Research on the application of new technology for

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Research on the application of a new process for repairing defects in aluminum alloys

[Abstract]: This paper proves that the new process is very effective in repairing local defects in cast aluminum alloy or machining by analyzing the metallographic structure of ZL115 samples repaired with a polymetallic defect repair machine and the external pressure strength test of the actual workpiece repaired. The metal structure at the repair site is dense, and the repaired Angang Steel is the world's largest hydraulic support. The prominent feature is that it will not cause workpiece deformation in the production process of high-strength plates

key words: aluminum alloy defect repair

Description: in another tensile experiment, we have concluded that the tensile strength of repairing defects with cast aluminum defect repair machine meets the requirements of relevant national standards, and some data and theories involved in this experiment will not be repeated

1. Experimental purpose

most of the parts of low-pressure thin-walled containers produced in batch use ZL115 differential pressure casting blanks. Although the castings have been tested by X-ray flaw detection, due to the limitations of structural characteristics and the different sensitive directions of the size and distribution of casting defects, some defects cannot be clearly shown on the X-ray film. They are often found after machining. Some penetrating pores with small diameter can only be found after the pressure test, which is expected to reach 24million vehicles this year. This has seriously affected product quality and production progress, and even caused huge economic losses. Some conventional welding repair methods can not meet the technical requirements well. In order to verify the effectiveness of this new process in solving the sealing and appearance defect repair of cast aluminum parts after machining, and to find a new way to repair the defects of cast aluminum parts, we have carried out a large number of process tests and necessary inspections, and expect to form a set of correct process procedures for defect repair of cast aluminum parts after the completion of the test, which can be used to guide the specific operation of defect repair

2. Defect repair mechanism of polymetallic repair machine

due to the strong chemical activity of aluminum and its alloys, it is very easy to form oxide film on the surface, and most of them are refractory (for example, the melting point of Al2O3 is about 2050 ℃, and the melting point of MgO is about 2500 ℃). In addition, aluminum and its alloys have strong thermal conductivity, which is easy to cause non fusion during welding. Because the specific gravity of oxide film is very close to that of aluminum, it is also easy to become an inclusion of weld metal. At the same time, aluminum and its alloys have large coefficient of linear expansion and strong thermal conductivity, which are prone to warpage and deformation during welding. This is a difficult problem when welding aluminum and its alloys. At present, the most commonly used argon arc welding in fusion welding relies on "cathode atomization" to break the oxide film, which cannot be regenerated under the protection of argon. However, when welding the aluminum alloy after heat treatment and strengthening treatment, the strength of the near seam area is greatly weakened, and warping deformation will inevitably occur

multi metal defect repair machine is mainly used to repair casting defects. It consists of inverter high-frequency + pulse power supply, welding gun that can make welding wire rotate at high speed and control part. The mechanism of repairing defects is: using high frequency + pulse voltage to break down the gas to form plasma gas, so as to produce an electric spark with a temperature of more than 6000 ℃. The electric spark will melt the fusible rotating electrode (i.e. welding wire) and the base metal in contact with it at the same time in an instant (-s). Relying on the combined action of the instantaneous high temperature, the mechanical friction between the rotating welding wire and the base metal and the rotating electric field force, the oxide film will be broken. Under the protection of argon, So that the oxide film can not be regenerated, so as to complete the metallurgical bonding between the welding wire and the base metal. Due to the short action time of electric spark, the base metal in direct contact with the welding wire is locally melted, and the aluminum has good thermal conductivity. The input heat is instantly diffused and lost to the air. The temperature rise of the matrix is almost not generated, so the matrix will not be deformed. The defects of precision aluminum castings are repaired after machining, without affecting the dimensional accuracy. After the repaired surface is filed, polished or machined, the appearance can be consistent with the substrate

3. Test plate making

test plate material: zl115

heat treatment state: quenching + incomplete artificial aging

welding wire: s311 standard aluminum silicon welding wire

sample form: test plate slotting, first opening and then repairing with cast aluminum defect repair machine, and then preparing metallographic samples

zl115 cast aluminum alloy and s311 welding wire chemical composition are shown in Table 1. Table 1 chemical composition of ZL115 and s311

brand code main elements (%) sicumgznmnsbfetialzalsi5zn1mgzl1154 2 0.. 651.. 8 0.. 25 allowance salsi-1s3114 00.300.050.100.05 0.800.20 allowance

4. Metallographic structure and analysis:

when argon arc welding is used to weld heat-treated strengthened aluminum alloy, the welding heat affected zone can be roughly divided into the following structural areas: 1) semi molten area (area prone to overburning); 2) Uneven solid melt area (coarse grain area of local decomposition of solid melt); 3) Softening annealing zone (over aged zone). The condition of the alloy in the semi melting zone has the greatest influence on the properties of the welded joint. The strength of welded structure basically depends on the microstructure change of semi melting zone. If overburning occurs in the semi melting zone, it will not only promote the formation of thermal cracks in this part, but also greatly reduce the strength and plasticity of the joint. In static tension test, the fracture of heat-treated aluminum alloy joints often occurs along the semi melting zone. The strength of the alloy in the semi melting zone shall not be greater than% of the strength of the alloy before welding

the uneven solid melt zone after the semi melting zone is characterized by uneven concentration of alloy elements around the grains, and the grains are relatively coarse

in the softening annealing zone after the uneven solid melt zone, it is easy to produce coarse grains and thick second phase inclusions

when repairing the defects of aluminum and its alloys, because the instantaneous pulse current is added between the welding wire and the base metal, the instantaneous ultra-high temperature is generated, which makes the welding wire and the base metal melt and crystallize instantaneously. It has no continuous molten pool, and the base metal has almost no temperature rise, so there is no heat affected zone. There is only one semi melting zone near the crack. Obviously, there will be no overburning in the semi melting zone, and there is no obvious change in the metallographic structure of the base metal. Figure 1 shows the microstructure of ZL115 base metal. Matrix is α (AL) solid melt, on which the strengthening phase silicon (SI) phase is distributed. Figure 3, figure 4 and figure 5 are the microstructures taken from the semi melting zone. It can be seen that the grain size in the semi melting zone has not changed significantly. We can see from Figure 3 that the silicon (SI) phase of the base metal has appeared in the weld. It can be seen from Figure 4 that the grain of the weld and the base metal grows continuously, that is, the dendritic crystal of the weld grows along the grain boundary of the base metal melting and perpendicular to the grain boundary of the base metal. Figure 5 shows that the weld metal and the base metal penetrate each other, and the dendritic crystals of the weld begin to grow from the silicon phase of the base metal. From Figure 3, figure 4 and figure 5, we can clearly see that the width of the semi melting zone is very small, only across grains. The grain size in the semi melting zone is not different from the base metal, and there is no coarseness. The scene of interactive crystallization is vividly displayed. At the same time, we should also note that the dendritic crystals of the weld metal are very fine and uniform. These phenomena are very consistent with the thermal cycle characteristics of the metal during the repair process

5. Casting defect repair test of actual workpiece:

there is one ф 540. The cast aluminum shell with a wall thickness of 6mm was found to have air leakage during the 0.3MPa air tightness test. Drill at the defective part ф 6mm deep 3mm hole, defect repair. Welding repair parameters: frequency 600Hz, duty cycle 45%, output amplitude 41%, speed 79%. The shell was tested for 0.3mpa

air tightness and 0.9 MPa external pressure strength for 10 times, and no leakage was found

6. Test results and analysis

the test results show that the new process is not like the general fusion welding to form a molten pool when repairing. It relies on the electric spark to melt the rotating electrode, that is, the welding wire, and at the same time, the contact part between the base metal and the welding wire produces local melting to form the metallurgical combination of molten metal. The weld is a very fine dendritic casting structure, and the matrix is also a casting structure. The difference between the two is that the crystallization direction is different. There is no weakening of the strength of the base metal by the heat affected zone. Because the temperature rise of the base metal is very small, the semi melting zone has no overburning phenomenon, the grains are small, there is no uneven solid melting zone and softening annealing zone in argon arc welding, and there will be no hot cracks and the phenomenon of strength and plasticity decline. Because this welding repair process does not have a molten pool, the molten metal melts and crystallizes instantaneously, and the time is very short (MS level). Although there is an instantaneous high temperature, it will not cause changes in the accuracy of the base metal, However, since the deposited metal is melted piece by piece, crystallized and solidified piece by piece, the maximum size shall not be greater than mm. Whether the dense combination between pieces is related to the operator's operation skills

7. Conclusion

according to the above experimental results and analysis as well as the data results of our previous tensile and hardness experiments, it is proved that during the welding repair process, the workpiece does not rise in temperature, does not deform, and the matrix strength is weakened slightly, which meets the relevant national standards. The metal at the welding repair point is dense, without burning marks, and the repair material is macroscopic consistent with the base metal. We believe that it is completely feasible to use this welding repair method to repair the defects of cast aluminum. We have sufficient reasons to define this repair method as a new type of fusion welding method. This new type of fusion welding method has an extremely broad application prospect

about the author: Zhang Lingui (1944.8 -) male, Han nationality, Henan Province, is now a senior engineer and chief technologist of Zhengzhou 713 Research Institute. He graduated from Xi'an Jiaotong University in 1969 and has been engaged in professional technical research for 35 years. He has won a ministerial second-class achievement award, a provincial excellent achievement award, a national service invention patent, and more than 20 non patented process inventions and innovations, He has published 12 academic papers in national first-class academic journals and more than 20 papers for provincial, ministerial and national academic exchanges. Its research results have created huge economic and social benefits. He was recorded in the dictionary of contemporary Chinese inventors. (end)

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