unfortunately difficult or practically impossible venture to not only TIG- but generally arc-weld Alloy 7075 without cracking.
The reasons are being founded on real complex metallurgical reactions of the alloy in case of melting, e.g. by arc welding.
Due to these reactions the alloy or the joint been created, is going to be strongly crack susceptible in the area of the "Partially Melted Zone" (PMZ). 7075 - aluminum alloy composition is based on the combination of Al + Zn + Mg + Cu + (Cr) + (Fe) and thus the reactions of separation between these elements, and subsequently building up complex and mostly brittle compounds while cooling down from melting temperature, are nearly impossible to be governed in a controlled way. Therefore in the very most cases of information, the Alloy 7075 is indicated as "impossible to weld".
Additionally to these critical factors you have to weld a "mold" as you say. The alloy 7075 is extraordinary suitable for tools like molds etc. due to its excellent mechanical properties. But just as long as the original microstructure has not been changed e.g. by thermal impact etc.
I assume that the original microstrucure has been changed while the mold's duty.
Very extensive and advanced investigations regarding the metallurgical mechanisms of crack susceptible aluminum alloys (also 7075) particularly in the PMZ, have been performed by HUANG and KOU, see also under:http://files.aws.org/wj/supplement/10-2002-HUANG-s.pdf
which is one of several reports of the named authors concerning metallurgical aspects of weld-cracking in aluminum alloys.
In Austria (Technical University of Graz) investigations have been conducted in welding 7075 by using friction welding with, as far as I know, relatively good results.
However, my personal philosophy in any case of welding "impossible to weld" materials is, that only the practical trial will bring the predication about whether the material is really impossible to weld. In Germany we say: "Nur der Versuch macht klug." - what can be translated with "Only the attempt, makes the wise man becoming a wise man." And as you have mentioned, your customer has said that the material has been welded in the past, perhaps he does also know a bit more to say "how" it has been welded.
But perhaps have other fellows in the forum specific experiences in (successful?) welding this difficult alloy? Would be very interesting to see and read...
Regards from Germany