Ann, you write a lot about light weight materials. As I have often commented, many cars on the road today have a major weight problem. As I write this I observed a Mercedes Benz ML 350. So, I looked it up. The four wheel drive version has a curb weight of 5,023. Now, it is not that big, and it is not armored, as far as I can tell. Yet, this mid-size SUV is very heavy. Considering that almost no one would drive this expensive a vehicle (over $50K base MSRP) off road, it seems that there is a lot of excess weight running around. Perhaps they could lighten up with aluminum.
Lou, in another blog here on Bayer's plastic sandwich car trunk lid http://www.designnews.com/author.asp?section_id=1392&doc_id=268704 some of us were talking recently about the greater weight of European vehicles vs American ones. Although there is an overall lightweighting effort going on and aluminum has replaced steel in many vehicles, it's also true that some people just want heavier vehicles. In my rural neighborhood, for example, I see lots of people driving those domesticated versions of Humvees.
If their own datasheets are to be believed, it seems like Alcoa may be being a bit modest when they say that the ArmX alloy is 20% stronger than 5083-H131. The datasheets suggest that the ArmX material may be as much as 200% stronger than 5083-H131. That's pretty amazing.
One thing I'm confused about is the nomenclature. The new alloy is called 5456-H151. The 5xxx designation indicates that the primary hardening element is magnesium. But Alcoa's literature says that it is "based on the proprietary 7085 alloy." The 7xxx designation indicates that the primary hardening element is zinc.
7xxx-series aluminum alloys typically have the highest strength, but less-than-stellar corrosion resistance. In particular, they tend to be susceptible to stress corrosion cracking. 5xxx-series aluminum alloys have the best corrosion resistance.
Any word on how the corrosion resistance of this new alloy compares to 5083-H131?
From what I know about traditional armor plate, two different types of steels (a softer outer steel to slow down the projectile, and a harder steel to stop the projectile) are welded together and then the assembly is rolled into a thin form. (Source: I worked two summers in a steel mill that manufactured armor plate.)
Is Aluminum armor plate also made in the same way, with two different types of Aluminum sandwiched together? That might explain why they say it's based on ArmX, but is described as 20% stronger instead of 200% stronger. Maybe the other alloys used to make the Aluminum armor plate aren't as strong?
Regarding both the 7085 alloy and possible armor construction, here's a 2002 press release describing the "new" alloy's use in blast-resistant (note that's *not* blast-proof) Fortress cargo containers for airlines: http://www.alcoa.com/global/en/news/news_detail.asp?newsYear=2002&pageID=222034673 "The Fortress Container uses hardened aluminum alloys for both the frame and skin." "The aluminum container structure is designed to resist pressure loads from an explosion, while an interior Kevlar lining provides protection from blast fragments. In designing the Fortress Container, Alcoa used its experience gained in developing aluminum armor for the Bradley Fighting Vehicle, the HMMWV (Humvee) and other military applications." How much this tells us about the armor plates, vs the cargo container panels, is an open question, but it sounds like their construction is similar. Here's some detailed info about the alloy. I couldn't extract the link, but if you Google this it should come up: ALCOA_7085-T7452_Die_Forging_green_letter_Ed_3_August_2006.pdf
I'm sure cost is the reason for not choosing an exotic aluminum alloy. In a military vehicle the pricetag is not very important when compared to performance, but in a consumer automobile the cost will be considered prior to performance. The only exception would be if the more expensive component solves other problems, like fuel economy requirements.
I think tekochip is right. The fact that aluminum costs many more times than steel is a hindrance in many automotive applications. So far, its main lightweighting use cases have occurred in aerospace and the military, and truck and trailer wheels, as we covered here: http://www.designnews.com/author.asp?section_id=1392&doc_id=251380
The U.S. Navy also experienced problems with aluminum when the USS Stark (frigate ship) was struck by two Exocet missiles in the Persian Gulf in 1987...killing 37 sailors. The aluminum melted in many places affected by fire, including aluminum stairs and ladders, hampering escape and damage control.
All aluminum has a rather low melting point of around 1,200 degrees F (660 deg C), and the mechanical properties of aluminum are severely compromised by temperatures of only 400 degrees F (205 deg C).
The idea of using aluminum alloys to make arm or plates is, in my view, particularly important because of 2 reasons. First, we all know that aluminum weighs a lot less than speed. Thus the humvees will be lighter and easier to transport. The greater strength of this allow is also great for battlefront scenarios. But, on the downside, the reduced weight means that other reinforcements have to be added if it is to withstand blasts without flipping the humvee.
The article mentioned brake calipers. Several years ago, I worked on a project with TARDEC to replace cast-iron brake calipers with high-strength aluminum brake calipers for a military vehicle (not this one). Doing so reduced the vehicle weight by literally hundreds of pounds.
I'm not sure where that project went, since I left the company soon after. My lab technician went on to get a master's degree out of the experience, while I'm still working (slowly!) on mine.
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