Difficulties in welding aluminum alloy materials

Aluminum and aluminum alloys are widely used in the welding structure of industrial products because of their low density, high strength, high thermal conductivity, strong corrosion resistance, outstanding physical and mechanical properties. For a long time, due to the improper selection of welding methods and welding technical parameters, the aluminum alloy parts are severely deformed due to excessive stress accumulation after welding, or due to defects such as weld porosity, slag inclusion, and incomplete penetration, resulting in weld metal cracks or The loose raw materials have severely affected the quality and function of the products.

1. Characteristics of aluminum alloy materials

Aluminum is a silvery white light metal with outstanding plasticity, high electrical conductivity and thermal conductivity, together with resistance to oxidation and corrosion. Aluminum is very simple to oxidize to form a film of Al2O3, which simply causes inclusions in the weld and then damages the continuity and uniformity of the metal, reducing its mechanical function and corrosion resistance. The chemical composition and mechanical function of common aluminum alloy base materials and welding wire.

2. Welding difficulties of aluminum alloy materials

(1) Very simple oxidation. In air, aluminum is simply oxidized to form a fine aluminum oxide film (thickness of about 0.1-0.2 μm) with a high melting point (about 2050 ° C), far exceeding the melting point of aluminum and aluminum alloy (about 600 ° C). . The density of alumina is 3.95-4.10g/cm3, which is about 1.4 times that of aluminum. The appearance of alumina film is easy to adsorb moisture. When welding, it prevents the fusion of essential metals, and it is very simple to form shortcomings such as pores, slag inclusion and unfusion. Causes the weld function to decrease.

(2) It is prone to stomata. The primary cause of pores in the welding of aluminum and aluminum alloys is hydrogen, because liquid aluminum can dissolve a lot of hydrogen, while solid aluminum hardly dissolves hydrogen, so when the temperature of the molten pool is rapidly cooled and condensed, hydrogen does not come out, simply The collection in the weld constitutes a pore. Hydrogen holes are currently difficult to avoid completely, and hydrogen has a lot of origins, such as hydrogen in an arc welding atmosphere, aluminum plates, and the appearance of welding wire to absorb moisture in the air. Practice has proved that even if argon gas meets the requirements of GB/T4842, the purity reaches 99.99% or more, but when the moisture content reaches 20ppm, it will also show many fine pores. When the relative humidity of air exceeds 80%, the weld will be significant. Presenting stomata.

(3) The deformation of the weld and the tendency to form cracks are large. The coefficient of linear expansion and the rate of crystallization of aluminum are about twice as large as that of steel, and it is prone to large internal stresses of welding deformation. For structures with higher rigidity, the occurrence of thermal cracking will be promoted.

(4) The thermal conductivity of aluminum is large (pure aluminum 0.538 cal/cm.s. °C). It is about 4 times that of steel. Therefore, when welding aluminum and aluminum alloy, it takes more heat than welding steel.

(5) Burning loss of transpiration of alloying elements. Aluminum alloys contain low-boiling elements (such as magnesium, zinc, manganese, etc.). Under the high-temperature arc effect, they are simply transpiration and burned, and then the chemical composition of the weld metal is changed to reduce the weld function.

(6) Low temperature strength and low plasticity. The strength and ductility of aluminum at low temperatures are low, damaging the formation of weld metal, and sometimes simply forming the appearance of weld metal collapse and weld penetration.

(7) No color change. When aluminum and aluminum alloys are changed from solid to liquid, there is no significant color change, making it difficult for the operator to grasp the heating temperature.

3. Technical methods for welding aluminum alloy materials

(1) Pre-weld preparation

Use chemical or mechanical methods to strictly align the outer oxide film on both sides of the weld bevel.

Chemical cleaning is to clean the surface of the workpiece with alkali or acid. The method can remove the oxide film and remove the oil. The specific technical process is as follows: the sodium hydroxide solution with a volume fraction of 6% to 10% is immersed at 70 ° C for 0.5 min. → Washing → Nitric acid with a volume fraction of 15% is immersed for 1 min at room temperature for neutralization treatment → water washing → warm water washing → boring. The washed aluminum alloy has a matt silver white appearance.

Mechanical finishing can be done with pneumatic or electric milling cutters. Scrapers, trowels, etc. can also be used. For thinner oxide films, a 0.25mm copper wire brush can also be used to remove the oxide film.

Immediately after finishing the welding, if the placement time exceeds 4h, it should be sorted from the beginning.

(2) Determine the installation clearance and the positioning welding distance

During the welding process, the aluminum plate is heated and expanded, causing the gap of the weld bevel to be reduced. If the gap is installed before welding, if it is too small, the groove of the two plates will be stacked during the welding process, and the unevenness and deformation of the plate surface after welding will be added. On the contrary, if the installation clearance is too large, the welding is difficult and there is a burn-through. The proper positioning welding distance can ensure the required positioning welding gap. Therefore, selecting a suitable installation clearance and positioning welding distance is a useful method to reduce the deformation.

(3) Select welding equipment

At present, there are many varieties of welding products on the market. Generally, it is preferred to use TIG welding (ie TIG welding). It is a welding method for melting the base metal and filling the wire by the arc heat generated by the tungsten electrode and the workpiece under the maintenance of argon gas. When the welder is working, because the polarity of the communication current is periodically changed, the half wave is positively connected in DC and the half wave is reversed in DC in each cycle. The positive half-wave time tungsten can emit the satisfied electrons without overheating, which is conducive to the stability of the arc. The reverse half-wave time of the oxide film produced by the surface of the workpiece is simply sorted out to obtain a weld with excellent appearance and excellent appearance.

(4) Select the welding wire

Usually 301 pure aluminum welding wire and 311 aluminum silicon welding wire are used.

(5) Select welding methods and parameters

Usually done by left welding, the torch and the workpiece are at an angle of 60°. When the welding thickness is 15 mm or more, it is performed by the right welding method, and the torch and the workpiece are at an angle of 90°.

When the welding wall thickness is more than 3mm, the V-shaped groove is opened, the angle is 60°~70°, the gap is not more than 1mm, and the multi-layer welding is completed. When the wall thickness is less than 1.5 mm, the groove is not opened, no gap is left, and no filler wire is added. When welding the pipe butt joint, when the pipe diameter is 200mm and the wall thickness is 6mm, the tungsten electrode with a diameter of 3~4mm should be used, with a welding current of 220~240A, and a filler wire with a diameter of 4mm, with 1~2 layers. Soldered.