In the power system, a large amount of conductive material needs to be consumed. The terminal of many devices is a copper material, and the overhead wire is mostly aluminum. Therefore, the wire made of aluminum material is widely used. If the two are directly connected, the contact resistance may be large. When the equipment is operated for a long time, overloaded or short-circuited, the connection will rapidly heat up and the heat will be transferred to the power equipment. SEO, such as a transformer, will cause burned contacts, phase-to-phase operation and blackouts, and will result in burning of equipment and equipment. Explosions, fires, etc. Therefore, copper-aluminum transition joints welded by special processes are generally used in power systems. In spite of this, the current level of technology makes it impossible for the weld to be perfect. As long as there is a small gap, it may be invaded by air, moisture, etc., oxidation occurs, the contact resistance increases, and the operation process generates heat to further increase the surface of oxidation. The expansion of copper-aluminum transition joints resulted in a decrease in strength and fracture, as shown in Figure 1. In addition, the increase of the contact resistance will also lead to the reduction of the short circuit current of the circuit, prolong the operation time of the short circuit protection device, or hinder the action of the short circuit protection device, greatly threatening the safety of the power supply system. In actual operation and maintenance, the breakage of copper-aluminum transition joints is very common, especially in heavily polluted areas, such as some coastal cities in the Pearl River Delta, and there are 4-5 similar repairs every month in large quantities. How to prevent the oxidation of copper-aluminum transitional joints and SEO fracture is an urgent problem to be solved.
1 fracture analysis of copper-aluminum transition joints
In order to find the cause of the fracture of the copper-aluminum transition joint from the source, the motor shell must conduct a comprehensive analysis of the causes of the fracture and summarize it in the following categories.
1.1 Welding Process
In copper-aluminum welding, the potential difference between aluminum and copper electrodes is large and the welding temperature is too high to easily cause corrosion of aluminum. The brittle materials produced by aluminum and copper interdiffusion during welding make the performance of joints unstable and reduce the joint strength and corrosion resistance. These adverse effects will directly lead to the shortened service life of copper-aluminum transition joints and accelerate the progress of the fracture.
1.2 Chemical reactions
When the contact surface of copper and aluminum metal and the moisture in the air, carbon dioxide and other impurities are easily formed under the electrolyte, thus forming a battery with aluminum as the anode, copper as the cathode, so that the aluminum produces electrochemical corrosion, resulting in copper The contact resistance of the aluminum connection increases, resulting in thermal oxidation. When a certain degree is reached, the weld surface breaks.
1.3 coefficient of expansion
The thermal expansion coefficient of copper and aluminum differ greatly. The coefficient of thermal expansion of aluminum is about 36% greater than that of copper. When the transition joint heats up, the copper material is squeezed, but it cannot be fully recovered after cooling. In this way, after a long period of time, Google’s rankings are subject to long-term changes in temperature and temperature (such as power and power outages, large loads and small loads, alternating hot and cold weather), and tend to create large clearances at the contact surfaces. Affecting the contact area, resulting in poor contact and thus increased contact resistance. At the same time, due to loose contact, the joints enter the air due to loose contact, resulting in oxidation of the aluminum wires to form aluminum oxide. Although the oxide layer of aluminum oxide is very thin, its resistance value is high, the contact resistance at the joint is greatly increased, the connection site is prone to heat, the oxidation is intensified, and the strength of the joint is reduced.
1.4 Influence of External Force The external force influence of copper-aluminum transition joint mainly comes from two aspects, one is the force during construction and the other is the traction of the wire in daily operation.
1.4.1 Construction Force
At present, when the copper-aluminum transition joint is actually constructed, it is connected by crimping the aluminum wire at the end of the aluminum material, and the copper material end is bolted directly to the terminal post. If the copper material is fixed first and then the aluminum wire is crimped during construction, the huge external force generated during the crimping process can easily make the weld surface of the copper-aluminum joint appear void. Although this tiny gap may not be visible to the naked eye, The normal operation of the copper-aluminum transitional joints lays a serious hidden danger.
2 The improved analysis of copper-aluminum transition joints found that if the mixer is not applied to the connection sequence of copper-aluminum transition joints or the construction is not standardized during the construction process of applying copper-aluminum transition joints, the joint surface of copper-aluminum transition joints can be easily damaged. , causing security risks, in the long run, the hidden danger will continue to enlarge, forming a vicious cycle, eventually leading to the fracture of copper and aluminum joints. To this end, the construction process should be strictly regulated. Specific operations should be done strictly in the following aspects: • Select the appropriate type of copper-aluminum transition joints; • Measure the resistance of the copper-aluminum transition joints to ensure that the resistance values ​​meet the standards; • Place the aluminum end into aluminum wire and use crimping pliers Crimping. The copper-aluminum transition joints are suspended during crimping and cannot impact on the surrounding objects to avoid the influence of external forces. • After confirming that the aluminum end is crimped, the copper end of the copper-aluminum transition joint is inserted into the terminal and fastened. During the bolting process, the copper-aluminum transition joints are fixed by hand to prevent the copper-aluminum transition joint from being stressed by rotation.
Optical Microscopy
In optical microscopy different filters are used to improve contrast and emphasize specific features based on material properties. This can be achieved with magnifications typically ranging from 2.5 times up to 1,000 times. In materialography, reflected light is the most commonly used type of light optical microscopy. Transmitted optical microscopy is also used, but mainly for mineralogy specimens.
Stereo Optical Microscopy
The stereo microscope is an optical microscope variant, designed for low magnification observation of a specimen, using the light reflected from the specimen surface.
Scanning Electron Microscopy
A scanning electron microscope (SEM) is a type of electron microscope that produces images of a specimen by scanning the specimen surface with a focused beam of electrons. The electrons interact with the atoms in the specimen, producing various signals that can be translated into information about the surface topography and the composition of the specimen.
Transmission Electron Microscopy
Transmission electron microscopy (TEM) uses a beam of electrons transmitted through an ultra-thin specimen and that interacts with the specimen as it passes through it. Generated signals can be translated into various types of information, including information on the type and orientation of individual crystals.
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