Tig welding aluminum

By Seldom

Date 03-07-2002 23:23

SB209 Gr6061 - SB209 Gr5052, GTAW 1/16"-1/2"= good old ER5356 with a 300 degree preheat is one procedure for sure.

By les Date 03-08-2002 14:06

Thanks for the information. I'm a little concerned about the preheating, is this recomended because of the use of different alloys? Some of the people that I work with, will preheat aluminum and I have never seen any ill effects from it. On the other hand I have read only bad news about preheating aluminum, (sensitizing it to cracking etc.).

Thanks again for a very rapid responce to my question.

Les

By Seldom

Date 03-08-2002 14:48

Hi Les,
Yes, there's a valid concern with the excessive heating of 6061-T1 materials and developing hot cracking. The 300 degree preheat was used to do the PQR on 1/4" material and most importantly, is also the maximum interpass temperature allowed. So the reasoning is if it's safe for the interpass, so should it be for the preheat temperature!

In addition, it's essential when welding 6XXX series that adequate dilution (alloying) of the base material and filler metal occurs. This is described throughout technical publications but was impressed upon me by an old friend that's an experienced metallurgist. NEVER "wash-pass" (without adding filler) a 6XXX series weld!

That’s the best I can do for you Les. I’m certain that either Dr. Crisi or Niekie3 could explain it in far greater detail though.
Good luck.

By Niekie3 (***)

Date 03-08-2002 22:28

When welding Al, you essentially always end up with a weld that is substantially weaker than the parent metal.

In the case of the heat treatable alloys such as 6061, the strength is obtained by a precipitation hardening mechanism. Typically small amounts of Cu is added to achieve this precipetation hardening. To achieve the hardening, the material is typically quenched from a solution heat treatment temperature and then "aged" by applying a moderate temperature to it, (Typically 300 - 350°F for this alloy) to help form the precipitates. If this temperature is too high, or held too long, the material "over-ages" which leads to a weaker material. It is for this reason that welding will always result in a weaker HAZ, because you will always have an over-aged material. It is also for this reason that you want to keep the pre-heat low.

In the case of the non-heat treatable alloys, strengthening is usually obtained by strain hardening. (Cold deformation.) When the material is heated up, the effect of the strain hardening is lost due to recrystallization and simmilar effects. That is why welding will laways leave a weakened zone in the HAZ of these alloys. That is why a pre-heat in these materials will result in lower strengths and wider HAZ's, so you want to keep the pre-heats low.

What is the moral of the story? - Make sure your welds are not in highly stressed areas of your components. The design philosophy in Al is somewhat different to that of steel and other alloys where you can get a joint efficiency of 1.

The story with the addition of filler being important is so because:

1) Some compositional ranges of alloying additions are very crack sensitive. The filler is needed to move the weld away from these ranges.
2) Si has a very important effect on the solidification range of Al. The wider the solidification range, the bigger the chances of hot cracking. At 12% Si, the solidification range is basically zero. It is for this reason that many casting grades of Al have around 12% Si.

Hope this helps

Regards
Niekie Jooste

By Lawrence (*****)

Date 03-08-2002 16:37

ASM handbook Vol. 6 agrees with Seldom in that both Heat treatable Aluminum alloys (6061) and Non heat treatable aluminum-magnesium alloys (5052) can be successfully joined with 5356.

Now as for technique, Vol. 6 states that when joining 6xxx series, "When 4043 filler is used, the weld metal should possess a minimum of 50% 4043" in comparison. "With 5356 the weld metal should consist of 70% 5356". "Joints in 6xxx series alloys are often beveled or spaced to permit the excess filler alloy dilution. This gives great authority to Seldom's admonition to avoid wash passes.

Speaking to heat input. Factors such as chill bars, interpass cooling and welding technique all will have an effect on the degree of softening that results from welding heat-treatable alloys. Preheat, lack of interpass cooling, and slow weaving weld passes all increase peak temperature and extend high temperatures farther into the weld, as well as increase the time at these high temperatures so as to reduce the mechanical properties in the heat affected zone.

The 5052, in the heat affected zone adjacent to the weld will be completely annealed, a few seconds above 650F will remove any effect of work hardening for 1-1 1/2 inches in any direction.

Not knowing the geometry, thickness or intended service of what your joining its hard to predict what need for preheat you might or might not have. But interpass temps will be critical in any case.

Tell us more and we will tell you more

Lawrence