6al 4v titanium microstructure question.

[mfurches]

Is the microstructure of annealed 6AL 4V titanium fairly uniform? I have been told that when machining it I need to start with a much larger piece of stock and machine the piece down in order to get past irregularities in the microstructure. Has anyone ever heard of this?

 

[TVP]

I think what you have been told is in reference to the formation of "alpha case". This is the surface layer that develops when titanium is exposed to oxygen at high temperature, somewhat analogous to decarburization in steel. Otherwise, the microstructure of properly annealed Ti-6Al-4V should be uniform.

 

[mfurches]

Im familiar with the alpha case , however, that isnt what im referring to. From what i read on keytometals .com " mill annealing is a general purpose treatment given to all products. it is not a full anneal and may leave traces of cold or warm working in the microstructure of heavily worked products, particularly sheet. " The titanium im buying is described as annealed in the specs but im wondering how i know if it is a true anneal or just this basic mill annealing they are referring to.

 

[CoryPad]

If the product is exposed to an incomplete annnealing process, the traces of strain would be located in the center of the part (where it is coldest) and would not be removed by machining.

 

[mfurches]

Thanks for the replies. I appreciate the help. I wonder if anyone could helm me understand the terms bimodal and lamellar as it relates to the microstructure of titanium.

 

[TVP]

Titanium has two crystallographic phases, the hexagonal close packed (hcp) "alpha" phase and the body centered cubic (BCC) beta phase. HCP is the low temperature phase which transforms to beta phase at high temperature (usually around 1000 C). Depending on the composition and thermomechanical process history, both phases can be present. Bimodal means that that primary alpha is present as grains with an equiaxed structure while the beta phase has transformed into a lamellar structure consisting of alternating plates of alpha & beta. A fully lamellar microstructure can be produced by heating above the beta transus followed by slow cooling into the alpha + beta field. The following links have excellent images and descriptions:

http://www.georgevandervoort.com/mic_met_pdf/Titanium_and_Its_Alloys.pdf

http://www.lbl.gov/ritchie/Library/PDF/MTransLamellar.pdf