What are the advantages in using magnesium alloy instead of aluminum a 3

[Tobalcane]

I've started doing vibration fixtures for electronic equipment. I learned that many people are using Magnesium alloy instead of aluminum. Can anybody tell me what are the advantages in using magnesium?

Thank in advance for your insight!

 

[EnglishMuffin]

Magnesium alloy is usually touted as having a higher specific strength than aluminum alloy, but of course it depends somewhat on which alloys you pick. Although magnesium alloys have a considerably lower density, specific stiffness of magnesium alloys is about the same as aluminum alloys, so it's not immediately obvious what the advantages would be from a vibration point of view, at least to me. However, magnesium alloy is also supposed to provide higher damping capacity than aluminum alloy, so perhaps that is one reason. On the other hand, most of the damping present in structures has nothing to do with the material properties, but comes from other sources, such as bolted joints etc. Then again, because of magnesium's higher specific strength, it may be possible to use thinner sections than would otherwise be practical, and thereby obtain better dynamic stiffness of components than you could with aluminum. I have read that the use of magnesium alloy in aerospace applications is declining, reputedly because of corrosion problems. If there was that big of an advantage to magnesium over aluminum, you would think it would show up in that field. But it's use in automotive is on the increase. Then there is the economic aspect, which may currently favor magnesium, much of which comes from Africa. That may explain some of it's current popularity. There may also be some castability advantages with magnesium alloy, but I'm not sure about that.

 

[brad]

It's the same reason as why aluminum is used instead of steel--for problems which deform in bending, a greater stiffness can be gained using a lighter material.

For beam stiffness and minimum, the defining relationship which dictates the optima is (E^0.5 / rho).

Material E (GPa) rho(Mg/m^3) ratio
Steel 200 7.85 1.8
Aluminum 70 2.8 3.0
Magnesium 41 1.75 3.7

So for the same mass, aluminum in bending can be as high as 1.7 times (3.0/1.8) stiffer than steel, but magnesium can be more than 2x (3.7/1.8) times stiffer than steel.

Hopefully this post makes sense.

Brad

 

[GregLocock]

EM is right about the damping, at least in the prototypes I've seen. The inherent material damping does affect the higher frequency modes, so for things like gear whine it is better to use magnesium than aluminium, so long as they are the same stiffness. However, usually 'they' are going for a weight reduction, and 'they' compromise on stiffness by using the same section thickness as aluminium. So we end up with a more flexible part that does not perform as well, dynamically.



Cheers

Greg Locock

 

[EnglishMuffin]

Brad:
Regarding the specific stiffness (E/density) I think it depends very much on which alloys you pick. If you look at this link, for example, the specific stiffnesses for a particular pair of aluminum and magnesium alloys are virtually identical - or at least very much closer than you are claiming. Unless of course the link is incorrect, which wouldn't surprise me. (It's Chinese).

http://www.wahhong.com.tw/engpage/m3.htm

 

[EnglishMuffin]

By the way, I've just discovered this thread, which I was completely unaware of until now, (using the AltaVista search engine directly on the web) in which Mr. Locock's opinion seemed (at least then) to be very similar to mine in regard to specific stiffness of magnesium versus aluminum. And he also mentioned the damping on that occasion.

Magnesium transmission casings
thread78-41560

 

[EnglishMuffin]

Brad - sorry - ignore my last comments - I didn't read your post carefully enough - I thought that last column related to specific stiffness.

 

[EnglishMuffin]

Brad: (I'm going to shut up after this!) :
I do think it is should be pointed out that if you have, say, a cantilever beam which is required to be equally stiff in all directions, then there is no advantage in using magnesium from the point of view of natural frequency - because in that case the natural frequency is simply proportional to the square root of specific stiffness, or (E/rho)^0.5 , and magnesium is then actually the worst of all using your figures. It is only when you are interested in preferentially stiffening the beam in one direction that you get the E^0.5/rho relationship.

 

[CurlyJon]

The primary reason I have most often heard is that you can get the same, or better, dynamic characteristics from a magnesium alloy fixture with lower weight. Since a shaker system has a specific force limit, the total weight may limit the available acceleration capability. When testing small, light weight components, we used to use aluminum fixtures because they were less expensive and weight was not a limitation.

CurlyJon

 

[ravide]

hi guys,
Can i just ask one question? In the case of most efficient vibration transmitter, does it follow that aluminium alloy would be the best material to use since it has low material damping?

Thanks,

regards,

 

[CurlyJon]

I think the transmission, other than near resonance frequencies, is more affected by stiffness than damping. I am more expert at measurements than at structural dynamics, so maybe someone else can give better information.

CurlyJon

 

[izax1]

I think that if we are talking about vibration fixture, several parameters needs to be considered. But first, the specific stiffness (ratio E/rho) is almost equal for Steel, Titanium, Aluminum and Magnesium. So the same geometric design in any of these materials will give the same natural frequency.

But this is not the issue in fixture design, because you are attaching (for the fixture) a lumped mass. In this case, it is mainly the E that will contribute to the stiffness. So in that sense, steel would be the choice. Due to weight limitations of the shakers this is not always possible, and hence a lower density material is used. In order for a Al fixture with a mass attached to it to have the same natural frequency as a steel fixture, the geometry neeeds to be altered. (Cross sections/Area moments of Inertia) So the same goes for Mg. Even lower E requires even higher geometrical stiffness for the same natural frequency as the steel fixture with the same mass attached.

If the design is clever, mass of a Al or Mg fixture will be lower than the steel.

I think the material damping in Al or (even higher in) Mg will have a small influence. The alloying components will tend to reduce the initially good damping properties of pure Al or Mg.

Bernt

 

[brad]

EnglishMuffin--
Sorry I didn't reply earlier (very little surfing lately).

Again, my source is M. Ashby for the values stated (I apologize for lack of citation). Ashby's text is a highly-respected text for materials in design (and he's British, so he must be right ). The E^0.5/rho is can be thought of as "specific stiffness" for problems dependent on area inertial terms (I/J), rather than area.

Specific stiffness of most structural metals ("area&quot is pretty close, which is why steel is usually used for axial/pure shear loads (it's cheap and just as good).

Regarding your last post: my stated relationship holds whether it is one direction or both directions of a cantilever, presuming one can use a "box" section (I think your statement is presuming a solid section, and I will agree with you that there is no benefit under that scenario).

Cheers,
Brad

 

[EnglishMuffin]

Brad: Yes, I was considering only a solid section, and I still believe for that case that your quoted relationship holds unidirectionally if you maintain the same mass and allow the cross section to change only in that direction. But I didn't think about a box section. How are you defining the wall thickness exactly ? Would you care to show how you derive the relationship in that case ?

 

[brad]

EM--
My idea was poorly communicated in my last post. You are correct--for unidirectional cantilever loading, the relationship holds whether a solid section or box/pipe section. For bi-directional cantilever loading, a "hollow" section must be utilized to benefit.

The formal citation: Michael F. Ashby, "Materials Selection in Mechanical Design". He describes the means to derive this (and gives great charts on these relationships). I will work to derive and post this in a coherent manner; it's been a few years since I did the derivation.

I'll endeavor to post this in the next few days.

Cheers,
Brad

 

[EnglishMuffin]

Thanks - I have seen that book mentioned before on this web site. It seems like a book I ought to get. My question is essentially how one defines the wall thickness of the box section. It obviously cannot be completely arbirary, since it's maximum possible value would imply a solid, for which we seem to agree that the relationship does not hold for the multi-directional case, and it's minimum is zero, which implies a non-existent object.