EBW of AA 6061 T651 aluminium alloy cold plates for the space guinea pig living unit cooling system

Abstract

This paper describes the activities conducted in the development of the electron beam welding (EBW) process for two cold plates, designed by Thales Alenia Space and made from AA 6061 T651. This special welding technology is made necessary by the need to minimize welding-induced deformations. Cold plates are the main components of the mice drawer system (MDS) payload cooling system. The development and manufacture of this component, funded by Italian Space Agency and Thales Alenia Space, as the industrial supplier, will make it possible to conduct several space research programmes. The MDS is necessary to maintain the temperatures required, both for the electronic control units and for the mouse habitat (cooling of the recirculation air and drinking water for the guinea pigs), during shuttle transportation and during conducting experiments onboard the International Space Station (ISS). The cold plates are made by assembling two aluminium plates of variable thickness with fins, worked by electroerosion, enclosing a coil made from AISI 316 for the water cooling system. The weld geometry may be likened to a butt joint with partial penetration along the 1700 mm perimeter of the plates. The oversize of the assembled component after welding (488 × 356 mm plates) was only 0.5 mm, considering a planar tolerance for the finished component of 0.05 mm. The objective is thus to minimize distortion and at the same time maximize penetration depth so as to reduce weld stresses. The main problems encountered have been in relation to defining weld parameters in order to eliminate porosity and eliminate problems associated with ‘flash’ (interruption of the electron beam, and hence welding) as seen in the treated aluminium alloy. Destructive analysis and NDT for qualification of the WPS have been aimed at evaluating weld efficiency and permissible stress. Draw tests, conducted on partial penetration test joints with a penetration depth of 5^0/+1 mm, have shown a linear strength of approximately 1 kN/mm, corresponding to approximately 154 MPa for the sections analysed. This strength is markedly superior than the stresses calculated by FEM analysis, also in the case of overloads arising under emergency conditions (85.2 MPa). Using the final parameters, summarized in WPS–EBW (2006)001, two cold plates have been welded, soon to be delivered to the ISS.

Keywords:

• aerospace
• Al–Mg–Si alloys
• aluminium alloys
• cooling
• EB welding
• finite element analysis
• mechanical properties
• mechanical tests
• plants
• rockets
• stress analysis