Aluminum alloy welding is difficult?
 

Aluminum alloy is one of the most widely used non-ferrous metal structural materials in the industry and has been widely used in aviation, aerospace, automotive, machinery manufacturing, marine and chemical industries. With the rapid development of the industrial economy, the demand for aluminum alloy welded structural parts is increasing, and the research on the weldability of aluminum alloys is also deepened. At present, aluminum alloy is the most widely used alloy, and the welding of aluminum alloy is also a very important part of the welding staff. Therefore, the welding and cutting alliance Xiaobian collects and popularizes the welding knowledge of aluminum alloy, and hopes to live up to everyone's expectations.

1

Type of aluminum alloy

There are two types of pure aluminum smelting products and pressure processed products. The former is represented by chemical composition Al, and the latter is represented by Chinese Pinyin LV (aluminum, industrial). Aluminum alloy can be divided into two major categories: deformed aluminum alloy and cast aluminum alloy according to the processing method:

The deformed aluminum alloy can withstand pressure processing. It can be processed into aluminum alloy materials of various shapes and specifications. Mainly used in the manufacture of aviation equipment, building doors and windows. The deformed aluminum alloy is further divided into a non-heat treatable reinforced aluminum alloy and a heat treatable reinforced aluminum alloy. The non-heat-treated reinforced type cannot be improved by heat treatment to improve the mechanical properties, and can only be strengthened by cold working deformation. It mainly includes high-purity aluminum, industrial high-purity aluminum, industrial pure aluminum, and rust-proof aluminum. Heat-treated reinforced aluminum alloy can improve mechanical properties by heat treatment such as quenching and aging. It can be divided into hard aluminum, wrought aluminum, super-hard aluminum and special aluminum alloy.

Cast aluminum alloy can be divided into aluminum-bismuth alloy, aluminum-copper alloy, aluminum-magnesium alloy, aluminum-zinc alloy and aluminum rare earth alloy according to chemical composition. Among them, aluminum-bismuth alloy has eutectic bismuth aluminum alloy and eutectic bismuth aluminum alloy. Crystalline aluminum alloy, cast aluminum alloy used in the as-cast state.

Deformed aluminum alloy series:

One series: 1000 series aluminum alloys represent 1050, 1060, 1100 series. The 1000 series is the most aluminum-containing series in all series. The purity can reach above 99.00%. Because it does not contain other technical elements, the production process is relatively simple and the price is relatively cheap. It is the most commonly used series in the conventional industry. Most of the circulation in the market is the 1050 and 1060 series. The 1000 series aluminum plates are based on the last two Arabic numerals to determine the minimum aluminum content of the series. For example, the last two Arabic numerals of the 1050 series are 50. According to the international brand nomenclature, the aluminum content must reach 99.5% to be qualified products. China's aluminum alloy technical standards (gB/T3880-2006) also clearly stipulates that the aluminum content of 1050 reaches 99.5%. The same reason, the aluminum content of the 1060 series aluminum plate must reach 99.6% or more.

Second series: 2000 series aluminum alloy stands for 2024, 2A16 (LY16), 2A02 (LY6). The 2000 series aluminum plates are characterized by high hardness, with the highest content of copper, about 3% to 5%. The 2000 series of aluminum rods are aerospace aluminum and are not commonly used in conventional industries.

Three series: 3000 series aluminum alloy is mainly composed of 3003 and 3A21. China's 3000 series aluminum sheet production process is relatively good. The 3000 series aluminum rods are mainly composed of manganese. The content is between 1.0% and 1.5%, which is a series with better anti-rust function.

Four series: 4000 series aluminum rods represent 4A01 4000 series aluminum sheets belong to the series with high enthalpy. Usually the cerium content is between 4.5 and 6.0%. It is a building material, mechanical parts forging materials, welding materials; low melting point, good corrosion resistance, product description: with heat and wear resistance.

Five series: 5000 series aluminum alloy represents 5052, 5005, 5083, 5A05 series. The 5000 series aluminum rods belong to the more commonly used alloy aluminum plate series. The main element is magnesium, and the magnesium content is between 3% and 5%. Also known as aluminum-magnesium alloy. The main features are low density, high tensile strength, high elongation and good fatigue strength, but they cannot be heat treated. The weight of aluminum-magnesium alloy is lower than other series under the same area. It is also widely used in conventional industries. In China, 5000 series aluminum plates belong to one of the more mature aluminum plate series.

Six series: 6000 series aluminum alloy represents 6061, mainly contains two elements of magnesium and antimony, so it concentrates on the advantages of 4000 series and 5000 series. 6061 is a cold-processed aluminum forged product, suitable for applications with high corrosive and oxidative requirements. . Good workability, easy coating and good processability.

Seven series: 7000 series aluminum alloy represents 7075, mainly containing zinc. Also belonging to the aviation series, it is an aluminum magnesium zinc copper alloy, which is a heat treatable alloy. It belongs to super hard aluminum alloy, has good wear resistance, and also has good weldability, but has poor corrosion resistance.

Eight series: 8000 series aluminum alloy is more commonly used for 8011. It belongs to other series. Most of the applications are aluminum foil. It is not very common for producing aluminum rods.

Nine series: 9000 series aluminum alloy is a spare alloy.

Cast aluminum alloy series:

Cast aluminum alloy (ZL) is divided into four categories according to the main elements other than aluminum in the composition, namely, copper, magnesium and zinc. The code numbers are 100, 200, 300 and 400 respectively.

High strength aluminum alloy:

High-strength aluminum alloy refers to aluminum alloy whose tensile strength is greater than 480MPa, mainly for anti-rust aluminum alloy, hard aluminum alloy, super-hard aluminum alloy, wrought aluminum alloy and aluminum lithium alloy in pressure processing aluminum alloy.

2

Aluminum alloy welding

The main welding processes for aluminum alloy materials are TIG, TIG, gas welding MSG, friction stir welding FSW, resistance spot welding, etc.

Aluminum alloy welding protection measures

(1) Chemical and mechanical methods are used to remove oxides from the groove and surrounding parts of the workpiece and the surface of the wire before welding. The order is chemical cleaning first, followed by mechanical grinding.

(2) Use a qualified protective gas for protection during the welding process.

(3) In the gas welding, the flux is used, and the oxide film on the surface of the molten pool is continuously scraped by the welding wire during the welding process.

Difficulties in welding

(1) Extremely easy to oxidize. In the air, aluminum is easily oxidized to form a dense aluminum oxide film (thickness of about 0.1 to 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 surface of the alumina film is easy to adsorb moisture. When welding, it hinders the fusion of the basic metal, and it is easy to form defects such as pores, slag inclusions, and unfusion. Causes a decrease in weld performance.

(2) It is easy to produce pores. The main cause of pores in the welding of aluminum and aluminum alloys is hydrogen. Since liquid aluminum can dissolve a large amount of hydrogen, solid aluminum hardly dissolves hydrogen. Therefore, when the temperature of the molten pool is rapidly cooled and solidified, hydrogen does not come out easily, and it is easy to weld. The pores gather to form pores. Hydrogen holes are difficult to avoid completely. There are many sources of hydrogen, such as hydrogen in an arc welding atmosphere, aluminum plates, and oxide film on the surface of the wire to absorb moisture in the air. Practice has proved that even if argon gas meets the requirements of GB/T4842, the purity is above 99.99%, but when the moisture content reaches 20ppm, a large number of tight pores will also appear. When the relative humidity of the air exceeds 80%, if no heating is taken When the measures are taken, the welds will have obvious pores. At the same time, the use of small current slow welding, increase the cooling time of the weld, and use the wire arc for the agitation of the molten pool, can better help the gas out of the molten pool.

(3) The deformation of the weld and the tendency to form cracks are large. The coefficient of linear expansion and the rate of crystal shrinkage of aluminum are about twice as large as that of steel, which tends to cause large internal stresses of welding deformation, and the structure with higher rigidity will promote the generation of hot cracks.

(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 consumes more heat than welded steel.

(5) Evaporation of alloying elements. Aluminum alloys contain low-boiling elements (such as magnesium, zinc, manganese, etc.), which are easily evaporated and burned under the action of high-temperature arc, thus changing the chemical composition of the weld metal and degrading the weld performance.

(6) Low temperature strength and low plasticity. The strength and plasticity of aluminum at low temperatures are low, which destroys the formation of weld metal and sometimes causes the weld metal to collapse and weld.

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

3

Defect repair

In the production process, aluminum alloys are prone to casting defects such as shrinkage cavities, blisters, pores and slag inclusions. How to repair defects such as blowholes in aluminum alloy castings? If it is repaired by equipment such as electric welding or argon welding, it is easy to cause side effects such as thermal deformation due to large heat release, and the repair welding requirements cannot be met.

The cold welding repairing machine utilizes the high-frequency electric spark instantaneous discharge and the non-thermal surfacing principle to repair the casting defects. Since the area affected by the cold welding heat is small, the substrate is not annealed and deformed, and no cracks, no hard spots, and hardening are generated. Moreover, the welding strength is high, and the re-solidification of the filler and the substrate after melting at the same time, the combination is firm, and the grinding, milling, boring and the like can be processed, and the compaction does not fall off. Cold welding repair machine is an ideal method to repair small defects such as aluminum alloy pores and sand holes.

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