Strong Oxidation Capacity
The affinity between aluminum and oxygen is very strong. It is easy to combine with oxygen in the air to form a dense and solid Al2O3 film with a thickness of about 0.1 μ m. The melting point is as high as 2050 ℃, far exceeding the melting point of aluminum and aluminum alloys, and the density is very large, about 1.4 times that of aluminum. In the welding process, alumina film will hinder the good bonding between metals and easily cause slag inclusion. The oxide film will also absorb water, which will promote the formation of pores in the weld during welding. These defects will reduce the performance of welded joints. In order to ensure the welding quality, the oxide on the surface of the weldment must be strictly cleaned before welding, and reoxidation must be prevented during the welding process, so as to effectively protect the molten metal and the metal at high temperature, which is an important feature of aluminum and aluminum alloy welding. The specific protection measures are:- The method of removing the oxide on the surface of the groove or around the workpiece by chemical welding wire;
- Qualified shielding gas shall be used for protection during welding;
- In gas welding, flux is used, and the oxide film on the surface of the molten pool is continuously broken with welding wire during the welding process.
Aluminum Has High Thermal Conductivity And Specific Heat And Fast Thermal Conductivity
Although the melting point of aluminum and aluminum alloys is much lower than that of steel, the thermal conductivity and specific heat capacity of aluminum and aluminum alloys are large, more than twice that of steel. During the welding process, a large amount of heat energy is rapidly transmitted to the interior of the base metal. In order to obtain high-quality welding joints, heat sources with concentrated energy and high power must be used, and sometimes preheating and other process measures need to be adopted to realize the fusion welding process.Large Coefficient Of Linear Expansion
The linear expansion coefficient of aluminum and aluminum alloy is about twice that of steel, and the volume shrinkage during solidification is 6.5% – 6.6%, so it is easy to produce welding deformation. The effective measure to prevent deformation is not only to select reasonable process parameters and welding sequence, but also to adopt appropriate welding tooling, especially when welding thin plates. In addition, when some aluminum and aluminum alloys are welded, they tend to form crystalline cracks in the weld metal and liquefied cracks in the heat affected zone. Thermal cracks often occur in the brittle temperature range due to excessive internal stress. This is one of the most common serious defects in the welding of aluminum alloys, especially high-strength aluminum alloys. In the actual welding site, the measures to prevent such cracks are mainly to improve the joint design, select reasonable welding process parameters and welding sequence, and adopt welding filler materials suitable for the characteristics of base metal.Easy To Form Pores
Pores in welded joints are very easy defects in the welding of aluminum and aluminum alloys, especially in the welding of pure aluminum and antirust aluminum. Hydrogen is the main cause of porosity in aluminum and aluminum alloy welding, which has been proved by practice. The source of hydrogen is mainly the water in the arc column atmosphere and the water adsorbed by the welding material and base metal. The water adsorbed by the oxide film on the surface of welding wire and base metal often occupies a prominent position due to the generation of weld pores. The liquid molten pool of aluminum and aluminum alloy is easy to absorb gas. A large amount of gas dissolved at high temperature decreases sharply when solidified by liquid. It is too late to precipitate in the process of cooling and solidification after welding, and accumulates in the weld to form pores. In order to prevent the generation of pores and obtain good welded joints, the source of hydrogen shall be strictly controlled. The water content of the welding materials used (including welding wire, welding rod, flux and shielding gas) must be strictly limited before welding, and drying treatment shall be carried out before use. The cleaned base metal and welding wire should be welded within 2-3 hours, no more than 24 hours at most. During TIG welding, large welding current and high welding speed shall be selected. During MIG welding, large welding current and slow welding speed shall be selected to improve the existence time of molten pool. When welding Al Li alloy, strengthening the front and back protection, cooperating with groove scraping and removing oxide film can effectively prevent pores.Welded Joints Soften Easily
When welding heat treatable strengthened aluminum alloy, due to the influence of welding heat, the heat affected zone in the welded joint will soften, that is, the strength will be reduced, which will deteriorate some mechanical properties of the base metal near the seam. The same is true for cold work hardened alloys, which weakens the performance of the joint, and the greater the welding line energy is, the more serious the performance reduction procedure is. In view of such problems, the measures taken are mainly to formulate the welding process conforming to specific materials, such as limiting welding conditions, adopting appropriate welding sequence, controlling preheating temperature and interlayer temperature, post weld heat treatment, etc. For aluminum alloy whose post weld softening cannot be recovered, it is best to adopt annealing or welding in solid solution state, and then carry out heat treatment after welding. If post weld heat treatment is not allowed, energy concentrated welding method and small line energy welding shall be adopted to reduce the reduction of joint strength.Evaporation And Burning Loss Of Alloying Elements
Some aluminum alloys contain alloying elements with low boiling point, which are easy to evaporate and burn at high temperature, which changes the chemical composition of weld metal and reduces the performance of welded joints. In order to make up for these burning losses, welding wires or other welding materials containing these boiling point elements higher than the base metal are often used while adjusting the process.Aluminum Has Low Strength And Plasticity At High Temperature
The strength of aluminum at 370 ℃ is only 10MPa. When welding, the weld will be poorly formed or even collapse or burn through due to the inability to support the liquid metal. In order to solve this problem, backing plates are often used when welding aluminum and aluminum alloys.The Corrosion Resistance Of Welded Joint Is Lower Than That Of Base Metal
The corrosion resistance of heat-treated strengthened aluminum alloy (such as duralumin) joints decreases obviously. The more uneven the joint structure is, the easier the corrosion resistance is to be reduced. The purity or compactness of weld metal also affects the corrosion resistance of the joint. With more impurities, coarse grains and precipitation of brittle phases, the corrosion resistance will be significantly reduced, resulting in not only local surface corrosion but also intergranular corrosion. In addition, for aluminum alloy, the existence of welding stress is also an important factor affecting the corrosion resistance. In order to improve the corrosion resistance of welded joints, the following measures are mainly taken:- Improve the non-uniformity of joint structure and composition. It is mainly through the welding material to alloying the weld, refine the grain and prevent defects; At the same time, adjust the welding process to reduce the heat affected zone and prevent overheating and post weld heat treatment.
- Eliminate welding stress, such as local surface tensile stress, which can be eliminated by local hammering.
- Take protective measures, such as anodic oxidation treatment or coating.
No Color Change, Which Brings Difficulties To Welding Operation
When aluminum and aluminum alloy are welded from solid state to liquid state, there is no obvious color change, so it brings many difficulties to the operator in the welding process. Therefore, welders are required to master the heating temperature during welding, use flat welding as far as possible, and start (extinguish) arc on the arc striking (extinguishing) plateBe-cu aluminum machining china prototype company service include : aluminum cnc turning, aluminum milling service, aluminum extrusions, 5 axis machining aluminum, swiss machining aluminum, aluminum guide,aluminum products