Here we present a protocol to obtain a novel Mn-Cu-based alloy with excellent comprehensive performances by a high-quality smelting technology and reasonable heat treatment methods.
Manganese (Mn)-copper (Cu)-based alloys have been found to have damping capacity and can be used to reduce harmful vibrations and noise effectively. M2052 (Mn-20Cu-5Ni-2Fe, at%) is an important branch of Mn-Cu-based alloys, which possesses both excellent damping capacity and processability. In recent decades, lots of studies have been carried out on the performance optimization of M2052, improving the damping capacity, mechanical properties, corrosion resistance, and service temperature, etc. The major methods of performance optimization are alloying, heat treatment, pretreatment, and different ways of molding etc., among which alloying, as well as adopting a reasonable heat treatment, is the simplest and most effective method to obtain perfect and comprehensive performance. To obtain the M2052 alloy with excellent performance for casting molding, we propose to add Zn and Al to the MnCuNiFe alloy matrix and use a variety of heat treatment methods for a comparison in the microstructure, damping capacity, and service temperature. Thus, a new type of cast-aged Mn-22.68Cu-1.89Ni-1.99Fe-1.70Zn-6.16Al (at.%) alloy with superior damping capacity and high service temperature is obtained by an optimized heat treatment method. Compared with the forging technique, cast molding is simpler and more efficient, and the damping capacity of this as-cast alloy is excellent. Therefore, there is a suitable reason to think that it is a good choice for engineering applications.
Since the Mn-Cu alloys were found by Zener to have damping capacity1, they have received widespread attention and research2. The advantages of Mn-Cu alloy are that it has high damping capacity, especially at low strain amplitudes, and its damping capacity cannot be disturbed by a magnetic field, which is quite different from ferromagnetic damping alloys. The high damping capacity of Mn-Cu-based alloys can be mainly attributed to the movability of the internal boundaries, mainly including twin boundaries and phase boundaries, which are generated in the face-centered-cubic-to-face-centered-tetragonal (f.c.c.-f.c.t.) phase transition under the martensite transformation temperature (Tt)3. It has been found that Tt depends directly on the Mn content in the Mn-Cu-based alloy4,5; that is, the higher the Mn content, the higher the Tt and the better the damping capacity of the material. The alloy, which contains more than 80 at% manganese, was found to have high damping capacity and optimum strength when quenched from the solid-solution temperature6. However, the higher Mn concentration in the alloy would directly cause the alloy to be more brittle and have a lower elongation, impact toughness, and a worse corrosion resistance, which means the alloy will not meet the engineering requirements. Previous research findings revealed that an aging treatment under suitable conditions is an effective way to reconcile this problem; for instance, Mn-Cu-based damping alloys containing 50 - 80 at% Mn can also obtain a high Tt and favorable damping capacity by an aging treatment in the appropriate temperature range7. This is due to the decomposition of the γ-parent phase into nanoscale Mn-rich regions and nanoscale Cu-rich regions while aging in the temperature range of the miscibility gap8,9,10, which is considered to improve Tt of this alloy along with its damping capacity. Clearly, it is an effectual method which can combine high damping capacity with excellent workability.
M2052 alloy used for forging forming, a representative Mn-Cu-based high-damping alloy with medium Mn content developed by Kawahara et al.11, has been extensively studied in the last few decades. Researchers found that M2052 alloy has a good sweet spot between damping capacity, yield strength, and workability. Compared with the forging technique, casting has been widely used so far due to the simple molding process, low production costs, and high productivity, etc. The influential factors (e.g., oscillation frequency, strain amplitude, cooling velocity, heat treatment temperature/time, etc.) on the damping capacity, microstructure, and damping mechanism of M2052 alloy have been studied by some researchers12,13,14,15,16,17,18. Nevertheless, the casting performance of M2052 alloy is inferior, for instance, a wide range of crystallization temperature, the occurrence of casting porosity, and concentrated shrinkage, eventually resulting in the unsatisfactory mechanical properties of the castings.
The purpose of this paper is to provide the industrial field with a feasible method of obtaining a cast Mn-Cu based alloy with excellent properties which can be used in machinery and in the precision instruments industry to reduce vibration and ensure the product quality. According to the effect of alloying elements on the phase transformation and the casting performance, Al element is considered to reduce the γ-phase region and the stability of the γ phase, which can make the γ phase more easily transform into a γ' phase with micro-twins. Moreover, the solution of Al atoms in the γ phase will increase the strength of the alloy, which can improve the mechanical properties. Also, Al element is one of the important elements which can improve the casting properties of Mn-Cu alloy. Zn element is beneficial to improving the casting and damping properties of the alloy. Finally, 2 wt% Zn and 3 wt% Al were added to the MnCuNiFe quaternary alloy in this work and a new cast Mn-26Cu-12Ni-2Fe-2Zn-3Al (wt%) alloy was developed. Furthermore, several different heat treatment methods are used in this work and their distinct effects are discussed as follows. The homogenization treatment was used to reduce dendrite segregation. The solution treatment was used for impurities immobilization. The aging treatment is used for triggering spinodal decomposition; meanwhile, the various aging times are used for seeking out the optimizing parameters for both excellent damping capacity and a high service temperature. Ultimately, a preferable heat treatment method was screened for superior damping capacity, as well as a high service temperature.
It turns out that the maximum internal friction (Q-1) and the highest service temperature can be achieved concurrently by aging the alloy at 435 °C for 2 h. Because of the simplicity and efficiency of this preparation method, a novel as-cast Mn-Cu-based damping alloy with excellent performance can be produced, which is of important practical significance for its engineering application. This method is particularly suitable for the preparation of casting Mn-Cu-based high damping alloy which can be used for vibration reduction.
1. Preparation of the Raw Materials
- Weigh all the required raw materials with an electronic scale by mass percentage (65% electrolytic Mn, 26% electrolytic Cu, 2% industrial pure Fe, 2% electrolytic Ni, 3% electrolytic Al, and 2% electrolytic Zn), as shown in Figure 1.
NOTE: All these raw materials were commercially available.
Figure 1: Presentation of raw materials. The materials used include 65 wt% electrolytic Mn, 26 wt% electrolytic Cu, 2 wt% industrial pure Fe, 2 wt% electrolytic Ni, 2 wt% electrolytic Zn, and 3 wt% electrolytic Al. Please click here to view a larger version of this figure.
2. Melting and Casting Process
NOTE: The detailed steps of sand casting are shown in Figure 2.
Figure 2: Sand casting and molding steps. The main process includes pattern-making, mold-making, and a casting operation. Please click here to view a larger version of this figure.
- To prepare patterns, make patterns according to the product drawing, and make sure that the size of the pattern is expanded to a certain extent to be liable for shrinkage and machining allowances.
NOTE: The pattern material used in this work is wood ( Figure 3) because a wood pattern is light, easy to work, and has a low cost and short production cycle.
Figure 3: Patterns used in the casting mold. These wood patterns were used to obtain the shape of the castings. Please click here to view a larger version of this figure.
- To prepare the molding sand, mix together the quartz sand with 4% - 8% sodium silicate.
NOTE: The sand diameter is about 0.4 mm and the particles are uniform.
- Complete the main molding process by hands.
- First, put two patterns in the molding flask.
- Then, roll over the flask after ramming the molding sand around the patterns and withdraw the patterns from the sand.
- Finally, brush the surface of the sand mold with casting coating for improving the casting surface quality and reducing casting defects.
NOTE: The molded sand mold is shown in Figure 4.
- To obtain a dry sand mold, put the mold in an oven at 180 °C and bake it for more than 8 h before casting to enhance its strength and permeability, facilitate the melt filling, and ensure the quality of the casting products.
Figure 4: The molded sand mold. It has two cavities and its surface has been covered with a coating. Please click here to view a larger version of this figure.
3. Induction of Melting
NOTE: Use a medium-frequency vacuum induction melting furnace.
- Open the furnace lid, put 20.8 kg of Mn, 8.32 kg of Cu, 0.64 kg of Ni, 0.64 kg of Fe, 0.64 kg of Zn, and 0.96 kg of Al materials in the crucible successively, and cover the materials with cryolite at last.
- Take out the casting mold from the oven and put it in the furnace; adjust its position for a successful pouring. Close the lid, vacuum the furnace, and then open the heat distribution system to start melting the alloy.
- When the metals start to melt, fill the furnace with argon to a 93-KPa negative pressure, to inhibit the splashing of the molten metal.
- After the alloy has melted, refine it for several minutes to reduce the harmful impurities and gas content.
NOTE: The melting procedure often includes smelting and refining.
4. Casting the Alloy
- Pour the molten metal smoothly into the casting mold after the refining process.
- After the molten metal is completely solidified, break the vacuum and take out the casting mold.
- Remove the castings from the casting mold when the temperature of the mold drops to a low level.
5. Pretreatment of the Castings
NOTE: The macrophotograph of the molded part is shown in Figure 5.
- Cut specimens from the casting by using a linear cutting machine.
NOTE: The specimens for the X-ray diffractometer (XRD) measurements and the metallographic observation are in 10 x 10 x 1 mm3. The specimens for the dynamic thermomechanical analysis (DMA) possess a dimension of 0.8 x 10 x 35 mm3.
Figure 5: The molded parts in the sand mold and the removed parts. Two castings were molded at one time. Please click here to view a larger version of this figure.
6. Heat Treatment
- Divide the polished specimens into seven groups and keep specimen #1 free of treatment, maintaining an as-cast state for comparison. Put the others in a box-type resistance oven for different heat treatments.
- Homogenize specimens #2 and #5 at 850 °C for 24 h and, subsequently, quench them in cool water before aging them at 435 °C, specimen #2 for 4 h and specimen #5 for 2 h.
- Solution-treat specimens #3 and #6 at 900 °C for 1 h and, subsequently, quench them in cool water before aging them at 435 °C, specimen #3 for 4 h and specimen #6 for 2 h.
- Age specimens #4 and #7 at 435 °C for 4 h and 2 h, respectively.
7. Damping Capacity Test
- Use a Dynamic Mechanical Analysis (DMA) to measure the damping capacity of the specimens17.
NOTE: The test mode is strain sweep at room temperature.
- During the test, detect the phase angle δ between the stress and the strain (as shown in Figure 6).
- Characterize the damping capacity by Q-1, which can be determined by the following formula.
Q-1 = Tan δ
Figure 6: The fixture construction and testing principle of the DMA. ( a) This panel shows the double cantilever fixture of the DMA. ( b) This panel shows the relationship of the applied sinusoidal stress to the strain and the resultant phase lag. The values of the lag between the stress and the strain, as well as the modulus, can be calculated by formulae. Please click here to view a larger version of this figure.
8. Sample Characterization
- Electrolytic polishing and metallographic observation
- For a dendrite microstructure observation, etch all specimens for about 1 min in a mixed solution of perchloric acid and absolute alcohol at 1:27.
- Then, clean the specimens with acetone, dry the sample with a blower, and observe the dendritic structure with a metallographic microscope.
- Phase structure characterization
- Characterize the phase structure and the lattice parameters of the specimens by X-ray diffraction (XRD) with CuKα radiation12,22.
NOTE: Use a scan speed at 2°/min. Before the XRD measurement, prepare the specimens carefully by removing any surface stress.
- Characterize the phase structure and the lattice parameters of the specimens by X-ray diffraction (XRD) with CuKα radiation12,22.
Figure 7 shows the dependency of the damping capacity on the strain amplitude for the as-cast MnCuNiFeZnAl alloy specimens #1 - #7 and as-cast M2052. The results show that the damping capacity of specimen #1 is higher than that of cast M2052 alloy (as shown in Figure 7a) and the traditional forged M2052 high-damping alloy mentioned in previous articles20,21. Moreover, the damping capacity of the original as-cast MnCuNiFeZnAl alloy can be further improved by, subsequently, homogenization-aging, solution-aging, and aging treatments (as shown in Figure 7b and 7c), among which an aging treatment for 2 h can lead to the highest damping capacity. When the strain amplitude ε is 2 x 10-4, the Q-1 values of specimens #1 - #7 are listed in Table 1. In addition, when comparing specimen #4 with specimen #7, it was found that the Q-1 can be significantly improved by a shorter aging time (as shown in Figure 7d). Furthermore, sand casting and aging for 2 h are simpler, economical, and efficient, compared to forging.
Figure 7: The dependency of Q-1 on strain-amplitude for the as-cast MnCuNiFeZnAl alloy specimens #1 - #7 and as-cast M2052. For the measurements of the strain-amplitude dependence of Q−1, the testing frequency and temperature were 1 Hz and 25 °C, respectively. This figure has been modified from Liu et al.18. Please click here to view a larger version of this figure.
|Q−1||3.0 × 10−2||4.7 × 10−2||4.9 × 10−2||3.9 × 10−2||4.5 × 10-2||4.7 × 10-2||5.0 × 10-2|
Table 1: The Q-1 values of specimens #1 - #7 when the strain amplitude ε = 2 x 10-4.
Figure 8 shows metallographic micrographs of as-cast MnCuNiFeZnAl alloy specimens #1 and #5 - #7. There will form a serious dendritic segregation during the process of slow cooling in the casting molding for the slow diffusion rate of Mn atoms between Cu atoms, which eventually leads to the formation of a dendritic microstructure. Since Mn is more susceptible to corrosion than Cu, the dark regions in the observed dendritic structure are Mn-rich dendrites, which are a few millimeters long and several micrometers wide, while the light regions are Cu-rich regions. When the temperature decreases, the Mn-rich dendrites mainly precipitate from the liquid phase of the Mn-rich regions, and then the Cu-rich intervals form between them. By comparison, the dimensions of the dark Mn-rich dendrites of specimen #5 are significantly smaller than those of specimen #1, which indicates that the dendrite segregation of specimen #5 was weakened to some extent. Similarly, the dendrite segregation of specimen #6 was weakened to some extent too but was still slightly better than that of specimen #5 due to the shorter holding time during the solution-aging treatment. However, there is no distinctive difference in the dendritical microstructures of specimens #7 and specimens #1. These results represent that the homogenization-aging and solution-aging treatments can weaken the macroscopic Mn segregation, but the direct aging treatment has no obvious effect on it. These conclusions can also be drawn from the compositional EDS analysis. Before a spinodal decomposition, the Mn content in the Mn-rich dendrites of as-cast MnCuNiFeZnAl alloy was 79.23 at% on average, and the Mn content was significantly reduced to 68.20 at% after homogenizing the sample at 850 °C for 24 h and to 73.42 at% after a solution treatment at 900 °C for 1 h.
Figure 8: Metallographic micrographs of as-cast MnCuNiFeZnAl alloys subjected to different heat treatments. The different dendritic structure of different specimens can be seen. This figure has been modified from Liu et al.18. Please click here to view a larger version of this figure.
According to the temperature-dependent damping capacity curve, the damping capacity decreases rapidly as the temperature rises. The temperature at which the damping capacity is drastically decreased is usually defined as the service temperature, which is one of the most pivotal indicators for damping alloys being used in the engineering area. The service temperatures of specimens #1 and #5 - #7 are listed in Table 2. It can be seen clearly that the aging at 435 °C for 2 h can cause the optimal service temperature.
|Service temperatures (°C)||43||50||55||70|
Table 2: The service temperatures of specimens #1 and #5 - #7.
The high damping capacity of Mn-Cu-based alloy is related to the γ' phase produced in an f.c.c-f.c.t martensitic transformation. Normally, the amount of the γ' phase is related to the Mn content. A large number of scholars7,22,23,24 have studied the relationship between lattice parameters, lattice distortion, and Mn content in Mn-Cu-based alloys. According to the c/a values of specimens #1 and #5 - #7, the Mn content in nanoscale Mn-rich regions of each specimen after the spinodal decomposition can be estimated by using the formula mentioned by Zhong et al.17. The CMn of specimens #1 and #5 - #7 are 84.18 at%, 84.75 at%, 85.08 at%, and 85.35 at%, respectively, in nanoscale Mn-rich regions after spinodal decomposition. Obviously, specimen #7 has the highest CMn, which means that the as-cast MnCuNiFeZnAl alloy has the superior damping capacity and, simultaneously, a higher service temperature by aging at 435 °C for 2 h.
The relationship between the lattice distortion (a/c-1), Q-1 (at a strain amplitude of ε = 2 x 10-4), and the service temperature of as-cast MnCuNiFeZnAl alloys subjected to different heat treatments, corresponding to specimens #1 and #5 - #7, is plotted in Figure 9. Evidently, the lattice distortion is directly proportional to the Q-1 and service temperature; namely, the greater the lattice distortion, the better the damping capacity and the higher the service temperature.
Figure 9: The relationship between the lattice distortion (a/c-1), Q-1 (ε = 2 x 10-4), and the service temperature of as-cast MnCuNiFeZnAl alloys subjected to different heat treatments. This figure has been modified from Liu et al.18. Please click here to view a larger version of this figure.
To ensure that this kind of as-cast Mn-Cu-based alloy possesses both superior damping capacity and excellent mechanical properties, it is necessary to ensure that the castings have a stable chemical composition, a high purity, and an excellent crystal structure. Therefore, strict quality control is necessary for the smelting, pouring, and heat treatment processes.
Firstly, it is necessary to choose the proper ingredients for the alloy. It should be considered that the added alloy elements can promote the decomposition of the γ-parent phase, which will help to produce more martensite micro-twins25. In addition, certain alloy elements also need to be considered to improve the mechanical and casting properties. The final alloy will then combine superior damping capacity and excellent mechanical properties.
Secondly, a reasonable melting process is necessary, which is connected to the casting characteristics of the alloy. The following key points should be considered in the melting process of cast Mn-Cu-based alloys: (1) Feed the metallic raw materials in the crucible in sequence by adding the high-melting-point alloy first and then adding the low-melting-point alloy, to prevent serious burning loss. (2) Adopt a vacuum melting method to ensure that the gas and the impurity contents in the alloy are low. At the same time, the inert gas is injected into the furnace to control the pressure and reduce the volatilization of the metal liquid during the vacuum melting. (3) When there are no more bubbles escaping from the surface of the molten metal, it enters the refining period. The purpose of the refining period is to remove any gas and volatile inclusions.
The more important step is the choice of heat treatment process. After obtaining the as-cast MnCuNiFeZnAl alloy with an excellent performance, a heat treatment process suitable for this alloy is also selected to further improve its damping capacity. Through analyzing the experimental results, it is found that the damping capacity can achieve its extreme value by a short-time aging treatment. The final heat treatment process for as-cast MnCuNiFeZnAl alloy is very simple and effective.
Finally, an optimization solution can be achieved for a new cast Mn-22.68Cu-1.89Ni-1.99Fe-1.70Zn-6.16Al (at%) alloy through investigating the effect of heat treatments on the damping capacity and service temperature. That is, the greatest degree of nano-Mn segregation can be achieved by an aging at 435 °C for 2 h, which results in Tt increasing, eventually significantly improve the damping capacity (Q-1 = 5.0 x 10-2) and the service temperature (70 °C), compared to the original as-cast alloy.
Although this method is just used for casting molding Mn-Cu-based high-damping alloy, it has the following advantages, such as cheaper modeling materials, a simpler mold manufacturing process, and higher damping capacity and mechanical property of products, etc. Besides, this method is suitable for different batches of production, for both small-batch production and mass production. Accordingly, this method is of great significance to improve the effect of vibration reduction, and it helps to widen the scope of its industry application. Because of the advantages of this method, it can replace forging technology to produce high-damping products in some areas.
The authors have nothing to disclose.
We give thanks to the financial support of the National Natural Science Foundation of China (11076109), the Hong Kong Scholars Program (XJ2014045, G-YZ67), the "1000 Talents Plan" of the Sichuan Province, the Talent Introduction Program of Sichuan University (YJ201410), and the Innovation and Creative Experiment Program of Sichuan University (20171060, 20170133).
|manganese||Daye Nonferrous Metals Group Holdings Co., Ltd.||DJMnB||produced by electrolysis|
|copper||Daye Nonferrous Metals Group Holdings Co., Ltd.||Cu-CATH-2||produced by electrolysis|
|Nickel||Daye Nonferrous Metals Group Holdings Co., Ltd.||Ni99.99||produced by electrolysis|
|Iron||Ningbo Jiasheng Metal Materials Co., Ltd.||YT01||industrial pure Fe|
|Zinc||Daye Nonferrous Metals Group Holdings Co., Ltd.||0#||produced by electrolysis|
|Aluminum||Daye Nonferrous Metals Group Holdings Co., Ltd.||Al99.90||produced by electrolysis|
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