Watanabe Experts

[smy0003]

I thought the problem was that magnesium became porous and lost a few psi of pressure a week?

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[sailaholic]

Hks it all depends on the alloy and heat treatment for all 3 materials.

Can't really comment on the porosity but I'd be surprised. Magnesium is much harder to cast though given it burns in air. So maybe if there is porosity issues it's to do with quality of the casting not the material.


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[hks_kansei]

Hks it all depends on the alloy and heat treatment for all 3 materials.

Certainly it does, and even manufacturing method would come into it (the whole cast vs forged aluminium wheels)

I'm just going more from anecdotes of Magnesium wheels tending to get stress fractures around the wheel nuts and spokes, vs alloy/alu where you only really hear of cracks in cheaper made wheels (or impact)

vs steel where you can drive headlong into a brick at 100kmh and it'll just go out of round a bit.

[93_Clubman]

See Babs 2nd last post page 1 with yellow highlights:
viewtopic.php?f=44&t=26447&p=332023&hilit=magnesium#p332023

[StillIC]

Hks it all depends on the alloy and heat treatment for all 3 materials.

What depends on the alloy and heat treatment?
In regard to the alloy:
Whether the wheel has a limited life? Yes for Al, arguably. No for steel as any old steel can have an infinite life. Probably no for Mg as they all seem to have a limited life.
The length of life of the wheel? Yes for Al. No for steel, see above. Yes for Mg.
The endurance limit, if relevant? Yes for Steel. Not relevant for Mg. Arguably for Al.
The failure mode? Yes to all.

In regard to the heat treatment:
Whether the wheel has a limited life? Not sure for Al, but heat treating can make it stronger. No for steel as any old steel can have an infinite life. Probably no for Mg as they all have a limited life.
The length of life of the wheel? Probably for Al. No for steel, as can be infinite. Probably for Mg, I think.
The endurance limit, if relevant? Yes for Steel. Not relevant for Mg. Probably for Al.
The failure mode? Yes to all, I think.

There are so many other different properties of metals that can be changed with different alloys and/or heat treatment, but I am unwilling to explore them all here.

Edit: Talk about a can of worms!

[sailaholic]

I was primary referring to failure mode in the comment to hks. Steel will snap/tear just as badly as cast aluminium depending on loading.

You've hit most of the stuff on the head. Though you can still change the life of steel wheels based on grade and temper. All depends if you're below the SN curve or not.

We have had steel wheels fail at work. Fatigue in the spokes (Sunraysia style) so now buy landcruisers with factory alloys and have no issues.

My thesis was on the change in mechanical properties of a novel titanium alloy for human implant use.

With varied heat treatment I could change elongation to failure between 6 and 60% for memory with corresponding changes you would expect is the various properties but also a change in elastic modulus! But that was due to a stress induced phase change in the microstructure.


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[StillIC]

We have had steel wheels fail at work.

With enough stress and/or enough cycles, anything will fail, as I am sure you know. If max stress is kept below the endurance limit, then all OK (as you know).

..... but also a change in elastic modulus! But that was due to a stress induced phase change in the microstructure.

This goes against all I have learnt, in that elastic modulus is an intrinsic property unaffected by heat treatment, loading or other post manufacture processes.

Could it be that a change in the cross sectional area of the test pieces you had, due to significant elongation (60% !!), made it seem as if the elastic module was changing?? That is, with elongation comes a reduction in cross section, as a function of Poisson's ratio, which altered the calculated elastic modulus??

[sailaholic]

[quote="StillIC]
This goes against all I have learnt, in that elastic modulus is an intrinsic property unaffected by heat treatment, loading or other post manufacture processes.
quote]

Yeah...that is the traditional learning, and boy did I get roasted over it in my first thesis presentation. "Modulus is set by god". Unfortunately I had been spending all my effort getting the testing done and had not done much of the lit review at that stage and couldn't refute the statement.

A change in modulus with titanium alloys (not pure or close to pure (which is most commercial Ti) is actually reasonably well documented when the heat treatment causes / traps the titanium as a beta or near beta phase alloy. The Beta phase alloy actually has different cubic structure (discorded base centered cubic) compared to the alpha phase of orthorhombic martensite. It's this change that allows modulus to change. Some samples actually had two yield points(!) showing stress could cause phase change and had been shown to be reversible which gave the possibility of memory shape alloys.

It was a pretty funky alloy, 4 major alloying elements (Nb, Zr, Mo, Sn) for a total of 32%. Combination was specifically dreamed up to try and achieve a very low modulus to imitate human bone.

Had to go and find my copy of the thesis, so can clarify, same alloy, different heat treatments. UTS 970Mpa, YS of 930, Modulus of 76Gpa and ~6% elongation to failure to UTS of 556MPa YS of 350Mpa Modulus of 52Gpa and 47% elongation to failure.

Was pretty cool sh*t but a bit of a head spin at the time.

[StillIC]

Some samples actually had two yield points(!) showing stress could cause phase change and had been shown to be reversible which gave the possibility of memory shape alloys.

Ahhhh...shape memory alloy. Of course! It didn't even occur to me until I read it in your post. A colleague of mine invented a shape memory alloy heat engine, the only prototype of which I have sitting on my desk at work. Unfortunately for him, some German researchers invented the same thing about 20 years earlier. I think it can be made cheaper than some traditional external combustion heat engines, but we have no likelihood of funding to ever develop it to find out. But there is no way I can throw it out. I think I am way off topic now.......