The Influence of Cooling Rate on the Damping Characteristics of the ZnAl4Cu1 Alloy

The paper presents the results of damping coefficient tests on the ZnAl4Cu1 alloy (ZL5). The damping coefficient has been calculated on the basis of specimen measurements obtained with the use of the signal echo method. The method consists in passing an ultrasonic wave through the tested material. The ultrasonic wave from a transmitting and receiving head passes through a specimen, bounces off its bottom surface and comes back to the measuring head in the form of a signal echo. The difference in the signal intensity between the first and the second echo in relation to the distance travelled by the ultrasound wave is a value of the material’s damping characteristics. The specimens were cast into three molds made of different materials, i.e. green sand, plaster and metal. The thermophysical properties of these materials are different, affecting the rate of heat absorption from the cast. Three series of specimens have been obtained which have different cooling rates. The specimens were then subjected to ultrasound and microscopic tests to assess the alloy structure. The internal alloy structure affects its damping properties to a great extent.


INTRODUCTION
Zinc-aluminum alloys, as well as ternary zinc-aluminum-copper alloys, can be used in less loaded machine parts and structural components. Due to their relatively low melting temperature, the energy consumption necessary to produce components from those alloys is reduced while they still maintain good corrosion resistance and high damping properties. Zinc alloys have the ability to dampen vibration, placing them in the group of HIDAMETS (High-Damping Metals), together with cast iron and bronzes [1,2]. A parameter indicating the damping properties of a specific material is the damping coefficient α. The coefficient's value depends on the type and composition of the alloy. In general, the damping properties of a specific alloy are mostly determined by its internal structure [3,4]. The value of the damping coefficient of an alloy with a specified chemical composition is constant. It can be changed, however, in the process of alloy modification, a process which usually affects the volume and number of precipitation of a specific compound or the size of grain. The inoculant's impact on the alloy structure is reflected in the change of the damping coefficient value of a specific alloy [5][6][7]. The cooling rate, apart from modification process, can also affect the macro and microstructure of the alloy by the changing shape and grain size of the phases [8][9][10]. The microstructure of an alloy influences its properties. The grain size and structure of the alloy components affect the ability of the alloy to dampen vibrations. A coarse-grained structure with large, branched dendrites also exhibits a better corrosion resistance tendency than finer grain structures. It is most often caused by corrosion centers at the grain boundaries. The more boundaries, the more places where a significant difference in corrosion potential can occur [11][12][13]. Zinc-aluminum based alloys, due to their lower component and production costs, often replace bronze in wear-resistant components. They work well for less loaded components operating at lower temperatures. The tribological properties of zinc alloys decrease at temperatures above 100 o C. The addition of copper increases the hardness, strength and wear resistance of Zn-Al alloys up to a content of 2-wt.%. Changes in the size of ingredients in the alloy structure also affect the wear resistance. Zinc-aluminum alloys with a finer structure display higher wear resistance and hardness [4,14,15].
This paper describes an attempt to measure the damping coefficient of the ZnAl4Cu1 (ZL5) alloy solidified at three different cooling rates. In order to obtain three different cooling rates, the tested alloy was cast into molds made of various materials characterized by different thermophysical properties. The https://journals.agh.edu.pl/jcme damping coefficient indicates the material's ability to absorb and scatter vibration. In perfect materials, the vibrating wave passes the material without any loss of its energy. In particular castings which are relatively heterogeneous in terms of their internal structure, the vibrating wave becomes attenuated. It is partially absorbed by the material and converted into other types of energy, mainly thermal, but primarily the energy of vibrations is dispersed on compounds of the alloy structure. The paper describes a test in which an ultrasonic wave has been used to determine the damping properties of the ZL5 alloy. The principle of ultrasonic wave measurements is based on physical properties of the materials being tested and also on the properties of the ultrasound waves themselves. Ultrasonic waves are very well propagated in solids such as metals. Examining an ultrasonic wave which has passed through the material or has been reflected from the specimen provides information regarding to any irregularities in material continuity. At the beginning of the usage of ultrasounds in foundry engineering, ultrasonic waves were only used to locate defects in castings. With the progress of technology, they were found to be useful in measurements of material thickness, determining microstructure and examining the ability of a specific material to dampen vibrations. Due to the use of an ultrasonic wave, tests of damping properties can be performed in a simple and fast manner. Transmitter--receiver heads are characterized by small diameters, facilitating the testing of small specimens. In order to eliminate the signal loss, the surface of a tested specimen should be smooth and even. At the point of contact between the transmitter--receiver head and the specimen, a coupling liquid needs to be used to transmit the transverse and longitudinal ultrasonic waves to the test material. The correct selection of the wavelength is another important factor. If the emitted wavelength from the measuring instrument is not matched to the diameter of the average grain, structural noise may occur, especially in the case of large grains. Structural noise is often the source of interference in damping measurements. To prevent those phenomena, it is recommended to use ultrasonic waves at least six times longer than the average grain size of the tested material [16,17].
The aim of the research is to determine the influence of the cooling rate of the ZL5 alloy on its microstructure and vibration damping properties. Typically, the literature finds the damping properties of zinc alloys cast into a metal mold and occasionally cast into one type of mold and inoculated. There are no comparisons of the test results for the damping properties of a specific alloy depending on the mold material. The mold material determines the cooling rate of the alloy, and thus affecting its properties. The damping properties depend on the alloy microstructure. The microstructure can be shaped by modifying or changing the cooling rate. In the case under consideration, it is proposed to determine the damping properties of the alloy for three different cooling rates. Alloys of this type are often cast into permanent molds, but also into sand molds. The use of a plaster mold in this case is intended to extend the results by a slow cooling. The type of mold material determines the cooling rate of the casting. The article shows the relationship between the cooling rate and the damping properties of the alloy which may be technologically useful.

EXPERIMENTAL PROCEDURE
The ZL5 alloy of composition shown in Table 1 was melted in a resistance crucible furnace in a graphite crucible. The furnace was heated to the temperature of 500°C. The batch was 3 kg of ZL5 alloy. After melting, the alloy was poured into three different types of molds ( Fig. 1): • a classical dry green sand mold, • a plaster mold made of casting jewelry gypsum, • a metal mold made of common steel preheated to a temperature of 100°C.
The molds had a cylindrical cavity with a diameter of 40 mm and a height of 100 mm. Two specimens were obtained from each type of the mold. During pouring and solidification of the cast, temperature measurements were taken. Data collected in such a manner was subsequently used to determine cooling curves and cooling rates. The temperature was taken with the use of a type K thermocouple placed in the cast axis. The thermocouple was located at the mid-height of the cast.
After cooling, specimens with a diameter of 40 mm and a height of 30 mm were cut out from the selection beneath the thermocouple of the castings.  The specimens were ground with a 1000 grit abrasive paper and then tested to determine the damping coefficient. Ultrasound tests were performed with the transmitter-receiver head from the Krautkramer 2000 ultrasound testing kit. To improve contact between the transmitter-receiver head and the specimen surface, a paraffin oil was used (Fig. 2).
A longitudinal ultrasonic wave with a frequency of 1 MHz was applied and the specimens were tested using the echo method. A signal emitted from the transmitter-receiver head passed through the specimen bounced off its bottom and came back to the measuring head. As a result of signal dispersion and absorption by the compounds of alloy structure, it returned in the form of a weakened echo (Fig. 3).
For each two specimens of a series, 10 measurements were performed which was necessary due to the heterogeneous structure of such material as a casting. The specimens were examined at a temperature of 22°C. After the collection of all the data, the average damping coefficient α was calculated for each specimen (Eq.(1)). After the ultrasound tests, specimens were polished and etched in the Palmerton's reagent [18] for 20 s. The etching revealed the alloy structure which was observed via brightfield microscopy.

RESULTS OF THE INVESTIGATION
The data collected during solidification in the molds made of different materials has been presented in the form of cooling curves to be seen in Figure 4. The cooling rate of the casting for each specimen has also been determined. The data was selected in the range from the moment of pouring the mold to the moment of the start of the solidification process. In the metal mold heated to the temperature of 100°C, the alloy cooled down at the rate of approx. 9.3 K/s. In the case of the green sand mold with a quartz sand matrix, dried up before casting, the cooling rate was approx. 1 K/s. The lowest cooling rate was observed in the casting gypsum mold and it was approx. 0.37 K/s. The cooling rate is clearly visible from the slope of the cooling curves. The greater the slope and the shorter the time to reach a given temperature, the faster the cooling rate. This is clearly seen in Figure 4, when the curve for samples cooling the fastest (metal form) and the curve for samples cooling the slowest (plaster mold) are compared. In order to illustrate the shape of the curve cooling at high speed in a metal form, an additional graph is provided in Figure 4.
The calculated damping coefficient α values are shown in Figure 5 and in Table 2.   The measured results indicate that the best damping properties have been achieved for the specimens in the gypsum mold, which solidified under the slowest heat removal condition. The lowest value of damping coefficient α was achieved for measurements performed on specimens cast in the metal mold, i.e. where the cooling rate was the highest. The difference between the best and worst result of damping properties is 9.17 dB/m. A hardness test was also performed for the tested samples. Hardness was measured by means of the Brinnell method. The diameter of the indenter was 2.5 mm. The applied force is 147 N. The hardness measurements results are presented in Table 2. As can be seen, there is no direct correlation between the cooling rate, the damping factor, and the hardness. Only for the highest cooling rate we observe the lowest value of the damping coefficient and the highest hardness. It is undoubtedly related to the presence of the finest alloy microstructure (Fig. 6).
Microscopic examination resulted in obtaining micrographs for the alloy based on different cooling rates. Microstructures are shown in Figures 6-8 in magnification 50× and 200×. The figures show the bright dendrites of the η phase (solid solution) against the background of the eutectic phase (α + η) visible by the lamellar grain pattern. The components of the microstructure of the ZnAl4Cu1 alloy poured into a dried green sand mold in magnification 200× show on Figure 9. As can be seen in the photos, the cooling rate affects all the components of the microstructure under different magnification. For the highest cooling rate of 9.3 K/s obtained in a metal form, we observe the smallest dendrites of the n solution and also the greatest fragmentation of the eutectic. Along with the reduction of the cooling rate to 0.37 K/s obtained in the gypsum form, we observe the largest dimension of the solution and eutectic dendrites. For the cooling rate of 1 K/s obtained in the dried sand form, the microstructure components are of average size. The cooling rate is determined by the type of mold material, which can also be clearly seen in the graph of the cooling curves (Fig. 4). The cooling rate directly affects the shape and size of the separate phases. This translates directly into the ability of the alloy to damp vibrations, which is presented in the comparison of the cooling rate and the values of the vibration damping coefficient in Table 2.  https://journals.agh.edu.pl/jcme

CONCLUSIONS
The ZnAl4Cu1 alloy cast into molds made of green sand, casting gypsum and steel solidified at different cooling rates. It can be perfectly seen on the distribution of cooling curves and is also evident from the calculated cooling rates. A low cooling rate obtained in the gypsum mold amounting to 0.37 K/s reflected for the values of the damping coefficient, which reaches to the highest value of 147.50 dB/m. In the gypsum mold we also have the most extensive phase η dendrites here, and the hardness here reaches an average value of 97 HB. The highest cooling rate was obtained for specimens solidifying in the metal mold, that is 9.3 K/s and the high cooling rate influences the microstructure. As we can see in the pictures (Figs. [6][7][8], the finest microstructure is in the sample of the metal mold (Fig. 6). For the specimens solidifying in the metal mold, the damping coefficient is α = 138.33 dB/m, the lowest value of all the samples. This means that the ZL5 alloy, which cools at a high cooling rate, has the lowest damping properties. However, the highest hardness is obtained here, amounting to 104 HB. This is the lowest value of a damping coefficient in this series of experiments. The specimens cast into the mold made of green sand, solidified at the cooling rate of 1 K/s. The value of the damping coefficient is 141.67 dB/m, and is between the aforementioned results for the plaster and steel molds. By controlling the cooling rate, we are able to influence the structure of the alloy, which affects its ability to subsequently dampen mechanical vibration. The results of the obtained damping coefficient α indicate that the ability to dampen vibrations increases for the tested ZL5 zinc alloy as the cooling rate decreases.