alum weld help

[WillieB]

If I understand, the center of the joint made up of 4943 filler is very strong, but heat affect dramatically weakens base metal

[H80N]

If I understand, the center of the joint made up of 4943 filler is very strong, but heat affect dramatically weakens base metal

But isn't that anealing of the base metal at the HAZ the same regardless of the filler metal used..??...

[WillieB]

It appears that way. It does seem to me that 4943 requires fewer BTU to wet, maybe it is quicker, reducing the width of HAZ.

[WillieB]

I make the point often that well designed items exploit diagonal bracing, and place a weld not at the center of a member, but at its end where stress is at its least. Essentially the way post and beam buildings are built. They are incredibly strong despite relative weakness at the joints. I once saw a Caterpillar bulldozer trying to knock down an 1800 timber frame house. after knocking the stone foundation from beneath it he pushed it across a meadow before he was able to break each corner post out and destroy the house.

In an aluminum frame, softness at the joint allowing minor deformation before cracking might contribute to the structural strength.

[OscarJr]

I make the point often that well designed items exploit diagonal bracing, and place a weld not at the center of a member, but at its end where stress is at its least.

You'd be surprised. For beams with rigidly fixed ends, beam deflection peaks in the center for uniform loading, but the stress is actually greatest at the ends, even though they are immovable.

Here is a stress/deflection graph of a 30in hollow circular beam, uniformly loaded along it's length, rigidly supported on both ends.

• The deflection curve has units of inches on the X-axis from 0 to 30, and units of inches in the Y-axis directly reflecting actual deflection figures (in this model at least)
• The stress curve has units of length just like the deflection curve, but the Y-axis has to be scaled down by a factor of 2E(-6) in order to be visible within the plot window along-side the deflection curve
• Notice how even though maximum deflection occurs in the center of the beam, the maximum stress "felt" is at the ends of the beam (at "0" and "30" inches, the ends of the beam)

Note: Even if the material Yield/UTS/Young's Modulus is changed, the fact remains that the equations describing these phenomenon remain unchanged---the stress curve remains parabolic in shape, and the deflection curve is still 4th-order Quartic. Parabolic stress necessitates relative maxima at end-points, quartic deflection necessitates zero deflection at end points and a relative maximum in the center (for uniform loading).

Basically, for uniform loading, the deflection along a beam does not always dictate the relative stress at those same points.

[ryanjones2150]

alum weld help

That's funny, I was just about post that exact thing. Weird.

[OscarJr]

That's funny, I was just about post that exact thing. Weird.

I suppose great minds think alike

Also interesting to note that [in this particular scenario] the stress curve has two roots (solutions on the X-axis) that correspond to zero stress. Of course the reality is that a mathematical model can only approximate true reality, but none the less there are two points on a rigidly-fixed tubular beam that experience almost nil stress, which is a consequence of uniform loading on it.

[WillieB]

Way too complex a graph for me to understand, I suspect you intended it to be. I do know a plank supported on two sawhorses one at each end, loaded with bricks until overloaded to failure, won't break at one end. It'll break near the middle.

[ryanjones2150]

alum weld help

Ha! But do you have a graph to prove it?!

[OscarJr]

Way too complex a graph for me to understand, I suspect you intended it to be. I do know a plank supported on two sawhorses one at each end, loaded with bricks until overloaded to failure, won't break at one end. It'll break near the middle.

No, it was not intended to be "too complex to understand". Just a visual representation of the bullet'ed statements regarding stress and deflection.

A plank on two sawhorses is a different scenario where a beam is simply supported from underneath on both ends, not with rigidly fixed ends. That scenario does not apply to a beam with rigidly fixed ends, which is what diagonal bracing would be. Correct, a plank on sawhorses will break in the middle with enough load, so long as it has a constant cross-section:



Rigidly clamp the ends, and it changes everything. Which brings us back to exactly what 'diagonal bracing" would be considered. The member acting as a diagonal-brace is a beam with rigidly fixed ends, hence the "plank supported on sawhorses" analogy does not apply.

[WillieB]

I haven't your level of knowledge in engineering. My point was that a bit of softness at the joint of a truss isn't necessarily destroying the strength of a well designed object.

The simplest of tables; a top with four legs welded to it will likely bend because the weld adjacent metal is annealed. Placing a diagonal brace of even 6" length will dramatically strengthen the table. Now the annealing at the weld is less a problem.

A bicycle, using a truss design isn't objectionably weakened by annealing near the weld.

Perhaps because of the nearby annealing of aluminum, the added strength of 4943 filler is of little value.

[WillieB]

Steel building or bridge frames have joints subject to motion fastened with elaborate fabricated intersections pinned with bolts, or rivets. This offers the sawhorse effect moving the stress away from the weld.

[H80N]

.......
Perhaps because of the nearby annealing of aluminum, the added strength of 4943 filler is of little value.

Willie

are we discounting the option of heat treating the whole welded assy and that some alloys like 6061 will age harden... regaining some of it's strength...??

[WillieB]

Willie

are we discounting the option of heat treating the whole welded assy and that some alloys like 6061 will age harden... regaining some of it's strength...??

Again, I'm a little vague about those facts, I understand expensive fabrications like Zancanato Cycles are heat treated to regain temper. I believe it is impractical for amateur fabricators like me to consider. Age hardening sounds pretty passive, I like that. Tell me more.

[H80N]

Again, I'm a little vague about those facts, I understand expensive fabrications like Zancanato Cycles are heat treated to regain temper. I believe it is impractical for amateur fabricators like me to consider. Age hardening sounds pretty passive, I like that. Tell me more.

Willie

I am by no means an expert on the subject either.. but I do understand that Aluminum Alloys 355, 356, 6061, 6063, 7075, and 2024 can be age hardened.

Here is a short thread from WW that gives us some info

http://weldingweb.com/showthread.php?248021-how-to-reharden-aluminum-after-annealing-it

Maybe somebody with more comprehensive knowledge of the subject can chime in...

AND... we have managed to wander way off topic yet again..

But it is a very interesting and probably useful offshoot....
(is an observation...NOT complaint....)

Here is a link to very much more comprehensive Aluminum info...
the water gets deep very fast...
Aluminum Alloy Property Data from Matweb

http://matweb.com/reference/aluminum.aspx