Strong material welded to weaker material

[greencircle]

Guys,

I need some help here.

Is Alum 7046 extruded tube weldable to A356 casting? If so any of you know filler material?

Do you see an issue for a 30ft (4ft casting + 26 ft pole) assembly. My concern is that the pole is have a yeild strength of 60ksi & the casting is 12ksi.

Normally the pole would be Alum 6061 (45ksi).But for that assembly height & windloading, the assembly would not meet the CSR. So planning on using stronger alloy.



I am planning on using the 0.6Y as allowable & check for the loading. Do you see any concerns?

Thanks
GC

 

[drawoh]

greencircle,

Is your soft A356 casting at the bottom of your structure where the maximum stress is?

Why not lose the casting, and weld on flanges of strong material?

Is there a high strength aluminium weld filler? I recall seeing yield stress at the weld of around 20ksi. I cannot find specs for aluminium 7046. Aluminium 7075-0 (annealed) has a yield stress of 15ksi. Can you heat treat this thing when you are done?

--
JHG

 

[greencircle]

Yes the sand cast part is a Pole Base. Flanging is a Good point. But not sure about heat treatment. Even if some can do, the overall size is still an issue.
I am think about leveraging the telescopic joint. The Cast base has a tall neck & the pole can inserted into this neck. A good 3 feet of overlap be made with 2 welded connections. one at the each end of the overlap.

 

[JStephen]

With a lot of the aluminum alloys/tempers, if you weld to them, the allowable stress near the weld is a lot lower than the allowable stress of the native material. So the 0.6Fy wouldn't necessarily be appropriate.
Is fatigue design required?
There is an Aluminum design manual that covers a lot of this.

 

[drawoh]

greencircle,

I was thinking that heat treat was not an option for you. High strength aluminiums are heat treated. Medium strength aluminiums are heat treated or work hardened. Material around a weld is annealed.

I see no advantage from a high strength aluminium alloy. Your weak point is at the welds. A 6061-T6 extrusion should work fine for you. Consider fabricating your tube out of 5086. If you toss out the casting and fabricate a large flange with gussets, you will provide your weak welds all sorts of leverage.

--
JHG

 

[greencircle]

Heat treatment is not an option because of the overall assembly height.

Your weak point is at the welds

Do you think this weak point (HAZ) would be some percentage of Yield strength of the material. If that's the case then the HAZ for 7046 should be much stronger than for HAZ of 6061. I couldn't find a resource for this data because 7046 has a yield strength of 60ksi.

 

[drawoh]

greencircle,

If this thing is in any way safety critical, you would make it from material you have specifications for. It helps if your fabricators can get their hands on it. Aluminium 7075-T6 is heat treated and it has a yield stress of 73ksi. Annealed (welded), it is called 7075-0, and the yield stress is 15ksi. I don't think there are rules of thumb here to help you. I am looking at Matweb here.

I have not looked this up just now, but I recall reading somewhere that 7075 is not recommended for welding. I know nothing about 7046.

--
JHG