Welding and lidding of CE alloys
Being aluminium-based, all the controlled expansion alloys (CE alloys) can be fusion-welded using MIG, TIG or laser techniques.
MIG and TIG welding
All the alloys can be welded together using standard aluminium alloy welding conditions. At present, little detailed work has been carried out on welded CE alloy assemblies but it has been noted that the weld tends to be a little weaker than the parent CE alloy (as it is for standard aluminium alloys) and similar stress relieving procedures (as for 4000 or 6000 series aluminium alloys) are recommended. Further work on this method of joining is planned. Contact us for up-to-date information.
The CE alloys can also be joined using pulsed-laser techniques. At present, this method can only be used reliably for alloys up to 50%Si (CE11). At higher Si contents, the extremely rapid temperature cycling in the heat-affected zone (HAZ) during laser welding may cause the silicon phase to fracture, leading to a region of weakness close to the edge of the weld pool. This pre-cracked region can then lead to gross cracking of the weld under cyclical loading caused by thermal cycling, particularly if the package body and lid are of different compositions.
Alloys up to 50% Si can be successfully welded and commercial packages are currently being produced in CE13F and CE11F alloys. However, depending on the package size and design, the following should be noted:
- Design: large radii are desirable at the internal corners of the package / lid interface.
- Pulsed laser: the use of a shaped pulse with a relatively low heating rate is advantageous in minimizing any cracking which can occur if the heating rate is too rapid, particularly with CE11.
- Lid material: to minimize thermal mismatch during solidification and cooling, the use of CE alloy lids is recommended: (e.g. CE11F packages – use CE17F, CE13F or CE11F lids, CE13F packages – use CE17F or CE13F lids, CE17F packages – use CE17F lids.)
Resistance welding (seam sealing)
The high thermal conductivity of the CE alloys combined with their low melting point means that it is extremely difficult to control the seam-sealing process and contain the heat to allow the formation of a good hermetic weld to a gold plating without causing the CE alloy to melt so destroying the integrity of the plating and hence the weld. Under optimized conditions, satisfactory welds have been produced but as yet this must be considered to be a process under development.
Packages that can be seam sealed require a top ring to be soldered into place. This has been demonstrated using Kovar* on CE7 and steel on CE11 (all gold plated). Hermetically sealed packages were produced in both cases.
* Kovar is a trademark of Carpenter Technology Corporation.