Effect of solution/precepitation hardening on Alum 6061 T6

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Guys,
We have a alum welded assembly. Basically 2 parts welded together.

Part 1 : A356 at T4 condition
Part 2 : Alum 6061 T6 condition

After welding them, there is a HAZ developed. We are trying to heat treat and bring the temper to T6 at HAZ. Now, I have learnt that there are 2 stages in this heat treatment process.

Stage 1 : Solution/ Precipitation hardening. This process will bring the temper to T4 condition (I guess)
Stage 2 : Artificial Aging. This process involves rapid quenching after heattreatment. I learnt there are couple of different time/temp configurations.

How ever my question is; Since one of the part2 is already at T6 before welding. After weld this part2 will actually have 2 tempers (T6 all over except T0 at HAZ).

What happens to the T6 region in Stage 1 ? What happens to T4 region in stage 1?
Can some one throw some light this for me please.

Thanks

 

[metman]

Solution treatment and aging are two separate processes.

1. Solution treatment is performed by heating and then quenching. This puts the Aluminum alloy in the "O" condition which is metastable and ready to be age hardened (precipitation treatment).
2. Preciptation treatment or age hardnening is performed by a lower temperature process over a longer perion of time.

Someone will give you the process temperature and times for these two processes.

Design for RELIABILITY, manufacturability, and maintainability

 

[tbuelna]

It should be no problem to have the A356 castings heat treated to a -T6 condition since this is routine. After welding you can have the weldment stress relieved and heat treated/aged to a -T6 condition.

It is also common practice to repair defects in A356-T6 aluminum sand castings using welding, and then stress relieve and re-heat treat the repaired casting to a -T6 condition. As long as you use the correct filler alloy when welding, there should be no problem with heat treating the A356/6061 weldment to a -T6 condition, including the weld HAZ.

 

[swall]

As I understand your question, you wish to weld a 6061-T6 wrought (extruded?) piece to a A356 casting in a "T4" condition. (T4 is an undefined temper for 356 alloy). In a nutshell, the 6061 material will revert to a cast structure (roughly condition F)in the fusion zone of the weld and an over aged (T7 or lower strength) condition in the HAZ of the weld. The A356 casting will revert to condition F in the fusion zone and a T7 or lower strength in the HAZ. If you wish to solution treat and age the weldment you will note that there is a mismatch in the solution temperatures of 6061 (985F) and A356 (1000F). There is also a mismatch in the ageing temperatures of these two alloys. 356 is aged at 310F and 6061 extruded material is aged at 350F. To deal with the mismatches, you will need to solution treat at 985F. This is due to the copper content of the 6061 alloy--if you attempt to solution treat this alloy at 1000F, you run the risk of eutectic melting. For ageing, if you age at 350F, you will overage the A356 material. You will end up with properties between that of a T6 and T7 treatment.