6061 alum or 7075 alum

[neilpiper]

I currently use 6061 for machining my parts, what would be the advantage of using 7075? How much more money are we talking about?

I also anodize my parts afterwards, will i get a different color anosizing if i go with 7075 ?

 

[swall]

7075 is much higher strength, but if the 6061 parts are strong enough, there is no advantage. 7075 is more expensive. The anodize film color, if clear rather than dyed, will be a different shade of gray.

 

[NickE]

In what condition are you machining the 6061, and what is the proposed condition of the 7075?

What types of parts are these?

Are parts failing?

Material changes kind of follow the "If it aint broke, dont fix it." rule. 7075 is generally a stronger alloy than 6061.

A quick review of the ASM metals reference book gives:

T6 condition
General Corrosion:  6061>7075
SCC:                6061>7075
Cold Workability:   6061>7075
Machinability:      6061<7075
General Weldability:6060>7075

The above is a pretty general guide though.

 

[neilpiper]

ok, yes parts are showing stress cracks.... the parts are cnc machined, instead of making the parts thicker (more material) i am thinking of using 7075 to keep the weight the same but increase the strength.

so how muich stronger is 7075 than 6061?

 

[Kwan]

7075-T6 tensile strength is around 76 ksi. 6061-T6 is around 42 ksi. The actual values vary with grain direction and form (extrusion versus plate for example).

7075-T6 is generaly used for compression type load paths and 2024-T6 for tension loads. This is because 7075 is superior for compression and 2024 has good crack growth properties. I'm being very general here.

Be aware though because 7075-T6 is prone to stress corrosion cracking. You have to understand your cracks. Why are they forming? Is it due to a high stress level under operating condition (fatigue)? Or due to ssc, or is the part welded? If you describe your part more on this forum the material experts here can point you in the correct direction. Where is the crack? Is it in a weld? Located at a fastener hole? etc.

 

[neilpiper]

ok here is an example of a part that i am talking about.

http://www.breaksk8.com/engtips/part1.jpg

http://www.breaksk8.com/engtips/part2.jpg

http://www.breaksk8.com/engtips/part3.jpg

5 inches in length, 1 inch high and 1 inch depth

Lets say that there is pressure being put on it lengthways, would it be better to have 7075 or 6061 or another type of alum?

 

[MikeHalloran]

Is the pressure on the part trying to make the part longer? As in, force applied to cross pins riding in the slot at either end?

Where do the cracks appear?




Mike Halloran
Pembroke Pines, FL, USA

 

[neilpiper]

no trying to make the part shorter i guess, thats how the force is being applied...... the slot would just be for looks.

 

[TVP]

If exposure to corrosive environments (chloride ions, etc.) is minimal, then 7075-T6 offers a fairly sizable increase in strength over 6061-T6. Anodizing the entire part and then sealing it should offer adequate resistance in normal atmospheric environments. I would change to 7075 if I could exclude stress corrosion cracking as a possbility.

 

[Kwan]

You can use 7075-T73 to minimize SCC further. I like to shot peen 7075-T6 anytime grain is opened (machining).

You have to try to determine if the crack is from SCC or is it a fatigue crack. This will help you choose the proper material. If it is SCC be aware that 7075-T6 might make the situation worse.

 

[neilpiper]

no chemicals whatsoever touch this part, so would you agree that 7075 is the way to go?

 

[TVP]

It's not just chemicals in the conventional sense, but also exposure to rain and other wet environments that can pose a problem. If salt is used for road de-icing, and this part is exposed to it, then SCC is a real possibility. If you are sure that Cl- ions are not going to be a problem, then yes, 7075-T6 is a good option. Kwan's suggestion of shot peening priot to anodizing is a good one if the resultant surface finish is acceptable.

 

[neilpiper]

I have never heard of shot peening, can you explain?

 

[swall]

Shot peening is a process for improving the surface stresses in a metal component. Think of grit blasting or sand blasting, which are uncontrolled processes that affect the surface of the metal. Shot peening is somewhat similar, but is a controlled process, utilizing round steel shot to impinge on the surface.This creates little dents and leaves the metal surface in a state of residual compression, which makes it more difficult for fatigue cracks and SCC cracks to initiate, since they initiate due to tensile stresses. In other words, the residual compressive stesses created by shot peening cancel out a portion of the applied tensile stesses that could cause a crack to form.

 

[TVP]

Take a look at this website for more information on shot peening:

http://www.metalimprovement.com