Comprehensive And Detailed Explanation Of Welding Procedures For Aluminum And Aluminum Alloys

ONE. Welding materials:

1). The purity of argon gas for welding is ≥99.99%, the dew point is ≤-55℃, and it should meet the requirements of GB/T4842 or GB10624. It should not be used when the pressure of bottled argon is less than or equal to 0.5Mpa. (The nitrogen content in the argon gas is greater than or equal to 0.04%, otherwise light yellow or grass green magnesium nitride and pores will be produced on the surface of the weld; the oxygen content is greater than or equal to 0.03%, otherwise dense black spots and arcs may be found on the surface of the molten pool. Unstable and large splash; water content ≥ 0.07%, the molten pool will boil and produce pores in the weld).

2). Manual tungsten argon arc welding electrode adopts cerium tungsten electrode. The diameter of the electrode should be selected according to the size of the welding current (usually a tungsten electrode that is one size larger than the specification required by the welding current), and the end of the electrode should be hemispherical (the method of making the hemisphere: use a size larger than the specification required by the welding current) For the No. 1 tungsten electrode, grind the end into a tapered shape, hold the electrode vertically, use a current 20~30A larger than the current required by the tungsten electrode to start an arc on the test plate and maintain it for a few seconds, the tungsten electrode tip is Hemispherical. If the tungsten electrode is polluted by aluminum, it must be re-polished or replaced; when it is slightly polluted, the current can be increased to make the arc burn on the test plate for a while, which can burn off the pollutants):

3). When using MIG to weld aluminum alloys, since the aluminum welding wire is relatively soft, in order to avoid biting the welding wire, the wire feeding wheel with gears is not allowed, and the push wire type should not be used; the wire feeding hose is not allowed to use a spring tube instead. Use polytetrafluoroethylene or nylon, otherwise the hose will be polluted or blocked due to grinding. MIG usually uses DC reverse polarity.

4). The main function of the flux is to remove the oxide film and other impurities. When using, it can be made into a paste with anhydrous alcohol or directly put the flux powder on the groove and both sides. When welding fillet welds, those fluxes that are easy to remove slag after welding should be selected; fluxes for aluminum-magnesium alloys should not contain sodium components.

5). When welding different grades of aluminum materials, when the drawings and processes are not specified, the welding wire material should be selected according to the base metal with better corrosion resistance and lower strength level. When welding corrosion-resistant aluminum alloys such as aluminum-magnesium alloys or aluminum-manganese alloys, it is advisable to use welding wires with magnesium content or manganese content similar to or slightly higher than the base metal. The welding wire can be selected from GB/T10585 "Aluminum and Aluminum Alloy Welding Wire", and can also be selected from GB/T3190 "Chemical Composition of Deformed Aluminum and Aluminum Alloy" and GB/T3197 "Aluminum and Aluminum Alloy Wire for Welding Rod"

Table 1 Welding wire for aluminum welding of the same grade

Table 2 Welding wire for dissimilar aluminum welding wire

Table 3 Selection of welding wire for different material and performance requirements

Note: ER aluminum welding wire is American standard ANSI/AWS A5.10

TWO. Welding Welders:

They should pass the examination in accordance with the provisions of Appendix A of the "Rules for the Examination of Welders of Aluminum Containers".

THREE. Preparation before welding:

1). Mechanical methods (including shearing) should be used in the processing of aluminum grooves. If the plasma flame method is used, no less than 3mm should be removed from the discolored part by mechanical processing. The surface of the grooves after processing should be flat and smooth, without cracks, splits Layers, inclusions, burrs, flash, and oxide colors. The surface of the groove should have a silvery-white metallic luster; if necessary, 100% PT should be carried out on the groove and within a range of not less than 50 mm on both sides.

2). The surface of the welding wire, groove and its sides within a range of not less than 50 mm must be cleaned (including removing the surface oxide film, scale, pollution and unqualified oxide color). Grinding can be done with φ0.15~0.2 stainless steel wire brush, metal grinding head (ie electric milling cutter), manual disc milling machine, file (should be a special arc-shaped file for aluminum), scraper and acetone (white cloth dipped in acetone should be clean, do not Use cotton cloth or cotton yarn to avoid lint when wiping), but it should be noted that these tools should be cleaned before use, and care should be taken not to press the oxide film into the base material during cleaning, so do not use too much force when cleaning ; However, it is not allowed to use a grinding wheel or ordinary sandpaper for grinding, because the aluminum material is very soft, resulting in the sand particles remaining in the aluminum material, and defects such as pores and slag inclusions are likely to occur after welding.

3). For outsourced hot-processed parts, such as heads, etc., in principle, 100% PT should be performed on the surface of the aluminum material after entering the factory, and if necessary, RT should be performed on the undetermined parts

4). The surface of the welding wire can be scrubbed with a stainless steel wire brush or clean oil sandpaper; the welding wire with thick oxide skin on the surface needs chemical cleaning after grinding before welding. Chemical cleaning: soak in 70℃, 5%~10% NaOH solution for about 0.5~3min, then rinse with clean water, then soak with about 15% HNO3 solution at room temperature for about 1~2min, rinse with warm water, and then use a hand-held A hair dryer (can not use an air compressor, because there is water and oil in the air) dry it and then put it in an oven to dry at 100 °C before use. This method can also be used for aluminum.

5). The cleaned welding wire and welding parts should be kept clean and dry, and the welding parts should not be touched or blown by hand. Welders generally wear white welding gloves, and do not wear dirty gloves because they are afraid of trouble; Clean up, and local pollution can be cleaned up again; it is best to cover both sides of the groove with white paper. Generally, welding should be done immediately after mechanical cleaning. If it is not welded within 4 hours after cleaning, it should be cleaned again before welding.

6). The assembly of the weldment should be accurate. If the assembly is not good, you should consider replacing the parts instead of forcibly assembling to avoid excessive stress. Before the formal welding, the groove size should be checked, and welding can be carried out only after passing the test.

7). The welding wire selected for tack welding and the technological measures taken are the same as the welding process.

8). The tack weld shall not have defects such as cracks, pores, and slag inclusions, otherwise it must be removed and rewelded. Re-welding should be carried out in the nearby area instead of spot welding in the original place; when the butt weld gap is not specified in the process, it can be 2~4mm.

9). For the tack weld that will melt into the permanent weld, it must ensure penetration and remove the oxide layer on its surface (only silver-white is allowed), and make the transition of both ends of the weld smooth to facilitate arcing, otherwise it should be trimmed. When welding on cold parts, the arc should stop for a while at the beginning of welding, and when the edge of the base metal begins to melt, wire welding should be added in time to ensure the penetration of the beginning welding point.

10). When welding the longitudinal seam, the arc ignition plate and the arc escape plate must be placed at both ends of the weldment. The arc ignition plate and the arc escape plate are made of aluminum of the same grade and thickness as the welded part. Try to avoid arc craters when welding circumferential seams.

11). During the welding process, the positioning welding spot is cracked, resulting in the dislocation of the plate edge or the change of the gap, the welding should be stopped immediately, and the welding can be continued after the repair.

12). During the welding process, the black ash and oxides on the surface of the upper weld should be cleaned with a wire brush first. When welding, pay attention to the crater, that is, the arc end. The arc can be started 10~20mm away from the welding start end, and then quickly return to the starting end for welding. The first layer is welded in a straight line. In order to obtain a good shape, the other layers can be swung laterally during welding and stopped a little on both sides to facilitate fusion.

13). Aluminum is prone to welding deformation and collapse during welding, so fixtures and backing plates should be made in a targeted manner before welding. When using a fixture, the front and back sides of the parts generally need to be clamped, and the rigidity and clamping force of the fixture should be moderate, because if the fixture is too small, the deformation control effect cannot be obtained. The tightening force should be 350Kg/100mm. The soft aluminum fixture can be carbon steel or stainless steel, which can slow down heat dissipation; the reinforced aluminum material can be made of aluminum fixture, which can enhance the heat dissipation. Longitudinal seam clamps can be used with piano keys, and circular seam can be used with hydraulic bulging clamps. When assembling the longitudinal seam, the gap can be appropriately increased so that there is room for shrinkage after welding; the annular seam (including circular flanges, flanges, etc.) should be left with some reversed staggered or pulled edges, because the flange will collapse and deform after welding. The material of the backing plate is generally stainless steel or carbon steel, and the backing plate can be made of graphite for the welding of aluminum materials with low requirements. The choice of backing plate material should also consider the effect on the cooling rate of the weld. When the aluminum plate is thick or the backing plate assembly gap is large, the gap can be sealed with clay mud and removed after welding. The size of the backing plate can be as follows:

Four, welding requirements:

1. Licensed welders should weld according to the welding procedure documents and other documents.

2. When one of the following situations occurs in the welding environment, effective protective measures should be taken, otherwise welding is not allowed:

1) The welding environment is not clean, with dust and smoke;

2) The wind speed of the welding environment is greater than or equal to 1.5m/s;

3) The relative humidity of the welding environment is greater than 80%;

4) Outdoor work in rain or snow;

5) The soldering temperature is lower than 5℃;

3. The welding of aluminum products is located in a special site, and the site should be laid with rubber or flannel; when welding, it should be kept away from ventilation openings and doors and windows to avoid affecting the gas protection effect.

4. The welding of aluminum by manual tungsten argon arc welding generally uses AC to produce the effect of cathode atomization; the melting electrode argon arc welding uses DC reverse connection. When DC welding is used due to equipment limitations, there is usually a layer of oxide film or even black ash on the surface of the weld, which can be wiped off with a wire brush or a rag. The white film formed on the surface of the weld due to flux residue or oxidation can be wiped off with a wire brush or a rag with hot water.

5. Preheating before welding: Due to the strong thermal conductivity of aluminum, when the thickness of manual tungsten argon arc welding is more than 10mm, it should be preheated before welding, but it should not exceed 100 °C, and the interlayer temperature during welding should not exceed 100°C. It can be heated with flame or far-infrared board depending on the specific situation.

6. During the welding process, the filling point of the welding wire should not be located directly under the arc, but should be located at the edge of the molten pool, about 0.5~1.0mm away from the center line of the arc, and the filling point of the welding wire should not be higher than the surface of the molten pool or in the arc. Swing downward and horizontally to avoid affecting the melting of the base metal, destroying the gas shield and oxidizing the metal; when the welding wire is withdrawn, do not expose the end of the welding wire to the gas protection zone, so as to prevent the end of the welding wire from being oxidized and then fed into the molten metal. pool. During welding, if the tungsten electrode touches the weld metal, the welding should be stopped immediately, the contamination should be removed with a metal grinding head, and the tungsten electrode should be reground; before welding or during the welding process, the oxidized part of the end of the welding wire should be cut off before welding.

7. A welding seam should be welded as much as possible at one time. When re-welding is necessary after stopping welding halfway, it should overlap by 10~20 mm. For multi-layer welds, before proceeding to the next weld, the surface color of the previous weld is checked, and only silver-white is allowed; and defects such as surface contamination and slag inclusion are thoroughly removed. The arc crater should be filled, and the arc connection should be fused and penetrated. Generally, the arc extinguishing method adopts the stack height arc extinguishing method: when the arc is closed, the arc is raised at a constant speed, and the filling wire is accelerated at the same time until the arc is extinguished, so that the welding seam at the arc extinguishing place is partially protruded, and the excess height is polished if necessary. This arc extinguishing method works better when there is an attenuation device on the welding machine.

9. When the thickness of the D-type welding seam between the nozzle and the shell is less than or equal to 12mm, the shell should generally be butt welded, and the edge height should be 25~30mm.

10. The weld thickness t of Class C and D joints shall not be less than 0.7 times the smaller value of the thicknesses δ1 and δ2 of the members on both sides of the fillet weld, and shall not be less than 3mm, unless otherwise specified in the drawing. The bottom does not exceed 10 mm (as shown in the figure below); when the lining or composite plate is composited with a cover plate lapped fillet weld, the thickness of the cover plate constitutes the side length L2 of the fillet weld. The limit of side length, when L2≤4mm, should require t≥0.7 L2.

11. The welding seam and base metal of Class C and D joints should have a smooth transition.

12. The surface of the weld and heat affected zone should be 100% inspected, and there should be no defects such as cracks, unfusion, pores, arc craters, slag inclusions and spatters, and there should be no arcing points outside the weld.

13. There should be no undercut on the surface of the welding seam of the aluminum pressure vessel. The undercut depth of the weld surface of the atmospheric pressure vessel shall not be greater than 0.5mm, the continuous length of the undercut shall not be greater than 100mm, and the total length of the undercut on both sides of the weld shall not exceed 10% of the length of the weld.

14. Welding sequence of heat exchange tube of heat exchanger: tube sheet assembly (reserved length of heat exchange tube should be no less than 4mm for later machining); cleaning of tube end and tube sheet; tube expansion at one end; welding of one tube sheet; Machining the unwelded end of the heat exchange tube; expanding the tube; cleaning the tube end and tube sheet; welding the unwelded side of the heat exchange tube. When welding, the first layer is generally not wire welded (whether the first layer is subjected to PT or air tightness testing according to regulations), but other layers should be wire welded.

15. Aluminum welding should pay attention to the larger blunt edge of the groove, generally 2~6mm. For joints with backing plates, the blunt edge can be appropriately reduced. When the aluminum plate is thin, butt welding should be considered for edge butt welding, such as edge and no wire welding when it is less than or equal to 3mm. Aluminum welding can be used for double-sided welding at the same time, with or without wire on the back, depending on the specific situation.

Five, weld repair:

1. For the defects that need to be repaired, the causes should be analyzed, improvement measures should be proposed, and the welding repair process should be prepared (the processing equipment should be considered to prevent welding deformation) before the repair can be started (the following table shows the weld defects and cause analysis in general);

2. The number of repairs on the same part of the weld should not exceed 2 times.

3. Generally use a metal grinding head to remove defects and bevels. If the defects are deep, the specified manual milling can be used first, and then the groove surface can be polished with a metal grinding head. The groove should have a uniform width, a smooth surface and a groove that is convenient for welding, and the two ends of the groove should have a certain slope (generally not more than 1:4).

4. Defects need to be removed before repair, generally 100% PT confirmation should be carried out (if the defect is a crack, 100% PT must be carried out). When repairing welding, you only need to remove all the defects, do not dig too deep, and perform RT if necessary to confirm that the defects have been completely removed. During repair welding, do not fill the wire first, and use the arc to melt the metal in the repair welding area; after filling the wire, the arc should be slightly biased towards the welding wire to prevent overheating of the metal in the repair welding area. time to facilitate the escape of hydrogen bubbles from the molten pool.

5. The performance and quality requirements of the repaired weld should be the same as the original weld;

6. The main weld defects of aluminum welding are pores and cracks:

 6.1 Causes of pores: Aluminum welding pores are mainly formed by hydrogen entering the welding pool. The sources of hydrogen include: materials, welding wire, shielding gas, wire feeding mechanism, welder's gloves, and high ambient humidity, such as contamination of the welding wire, oxide film on the material and the welding wire itself, oil or sweat on the wire feeding mechanism, etc. Precaution:

 6.1.1 The hydrogen content in the material and welding wire is less than or equal to 0.4mL/100g;

 6.1.2 Oil stains and oxide films should be removed from the surface of the welded parts, and the storage time should not exceed 4h. After the surface is cleaned, cover the groove and both sides with dry, clean and lint-free objects;

 6.1.3 The welding wire should be polished as much as possible, otherwise the treatment method is the same as above;

 6.1.4 Impurity content in protective gas: H2≤0.001%; O2≤0.02%; N2≤0.1%; H2O≤0.02%;

 6.1.5 Shielding gas pipeline: stainless steel or copper pipes are generally used, and plastic pipes are used for hoses instead of rubber and other easy-absorbing hoses; When the protective gas is heated, the pipeline should be purged;

 6.1.6 Wire feeding mechanism: there should be no oil pollution, the wire feeding tube should be made of PTFE or nylon, and the pollution and condensed water in the tube should be cleaned before welding;

 6.1.7 On-site environment: the temperature should not exceed 25°C, the relative humidity should not exceed 50%, and the environment should be kept clean;

 6.1.8 Welders: work clothes should be as white as possible, so that pollution can be detected and cleaned up in time. When welding, pay attention to sweat and oil stains so as not to contaminate the weldment again;

 6.1.9 Test welding should be performed on the test plate before welding to check whether the shielding gas and piping are qualified.

 6.1.10 Precautions for welding procedures: use double