Corrosion of Additvely Manufactured Magnesium Alloy, WE43

Sam Ortgies Author

08/04/2020 Added

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Corrosion behavior of WE43 that has been produced using additive manufacturing compared to wrought samples.

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[00:00:00.000]Hello, I am Sam Ortgies and I am going to present
[00:00:03.090]the corrosion of Additively Manufactured Magnesium Alloy, WE43
[00:00:08.620]Now WE43 is the only FDA approved, medical grade magnesium alloy
[00:00:14.400]and that’s important for our motivation for this research.
[00:00:19.011]So currently there is a problem known as stress shielding with medical implants.
[00:00:23.259]Now stress shielding is when your implant is stronger than your bone.
[00:00:27.792]So, the loads that are applied to your leg gets transferred
[00:00:31.859]into the implant and your bone grows back weaker than before.
[00:00:35.866]Now this can cause problems in the future because if that implant ever
[00:00:40.000]has to get taken out, the bone is
[00:00:42.150]no longer strong enough and can break causing more problems in the future.
[00:00:50.020]So, what can happen, like this photo,
[00:00:53.455]is we have an implant that goes in and then after a while,
[00:00:57.113]we can see, the bone degrades away leaving the implant.
[00:01:01.669]And this is because the implant is stronger than bone so your body will
[00:01:05.756]naturally get rid of the stuff that isn’t needed, which in this case is the bone.
[00:01:12.443]So, after 2 years, you can see that there
[00:01:14.681]is a decrease of about 4mm of bone which is not very beneficial to
[00:01:18.794]the patient
[00:01:21.629]Another reason that magnesium is of interest, is because of its biocompatibility
[00:01:26.409]and its corrosion. So, Magnesium is naturally
[00:01:30.990]occurring in the body which means if it corrodes in the body, your body is not going to reject.
[00:01:38.252]It’s actually going to accept it and you can process
[00:01:40.622]that magnesium out like it does any other metals in your body.
[00:01:49.412]So, what we can do is make an implant that over the course of
[00:01:50.838]x amount of time, degrades away and is fully gone and you are
[00:01:57.011]just left with your bone. And this is good for some implants that
[00:02:01.072]have to be taken out.
[00:02:03.092]This eliminates the need for a second surgery which is good for the
[00:02:06.106]patient and no one want to pay for two surgeries.
[00:02:10.826]What we are doing in this is,
[00:02:13.493]we did Additive Manufacturing, which is a 3D printing process,
[00:02:19.566]and we wanted to look at the corrosion of our samples
[00:02:24.843]and compare it to cold rolled WE43.
[00:02:30.338]So what we did is we used powder bed fusion,
[00:02:32.938]which is a form of 3D printing, and what it is is it has a platform and it spreads
[00:02:39.917]an even layer of powered, then a laser comes from above and melts the powder
[00:02:43.227]in the designated locations. The table drops down slightly and the process repeats itself.
[00:02:52.865]And so, in this case we can see in the upper right-hand corner our corrosion samples
[00:02:57.384]being printed using our Lumex Avance-25 AM machine.
[00:03:04.595]So, like I said we are using WE43 metal,
[00:03:08.595]it is in the form of a powder.
[00:03:10.178]Between 20 and 35 microns is where most of this powder size falls, its fairly spherical,
[00:03:18.365]which is all things we want to look at because it affects the build part and build quality
[00:03:22.549]that we are going to get. So, we have good powder.
[00:03:26.713]This came from Luxfer Mel Technologies and it has a good distribution of what we are looking for.
[00:03:34.383]So, to test the corrosion we did hydrogen evolution
[00:03:39.362]and we choose hydrogen evolution because
[00:03:42.342]it has a one to one molar ration between the amount
[00:03:45.742]of hydrogen evolved from the sample and the amount of magnesium
[00:03:48.862]that is corroded away. So, by measuring the amount of hydrogen,
[00:03:53.248]or the amount of hydrogen that we capture,
[00:03:55.258]we know exactly how much magnesium alloy was corroded out.
[00:04:01.893]The solution that we used was a hanks balanced salt solution and
[00:04:07.105]we used it in deionized water. Our samples were originally 3mm thick
[00:04:11.105]with a diameter of 38mm but the thickness ended up changing.
[00:04:18.062]So our test setup we had a CO2 tank
[00:04:19.849]with our Hanks Balance Salt Solution,
[00:04:22.706]a hot plate and a mixer so we can keep temperature
[00:04:25.433]of the solution at body temperature and a
[00:04:27.993]pH probe so we could constantly monitor the pH
[00:04:31.993]of the solution and we tried to keep it between 7 and 8 pH.
[00:04:36.314]This was all to simulate body conditions.
[00:04:38.875]And a temperature around 37 degrees Celsius,
[00:04:41.475]again to maintain body conditions or as close as we can get.
[00:04:45.943]The CO2 was to decrease the pH when it got up close
[00:04:49.954]to 8 pH or a pH of 8 and it would bring it down to a pH of 7
[00:04:58.347]So, what we did was we had these
[00:05:00.627]custom-made graduated cylinders that our sample
[00:05:03.777]would rest inside and as the solution degraded
[00:05:09.496]or corroded the sample, it would capture the
[00:05:12.021]amount of hydrogen that was evolved.
[00:05:17.474]The temperature and pH were constantly monitored
[00:05:20.284]in order to get the best results that we could.
[00:05:23.937]And we took measurements every 15 minutes.
[00:05:27.374]So, like I said earlier, we did have to change our sample
[00:05:30.149]a little bit from the 3mm thick to 1mm thick.
[00:05:33.436]This was due to the availability of the wrought sample;
[00:05:37.216]we could only find WE43 to compare it against in a 1mm thick sheet.
[00:05:44.789]So, we adjusted our procedures a little and instead
[00:05:49.707]of corroding the whole 3D structure, we actually mounted
[00:05:53.267]it in epoxy and pvc in order to just expose one face to
[00:05:57.984]the solution and that allowed us to get comparative data
[00:06:01.984]between the wrought sample and the Additively Manufactured
[00:06:07.151]sample because we have the same surface area exposed to the solution.
[00:06:12.679]We also changed into using video recording.
[00:06:15.403]So, we recorded our whole corrosion process
[00:06:21.131]and we post processely got the measurements for our hydrogen.
[00:06:30.158]So, we wanted to look at the density
[00:06:32.300]because the position inside the build chamber
[00:06:34.769]does affect how our sample corrode.
[00:06:37.909]So, what we saw was the sample farther away
[00:06:41.389]from the door of the machine ended up being denser
[00:06:45.389]than those close to the door.
[00:06:47.625]The ones here, samples 4 and 5 which
[00:06:49.835]were closest to the door, we saw delamination
[00:06:52.515]and a lot of defects within the part.
[00:06:56.556]This will affect the corrosion because there are
[00:06:59.496]defects and more surface area exposed to the solution.
[00:07:03.806]Whereas our more dense samples, there is less surface area
[00:07:07.561]because there is not as many pores in the sample,
[00:07:11.561]so we expected these to corrode slower.
[00:07:15.183]But position on the build plate of our samples did have
[00:07:19.032]and affect and our highest density gave us in a position right around the center of our build plate
[00:07:30.360]So, what this is showing is how we mounted
[00:07:35.495]our build plates on our build platform.
[00:07:37.305]Our corrosion discs were in the portrait orientation
[00:07:43.228]on the build platform and our argon flow was left to right
[00:07:46.604]and our build direction was right to left.
[00:07:49.027]This was due to our soot formation due to vaporization of powder.
[00:07:54.465]We wanted the argon air to carry off the soot in a way away
[00:07:58.981]from where we going to print. This wasn’t perfect.
[00:08:04.609]We did see some soot buildup on the platform from the powder
[00:08:08.609]that was going to be printed and we do expect
[00:08:12.609]this is one of the reasons why we got delamination
[00:08:15.839]on some of the samples and high vaporization of our powder.
[00:08:22.215]So now to the corrosion of our samples.
[00:08:26.095]For our wrought sample, we had fairly low evolved hydrogen
[00:08:30.540]amount the we gained. Each time we had a low pH, around 7,
[00:08:34.540]we had a steep increase in our slope of our evolved hydrogen.
[00:08:40.279]But this was consistent between not only our
[00:08:44.279]wrought sample but our AM sample.
[00:08:46.769]As we can see, we maxed out at around 20 mL/cm2 of evolved hydrogen.
[00:08:55.245]Whereas, for our printed result,
[00:08:57.925]this is our AM sample, we had around 60 mL/cm2
[00:09:00.775]for our max. And again,
[00:09:02.275]we can see that around 7 pH
[00:09:04.105]we have that very steep slope of evolved hydrogen.
[00:09:09.883]But this shows that we are seeing a very drastic difference
[00:09:13.883]between our AM sample and our wrought sample.
[00:09:18.961]When looking at our corrosion rate we can see that
[00:09:22.078]our printed sample has a much higher corrosion rate than our wrought sample.
[00:09:27.988]This is good information to have for our first step
[00:09:34.322]into looking at the use in biomedical and the biomedical field.
[00:09:39.786]So, these high corrosion rates were over 10 times
[00:09:43.038]the amount of hydrogen as seen in literature.
[00:09:45.948]This was due to our solution being slightly different
[00:09:48.165]to that in literature.
[00:09:49.838]And so, in our future work we at some point,
[00:09:53.068]would like to mirror exactly what other people
[00:09:57.773]in literature have for their solution.
[00:10:00.693]Since our solution is slightly different, comparing our results to literature is kind of hard,
[00:10:06.392]but we can, we do have comparative analysis between a wrought sample
[00:10:12.067]in the same solution as our printed sample from additive manufacturing.
[00:10:19.272]So, in summary we investigated how WE43 effects
[00:10:28.063]or is printed with Powder Bed Fusion.
[00:10:31.543]We had decent density with our highest density at 1.843 g/cm3
[00:10:36.616]which is around 99% dense.
[00:10:39.876]We looked at the corrosion rate and ours was 10 times
[00:10:42.596]higher that of literature, but again,
[00:10:44.928]it was due to a difference in solution.
[00:10:50.595]Our, when we compare just our samples with samples,
[00:10:54.595]we corroded that were wrought, we had about 3 times faster
[00:10:58.595]compared to that of wrought our printed was.
[00:11:05.588]So again, we had a higher corrosion rate as compared to
[00:11:09.588]a wrought sample which is a good first step.
[00:11:14.568]For our future step we want to look at EBS
[00:11:16.528]analysis of WE43 powder and the part.
[00:11:19.988]What this will give is the chemical composition of
[00:11:22.688]powder to see if the powder oxidized with could
[00:11:25.769]explain the high vaporization and to see if the part itself so
[00:11:29.429]we can confirm or reject the idea that this is still WE43.
[00:11:35.786]If some of the underlying composition of the powder
[00:11:39.786]is different than WE43 this might not be able to be
[00:11:43.786]used in medical applications because it is no longer WE43.
[00:11:49.943]So, we want to look at that. Preform corrosion test in a solution
[00:11:56.981]that we can compare with literature.
[00:11:58.971]We also want to look at the corrosion crack propagation throughout our samples.
[00:12:08.264]That’s it. Thank you for your time

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Corrosion of Additvely Manufactured Magnesium Alloy, WE43

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