turer’s specification and the outcome of this result is that ZP131 is not only suitable as a mould material for casting light materials, it gives better surface quality without any loss of other properties. Further, from the plots of Fig. 6, it is clear that the best combination of factors for the most fa- vourable surface roughness is A356, ZP131, MAGCOAT and a pouring temperature of 690C (1274F).
The lower surface roughness obtained from ZP131 moulds can be attributed to the lower and more uniform grain size of gypsum particles. ZCAST 501, on the other hand has dis- proportionately larger sand grains and also suffers from easy mould wall erosion due to relatively lower bonding strengths. The degree of the spherical curvature of the added olivine sand, which is basically a crushed rock, is detrimental to the as-cast surface roughness. Scanning Electron Microscope (SEM) photomicrographs showed highly angular sand grains as shown in Fig. 7, surrounded by the finer gypsum powder.
The confidence level of the mould coating parameter shown in Table 3 is slightly higher than the common acceptable levels, but considering the S/N ratios, it is evident that mag- nesium oxide-based Magcoat product was the best suited in terms of achieving the best surface roughness (Fig. 7). A356 scored the best when it comes to the surface roughness and both magnesium grade alloys seemed to have undergone some mould-metal reaction. The propensity of Mg alloys to oxidise is governed by the Gibb’s free energy change, and the application of a mould coating such as MAGCOAT can reduce this reaction by increasing this free energy.
percent elongation
ANOVA results on the percent elongation are presented in Table 5 and it is evident that the alloy factor came out to be the most significant, at 99 % confidence level. It may be re- iterated that the analysis across different alloys is only meant to find if all the alloys considered are exhibiting the expected levels of ductility when cast in digitally printed moulds. While SC1 gave the maximum percent elongation, A356 fell short again, from the normal expected ductility levels probably due to the same reasons as explained previously, such as improper degassing and possible contamination from the crucible. In comparison, both mould coating and mould material contrib- uted minimally to the overall variance and are pooled as ex- perimental errors. For completeness however, the S/N ratios of these factors are also shown in Fig. 8. The best combina- tion of other factors in favour of percent elongation is ZP131, MAGCOAT and a pouring temperature of 690C (1274F).
In terms of percent elongation, interestingly, SC1 outper- formed AZ91. Alloy SC1 is a special alloy developed by the Australian Research Institute CAST, to increase the creep properties of magnesium in high temperature applications. Additions of special alloying elements such as zinc, lantha- num, cerium, and zirconium create a completely different secondary phase which is intended to act as a more rigid
International Journal of Metalcasting/Summer 2011
interlocking medium to resist grain movement.28 The in-
creased ductility in this case perhaps is due to this interlock- ing secondary phase, as against the brittle secondary phase Mg17Al12, common with the alloys of the AZ series.
hardness
ANOVA and evaluation of S/N ratios considered in the same lines as above showed that the alloy factor is the most sig- nificant at 95% confidence level and within the alloys inves- tigated, A356 castings attained the highest hardness values, followed by AZ91. Mould material was the second ranked factor, with ZCast 501 moulds resulting in the highest hard- ness, followed by ZP131 and silica foundry sand moulds. Mould coating and pouring temperature had the least sig- nificance on the hardness and hence were pooled as experi- mental error. In terms of actual values, A356 samples gave an average hardness value of 59.62 HB, closely followed by AZ91 at around 58.06, while SC1 was the softest, at 48.17 HB. These measured values of hardness are also close to the values reported in the literature at 60 HB for A356 and 50-65 HB in the case of AZ91HP.29
overall Results
The overall trend of the experimental results is presented in Table 6, with factors ranked according to their relative statistical significances in influencing different responses. While this is a useful comparison of the performance of dif- ferent alloy systems when rapidly cast using the 3D printed moulds, the overall observation is that 3D printed moulds can be effectively used for casting light metals and alloys and the casting characteristics are not much different from other processing routes. Within the combination of fac- tors however, there are differences from one alloy system to the other. Keeping in view, certain limitations with the aluminium castings and within the experimental ranges and, AZ91HP and SC1 exhibited expected UTS and ductility lev- els respectively, while ZP131 is the most favourable mould
Figure 7. An SEM photomicrograph of ZCast 501: Large and angular olivine sand grains are surrounded by finer gypsum plaster.
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