Method of producing a copper-germanium-boron master alloy and its use in making silver-copper alloys.

Most of the largest manufacturing jewellery companies in the world are located in South East Asia or India. Governments in that region levy high taxes on imported silver and semi-finished silver products. Manufacturers therefore prefer to purchase a master alloys for their jewellery production because they attract much less import duty. Master alloys consist of all the elements in a silver alloy, except the silver. The jewellery manufacturer then melts the master alloy with silver he has sourced from within his own country. The difficulty in supplying master alloy is that the conditions under which it is melted may vary widely. If the conditions are not optimal there is a danger that some of the more reactive elements in the alloy may be lost. Silver-copper-germanium alloys always require a grain refiner in the form of boron. Boron easily reacts with oxygen and it is necessary to ensure a generous level of boron is available in the master alloy. This is to allow for any loss during subsequent re-melting and investment casting.
Boron is normally added to alloys in the form of it’s own 98% copper, 2% boron master alloy. CuB master alloys are not easy to use and are prone to give hard spots in the final alloy unless used sparingly. Hard spots can cause finishing defects and are a serious problem because they are often not discovered until the final stages of production. Patent EP1794335 describes a new method for adding boron to alloys in the form of borane gas or using the chemical, sodium borohydride. Using sodium borohydride, boron levels of 80ppm are typical, with levels of 800ppm possible. No hard spots are produced. Using copper/boron master alloy the level of boron in the finished alloy is less than 20ppm.
A further advantage of adding boron in the form a gas or chemical is that no extra copper is added at the same time, therefore the main composition does not have to be re-balanced. This is used to advantage on continuous casters for making strip. In this process the alloy remains liquid for up to one hour. Depending on the casting conditions the possibility exists that the boron levels may drop because of this long exposure. It is quite a simple matter using sodium borohydide to restore the boron. This is done by inoculating the melt with sodium borohydride tablets at set intervals.