MENG Xiaomin, ZHAO Dong, MAJID Shaker,3
(1. Chongqing 2D Materials Institute, Chongqing 400701, China; 2. Chengdu Lianyu Precision Machinery Co., Ltd., Chengdu 611730, China; 3. Lehrstuhl Für Physikalische Chemie II, Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058, Erlangen, Germany)
Abstract: The relationship between the microstructure and the practical performance of two different copper-beryllium alloys including their lifetime has been investigated. Herein, two valves made of two different alloys with very similar compositions and the same heat treatment methods were investigated by various standard techniques including metallography, X-ray diffraction, chemical composition, microhardness, and thermal conductivity measurements. Although both alloys experienced the same heat-treatment processes, they revealed different thermal and mechanical properties due to the minor difference in their chemical composition.The alloy providing a longer lifetime (40% more) as the tip had a higher thermal conductivity of 280.3 W(m·K)-1 (about two times that of the other alloy). Regarding the metallography images and the measured thermal conductivity values of the alloys, the extended lifetime of the nozzle with the optimum performance is ascribed to its biphasic microstructure and the minor grain boundaries and interfacial thermal resistance. And important difference in the chemical composition was investigated in this study.
Key words: crystal structure; grain boundaries; metals and alloys; thermal properties; needle valve
Copper-beryllium alloy, which is widely used in aerospace[1], automotive[2], and electronics[3]due to its excellent fatigue resistance[4], high thermal conductivity[5], machinability[5], and excellent wear resistance[6]is a proper candidate for the fabrication of the needle valve of molds[7]. Nowadays, the mold industry has a great demand for copper-beryllium alloys, mainly due to their good casting properties,easy welding and repairing, absence of strength loss,rust resistance, facile maintenance, and stable thermal conductivity[8]. This type of alloy is an age-strengthened alloy, and its structure can be improved by optimizing the chemical composition and heat treatment, thereby improving the service performance of the part[9]. Thus,the processing history, microstructure, physicochemical properties, and composition of this category of alloys are of high significance in determining their operating performance in working conditions. To be specific,an optimized microstructure of an alloy can enhance its thermal conductivity. As a result, the alloy with a higher thermal conductivity as a needle tip possesses an extended lifetime.
Copper-beryllium alloys have been extensively reported for use in multiple fields[10]; however, they lack minute scientific investigation as needle valves.To date, the relationship between the microstructure of copper-beryllium alloys as high-end needle valve mold nozzle tips and their practical performance including lifetime has not been investigated. Hence, in this study,it is tried to investigate the effect of the alloy properties used in the needle valve on its performance in industrial applications. Herein, two different needle valves made of Protherm Cu-Be (PCB) and C17200 Cu-Be (CCB)alloys were utilized without further aging or thermal treatment. During the operation, it was found out that the operation cycle of these two alloys despite their same design, appearance, and chemical structure was significantly disparate. Aiming at this phenomenon, this article has analyzed and compared the needle valves to explore their differences and provide reliable ideas for the later improvement of this type of alloys.
2.1 Materials
The materials were provided by Chengdu Lianyu Precision Machinery Co., Ltd. Cu-Be needle valves were purchased from Protherm and Yixin. Distilled water, concentrated nitric acid, and other materials were also provided by local companies.
2.2 Heat treatment
The heat treatment methods of both alloys are the same in a furnace under air atmosphere and 10 ℃·min-1heating rate. The heat treatment started by annealing at 800 ℃ for 30 min to obtain solid solutions. Then, the samples were removed from the furnace to be watercooled followed by immediately placing them at 320 ℃for 4 h to be aged. Finally, they were naturally cooled down to room temperature.
2.3 Characterization
The chemical composition and crystallographic structure of the samples were analyzed with an inductively coupled plasma emission spectrometer (ICP,Varian 710-ES, USA) and an X-ray diffractometer(XRD of Shimadzu, Japan; the scan range is 30-90°, scan rate is 2°/min). The samples were sanded,polished, and etched in a 20% aqueous nitric acid solution at 25 ℃ to observe their metallographic structure employing a Zeiss optical microscope (Axio Scope A1). The thermal conductivity was tested by Light Flash Apparatus (NETZSCH Germany, LAF467)in the air atmosphere and the test temperatures are 30,100, 200, and 300 ℃. The equipment model used for evaluating microhardness was MHV2000, with 100 g loading force at 15 s.
Table 1 reveals the chemical composition of PCB and CCB alloys used in the needle tips (Fig.1(a)). The content of Be and Ni in PCB alloy and CCB alloy were quite close. The weight ratio of Be was 0.352% and 1.975%, and the content of Co was 0.038% and 0.05%in PCB and CCB alloys, respectively. The largest elemental difference of additives was related to Ni.PCB had 1.6% more than CCB (0.06 wt%). Overall,the added elements to the needle tip in PCB and CCB alloy were 2.0% and 2.095 wt%.
Fig.1 (a) Nozzle-valve needle and Be-Cu alloy tip; (b) XRD patterns of PCB and CCB alloys; (c) magnified XRD curves
Table 1 Chemical composition of the needle valve tips/wt%
The digital image of a needle valve is shown in Fig.1(a). Fig.1(b) and (c) demonstrate the phase structure of the two samples obtained by XRD. PCB and CCB alloys both containα-Cu phase (43.5°, 50.4°,and 74.0°), while only PCB has a minor amount of Be-Ni phase (Fig.1(c))[11]. Meanwhile, a slight sign of aγphase is observable only in CCB, as shown in Fig.1(c)(small peak at 47.6°).
The metallographic structure of PCB and CCB alloys are exhibited in Fig.2(a)-2(d). PCB alloy showed a biphasic structure composed ofα-Cu and BeNi phase(Fig.2(a)), whereas CCB consisted ofα-Cu (Fig.2(c))and obvious grain boundaries. The magnifications of the metallography microscope images provide a much clear observation of phase distribution. Fig.2(b)demonstrates that in PCB, BeNi phase is uniformly distributed in theα-Cu substrate. On the other hand, CCB alloy is mainly made up ofα-Cu with distinguishable phase boundaries as shown in Fig.2(d),and the distribution of grain boundary distributes in the network. In addition, theγphase is mainly situated at the coarse grain boundaries[12].
Fig.2 Metallographic morphology of (a-b) PCB, and (c-d) CCB alloys (the samples were etched in concentrated nitric acid
Beryllium copper alloy is a typical agestrengthened alloy, and the solubility of Be in Cu at room temperature is only 0.2%[13]. In the CCB alloy,the Be content is 1.975%, which is a high content for beryllium copper alloys. In the aging stage of the heat treatment process, a minor amount ofγphase was formed and mainly distributed in the grain boundaries.In addition, because of the high content of Be in CCB alloy, the holding time of solution treatment was not sufficient, thereby, under these conditions, forming coarse grain boundaries is unavoidable[12]. The Be content of PCB alloy is only 0.352%, while the Ni content is as high as 1.6%. These two elements promote the precipitation of BeNi phase in the aging process of the alloy, which is continuously distributed in theα-Cu matrix.
The Vickers hardness value of PCB was significantly lower than that of CCB, and their average values were 227.88 and 355.33, respectively (Table 2). The coarse reticular grain boundary structure and the precipitation of the hardened phase,γ, make the hardness of CCB alloy significantly higher than that of the PCB alloy.
Table 2 Thermal conductivity of PCB and CCB alloys at different temperatures/(W/(m·k))
Fig.3 a and Table 2 display the thermal conductivity values of the samples measured at multiple temperatures. The thermal conductivity of PCB alloy on average was 2.28 times higher than that of the CCB alloy. The thermal conductivity of the two samples increased slightly and linearly with the rise of temperature. As shown in Table 2, the thermal conductivity of PCB alloy was the lowest at 30 ℃,242.7 W (m·K)-1; as the temperature raised to 300 ℃,the thermal conductivity value also enhanced up to 280.3 W (m·K)-1. Moreover, Fig.3(b) demonstrates the schematic relationship between the thermal resistance and thermal conductivity of the probing alloys.
Fig.3 (a) Thermal conductivity of PCB and CCB at different temperatures; (b) Schematic relationship between the material’s resistance and thermal conductivity
The factors affecting the thermal conductivity of metal materials include the distribution of alloying elements, crystal defects, and the second phase[14,15].The lattice distortion caused by the solid solution alloying elements has a greater scattering effect on free electrons in the alloy than the scattering effect of the second relative electron[16]. During heat treatment, at the solid solution temperature (800 ℃), more Be elements dissolve into the copper matrix, resulting in stronger lattice distortion. In the process of rapid cooling to room temperature, the dissolved Be will not completely precipitate, and the lattice distortion formed during the solid solution process also exists, and it is more severe in CCB alloy than in PCB because Be content in CCB is much higher than that of the PCB[14,15]. In addition,the microstructure of CCB alloy is mainly composed of coarse grains and a connected network with grain boundaries, and a small amount of strengthening γ phase distributed at grain boundaries, which increase the interfacial thermal resistance of heat flow during the transfer process. However, the structure of PCB alloys is quite different, mainly consisting of a softα-Cu matrix and a more continuously distributed BeNi phase.Besides, for PCB alloys, the precipitation of BeNi phase can effectively reduce the lattice distortion of the Cu matrix, making the thermal conductivity of the alloy closer to that of the Cu matrix, thanks to the optimum content of Be. Therefore, the thermal conductivity of PCB alloy is better than that of CCB alloy[17,18].
Although the heat treatment process of both PCB and CCB needles were the same, they exhibited dissimilar phase structure, metallographic morphology,and thermal conductivity. These differences are ascribed to the difference in their chemical compositions, which leads to the extended lifetime of the PCB alloy under working conditions.
The thermal conductivity of the metal samples is majorly affected by the solid solution alloys and their content as well as their interfacial thermal resistance.The greater solid solubility results in the more serious lattice distortion of the solid solution and the extended grain boundaries strengthen the electron scattering effect, thereby, leading to lower thermal conductivity.Since thermal conductivity is a key factor in determining the lifetime of the tip, the tip with a higher thermal conductivity also had a 40% prolonged lifetime.
Conflict of interest
All authors declare that there are no competing interests.
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