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Metal Casting Technologies : June 2010
34 www.metals.rala.com.au TECH N ICAL FEA TURE TECHNICAL FEATURE 2) For a given population of nucleant particles, all terms are constant except for Q. Thus, Equation 1 simplifies to d = a + b/Q for a particular set of casting conditions. In a recent study  of aluminium alloys it was found, Figure 2, that both the values of 'a' and 'b' decreased with increasing cooling rate, Ṫ, which means that both the proportion of active nucleants involved in grain refinement and the rate of development of constitutional undercooling increased. However, the change in the b value was more pronounced. As a result Equation 2 was further developed to Equation 3 where b" is a fitting constant. Figure 2 indicates that the grain size can be approximately halved by increasing the cooling rate from 0.3oC/s to 15oC/s . When a melt is cast at low superheats into chill moulds or on a chill plate, nuclei generally form at/or near the mould wall in highly localised undercooled regions, and then are transported into the centre of the melt. Survival of these nuclei is also dependant upon the undercooling in the melt leading to substantial grain refinement [20, 21]. To illustrate the effect of superheat on grain size the same set of alloys used to produce Figure 2, were also cast into a steel die at two different pouring temperatures. Reducing the superheat had a different effect to cooling rate. Whilst the effect of increasing cooling rate is to reduce both the 'b' value and the 'a' value (Figure 2), reducing the superheat was found to reduce the 'a' value whilst the 'b' value remained essentially constant (Figure 3). In other words, reducing the superheat led to a substantial increase in the proportion of nucleant particles that are able to nucleate grains for all alloy compositions. Based on Figure 3, Equation 2 becomes The positive effects of dynamic conditions on the solidification of metals have been recognised for many years. Ultrasonic treatment, in particular, is one simple means that has been shown to be effective for various alloys . Acoustic cavitation is essential to ultrasonic grain refinement. Accordingly, the ultrasonic intensity applied needs to exceed a certain cavitation threshold which depends on alloy chemistry. Figure 4 shows the influence of the ultrasonic intensity, measured by the square of the ultrasonic amplitude (A), on grain refinement of Mg-Al alloys. The cavitation threshold varied from A2 = 1 μm2 for the Mg-8%Al alloy to approximately A2 = 6.52 μm2 for the other three alloys. Figure 5 re-plots the grain size data in Figure 4 vs. 1/Q for A2 ≥ 6.52 μm2, where the Mg-8%Al alloy was excluded from the plot for the case when A2 = 6.52 μm2, which marks the approximate cavitation threshold for the other three alloys. The slopes of the lines of best fit corresponding to A2 = 152, 202, 252 μm2 in Figure 5, are essentially the same while the intercept decreases. This revealed that increasing the ultrasonic intensity above the threshold refines the grain structure mainly by activating more nucleant particles but the potency of the particles is little changed. On the other hand, the noticeable change in the slope of the line of best fit from A2 = 6.52 μm2 to A2 = 152 μm2 and beyond signifies the transition from underdeveloped cavitation to developed cavitation and thus Figure 5 can be used to help determine the cavitation threshold. Figure 2: Grain size plotted against 1/Q for four alloys (1050, 5083, 6060 and 6082) for a TiB2 addition of 0.005% and Ti contents varying from 0 to 0.05% at cooling rates ranging from 0.3 to 15 C/s . Figure 3: Grain size measurements for the alloys used to develop Figure 2, cast into a steel mould preheated at 300 C. The alloys were cast at superheats of 35 and 65 C .