Either spot, seam or projection welding, the welding surface must be clean before, to ensure joint quality and stability.
Mechanical cleaning methods cleaning and chemical cleaning sub-two. Mechanical cleaning methods are commonly used blasting, shot peening, polishing and so on with gauze or wire brush.
Different metals and alloys requires the use of different cleaning methods. Described as follows:
Aluminum and its alloys on the surface cleaning is very strict, due to aluminum on a strong chemical affinity of oxygen, just cleaned surface will soon be oxidized to form aluminum oxide film. Therefore, the surface cleaned before welding is allowed to remain in the time is strictly limited.
Aluminum oxide film is mainly used to remove chemical methods in alkaline solution to the oil and washed the workpiece into the phosphoric acid solution is corrosive. To slow down the growth rate and the new film new film filled pores in the purification of corrosion at the same time. Purification of the most commonly used agents are potassium dichromate and sodium dichromate (see Table 1). After purification in addition to the oxide film is not at the same time, resulting in excessive corrosion of the surface.
Corrosion after washing, and then the lighting solution in nitric acid treatment, after washing again. After washing temperature of 75 â„ƒ in the dry room dry, utilize hot air dryer. This clean-up after the workpiece before welding can be maintained in 72h.
Aluminum can also be mechanically cleaned. If using gauze No. 0-00, or with electric or pneumatic wire brush and so on. However, in order to prevent surface damage, wire diameter must not exceed 0.2mm, wire length should not be less than 40mm, the brush pressing force on the workpiece must not exceed 15-20N, and must be cleaned no later than within 2-3h welding.
In order to ensure the stability of the welding quality, the factory is currently in the chemical cleaning up after the wire brush to clean up before welding and then overlap the inner surface of the workpiece.
Aluminum clean-up required to measure the release of two aluminum alloy workpiece total resistance between two electrodes, R. Method is to use a dedicated device similar to spot welding, an electrode above the insulation on electrode holders, in two specimens pressed between the electrodes, so the R value can be measured most objectively reflects the quality of surface cleaning. For LY12, LC4, LF6 aluminum should not exceed 120 micro-ohms R, just clean up after the 40-50 micro-R generally in Europe, for better conductivity LF21, LF2 aluminum alloy and sintering the material class, R must not exceed 28 -40 micro-ohms.
General use of chemical cleaning of magnesium alloy, the corrosion solution and then purified in the chromic anhydride. This treatment will form a thin dense surface oxide film, it has stable electrical performance can be kept 10 days and nights or longer, the performance remained almost unchanged. Magnesium can also use a welding wire brush cleaning.
Copper alloy can be processed in nitric acid and hydrochloric acid, then neutralize and remove welding residue.
Stainless steel, high temperature nickel alloy welding wire, keeping the height of the surface cleanliness is very important, because the oil, dust, paint the presence of sulfur can increase the possibility of embrittlement, so that the joint defects. Laser cleaning methods are available, shot peening, steel wire brush or chemical corrosion. For the particularly important parts, sometimes with electrolytic polishing, but this method is complicated and low productivity.
Titanium oxide, in hydrochloric acid, nitric acid and sodium phosphate mixed solution to remove corrosion depth. Can also use a wire brush or shot peening.
Low carbon steel and low alloy steel in the atmosphere, corrosion resistance is low. Consequent, these metals in the transport, storage and processing of oil is often used corrosion protection. If the surface is not coated with oil or other plant of the stolen goods contaminated with undesirable conductive material, the pressure of the electrode, the film easily pushed open, will not affect the joint quality.
Steel supplier states are: hot rolled, not pickled; hot rolled, pickled and oiled; cold. Not pickling hot-rolled steel welding, must be sand blasting, shot peening, or removed by chemical methods of corrosion oxide, sulfuric acid and hydrochloric acid in solution, or phosphate-based solution of urea but contains sulfur corrosion in the the latter component can be effectively coated the same time oil and corrosion.
A coated steel plate, with few exceptions, generally do not have a special clean-up can be welded, aluminized steel is required or chemical corrosion with a wire brush cleaning. Steel plate with a phosphate coating, the surface resistance to the ground electrode will be high pressure, the welding current can not level. Only the higher pressure can be welded.
2) The current waveform of the best features of a slow rise slow down;
3) can precisely control the process parameters, and is not voltage fluctuations;
4) to provide price-shaped and saddle-shaped electrode pressure;
5) The head of the inertia and friction is small, the electrode with good mobility.
The current domestic use are mostly 300-600KVA DC pulse, frequency and phase secondary rectification welding machine, the individual to 1000KVA, all have these characteristics. There is also single-phase AC welding machine, but only in unimportant parts.
Aluminium welding electrode should be used for Class 1 the electrode alloy, spherical end, in order to facilitate solid nugget pressure and heat.
As the current density there is large and the oxide film, aluminum spot welding, it is prone to electrode adhesion. Electrode not only affects the appearance of the adhesive quality, but also reduced due to reduced current strength of the joint. This is subject to frequent trimming electrode. Finishing order of the electrode after each piece points welder and welding conditions, welded metal model, clean condition, with or without current waveform modulation, the electrode material and the cooling conditions and other factors. 5-10 points usually aluminum welding, spot welding LF6, LY12 when 25-30 points.
LF21 aluminum low intensity anti-permeability, ductility after welding better, no crack, usually fixed electrode pressure. Hard aluminum (such as LY11, LY12), super-hard aluminum (such as the LC4, LC5), high strength, ductility is poor, can easily crack, must adopt price-shaped curve of the pressure. But for thin pieces, using a large welding pressure or pulse with a slow cooling double-pulse heating, the crack is not inevitable.
Shaped by price pressure, the forging force behind a very important moment in power, usually 0-2 weeks. Forging force increases too soon (before the power failure), equivalent to increasing the welding pressure, will affect the heating, resulting in lower joint strength and volatility. Forging force increase is too late, the nugget has been formed during the cooling crystallization of crack, plus forging force have been. Sometimes in advance of forging force exerted on the power point, because the delayed action of electromagnetic valve, or pneumatic forging force is not caused by increased flow is slow, do not apply early enough to prevent cracking of the reason.