EB Weld Problems

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We are struggling with excessive spatter, expulsion, and a severely uneven underbead while welding Titanium 6Al-4V in an older Sciaky machine using a low voltage French gun. The weld thickness is .160" and the circumfrential weld measures approx. 135". There is no evidence of outgasing but we are routinely experiencing beam shutdown during various aspects of the process. This usually occurres during either the penetration pass or the cosmetic pass. Machine is runnning at 50Kv and 40 IPM travel yielding approx. 23000 Joules of energy for the penetration pass. Our test on plate showed acceptable characteristics but the part is posing problems. The plate was AMS4911 and the part is cast. Any ideas??
Reply:From what you say, sounds like the casting is outgassing, where the plate did not, and this causes spatter, expulsion, and raises the the vacuum level in the chamber to where the beam shuts down.Is this weld done very near sharp focus?   It may helpful to defocus to a larger beam size, and slow down the travel, making a larger keyhole.  Maybe this would allow outgassing to occur more freely, without building pressure under the backfilling metal, and blowing this metal out.I had a 0.25" deep EB weld on 21-6-9 (high nitrogen stainless) that initially had hundreds of large irregular voids with a sharp focus/high speed (50 ipm) weld.  The problem was minimized (a few small voids) by defocusing to a larger beam size, slowing down to about 20 ipm, and adjusting beam current to get back to the desired weld depth.  The theory was that the larger keyhole backfilled more freely, where the narrower keyhole formed coldshut voids from irregular fluid flow.Are you aware of the modifed faraday cup device used to measure electron beam size?  With this technique, you can chart how you're focus coil current correlates to beam size, and thereby know you are getting the correct beam size after the filament is changed or whatever.
Reply:Thank you for your response!! Our monitoring system has not shown loss of vacuum to a degree that would shut the beam down. However, I was considering adding a secondary vacuum monitoring device separate from the controller to verify outgassing. By the way, I am not a weld engineer and am trying to learn and understand this process on the fly. Your idea concerning the keyhole size is intriguing and I had already thought of slowing the feedrate down. My technician simply wants to increase the beam current to make the weld zone hotter. According to our customer's specs. for the job, we are already 6000 Joules higher than the energy they are using to do the same weld. Yes, the penetration pass is done at sharp focus at the surface of the material with the gun positioned 7" from the work. I am not familiar with the modified Faraday cup, where might I find more info. on it?
Reply:Originally Posted by sfay...using a low voltage French gun.
Reply:Originally Posted by ZTFabFrench?? I'm surprised it didn't just throw itself on the ground and give up!!  I think it might be the flux capacitor. I'd crank that bad boy up to 1.21 gigawatts and let 'er go. On a serious note, pulser is definitley the man to be talking to as I am obviously not!  (...and I mean no disrespect to French people)- Paul
Reply:Regarding the Faraday cup, I suggest trying to contact one of the few remaining EBW equipment manufactures, it appears sciaky and PTR may have some info.This fellow John Elmer developed a fancy version with software to generate a 3-D image of the beam profile, but in its simplifies form, it is a 3" diameter copper cup, 3" tall, with a adjustable tungsten plates on top to produce a narrow slit that the beam is swept across with the beam deflection.  the cup capture the electrons as it sweeps across, and a signal from the cup is then processed along with slit size and speed, to determine the current density, or beam size (something like that).  Don't know the details of how this is done, but it is simple compared to the Elmer method, and still a very valid measurement.Here is some info I found in a web search:http://files.aws.org/wj/supplement/Elmer12-01.pdfhttp://www.llnl.gov/str/Pat197.htmlhttp://www.sciaky.com/7.htmlhttp://www.osti.gov/energycitations/...osti_id=372601http://www.ptreb.com/Capability.asp?...ID=55&Cat2ID=7
Reply:Clean Clean Clean...titanium is the worst for problematic welds due to impurities on the metal. Does the gun shutoff occur at the same vac level every time?Last edited by JonPfen; 10-29-2015 at 12:54 PM.