Welding and technical points of aluminum stamping parts
1. Aluminum stamping parts of the welding process
1: argon gas purity ≥99.99%, dew point ≤-55℃
Reasons why it is not suitable to use the bottled argon gas when the pressure is ≤ 0.5mpa:
- Nitrogen content ≥0.04%, light yellow or grass green MgN and pores are produced on the weld surface.
- Oxygen content ≥0.03%, dense black spots, arc instability and large spatter appear on the surface of the molten pool.
- Water content ≥0.07%, the molten pool will boil, and pores will be formed in the weld seam.
2: Cerium tungsten electrode – manual argon tungsten arc welding electrode
The diameter of the electrode is generally one size larger than the specification required by the welding current, and the end is hemispherical:
Tungsten extreme grinding into a cone, vertical clamping. Arc the test plate with a current 20~30A larger than the required current and maintain it for several seconds. Tungsten ends form hemispherical heads.
If the tungsten electrode is contaminated with aluminum, it must be re-polished or replaced. In the case of slight pollution, the current is increased to make the arc burn for a while in the test plate, burning off the pollutants.
3: Requirements for wire feeding equipment for MIG welding
Because the aluminum welding wire is relatively soft, in order to avoid biting the welding wire, the wire feeding wheel is not suitable for the wire feeding wheel with gear, should not use push wire type.
Wire feeding hose is made of PTFE or nylon instead of spring tube. Otherwise grinding results in contamination or blockage of the hose.
Importantly, MIG usually USES dc reverse polarity.
4: flux selection
The main function of flux is to remove oxidation film and some other impurities, use can be mixed into a paste with anhydrous alcohol or directly put the flux powder in the groove and on both sides.
Choose flux that can remove slag easily after operation
Choose aluminum magnesium alloy flux without sodium.
5: heterogeneous aluminum welding, welding material selection
When welding different grades of aluminum, when the drawings and process are not specified, choose the welding wire material according to the base material with better corrosion resistance and lower strength level.
Table 1. Wire same with base material
Table 2. Welding wires for heterogeneous base materials
Table 3. welding wires selected according to different materials and properties
2. Preparation before welding
- Aluminum groove processing shall adopt mechanical method (including shearing), and the surface of the groove shall be silver-white metallic luster; If necessary, carry out 100%PT on groove and both sides within the range of no less than 50 mm.
- The welding wire, groove surface and its two sides must be cleaned within the range of no less than 50 mm, to remove the surface oxidation film, scale, pollution and unqualified oxidation color. Pay attention not to press the oxide film into the base material during cleaning, so the cleaning should not be too hard; Do not use grinding wheel or ordinary sandpaper, because the aluminum is very soft, easily lead to the sand in the aluminum. Defects such as porosity and slag will be produced after welding.
- The surface of welding wire can be swabbed with stainless steel wire brush or clean oil sand paper; If the surface oxide skin thick wire, before use, first after grinding, also need chemical cleaning.
Chemical cleaning: put the welding wire into the NaOH solution of 5%~10% at 70℃, soak for 0.5~3min and then rinse with water. Then put in about 15% HNO3 solution, soak at room temperature for about 1~2min and rinse with warm water. Finally, use a hand-held hair dryer (cannot use air compressor, because there is water and oil in the air) to blow dry, and put it into the oven to dry at 100℃.
- Clean welding wires and welding parts should be kept clean and dry. Do not touch or blow the welding parts with your hands. Welders usually wear white welder’s gloves. Do not wear dirty gloves for fear of trouble.
Clean up before welding. Local pollution can be cleaned up again. It is best to cover both sides of the groove with white paper.
General mechanical cleaning should be immediately after welding, if there is no welding within 4h after cleaning, before welding should be cleaned again.
- Assembly of welding parts should be accurate. If the assembly is not good, consider changing the parts, not be forced to assemble welding parts, to avoid causing too much stress. Check the groove size before welding, and solve the welding after passing.
- When assembling welding parts, try to avoid positioning welding at stress concentration places (such as welding seam intersection and corner on workpiece, etc.).
The position weld length and spacing table are as follows :(mm)
- Aluminum stamping parts are prone to deformation and collapse during welding, so fixture and backing plate need to be made specifically.
The front and back of the parts need fixture clamping, moderate adjustment of fixture rigidity and clamping force. If the adjustment is too small to reach the effect of deformation control, too large weld restraint is too strong easy to crack.
The best clamping force is 350Kg/100mm.
Soft aluminum fixture can be used carbon steel or stainless steel, slow down heat dissipation; Aluminum fixture can be used to strengthen the heat dissipation.
The clamping device for longitudinal seam can be keyboard type, and the clamping device for circular seam can be hydraulic expansion. The gap can be increased appropriately when the longitudinal seam is assembled, so that there is room for contraction after welding. Circular joints (including circular flanges, flanges, etc.) are left with reverse wrong edges or flanges, because the weld flange will collapse and deformation.
Backing plate material is generally stainless steel or carbon steel, aluminum welding requirements are not high can be made of graphite backing plate. The influence on weld cooling speed should also be considered when selecting backing material. When the aluminum plate is thicker or the mounting gap of the backing plate is larger, the gap can be sealed with clay and removed after welding.
3. Welding requirements
- Manual argon tungsten arc welding is generally used to process aluminum, so as to produce the effect of cathode atomization; Argon arc welding of molten electrode USES dc reverse connection.
When using dc welding, the weld surface is generally a layer of oxide film or even black ash, then can be wire brushed or cloth wiped.
On the weld surface as a result of flux residue or oxidation of the formation of white film, can be brushed with steel wire or cloth dipped in hot water.
- Preheat before welding:
Aluminum has strong thermal conductivity, so when the welding thickness is greater than 10mm, it should be preheated before welding, but not more than 100℃; The temperature between layers should not exceed 100℃ during welding. It can be heated by flame or infrared ray plate according to specific conditions.
- In the welding process, the filling point of the welding wire is not directly below the arc, but at the edge of the pool, about 0.5~1.0mm from the arc center line. The filling point of the welding wire is not higher than the surface of the molten pool or it swings laterally under the arc, so as to avoid affecting the melting of the base material and damaging the gas protection and causing metal oxidation.
When the welding wire is withdrawn, do not expose the end of the welding wire to the outside of the gas protection zone, to avoid the end of the welding wire is oxidized and sent back into the weld pool. When welding, if the tungsten electrode encounters the weld metal, the welding shall be stopped immediately, the metal grinding head shall be used to remove the pollution, and the tungsten electrode shall be repaired. No matter before welding or during welding, it is necessary to remove the oxidized part at the end of the welding wire before welding.
- Try to finish the welding of one weld at a time. If stop the welding halfway and re-weld, overlap the welding mark by 10~20 mm. The surface color of the multi-layer weld shall be silver white before the next weld. Thoroughly remove surface contamination, slag and other defects.
The arc pit shall be filled and penetration shall be required at the arc joint.
Generally, arc extinguishing method is adopted by stacking high arc extinguishing method: after welding, arc closing at a uniform speed to raise the arc, and at the same time to accelerate the filling of welding wire, until the arc is extinguished, making the welding seam at the arc extinguishing part protruding, and polishing excess height when necessary. When there is an attenuating device on the welding machine, this method is more effective.
4. Matters needing attention during welding
- In the process of welding, if the locating welding spot cracks, causing dislocation of plate edge or change of gap, the welding shall be stopped immediately, and the welding can be continued after repair.
- The locating welding seam shall not have defects such as cracks, pores and slag, or the rewelding shall be removed. Rewelding should be done in the vicinity rather than in situ. When the gap of butt weld is not specified, the standard of 2~4mm can be followed.
- When welding the longitudinal seam, arc starting plate and arc backing plate must be placed at both ends of the welding part. Arc starting plate and arc backing plate are made of the same aluminum as the welded part.
- For positioning welds that will melt into permanent welds, it is necessary to ensure the penetration of welds and remove the oxide layer on the surface (only silver white is allowed), and smooth transition of both ends of welds to facilitate arc welding, otherwise, it is necessary to repair. When welding cold parts, the arc starts to stay for a little while, and when the edge of the base material starts to melt, the wire is added to weld in time to ensure the welding penetration.
- In the welding process, the black ash and oxide on the upper weld surface should be cleaned with steel wire brush first.Welding process, pay attention to the treatment of the crater, that is, the arc.Arc starting can be 10~20mm away from the starting end of welding, and then back to the starting end of welding quickly. The first layer adopts linear welding. In order to obtain good formation, other layers can swing laterally and stop at both sides for fusion.
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