Have anybody ever heard of aluminum rods being heated up to 400 degress and then vacuumed to remove gas pockets? If so does this make the rod stronger?
I rather doubt there would be any detectable improvement. Heating may even blister or reduce the strength of the aluminum. 400oF or C? What is the alloy and temper? Are these cast or forged rods?
1) In principle, this process can remove hydrogen but not air or nitrogen.
2) In practice, the US Patent below indicates that degassing must be preceded by coating (plating or hot spraying) with nickel or chromium. This eliminates the alumina film from the aluminum surface.
3) Vacuum isn’t necessary, just dry air or inert gas of negligible H2 partial pressure.
4) A process such as forging is needed to consolidate the empty voids after degassing.
5) If desired, the nickel or chrome coating can be removed by anodic stripping (reverse plating) in sulfuric acid.
References: US Patent No. 2995478 “DEGASSING ALUMINUM ARTICLES” (1961).
Note: In order to view the TIFF image file of the patent, I first had to install AlternaTIFF, a free web browser add-on (via a link on the US Patent Office site).
J. H. O’Dette, “Blister Formation in Rolled Aluminum,” Transactions of AIME, vol. 209 p. 924-929 (1957).
W. Eichenauer, K. Hattenbach and Z. Pebler, “The Solubility of Hydrogen in Solid and Liquid Aluminum,” Zeitschrift für Metallkunde vol. 52 p. 682-684 (1961).
D. E. J. Talbot and D. A. Granger, “Secondary Hydrogen Porosity in Aluminum,” J. Inst. Metals vol. 92 p. 290-297 (1963/1964).
The above references are from Aluminum, vols. I and III, Kent Van Horn (ed.) American Society for Metals (1967). I expect that more recent researches have been conducted, but didn’t find any in the 1984 book below.
“In addition to causing primary porosity in casting, hydrogen causes secondary porosity, blistering, and high-temperature deterioration (advanced internal gas precipitation) during heat treating. It probably plays a role in grain boundary decohesion during stress-corrosion cracking.”
Hence, the level of hydrogen in liquid aluminum is normally reduced by purging with hydrogen-free gases or by vacuum degassing before casting.
– Aluminum: Properties and Physical Metallurgy, J. E. Hatch (ed.), p. 230 ASM (1984).
Any actual evidence of porosity, e.g., from a part sliced up with a wafering blade?
If so, the porosity is more likely to be air (which are mostly N2, as the O2 oxidizes the walls of the bubbles) entrapped in the casting mold. Heating to a sufficiently high temperature can cause blistering if these bubbles are just below the skin of the casting.
Some metallurgical testing labs and aluminum foundries can analyze for hydrogen.
If the parts have been forged, there shouldn’t be any porosity left, anyhow.
Do you have a link for the vacuum degassing vendor?
Hope this helps. Please furnish the requested info.
Ken
Hot Isostatic Pressing, or HIP, is commonly performed on aluminum castings to remove porosity, which improves properties such as fatigue strength and pressure tightness. Try the following links for more information on the process:
Since the rods are forged, there shouldn't be porosity; thus, no benefit from the proposed vacuum degassing. In fact, the rods would overage from the T6 temper (although this may occur during use, anyway).
Does the vendor have a website? I would like to evaluate any metallurgical claims a little more thoroughly.
For cast material, the Densal II + T6 process in TVP's link seems quite beneficial. The biggest benefit would be for the poorest castings; perhaps selected results are shown.
This is probably not new to you, but Al and its alloys do not have what is known as a "fatigue limit" like steel does. Given enough stress reversals/variation, they will eventually crack. Best to Zyglo them as frequently as possible.
The oft quoted claim that aluminum alloys do not possess an endurance limit is controversial at best. It's one of those things that gets repeated over and over until everyone thinks that it is a fact. It is certainly true that iron alloys are almost unique in possessing a clearly defined knee in their S-N curves because of dislocation pinning behavior. But endurance limits are sometimes quoted for Aluminum : see for example
which claims a limit of 15000 psi for their improved 356 aluminum alloy.
It would be more true to say that Aluminum alloys do not have a clearly defined endurance limit, and consequently whatever limit that may exist is very hard to discover since it would take a prohibitively long time to determine it. However, even aluminum alloys can exhibit a knee in their S-N curves for some types of fatigue : see for example
To add to what EnglishMuffin wrote, Ford has sponsored recent research on very high cycle fatigue of aluminum (109-12 cycles). The testing has been performed in Austria at the Institute of Meteorology and Physics (H. Mayer and S. Stanzl-Tschegg) and at the University of Michigan (M. J. Caton and J. W. Jones), and the results indicate an endurance limit, it's just that the limit is only obtained at very high cycles and at fairly low stresses. For more information, check out the following references:
"Demonstration of an Endurance Limit in Cast 319 Aluminum", Metallurgical and Materials Transactions A, Volume 34A, January 2003, pp. 33-41.
"Understanding Very High Cycle Fatigue of Metals", International Journal of Fatigue, Volume 25, 2003, pp. 1101-1107.
