Aluminum alloy welding
The process of selecting an aluminum alloy wire can only be done after a thorough analysis of the many variables involved in the welding of aluminum parts and their application, in order to select the most suitable alloy filler. The first thing that must be considered is the type and chemistry of the base material to be welded, and the second thing to be considered is the performance requirements of the welded part. Finally, the selection of alloy filler is an integral part of the development and evaluation of suitable welding process parameters.
the preparation before welding.
1. Aluminum bevel processing should be used mechanical methods (including shear), such as the use of plasma flame method processing should be mechanically processed from the discoloration part to remove not less than 3 mm, the surface of the bevel after processing should be flat and smooth, there should be no cracks, delamination, inclusions, burrs, flying edges and oxidation color. The bevel surface should be silver-white metallic luster; 100% PT for the bevel and not less than 50 mm on both sides when necessary.
2. Surface cleaning (including removal of surface oxide film, scale, pollution and unqualified oxide color) must be carried out on the surface of welding wire, bevel and not less than 50 mm on both sides. Grinding available φ0.15 ~ 0.2 stainless steel wire disc brush, metal grinding head (i.e. electric milling cutter), manual disc milling machine, file (should be aluminum special arc file), scraper and acetone (dipped in acetone white cloth should be clean, do not use cotton cloth or cotton yarn to avoid wiping out the lint), but should be noted that these tools should be cleaned before use, cleaning should also be careful not to oxidation film to press into the base material, so do not use too much force when cleaning. Therefore, do not use too much force when cleaning; but do not allow the use of abrasive wheels or ordinary sandpaper grinding, because the aluminum is very soft and lead to sand particles in the aluminum, easy to produce defects such as porosity and slag after welding.
3. For outsourced hot processing parts, such as head, etc., in principle, the surface of aluminum should be 100% PT after entering the factory, and if necessary, RT should be carried out on the parts that cannot be determined.
4. The surface of welding wire can be scrubbed with stainless steel wire brush or clean oil sandpaper; chemical cleaning is also required for the wire with thicker surface oxide after grinding before welding. Chemical cleaning: with 70 ℃, 5% ~ 10% NaOH solution soaked 0.5 ~ 3min or so after rinsing with water, followed by about 15% of HNO3 solution at room temperature soaked about 1 ~ 2min after rinsing with warm water, and then hand-held blowers (can not use air compressors, because there is water and oil in the air) blow dry and then put into the oven 100 ℃ drying can be used. For aluminum can also refer to this method.
5. clean wire and welding parts should be kept clean and dry, no hand touching and mouth blowing welding parts, welders generally wear white welder’s gloves, do not wear dirty gloves for fear of trouble; pollution is strictly prohibited before welding, otherwise it should be cleaned again, local pollution can be partially re-cleaned; best covered with white paper on both sides of the bevel with. General mechanical cleaning should be immediately after welding, such as cleaning within 4h after not welding, before welding should be re-cleaned.
6. Weld assembly should be accurate, if the assembly is poor, should consider changing parts, and shall not be forced to group to avoid causing excessive stress. In the formal welding before the bevel size should be checked, qualified before welding.
7. Positioning welding selected wire and take the same process measures as the welding process.
8. Welded parts grouping in the stress concentration (such as weld crossings and corners on the workpiece, etc.) to avoid positioning welding, positioning weld length and spacing can be according to the following table: (mm)
Thickness of base material
Positioning welding spacing
Positioning weld length per section
<3
3~6
＞6
Joint pipe or flange
40~50
50~60
50~80
2~4 points
4~6
5~10
10~12
3~8 per point
9. Positioning weld shall not have cracks, porosity, slag and other defects, otherwise it must be removed and rewelded. Rewelding should be carried out in the nearby area, and do not spot weld in the original place; butt weld gap in the process is not specified, can be 2 ~ 4mm.
10. For the positioning weld that will melt into the permanent weld must ensure that the weld through and remove its surface oxide layer (only silver-white), and make a smooth transition at both ends of the weld to facilitate the arc, otherwise it should be trimmed. When welding on cold parts, the arc should be slightly in the beginning of the welding to stay for a while, when the edge of the base material begins to melt, and then timely fillet welding to ensure that the beginning of the welding point weld through.
11. When welding longitudinal seam, must be placed at both ends of the welded parts to lead the arc plate and back arc plate, lead the arc plate and back arc plate with the welded parts of the same grade and thickness of aluminum. When welding the annular seam, try to avoid producing an arc pit.
12. Positioning welding joints cracking during the welding process, resulting in plate edge misalignment or gap changes, should immediately stop welding, after repair before continuing welding.
13. In the welding process, should first steel wire brush to clean the upper weld surface of the black ash and oxide. Welding attention to the handling of the fire mouth, that is, the closing arc. Lead arc can be 10 ~ 20mm from the beginning of the welding, and then quickly back to the beginning of the welding, the first layer of straight-line welding, in order to obtain a good shape, the other layers of welding can swing laterally and stop a little on both sides in order to fuse.
14. Aluminum welding deformation and weld easy to produce collapse, so before welding should be targeted to produce fixtures and pads. The use of fixtures, the general parts need to be clamped front and back, and the rigidity of the fixture and the size of the clamping force to be moderate, because too small to take the role of controlling deformation, too large the weld constraint is too strong to lead to cracking of the weld, clamping force by 350Kg/100mm appropriate. Soft aluminum clamps can be carbon steel or stainless steel, can slow down heat dissipation; reinforced aluminum available aluminum manufacturing clamps, which can enhance heat dissipation. Longitudinal seam clamps can be used in piano key type, and ring seam can be used in hydraulic expansion clamps. The longitudinal seam can be assembled with an appropriate increase in clearance so that there is room for shrinkage after welding; the annular seam (including round flange, flange, etc.) is left with some reverse wrong edge or wrenching edge because the flange will collapse and deform after welding. The pad material is generally stainless steel or carbon steel, not demanding aluminum welding available graphite pad. The choice of pad material should also consider the impact on the cooling rate of the weld. When the aluminum plate is thicker or pad assembly gap is larger, available clay mud to seal the gap, after welding to remove. The size of the pad plate can be according to the following figure.
Picture
Material and size
A
B
C
Stainless steel or carbon steel
Graphite
Fourth, welding requirements.
1. Licensed welders should be in accordance with the welding process documents and other documents welding.
2. In the welding environment in one of the following cases, effective protective measures should be taken, otherwise not allowed to weld.
1) the welding environment is not clean, dust, smoke.
2) welding environment wind speed greater than or equal to 1.5m / s.
3) welding environment relative humidity greater than 80%.
4) outdoor work in rain and snow.
5) welding temperature below 5℃.
3. Aluminum products welding is located in a special site, the site should be paved with rubber or fleece; welding should be away from the vents and doors and windows to avoid affecting the gas protection effect.
4. Welding aluminum with manual tungsten arc welding generally use AC in order to produce the role of cathode atomization; fusion electrode argon arc welding with DC reverse connection. When using DC welding due to equipment limitations, the surface of the weld generally has a layer of oxide film or even black ash, which can be wiped away with a wire brush or rag. The surface of the weld due to flux residue or oxidation and the formation of white film available steel wire brush or rag Bun hot water to wipe away.
5. Preheating before welding: Due to the strong thermal conductivity of aluminum, so the general manual tungsten arc welding greater than 10mm thickness, welding should be preheated before welding, but not more than 100 ℃, the interlayer temperature does not exceed 100 ℃ when welding. Depending on the specific situation with a flame or far-infrared plate for heating.
6. In the welding process, the filling point of the wire should not be located directly below the arc, but should be located at the edge of the molten pool, about 0.5 ~ 1.0mm from the arc centerline, the filling point of the wire shall not be higher than the surface of the molten pool or swing laterally under the arc to avoid affecting the melting of the base material, the destruction of gas protection and metal oxidation; wire retraction do not make the wire is not exposed outside the gas protection zone, so as to avoid the wire is not oxidized and then sent into the molten pool. When the wire is withdrawn, do not make the unend of the wire outside the gas protection zone, so that the unend of the wire is not oxidized and then sent into the melt pool. When welding, if the tungsten electrode touches the weld metal should immediately stop welding, use the metal grinding head to remove pollution, and repair the tungsten electrode; whether before welding or during the welding process, should first remove the end of the wire has been oxidized part of the weld again.
7. a weld should try to finish welding at once, have to stop midway after re-welding, should overlap 10 ~ 20 mm. multi-layer weld before the next weld, the previous weld surface color inspection, only silver white; and thoroughly remove surface contamination, slag and other defects. The arc pit should be filled and the arc joint should be fused and welded through. General arc extinguishing using the stacked high arc extinguishing method: close the arc at a uniform rate to raise the arc, while accelerating the filler wire until the arc is extinguished, so that the arc extinguished weld local projection, if necessary, grinding over the residual height. In the welding machine with attenuation device, this arc extinguishing method is more effective.
8. A, B type joint weld residual height, residual height difference and width difference between the following table mm.
Weld position
Residual weld height
Difference in weld height
Weld width difference
Tungsten arc welding
Melting pole argon arc welding
Manual welding, semi-automatic welding
Automatic welding, mechanized welding
0~3
0~5
0~2
0~3
0~2
Weld position other than flat welding
0~4
0~5
0~3
0~3
0~2
9. Take over with the shell of the D class welds when the shell thickness ≤ 12mm, the shell should generally wrench the edge of butt welding, wrench height of 25 ~ 30mm.
10. C, D class joint weld thickness t, no other provisions on the drawings, should not be less than the composition of the corner weld on both sides of the component thickness δ1, δ2 smaller value of 0.7 times, and should not be less than 3mm, in general not more than 10 mm (according to the following chart); in the lining or composite plate composite using the cover lap corner weld, the cover thickness constitutes a corner weld on one side of the length of L2 is generally thin, making Weld foot length is limited by the length of the side edge, when L2 ≤ 4 mm, should require t ≥ 0.7 L2.
Picture
11.C, D class joints of the weld and the base material should be a rounded transition.
12. The surface of the weld and heat-affected zone should be 100% inspected, there should be no cracks, unfused, porosity, arc pits, slag and spatters and other defects, there should be no arcing points outside the weld.
13. Aluminum pressure vessel weld surface should not be biting edge. Atmospheric pressure vessel weld surface bite edge depth should not be greater than 0.5mm, bite edge continuous length should not be greater than 100mm, the total length of both sides of the weld bite edge should not exceed 10% of the length of the weld.
14. Heat exchanger tube welding sequence: tube plate assembly (heat exchanger tube reserved length should be not less than 4mm, in order to machine later); tube end and tube plate cleaning; one end of the expansion; welding a side of the tube plate; machining the tube end of the unwelded heat exchanger tube; expansion; tube end and tube plate cleaning; welding the unwelded side of the heat exchanger tube. When welding the first layer is generally not add silk welding (whether the first layer of PT or gas tightness testing according to regulations), but other layers should be added silk welding.
15. aluminum welding should pay attention to the bevel blunt edge larger, generally 2 ~ 6mm. for the joint with the pad, the blunt edge can be reduced. When the aluminum plate is thin, butt welding should consider wrenching edge butt welding, such as when ≤ 3mm consider wrenching edge and not adding wire welding. Aluminum welding can be used on both sides at the same time welding, the back of the wire or no wire depending on the specific situation.
V. Weld rework.
1. defects that require rework should be analyzed to produce the reasons, proposed improvement measures, the preparation of welding rework process (should consider processing installed to prevent welding deformation) before starting rework (the following table is the general case of weld defects and cause analysis).
2. The same part of the weld should not be reworked more than 2 times.
3. generally use a metal grinding head to remove defects and open bevels. If the defect is deep, you can first use the prescribed manual milling and polishing, and then use the metal grinding head to polish the bevel surface. The bevel should be of uniform width, smooth surface and easy to apply welding groove, with a certain slope at both ends of the groove (generally not more than 1:4).
4. The defect needs to be removed before reworking, and generally 100% PT should be confirmed (if the defect is a crack, then 100% PT must be performed). Patch welding only the defect can be removed, do not dig too deep, RT for confirmation of defects have been completely removed if necessary. When filler welding first not filler wire, with the arc to melt the metal in the filler area; filler wire after the arc should be slightly biased toward the wire to prevent overheating of the metal in the filler area, the arc can move moderately reciprocally in the direction of the length of the filler weld to extend the melt pool existence time to facilitate the escape of hydrogen bubbles from the melt pool.
5. rework weld performance and quality requirements should be the same as the original weld.
6. The main weld defects in aluminum welding are porosity and cracking.
6.1 porosity causes: aluminum welding porosity is mainly formed by hydrogen entering the weld pool. Sources of hydrogen are: materials, wire, shielding gas, wire feeding mechanism, the welder’s gloves and environmental humidity is too high, such as wire is contaminated, materials and the wire itself oxidation film, wire feeding mechanism on the oil or sweat stains. Preventive measures.
6.1.1 The amount of hydrogen contained in the material and the wire ≤ 0.4mL /100g.
6.1.2 the surface of the welded parts should be removed from the oil and oxide film and stored for no more than 4 h. After surface cleaning, dry, clean, lint-free objects should be used to cover the bevel and both sides.
6.1.3 welding wire as far as possible using polished wire, otherwise the treatment method as above.
6.1.4 impurity content within the shielding gas: H2≤0.001%; O2≤0.02%; N2≤0.1%; H2O≤0.02%.
6.1.5 shielding gas piping: generally use stainless steel or copper pipes, hoses using plastic tubes and not rubber and other easy to absorb water hose; before welding should check the cooling water piping to ensure that no water leakage; when the environment is very humid, the shielding gas should be heated to blow the pipeline.
6.1.6 wire-feeding mechanism: no oil, wire-feeding tube using polytetrafluoroethylene tube or nylon, and cleaning the tube before welding pollution and condensation.
6.1.7 site environment: the temperature should not exceed 25 ℃, relative humidity does not exceed 50%, keep the environment clean.
6.1.8 Welders: work clothing as white as possible, so that pollution can be detected and cleaned up in time. Weld with care that sweat and oil stains do not recontaminate the welded parts.
6.1.9 Before welding should all be on the test plate test welding in order to check whether the shielding gas and piping qualified.
6.1.10 Preventive measures on the welding process: double-sided welding instead of single-sided welding; each weld thin than thick conducive to the escape of porosity; large diameter wire is conducive to reducing porosity; preheat before welding, slow cooling after welding; reduce the arc voltage, increase the current, reduce the welding speed, conducive to reducing porosity.
6.2 Cracking causes: poor material weldability; the choice of welding wire did not consider the crack resistance; the structure of the constraint is too large. Preventive measures.
6.2.1 poor weldability of aluminum should be considered before welding its crack resistance and not just strength; if necessary, consider annealing the welded parts after welding, and then quenching and aging after welding.
6.2.2 Choose a wire with good crack resistance, such as a wire containing Si and other low melting point elements.
6.2.3 Minimize the constraint of the welded joint, and arrange the welding sequence reasonably so that the weld can have a transverse shrinkage margin to reduce the welding stress. There is no need to position the welding do not weld positioning welding, can double-sided positioning welding do not single-sided positioning welding.
6.2.4 Try to change the single-sided welding to double-sided welding.
VI. Flaw detection requirements.
1. 100% RT shall be carried out after head seam welding, and then 100% RT or 100% PT shall be carried out after forming according to regulations.
2. 100% RT is generally carried out for A and B type welds.
3. 100% PT for C and D type welded joints on containers.
4. 100% PT is carried out on aluminum surface filler welds.
5. Temporary fixed connection welds of aluminum clamps and tensioners are removed
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