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Mr. Meeseeks, in my experience designing with aluminum and steel, a major consideration is fatigue. You mentioned it, but steel, if stressed below some amount has a fatigue life of approximately infinity. Aluminum, at least the 5000 and 6000 alloys we used, experience reduced life even at low stress levels.
Post and explain some S-N curves for the non-engineers among us.
Thus I would have no qualms with aluminum for non-stressed parts, the body for example. Indeed, I noted my 2008 hood is aluminum when I tried to hang my magnetic strap voltmeter on it.
Parts that carry a load ... the frame or suspension parts for example, would not seem, TO THIS OLD GUY, as good for aluminum construction. And springs are definite poor choices to me.
What parts did Ford switch from steel to aluminum?
Mr. Meeseeks, in my experience designing with aluminum and steel, a major consideration is fatigue. You mentioned it, but steel, if stressed below some amount has a fatigue life of approximately infinity. Aluminum, at least the 5000 and 6000 alloys we used, experience reduced life even at low stress levels.
Post and explain some S-N curves for the non-engineers among us.
Thus I would have no qualms with aluminum for non-stressed parts, the body for example. Indeed, I noted my 2008 hood is aluminum when I tried to hang my magnetic strap voltmeter on it.
Parts that carry a load ... the frame or suspension parts for example, would not seem, TO THIS OLD GUY, as good for aluminum construction. And springs are definite poor choices to me.
What parts did Ford switch from steel to aluminum?
I'm at home and I don't have my laptop so I free hand drew something for you, I wish I had something more professional and accurate at the moment.
For those just reading along, gbynum asked for an S-N curve comparing Aluminum alloys and steel alloys. An S-N curve is simply a fatigue chart where S is the stress (y-axis) and N is the number of cycles to failure (X-axis). As you can see I've pointed out steel has an endurance limit, or in your words; practically infinite fatigue life, where aluminum does not.
What this means (for readers generally, I assume gbynum knows) is that once below a particular stress level you will not see the steel fail by cyclic load alone. Aluminum does not have this so if tested enough, from the same repeated load, it will eventually fail.
Both metals will have their advantages and disadvantages however there are many other factors to consider when using a material for your suspension than just fatigue. You can have a steel alloy with such a high tensile strength that it never fails under cyclical loading but higher tensile strength of steel also comes with decreased toughness and a higher sensitivity to its environment. So before ever failing from fatigue your steel components *could* instead fail from corrosion pits, cracks, and other inclusions that can propagate quickly. I've worked in the railroad industry where steel tracks fail all the time due to constant vibrations and loading when these types of defects show up.
Even with that I see your point and I agree that switching to aluminum on non-stressed or non critically load bearing parts is safer and easier to "trial and error" and much more consideration should be taken for things like the frame and suspension.
I don't know about Ford specifically (where am I?) but the automotive industry as a whole has been looking into aluminum bodies/frames/suspension for weight, fuel economy, and - in suspension specifically- noise reduction. It is true that if you were to, say, hit a curb hard enough or get into a wreck the aluminum is more likely to break when the steel is more likely to bend.
Basically you're not unsafe with aluminum suspension. Are you better off...it depends what you want out of it. Seems like you have a trade off. Do you like that your steel suspension can theoretically last forever, or do you prefer the light weight benefits of the aluminum? I don't think one or the other is inherently a bad thing.
TL;DR: Steel could last longer than the aluminum. Aluminum weighs less (better fuel economy) than the steel.
As an aside, gbynum, I'd really like to pick your brain sometime about your design experience and the results you've seen as "an old guy" as opposed to theoretical textbook expected results.
I'm at home and I don't have my laptop so I free hand drew something for you, I wish I had something more professional and accurate at the moment.
For those just reading along, gbynum asked for an S-N curve comparing Aluminum alloys and steel alloys. An S-N curve is simply a fatigue chart where S is the stress (y-axis) and N is the number of cycles to failure (X-axis). As you can see I've pointed out steel has an endurance limit, or in your words; practically infinite fatigue life, where aluminum does not.
What this means (for readers generally, I assume gbynum knows) is that once below a particular stress level you will not see the steel fail by cyclic load alone. Aluminum does not have this so if tested enough, from the same repeated load, it will eventually fail.
Both metals will have their advantages and disadvantages however there are many other factors to consider when using a material for your suspension than just fatigue. You can have a steel alloy with such a high tensile strength that it never fails under cyclical loading but higher tensile strength of steel also comes with decreased toughness and a higher sensitivity to its environment. So before ever failing from fatigue your steel components *could* instead fail from corrosion pits, cracks, and other inclusions that can propagate quickly. I've worked in the railroad industry where steel tracks fail all the time due to constant vibrations and loading when these types of defects show up.
Even with that I see your point and I agree that switching to aluminum on non-stressed or non critically load bearing parts is safer and easier to "trial and error" and much more consideration should be taken for things like the frame and suspension.
I don't know about Ford specifically (where am I?) but the automotive industry as a whole has been looking into aluminum bodies/frames/suspension for weight, fuel economy, and - in suspension specifically- noise reduction. It is true that if you were to, say, hit a curb hard enough or get into a wreck the aluminum is more likely to break when the steel is more likely to bend.
Basically you're not unsafe with aluminum suspension. Are you better off...it depends what you want out of it. Seems like you have a trade off. Do you like that your steel suspension can theoretically last forever, or do you prefer the light weight benefits of the aluminum? I don't think one or the other is inherently a bad thing.
TL;DR: Steel could last longer than the aluminum. Aluminum weighs less (better fuel economy) than the steel.
As an aside, gbynum, I'd really like to pick your brain sometime about your design experience and the results you've seen as "an old guy" as opposed to theoretical textbook expected results.
Thanks for your time - and informative, factual posts. I don't have any specific questions yet, but appreciate the info and will be checking into this thread regularly.
What are your thoughts on suspension components like control arms and track bars being aluminium and how would they compare during normal use to steel? (Normal use being standard driving and not being in an accident or being folded over a rock while off-roading)
What are your thoughts on suspension components like control arms and track bars being aluminium and how would they compare during normal use to steel? (Normal use being standard driving and not being in an accident or being folded over a rock while off-roading)
I really think it depends how it's designed. As mentioned before steel is stiffer than aluminum [by weight]. So sometimes you'll see parts, like the control arm for instance, that have a different geometry than a steel one and they do this to increase the stiffness. So basically if they are well designed, with the differences of the metal properties in mind, then I'd trust it. Here is a side by side so you see what I mean about the geometry:
Day-to-day aluminum suspension parts *should* result in better handling and reduced noise and vibration BUT I don't know this to be 100% true or how noticeable it would be to the driver. Also with the lighter weight they should reduce the force on impact loading and hopefully last longer. Also being so corrosion resistant means they won't rust out and they look shinier longer.
There is also something known as the "gyroscopic phenomenon". Really just the torque action from the steering system to the wheels. Heavier components directly affect the torquing action which is why they believe have lightweight components will result in better handling.
Again, I don't actually know how this was tested and what the results were. Apparently Volvo tested this out on an entire aluminum rear axle system years ago and claim "better handling and reduced noise" but I'll reserve my personal judgment until I see some numbers.
Of course if you're going to use aluminum suspension parts you have to consider the material of the other connecting parts as well (bushings, fittings, etc.). And these may be more difficult to come by.
As a whole I don't believe the automotive industry ever decided to make the switch to aluminum because it's "better" than steel. I think their hands were tied to come up with a lighter solution to reduce emissions when newer safety features and luxury features are weighing the vehicle down. Luckily they are seeing a lot of additional benefits to the aluminum besides weight reduction - corrosion resistance, reduced impact load, less noise, etc.
Steel parts are inherently stiffer, stronger, and cheaper albeit heavier. I think the automotive industry is trying to circumvent this by making sure the aluminum alloy is designed in such a way it can make up for where it's lacking.
How is Ford addressing the potential for corrosion due to electrolysis?
The Brill Bullet interurban cars are considered the forebear of the modern high speed trains such as Amtrak's Acela. Those cars were one of the first (if not the first) use of aluminum for structural components of a rail vehicle in 1931. Though they lasted nearly 60 years in revenue service, they suffered from corrosion due to electrolysis where the aluminum was in contact with steel at various locations.
The Bullet cars did achieve their major design goal of high speed operation and reduced power consumption through the use of aerodynamics and weight reduction, but the ride quality never met expectations, partly due to the reduced weight.
A single car, #203, was built using steel, to the same design, after the original aluminum 203 was destroyed by a fire shortly after being placed in service. That car had noticeably different ride quality than the others due to it's heavier weight.
Other factors contributing to the ride quality issues were related to the truck (suspension) design.
Having operated those cars near the end of their service life, and the sole operational survivor in preservation, I have seen the corrosion first hand.
That is the main reason I remain skeptical of major use of aluminum in cars and trucks.....
While I can't speak for Ford specifically, as Martian pointed out, I do know a bit about electrolysis and a bit about trains.
Firstly, as you must know, I can see how with "hunting" and switches and frogs, a train car being of a lighter weight can be a huge disadvantage. Typically In large machinery applications, such as railroads and mining, heavier is almost always better as some parts of the track structure rely on weight alone. Modern vehicles differ greatly from how a train is meant to run on tracks so I won't compare the ride quality in that application.
But to your main point, the corrosion seen between two dissimilar metals in contact is being counteracted by using barriers (typically plastic). This really is a big problem when you have aluminum parts contacting steel parts and put under stresses and loads. I briefly mentioned that with aluminum suspension parts you have to consider the connecting parts such as fasteners and to pay attention to the ball joints. A stock aluminum control arm for example will have composite inserts for the socket as opposed to metal ones.
Another prevention method well used in my industry is anodizing. The aluminum is coated to create a more corrosion resistant surface. You'll typically see anodized aluminum in steering knuckles. There are other types of coating cheaper than anodizing as well.
Other components may require a corrosion resistant paste or composite shims. I imagine this is what the automotive industry is using to counteract electrolysis.
And to your point, I believe many aluminum parts were being utilized before the companies reported the accelerated corrosion and now the steps mentioned above being taken is a recent thing.
You're right to be concerned about it and I'm definitely not saying these methods are better than just using steel on steel, because I doubt they are, but as far as I know these are the preventative steps the industry SHOULD be utilizing to reduce corrosion.
TL;DR The positives of aluminum being lightweight and of similar strength to steel can be offset by the corrosion between two dissimilar metals. Barriers, such as plastic components and coatings, are being incorporated to counteract this.
First, I really like this thread - thanks for sharing your knowledge MrMeeseeks!
My question has to do with the finish & painting of aluminum skins. I work in aerospace and see plenty of aluminum parts but few have the sort of finish an automotive body part has. There are also special considerations when applying paint to aluminum. What are they doing to make the aluminum skins equivalent to the steel ones in appearance?
10-14-2016, 08:35 PM
