Aluminum and aluminum alloy because of the unique physical and chemical properties, so the welding process will produce a series of difficulties, specifically, the following main points:

First, the strong oxidation

Strong affinity for aluminum and oxygen in the air combines with oxygen to generate highly dense and strong AL2O3 film thickness of about 0.1μm, high melting point 2050 ℃, far more than the melting point of aluminum and aluminum alloy, and the dense about 1.4 times of aluminum. In the welding process, the alumina film will hinder a good combination between metal and could easily lead slag. Oxide film will be adsorbed moisture, seam welding will lead to generate pores. These defects will reduce the performance of welded joints. In order to ensure the welding quality, rigorous cleaning before welding weldment surface oxides, and to prevent re-oxidation during the soldering process, the molten metal and the metal at high temperature for effective protection, which is aluminum and aluminum alloy welding is an important characteristics. Specific protection measures are: 1, before Welding Electrode by mechanical or chemical method to clear the workpiece and the groove and the wire around the part of the surface oxide;

2, the welding process to be used to protect a qualified protective gas;

3, in the welding, the use of flux, constantly in the process of welding wire prick the pool surface with the oxide film.

Second, the aluminum thermal conductivity and specific heat, thermal conductivity fast

Although the melting point of aluminum and aluminum alloy lower than steel, but aluminum and aluminum alloy thermal conductivity, heat capacity are large, more than twice as large as steel, the welding process a lot of heat is rapidly transmitted to the base metal inside, in order to obtain high-quality welded joints, must focus on energy, power heat source, and sometimes requires the use of preheating and other process measures, in order to achieve welding process.

Third, the linear expansion coefficient

Aluminum and aluminum alloy steel linear expansion coefficient of about 2 times the volume contraction during solidification rate of 6.5% -6.6%, so easy to produce welding deformation. Effective measures to prevent the deformation is in addition to choose reasonable parameters and the welding sequence, the use of appropriate welding tooling is also very important, especially when welding sheet metal. In addition, some aluminum and aluminum alloy welding, weld metal in the formation of crystals in the crack orientation and the formation of liquid in the heat affected zone are greater tendency to crack, often due to excessive internal stress in the brittle temperature range hot cracking. This is aluminum, especially when high-strength aluminum alloy welding one of the most common serious defects. In the actual welding site measures to prevent such cracks are mainly designed to improve joint, choose the right welding parameters and welding sequence, the base material used to adapt the characteristics of welding filler materials.

Fourth, easy to form pores

The pores in the welded joints of aluminum and aluminum alloy welding defects apt, especially aluminum and rust-proof aluminum welding. Hydrogen is generated when welding aluminum and aluminum alloy main vent, which has been proven by practice. Hydrogen sources, primarily the arc atmosphere, water, welding material and base metal adsorbed water, in which wire and base metal oxide film of adsorbed water to produce weld porosity, often occupies a prominent position.

Aluminum and aluminum alloy bath is easy to absorb liquid gases at high temperatures a large number of gases dissolved in the liquid solidifies, the solubility of a sharp decline in the solidification process of cooling after welding too late to precipitation, while the formation of pores in the weld together . In order to prevent the generation of pores in order to get a good solder joints, the source of hydrogen should be strictly controlled, before welding must be strictly limited by the use of welding materials (including wire, welding rod, flux, shielding gas) and water content before use for dried. After cleaning the base metal and wire welding is best completed in 2-3 hours, no more than 24 hours. TIG welding, the welding current with a large selection of high welding speed. MIG welding, use a large welding current slow welding speed, in order to enhance the presence of bath time. Al-Li alloy welding, to enhance the positive and back protection, with the groove scraper to remove the oxide film profile, which can effectively prevent the pores.

Fifth, welded joints is easy to soften

Welding of aluminum alloy can be heat strengthened, due to the impact of heat welding, heat affected zone in welded joints will be softened, that reduces the strength, the seam zone near the base metal parts of the deterioration of some mechanical properties. For work hardening of the alloy, too, so that joint performance weakened, and the greater the welding heat input, reduced performance of the program is also more serious. For such problems, mainly the development of measures to meet specific material welding process, such as restrictions on welding conditions, welding sequence appropriate to control the preheating temperature and interpass temperature, post weld heat treatment. For the welding of aluminum alloy softening can not be restored, preferably in a solid solution state annealing or welding, heat treatment after welding, after welding is not allowed if the heat treatment, the use of energy should be concentrated in a small heat input welding methods and welding, to reduce the joint strength decreased.

Sixth, evaporation and burning of alloying elements

Some aluminum alloys containing low boiling point elements that easily evaporate at high temperature burning, thus changing the chemical composition of weld metal, reducing the performance of welded joints. To compensate for these burning in the adjustment process, while often used with these elements in the boiling point higher than the base metal wire or other welding materials.

Seven, the strength of aluminum at high temperatures and low plasticity

Aluminum at 370 ℃, the strength of only 10Mpa, welding will live because they can not support the liquid metal leaving the poor weld, and even the formation of collapse or burn, in order to solve this problem, when welding aluminum and aluminum alloy plate to be used often.

Eight, the corrosion resistance of welded joints is lower than the base metal

Heat strengthened aluminum alloy (such as hard aluminum) to reduce the corrosion resistance of joints is obvious, the joint organization, the more uneven, the more easily reduced corrosion resistance. Weld metal also affects the purity or density of the corrosion resistance of joints. Impurities, coarse grains and the precipitation of brittle phases such as, corrosion resistance will be decreased, not only from the local surface corrosion and intergranular corrosion often, in addition to aluminum, the presence of welding stress corrosion resistance is also an important influence factors.

In order to improve the corrosion resistance of welded joints, mainly to take the following measures:

1, to improve the joint composition of non-uniformity. Mainly through the welding material to weld alloy, grain refinement and to prevent defects; while adjusting the Cast Iron Welding Rod process to reduce the heat-affected zone, and to prevent overheating, post weld heat treatment.

2, the elimination of welding stresses, such as local surface tensile stress using a local approach to eliminate hammer.

3, to take protective measures, such as taking anodized or coating.

Nine, no color change to the welding operation difficult

When welding aluminum and aluminum alloy from a solid into a liquid, there is no obvious color change, so in the welding process has brought many difficulties to the operator. Therefore, the required master welder welding the heating temperature, try to use flat welding, in the lead (quenching) arc board cited (put out) arc.