As always you raise an excellent technical point about the solidus
and liquidus values. To define the terms we are discussing; the
solidus is the temperature below which an alloy is completely solid,
the liquidus is the temperature above which an alloy is completely
liquid. The temperature range between these two points is the melting
range of the alloy where there exists a mixture of solid and liquid
phases; this is sometimes termed the ‘mushy zone’.
What we need to consider when thinking about the different 'fusing’
characteristics of traditional sterling silver and Argentium silver
is what I would call the ‘effective’ melting range of an alloy. If we
step back from silver alloys and consider the difference between the
effective working temperatures of silver-copper-phosphorus brazing
alloys and silver-copper-zinc-tin brazing alloys (here I am using the
AWS definition of brazing alloys which is “a joining process that
produces coalescence of materials by heating them to the brazing
temperature in the presence of a filler metal having a liquidus above
450 C (840 F) and below the solidus of the base metal. The filler
metal is distributed between the closely fitted faying surfaces of
the joint by capillary action”) then my use of the term 'effective’
melting range may become clearer.
In EN 1044:1999 alloy CP105 (2% silver, 91.7% copper, 6.0%
phosphorus) has a defined solidus of 645C and a liquidus of 825C.
However for all the CP range of brazing alloys this standard also
gives a working temperature, for this alloy 740C, which is
significantly below the liquidus, i.e. this brazing alloy works
producing effective joints at temperatures well below the point where
it is fully molten.
In comparison the EN 1044:1999 alloy AG 103 (55% silver, 21% copper,
22% zinc 2% tin) has a defined solidus of 630C and a liquidus of
660C. However there is no working temperature specified and any
general guidance on best brazing practice would be that an effective
working temperature would be at or just above the liquidus at 660C.
The reason for the CP alloy family having a working temperature
specified in the standard is that it was recognised that the majority
of the alloy was completely molten at a temperature well below the
liquidus of the alloy and in effect there was no need to overheat the
alloy by using joining temperatures above 825C. In comparison the AG
alloy family are not sufficiently molten or fluid at temperatures
below the liquidus of the alloy and require heating to slightly above
the liquidus temperature to form an effective joint.
So, what I have tried to do in this round about way is to illustrate
the point that James has made that although the solidus and liquidus
give you a melting range, at different points within this melting
range, dependent on the composition of the alloy, there may be a
significant difference in the total amount of liquid phase compared
to solid phase.
I have spent may years looking at different cross-sections of
traditional sterling silver alloys to assess their metallurgical
soundness (freedom from phosphides, oxides etc) and I can say that
commercial sheet and wire products generally have very little
copper-rich second phase present. In comparison Argentium silver
alloys do have a greater proportion of their overall structure
constituted from the copper-germanium-rich second phase. I cannot
definitively prove this final statement, but in my opinion the
Argentium silver alloys have a sufficient amount of molten material
present at just above the solidus temperature to facilitate the
’fusing’ operation and by contrast traditional sterling silver alloys
need to be heated to just below the liquidus to obtain the same
reaction. So this would be why I would term Argentium silvers as
having a wider ‘effective’ melting range than the traditional
sterling silver alloys and I can say that we are continually working
towards a better understanding of this phenomenon.
So having gone through all of the above I could have simple said, I
agree! More liquid phase present at the lower temperatures of the
A couple of other comments; first I have not put all the temperature
conversions in above so for those that are not bilingual;
645C=1193F; 825C=1517F; 740C=1364F; 630C=1166F; 660C=1220F. Secondly
where I have used the term ‘fusing’ above I have used quotation
marks, this is to prevent me being told off for not calling this
process Transient Liquid Phase Diffusion Bonding!