| In view of non-precious metal ceramic alloys' disadvantages,i.e.high fusion temperatures resulting in poor castability and producing easily casting defectsjlarge casting shrinkage and distortion making prostheses fit difficultly and existing accurate problems;extra hardness leading to poor workability and difficulty of finishing and polishing those restorationsjblack metal background after pre-oxidation affecting the color of porcelain prostheses;more important aspect, biological compatability of these kinds of alloys being questionable; As well as with the rapid development of national economy and the increase of the living level of people,dentists and patients both long for higher qualfty,more reliable and heathful restorations. Porcelain-fused-to-precious metal ceramic alloy prostheses meet these needs in clinic. Because of precious metals having steady chemical property,adequate mechanical property,good biocompatability and exceedingly good workability,they have been being conventional metal materials in dentistry prosthodontics. But in our country,up to now.precious metal ceramic alloys need import whole, and price is still only restrictive factor for wide-ranging application in clinic. How to reduce cost? The fundamental way is to develop the precious metal ceramic alloy system made in China. It need dentists and metallurgists make great long-term efforts. While the simple and convenient actual method is to make full use of the imported precious metal ceramic alloys. Recently in our department, AURA systems imported from Heraeus Company are being applied in clinic.The treatment outcomes are very satisfied and economic results are very better,however there are still many detail problems. For example, may the surplus or excess alloys ( buttons and sprues )reuse? can or not especially the metal-ceramic bond strength after reuse the surplus alloys meet the clinic? This paper researches these problems in order to provide the theoretical bases for clinical application and development of this kind of the alloys.In this investigation, first, castings produced with new alloy (manufactures' product Au-Pt alloy) under the different sprue designs conditions have been anah/sed,examined and tested,aimed to find the most reasonable sprue design for forming denser microstrctures. And the structural analysis of the as-cast microstructure of this alloy were studied preliminarily. The focal point is on the effect of recast the same alloy on the as-cast microstructure, mechanical property and metal--3-ceramic bond strength. Then, referring to manufactures' advice for the reuse surplus alloy by adding one-third new alloy,and following to simulate actual prosthodontic laboratory conditions, the reuse way is to add one-third new alloy into two-thirds last generation surplus afloys,up to four generation round. The effect of this dilution of old with new alloy on the above-mentioned properties of the resulting castings were investigated,and compared with the clinical requirements. Results and conclusions are as follows:1. By horizontal spruing system, castings had no shrinkage cavity and porosity, and on denser state.2. As-cast microstructures of Bio Herador N gold ceramometal alloy showed two phases. The basic grains were sepherical and refined,brown-yellow color. The second precipitation phase shaped irregularly,scattered in the boundaries of the basic grains. And by Energy Disper-sive X-ray (EDX) analysing, it's enriched Pt and Ru: the content of platinum (64.63%)were five times than the labelled content(11.5%); ruthenium is twenty times (8.88% vs. 0.4%).It showed that there are serious Pt and Ru microsegregation in as-cast microstructure of this alloy and mainly located in boundaries.3. After recasting the same alloy no adding new alloy each time, in metallography from one to four generation castings,the structure became loose gradually,and shrinkage porosity became more and more,and shapes of basic grains became irregularly gradually. In mechanical property, the ultimate tensile strength from I to IVca...
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