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What are usual welding problems for 7075 aluminum strip?
- What are usual welding problems for 7075 aluminum strip?
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What are usual welding problems for 7075 aluminum strip?
KamorkasOa > 05-10-2018, 09:11 AM
Processing technology of 7075 aluminum strip has developed rapidly with continuous market demand. However, problems still arise in practical manufacturing process. Owing to the fact that aluminum is featured by strong oxidation property, low melting point, high heat conductivity, big coefficient of liner expansion and large latent heat of fusion, workers may meet some technical problems in the welding process
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The liner expansion coefficient of 7075 aluminum strip is twice that of carbon steel or low alloy steel. As a result, it has shrinkage, porosity, cracks and high internal stress during the welding process. This problem can be solved by adjusting welding stick composition and soldering process. Aluminum silicon sticks can be used for welding of alloys except 5000 series of aluminum alloys. When the silicon content surpasses 0.5%, 7075 aluminum strip has larger hot cracking tendency, which decreases with increase of silicon content. In accordance with our manufacturing experience, the best silicon content should be between 5% and 6%. If we choose aluminum silicon sticks with 4.5% to 6% silicon content, there is great possibility that the strip will not crack at all. Besides, 7075 aluminum strip gets oxidized easily and produces Al2O3 during the welding process. Al2O3, with high melting point and stable properties, proves hard to be removed. Therefore, the surface of the strip must be cleaned completely with chemical or mechanical methods before it’s welded. Alternating current power supply should be applied for argon tungsten-arc welding to clean Al2O3 by the negative pole. With gas welding there are special cleaning agents.
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RE: What are usual welding problems for 7075 aluminum strip?
askaluadmin > 05-10-2018, 09:16 AM
With the continuous development of aluminum profiles, aluminum processing technology has also been developing rapidly. However, because aluminum has many physical and chemical properties, such as strong oxidation, low melting point, fast heat conduction, large coefficient of linear expansion and high melting latent heat, the following common problems are easy to appear when selecting welding methods.
1, aluminum and aluminum alloys can dissolve large quantities of hydrogen in liquid, and hardly dissolve hydrogen in solid state.
In the process of solidification and rapid cooling of welding pool, hydrogen is not enough to overflow, and hydrogen pores are easy to form. Moisture content in arc column atmosphere, welding material and moisture absorbed on the surface of mother material are important sources of hydrogen in weld metal. Therefore, the source of hydrogen must be strictly controlled to prevent stomatal formation.
The 2 and 7075 aluminum strips are easy to oxidize in air and during welding. The alumina (Al2O3) produced is very high melting point, very stable, and is not easy to remove.
Before welding, chemical or mechanical methods should be used to strictly clean the surface and remove the oxide film on the surface. During argon tungsten arc welding, the AC power is selected, and the oxide film is removed by "cathode cleaning". During gas welding, the flux is used to remove the oxide film. When the thick plate is welded, the heat of welding can be increased, for example, the heat of the helium arc is large, the mixture of helium or argon helium is used to protect, or the large standard melting electrode gas shielded welding is used. The "cathode cleaning" can not be needed in the direct direct connection.
3. The thermal conductivity and specific heat capacity of aluminum and aluminum alloys are about two times higher than those of carbon steel and low alloy steel.
During the welding process, a large amount of heat can be quickly transmitted to the inner metal of the base metal. Therefore, when welding aluminum and aluminum alloy, the energy is consumed outside the molten metal pool, and more heat is unnecessary to be consumed in the other parts of the metal. The consumption of this useless energy is more significant than the welding of steel, in order to obtain high quality. For welding joints, energy sources with concentrated energy and high power should be used as far as possible.
The linear expansion coefficient of 4, 7075 aluminum strip and aluminum alloy is about two times of that of carbon steel and low alloy steel.
When the 7075 aluminum weld pool is solidified, shrinkage, shrinkage, hot crack and high internal stress are easily produced. In production, the measures of adjusting the composition of welding wire and welding technology can be used to prevent the hot crack. In the case of corrosion resistance, aluminum silicon alloy welding wire can be used to weld aluminum alloy except Al Mg alloy. When silicon content is 0.5%, the tendency of thermal cracking is larger. With the increase of silicon content, the crystallization temperature range becomes smaller, the fluidity increases significantly, the shrinkage rate decreases, and the thermal cracking tendency decreases accordingly. According to the production experience, when the silicon content is 5% to 6%, no hot cracking will occur, so the use of SAlSi (silicon content 4.5% ~ 6%) welding wire will have better crack resistance.