Material Compatibility (galling)


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    Default Material Compatibility (galling)

    Lets say I want to make a cylinder out of aluminum (6061-t6), and a piston out of aluminum too (7075-t6). Will these gall? I don't want to make the pistons out of plastic (nylon, derlin, or teflon), and there is no room for a sleeve. Iron, steel, and brass are too heavy.

    Are there any charts that I can find that show how two different materials rubbing together will react?

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    The general 'rule of thumb' is that metals running together should be dissimilar and one should be softer or weaker than the other.

    There are some situations where the 'rule' can be broken fairly safely:

    Brass running on brass, or bronze on bronze, is normally quite okay and sometimes with some bronzes it is not even necessary to use lubrication. The mechanism inside old clocks was often made from a bronze that did not require lubrication and oiling them could cause accelerated wear b ecause the oil collected dust.

    Hardened steel running on hardened steel, with the proper type of lubrication is also okay; obviously because it occurs in gearboxes and transmissions in nmotor vehicles. But in this case the surface finish has to be very good and the correct lubricant is essential.

    The reason for having differences in strength and softness is because whenever two metals are sliding past each other it is the high spots that hit. And extremely high contact pressures can occur betrween these high spots; high enough to create a hot spot at a temperature high enough that micro-welds can form. This is less likely to happen between dissimilar metals and when it does the weld fractures readily. Sometimes the fracture is in the harder metal, the localized high temperature hot spot that caused the micro-weld can make it brittle, when this happens the minuscule particle that breaks off embeds into the softer material and does not cause problems. Sometimes the weld tears out of the softer metal, but again this does not cause a problem because the fragment of softer metal gets smeared out into the surface roughness of the harder metal. This process of welding and breaking and smearing eventually results in the high spots in both metals being rubbed off or smeared out and this is the running in process.

    However, when the two metals are identical or very similar things can go wrong very badly. The micro-welds still form, but now the material strength is the same on both sides. This means that there is a chance that tearing will occur on both sides and instead of a little particle breaking off and getting embedded in a softer material it is possible that a large particle, larger than the clearance between the moving parts, tears off and gets wedged in the clearance...causing more welding and tearing until everything just galls up solid.

    It is possible to get some aluminum alloys that will run against each other okay. I think some of the high silicon casting alloys can run with some of the formable alloys; think about forged pistons running in an aluminum cylinder on cheap little engines. But I think trying to run 6061 against 7075 is risky. It is possible this combination would work okay if one or both was hard-coat anodized but some experimentation may be a good idea.

    An open mind is a virtue...so long as all the common sense has not leaked out.


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    I don't know the answer, but that is an interesting combination. The difference in properties between those two alloys is nearly as much as between many steel alloys and the 7000 series Al.

    I assume you do not have room in your design for piston rings.

    With a cylinder of 6061 T6, this is not going to take a great deal of abuse anyway, so I am assuming that the application is not that severe (not an endurance race car).

    It actually is not that hard to coat up the wall - either alodine or anodize or nitride it. I am not sure if you can chrome or nickel plate Al cyclinder walls, but I am thinking the answer is yes. If you end up plating the walls, then I would try to use 7000 series for the entire setup if you can - great stuff.



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    Thanks for the feedback! Interesting.

    I guess I'll have to experiment. I know 7075 is harder (or stronger?) than 6061. I would use something else for the piston but I don't know what, it has to be light, or as light as aluminum. It's going to be a air powered motor, so the lighter the better, and I don't want to use piston rings because I want as little friction as possible, and because, as harryn said, it probably won't stand up to rings. Titanium would probably work, but too bad it costs an arm and a leg.

    I made this compressor out of cast iron and 6061, and I'm going to make another similar one out of aluminum, but run it backwards as a motor.

    Attached Thumbnails Attached Thumbnails Material Compatibility (galling)-cimg1395-jpg   Material Compatibility (galling)-cimg1397-jpg  


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    Quote Originally Posted by skmetal7 View Post
    Thanks for the feedback! Interesting.

    I guess I'll have to experiment. I know 7075 is harder (or stronger?) than 6061. I would use something else for the piston but I don't know what, it has to be light, or as light as aluminum. It's going to be a air powered motor, so the lighter the better, and I don't want to use piston rings because I want as little friction as possible, and because, as harryn said, it probably won't stand up to rings. Titanium would probably work, but too bad it costs an arm and a leg.

    I made this compressor out of cast iron and 6061, and I'm going to make another similar one out of aluminum, but run it backwards as a motor.
    matweb.com is your friend

    btw, air powered motor.... can you use an o-ring?



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    Default friction

    I occurs to me there will be much less friction using rings than full surface of the aluminum. I think that is why rings were incorporated in the first place. Plus the added feature of a cheap replacement product down the road.

    JMO
    Bob



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    I've made collets with a 0.5mm pitch thread out of 7075-T6 and the collet nut out of 6061-T6. Ive never had any gauling problems. I'd imagine if a thread would not gaul then an arc surface with light pressure would not as well.
    http://i54.photobucket.com/albums/g1...2/100_0653.jpg

    Now wear is another issue. If you anodize either the piston or cylinder I would be concerned that you are more closely matching the surface hardness of the two, thus bringing gaulling back into your woes. One advantage to wieght of a piston is the inertia, right? maybe? Brass360 would be a good choice??



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    It will Gall, garanteed.
    http://en.wikipedia.org/wiki/Galling
    Galling is caused by the oxides of two similar materials rubbing together, pressure makes it much worse. Chrome on chrome, aluminum on auminum, titanium on titanium. I made a 6061 spinning fixture once when I was younger and tried metal spinning some aluminum sheet, JUNK! Oh well, learned exactly what galling is.



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    You can assemble like materials without galling. You must ensure that they have a different hardness. Like the above wiki link,, "It does not occur when joining dissimilar materials (for example threading 18-8 stainless into 17-4 stainless)"
    Hardness is the key.

    Aluminum 6061-T6
    Hardness, Brinell 95.0

    Aluminum 7075-T6
    Hardness, Brinell 150

    Oh yeah, matweb is sweet!



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    I can tell you that 2024 and 6061 won't work

    Mechanical property data for
    Aluminum 6061 T6 Plate Bare

    Property Value Notes
    Brinell hardness 95 500 kg load; 10mm ball
    Elongation
    % in 2 inches 12 1/16" thick sample
    Elongation
    % in 2 inches 17 1/2" thick sample
    Endurance limit
    KSI 14 Based on 500,000,000 cycles of completely reversed stress using the R.R. Moore type of machine and specimen
    Modulus of Elasticity
    KSI x 10^3 10 Average of tension and compression moduli. Compression modulus is about 2% greater than tension modulus
    Ultimate
    KSI 45
    Ultimate shear strength
    KSI 30
    Yield
    KSI 40


    Mechanical Properties
    Mechanical property data for
    Aluminum 2024 T3 Sheet BARE

    Property Value Notes
    Brinell hardness 120 500 kg load; 10mm ball
    Elongation
    % in 2 inches 18 1/16" thick sample
    Endurance limit
    KSI 20 Based on 500,000,000 cycles of completely reversed stress using the R.R. Moore type of machine and specimen
    Modulus of Elasticity
    KSI x 10^3 10.6 Average of tension and compression moduli. Compression modulus is about 2% greater than tension modulus
    Ultimate
    KSI 70
    Ultimate shear strength
    KSI 41
    Yield
    KSI 50

    Why not just make a steel liner? Turn it out of pipe and hone it?



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    wow thanks for the replies!

    yeah i think ill be able to fit a cylinder liner in there if i move things around a bit. I was practicing making one out of steel tube and honed and it seems like it might work.



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    Quote Originally Posted by skmetal7 View Post
    wow thanks for the replies!

    yeah i think ill be able to fit a cylinder liner in there if i move things around a bit. I was practicing making one out of steel tube and honed and it seems like it might work.

    I really think that is your best bet. Out of every aluminum block engine, piston pump, ect I've ever seen, every one has an iron bore. I doubt they would use liners if they could save money, especially on something small like a weed-wacker.



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Material Compatibility (galling)
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