Aluminum and aluminum alloy materials are low density, high strength, high thermal conductivity, and strong resistance to corrosion. They have excellent physical and mechanical properties, and are therefore widely used in structures’ welding for industrial products.For a long time, due to the improper selection of welding methods and welding technical parameters, the aluminum alloy parts were severely deformed due to stress concentration after welding, or due to defects such as pores, slag inclusions, and incomplete penetration of welding seams,it will result in raw material cracks or loosens, which has severely affected the quality and function of the aluminum welding products.
1.Aluminum alloy material characteristics
Aluminum is a silver-white light metal with excellent plasticity, high electrical conductivity and thermal conductivity, together with the ability to resist oxidation and corrosion.Aluminium oxide is very simple and then produce aluminum oxide film, which simply causes inclusions in the welding seam, and then damages the continuity and uniformity of the metal, reducing its mechanical function and resistance to corrosion.
2 Aluminum welding and welding of aluminum alloy difficulties
(1) Very simple oxidation.
In the air, aluminum is simply oxidized to produce a fine aluminum oxide film (thickness about 0.1-0.2μm) with a high melting point (about 2050℃) far beyond the melting point of aluminum and aluminum alloys (about 600℃). The density of alumina is 3.95-4.10g/cm3, which is about 1.4 times as that of aluminum. The appearance of alumina film is easy to absorb moisture.When aluminium welding is processed, it impedes the fusion of the basic metal. It is simple to appear defects such as blowholes, slag inclusions, and non-fusion, which leads to a reduction in the function of the aluminum welding seam.
(2) Prone to appear blowholes during aluminum welding.
The primary cause of blowholes in aluminium welding or welding of aluminium alloy is hydrogen, because liquid aluminum can dissolve many hydrogen, while solid aluminum hardly dissolves hydrogen, so when the bath temperature rapidly cools and condenses, hydrogen is too late to escape, and it is easy to gather in welding seam then become blowholes. Practice has proved that even if the argon gas meets the GB/T4842 standard, the purity reaches more than 99.99%, but when the moisture content reaches 20ppm, many blowholes will appear. When the relative humidity of the air exceeds 80%, air holes will appear in welding seam.
(3) The welding deformation and the tendency to form cracks is large.
The coefficient of linear expansion and the rate of crystal reduction of aluminum are approximately twice as large as those of steel, and the internal stress of larger welding deformation is apt to occur, and the structure with greater rigidity will promote the occurrence of hot cracks.
(4) The thermal conductivity of aluminum is large (pure aluminum 0.538 card/Cm.s.℃) about four times as that of steel, so aluminum welding and welding of aluminum alloys require more heat than steel welding.
(5) Transpiration loss of alloying elements.
Aluminum alloy contains low-boiling elements (such as magnesium, zinc, manganese, etc.). Under high-temperature arc, it is easy to evaporate and burn, and then changes the chemical composition of the welding metal, so that the weld function is reduced.
(6) High temperature strength and low plasticity.
Aluminum has low strength and plasticity at high temperatures, which damages the forming of the welding metal, and sometimes simply constitutes the collapse of the welding metal and the appearance of the welding perforation.
(7) No color change.
When aluminum and aluminum alloys change from a solid state to a liquid state, there is no noticeable color change, making it difficult for operators to grasp the heating temperature.
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