The Material Science of Metal 3D Printing

this episode of real engineering is brought to you by brilliant a problem solving website that teaches you to think like an engineer one of the first things you learn in mechanical engineering is how to design your inventions in a way that is possible to manufacture and assemble this is a skill that takes time to learn primarily by working with machinists to look at your design and laugh at the incompetence of this young college kid from placing fastener holes and inaccessible locations to placing sharp inner corners that no milling machine can achieve so much of our engineering capabilities are dictated by what we can manufacture and every time a new method of manufacturing is invented it oh sure Zin new technologies once deemed impossible just as the simple cylinder boring machine facilitated the Industrial Revolution 3d printing may now be opening doors to new designs complicated hollow structures are now possible allowing designers to integrate cooling ducts directly into parts an incredibly useful tool for high-temperature parrots like turbine blades and rocket nozzles we can perform something called topology optimization where we use finite element analysis a type of stress simulation to tell us exactly where material is needed allowing us to generate the perfect structure for our application similar to how hollow bones are formed and thus allowing us to save on weight helping lightweight vehicles to gain even more performance often parts are machined down from joint blocks of raw material to their final form in aviation we measure this waste with the body to fly ratio which divides the weight of the final part by the weight of the raw material it was manufactured from imagine taking a material like titanium alloy which can cost upwards of $30 per kilo and then throwing away 90% of it in the manufacturing process needless to say this is a massive source of increased costs that 3d printing could help reduce all of these benefits can come together to unshackle engineers to form the perfectly shaped objects and perhaps one of the most interesting applications of this is this incredible 3d printed aerospike rocket engine that has the corporated liquid cooling channels directly into the rocket nozzles interior and shaped everything optimally to provide a highly efficient rocket nozzle that can operate efficiently at many different altitudes but even with all these amazing applications we rarely see 3d printed parts outside of prototyping applications like this it's clear that 3d printing can take human manufacturing to its next evolution but what's holding it back as usual the first problem is cost if we plot the price of a 3d printed parrot as a function of the number of parts created it would look something like this its price will be dominated by initial machined costs and that line will only marginally trend downwards as we print more parts due to the insane time it takes to print a single parrot after all we are essentially welding hundreds of kilometres of metal powder together to scale up our manufacturing we need to buy additional machines which will not lower our cost this turns our traditional economies of scale on its head take an injection molded part early on the cost will be dominated by the cost of creating an expensive mold needed to form the part but once that is finalized this machine can churn out piece after piece in rapid succession we mostly just have to wait for the plastic to cool down before we can eject it from the mold and restart the process this results in a graph that looks something like this where our cost per pair rapidly decreases as we build more soon becoming dominated by the material costs this means that it only makes economic sense to use 3d printing for parts that fall behind this break-even point which is why it is used so frequently in rapid prototyping if we can reduce the raw material cost with better supply and decrease 3d printer machine costs we can lower this line and open up more parts to being replaced by 3d printing that is gradually happening as the cost of these machines lower in large part due to patents expiring in the last 5 years however it's not just cost preventing 3d printed parts from entering the market this month I spoke with Professor Roger Reed the founder of ox met a company taking on the challenge of developing metal alloys and printing techniques to improve the material properties of these additive manufactured parts to get a better idea of the material science that prevents 3d printed parts from being approved for even specialized small batch applications we have thousands of years of experience mostly through trial and error of learning how manufacturing techniques affect the material properties of the metals we use from learning how to tailor carbon content during iron ore smelting to learning how each hammer blow can affect the crystalline structure of the metal in particular we've learned how the exact way we heat and cool metal affects its material properties as a result of its internal crystalline structure but additive manufacturing throws away much of the techniques we've developed forcing us to build much of our understanding up from scratch and develop completely novel techniques for studying and optimizing our material properties one of the key areas of research in this regard is improving 3d printed metal fatigue life the fatigue life is a measure of how many cycles of stress a part can sustain before breaking because materials can fracture even below their ultimate strength if you cycle them at a lower stress for extended periods this affects every metal and as the reason continual maintenance is always needed for machinery we can visualize a materials fatigue strength by plotting on an SN curve which places the magnitude of the alternating stress on the y-axis and the number of cycles it survived on the y-axis for traditionally machined titanium it looks something like this whereas for 3d printed parts it looks more like this but simply 3d printed parts fail much sooner stopping many of the parts from being approved for applications they are best suited for like aviation so why does this happen first we need to understand what causes fatigue fractures the primary cause of these fractures is crack growth where small voids and imperfections within the parish can for stress to divert and pie in sharp corners and thus exceed the metals strength locally and cause the crack to grow the more imperfections present the more likely your fatigue life is going to suffer and 3d printed materials tend to have a lot of imperfections we got a clearer look at why this happens when researchers used high-speed synchrotron x-ray imaging to get this phenomenal footage of the laser melting process which revealed many of the phenomenon resulting in imperfections here we have a powder bed of iron nickel alloy called invar 36 which has been turned into a powder by blasting a stream of molten metal with a high-pressure gas this process is called atomization as the laser moves across the powder bed it melts it essentially forming a weld line you can see that this layer tended to dig into the powder bed creating a track that varies in height these sort of imperfections means the final product needs a surface machining to create a quality part although it's important to note that this study was specifically studying something called an overhang condition where the parent has no structure below it and has to build on the loose bed of powder instead as the laser matches on the powder in front of it gets blown away meaning the laser no longer has metal powder to melt in that region and instead forms new beads of molten metal ahead of the original track which eventually coalesce with the original finally we see some worrisome behavior as the laser reaches the end of this track as the molten metal begins to cool we see pores begin to form in the upper surface of the track the exact kind of imperfections that could allow crack growth to occur in the future this study also varied several factories like laser speed and laser power to study their effects on the melt tracks properties here they increase the speed of the laser to a point where the metal particles did not have enough time to heat up and coalesce in another experiment they investigated the interaction of two melt rocks here you can see more pores forming as a result of overhangs trapping gas and yet more porous form in the same manner as before as we reach the end of the track clearly this process is much more complicated than just melting some metal powder together and in the end the final products that come directly out of a 3d printer are far from finished and need a significant amount of post-processing for example we can help close these pores by using a method called hot isostatic pressing where we apply heat and very high ISO static pressure which just means the pressure is the same in all directions this maintains the overall part shape but compresses and heats the part up enough to close those pores to improve our fatigue strength but not enough to compete with traditionally machined parts this of course pushes the cost bar higher making 3d printing again less attractive for applications outside of rapid prototyping and we have yet more material property issues to address we explored the science of forging with my friend Alec Steele in a previous video we learned how the internal crystal grain structure is one of the most influential factors in determining and materials final material properties we can control the materials hardness and ductility by simply heating and cooling it in a particular way typically when a piece of molten metal is cooled crystals grow at random from individual nucleation sites and form crystal grains the size and structure of these grains dictates so many of the metals final material properties and we've learned over thousands of years of metal forging how to get the best out of our materials once again additive manufacturing throws much of this knowledge out the window leaving us to start from scratch we have learned that 3d printed materials tend to have these columnar grains that rise up in the direction of the print and the grains tend to follow the direction of the laser forming directional grain structures that can almost be thought of like the grains in a piece of wood this means that how the laser mousse has a massive effect on the material properties of the material and thus we can use this to our advantage by tailoring our laser scan strategy one of the most common laser scan strategies is the island scan strategy where a pattern like this is formed creating 5 millimeter of laser trackpads oriented perpendicularly to each other these islands are formed in a random sequence this scan strategy developed by concept laser was created to alleviate residual stresses that form as a result of uneven heating and cooling within the metal which can decrease the part's overall strength just another factor designers have to consider and often requires the part to be placed in an oven after printing to help alleviate residual stresses however one study found that this scan strategy has some unique effects on the grain structure creating those aforementioned vertical grain structures with fine grain boundaries between each island and these fine grain boundaries had a high density of cracks which again can grow and cause fatigue failure there are of course alternative laser scan strategies like this helical one other researchers are attempting to use thermal and other specialized cameras inside the building to observe the phenomenon like pore formation and informed the laser exactly how to operate with machine learning to maximize material properties well I don't see this manufacturing technique ever being used for low-cost high-volume parts where other manufacturing techniques are much better suited if we can improve the fatigue life of these metals we could start seeing them appear in more applications like that incredible 3d printed aerospike engine we saw earlier this is a very new area of research that could use more ice just as I learned how to design to get the best out of carbon fiber composites and molded plastics over the course of my university life and industry experience we are now seeing young engineers beginning their education with this form of design in mind allowing them to create designs that were once deemed impossible I believe there is going to be a fascinating meeting of material science and machine learning in the space to customize laser scanning patterns for particular parts and allow the machine to spot and fix defects as they happen and I would imagine the overlap of material scientists and machine learning coders is a small pool of people at the moment so perhaps this could be a career path for you and he could start working towards it right now by 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Views: 1,700,559

Rating: 4.9416552 out of 5

Keywords: engineering, science, technology, education, history, real

Id: fzBRYsiyxjI

Length: 14min 59sec (899 seconds)

Published: Sat Oct 26 2019

Reddit Comments

[u/Salsa_Z5]

Roger is a giant in the world of Ni superalloys. Was really hoping he would have an interview in the video and talk about some of the metallurgical issues.

[u/sicutumbo]

Could you run the laser over the part again, but at lower intensity, so that it heats up enough to remelt but not be affected by the powder being blasted away? I think that would make the structure have less defects without any secondary process to dramatically increase costs. Maybe have a second, less powerful laser instead so that you aren't doubling the time it takes to make a part.

[u/ThunderFuckMountain]

I have a few things to add on to this video, which were touched on briefly, but not necessarily in the scope of the video. 3D printing *is* a new process and we're learning more about it, but for some parts, it is definitely a cost benefit.

Consider both RelativitySpace and SpaceX -- both of these companies currently 3D print their rocket chambers. These get very hot during operation, and you could definitely destroy the chamber within seconds if you don't have a way to cool them. Conventionally, the process is that you have to mill the cooling channels. The larger the chamber, the more channels you have to mill. Sometimes these channels can have bifurcations which can even further increase machining time. Then, you have to close out the channels somehow.

1, you can try electronickel plating, a process that takes a long time (and requires a large bath).

2, you can try laser cladding (a process similar to DMLS - requires a machine).

3, you can try to "shrink fit" a jacket onto the chamber - but if your tolerances aren't exactly right, the jacket will not properly bond to half of the channels.

Something that was touched on in the video was the cost and number of machines required to make 3D printing sustainable, and used the chambers as an example -- then proceeded to use injection molding as a counterexample for kicking out parts quickly. Try to MIM a bunch of finished chambers at one time -- go ahead, I'll wait.

The fact is, making these chambers with conventional manufacturing techniques also takes a long time, and also requires multiple machines (for each process step) if you want to increase part output. For these chambers, if speed is of the most importance (think about it -- delaying launch dates ALSO costs money), I think 3D printing is the way to go. Hell, for a lot of smaller parts, 3D printing could save a lot of money.

Of course, you wouldn't send your model of a conventionally machined part and ask the vendor to print it. The part should be designed for 3D printing (and hopefully, with the idea of no machining to light machining afterwards -- otherwise you haven't done the process its justice). As you would design a part for conventional manufacturing, you would design it for 3D printing -- perhaps you could include a non-solid "in-fill" to reduce part weight while still retaining overall strength. You wouldn't design a part to have an "overhang", where it has to build solid material on top of loose powder; and if you did, proper support structure would be put in place so that it could be built effectively. To maximize output, you could include multiple parts on the same build plate (and they don't even have to be the same part) -- the video showed a few frames of impellers being stacked on a build plate, but didn't really talk about this point.

Also, The stress relief process is so extremely important in all 3D printed parts. When you print, you have to attach the part to something (a build plate) and in the process of fusing the layers together you create so many internal stresses. The video talked about trying to remove internal stresses with different scan structures (5 mm squares are pretty big, if you think about the fact that the powder grains are 40µm big in diameter), but the most important part is the annealing. Obviously, you have to separate the part from the build plate at some point, but if you try to bandsaw or EDM the part off, parts (especially larger parts) are more likely to "potato chip" or warp.