Ultrasonic alloys

Materials World magazine
2 Oct 2011

Casting techniques for metallic objects have been enhanced by incorporating sound treatment. Zohair Sarajan, from Islamic Azad University and Farzad Karami Sheikhiabadi from Shahid Sadughi Technical School, both in Iran, outline how applying noise to the molten metal increases density and strength.

Controlling the microstructure in the casting of components made by aluminium-silicon is an important challenge faced by the foundry industries. It includes controlling parameters such as grain size, porosity and its distribution, morphology of metallic and non-metallic secondary phases available in microstructures, and macro and micro segregations.

The effect of ultrasonic frequency and time on the size of alpha alumina (α-Al) primary solid phase and density on an Al-6wt.%Si alloy has been researched by the authors. This has been done using a power ultrasonic field with the frequency and times of 16, 18, 20 and 22KHz and SEC. It was observed that the increase in ultrasonic time up to 22 seconds and ultrasonic frequency up to 22kHz led to a decrease in the size of α-Al phase (grain refinement), modification of silicon phases and an increase in density and tensile strength.

An ultrasonic treatment is applied in metallic melts during their casting in order to obtain a non-dendritic and fine-grain solidification microstructure due to its mass transfer and energy transfer abilities. The primary aluminium dendrites nearly disappeared and the grain size reduced, which might be because of the remelting of dendrite roots, or the fracture of dendrite arms by the acoustic streaming forced convection.

The modification of the silicon phase is of interest, since fine silicon phase along with fine primary aluminium grains improves mechanical properties and ductility. With the increase in ultrasonic frequency and times there is an increase in intensity of flows in front of solid-liquid interface and effective times for penetration of the liquid into inter-dendrite regions. Consequently, shape factor of primary solid particles increases and changes to a rosette shape.

Sound and temperature effects

The ultrasonic treatment equipment used in this experiment consists of temperature and power control systems and includes a horn. Here, it was used on top of the mould. During the course of the ultrasonic treatment of a metal melt, the ultrasonic wave emitted from the transducer and passed through the acoustic horn was directly propagated into the melt. The temperature of the ultrasonic treatment of the metal melt was monitored with a thermocouple.

The sizes of various phases were measured statistically using the Image Analyzer. One-and-a-half kilogrammes of aluminium-6wt.%silicon alloy was put in a silicon carbide crucible and melted by an electric resistance furnace. When melt temperature reached 700°C, ultrasonic was turned on and the molten alloy poured into the preheated mould at 360°C. Immediately after pouring, the liquid metal was stirred slightly under a hot argon atmosphere at about 100ºC and at maximum flow of 2,360cm3/min. When ultrasonic treatment was stopped, the mould was air-cooled.

Building a picture

The images in Figure 1 show the microstructure of a cast sample without ultrasonic columnar dendrites. The SEM images in Figure 2 show the microstructure of samples ultrasonically treated at 16KHz for 16, 18, 20 and 22 seconds. Comparing both sets of images reveals that ultrasonic treatment of the melt at 16KHz has a minor effect on the size of α-Al phases, and large columnar dendrites are still available in microstructures. Comparing the SEMs (Figure 2 [a] and [b]) shows that increasing ultrasonic time to 18 seconds did not noticeably diminish the size and number of columnar dendrites. By further increasing the time to 20 seconds, (Figure 2 [c]), the length of columnar dendrites decreased by up to 250μm, however, the size of α-Al phase was still too large.

The microstructures of samples ultrasonically treated at 22KHz for 16-22 sec are shown in the micrographs (Figure 3a-c). Images b and c show that increasing ultrasonic time leads to an increase in the number of individual α-Al phases with non-dendritic morphology, and more grain refinement is created.

The graphs (Figure 4) show variation in size of α-Al phase against ultrasonic time and frequency. It is evident that increases in ultrasonic frequency (16-22kHz) and ultrasonic time (16-22sec) lead to a decrease in size and diminished dendritic morphology of the α-Al phase.

Ultrasonic energy influences the final microstructure in two ways. Cavitation periodic tensionpressure forces are created in liquid elements by these waves. During a half-cycle of tension, cavitation occurs. During growth of these cavities, their surface temperature starts to decrease and new, solid particles nucleate in these local regions.

Ultrasonic energy leads to force convections in the liquid and dendrite arms fragmentation. The induced flows exert external force towards dendrite arms to rearrange it in flow direction. These viscous drag forces lead to detachment of dendrite arms. By flows, detached dendrite arms move toward the liquid and act as new sources of nucleation. With further fragmenting of dendrite arms, nucleation rate increases and more grain refinement occurs that is evident from the images (Figures 2 and 3). Density of an ultrasonic untreated sample was 2.40g/cm3.

The porosities in an ultrasonic untreated sample were generally located at interdendrite regions between dendrite arms or in the hypo eutectic regions, as shown in the micrographs (Figure 5 a-b).

The graph (Figure 6) shows the variation of density against ultrasonic frequency and time. At 16KHz and one second, the density of vibrated alloy increases to 2.47g/cm3, and a further increase in ultrasonic time up to 22 seconds leads to increase in density of vibrated alloy to 2.52g/cm3.

However, formed bubbles that move from interdendrite regions towards remaining liquids during solidification do not have sufficient time to exit and rise to the top of the mould. Therefore, they remain in the non-solidified liquids and will form large porosities in the hypo eutectic regions (see figure 5a). Ultrasonic treatment is then employed, along with leads, to decrease the volume and amount of shrinkage pores in the microstructure (see figure 5b).

The results show that application of ultrasonic treatment led to an increase in grain refinement and density of Al-6wt.%Si alloy. Extending ultrasonic time up to 22sec led to a decrease in the size of α-Al phase for all ultrasonic frequencies. Increasing ultrasonic time and frequency led to a rise in the density of Al-6wt.%Si alloy. It was observed that mass feeding of liquid phase and melt degassing because of applying ultrasonic treatment has a strong effect on the density of alloy. During this process, the silicon phases are broken into pieces and the considerable modification of microstructure can be achieved. These are due to the mechanical effects, cavitation effects and acoustic streaming effects induced by the power ultrasonic field.  

Further information

Zohair Sarajan, Department of Materials Science and Engineering, Islamic Azad University-Yazd Branch, Safaeeyeh, Yazd, I.R of IRAN, P.O.Box 89195/155. Tel: +98 351 8219223, Email: zohairsarajan@iauyazd.ac.ir