Investment casting of gold jewellery

POROSITY IN CASTINGS 0 1 3 4 ITS CAUSES 0 1 3 4 PREVENTION 0 1 3 4 Dieter Ott 0 1 3 4 Christoph J. Raub 0 1 3 4 0 Consultant, International Gold Corporation , Johannesburg, Republic of South Africa 1 Forschungsinstitut fur Edelmetalle und Metallchemie , Schw 2 bisch Gmund, Federal Republic of Germany William S. Rapson (in part) 3 GoldBull., 1985, 18, (3) 4 GoddBull., 1985, 18, (3) Porosity is one of the important causes for the rejection of jewelle, y items cast by the lost waxprocess. Ifitisofthesbrrnkagetype, thenthereisaconsiderablereseivoirofknowledge, arising f om experience in the casting of dental gold and other gold alloys, which can be drawn upon to overcome theproblem. If it isgas porosity, howevet; the caster is faced with thefact that therearespecialfeatures in regardto its occurrencein caratgoldcastingsabout which there isstilllittle information. This article addresses both these situations. It not only reviewsavailableinformationonshiinkageporosityindentalgoldalloys, butalsopresents an account of investigations carried out on certain hitherto unstudiedaspects ofgasporosity in caratgold castings. In particular itfocusses upon the role played by reactions between the metals being cast and the calcium sulphate-bondedinvestments which are generally usedforproduction of the moulds. - Shrinkage Porosity This manifests itself in the form of irregularly shaped voids which can be either small and fairly evenly distributed throughout the castings (microscopic porosity) or larger (macroscopic porosity) and confined to certain parts of the metal. Shrinkage porosity of the first type does not pose serious problems. It tends to occur in castings of small dimensions or in castings made using relatively low mould temperatures. Under both these conditions solidification occurs evenly and rapidly. Shrinkage porosity of the second type may cause rejection of castings. It tends to occur in larger castings whenever free flow of molten metal into any part of the solidifying casting in the mould is blocked. Thus under ideal conditions, after the metal immediately in contact with the investment surface has solidified, the contraction which follows cooling and, in particular, crystallization of the rest of the metal should be compensated for by flow of fu rther molten metal into the dendritic massfrom the sprue and its reservoir. Such conditions are, however, not simply achieved. In the early days of dental casting, shrinkage porosity near to the point of attachment of the sprue was not an uncommon defect in castings. It was the subject of a number of investigations which demonstrated that it was caused by solidification of the melt in the sprue cutting off the supply ofinelt to the main casting while it was still undergoing solidification and contraction. It could be avoided by appropriate attention to the melt temperature and to the design and dimensions of the sprue and its reservoir. Even with sprues of good design, it was nevertheless found that localized shrinkage porosity still persisted in some castings near to portions of their surfaces. This subsurface porosity was ultimately found to be associated with slow cooling of the metal near certain areas of the mould surface. Such slow cooling apparently arises through selec tive heating of the surface of the investment over these areas, either because of the pattern of flow ofmolten metal into the mould during casting or in centrifugal casting owing to an increase of the rate of heat transfer to the investment over these areas as a result of selectively increased hydrostatic pressure of the melt (16). Under these conditions, pockets of molten metal persist in these areas aftersolidification elsewhere is complete, with the result that localized shrinkage porosity or su b-surface porosity d evelops. In the case of shrinkage porosity, the pores may contain gases released from solution in the melt during its cooling and solidification. However, no account of any specific studies in this connection has been found, though the effects of hydrogen upon the shrinkage porosity of aluminium-copper and aluminiumsilicon alloys have been studied (22). Gas Porosity This manifests itself in the form of gas-containing pores which are rounded orspherical in shape, and which have therefore clearly been formed in the melt prior to its solidification. It can arise in a variety of ways: for example, by release of gases from solution in the melt as it cools, as a result of `back pressure' of gases in the mould if the investment is not sufficiently permeable, by inclusion as a result of turbulent flow of the melt while pouring, and by generation of gases either directly or indirectly as a resu It of reactions between the melt and the investmentmaterial. Although observed, it seems never to have posed acute problems in the casting of dental gold alloys. Porosity in jewellery Gold Alloy Castings In modern practice, porosity in 18- and 14-carat gold-silvercopper alloy castings normally takes the form of microcrystallization voids (shrinkage porosity) or gas bubbles in the metal (gas porosity). Figure 1 is across section vertical to the surface ofan 18-carat gold alloy with typical crystallization voids of dendritic origin. Figure 2 is a scanning electron microscope picture of a fracture surface of the alloy. It shows the characteristic dendritic growth structure of the crystallites which results in the formation of microcrystallization voids. It wil l be apparent from the brief review above that knowledge of this type of porosity, and of methods for its prevention, is considerable and probably adequate for dealing with manifestations of it which arise in castings of gold jewellery. This is currently not the case in respect of gas porosity which is found, in particular, in castings with considerable strength in their outside layers and occurs frequently concentrated in a zone just below the surface of the casting (see Figure 3). It may be responsible therefore at least in part for subsurface porosity, (2, 10, 24, 25). The understanding of the occurrence and prevention of gas porosity in cast gold jewellery has been made difficult by its sporadic and often non-reproducible appearance in castings made using both centrifugal and vacuum-assisted casting techniques. One widely held view is that the gas bubbles or pores are the result of the entrainment of air in the turbulent stream of metal when it is opoured into ^the mou:ld.Their app^earance, however, does not accord with this view, since their surfaces are without exception unoxidized. Any direct contact with atmospheric oxygen is therefore excluded. Thus, a thorough investigation of the effect appeared necessary. Fig. 1 Micro-crystallization voids in a cross section, taken vertical to the surface, of an 18-carat yellow gold casting Magnification: 50x Fig.2 SEM picture ofthefracturesurfaceofthe18-caratyellowgoldcastingshown in Fig. 1 showing the characteristic dendritic growth struc (...truncated)