**3. Results and Discussion**

Reference samples (a number of those had been produced) showed a coarse columnar grain structure as shown in Figure 2 with the average grain size 630 μm.

First experiments when the rod was introduced without or with ultrasound in the temperature ranges 740 to 715 ◦C and 740 to 710 ◦C did not produce any significant results with a very coarse mixed columnar and equiaxed structure formed (these results are not presented here). This confirmed the importance of the temperature range needed for the desired effect. Although the introduced rod was quick to dissolve, the relatively high temperature apparently prevented the survival of solid fragments as well as facilitated deactivation of solid inclusions (only 10–15 ◦C above liquidus is needed for that [9]).

(a) **Figure 2.** *Cont.*

**Figure 2.** Macro (**a**) and micro (**b**,**c**) grain structure of a reference sample cast at 670 ◦C in a TP1 mould. The positions of photos in (**b**) and (**c**) with respect to the sample cross-section are marked in (**a**) (same positions have been used hereafter).

Following experiments were performed in the temperature ranges 720 to 670 ◦C and 710 to 670 ◦C. The decrease in the introduction temperature yielded a remarkable change in the grain structure even when the rod was introduced without ultrasonic vibrations, with the average grain size 250 μm, Figure 3. It can be noted that a slight decrease in the starting melt temperature gives some additional grain refining effect (228 μm), most probably due to the better survival chances of crystal fragments and active inclusions.

**Figure 3.** Effect of rod introduction into the melt in the temperature range 720 to 670 ◦C (**a**,**c**) and 710 to 670 ◦C (**b**–**e**) on the grain structure of the alloy cast in a TP1 mould.

Ultrasonic oscillations applied to the rod further refined the grain structure as illustrated in Figure 4. This might be due to the better distribution of crystal fragments in the melt as well as to the additional activation of insoluble inclusions by ultrasonic cavitation [13,17]. At the same time the grain size obtained upon rod introduction with ultrasonic oscillations in a wider temperature range was finer than in a narrower temperature range, i.e., 123 μm and 165 μm, respectively. As ultrasonic oscillations, in addition to accelerated rod dissolution, may facilitate additional mechanisms of grain refinement such as activation of inclusions and fragmentation of solid crystals with their effect being time-dependent, therefore, the longer the ultrasonic cavitation and vibration works, the better the result [13]. Hence, a slight increase in the rod introduction time provided by using a wider temperature range is beneficial for the grain refinement.

**Figure 4.** Effect of rod introduction with ultrasonic vibrations into the melt in the temperature range 720 to 670 ◦C (**a**,**c**) and 710 to 670 ◦C (**b**–**e**) on the grain structure of the alloy cast in a TP1 mould.

The standard procedure of grain refinement of aluminium alloys is the addition of an AlTiB master alloy. In order to compare the grain refining effects that we have achieved with the same-alloy rod introduction, we did additional experiments with the standard Al5Ti1B grain refining master alloy addition (0.2% of master alloy). Two experiments were performed, i.e., addition of the grain refiner to the base alloy and same-alloy rod introduction into the grain refined master alloy. The obtained grain sizes were rather small (112–123 μm), Figure 5, and comparable with those obtained upon same-alloy rod introduction with ultrasonic oscillations, Figure 4. Although the addition of a rod to the grain refined alloy gave a slight additional decrease in the grain size (112 μm vs 123 μm), it was not statistically significant. These results demonstrate that under given cooling conditions (TP1 mould) the addition of 0.2% grain refiner as well as the addition of the same-alloy rod gives the maximum

number of solidification sites that can be realised and produce grains. Therefore the achieved grain size 112–123 μm is the smallest possible and does not depend on the way how the active solidification sites are introduced.

(a)

(c)

**Figure 5.** Effect of Al5Ti1B grain refiner addition and same-alloy rod introduction into the melt in the temperature range 710 to 670 ◦C on the grain structure of the alloy cast in a TP1 mould: (**a)**, macrostructure; (**b**,**c**) microstructure.

(a) (b)

Figure 6 summarises the results of grain size measurements for all studied cases.

**Figure 6.** Grain sizes achieved in experiments: (**a**) the effects of the same-rod introduction, ultrasonic oscillations and the temperature range of introduction and (**b**) comparison of the same-alloy rod introduction with the standard grain refinement by master alloy addition.

This work has a limited scope of demonstrating the grain refining effect of the introduction of a same-alloy solid material into the melt facilitated by ultrasonic vibration of this rod. We based our discussion on the known mechanisms of suspension casting that include the introduction of active substrates and rapid undercooling of the melt [2–5] and known mechanisms of ultrasonic melt processing that include (in relation to the observed effects) accelerated dissolution and mixing [14] facilitated by fragmentation of solid suspended particles [13]. The combination of these effects produced a remarkable grain refinement effect, comparable with the standard procedure of grain refinement. The following research should focus on the further optimisation of the procedure with respect to the temperature range, processing time, amount of the introduced solid material and further cooling conditions during solidification. The effect of ultrasonic vibrations on the melt degassing on the one hand and on the introduction of potential oxides with the solid material on the other hand, should be further explored.
