Hi all, I am involved in welding of aluminum 2219 alloy. I have heard that if you want your weld zone to be of same strength as of base metal then your weldment mmust be subjected to complete resolution treatment followed by aging. But, recently I went through some literature regarding aluminum welding. There was a table of strengths of different aluminum alloys as welded and after complete resolution treatment and aging. For aluminum 2219 alloy, T8 and T6, as welded was around 35 ksi, but after complete heat treatment the strength was not matching the BM strength, it was approx 8 ksi less.
My question is why can't we achieve exactly equal to BM even if we are using heat treatable filler rod 2319? Will it always be the case that after complete heat treatment we won't match BM or I'm missing something?
Thank all in advance for reading and responding.
Well, I see the 2219 is a heat treatable aluminum alloy provided as a wrought or forged product. The heat treatment consist of heating the metal to about 1000 degrees F followed by artificial aging between 300 and 400 degrees F. As such, the alloy is precipitation hardened, which isn't really precipitation, but more of a clumping mechanism. The clumping mechanism I am referring to is where the alloying constituents are "pulled" from the supersaturated solution and form small clumps or clusters in the atomic lattice. I don't know whether the base metal is rolled or forged after the heat treatment or before, but it would make sense to do a final roll after heat treatment to gain the benefit of stain strengthening, i.e. further distortion of the atomic lattice. Any mechanism that distorts the atomic lattice will strengthen the base metal. The distortion can come from cold work, alloying, etc.
In the case of welding, the alloying constituents do precipitate, i.e. diffuse to the grain boundaries. The precipitation of the alloying elements to the grain boundaries, i.e., over aging, weakens the base metal. The post weld heat treatment forces the alloying elements back into solution, the base metal is aged once again by heating to a low temperature to trigger the "clumping" action, but there is no cold working to further strengthen the metal.
As is the case with the heat treatment of ferrous metals, the strongest response to heat treatment occurs with virgin material. Repeated heat treatments produce less dramatic results. The same may be true with the heat treatable aluminum alloys.
This is what may be going on, I can't say without knowing exactly what processing steps are involved in the manufacture of the base metal. Without all the details, I can only surmise what the exact mechanism is. And as you point out, 2319 has almost the same chemistry as the base metal, but there are minor differences and it no doubt responds to the heat treatment in a slightly different manner.
Best regards - Al
We weld in annealed form followed by complete heat treatment, solution treatment plus quenching. We have a qualified procedure but the problem lies at heat treatment facility, they say whenever u send final product for heat treatment then send samples along with them, samples and final product are heat treated together, all the testing is carried out on samples to find the mechanical properties after heat treatment. The properties of sample materials are assumed to be the properties of final product. Not surprisingly, we always match the mechanical properties on the basis of samples of 2219 material.I contradict with heat treatment officials on two grounds.
1) Since response of weld metal is diffrent to heat treatment so rather then sending the sample of material you should demand welded sample in an annealed form like a final product, both should be heat treated together and testing should be done on the samples.
2) why should we send you samples everytime along with our product? Involve an inspector who could monitor the thermal cycle parameters everytime which will ensure repeatable mechanical properties.
What do you say on the current procedure followed by heat treatment officials? Also which code/ specification can tell thermal cycle information of 2219-O to 2219 T6
I would check literature published by the Aluminum Association and I would check literature published by the American Society for Materials. ASM publishes several books on aluminum, its alloys and heat treatments. The Aluminum Association publishes specifications on the various alloys, their designations, mechanical properties, and I would assume they also specify the different heat treatments used to achieve the necessary mechanical properties.
Best regards - Al
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