Metal Casting Technologies : MCT-3RDQRT-2017
28 www.metals.rala.com.au METAL Casting Technologies 3rd Quarter 2017 29 FIGURE 6. Deep etching of Al – 7%Si – 0.3%Mg alloy showing eutectic Si and intermetallics in (a) conventionally cast (b) semi-solid cast via sloped cooling plate method.  TECHNICAL FEATURE l Fractography of shell moulded  and other resin bonded sands  to study the effects of type of resin and sand and curing variables, etc. on mould or core strength. l Characterization of investment moulding materials such as zircon, zircon flour and molochite, dried slurry and the ceramic shell moulds in the green state, after firing and after pouring . This study concluded that the bonding developed during firing is mainly due to mechanical keying of the refractory particles and polymerized silica gel. A more recent study has examined the behaviour of face-coats during shell firing and pouring of investment moulds used to produce TiAl alloys . l Examination of the suitability of sands from new or alternate sources such as evaluation for use as core sand to reduce imports  or for specialized uses, e.g. for 3D printing of moulds and cores . l Effects of thermal and mechanical reclamation processes on the sand surfaces and possible effects on mechanical and fracture properties during re-use . Figure 7 shows how the SEM can be used in a study of the effect of curing temperature on the tensile strengths of Shell moulded test-pieces . When under-cured at 160oC, as shown in view (a), the bond adhesive strength was not developed and bonds were pulled off the sand grains. In the over-cured condition, at 300oC, shown in view (c), the bond bridges were broken but sand grains remained intact. In view (b), the fracture surface of the sample cured at 240oC, the recommended temperature, reveals that the bonds did not break. The interface between the sand and resin tended to fail and some grains of sand were also broken. Likewise, Figure 8 shows how the effect of type of resin to be used in Shell moulding can be evaluated . This study showed that the strength developed during curing could be related to the relative diameters of the resin bond-bridges between sand grains. In examining the fracture surfaces of standard test pieces, the smaller bond contact areas (mean bridge diameter of 60μm) shown in view (a) gave less than half the measured bend strength compared to those in view (b) where the mean bond bridge diameter was 90μm. In the examples shown in Figures 7 and 8 the samples were given very thin Au coatings to facilitate examination in conventional SEM instruments. However as mentioned earlier coating may not be necessary. By using a variable pressure/environmental SEM often referred to as an ESEM it is possible to study not only uncoated specimens but also non-dried material which may contain water or liquids or other out-gassing content. In an ESEM the specimen chamber environment can be controlled as air, water vapour, nitrogen or argon. The ESEM has enabled the study of clay bonded moulding sands in the natural state, i.e. without drying, to give clearer information on agglomeration and bonding and on the effects of heating and re-wetting cycles in greensand . Although ESEM is mainly used for bio-materials it can be used for a variety of studies on wetting, drying, freezing, crystallization, etc. Special stages can be used for heating and cooling and it is also possible to introduce liquids to the specimen to study in-situ metallic corrosion. Improving quality and the environment SEM observation combined with microanalysis has proved to be a powerful tool in characterizing the nature of castings defects such as blowholes, pinholes, inclusions, shrinkage, cracks, veining, etc. in cast iron products [30-32] and in the study of entrainment defects such as oxide bi-films in Al alloys . Automated SEM/EDS has been developed to provide techniques for rapid and representative characterization of non-metallic inclusions in steel ingots and castings to provide information not only about size, shape and distribution but also about the elemental content within inclusions [34, 35]. Automated feature analysis as used for inclusion assessments can also be applied to other forms of microstructure evaluation and for surface quality measurements, etc. FIGURE 7. Fractography of Shell Moulded test pieces to study the effect of curing temperature . Views are at x100 approx. (a) Cured at 160oC – under-cured. (b) Cured at 240oC – correctly cured. (c) Cured at 300oC – over-cured. A C B A B Induction Melting Systems Induction OEM for: SHAPING THE FUTURE OF INDUCTION Global Sales & Service www.AjaxTocco.com World Headquarters 1745 Overland Avenue Warren, Ohio USA 44483 +1-330-372-8511 +1-330-372-8608 Fax 24 / 7 Customer Service: 800-547-1527 z Coreless & Channel Furnaces z Power Supplies z Ferrous & Non-ferrous z Melting, Holding & Duplexing z Retrofits & Rebuilds z Coil Repair z Parts & Service z Installation & Construction Service, Support & Manufacturing A B FIGURE 8. Fractography of Shell Moulded test pieces to study the effect of type of resin on strength . (a) Resin A gave smaller areas of bond bridge attachment to sand grains and lower strength. (b) Resin B gave larger attachment areas and higher strength.
MCT DEC 2017 (4TH QRT)