they are inhaling air depleted of oxygen and as a result this can have a serious impact. The human body is used to atmospheric percentages of oxygen (21%) and values below 19.5% can be harmful and are defined as oxygen-deficient per OSHA [3]. Thus, any user of nitrogen or argon gas (and this applies not just to any process using inert gases), especially in small spaces such as a closed room, needs to be aware of this risk and protect against it.

2.4 Environmental Impact

A key challenge with powder based processes lies in collecting and disposing the stray or “fugitive” powder from different locations such as the tool, PPE, containers and vacuum systems into temporary storage, during which the above risks of fire/explosion and inhalation remain. Additionally, the storage typically results in loose powder and solid waste as well as water with powder particles, both of which need to be disposed into the outside world and could pose an environmental hazard. I will discuss this further in a future post, when I attempt to look at some of the environmental aspects around this technology.

Fig 5. Metal powder settled at the bottom of the water column in the wet separator (vacuum cleaner). Where does it go next?

3. Disclaimers

  • This is intended to supplement the supplier training you must receive before using the equipment and not meant to replace it – in case of conflicting information, your supplier’s training and equipment requirements override any discussion here. PADT assumes no legal responsibilities for any decisions or actions taken by the readers of this document.
  • My personal experience derives specifically from the use of Laser-based metal 3D printing tools, specifically Concept Laser’s MLab Cusing R equipment. I expect majority of this information to be of use to users of other laser based powder bed fusion metal systems and to a lesser extent to Electron Beam systems, but have no personal experience to vouch for this.
  • Local, state and federal regulations vary, and are important – partner with your local fire marshal (or equivalent authority) as a starting point and take them along with you every step of the way. If in the US, familiarize yourself in particular with OSHA’s guidance on dust explosions [4] and NFPA 484 [5], the National Fire Protection Association’s standard for combustible metals (links below).

4. References

  1. J.M. Benson, “Safety considerations when handling metal powders,” Southern African Institute of Mining and Metallurgy, 2012
  2. R. G. Goldich, “Fundamentals of Particle Technology,” Chapter 15, Midland IT and Publishing, UK, 2002
  3. OSHA on Oxygen Deficiency
  4. OSHA’s Guidance on Dust Explosions
  5. National Fire Protection Association’s standard for combustible metals, NFPA 484
  6. D. Bhate, “Reactive and Non-Reactive Metal Alloys in Laser-based Powder Bed Fusion,” PADT Blog Post, 2016

Acknowledgements

Thank you to Perry Harlow-Leggett, the AM team at UL whose articles and webinars I have benefited from, and all the folks behind the scenes at OSHA and NFPA.

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Continue to Part 3B here, where I address mitigation strategies to address the risks described in this post. In the meantime, please read my prior posts below if you haven’t already, or send your inputs to me via message on LinkedIn. Thank you!

Be Alert and Always Inert – The importance of an inert environment with Metal 3D Printing (Video)

Metal 3D Printing is one of the more exciting areas of additive manufacturing, and we are learning a lot about how to safely operate our new system.  Our very own Dhruv Bhate, PhD shared those lessons learned in this new video:

The video stresses the importance of keeping an inert environment to keep part quality and to ensure a safe operating environment. Our Concept Laser Metal 3D Printer uses high powered lasers to melt metal powder one layer at a time to build 3D Parts. This process produces soot that is highly flammable.

Dhruv shows the process we use to break the part from the machine, clean the chamber of soot, and replace the filter that captures the soot.

To learn more about PADT and how we can help you with your 3D Printing, product development, or Numerical Simulation needs, please visit www.padtinc.com

Use this link to see all of our blog posts on Metal 3D Printing

Reactive and Non-Reactive Metal Alloys in Laser-based Powder Bed Fusion

One of the first concepts you come across in metal 3D printing is the notion of reactivity of the powder metal alloys – in this post, I investigate why some of these powder alloys are classified as reactive and others as non-reactive, and briefly touch upon the implications of this to the user of metal 3D printing tools, scoping the discussion to laser-based powder bed fusion. Ultimately, this boils down to a safety issue and I believe it is important that we, the users of these technologies, truly understand the fundamentals behind the measures we are trained to follow. If you are looking to get something chemical etched visit https://interplex.com/technology/process-capability/chemical-etching/.

Figure 1 below is indicative of the range of materials available currently for the laser-based powder bed fusion process (this selection is from Concept Laser). I have separated these into non-reactive and reactive metal alloys. The former includes steels, Inconels, bronze and CoCrW alloys. The reactive metal alloys on the other hand are Aluminum or Titanium based. The question is: what classifies them as such in the context of this process?

Figure 1.
Figure 1. Typical metal alloys available for the laser-based powder bed fusion process (from Concept Laser), classified as Non-Reactive and Reactive

Reactivity in this process really pertains to the likelihood of the alloy in question serving as a fuel for a fire and/or an explosion, which are two related but distinct phenomena. To truly understand the risk associated with powder metals, we must first understand a few basic concepts.

1. Fire and Explosion Criteria

Figure 2 is a commonly used representation of the criteria that need to be met to initiate a fire (fuel, oxygen and an ignition source) and an explosion (the same three criteria for a fire, plus a dust cloud and confined space). When handling reactive metal alloy powders, it is important to remember that two of the three requirements for a fire are almost always met and the key lies in avoiding the other criterion. When not processing the powder in the machine, it is often subject to ambient oxygen content and thus all precautions are taken to prevent an ignition source (an ESD spark, for example). When the metal is being processed with a high power laser, it is done in an inert atmosphere at very low Oxygen levels. This thought process of appreciating you are one criterion away from a fire is useful, if sobering, to bear in mind when working with these powders.

fireexplosion
Figure 2. The fire triangle and explosion pentagon

2. Terms Used to Describe Fire and Explosion Risk

There are several terms used to describe fire and explosion risk. I have picked 5 here that tie into the overall “index” I will discuss in the following section. All these parameters are in turn functions of the material in question, both with regard to its composition and its size distribution and are co-dependent. These definitions are adapted from Benson (2012) and Prodan et al. (2012).

  • Fire Related: These two terms describe the sensitivity of a metal dust cloud to ignition.
    • Ignition Temperature: This is the lowest surface temperature capable of igniting a powder or dust dispersed in the form of a dust cloud
    • Minimum Ignition Energy: This measures the ease of ignition of a dust cloud by electrical and electrostatic discharges.
  • Explosion Related: These terms describe the severity of an explosion arising from a fire once ignited.
    • Minimum Explosion Concentration (MEC): This is the smallest amount of dust which when suspended in air, under a set of test conditions, will initiate an explosion and propagate even after the action of the ignition source has ceased.
    • Maximum Explosion Pressure: This is a measure of the highest pressure that occurs during of an explosion of a flammable mixture in a closed vessel.
    • Maximum Rate of Pressure Rise: This is the maximum slope of the pressure/time curve during a flammable mixture explosion in a closed vessel.

3. Index of Explosibility

Having defined these terms, the question is how they can be tied together to give some sense of the hazard associated with each metal powder. I came across a 1964 US Bureau of Mines study that defined an Index of Explosibility as a measure of the hazard risk posed by powder metal alloys. The index represents both the sensitivity of the powder to ignition, and once ignited, the severity of the resulting explosion. Since this is a subjective metric, it is normalized by comparison against a “standard”, which was selected as Pittsburgh coal dust in the 1964 study. Importantly though, this normalization enables us to do qualitative comparisons between metal powders and have some sense of the hazard risk posed by them. Figure 3 is the equation reproduced from the original 1964 report and shows how this term is estimated.

Index of Explosibility (US Bureau of Mines study, 1964)
Figure 3. Index of Explosibility (US Bureau of Mines study, 1964)
Figure 3. Particle size has a significant impact on explosibility
Figure 4. Particle size has a significant impact on explosibility

The study also showed how the index was a direct function of particle size. Most powders for 3D metal printing are in the 20-100um range, and as shown in Fig. 4 for atomized Aluminum, the risk of an explosion increases with reducing particle diameter. 

The authors tested a range of metals and computed the different variables, which I have compiled anew in the table in Figure 5 for the ones we are interested in for metal 3D printing. The particle sizes in the 1964 study were ones that made it through a No. 200 sieve (less than 75 microns), but did not include sub-micron particles – this makes it an appropriate comparison for metal 3D printing. It is clear from the Index of Explosibility values, as well as the Cloud Ignition Temperatures in the table below why Aluminum and Titanium are classified as reactive metals requiring special attention and care.

Figure . Explosibility study findings from US Bureau of Mines study (1964)
Figure 5. Index of Explosibility comparison for selected metal alloys, adapted from US Bureau of Mines study (1964)

4. Implications for Metal 3D Printing

So what does this mean for metal 3D printing? There are three things to be aware of that are influenced by whether you are working with non-reactive or reactive alloys – I only provide a general discussion here, specific instructions will be provided to you in supplier training and manuals and must be followed.

  • Personal Protective Equipment (PPE): There are typically two levels of PPE: standard and extended. The standard PPE can be used for non-reactive alloy handling, but the reactive alloys require the more stringent, extended PPE. The main difference is that the extended PPE requires the use of a full bunny suit, ESD grounding straps and thermal gloves.
  • Need for Inert Gas Handling: Many tasks on a metal 3D printer require handling of powder (pouring the powder into the chamber, excavating a part, cleaning the chamber of powder etc.). Most of these tasks can be performed in the ambient for non-reactive metal alloys with standard PPE, but for reactive alloys these tasks must be performed in an inert atmosphere.
  • Local authority approvals: It is important that your local authorities including the fire marshall, are aware of the materials you are processing and review and authorize their use in your facility before you turn on the machine. Local regulations may require special procedures be implemented for preparing the room for use of reactive metal alloys, that do not apply to non-reactive metals. It is vital that the authorities are brought into the discussion early on and necessary certifications obtained, keeping in mind that reactive metal alloy use may drive additional investment in safety measures.

5. Conclusion

Safe operation of metal 3D printers requires installation of all the necessary safety equipment, extensive hands-on training and the use of checklists as memory aides. In addition to that, it helps to connect these to the fundamental reasons why these steps are important so as to gain a clearer appreciation of the source of the hazard and the nature of the risk it poses. In this article I have tried to demonstrate why reactivity in metal 3D printing matters and what the basis is for the classification of these metal alloys into reactive and non-reactive by leveraging an old 1964 study. I wish to close with a reminder that this information is meant to supplement formal training from your equipment supplier – if there is any conflict in the information presented here, please revert to your supplier’s recommendations.

Thank you for reading; stay safe as you innovate!