Investigation of Ultra-High Vacuum Compatible Weld Joints of AA5083 and AA6061 Materials for Synchrotron Radiation Source

Aluminum alloys are used as construction material for ultra-high vacuum (UHV) chambers of synchrotron radiation sources (SRS) due to UHV compatibility, high thermal conductivity and non-magnetic requirements. Aluminum alloy AA5083-H321 has been used as material of construction for UHV dipole chambers of Indus-2 SRS at RRCAT. Weld joints of UHV chambers require helium leak tightness less than 2E-10 mbar center dot l/s. Weld joint design and selection of right base and filler materials need special care in obtaining leak tight joints of vacuum chambers by gas tungsten arc welding (GTAW). GTAW is used in the fabrication of the present dipole chambers which are welded in lip-fillet configuration. One of the in-service dipole chambers has developed leak over time at few locations and has been repaired in-situ. This failure due to fine cracks developed in the weld region of the dipole chamber can be attributed to either material of construction or weld design. In this study, GTAW welding of similar base alloys viz. AA5083 to AA5083 with filler rod ER5183 and AA6061 to AA6061 with filler rod of ER4047 has been carried out. It reports the investigations carried out with respect to their GTA welds fabricated in two different weld joint designs. Computational studies were also carried out to obtain residual stresses in weld joints using finite element analysis (FEA). The GTA weld joints are subjected to various quality tests including helium leak test, thermal cycling and metallographic examination. Helium leak tightness (2E-10 mbar center dot l/s) was achieved in both the welds of AA5083 (ER5183 filler) and AA6061 (ER4047 filler), however, fine cracks are seen at the root of AA5083 welds whereas welds of AA6061 were free of such defects. Based on these investigations, weld-joint design and material of construction (including filler material) are proposed for new dipole vacuum chambers of Indus-2.