Aluminum castings - Surface color modification after heat treatment 1

[customcastings]

Hello there.

I am trying to find out why aluminum castings suffer a color modification on its surfaces after heat treatment (T6/quenching). We use Foseco's dycotes in our dye casting process, and I wondered if these substances would react with the aluminum oxides on the parts surface, therefore generating different oxides (that will bring a different color to the casting).
The coloration is not homogeneous over the castings surfaces - even some cavities that are unreachable by tools and (dirty) gloves have a different color pattern too.

I appreciate any help/comment on the topic.

eSchein

 

[TVP]

Color will vary based on the chemical composition (silicon, etc.) and surface contamination (die coating/lubricant, dirt, oil, etc.). The solidification pattern can even have an effect, but the first two are more significant. You may want to investigate a thorough cleaning procedure prior to or after heat treating if the optical quality of the surface is critical.

 

[etch]

copper contents can also have smlight colouration, and also any free iron, from ladels etc

 

[kenvlach]

Agree with preceding comments.
The surface of any metal being heat-treated should be first be cleaned to avoid unwanted reactions with contaminants. However, some discoloration is typical of diecast Al alloys due to alloy segregation, and it is accentuated by heat treatment.

Re Foseco's Dycote lubricants/dressings for aluminum diecasting: Most contain colloidal graphite and will cause discoloration. However, Dycote 670 contains an aluminum paste and is specifically advertised to produce stain-free castings. Contact Foseco at

http://www.foseco.com

What alloy and diecasting process are you using?
A T6 heat treatment is unusual for diecastings: a) solutionizing is impossible for most alloys due to limited solid solubility and b) solutionizing temperatures can cause blistering of pressure diecastings due to gas porosity. Are you perhaps only artificial aging (T5)?

 

[HiGroove]

This may be over simplifying your problem but have you checked the cleanliness of your quench water ? And or the hardness of the water ?

I have had problems with AC2B aluminum alloy and discolored parts.

 

[customcastings]

The water we use is pretty clean - comes straight from the public pipelines, and we replace the tanks with clear water often.

eSchein