Alum. 6061 vs 7075

[nolimitmfg]

We are currently manufacturing a part out of 6061. The part is very similar to a roller lifter for an automotive engine in shape, and how it must perform. The part is being stressed in compression and also slightly side loaded. There is also lash in the assembly to make up for heat. The part runs in gravety fed oil. It is also hard anodized with a teflon type coating over top of the anodize. We have not had any significant failers, but the customer is asking us to use 7075 instead. Would there be any drawbacks to using the 7075 material such as possibly being more brittle. My concern is the two ears that hold the roller bearing could fatigue and snap off.

 

[swall]

7075 is more expensive than 6061. In my opinion, this is a case of "if it ain't broke, don't fix it". Your stated low incidence of failure suggests that there is no inherent stress problem with the part. I would leave it alone unless the customer absolutely insists on the change.

 

[nolimitmfg]

The customer is really insisting on this and cost doesn't seem to be a deteriant. I just want to make sure we do not create a new problem.

 

[TVP]

I agree with swall. IF your customer is insistent, then perform a small trial. 7075 will not perform worse than 6061 under your stated loading conditions. The only concerns I would have are related to manufacturing. Is the part crimped or otherwise deformed at room temperature as part of an assembly process? Is the part completely machined from bar, or is it finish machined from a formed/forged blank? 7075 has limited ductility at room temperature, especially in the T6 temper.

 

[nolimitmfg]

It is completly machined from bar. It is a T6 temper.

 

[Screwman1]

One possible consideration is the susceptibility of T6 7075 to stress corrosion cracking under sustained tensile loads. I don't know what the service conditions are ,but it is one thing to consider.

 

[mfgenggear]

nolimitmfg

what is the customers rationale for wanting 7075?
I would make analysis of both materials, I would sort of throw it back onto their lap.
and ask for a letter or email to remove your self as not responsible if a problem should arise.

HTH
mfgenggear

 

[nolimitmfg]

The customer's thought was, that the ears holding the roller axle were possibly mushrooming or pounding out over time, but there was no real evidence. If any did occur, it was because the bore was slightly undersize, causing the lifter pieces to stick. So we had to adjust the machined tolerance for the bore deviations.

The part is in service for about 9hrs a year. I would say it should run for about 2-3 years before needing possible replacement. The part is returned by a spring with any where from 150-1000 ft lbs. It is making 243 cycles up and down in about 13 seconds. A total of 9750 cycles over the course of a year. So there is sort of a pounding going on.

So will the compression, then non compression of the part cause stress cracking quicker than the current 6061 material vs 7075. The 7075 I believe will resist the mushrooming effect, but will it have a greater chance of stress chacking?

 

[mfgenggear]

nolimit

I uploaded the material data sheets for each material & compared the physical properties.
the 7075 was more impressive. however I would go the expert material engineers.
as stated above do an actual test.
I am an amature material engineer, i am not an expert. but these are my thoughts
I do see there is 1.5 x difference in hardness, the elongation is basicly the same. fracture toughness is equal.
if the customer wants the better wear & structural I would give it to them.

HTH
Mfgenggear