Metal Casting Technologies : MCT SEP 2018 (3RD QRT) WHOS WHO OF METALS
28 www.metals.rala.com.au METAL Casting Technologies 3rd Quarter 2018 29 acts as a poisoning element (as do P, Ca and Bi) with respect to subsequent modification treatments using Na or Sr. Sb tends to form finer eutectic Si after a longer holding time of the melt than for shorter holding times [35-37]. It is also found that the refining effect of Sb is improved when Mg is present in the alloy . Compared to Sr treatment the mechanism of Si refinement by Sb has received less research attention. It is suggested that Si is refined due to its increased nucleation at AlSb and Mg2Sb2 clusters [36,38] while other work using 0.5% Sb addition has claimed that AlSb clusters becoming eliminated at melt holding times longer than 1hr. leading to the formation of a large number of Al-rich nano-particles which act as precursors for Si nucleation. This change with time is said to result in a change in eutectic Si from fine lamellar to shorter rod-like shapes . Interaction Effects Because of the deleterious effect of Sb on treatments with Na or Sr the interaction behaviour of these and other elements in the melt has become an important practical research issue in that when melts are using recycled scrap or some grades of secondary ingot residual incompatible modifier/refiner elements may be present [39-42]. Using thermal analysis, it has been shown that the presence of Sb appears to neutralize the modifying effect of Sr on eutectic Si. Sb in Sr treated melts increased nucleation and growth temperatures. The modification effect of Sr was said to lost when the Sr/Sb ratio in the melt was below 0.5 . Interaction in melts between Na, Sr and Sb that causes loss of modification is thought to be due to the formation of a number of intermetallic phases including Sr2Sb, Mg2Sb2Sr, Na3Sb, etc. [39, 42]. Interaction effects between Sb and Sr may nevertheless prove useful in refining the form of primary Mg2Si in Al/Mg2Si and in Mg/Mg2Si alloys [43,44]. Mg2Si, with a high melting temperature, low density, high hardness and low expansion, is a useful reinforcing phase for light alloys to improve mechanical properties at ambient and higher temperatures, notably creep resistance. A 0.2wt% addition of Sr-Sb was shown to reduce the size of primary Mg2Si from 50 to 20μm . A similar refining effect on primary Mg2Si has been obtained by treatment with 0.5%Y-Sb addition . In Mg-12Al-0.7Si alloy 0.5wt%Sb treatment was also seen to refine the Mg17Al12 phase into discontinuous islands thus prevent the formation of damaging networks of this phase . Use of Antimony in Primary Aluminium Dendrite Refinement To achieve optimum microstructures and properties in hypoeutectic alloys such as Al-7Si-0.3Mg it is important to combine refinement of the grain size of the primary Al with effective modification of the eutectic Si [46-48]. However, B in Al-Ti-B grain refiners tends to react with Sr modifier to form SrB6 and this can reduce the grain refinement and modification effects. The modification effect has generally been more seriously affected with time after combined grain refiner and modification treatment. Additions of Sb have therefore been used to retain refined Si after holding in treatments based on combinations of Al-Ti-C and Sr + Sb . At short holding times eutectic Si was obtained in fine fibrous form (Sr modification) and at longer times in fine plate-like form (Sb refinement). is much less directional, taking place in a “pasty” mode leading to porosity becoming distributed throughout casting sections. In the latter case pores can exist close to cast surfaces resulting in reduced tool life during machining and inferior machined surface finish. It has also been observed that the oxide film population of Al-Si melts significantly affects modification and the distribution of porosity [7,16,26,27]. Work by Campbell et Al [e.g. 26] has shown that during melting, melt transfer and pouring oxide bifilms are introduced into liquid Al alloys. These oxide films, existing as folded-over, crumpled and entangled forms due to turbulence in the melt, can act as nucleating sites for gas and shrinkage pores during solidification thus reducing the mechanical properties and reliability of castings. They also influence modification effects: it is found that liquid metal with a high bifilm content requires 300-500ppm Sr for modification whereas clean metal requires less than 100ppm . Modification of metal with a high bifilm content was found to reduce the Weibull Modulus for ultimate tensile strength (i.e. increase variation in properties) compared to the unmodified condition, whereas modification of metal with a low bifilm content increased the Weibull Modulus for UTS . A study on the role of P and porosity formation in Al-Si suggests that in unmodified melts the Si is nucleated by AlP particles which had originally formed on oxide biofilms, the oxide films then becoming straightened by the plate-like growth form of the faceted Si phase . Modification is said to suppress any nucleating action of AlP on these films which then remain free from Si since they cannot act as nucleating sites. These free films are therefore not straightened and remain in crumpled forms which can then entangle to act as dams between primary Al dendrite arms thus blocking interdendritic flow of residual liquid and limiting feeding [7,16,26]. Work by Dahle et Al [14,25] did not consider the role of bifilms but has explained the increase in porosity in Sr modified Al-Si in terms of a non-directional solidification mode. Sr modification is shown to give rise to large equiaxed eutectic cells (often referred to as “grains” in the literature) which act as barriers to the flow of interdendritic feed metal . Unmodified structures were found to solidify in a directional manner forming a solid shell and contained a larger number of smaller eutectic cells. Sr is seen to have a more deleterious effect on soundness than Na [17,18] producing larger increases in pore size and number, and in overall %porosity. This increase in porosity may be due to a combination effect resulting from the presence of bifilms and from the non- directional solidification (pasty) mode of large equiaxed eutectic cells plus the potential for Sr to enhance oxidation or increase gas pick up. Use of Antimony to Refine Eutectic Silicon Since the refining effect of Sb treatment on the form of eutectic Si does not fade with time and also since the treatment has not been associated with problems of gas pick-up or dispersed shrinkage it has become used as an alternative to modification with Na or Sr in China, Japan and some European countries [4,24,29,30]. Sb additions of normally around 0.2% and sometimes up to 0.5% are used. Sb does not produce the fibrous coral-like structure that is seen in Na or Sr treated metal but instead produces a fine plate- or flake-like form , hence it is more accurately referred to as a “refiner” rather than a “modifier”. The refining effect of Sb is limited at lower cooling rates, as in sand castings, hence Sb treatment is more suitable for gravity and low pressure die-castings. As mentioned in the previous section it is believed that atoms of modifying or refining elements are absorbed at growth steps at the silicon-liquid interface causing the Si to undergo a high degree of impurity induced twinning. Research in the 1980’s  suggested that growth twins in Si are ideally encouraged when the ratio of the atomic radius of the modifier element to that of Si (rm/r Si) is 1.65. however, it was also noted that fibrous Si that was formed by rapid cooling (quench modified) was twin-free . Further study has shown that the value of the rm/r Si ratio does not relate to the practical effectiveness of a modifier element and has concluded that the interacting effects of cooling, nucleation and growth rates must also be considered in any modification models [32,33]. It has been observed that Ba, Ca and Eu treatments give fibrous Si just like Na and Sr, whereas Yb, Y and other RE metals give refined plate-like forms like Sb [20,29,32]. It is of note that out of 20 or so potential modifying elements Sb has by far the lowest rm/r Sii value at 1.24 with the other values ranging from 1.48 (for Lu) to 1.84 (for Sr) . Sr modification treatment increases eutectic cell size [14,28], whereas Sb treatment reduces eutectic cell size e.g. a 1000ppm Sb addition reduces the eutectic cell size from around 150- 220μm in untreated metal to 84μm . The finer cell size means lower restriction to the flow of feed metal and less chance of dispersed porosity. Sb treatment is persistent, Sb is not lost and is not easily removed from melts and as such it TECHNICAL FEATURE Beckwith Macbro Resin Coated Sands Contact : Rob Dalla Via 30 Devon Road Devon Meadows, Melbourne Telephone: +61 3 5995 4244 Mobile: 0417 332 723 Fax: +61 3 5995 5030 E-mail: email@example.com Website: www.beckwithmacbro.com.au All grades of resin coated sand used for shell molding and shell cores for ferrous and non-ferrous applications PRODUCTS n Range of resin strengths from 1.0% to 5.0%. n Silica, Zircon, Chromite coated sands or blended mixes. n Coated Sands of different AFS typically from 50-90 AFS. n Thermal Reclaimed Coated Sands. n Frac Sand. SERVICES n Full technical and trouble shooting service. n On-site shell core and shell molding facility to evaluate the product applications. n Laboratory facility to ensure product quality. FORSALESHELLCOREMACHINESSHELLMOULDMACHINES ASTUDYONTHEROLEOFPAND POROSITY FORMATION IN Al-Si SUGGESTS THAT IN UNMODIFIED MELTS THE Si IS NUCLEATED BY ALP PARTICLES WHICH HAD ORIGINALLY FORMED ON OXIDE BIOFILMS, THE OXIDE FILMS THEN BECOMING STRAIGHTENED BY THE PLATE-LIKE GROWTH FORM OF THE FACETED Si PHASE. USING THERMAL ANALYSIS, IT HAS BEEN SHOWN THAT THE PRESENCE OF Sb APPEARS TO NEUTRALIZE THE MODIFYING EFFECT OF Sr ON EUTECTIC Si. Sb IN Sr TREATED MELTS INCREASED NUCLEATION AND GROWTH TEMPERATURES.
MCT JUN 2018 (2ND QRT)
MCT DEC 2018 (4TH QRT)