Ann, this is an interesting twist on an old technology. Welding has been around for a long time. By improving the process new things are possible. Isn't it interesting what some of these engineers will come up with?
Thanks, naperlou. I thought the combination of technologies to improve this process was especially interesting. Seems like we're seeing more of that: combining different assembly or manufacturing-related techniques to solve new materials and/or process problems. For instance, yesterday's robots plus lasers in composite repair story http://www.designnews.com/author.asp?section_id=1392&doc_id=243715
Welding dissimilar materials -- especially dissimilar materials with very different thermal characteristics (sheet vs. foam) -- can be a big challenge. The closest I've come to this is inertia welding a hollow carbon steel tube to a solid stainless steel shaft. That's challenging enough, but it's child's play compared to the process described in this article.
It wasn't immediately clear to me how trimming the edges of the foam prevents intermetallic formation. I know that intermetallic formation can be prevented by keeping heating times short, and maybe precise alignment between the aluminum foam and the steel sheet prior to welding helps with this. The high heating and cooling rates made possible by laser welding might also help.
Thanks for weighing in on this subject, Dave, with your background in metals. From the description, I visualized lasers trimming away the aluminum foam so it doesn't contact the edges of the panel's top steel sheets, only its internal steel sheets. I saw it somewhat like the peanut butter that slops over the side of a sandwich. Here, the "peanut butter"--perhaps a slice of cheese is a better metaphor-- is cut back so it doesn't stick out that far. Then the "bread slices" are positioned so there's no gap and welded. This may be inaccurate, but that's what I thought it meant. A video sure would be helpful.
I wonder about repairs underway? When you are at sea, you are on your own when trouble occurs. Can repairs be made without a 6kW laser? How about standard aluminum welding processes? Can you store on board replacement sections that can be used to reinforce or replace?
And does the sandwich material come with pickles? Just curious.
As excited as I was by the headline, as someone who has spent a lot of time in the marine industry, once I read about a dissimilar metal construction I lost my enthusiasm. The main place where ships need to save weight is in the superstructure (to make them less "tippy") and that problem has been solved by using all aluminum construction. An explosion welded bar of steel/aluminum is used to provide a transition from the deck to the superstructure. Other than that, I have to agree with Warren that the ability to repair stuff easily at sea is paramount. Structures corrode, crack and come apart due to the constant barrage of vibration, sea spray and racking stresses. Probably best to keep this sandwich either in the air or on solid ground.
I also spend many years in the marine industry, and the dissimilar metal issue arose all the time. And the center of gravity would certainly rise, but surely this is taken under consideration.
i wonder where they are on using this new material? I am sure better minds than ours have figured out what the issues are. Just like they did on the Titanic, er I mean the center fuel tank on the 747s, er I mean the solid-state booster on the space shuttle, er, well, you know what I mean.
As the article states, the demonstrator, at least, was a marine gear unit foundation. I also thought about corrosion, but because the aluminum and steel aren't touching, there are no intermetallic phases, so there are, presumably, no corrosion issues. Thanks for the points about repair at sea. I can't speak to that, except to note that the laser welds occur only on specific welds, as stated in the article: butt welds and fillet welds. Again, a video sure would have helped.
@Ann: If the steel and aluminum are welded together, then I'm sure they are in intimate contact. (After all, putting two surfaces in intimate contact with one another is what welding is all about). The fact that brittle intermetallic compounds aren't formed just means that the heating and cooling take place rapidly, not that the aluminum and steel aren't touching.
It's true that some intermetallic compounds can contribute to corrosion, but the lack of intermetallic compounds doesn't mean that there is no possibility of corrosion. Aluminum is anodic to steel, so in the presence of an electrolyte (i.e. water), the aluminum would be expected to corrode. As redandgearhead pointed out, the key to preventing this would be sealing the part in order to prevent water entry.
It may also be worth pointing out that surface area of the foam may be very large -- this is why sponges are made out of foam, after all -- so even if galvanic corrosion occurs, the corrosion rate may be very slow. However, these would be good questions for LZH.
Dave, my understanding of intermetallic phases and corrosion problems was also that they are caused by welding steel with aluminum. From the description given in the press release, it sounds like that is not what occurs. Instead, it sounds like the researchers have avoided that by trimming the aluminum foam away so that only the outer "bread slices" of steel are welded together, and the aluminum foam internal layer is not welded, or exposed to water.
@Ann: Okay, I think I understand -- the steel is already bonded to the aluminum prior to welding. (The sandwich panels are probably made by roll-cladding steel with powdered aluminum along with a foaming agent, then foaming the aluminum, as described here). So the welds are steel-to-steel, and the aluminum is not exposed to the environment. This makes sense.
Ask the the guys maintaining the new LCS. These type of disimilar bond require active corrosion management to offset the potential created at the junction. Another option is to use a Ti Gr2 bridge piece at the junctions.
The first word that pops into my head when I hear the words dissimiar metal is corrosion. When I pull an aluminum wheel off my car which pushes against a steel brake disc, you can see the pattern of the contact patch because the contact patch is rusted.
I am fascinated by the technology to do this and how wavelength was important to the process. ("Say gurl, see if vermillion will work. Or maybe a chartreuse, I'm partial to greens.) I had a manufacturing rep in our design group when I first went to work (his name was Edsel Buick Dodge and I'm not kidding) who said we had some welders who were good enough to make welds which looked like they had welded aluminum to steel.
If there is no oxygen in the structure is there no corrosion? Is so, then keeping the honeycomb sealed would be really important. Water never sleeps, though.
I'm with ChasChas, my materials professors must be spinning in their graves.
I would very much like to see durability testing information on this material, and corrosion resistance information as well. I have never seen, nor ever heard of, aluminum not corroding in a saltwater environment. And steel encased aluminum foam would be a nice sealed environment where corrosion would not be found.
Also, I doubt that a steel plate with a foamed aluminum core would be stiffer than a solid plate of equal thickness, although it could be stiffer than a solid plate of equal weight. Such assertions do need to be qualified, you know.
The existance of intermetallic compounds is not where I see the problems, but rather the proximity of two materials that have such different potentials. Corrosion is probably unavoidable.
William, as the article states, using metal foam sandwiches to make ships lighter isn't a new idea. The metal foams are often, but not always made of aluminum. Since this research was conducted in Germany, it's possible that this practice is more common there than in the US. Since the aluminum is in the interior of the sandwich structure, corrosion is most likely less than it would be if it were on the outside. Regarding stiffness, it was bending stiffness being described, and the effect sounds like plywood vs a single-layer wood panel. LZH may have such testing info available. Please let us know if you find out more.
The explanation does make sense, and I can see that the probability of corrosion would be reduced. But I can also see that if the skin is penetrated during the following production stages, or after the product is delivered, that problems could occur. So a nondestructive method of checking these materials would be potentially worthwhile. So there is a chance for the NDT companies to sell a new product.
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