MIND OF METAL: POST PROCESSING
pre-processing … processing … POST PROCESSING
Perhaps you’ve designed an optimized titanium hip replacement. Or a mold with conformal cooling. No matter how extraordinary or how utilitarian, the point is: you are now printing with metal. New to you! (And kind of exciting.) You look at the part on the build plate. The product of all your hard work is finally ready to be put to use.
Hmm. Or maybe not quite ready. You may have some post-processing steps to consider.
Post-processing can be broadly broken down into three categories or steps:
Any one or all of these steps may apply to the printed metal part in order for it to suit its desired intent.
This step may seem a bit obvious; of course you need to remove the part from the printer before you can use it. However, it may not be quite as simple as taking a spoon out of the kitchen drawer. Extraction may involve:
In the case of SLM, BDJ and other powder bed methods, the part will be buried in the powder used to print the part. This ultra-fine powder needs to be treated with caution. Breathing or even touching it can be harmful, so you will need to exercise care to limit your exposure to it. You may also want to give thought to sieving and reusing excess powder.
Powder bed fusion processes like SLM fuse the created part to the build plate. Consequently, the part will need to be cut off. A bandsaw will work but if available, wire Electrical Discharge Machining may be preferable since the wire used is thinner than a bandsaw (0.20mm vs 0.45mm+) and the tolerances will be tighter to the build plate and therefore remove less material.
It is a good practice if possible to design metal parts with as few supports as possible because of the extra work required in their removal. However, if your structure required supports, this is the stage where you would normally remove them, though they can also be removed at the finishing stage for secondary processing.
Need for stress relief will depend on the build process. The heating and cooling metal undergoes during the build can lead to internal stresses within the created part. Although developments in control software have reduced the need for this step, it is sometimes necessary to relieve a part’s internal stress before it is removed from the build plate. This can be accomplished by placing the build plate in a heated oven or furnace, and then allowing it to cool gradually and uniformly. Although stress relief requires the application of heat, we categorize this step – if necessary – in part extraction specifically because it is occurs prior the removal of the part from the build plate.
While it can sometimes be viewed as an optional step, heat treatment can have very important some cases a necessary, even in optional cases metal 3D printed parts can benefit from a heat treatment. SLM components boast a 98%+ density and may not require as much heat treatment as BDJ components. However, heat treatment can still be critical for functional pressure or load bearing components. Heat treatment can involve:
When using binder jetting a completed print is considered ‘green’ and the binding agent must at least be in part removed prior to sintering. Debinding may involve heat or chemical treatment, but the goal is to remove as much of the binder as possible before further processing the print. This step is exclusive to binder jetting and involves the removal of the binding agent used to create the print by placing the component in a furnace and burning it out.
Subsequent to the debinding process, infiltration can be used to replace binding agents and reduce part porosity.
Sintering is the process of fusing particles together into one solid mass by using a combination of pressure and heat without liquefaction or melting. Sintering can be applied to a wide range of materials ranging from plastics to metal. As indicated earlier, when applied to metal prints annealing can reduce thermals stresses, reduce potential warping and distortions, reduce porosity and strength parts.
Annealing is subjecting a part or material to heat in order to increase its ductility and reduce the hardness. This change in hardness and ductility is a result of the reduction of dislocations in the crystal structure of the material being annealed. Annealing may result when subjecting 3D printed parts to a sintering process (in the case of binder jetting), or may be used to relieve stress for SLM/DMLS parts.
Hot Isostatic Pressing (HIP)
HIP is commonly used in the casting industry to improve the fatigue life of casted parts. It is a process that subjects metal components to elevated temperatures and isostatic gas pressure, and can be used to increase 3D metal part density. This process can be used on almost any metal part to increase part density to 100%.
Metal parts usually require further steps to bring about the desired finish. This is not always simply an aesthetic consideration; it can also impact the performance of fitted components. For example, complex interlocking and dynamic components require a smooth surface to reduce friction, but not at the expense of the unit’s integrity. (In such cases, it is very important that the part design accounts for the resultant reduction in wall thickness.)
Machining: It may seem a little counterintuitive, but CNC machining can be used to refine dimensionally crucial features (such as holes or threads) where tolerances need to exceed 0.005”. Micro-machining can improve the surface quality and fatigue strength.
Manual Finishing/Polishing: Manual polishing and finishing is laborious and time consuming and is more effective on softer metals than harder. There are a great many tools that can be used in this process including hand grinders, sandpaper, drills, carbide burrs, flapwheels and a host of other devices.
Sand (abrasive) blasting: Far more effective than mere sanding, sandblasting is a useful way to smooth out the surface finish of metal prints.
Plating: As the name implies, plating enhances the base print with a thin metal surface layer. This can be achieved via electroplating (electrolysis or dipping) and using any of a number of metals for plating.
Chemical Treatment: As you may be aware, this option is more commonly associated with polymer prints – anyone having taken time to subject an ABS print to acetone vapour, or PVB prints to isopropyl alcohol will understand. While not as common, metal applications include the application of citric or nitric acid baths to passivate stainless steel (i.e. give it greater resistance to oxidation).
PRE-PROCESSING FOR POST-PROCESSING
Does post-processing seem rather involved? Yes, there is no doubt that it can be. To have a realistic grasp of the time and costs associated with metal 3D printing, you will need to inform yourself as to precisely what processing your part will require.
You will want to consider whether pre-processing variables can be adjusted in order to mitigate the amount of post processing required.
Here are some considerations:
Design: The greatest strength of additive manufacturing is design freedom; therefore, adjusting the myriad possible variables and settings is the most obvious way to reduce need for post-processing.
One example is layer thickness. A thicker layer means a faster print, but more work will be required to smooth out the print shell. Conversely, the finer the layer, the slower the print but the finishing time is reduced.
Another example is print orientation. This can minimize the use of support structures, and account for any z-axis strength variations.
(Spoiler alert: our next installment in our Mind of Metal blog series will discuss design considerations in much more depth!)
Powder: Metal powder particles can vary in shape and range in size from 15-150μm. Generally speaking, finer powder increases print density and the print will be more exact and refined.
INSPECTION AND TESTING
These may also be part of the equation. Often these are not a variable, but a requirement when certification of parts requires adherence to industry specifications. Inspection and testing processes include:
- inspection and nondestructive testing using white/blue-light scanning
- dye-penetrant testing
- ultrasonic testing
- computed tomography (CT) scanning
- destructive testing of sample parts
- powder chemistry analysis
- material microstructure analysis
In addition to industry-specific testing, there is a growing body of additive manufacturing standards available through organizations like ASME and ASTM.
Our next post will continue our Mind of Metal series with a detailed look at design considerations for metal printing.
We would like to thank and note Eplus3D for their collaboration and input in the creation of this article.