Here is today’s other baffling science story: In its quest to save weight, Volkswagen is ripping aluminum out of plans and bills-of–material, to replace it – with steel. Not good old steel. They replace it with much better new steel. According to Reuters, “Volkswagen AG is using new high-strength steel to make cars lighter and comply with strict emissions rules, confounding forecasts that aluminum would be the metal of choice for reducing weight.”
High tensile steel is up to six times stronger than conventional steel, and helped Volkswagen reduce the new Golf’s weight by about 100 kg, while also saving money. “Aluminum is about a third of the weight of conventional steel but costs three times as much,” says Reuters (let them answer the fuming mails by irate nerds who insist that such a statement is utter nonsense.)
“Volkswagen is using high-strength steels in increasing amounts. It is a very cost effective way of reducing weight,” Armin Plath, VW’s head of materials research and manufacturing, told Reuters in an interview. “Using new innovations in steel engineering… it is possible to reduce weight without the use for more costly materials such as aluminum and carbon fiber.” Volkswagen uses hot formed advanced high strength and ultra-high-strength steel. Other companies also increasingly use these materials.
However, Volkswagen may have to change its mind after all. Said Plath:
“If you now want to go beyond what is currently achievable, then maybe it will be required to use other materials such as aluminum and fiber re-enforced plastics.”
I wonder what the less obvious downsides to high strength steel are? Corrosion? Repairability? Is that even a word?
Perhaps malleability is the word.
“Using new innovations in steel engineering”
Is this code for buying it from China?
High-strength (HSLA) steel such as 645A is relatively expensive and unlikely to be imported from China; steel foundries in Western countries can be competitive with materials like these.
And HSLA steel is less prone to corrosion than conventional steel, though naturally more so than aluminium.
I know nothing about the materials, but it seems that they gradually get cheaper. I noticed that builders of ultralights, who quite costs sensitive, started moving from 20xx aluminum to 6061-T6. It offers better strength and better corrosion resistance, requiring no anodizing (not that VW ever cared about corrosion resistance AMIRITE). Clearly this would only happen if better aluminum alloys became cheaper.
Nope these grades are produced right here in Dearborn Michigan. For all the facts about AHSS and UHSS check these two sites. http://www.smdisteel.org or http://www.worldautosteel.org
It’s possible that when high strength steel is deformed in an accident, it’s not so easy to straighten it out, so you have to replace rather than repair. That happened to my 98 Accord after it was rear-ended; the car was totalled.
I doubt your ’98 would have had much, if any, high-strength steel. More likely the rear-end collision bent the car and it was not repairable, even with standard steel.
Detroit X – apparently the downsides of HSS and UHSS are in cost and manufacturing processes. The manufacturing process requires higher heat and different forming apparatus than the more malleable low-carbon steel. Also, cycle time may be increased. At least, that’s if I’m remembering my presentations correctly, it’s been a few years since the automotive materials conference.
But, when compared to Aluminum (which also has manufacturing drawbacks) HSLA has a number of huge advangates; no bi-metal joining, better fatigue strength, etc. I believe that aluminum on a per gram basis is probably cheaper, but on a per assembled part basis might lose its advantage.
Ideally the best material will be used for the proper conditions – HSLA for shock towers, crash structures etc, something thin and lightweight for car parts that don’t experience much stress (carbon fiber) and other materials (aluminum, magnesium, etc) for the remainder… it’s a weight savings / cost increase / added value equation, and it’ll always be changing.
Indeed, the BIW is made of different types of AHSS… depending on what is needed in the different areas, including “old school” steel.
Industry journals will attest to that… heat treated high strength steel can only be laser cut for holes after the heat treatment, no stamping post heat treatment.
Econobiker, that’s not always true. Almost all of the EHS steels can be cold-worked after heat treat. Most of the strength increase comes from the increase in crystalline surface area rather than the heat treat. The UHS-class steels DO use significant heat treating to reach their strength, but cold-formability is generally alright with Sy’s below 700 MPa. Another process that is being used with these steels is hot-stamping to get more complex draws and to reduce part counts.
“I wonder what the less obvious downsides to high strength steel are? Corrosion? Repairability? Is that even a word?”
As a general rule, for any given material, the higher its strength, the lower its ductility. The less ductility, the less plastic (permanent) deformation it can undergo before it breaks. There are ways to get both, such as with alloying & refining grain size, but usually you trade one for the other.
Also as a general rule steels are stronger than aluminum alloys. Junk structural steels may have yield strengths ~36K, while similar quality junk aluminum may be half that. We routinely use steels in the 80K-120K range, and at that strength they still have plenty of ductility. Steels can be had all the way up to 200K or higher for the right applications. For comparison, the strongest aluminum alloys will top out in the 70-80K range, and at that point, they have little ductility left.
The result is that at strength, aluminum has rather poor ductility, and aluminum structures that are bent rarely are able to be bent back without cracking/breaking. Also, when cracks do appear in high-strength aluminum, it’s too hard to have decent fracture toughness so the crack easily & quickly grows. For example, a crack in a quality steel bicycle frame (not at Huffy) isn’t necessarily a big deal–it will slowly grow and the steel will bend before it completely fails, but a crack in an aluminum frame will usually fail catastrophically with little warning.
Another structural difference is fatigue limit. Put a low enough stress on most steels, and they will never fail from fatigue. Aluminums don’t do that. Any load will lead to fatigue with enough cycles. As a result, aluminum structures have to be designed more robustly.
As for corrosion, a high-strength version of a material will typically have worse corrosion characteristics, but leaving things in the rain is very mild compared to real corrosive environments like sour gas wells. Strong steel alloys cannot be used subsea around hydrogen because of embrittlement, but that’s never going to an issue for a car.
Another consideration is that specific strength (strength / density) is not necessarily the factor that determines how light a structure is. It will actually depend of the type of performance and geometrical constraints. The actual parameter may be strength^2 / density or strength / density^2. For the first, steels would blow away aluminums, but for the second, aluminums will win.
Gosh it’s nice to see informed comments. We’re using this new magic microalloyed steel (DOMEX 700MC) extensively in the American Exocet. The stuff really has no downside. Great toughness, ductility, compatibility with weld filler, excellent air-annealing characteristics in heat-affected zones, no preheat needed, excellent formability, etc. While the modulus is the same for just about all steels, the greatly increased strength is great for strength-driven designs and any applications where energy absorption is needed. The cost isn’t out of line, either. I love the future!
http://www.ssab.com/Global/Domex/Datasheets/en/421_Domex%20700%20MC.pdf
Very interesting article. Prompted me to find this site:
I don’t think malleability is a problem for the hot-formed austenitic steel talked about here (depending on quenching process), but it’s been ages since I was conversant in this topic and I’ll enjoy reading up on recent progress.
Plus, I’m thinking that structural components and not sheet metal are the applications referred to…. or I’m full of it… one way to find out, read up.
Wow, your use of “austenitic” made me remember of the iron-carbon phase diagram. What a trip down memory lane.
We got a materials lecture last semester and AHSS was one of the main dishes. What it’s been achieved at micro-structure levels is almost magic. There are some steels that behave in one way during normal conditions and have different properties during a crash.
You would be surprised at when the heat treatment happens and how much strength is picked during that process.
I can’t recall the site at the moment, but there’s a UK one which allows to compare dent resistance between aluminum and steel, and there’s yet another one that goes deep into the AHSSs.
Lots of companies are doing this. Steel is easier to fabricate and weld, and has much better fatigue performance than aluminum. And when you’re building cars instead of spacecraft, cost matters.
Until spacecraft can be built to allow automakers to utilize the lower wages of other planets’ populations, yes, cost will matter.
Wow, now talk about illegal aliens…! ;)
Aluminum has about 1/3 the density of steel. But you need to use more aluminum to equal the same amount of steel, so you gain some weight back in added material. if you’re using, say 40% more material to equal the strength of the steel part, you’re only saving about half the weight.
HSLA steel can be 20-30% lighter and is also stronger than the regular stuff, meaning you save up to 30% of the weight just by using the fancy steel, but you can match or exceed the weight savings of Al if you can use less of it. And you don’t end up with parts that dimple in a hailstorm (I’m looking at you, Miata hood and trunklid)
Yes, aluminum/aluminium lacks the flexural rigidity of steel. The classic example I remember from school, was the flexi-flyer frames on racing motorcycles that resulted when they first tried to switch the frames to aluminum, but left them the same in dimensional section and profile.
As a car enthusiast, bicycle enthusiast, and professional structural engineer by day, this topic is awesome, and so are the comments.
I haven’t given the materials question too much thought with respect to cars (yet), but when buying a road bike last summer I brushed up on my materials to make an informed decision.
As redav pointed out, steel is more ductile for a given level of strength than aluminum. Basically it has “give”. It springs back from impacts that are within its elastic zone, and is very forgiving. When designing structures, whether concrete or steel, we try to design on the basis of a ductile failure. This means you can see the failure coming, rather than something failing suddenly. When this is applied to say, a bike frame, carbon fibre (which we all know is very light) tends to have very high strength, but will fail in a very brittle manner. Aluminum has middling weight and middling charecteristics of strength to ductility. Steel is at the other end. Alot of people wont even look twice at a steel bike anymore, but they are still very good.
Another aspect is section. A deeper section adds strength. This is why I shaped girders exist. They add depth to the section and remove extra material where its not needed. This is why many bycicles of aluminum will tend to have oval shaped tubes.
High strength steel in cars is a good thing in my opinion. Steel is by far the easiest to work with, when compared to aluminum and FRP composites, and this is good for the end user. Where I work we have many guys certified to weld steel, only one to weld aluminum. I imagine its similar in your average frame shop.
So, are Ford screwed with their aluminum F-150, or are they not (in particular in light of redav’s comments)? I don’t care about VW, but this is important.
Redav may correct me, he seems to know his stuff!
My thoughts are that aluminum is more suited to a luxury car, like Audi does with the A8. Aluminum’s main weakness is its lack of “toughness” (which is an actual technical term in materials!), or, its ability to absorb energy. This strikes me as a bad feature in a pickup truck, but less of an issue in a luxo-barge.
As redav stated, with aluminum, to achieve the factor of safety similar to that with steel, you need a much higher ultimate strength. In bikes, they tend to do this by optimizing the section of the tubing. This is why very deep yet skinny oval tubes are prevalent on aluminum bikes. In all honesty, it strikes me that there is room to do this on a truck frame, but would you want to is the other question, in light of these high strength steels being available.
AS a side note, with one of these “optimized” aluminum frames (my term) you may end up actually having MORE cross sectional material per unit of length, but it would still be lighter due to aluminums density.
…and interesting enough, word is that Audi is looking at abandoning the A8s ASF (Audi Space Frame) architecture for the future D5 model (many years in the future).
“So, are Ford screwed with their aluminum F-150”?
I would say no. While I wouldn’t expect Al to be ideal for trucks, it shouldn’t be a show-stopper. Rather, different materials just introduce different problems that have to be solved, just like designing a FWD v. RWD car. They behave differently, so you have to do different things get them to do what you want.
To clarify davefromcalgary’s comments- In mechanical design, there are three groups of things that you can control: material properties, geometries, and loads. Undoubtedly, Ford is tweaking all three. – The right alloy, heat treatment or possibly even using a composite structure can improve fracture toughness (how much work does it take to get cracks to grow). – Making the frame thicker certainly makes it stronger, but even more important is stress concentration. It’s a lot like aerodynamics–the more smoothly load can ‘flow’ through the structure, the less it will tend to concentrate pressure at any given point. Points of high stress concentration is where cracks start. Simply improving the shape and surface finish dramatically increase fatigue life. – Load & lifespan are related exponentially. Each incremental decrease in load increases the life by an order of magnitude. If they are successful at isolating vibration from the frame (engine, road), it will last longer.
So, simply beefing up the design to make it (statically) stronger does work, but that’s not the only way.
As for bikes, I believe the main requirement is stiffness. It’s desirable to be compliant vertically (to ease out the bumps for the rider) while maintaining rigidity laterally (for steering & pedaling efficiency). Flattened tubes accomplish this. They also can be shaped to increase the weld area at the bottom bracket & head tubes, which does increase strength. Ovalizing & butting is a great way to distribute stresses over larger areas which reduces stress concentrations. (Bike tubing can have extreme shape changes on the insides of the tubes to give it the best directional stiffness & strength properties, but since it isn’t visible, few think about it.)
Excellent reply, and excellent point on stress concentrations. Shape is one factor, shaping process another. This is why for the last decade every truck redesign makes sure to clarify their truck uses hydroformed frame rails. It uses even pressure to bend the frame, causing less of these stress concentrations.
I have already seen trucks with prohibitions on drilling the frames due to a concern for stress concentration. All customer equipment must be installed using stock brackets on them. I suppose that may be coming to F-150 too.
Hydroforming probably addresses two different issues: creating good, smooth shapes to reduce locations of stress amplification and also to reduce residual stresses from the forming process.
Most likely not. At the scale that VW wants to produce MQB platform cars in, it makes sense to use steel, because the global savings will add up quickly.
Regarding the F150, it makes a difference what materials are used where. To use a more extreme example, look at a dump truck:
-Frame is a C channel steel. Lots of strength and flexibility. -Cab is aluminum. Light weight, not a lot demands on the material. -Hood is fiberglass or composite. Lightweight, impact resistant, flexible. -Engine, transmission, axles are mix of metals combined to cut weight but be durable. -Dump body varies to suit the load being hauled. Aluminum body good for hauling soft material like sand; steel body for harder materials like rocks or demolition waste. Aluminum body weighs less = higher payload and more revenue, but is not always suitable. Other types of truck bodies use steel, fiberglass, wood, aluminum, and various combinations of all.
Pickup trucks can be engineered to be a scaled down version of a dump truck; different materials for different needs within the same vehicle. The only issue might be the truck bed; aluminum might be too soft for some uses, but a durable bed liner could solve that problem.
now if there’s a press release calling it Rearden Steel I’ll be very amused
you’ll know if it’s the real steel by its bluish cast…
Where is the guy with the aluminum foil hat charging that this is all an Obsma conspiracy?
As a structural engineering in another industry who has worked with both steel and aluminum structure, I can only say that this isn’t surprising. Aluminum is less dense than steel, has about 1/3 the elastic modulus, a fixed fatigue life, and several other major disadvantages. It takes more aluminum to get the required strength that less steel can provide, in most applications, it’s really not that advantageous. In applications undergoing shock/vibration, it’s a frackin’ disaster.
I suspect a few generations of aerospace engineers would disagree with “frackin disaster”
Actually, you know about the limited life airplanes, like Piper PA-38? 11,000 hours and done, it’s unaiworthy anymore (there’s a life extension kit for another 6 to 7 thousand, but it costs more than the residual value of the airplane). That’s the aliminum fatigue in action.
And it can catch up with “old” airplanes they do not have life limits designed into them from the factory. The recent emergency grounding of Cessna 210 for the spar cap cracks is a great example. Awesome surprise for operators who had the airplanes taken out of revenue service.
Airframes usually end up with cracks in them at some point. It more of when then if.
Airframes have to be inspected between every single flight for cracks for a reason. Catastrophic failures with no warnings aren’t fun. Also the “glue” that is used to put together airplanes helps to mitigate AL risks. The biggest downside in my experience has to do with what happens when there is a fire.
For whatever its worth an auto body guy I know is dreading this high strength steel. He says he’s had to deal with it already and it is a lot harder to deal with. I dont know if I believe him though, he hates everything.
After reading all these comments I feel like I’m ready to start producing cannons. Once I discover flight, watch out.
Seems Honda did this with the new Accord:
“High-Strength Steel The 2013 Accord unit-body uses 55.8-percent high-tensile steel, more than in any previous Accord. In addition, 17.2-percent of the steel is now grade 780, 980 and 1,500 – extremely high grades that have never before been used in any Accord. This contributes to higher body rigidity and reduced weight, which directly benefits ride and handling, interior quietness, performance and efficiency and long-term durability.
The measured improvements in rigidity are significant. In static tests, bending rigidity is up 34 percent and torsional rigidity is up 42-percent compared to the previous-generation Accord. In dynamic tests, front lateral rigidity is up 16 percent and rear vertical rigidity is up 39-percent.” http://www.hondanews.com/channels/honda-automobiles/releases/2013-honda-accord-body
Man, just when Morgan had wood perfected!
And the C6 Corvette has balsa wood in its floor pan as a noise deadener!
Another significant point is the number of different materials used in the body. If each color in the above figure represents a different metal and/or forming process then it becomes clear how highly engineered the frame really is.
(Also nice to see the TTAC B&B has a good share of engineers and metallurgists!)
The choice of aluminum vs steel depends on many aspects of the part, the process, and the application. First, some general parameters. A typical Body in White (BIW) has on the order of 250 individual stamped parts. A steel-intensive BIW will weigh about 1000 pounds, and probably less. There are dozens of different grades used throughout the BIW, ranging from the lower strength 18,000 pounds per square inch (18,000 psi = 18 ksi = 125 MPa) to 10x that strength. The lowest strength steel is typically found on fenders which have a relatively complex shape. The highest strength steels are associated with components involved in crash protection. Skin panels are around 0.75mm thick (0.03 inch), while the structural components are up to 2.5mm (0.10 inch). The body style of a steel-intensive vehicle is usually a monocoque or unibody construction, which makes extensive use of stamped components. Unibody construction techniques cab be applied to aluminum-intensive vehicles, or a space-frame construction may be chosen. A space-frame will make much greater use of aluminum extrusions, and would have stamped aluminum sheets only for hang-on panels. The discussion below applies primarily to stamped components.
Aluminum is 1/3 the density of steel. Meaning that an identical part made of aluminum will be 1/3 the weight of an identical part made of steel. But… aluminum has 1/3 the elastic modulus of steel. This means that for a given shape, aluminum is 1/3 as stiff as steel, and that aluminum has 3x more springback than steel. (Springback is a measure of how well the stamped part stays in the desired shape.) Because aluminum is 1/3 as stiff as steel, it is necessary to increase the thickness of the aluminum version of the part in question. So what might be 1.8 mm in steel needs to be 2.2 mm in aluminum (as an example – the part functional requirements determines the necessary thickness increase.) Even with this increased thickness, the aluminum part will likely weigh less than a comparable steel part. But…On a per-pound basis, sheet aluminum is more expensive than sheet steel. For a given part, there is usually a cost-advantage for steel. This gets magnified because the steels that are available today have a much better balance of strength vs formability, meaning that they are higher strength yet can be formed to the desired shape. What this translates to is that what needed to be 1.8 mm made from previous generations of steel can now be made from 1.4 mm. The higher strength balances out the lower thickness. Of course, there is a cost premium for these steels on a per-pound basis, but since the part is now made from thinner steel, it is not necessary to buy as much. This minimizes the financial impact.
Now consider the amount of sheet metal thrown away. About half of the sheet metal purchased for a vehicle makes it on to the final product – and not because there is anything wrong with the quality. In the forming process, it is necessary to have additional metal around the perimeter of most parts to control the metal flow. This is called the binder and addendum. Although this metal is necessary to form the panel, it gets trimmed off the final product and winds up in the scrap chute. Then there is what is called “engineered scrap”. Think about the window opening area in a door. That area gets cut out in the final product. For every vehicle, there is on the order of 500 to 1000 pounds of scrap produced. If it is steel scrap, it can be bundled together and recycled. Multiple grades of steel can be mixed together and recycled with no constraints. However, the situation is different with aluminum. There are primarily 2 families of aluminum grades used in automotive construction – the 5XXX series and the 6XXX series. In general, the 5XXX series has higher strength while the 6XXX series has greater formability. Unfortunately, the elements that give 5XXX the higher strength are detrimental to the properties of 6XXX alloys. This means that instead of a manufacturing facility having one common scrap conveyer system (like that has been used for steel), if that facility will make aluminum-intensive vehicles, the scrap needs to be segregated. Of course it’s possible to do so, but this comes at a cost to the facility.
What about an actual assembled vehicle? Somehow, the parts need to be attached to each other. For steel, resistance spot welding has been used for many years. RSW of sheet aluminum is much more challenging – the electrical resistance of aluminum is much lower than steel (meaning it’s a better conductor), and there is a thin layer of aluminum oxide on the sheet surface that must be accounted for. So to get around these hurdles, a different approach is used all together – self piercing rivets is a common approach. In sufficient volumes, this could come close to being the same per-vehicle cost as spot welding.
Aren’t there aluminum-intensive vehicles on the road now? Of course. But look at the nameplates. They are probably the vehicles that are not what most would call mass-market. Because these vehicles cost more, it is easier for the manufacturer to recoup the additional manufacturing cost. As the selling price decreases, it becomes more challenging to get it to be cost-neutral. That is, unless the cost equation changes. And that’s where fuel economy and CO2 comes into play.
If the BIW can be made lighter, then there can be a reduction in the engine/powertrain which can lead to further reductions in the vehicle weight. These reductions are needed for automakers to meet the current and future CAFE / emissions requirements. While the lighter weight is great for meeting these constraints, remember that the automaker needs to also meet safety/crash requirements. For that, strength and stiffness are critically important. Every automaker will balance these requirements with the lowest manufacturing cost. Some companies will stay with steel-intensive vehicles (but make use of higher strength grades), some companies will incorporate as much aluminum (and other lighter-weight metals like magnesium) as possible, and others will expand their use of carbon fiber and other composites. But one thing is certain… every company will need to make significant changes in their parts, processes, and products to accommodate the regulations. Thanks for reading through this – it turned out much longer than I thought it would be. -Danny http://www.EQSgroup.com http://www.Learning4M.com
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