Aluminum Alloy 319 or 356 1

[JoeMaldo]

I'm been given a project to find an aluminum alloy that can be cast, machined, and coated. After the part is machined it has to be coated with a baked on phonolic coating at 500oF. Current material used is Al 319---the "critical" dimensions on the part do not shrink back to size after coating. What is the difference between Al 319 and Al 356-T51?

 

[sbi]

First off teh 356 has been heat treated, as where the 319 is as cast.
356 will pour in a thin mold much easyer, it is a softer alloy till treated
in as cast form 319 is better for machinablity than 356
Have you tried coating first before you machine? gives a nice look when done, with shiny aluminum on macined surfaces
Have you talked to you foundry? they should be able to help with this.


SBI
Central Ne.,USA

 

[JoeMaldo]

We have considered coating the part first and then machining the uncoated sections but we are still reviewing other options. I'm trying to figure out if there is an aluminum alloy that can withstand plastic deformation while it is being baked to 500oF (during coating process).

Design Problem: Find a material (Aluminum alloy / temper designation) that will sustain 500 degrees fahrenheit during the coating process after the part has been casted and machined.

The bores on the part are growing and not coming back to machining specifications. (using Al 319)

Any information on how to approach this problem would be appreciated.

Thanks.

 

[kenvlach]

Sounds like the coating bake is a 500^oF residual stress relief, when the yield stress (0.2% offset) drops to 5 ksi.

A) Try rough machining the bore and any other large volumes of material removal and/or critical dimensions, then performing a stress relief bake before finish machining. This is sometimes necessary for precision parts, even after T51 or T71 stabilization.

B) If Method A) is inadequate, you need to do a more complex thermal stabilization: stress relief bake, rough machining, cool to -150^oF for 2 hours, heat to 475^oF for 2 hours, then finish machine.

These temperatures are for 356. Look up stress relief and stabilization in an aluminum casting book (or ask your foundry). Also, the coating bake time is probably too short to completely stabilize your item vs. residual stress effects in service, so you need some additional thermal treatment, regardless.