(1) It is easily oxidized. In the air, aluminum is easily combined with oxidation to form a dense aluminum oxide film (thickness about 0.1-0.2μm), with a high melting point (about 2050 ° C), far exceeding the melting point of aluminum and aluminum alloy (about 600 ℃). The density of alumina is 3.95-4.10g / cm3, which is about 1.4 times that of aluminum. The surface of the alumina film is easy to absorb moisture. When welding, it hinders the fusion of the basic metal and easily forms pores, slag, Defects such as no fusion cause the weld performance to deteriorate.
(2) It is easy to produce stomata. The main cause of pores when welding aluminum and aluminum alloys is hydrogen. Since liquid aluminum can dissolve a large amount of hydrogen, and solid aluminum hardly dissolves hydrogen, when the temperature of the molten pool is rapidly cooled and solidified, the hydrogen does not have time to escape and is easy to weld. Gathered in the gap to form stomata. Hydrogen holes are currently difficult to avoid completely. There are many sources of hydrogen, such as hydrogen in the arc welding atmosphere, and moisture on the surface of the aluminum plate and welding wire. Practice has proved that even if the argon gas has a purity of 99.99% or more according to the GB / T4842 standard, when the moisture content reaches 20ppm, a large number of dense pores will appear. Stomata are apparent.
(3) There is a large tendency for weld deformation and crack formation. Aluminum's linear expansion coefficient and crystal shrinkage rate are about twice larger than steel, which is easy to produce large welding deformation of internal stress, the structure of greater rigidity will promote the occurrence of thermal cracks.
(4) The thermal conductivity of aluminum is large (pure aluminum 0.538 calories / Cm.s. ℃). It is about 4 times that of steel. Therefore, when welding aluminum and aluminum alloys, it consumes more heat than welding steel.
(5) Evaporation burning of alloying elements. Aluminum alloys contain low-boiling elements (such as magnesium, zinc, manganese, etc.), which are easily evaporated and burned under the action of high-temperature arc, thereby changing the chemical composition of the weld metal and reducing the weld performance.
(6) High temperature strength and low plasticity. At high temperatures, the strength and plasticity of aluminum is very low, which destroys the shape of the weld metal, and sometimes it is easy to cause the weld metal to collapse and weld through.
(7) No color change. When aluminum and aluminum alloys change from solid to liquid, there is no obvious color change, which makes it difficult for the operator to grasp the heating temperature.