Low melting carat gold brazing alloys for jewellery manufacture

A STUDY OF Ag-Au-Ge-Si ALLOYS G. Zwingmann The Au-Ag-Ge-Si system has been investigated as a source of cadmium free low melting carat gold hard solders but alloys of this type have been found to have restricted applicability. In particular, when applied to copper-containing alloys, brittle copper germanides and silicides are formed. - The brazing alloys which are used in the manufacture of gold j ewellery and related products must meet a number of requirements. Not only must the colour of the solder in each case match that of the alloys which are being joined as closely as possible, but its composition must be such that the final article conforms to hallmarking or related specifications. These vary from country to country and in Germany they permit in the case of nonjewellery articles a tolerance of 0.5 per cent, and in the case of gold jewellery a tolerance of 1.0 per cent in respect of the overall fineness or caratage stamped on the product. If it is desired to use a hard solder with a gold content lower than that of the alloys which are being bonded together, the gold contents of the Jatter must therefore be increased in order to compensate for the lower concentration of the gold in the hard solder. In general, however, it is preferred where possible to use a solder of the same fineness or caratage as the materials to which it is being applied. Accordingly, carat gold solders are normally sold in the same caratages (18, 14, 10, 9 and 8) and colours as are carat golds themselves. Additionally, however, manufacturers supply carat gold solders with a range of different working temperatures in each caratage and colour. This enables the craftsman or manufacturer to choose an appropriate solder in each instance, and in particular to use solders in appropriate succession when a number of soldering operations have to be carried out in close proximity to one another on the one article. To achieve the combinations of properties called for, base metals such as zinc and cadmium have usually been incorporated in Ag-Au-Cu alloys. Cadmium is a useful addition where low working temperatures are called for, but its use is not desirable because of the toxicity of the fumes to which it gives rise. The aim of the present study was to develop carat gold brazing alloys, for the manufacture of jewellery and precious metal wares in each of the recognised caratages, which were free of cadmium and which had working temperatures in the range 400-600 C. The information about their products which is released by suppliers of carat gold solders relates mainly to their melting behaviours or working temperatures. Working temperatures for 8 carat gold solders range approximately from 700C (first solders) to 640C (third solders), for 14 carat gold solders from 780C (first solders) to 670C (third solders), and for 18 carat gold solders from 820 C (first solders) to 700C (third solders). In no instance are solders with working temperatures below 600 C available. Data concerning the compositions of their solders are not normally released by manufacturers but it is apparent from the work of Lder (1) and from data made available by one manufacturer that most carat gold hard solders contain from 2.6 to 20 per cent Cd. Information is more fully available, however, concerning the compositions of gold solders applied industrially and Wuich (2) has described the application in the electronics industry of gold solders which melt at significantly lower temperatures than 600 C. Mention may be made for example of the Au-Sn solder containing 20 per cent Sn, which melts at 280 C, and of the eutectic Au-Ge alloy containing 12 per cent Ge, which melts at 356 C. Such alloys are, however, for the most part brittle and are not available in caratages which would make them suitable for jewellery applications. Although certain gallium bearing gold alloys (3) are of interest in this connection, no gold alloys have in fact been identified so far which can be adjusted in composition to the caratages of carat golds as used in j ewellery and which would have working temperatures when used as solders of between 400C and 600C. A survey of various binary systems (Hansen and Anderko (4), Elliot (5) and Shank (6)) reveals that most gold alloys which have sufficiently low liquidus temperatures are either composed predominantly of brittle intermetallic phases or else have gold contents far removed from those of the accepted carat golds. Where solid solutions are formed with gold, these have liquidus temperatures which are far too high. The Au-Ge and Au-Si systems are an exception to this rule, in that they are purely eutectic in character. The same is true of the Ag-Ge and Ag-Si systems. Since the Ag-Au system consists of a continuous series of solid solutions, it follows that no eutectic four phase equilibrium can develop in either the Ag-Au-Ge or Ag-Au-Si systems. A continuous binary eutectic curve traverses the diagram of each of these ternary systems between the two binary eutectic temperatures. In the Ag-Au-Ge system the eutectic liquidus temperatures lie accordingly between 651 C and 350 C, and in the Ag-Au-Si system between 810 C and 370 C. Fig. 1 Temperature of the 'liquidus of Au-AgGe-Si alloys as a function of their (Ge + Si) contents The phase relationships for the ternary Ag-Au-Si system have been investigated by Kuprina (7) and those of the Ag-Au-Ge system by Zwingmann (8). From their studies it may be deduced that eutectic alloys with finenesses of 333, 585 and 750 (i.e. caratages of 8, 14 and 18) should have liquidus temperatures either just inside or outside the desired upper limit of 600 C. Generally speaking, the eutectic temperatures of the Ag-Au-Ge alloys are significantly lower than those of the Ag-Au-Si alloys. On the one hand, however, the Ge-contents of the eutectic Ag-Au-Ge alloys are significantly higher than the Si-contents of the eutectic Ag-Au-Si alloys, so that the former alloys tend to be less workable, while on the other hand the Au-Si alloys (Wise (9), Predecki, Giessen and Grant (10), and Philofsky, Ravi, Brooks and Hall (11)) have a tendency to solidify in metastable forms which can also result in brittleness. It was decided therefore to investigate whether, in the quaternary Ag-Au-Ge-Si system, alloys with more suitable properties could be identified for use as carat gold hard solders. Course of the Investigations Three series of alloys were prepared containing respectively 33.3, 58.5 and 75 wt. per cent of gold. Within each series, and for a series of Ge contents of from 0-15.6 wt. per cent, Ag-Au-Ge-Si alloys of different Si-contents were subjected to thermal analysis and microscopical examination in order to determine firstly the liquidus temperatures and secondly the compositions of the eutectic alloys. In the case of selected alloys the effects upon the liquidus temperatures of further additions of Cu, Ga, In, Sn or Zn were studied. At the same time the behaviour of the alloy (...truncated)