Welding characteristics and methods of aluminum alloy tube

Aluminum alloy is widely used in various welding structure products because of its light weight, high strength, good corrosion resistance, non-magnetic, good formability, good low temperature performance and so on.

Using aluminum alloy instead of steel plate for welding can reduce the weight of the structure by more than 50%. Therefore, aluminum and aluminum alloys are widely used in aviation, aerospace, electrical and other fields, and more and more are also used in petrochemical industry.

The use of aluminum magnesium alloy (mainly: 5083, 5183, 5A02 equivalent to the old brand of LF2, LF4). However, aluminum and aluminum alloys are prone to oxidation, porosity, hot cracking, burn through and collapse during the welding process. This kind of material is recognized as materials of the difficult welding Welding, especially the welding of small diameter thin wall pipe is more difficult to master. Therefore, solving these welding defects of aluminum and aluminum alloy is a problem that must be solved in the construction process.

Physical and chemical properties and welding characteristics of aluminum and aluminum alloys

2.1 easily oxidized  

Aluminum has a strong affinity for oxygen. At room temperature, the aluminum surface can be oxidized into a dense AL2O3 film with a thickness of about 0.1~0.2 m. Although this layer of aluminum oxide film is relatively dense, it can prevent the continued oxidation of the metal, which is beneficial to the natural anti-corrosion, but it has brought difficulties to welding, because the melting point of aluminum oxide (2050 ℃) is far more than the melting point of aluminum (about 600 ℃), the proportion is about 1.4 times that of aluminum. In the welding process, it will hinder the fusion between metals, easy to form slag, and alumina film also adsorbed more water, welding will promote the formation of pores in the weld.

2.2 large thermal conductivity and specific heat capacity  

The thermal conductivity of aluminum is about four times that of steel. Therefore, when welding aluminum pipes, more heat is consumed than steel pipe welding. In order to obtain high-quality welded joints, energy-concentrated and high-power heat sources must be used.

    2.3 easy to form hydrogen hole   

The welding pores of aluminum and aluminum alloys are mainly hydrogen holes. Aluminum can absorb and dissolve a large amount of hydrogen in the liquid state, and its solubility is 0.0069 ml/g in the molten state and 0.00036 ml/g in the high temperature solidification state, with a difference of nearly 20 times. The thermal conductivity of aluminum is very large. Under the same welding process conditions, its cooling rate is 4 to 7 times that of steel, which accelerates the crystallization of metal. During the rapid cooling process of the welding molten pool, the solubility of hydrogen decreases sharply. At this time, a large amount of supersaturated gas is precipitated, and hydrogen is too late to precipitate in the weld metal to form pores. Therefore, when aluminum is welded, there is a great tendency for the weld seam to generate pores.

2.4 easy to form hot crack

The linear expansion coefficient and the crystallization shrinkage rate of aluminum are about double that of steel, which is easy to produce large welding deformation and stress. In addition, the adverse effects of some impurities or alloying elements will lead to cracks in rigid joints.

2.5 burn-through and collapse

When aluminum and aluminum alloys change from solid to liquid. Since there is no obvious color change, it is not easy to judge the temperature of the molten pool. When welding, often because the temperature is too high is not easy to be detected and lead to burn through or serious collapse.