Blistering during Heat Treatment of AA 6061

[HY Tan]

Hi,

My company is manufacturing aluminum 6061 forgings (forging temp ~500C). After forging, we are required to heat treat the part to T6 via solutionizing, quench and artificial aging.

One problem we are encountering is blistering at certain locations, blister size approx. 1-2mm diameter. The blisters do not appear at any stage during the forging process, only right after the solutionizing oven.

Do you guys have any idea about the cause of such blisters? I've tried solutionizing in an inert environment (N2) but to no avail. This seems to eliminate the possibility of hydrogen embrittlement from moisture? Could it be entrapped forging lubricants during our forging process within the material, which expands during solutionizing?

Thank you.

 

[EdStainless]

Have cross sectioned samples and prepared micros?
That is where you start.


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P.E. Metallurgy, Plymouth Tube

 

[HY Tan]

Hi Ed,

I've compiled some pictures. Here you go!

Just to clarify, the SEM picture on the top right corner shows a cavity rather than blister. It is in fact a protruded blister. It shows a cavity because I have sanded off the blister, exposing the underlying cavity.

Thank you.

 

[metengr]

These look like subsurface defects from forging. It could be your having grain flow issues related to geometry or type of lubricant during forging. If the aluminum material is sticking during forging you have internal fissures that if open to the surface oxidize and delaminate.

 

[HY Tan]

Hi Metengr,

Could these internal fissures rupture during solutionizing since I am heating up to 560C?

 

[EdStainless]

Sure, but they happened during forging. Even if you didn't HT and have them blister there would still be the internal defects.

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P.E. Metallurgy, Plymouth Tube

 

[swall]

You may have hydrogen carrying over from the cast ingot due to insufficient de-gassing of the melt.

 

[WKTaylor]

Guys... Your thoughts?

Hy Tan cited the forging temperature of 500C = 932F... which is roughly the solution HT temperature. This seems like an incredibly high temperature for forging 6061 for many reasons.

I would generally recommend cold [AQ or W temper] or warm forging 300-400F... but NOTHING close to HOT forging at 932F.

Regards, Wil Taylor

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[HY Tan]

Hi WKTaylor,

The reason we require 500C (~932F) is due to tonnage being too high if I drop the temperature. 500C is the oven setting temperature. I believe the part cools off by about 20-40C right before forging.

Do you believe the high temperature of forging to be a cause of these blisters? Or would they lead to some other currently unforeseen issues?

Thanks for your feedback!

 

[TVP]

480 C (900 F) is the maximum recommended forging temperature for 6061 (ASM HANDBOOK Volume 4). It is possible that the high temperature is contributing to the blisters. Grain growth can occur at temperatures above 400 C, especially if there is any prior cold working of the surface, and this irregular grain structure can cause local defects. I think swall's comment about hydrogen is something that should be further investigated. If this is a cast billet, rather than rolled/extruded bar, then excess hydrogen may be present that can definitely cause blistering upon subsequent heating. Can you have the cross-section mounted, polished, and etched to reveal the microstructure? I think that this would help to pinpoint the failure mechanism.