1 Aluminum and aluminum alloy pipe welding characteristics and methods

Aluminum alloy due to light weight, high strength, good corrosion resistance, non-magnetic, good formability and low temperature performance and other characteristics, is widely used in a variety of welding structure products, the use of aluminum alloy instead of steel plate material welding, structural weight can be reduced by more than 50%. Therefore, aluminum and aluminum alloys are widely used in aviation, aerospace and electrical fields, but also more and more used in the petrochemical industry. Puyang Zhongyuan Dahua new air separation equipment uses a large number of aluminum magnesium alloy (mainly: 5083, 5183, 5A02 is equivalent to the old brand LF2, LF4).

However, aluminum and aluminum alloys are prone to oxidation, porosity, hot cracking, burning through and collapse during welding. This kind of material is recognized as a more difficult welding material to be welded, especially the welding of small thin wall pipes is more difficult to master. Therefore, solving these welding defects of aluminum and aluminum alloys is a problem that must be solved during the construction process.

2. Physical and chemical properties and welding characteristics of aluminum and aluminum alloys

2.1 Easy oxidation

The affinity between aluminum and oxygen is very strong, at room temperature, the aluminum surface can be oxidized into a dense AL2O3 film with a thickness of about 0.1-0.2m. Although this layer of alumina film is relatively dense, can prevent the continued oxidation of the metal, which is beneficial to natural corrosion protection, but it brings difficulties to welding, which is because the melting point of alumina (2050 ° C) far exceeds the melting point of aluminum (600 ° C), and the specific gravity is about 1.4 times that of aluminum. In the welding process, it will hinder the fusion between the metals, easy to form slag inclusion, and the alumina film also adsorbs more water, and the welding will promote the formation of pores in the weld.

2.2 Large thermal conductivity and specific heat capacity

The thermal conductivity of aluminum is about four times that of steel, therefore, when welding aluminum pipes, more heat is consumed than steel pipe welding, in order to obtain high-quality welded joints, energy concentration and power must be used.

2.3 Easy to form hydrogen pores

The welding pores of aluminum and aluminum alloys are mainly hydrogen pores. Aluminum can absorb and dissolve hydrogen in a large amount in the liquid state, the solubility in the molten state is 0.0069ml/g, and in the high temperature solidification state is 0.00036ml/g, the difference is nearly 20 times. The thermal conductivity of aluminum is very large, under the same welding process conditions, its cooling rate is 4-7 times that of steel, so that the metal crystallization is accelerated, the solubility of hydrogen in the welding pool in the rapid cooling process, the solubility of hydrogen drops sharply, at this time, a large number of supersaturated gas is precipitated, and hydrogen is too late to precipitate in the weld metal to form pores. Therefore, when welding aluminum, the weld has a great tendency to produce pores.

2.4 Easy to form hot cracks

The linear expansion coefficient and crystallization shrinkage rate of aluminum are about twice that of steel, which is easy to produce large welding deformation and stress, coupled with the adverse effects of certain impurities or alloying elements, which will lead to cracks in rigid joints.

2.5 Burn through and collapse

When aluminum and aluminum alloys are transformed from solid to liquid, it is difficult to judge the temperature of the molten pool because there is no obvious color change. When welding, the temperature is often too high to be detected and lead to burn through or serious collapse.

3 Preparation before welding

3.1 Groove processing adopts mechanical processing method

The surface of the groove after processing should be smooth, free of burrs and flash edges. The form of the groove is generally V-shaped, without blunt edge, and the groove Angle is 70° ~ 75°. Butt welding with different wall thicknesses should have a transition section of 14°.

3.2 Preparation before welding

Before welding, the oil and oxide film within 30 ~ 50mm range of the welding wire, welded pipe groove and the inside and outside of the groove are removed, the removal sequence and methods are as follows: Use acetone or carbon tetrachloride and other organic solvents to remove the surface oil, and the removal range on both sides of the groove should not be less than 50mm. After removing the oil, the welding wire is used by chemical method, the groove is easy to use mechanical method, and the test tube is also used by chemical method to remove the surface oxide film.

The mechanical method is that the surface of the groove and its vicinity can be filed, scraped, milling or cleared with a stainless steel wire brush of about 0.2mm to expose the metal, the clearance range on both sides should be not less than 30mm from the edge of the groove, and the tool used for regular degreasing treatment.

The chemical method is to soak NaOH solution at about 70℃ 5 ~ 10% for 30 ~ 60 seconds, or soak NaOH solution at room temperature 5 ~ 10% for 3 minutes. Then soak in about 15% HNO3 (room temperature) for about 2 minutes and wash with warm water. Or rinse with cold water and allow to dry completely. The welding wire that has been reliably treated and has not been oxidized or contaminated can be used directly without the above treatment.

The cleaned groove and welding wire should not be stained again before welding, if there is no effective protective measures, welding should be performed within 8 hours, otherwise it should be cleaned again. When the pipeline group is aligned, the inner wall should be flush without burrs and particles, and the amount of the wrong side should meet b≤0.5mm. There is no liner welding seam inside, and the gap is required to be equal to zero as much as possible, especially in the upside-down welding part, and the inner wall of the pipe should be rounded by 1 to 1.5mm.

3.3 Considerations of the welder and others

The welding machine must be AC TIG welding machine, with steep drop external characteristics and sufficient capacitance, and has stable parameters, flexible adjustment and safe and reliable performance, should also have arc, arc stability and eliminate DC component device, the current on the welding machine, voltmeter should be qualified by the measurement department. Before the welding machine is used, check whether the ground is intact, whether the cooling water and air road are smooth, and its various functions must ensure that they can work normally. Welding sites should be kept clean, in addition to wind, rain and snow facilities, should also ensure that the relative humidity of welding ≤80%, environmental temperature > 5℃.

4 Welding process

4.1 Selection of welding materials

In principle, the welding wire is selected with the same composition of aluminum and aluminum alloy welding wire or strip, argon purity > 99.95%, as far as possible to choose large diameter welding wire. In the arc welding of Al-Mg aluminum alloy, it is usually recommended to use CB-AMr2, CB-AMr3, CB-AMr6, CB-AMr61, CB-AMr63, 1557, 1577 electrodes, and 01201 and 01217 welding electrodes for Al-Cu aluminum alloy.

4.2 Group bonding spot welding

Because the aluminum and aluminum alloy tubes conduct heat quickly and the molten pool crystallizes quickly, the group does not leave gaps and blunt edges, and should be avoided forcibly to reduce the residual stress after welding, and the length of the positioning weld is 10-15mm. The positioning welding position is at 7, 9 and 12 points of the pipe, and the positioning welding weld is often reserved as a formal weld, so problems should be dealt with in time.

Before welding, remove the black powder and oxide film on the positioning welding surface, and shape the two ends into a gentle slope. The welds do not need to be preheated. Test welding on the test plate before welding, and then proceed to formal welding after confirming that there is no porosity. Using high frequency arc starting, the arc point should cross the center line about 20mm, and stay still for about 2-3 seconds, and then in the case of ensuring welding penetration, the use of high current, rapid welding, the wire does not swing, the end of the wire should not leave the argon gas protection area. If leaving the argon protection zone, the end of the welding wire should be cut off, the Angle between the welding wire and the weld surface should be about 15°, and the Angle between the welding gun and the weld surface should be maintained between 80° and 90°.

In order to increase the argon protection area and enhance the protection effect, the large-diameter welding torch porcelain nozzle can be used to increase the argon flow rate of the welding torch. When there is a splash on the nozzle that significantly obstructs the flow of argon gas, the splash must be removed or the nozzle must be replaced. When the extreme part of tungsten pollution, irregular shape and other phenomena, must be trimmed or replaced. Tungsten pole should not be extended out of the nozzle, the control of welding temperature is mainly the control of welding speed and welding current.

The test results show that the high current and fast welding can effectively prevent the formation of pores, which is mainly due to the fast welding speed in the welding process, the melting metal is heated for a short time, and the chance of gas absorption is less. When the arc is closed, pay attention to filling the arc pit, reducing the solution pool, avoiding shrinkage holes, and the junction of the end point should be welded over 20~30mm. After arc stop, delay gas stop for 6 seconds, when the rotating aluminum and aluminum alloy pipe butt welding, the welding torch should be in a slightly up-slope welding position, which is conducive to welding through. When the bottom of the thick wall pipe is welded, it is not necessary to add welding wire, but the welding layer needs to add welding wire.

5 Welding Inspection

Surface and radiographic inspection of all welds is carried out according to HGJ222-92 "Technical Specification for Welding of Aluminum and aluminum Alloys".

6 Implementation Effect

Using the above welding technology, the problems of porosity and burning through in actual welding construction have been effectively solved, and the pass rate of welding inspection has reached 97%. Of course, there is also the problem of back molding, which mainly depends on the operator's feeling, and the technical requirements for the operator are high.