ABSTRACT: A company uses T U 1 oxygen-free copper rod to produce electric vacuum device. The surface of the copper rod bubbles during the packaging process. The causes of blistering were analyzed by means of low magnification test, metallographic examination, chemical composition analysis, sem and eds. The results showed that the tail of t u 1 Oxygen Free Copper Rod was not completely cut off during extrusion, and there were defects of subcutaneous shrinkage. Keywords: T U 1 Oxygen Free Copper Rod; Electric Vacuum Device; surface blistering; subcutaneous tail shrinking; TG-379 SYMBOL NUMBER: B article number: 1001-4012(2017)02-0144-03 electric vacuum devices are mainly used in the fields of high frequency and ultra-high frequency transmitting tubes, high voltage and ultra-high voltage switches, waveguides, magnetron, etc. , the reason for strict control of oxygen content in materials is that electric vacuum devices need to be heated and sealed in hydrogen, and the presence of oxygen will lead to the occurrence of hydrogen disease, which will lead to the destruction of devices in high vacuum environment, therefore, high purity oxygen-free copper must be used as raw material for electric vacuum copper. T U 1 oxygen-free Copper Rod was used as raw material in the manufacture of electric vacuum devices. It was found that the surface of the workpiece blistered during the packaging process and the product was scrapped. For a long time, oxygen-free copper and other red copper in the production process of foaming phenomenon has been puzzling the raw material manufacturers. In this paper, the possible causes of surface blistering of oxygen-free copper parts are discussed through comprehensive physical and chemical examination and analysis, and the improvement measures are given.

1. Physical and chemical examination 

1.1 low power examination

The surface morphology of the t-u 1 oxygen-free copper electro-vacuum device is shown in Fig. 1. The Arrow in Fig. 1 is the bubble. After turning the transverse face of the workpiece, it is corroded by 30% nitric acid solution, which can be observed by the naked eye

There are subcutaneous defects near the surface of the bar, and no other low-power defects are found. As shown in Fig. 2, Fig. 3 shows an enlarged appearance of the defects indicated by the Arrow in Fig. 2.

1.2 metallographic examination samples are taken along the transverse and Longitudinal sections of the workpiece. The specimens are prepared by metallographic examination and then corroded and observed under metallographic microscope. The microstructure of the workpiece is single-phase recrystallized, and there is no second phase between crystals, the microstructure is normal. Subcutaneous tailing defects can be observed on both transverse and Longitudinal sections of the workpiece, as shown in figures 4 and 5. The defects indicated by Arrow 1 in figure and the outer surface of the workpiece by Arrow 2 in figure.

The samples were annealed in hydrogen gas at the defective and non-defective parts respectively, and the grades were evaluated according to the metallographic examination method for oxygen-free copper content (Y s/t 335 x 2009) . After annealing with hydrogen, the defects cracked seriously, and the oxygen content grade was 5, as shown in Fig. 6. After annealing with hydrogen, the defects did not crack and the oxygen content grade was 1, as shown in Fig. 7.

1.3 chemical composition analysis the samples of the workpieces were analyzed by chemical composition analysis, in which the oxygen content was determined by the American likardine oxygen analyzer and the other elements were all analyzed by the American thermoelectric 4460 Direct Reading Spectrometer. The Analytical Results of the contents of the elements and the chemical composition of the oxygen-free copper as specified in Gb/t 523(1)2012《 grade and chemical composition of processed copper and copper alloys 》[3] are shown in Table 1. It can be seen that the content of all impurity elements in the workpiece is lower than the upper limit of the standard, especially the oxygen content is much lower than the standard.

1.4 The scanning electron microscope (SEM) image of the bottom of the work piece after the blister was opened is shown in Fig. 8. The surface of the work piece has obvious attachment. The attachment is analyzed by energy spectrum (EDS) , the results show that the attachment is mainly composed of oxygen, copper, aluminum and other elements, as shown in Fig. 9.

２. It is analyzed and discussed that the workpiece material is t u 1 oxygen Free Copper Bar. After the original bar is machined by vacuum high temperature brazing, there are many bubbles on the surface, the surface is smooth and full, and the surface is slightly elongated along the processing direction (Fig. 1) . After the low-power corrosion of the end surface of the workpiece, the microstructure is fine, and there is a discontinuous ring-shaped subcuticular shrinkage defect near the surface of the bar. A crack has been opened locally and formed (Fig. 2) . The outer side of the crack bulges outwards to form a bubble (Fig. 3) , no other low power defects were found. Shrinkage tail is a special defect on the end of extruded products. According to its position on the products, it can be divided into primary shrinkage tail, secondary tail and subcutaneous shrinkage tail. HYPODERMIC shrinkage tail is located on or near the surface of the product, the serious can form a surface layer. During the later stage of extrusion, the metal near the dead zone of the product is fractured due to the great shear stress caused by the violent slip, at which time the dirt such as oxide film and Lubricant on the surface of the remaining billet flows into the product and covers the surface or near the surface, so as to form hypodermic coiling defect [4]. The oxide inclusion was observed in the microstructure of the transverse section of the workpiece. The bubble section was partially opened and there was a gap in the middle of the crack (Fig. 4) Most of the defects in the Longitudinal section of the workpiece are a discontinuous black strip along the axial direction. There are also some cracks (Fig. 5) , and the outward bulge is also obvious. When the sample with hypodermic tail was heated to 840 °C and annealed at 20 min in hydrogen atmosphere, the grain boundary of the inclusion defect was cracked seriously (Fig. 6) . The oxygen content of the sample with hypodermic tail was graded 5 by Metallography, but the defect-free region was not cracked (Fig. 7) , the oxygen content grade obtained by metallographic examination is grade 1. Sem and eds analysis of the bottom of the bubble show that there is an attachment on the bottom of the bubble and the attachment is oxide inclusion (Fig. 8 and Fig. 9) . The results of chemical composition analysis showed that the contents of each component element met the standard technical requirements, and the oxygen content of raw materials was within the normal range. The results of low power examination showed that there were no other defects except subcutaneous tail-shrinking, and the structure of low power was normal. The microstructure of the workpiece is single-phase recrystallization, there is no second phase between crystals, and the microstructure is normal. It can be seen that, because there is a discontinuous subcutaneous shrinkage defect near the surface of the workpiece, the oxide inclusion in the subcutaneous shrinkage tail contains a large amount of oxygen element, which reacts with hydrogen to Form Water Vapor in high temperature package in hydrogen atmosphere, when the pressure exceeds the strength of the outer metal layer of the workpiece, the outer metal layer will be deformed and bulge out to form bubbles on the surface of the workpiece.

3. Conclusions and recommendations 

(1) because of the discontinuous subcutaneous shrinkage defect near the surface of the workpiece, the oxide inclusions in the subcutaneous shrinkage tail contain a large amount of Cuprous oxide or cupric oxide, it is the main reason that causes the blistering on the surface of the workpiece when it is heated in hydrogen.

 (2) the measures to reduce or avoid the tail shrinkage are: cutting off the tail, selecting reasonable extrusion temperature and rate, and improving the die to provide the metal uniform flow condition in the extrusion, reducing the turbulence flow can make the tail shrinkage greatly reduced; In addition, the use of peeling extrusion can also be reduced, light shrink tail, improve the quality of extrusion products.

 Source: Chinanews.com, by Li Xianghai