Abstract aluminum and aluminum alloy material having a low density, high strength, high thermal conductivity, high corrosion resistance, good physical properties and mechanical properties, which are widely used in industrial products, welded structures. For a long time, since the selection of welding methods and welding parameters properly, often welding defects, the paper describes the selection process steps such metal parts welding and welding parameters.
Key words aluminum welding process
Aluminum and aluminum alloy material having a low density, high strength, high thermal conductivity, high corrosion resistance, good physical properties and mechanical properties, which are widely used in industrial products, welded structures. For a long time, since the selection of welding methods and welding parameters incorrectly, resulting in welding aluminum parts due to stress too concentrated a serious distortion, or because the weld pores, slag, incomplete penetration defects resulting weld metal cracks or material osteoporosis, a serious impact on product quality and performance.
1 aluminum alloy Features
Aluminum is a silvery-white light metal, with good ductility, high conductivity and thermal conductivity, and also has the ability to resist oxidation and corrosion. Easy to produce aluminum oxide aluminum oxide film in the weld prone to inclusions, thereby undermining the continuity and uniformity of metal, reducing its mechanical properties and corrosion resistance. Chemical composition and mechanical properties of common aluminum base metal and wire are shown in Table 1.

2 aluminum alloy welding difficulty
(1) easily oxidized. In the air, the aluminum oxide combined with easy to produce a dense aluminum oxide film (thickness of about 0.1-0.2μm), high melting point (about 2050 ℃), far more than the melting point of aluminum and aluminum alloys (about 600 ℃ or so). Alumina density 3.95-4.10g / cm3, aluminum is approximately 1.4 times the surface of the aluminum oxide film of easy absorption of moisture, welding, it prevents fusion of the base metal, easy to form pores, slag, No fusion defects, performance degradation caused by welding.
(2) easy to produce pores. Aluminum and aluminum alloys mainly pores generated during welding is hydrogen, since a large amount of liquid hydrogen can dissolve the aluminum, which solid aluminum hydroxide is almost insoluble, so that when the bath temperature rapid cooling and solidification, too late to escape hydrogen, easy welding cracks aggregate to form pores. Hydrogen is currently difficult to completely avoid the holes, many sources of hydrogen, there is an atmosphere of hydrogen electric arc welding, aluminum welding wire adsorbed moisture in the air and so on. Practice has proved that, even if the argon gas by GB / T4842 standards, purity of 99.99%, but when the moisture content of 20ppm, there will be a lot of dense pores, when the relative humidity exceeds 80%, the weld will a marked pores.
(3) Weld tendency of large deformation and crack formation. Coefficient of linear expansion and shrinkage of a crystalline aluminum than steel is about twice as large, easy to produce high welding stress deformation of the rigid structure will cause heat cracks.
Thermal conductivity (4) large aluminum (aluminum 0.538 card /Cm.s.℃). About four times stronger than steel, and therefore, when welding aluminum and aluminum alloys, welding steel than to consume more calories.
Evaporation of burning (5) alloying elements. Aluminum containing low boiling point elements (such as magnesium, zinc, manganese, etc.), at a high temperature arcing, burning easily evaporate, thereby changing the chemical composition of the weld metal, weld performance.
(6) high-temperature strength and low plasticity. High temperature strength and ductility of aluminum is very low, undermining the weld metal forming, sometimes also likely to cause collapse of the weld metal and weld wear phenomenon.
(7) No color change. Aluminum and aluminum alloy from the solid state into liquid, no obvious color change, so that the operator is difficult to grasp the heating temperature.

3 process for welding aluminum alloy material
(1) welding preparation
By chemical or mechanical methods, strict clean the surface oxide film on both sides of the weld groove.
Chemical cleaning is the use of alkali or acid to clean the surface, the method can remove the oxide film, but also in addition to oil, the specific process is as follows: the volume fraction of 6% to 10% sodium hydroxide solution, at about 70 ℃ soak 0. 5min → washing → volume fraction of 15% nitric acid soak at room temperature 1min neutralization treatment → water washing → washing → drying temperature. After washing the aluminum alloy surface is a matte silver-white.
Mechanical cleaning can be pneumatic or electric cutter, but also the use of scraper, rasp and other tools, the thin oxide film may also be of 0.25mm copper wire brush grinding remove oxide film.
Cleared immediately after welding, if you place more than 4h, should be re-cleaned.

(2) determine the assembly clearance and tack welding pitch
Welding process, aluminum thermal expansion, resulting in reducing the gap weld groove before welding assembly gap if left too small, the welding process will cause the two plates overlap groove, the increased weld plate surface roughness and the amount of deformation ; on the contrary, the assembly gap is too large, the welding difficult, and there is the possibility of burn. Suitable tacking spacing can ensure the desired positioning of welding gap, therefore, select the appropriate assembly and tack welding gap spacing is an effective measure to reduce distortion. According to experience, different thickness of the seams reasonable assembly process parameters shown in Table 2.
(3) Select the welding equipment
Currently on the market more types of welding products, under normal circumstances should AC GTAW (ie, TIG welding). A welding method it is under the protection of argon gas, the use of tungsten electrode and the workpiece ask arc generated hot melt base material and filler wire. When the welding work, due to the polarity of the alternating current is converted in periodic in each half wave cycle of DCSP, half-wave DC reverse. Tungsten may be transmitted during the positive half-wave electronic access sufficient but Buzhi Yu overheating conducive to the stability of the arc. Reverse of the oxide film formed on the surface of the workpiece during the first half-wave can easily be cleaned up and get bright and beautiful surface, forming a good weld.
(4) Select the wire
Generally used in aluminum welding wire 301 and 311 aluminum silicon wire.
(5) Select the welding methods and parameters
Generally left welding method, welding torch and workpiece angle of 60 °. Welding thickness 15mm or more to the right welding method, welding torch and workpiece a 90 ° angle.
When more than 3mm wall thickness welded, open V-shaped groove, an angle of 60 ° ~ 70 °, the gap should not be greater than 1mm, in order to complete the multi-layer welding. Wall thickness of 1.5mm or less, without groove, no gap, without filling wire. When welding fixed pipe butt joints, when the diameter is 200mm, wall thickness of 6mm, should be with a diameter of 3 ~ 4mm tungsten electrode to 220 ~ 240A welding current, 4mm diameter filler wire, with 1-2 layers Welding End.