I do not believe that 6061 can be successfully welded with Electron Beam Process in a vacuum. 

Is this heat treated 6061? 4047 isnt a heat treatable filler, but 4643 is. This is only aplicable if You plan to heat treat after welding. The heat afected zone of heat treated 6061 will lose strength after welding, 6061-T6 will get down to 22-25 KSI ultimate. The HAZ usually is as weak or weaker than the filler. If You were working with soft 6061 I doubt the weld would be any weaker than the parent metal if You use a 4xxx or 5xxx filler. if the part will be hotter than 150f in use, don't use a 5xxx filler. 6061 often cracks if welded without filler metal. Heed Joe Kane's post, I don't know anything about electron beam welding.

Paul,

this is a quite interesting topic.

Unfortunately I am no expert in Electron-Beam Welding but however, I am very interested in, and fascinated by this process. Therefore I have taken the opportunity to talk to a real expert on the last IIW-Meeting in Dubrovnik Croatia and have passed on your question to him. He is the founder of a very well known German Electron-Beam-Equipment-Manufacturing Company and an Expert-Member of the IIW-Commission IV (Beam Processes). He told me the following:

·  Yes, normally by using EB-Welding under vacuum conditions and joining 6061-Alloys, a Silicon-alloyed filler metal - e.g. AA 4047 - is used for preventing cracking. This is - as well known - possible by "shifting" the alloy's composition from a cracking-susceptible region to a non-susceptible composition area. But...

·  He told me also that he personally is not a great supporter of this kind of technique. He instead uses rather a procedure where the Electron-Beam is used as an "additional" heat-source, flowing always a defined distance ahead the actual welding-zone. Due to the Beam is consisting of electrons and these again are almost free of mass, it can be magnetically deflected without "measurable" time-delay (quasi real-time) and thus being used for both, the melting of the base material and the preheating of an area in front of the actual joining-zone.

·  The great advantage by doing so is to be able to reduce the temperature gradient between the area having melting temperature and the base material having room temperature. This again enables him to reduce the height of shrinkage stresses being finally a major responsible factor for inducing cracks. And finally he can prevent to use an additional filler for achieving the same result. Sound and crack free Welding Joints. So he told me at least.

·  I thanked him for his explanations although they were a bit short in time - since we have spoken while a short coffee-break. Therefore I have tried to prepare a little sketch to visualizing what I have heard from him. I attach the "drawing" to this short response and of course I hope very much that I by myself have understood correctly what he has described. I have simply named the mentioned kind of procedure within the sketch, the "Flow-Mode", not knowing if there exists perhaps another term for this kind EB- or Beam-welding-sequence, respectively. Yes, now you might say - you will never find conditions like being drawn on the sketch (parts without clamping and able to move free of any restriction) but I have used this kind of representation only for emphasising the mechanisms between the thermal energy input and the interacting forces between extension and contraction. I request your understanding for this simplification.

·  Thus I guess there is at least one different method for EB-Welding AA 6061 without cracks and using a filler material like that one you have mentioned.

Best regards,
Stephan