"orange peel effect" in AW 6082 T4

[giamma109]

Hi,

I have some problems of orange peel effect in cold forging of AW 6082 T4 and I have a fracture in some areas. I think that the problem is only the grain growth...Is it correct?

thank you

 

[CoryPad]

Does the orange peel appear on the part after the solution heat treatment? That could be due to grain growth.

When does the fracture occur? During cold forging? During heat treatment? During use of the part? This information is needed to answer your question.

Regards,

Cory

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[giamma109]

The orange peel appears after the cold forging, and the fracture occurs during cold forging....

 

[CoryPad]

Orange peel is a surface texture effect caused by deforming an unusually large-grained material beyond its elastic limit. The large grain structure will reduce the plastic strain capacity, so fracture can occur easier than normal.

Are you using 6082-T4 as your starting material for forging? Or do you heat treat forgings to the T4 condition?

Regards,

Cory

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[giamma109]

I'm using 6082-T4 as starting material for forging

 

[TVP]

The orange peel effect is quite common in aluminium alloys, which is due to the grain growth that occurs during solution treatment or annealing. It is usually most prevalent when a critical amount of cold work (usually 5-15%) has been introduced prior to the solution treatment. The cold work provides the energy to drive grain growth after the microstructure recrystallizes.

Can you provide some additional details about the process? What is the product form for the slug? What is the prior thermomechanical history? Is this a multi-stage cold forging process, and if so, how many stages make up the sequence? Does the orange peel/fracture occur after cold forging of the initial slug, or after an intermediate annealing process? Also, what is the exact composition of the 6082 alloy?

 

[giamma109]

Hi TVP,

it's a bush, with covering of rubber, it's an anti-vibrating joint for automotive. It's a multistage cold forging process with 6 stages. The orange peel/fracture occurs after the second stage. You can find the composition of 6082 T4 in:

http://www.matweb.com/search/SpecificMaterial.asp?bassnum=MA6082T4

 

[TVP]

giamma109,

Thanks for the additional information. I am quite familiar with the elemental specification limits for alloy 6082. The reason I asked for the specific composition that you are currently using is because the orange peel effect is affected by the concentration of Fe, Mn, Cr, Ti, and V. Higher amounts of these elements will tend to improve the condition, because they inhibit the grain growth that can occur. However, excessive amounts will tend to reduce the fracture strain (ductility).

Do you currently purchase the raw material as a solution-treated bar (6082-T4) with a length of several meters or do you use a pre-cut slug? If you purchase the material as bar, do you cut the slug and perform some type of upsetting as the first step and then solution treat the upset slug in the temperature range of 525-540 C? If so, then this is the problem. If not, could you please provide some of the details that I asked about previously?

 

[CoryPad]

Is there a specific reason to use T4 treated material as your slug? Do you use wire or bar as the starting stock? Can you not use as-drawn material? Or, how about using strain relieved material or fully annealed material? Strain relief uses a temperature around 345 [°]C, while annealing uses a temperature of around 415 [°]C. Your T4 treatment uses a solution treatment of 530 [°]C, which encourages grain growth.

Regards,

Cory

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[giamma109]

I use wire (6082 O) with a length of several meters as starting stock, then i cut the wire and I make the T4 treatment (solution treatment of 530°C, quenching in cold water, then natural aging), after this treatment I use slugs for the tests of cold forging with a mechanical press SAKAMURA. I use T4 treated material as slug, because the T4 material is more ductile than T6 material.

 

[TVP]

giamma109,

Thanks for the additional information. The situation is quite clear now, and the orange peel problem is almost a textbook example. The initial wire has been fully annealed, which means that it was heated in the range of 415-440 C followed by slow cooling. This treatment produces a microstructure with coarse, widely spaced precipitate particles and results in minium hardness and flow stress.

Your process involves solution treating at 530 C after cutting the slug. This operation is not necessary, and is the cause of the orange peel. The reason it is not necessary is because the raw material has already been annealed (recrystallized) to its softest condition. It should be capable of very large strains (deformation) before requiring any subsequent annealing. Typically for this alloy reduction in area of < 95% (true strain of < 3.0) is possible. If you want to remove the strains caused by the cutting/shearing process, then you should only heat the slugs in the range of 345-375 C.

Depending on the final product form, you can either wait until the part has been formed through all 6 six forming operations to perform the solution treatment, or perhaps solution treat after the 4th or 5th forming step. The reason to perform the solution treatment before the last forming step is so that any distortion that occurs due to the quenching operation can be corrected in the last one or two forming steps.

 

[giamma109]

Thanks TVP,

If I understand the situation, I have mistaken the treatment to obtain T4 state from O state. If I use a really T4 raw material, I will not have the orange peel effect/fracture....is it right?

 

[CoryPad]

No, that isn't right. You should use the O temper material you have and begin forming it in that condition, DO NOT solution treat it. After forming, you can solution treat and age it if needed.

Regards,

Cory

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