[China Aluminum Industry Network] 1. Overview
Mass production of low-pressure thin-walled vessels, most of the parts use ZL115 differential pressure casting blanks. Although the castings were examined by X-ray inspection, due to the limitations of structural features and the different size and distribution of the casting defects, some defects were not clearly visible on the X-ray film. They were often found after mechanical processing, and some had small diameter penetrations. Stomata can only be found after stress testing. This has seriously affected the quality of products and production schedules, and even caused huge economic losses. Some conventional welding repair methods do not meet the technical requirements well. In order to verify the effectiveness of this new process in solving the problems of sealability and appearance defects repair after cast aluminum parts were machined, a new way of repairing defects in cast aluminum parts was found. We conducted a large number of process tests and necessary tests. And form a set of correct procedures for the repair of defects in cast aluminum parts.
2. Mechanism of defect repair of metal surface repairing machine
Due to the strong chemical reactivity of aluminum and its alloys, the oxide film is easily formed on the surface, and most of them are refractory (for example, the melting point of Al2O3 is about 2050°C, the melting point of MgO is about 2500°C) and the thermal conductivity of aluminum and its alloys. Strong, easy to cause non-fusion phenomenon when welding. Since the specific gravity of the oxide film is extremely close to that of aluminum, it is also likely to become an inclusion in the weld metal. At the same time, the linear expansion coefficient of aluminum and its alloys is large and the thermal conductivity is strong, and warpage and deformation are easily generated during welding. This is a difficult problem when welding aluminum and its alloys. At present, the more commonly used argon arc welding in fusion welding is based on “cathode atomization†to crush the oxide film and protect the oxide film from being regenerated under the protection of argon gas. However, when the heat-treated aluminum alloy is heat treated by welding, the strength in the near-slit region is greatly weakened, and warpage is unavoidable. The metal surface repairing machine is mainly used for repairing casting defects. It has an inverter high-frequency + pulse power supply, a welding torch which can make the wire rotate at a high speed, and a control part. The mechanism of repairing defects is: using high-frequency + pulse voltage to break down the gas to form plasma gas, thus generating an electric spark with a temperature up to 6000° C., and the electric spark will melt the rotatable electrode (wire) instantly (10-5 -10-6 seconds) and the base metal with which it is in contact are melted at the same time, relying on the combination of the instantaneous high temperature and mechanical friction of the rotating wire and the parent material and the rotating electric field force to break the oxide film and protect it under the protection of argon gas. The film cannot be regenerated to complete the metallurgical bond between the wire and the base metal. Because of the short spark action time, the base metal that is in direct contact with the welding wire is partially melted. The heat conductivity of aluminum is very good, and the input heat is diffused and dissipated into the air instantly. The temperature of the substrate hardly rises, so the substrate will not be deformed. Cast aluminum parts are repaired after machining, without affecting the dimensional accuracy. After repair, the surface is repaired or machined. The appearance can be consistent with the substrate.
3, repair experimental materials:
ZL115 heat treatment state: Quenching + incomplete artificial aging wire: S311 standard aluminum silicon wire Sample form: test plate slotted, open hole with a cast aluminum defect repair machine for repair after the preparation of metallographic specimens.
4, metallographic organization and analysis
When heat-strengthening aluminum alloys are welded by argon arc welding, the heat-affected zone can be roughly divided into the following tissue areas: 1) semi-melting zone (area that is prone to overheating); 2) uneven solid-melt zone (solid Coarse grain region where the melt is partially decomposed; 3) softened annealing zone (over-aging zone). The condition of the alloy in the semi-melt zone has a great influence on the performance of the welded joint. The strength of the welded structure basically depends on the structural change in the semi-melt zone. If the over-firing phenomenon occurs in the semi-melting zone, it not only promotes the formation of hot cracks in this area, but also greatly reduces the strength and the plasticity of the joint. Heat-treated aluminum alloy joints tend to fracture along the semi-melt zone during static tensile tests. The strength of the alloy in the semi-melt zone is not more than 50-70? The heterogeneous solid-melt zone after the semi-melt zone is characterized in that the alloy elements have uneven concentration around the crystal grains, and the crystal grains are relatively coarse. In the softened annealing zone after the heterogeneous solidification zone, coarse grains and reticulated second phase inclusions having a relatively large thickness are easily generated. When repairing the defects of aluminum and its alloys, the metal surface repairing machine generates instantaneous ultra-high temperature due to the transient pulse current between the welding wire and the base metal, instantaneously melting the welding wire and the base metal, and it does not have continuous melting. In the pool, there is almost no temperature rise in the base metal, so there is no heat-affected zone. It has only one semi-melting zone in the near seam zone. Obviously, there is no overheating in the semi-melt zone, and there is no significant change in the metallographic structure of the base metal.
5, the actual workpiece casting defect repair:
A cast aluminum housing with a Ñ„ 540 and a wall thickness of 6 mm was found to leak when tested for 0.3 MPa airtightness. Drill 6mm deep 3mm holes in the defect to repair defects. Welding parameters: frequency 600HZ, duty cycle 45%, output amplitude 41%, speed 79%, the shell has made 10 times 0.3MPa airtight and 0.9MPa external pressure strength test, no leakage was found.
6, results and analysis
The results show that the new process is not like the general weld formation welding pool in the repair, it is to rely on electric spark to melt the rotating electrode, the wire, while the parent metal and the wire contact part of the local melting to form a molten metal metallurgical combination. The weld is a very fine dendritic cast structure, and the matrix is ​​also a cast structure. The difference between the two is that the crystal orientation is different. There is absolutely no weakening of the base metal in the heat affected zone. Due to the small temperature rise produced by the base metal, there is no over-firing phenomenon in the semi-melting zone, and the grain size is small. There is no uneven solid-melting zone and soft annealing zone appearing in the argon solitary welding, and there will be no thermal cracking and strength. The phenomenon of plasticity declining, due to this weld repair process does not exist molten pool, melted metal instantaneous melting, instant crystallization, time is very short (μms level), despite the instantaneous high temperature, will not cause the parent material to change the inch accuracy, However, because the deposited metal is a piece of a piece of melted, solidified piece of crystal, whether the film and film can be densely combined with the operator's work skills have a certain relationship.
7. Conclusion
Based on the above results and analysis, as well as the results of our previous tensile and hardness tests, it was proved that during the welding process, the parts did not increase in temperature or deformation, and the strength of the matrix weakened to a very small extent. It was in accordance with the relevant national standards and the welding point was dense. , No burn marks, fill material and the parent material macro consistent. We believe that the use of this welding repair method for cast aluminum defect repair is completely feasible, and we have good reason to define this repair method as a new type of welding method. This new type of welding method is extremely broad. Promote application prospects.
Black Wire Mesh,Black Metal Mesh,Black Wire Fence,Black Metal Wire Mesh
ANPING HONGYU WIREMESH CO.,LTD , https://www.hongyufence.com