Aluminum 355-T6 Castings Welding Repair

[matt037]

I'm new here so here goes...
This is more of a general question regarding weld repair of 355-T6 aluminum castings. Here is a little background; the castings are heat treated to obtain maximum mechanical properties, they are then repaired by welding in order to fill holes and/or correct other damage. The welded area has lower mechanical properties than the surrounding material and this may cause a strength issue. What is the proper technique for getting the entire casting (including the welded area) back to the T6 temper? Can we re-heat treat the entire casting again (solution treating, quench, heat treating) without rendering it useless? Is there a danger of causing excessive grain growth by subjecting the casting material to a second heat treat cycle?

 

[swall]

The proper way to do this is to weld repair before heat treating to the T6 condition. But, unfortunately, many cases arise when this is not done,such as machinability considerations, etc. What we used to do (aerospace supplier, supplying 355 per AMS 4215) was to re-solution treat with a short cycle (4 hours) followed by the normal ageing cycle.

 

[matt037]

Thank you swall. What is the reasoning behind the short cycle (4 hr) solution treat, versus the suggested length of 10-12 hrs (ASM Metals Handbook)? Was this to control the grain growth in the casting or is there some other consideration here?

 

[swall]

In 355, the solution treatment accomplishes two purposes. First of all,it causes the copper and magnesium to go into solid solution to facilitate the subsequent precipitation hardening. Second, it refines the shape of the primary silicon particles, causing them to break up and round off. The latter is what takes up much of the time. So, in a re-solution treatment, that part can be skipped, thus shortening the cycle. The logical follow-up question then is, "since we got some melting and resolidification from the welding, don't we still need the longer time to refine the shape of the silicon particles in the weld zone?" No, not at all. Those particles will be extremely fine due to the rapid solidification and there is no benefit in the 10-12 hour solution time to refine them.