How to determine the welding current, time and pressure of spot welding parameters (high score for help)

Spot welding parameters of aluminum alloy

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Stainless steel spot welding parameters

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Equal spot welding parameters of low carbon steel

Spot welding method and technology 1. Spot welding method: Spot welding is usually divided into two categories: double-sided spot welding and single-sided spot welding. In double-sided spot welding, electrodes are supplied to the welding position from both sides of the workpiece. The typical double-sided spot welding method is shown in figure 1 1-5. A in the figure is the most commonly used way, when there are electrode indentations on both sides of the workpiece. In Figure B, the conductive plate with large welding area is used as the lower electrode, which can eliminate or reduce the indentation of the lower workpiece. Commonly used for spot welding of decorative plates. C in the figure is double-sided spot welding, that is, welding two or more spot welds at the same time. Transformers are used to connect electrodes in parallel. At this time, the impedance of all current paths must be basically equal, and the surface state, material thickness and electrode pressure of each welding part must be the same, so as to ensure that the current passing through each solder joint is basically the same. D in the figure is multi-transformer double-sided multipoint spot welding, which can avoid the deficiency of C. In single-sided spot welding, the electrode is fed from the same side of the workpiece. The typical single-sided spot welding method is shown in figure 1 1-6, where A is single-sided spot welding, and the electrode without solder joint adopts large diameter and large contact surface to reduce current density. In Figure B, there is single-sided two-point spot welding without shunt. At this time, all welding current flows through the welding zone. In figure c, there is shunt single-sided two-point spot welding, and the current flowing through the upper workpiece does not pass through the welding zone, forming an airflow. In order to provide a low resistance path for welding current, a copper pad is placed under the workpiece. In Figure D, when the distance L between two welding spots is large, for example, when welding a skeleton member and a composite plate, in order to prevent the composite plate from warping due to improper heating and reduce the resistance between the two electrodes, a special copper bridge A is adopted and pressed on the workpiece at the same time with the electrodes. In mass production, single-sided multipoint spot welding is widely used. At this time, a transformer can be used to supply power, and each pair of electrodes presses the workpiece in turn (Figure 1 1-7a), or each pair of electrodes can be supplied with a separate transformer, and all electrodes press the workpiece at the same time (Figure1-7b). The latter type has many advantages and is widely used. Its advantage is that each transformer can be placed in the nearest place to the connecting electrode, so. Its power and size can be significantly reduced; The process parameters of each solder joint can be adjusted independently; All solder joints can be welded at the same time, with high productivity; All electrodes press the workpiece at the same time, which can reduce the deformation; Multiple transformers are energized at the same time, which can ensure the three-phase load balance. Second, the selection of spot welding process parameters is usually based on the material and thickness of the workpiece, and refer to the welding condition table of this material. First, determine the shape and size of the electrode end face. Secondly, the electrode pressure and welding time are preliminarily selected, and then the welding current is adjusted to weld the sample with different currents. After checking that the nugget diameter meets the requirements, adjust the electrode pressure, welding time and current within a proper range, and weld and check the sample until the quality of the solder joint completely meets the requirements specified in the technical conditions. The most commonly used method for testing samples is tearing method. The symbol of high quality solder joint is that one piece of torn sample has a round hole and the other piece has a round boss. Thick plates or quenched materials sometimes can't tear out round holes and bosses, but the diameter of nugget can be judged by shearing fracture. If necessary, macroscopic measurement, tensile test and X-ray inspection are needed to determine the permeability, shear strength, shrinkage cavity and crack. When selecting process parameters with samples, we should fully consider the differences between samples and workpieces in shunt, the influence of ferromagnetic substances, assembly clearance and so on, and make appropriate adjustments. 3. Spot welding of different materials with different thicknesses When spot welding of different thicknesses or different materials is carried out, the nugget will be asymmetrical with its interface, but it will shift to the thick plate or the side with poor electrical and thermal conductivity. As a result of displacement, the penetration rate of thin parts or workpieces with good electrical and thermal conductivity will decrease, and the strength of welded joints will also decrease. The nugget deviation is caused by different heating and cooling conditions of two workpieces. When the thickness is not equal, the resistance on one side of the thick part is large, and the interface is far away from the electrode, which leads to more heat generation and less heat dissipation, resulting in the nugget tilting towards the thick part; When the materials are different, the materials with poor electrical and thermal conductivity are easy to generate heat and not easy to dissipate heat, so the nugget is also biased towards this material (see figure 1 1-8). The principle of adjusting nugget deviation is to increase the calorific value of thin plates or workpieces with good electrical and thermal conductivity and reduce their heat dissipation. The common methods are: (1) using strong conditions to increase the influence of contact resistance between workpieces and reduce the influence of electrode heat dissipation. An obvious example is that the capacitor energy storage welding machine can weld workpieces with large thickness ratio by using large current and short power-on time. (2) Use electrodes with smaller diameters and different contact diameters on one side of thin parts or workpieces with good electrical and thermal conductivity, so as to increase the current density on this side and reduce the influence of electrode heat dissipation. (3) Use a thin plate made of different electrode materials or copper alloy with poor thermal conductivity on one side of the workpiece with good electrical and thermal conductivity to reduce the heat loss on this side. (4) Use the process gasket to pad a gasket (thickness 0.2-0.3mm) made of metal with poor thermal conductivity on one side of the thin part or workpiece with good electrical and thermal conductivity, so as to reduce the heat dissipation on this side. The design of spot welding joints usually adopts lap joints and folded joints (Figure 1 1-9). A joint can be composed of two or more workpieces with equal or unequal thickness. When designing the spot welding structure, the accessibility of the electrode must be considered, that is, the electrode must be able to reach the welding part of the workpiece conveniently. At the same time, allowance, overlap, point distance, assembly gap, solder joint strength and other factors should also be considered. The minimum allowance depends on the kind, thickness and welding conditions of the metal to be welded. For metals and thin parts with high yield strength or under strong conditions, smaller values can be taken. The overlap is twice the margin, and the recommended minimum overlap is shown in table 1 1-2. Table 1 1-2 Minimum overlap of joints (mm)3 Thickness of thinnest plate, single-row solder joint and double-row solder joint.

Structural steel stainless steel and superalloy light alloy structural steel stainless steel and superalloy light alloy

0.50.8 1.0 1.2 1.52.02.53.03.54.0 89 10 1 1 12 14 16 182022 6789 10 / kloc-0/2 14 1665438 +0820 12 12 14 14 162024262830 16 182022242832364042 14 / kloc-0/6 1820222630343840 22222426303440464850

Point distance is the center distance between two adjacent points, and its minimum value is related to the thickness, conductivity, surface cleanliness and nugget diameter of the welding metal. Table 1 1-3 is the recommended minimum point distance. Table 1 1-3 Minimum point distance of solder joint (mm)3 Minimum thickness point distance of sheet metal parts

Structural steel, stainless steel and superalloy light alloys

0.50.8 1.0 1.2 1.52.02.53.03.54.0 10 12 12 14 14 16 18202224 8 10 / kloc-0/0 12 1265438+ 04 16 182022 15 15 15 15202525303535

The minimum point distance mainly considers the shunt effect. When strong conditions and large electrode pressure are adopted, the point distance can be appropriately reduced. When using thermal expansion monitoring or controllers that can change the current at each point in turn, and other devices that can effectively compensate the shunt effect, the point distance can be unlimited. The assembly gap must be as small as possible, because eliminating the gap with pressure will consume part of the electrode pressure and reduce the actual welding pressure. Uneven gap will make the welding pressure fluctuate, resulting in significant differences in the strength of each solder joint. Excessive clearance can also cause serious splashing. The allowable gap value depends on the stiffness and thickness of the workpiece. The greater the stiffness and thickness, the smaller the allowable clearance, usually 0. 1-2mm. The shear strength of a single solder joint depends on the nugget area at the interface between two plates. In order to ensure the strength of the joint, in addition to the nugget diameter, the penetration depth and indentation depth should also meet the requirements. The expression of penetration rate is: η=h/δ-c× 100% (see figure110). The permeability of the two plates is only allowed to be between 20-80%. The maximum permeability of magnesium alloy is only allowed to reach 60%. Titanium alloy is allowed to reach 90%. When welding workpieces with different thicknesses, the minimum penetration rate on each workpiece can be 20% of the thickness of the thinner part in the joint, and the indentation depth should not exceed 15% of the plate thickness. If the thickness ratio of two workpieces is greater than 2: 65,438+0, or when welding in inaccessible parts, the indentation depth can be increased to 20-25% by using a flat-headed electrode on one side of the workpiece. The figure 1 1- 10 shows the nugget size on the low magnification grinding plate. Under the tensile load perpendicular to the panel direction, the strength of spot welded joint is positive tensile strength. Because the acute angle formed between two plates around the nugget will cause stress concentration and reduce the actual strength of the nugget, spot welded joints are generally not loaded in this way. The ratio of normal tensile strength to shear strength is usually used as an index to judge the ductility of joints. The greater the ratio, the better the ductility of the joint. The joint strength formed by multiple solder joints also depends on the point spacing and solder joint distribution. When the point distance is small, the strength of the joint will be affected by shunt, and when the point distance is large, the number of solder joints that can be arranged will be limited. Therefore, only by considering the distance between points and the number of solder joints, the maximum joint strength can be obtained, and it is best to arrange multiple rows of solder joints in a staggered manner instead of a rectangular arrangement. Spot welding of common metals. Workpiece cleaning before resistance welding, whether spot welding, seam welding or projection welding, the surface of the workpiece must be cleaned before welding to ensure the stability of joint quality. Cleaning methods are divided into mechanical cleaning and chemical cleaning. Commonly used mechanical cleaning methods include sandblasting, shot blasting, polishing and using gauze or wire brush. Different metals and alloys require different cleaning methods. The brief introduction is as follows: aluminum and its alloys have strict requirements for surface cleaning. Because aluminum has a strong chemical affinity for oxygen, the newly cleaned surface will be oxidized to form an alumina film soon. Therefore, the time allowed to keep the surface clean before welding is strictly limited. The oxide film of aluminum alloy is mainly removed by chemical methods. After degreasing and cleaning in alkaline solution, the workpiece is put into orthophosphoric acid solution for corrosion. In order to slow down the growth rate of the new membrane, the pores of the new membrane are filled, and purification treatment is carried out at the same time of corrosion. The most commonly used purifying agents are potassium dichromate and sodium dichromate (see table 1). After purification, the oxide film will be removed without causing excessive corrosion on the surface of the workpiece. Rinse after corrosion, then polish in nitric acid solution, and rinse later. After cleaning, it was dried in a drying room at 75℃ and dried by hot air. The cleaned workpiece can be kept for 72 hours before welding. Aluminum alloys can also be cleaned mechanically. Such as the use of 0-00 gauze, or the use of electric or pneumatic wire brush. However, in order to prevent damage to the surface of the workpiece, the diameter of the steel wire shall not exceed 0.2mm, the length of the steel wire shall not be shorter than 40mm, the force of the brush pressing on the workpiece shall not exceed 15-20N, and welding shall be carried out no later than 2-3h after cleaning. In order to ensure the stability of welding quality, at present, most domestic factories use wire brushes to clean the overlapped inner surfaces of workpieces after chemical cleaning and before welding. After aluminum alloy cleaning, the total resistance R between two electrodes of two aluminum alloy workpieces must be measured. The method is to use a special device similar to spot welding machine, in which one electrode is insulated from the electrode clamp and two test pieces are pressed between the electrodes, so that the measured R value can most objectively reflect the quality of surface cleaning. LY 12, LC4, LF6 aluminum alloys, the r shall not exceed 120 micro-ohm, and the r after cleaning is generally 40-50 micro-ohm. For LF2 1, LF2 aluminum alloy and sintered aluminum with good conductivity, R shall not exceed 28-40 micro-ohms. Magnesium alloys are generally chemically cleaned and corroded in chromic anhydride solution, and then purified. After this treatment, a thin and dense oxide film will be formed on the surface, with stable electrical properties, which can last for 10 days and nights or longer, and the performance is almost unchanged. Magnesium alloys can also be cleaned with wire brushes. Copper alloy can be treated in nitric acid and hydrochloric acid, and then neutralized and removed the residue at the weld. When resistance welding stainless steel and superalloy, it is very important to keep the workpiece surface highly clean, because the existence of oil, dust and paint will increase the possibility of sulfur embrittlement, which will lead to defects in the joint. The cleaning method can be laser, shot peening, wire brushing or chemical etching. For particularly important workpieces, electrolytic polishing is sometimes used, but this method is complicated and has low productivity. The scale of titanium alloy can be deeply etched and removed in the mixed solution of hydrochloric acid, nitric acid and sodium phosphate. It can also be treated with wire brush or shot blasting. Low carbon steel and low alloy steel have low corrosion resistance in the atmosphere. Therefore, these metals are usually protected by corrosion-resistant oil during transportation, storage and processing. If the oiled surface is not polluted by stolen goods or other bad conductive materials in the workshop, the oil film is easily squeezed out under the pressure of the electrode, which will not affect the quality of the joint. The supply situation of steel is: hot rolling, no pickling; Hot rolling, pickling and oiling; Cold rolling. When welding hot rolled steel without pickling, it is necessary to remove the oxide scale by sandblasting, shot blasting or chemical corrosion, which can be carried out in sulfuric acid and hydrochloric acid solution or in a solution mainly containing phosphoric acid but containing sulfur and urea. The latter part can be effectively oiled and corroded at the same time. With a few exceptions, galvanized steel plates can be welded without special cleaning, while aluminized steel plates need to be cleaned by wire brush or chemical corrosion. The surface resistance of steel plate coated with phosphate will be very high, and the welding current cannot pass under the pressure of grounding electrode. Welding can only be carried out under higher pressure. Second, spot-welded galvanized steel plates can be roughly divided into galvanized steel plates and hot-dip galvanized steel plates. The former has a thinner coating than the latter. Type 2 electrode alloy is recommended for spot welding galvanized steel plate. 1 alloy can be used when the appearance of relative spot welding is very demanding. It is recommended to use a conical electrode shape with a cone angle of 120 degrees-140 degrees. When using the electrode holder, it is recommended to use a spherical electrode with a radius of 25-50 mm In order to prolong the service life of the electrode, a composite electrode embedded with tungsten electrode tip can also be used. The electrode body made of two electrode alloys can enhance the heat dissipation of the tungsten electrode tip. 3. Spot welding of low carbon steel The carbon content of low carbon steel is lower than 0.25%. Its resistivity is moderate, and the required welding machine power is not large; The plastic temperature range is wide, and the required plastic deformation can be easily obtained without using large electrode pressure; The content of carbon and trace elements is low, there is no high melting point oxide, and generally no quenched structure or inclusion is produced; The crystallization temperature range is narrow, the high temperature strength is low and the thermal expansion coefficient is small, so the cracking tendency is small. This kind of steel has good weldability, and its welding current, electrode pressure and electrifying time can be adjusted widely. Steel has good weldability, and its welding current, electrode pressure and electrifying time can be adjusted widely. 4. Because of the extremely fast cooling rate in spot welding of quenched steel, hard and brittle martensite structure will inevitably appear in spot welding of quenched steel, and cracks will occur when the stress is high. In order to eliminate quenching structure and improve joint performance, double pulse spot welding method of tempering after welding is usually adopted. The first current pulse of this method is welding pulse, and the second is tempering pulse. When using this method, we should pay attention to two points: (1) The interval between two pulses must ensure that the solder joint is cooled below the martensite transformation point Ms temperature; (2) The amplitude of tempering current pulse should be appropriate to avoid secondary quenching caused by the metal in the welding zone exceeding the austenite transformation point again. Examples of process parameters of double pulse spot welding of hardened steel are shown in the table below for reference: welding conditions of double pulse spot welding of 25CrMnSiA and 30CrMnSiA steels; Thickness (mm) Electrode end face diameter (mm) Electrode pressure (kn) Welding time (weeks).

1.0 1.52.02.5 5-5.56-6.56.5-77-7.5 1- 1.8 1.8-2.52-2.82.2-3.2 22-3224-3525-3730-40

Thickness (mm) Welding current (KA) Interval time (weeks) Tempering time (weeks) Tempering current (KA)

1.0 1.52.02.5 5-6.56-7.26.5-87-9 25-3025-3025-3030-35 60-7060-8060-8565-90 2.5-4.53-53.5-64-7

5. There are two kinds of spot welding aluminized steel plates. The first one is mainly heat-resistant, and its surface is coated with a layer of Al-Si alloy (containing Si6-8.5%) with a thickness of 20-25 microns, which can withstand the high temperature of 640 degrees. The second type is mainly corrosion-resistant, and it is a pure aluminum coating with a coating thickness of 2-3 times that of the first type. When these two kinds of galvanized steel plates are spot welded, good strength solder joints can be obtained. Because the coating has good electrical and thermal conductivity, it needs a large welding current. Hard copper alloy spherical electrodes should be used. The following table shows the welding conditions of spot welding of the first aluminized steel plate. For the second type, due to the thick coating, larger current and lower electrode pressure should be adopted. Welding conditions for spot welding of hot-dip galvanized aluminum sheet (mm) thicKNess (mm) electrode spherical radius (kn) electrode pressure (week) welding current (KA) shear strength (kn)

0.60.8 1.0 1.2 1.42.0 252550505050 1.82.02.53.24.05.5 9 10 1 1 12 14 1 8 8.79.5 10.5 12.065 438+03.0 14.0 1.92.54.26.08.0 13.0

Six, stainless steel spot welding stainless steel is generally divided into: austenitic stainless steel, ferritic stainless steel and martensitic stainless steel. Due to the high resistivity and poor thermal conductivity of stainless steel, compared with low carbon steel, smaller welding current and shorter welding time can be used. This material has high high temperature strength, so it is necessary to adopt high electrode pressure to prevent shrinkage cavity, crack and other defects. Stainless steel has strong heat sensitivity. Usually, short welding time and strong internal and external water cooling are used to accurately control heating time, welding time and welding current to prevent grain growth and intergranular corrosion in heat affected zone. It is suggested to use grade 2 or grade 3 electrode alloy for spot welding stainless steel electrode to meet the needs of high electrode pressure. The following table gives the welding conditions of stainless steel spot welding: welding conditions of stainless steel spot welding plate thickness (mm) electrode tip diameter (mm) electrode pressure (kn) welding time (weeks) welding current (KA)

0.30.50.8 1.0 1.2 1.52.02.53.0 3.04.05.05.06.05.5-6.57.07.5-8.09- 10 0.8- 1.2 1 .5-2.02.4-3.63.6-4.24.0-4.55.0-5.67.5-8.58 .5- 10 10- 12 2-33-45-76-87-99- 12 1 1- 13 12- / kloc-0/6 13- 17 3-43.5-4.55-6.55.8-6.56.0-7.06.5-8.08- 108- 1 1 1 1- 13

Seven, aluminum alloy spot welding aluminum alloy is widely used, divided into two categories: cold working strengthening and heat treatment strengthening. The weldability of aluminum alloy spot welding is poor, especially for aluminum alloy strengthened by heat treatment. The reasons and technological measures to be taken are as follows: (1) The electrical conductivity and thermal conductivity are high, and it takes a large current and a short time to reach enough heat to form nuggets; But also can reduce surface overheating, avoid electrode adhesion and diffusion of electrode copper ions to pure aluminum coating, and reduce the corrosion resistance of the joint. (2) If the plastic temperature range is narrow and the linear expansion coefficient is large, it is necessary to adopt large electrode pressure and good electrode follow-up to avoid cracks caused by excessive internal tensile stress during nugget solidification. For aluminum alloys with high crack tendency, such as LF6, LY 12, LC4, etc. It is necessary to increase the forging pressure to make the nugget have enough plastic deformation and reduce the tensile stress during solidification to avoid cracks. When the bending electrode can't bear large forging pressure, the welding pulse followed by slow cooling pulse can also be used to avoid cracks. For thick aluminum alloy, the two methods can be combined. (3) Before welding, the oxide film easily formed on the surface must be strictly cleaned, otherwise it will easily lead to splashing and poor nugget formation (the nugget shape is irregular when tearing, and the boss and hole are not round), which will reduce the strength of the solder joint. Uneven cleaning will lead to unstable solder joint strength. For the above reasons, a welder with the following characteristics should be selected for spot welding aluminum alloy: 1) It can provide large current in a short time; 2) The current waveform preferably has the characteristics of slowly rising and falling; 3) the process parameters can be accurately controlled, and are not affected by the voltage fluctuation of the power grid; 4) can provide valence electrode and saddle electrode pressure; 5) The inertia and friction of the nose are small, and the electrode has good follow-up. At present, most welding machines used in China are 300-600KVA DC pulse, three-phase low frequency and secondary rectification, and some of them reach 1000KVA, all of which have the above characteristics. There are also single-phase AC welding machines, but only for unimportant workpieces. 1 electrode alloy should be used as the electrode for spot welding aluminum alloy, and the end face is spherical, which is beneficial to compaction, nucleation and heat dissipation. Due to the high current density and the existence of oxide film, electrode adhesion is easy to occur in spot welding of aluminum alloy. Electrode adhesion not only affects the appearance quality, but also reduces the joint strength due to the decrease of current. Therefore, it is necessary to trim the electrode frequently. The number of points that can be welded after each electrode dressing is related to welding conditions, the type of metal to be welded, cleaning conditions, whether there is current waveform modulation, electrode materials and their cooling conditions. Usually, pure aluminum is spot-welded at 5- 10, and LF6 and LY 12 are spot-welded at 25-30. Impermeable aluminum LF2 1 has low strength, good weldability, and no crack after ductility, so fixed electrode pressure is usually used. Hard aluminum (such as LY 1 1, LY 12) and superhard aluminum (such as LC4 and LC5) have high strength, poor ductility and easy cracking, so valence curve pressure must be adopted. However, for thin parts, using large welding pressure or double pulse heating and slow cooling pulse, cracks are not inevitable. When the valence pressure is used, it is very important that the forging pressure lags behind the power failure time, which is generally 0-2 weeks. If forging is added too early (before power failure), the welding pressure will increase, which will affect the heating and lead to the decrease and fluctuation of solder joint strength. If the forging pressure is applied too late, cracks will form when the nugget cools and crystallizes, and the forging pressure will not help.