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Spot welding methods and processes

2015-4-8    來源:鍋爐輔機    作者:鍋爐配件廠家  閱讀:次  【打印此頁】

性福宝下载 First, spot welding methods classification

When butt welding pieces were fed, should follow the following principles: ① to shorten the length of the secondary circuit and reduce the area of the loop space included, in order to save energy; ② secondary circuit to minimize stretching ferromagnetic volume, particularly avoid stretching joints in welding different volume have a greater change, to reduce volatility of the welding current, ensure quality constant for each point (when using the AC power frequency).

 

 

性福宝下载 1. Single-sided spot welding all common welding are used in the program. From both sides of the weldment feed for small parts and large parts surrounding each spot welding.

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2. Single-sided spot welding electrode side up when the parts or components poorly larger when the secondary circuit is too long, can be used in the program. Weldment from unilateral feeding, consider adding a copper pad on the other side in order to reduce diversion and as a reaction fulcrum.

 

 

3. Double-sided spot welding from one side of the feeding points as possible in order to increase productivity while welding. Single feed often invalid shunting (Fig. 1f and g), a waste of energy, when the pitch is too small will not be welded. In some cases, such as the design allows between two points on the board a long, narrow gap punch (Figure 1f) can shunt current decline.

 

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性福宝下载 Although the double-sided spot welding 4. scheme can be implemented on a general-purpose welder, but the current difficult evenly distributed between the two points, two points more difficult to ensure consistent quality. And Figure 1j As a result of a push-pull feed manner, the upper and lower plates and uneven heating shunt phenomenon greatly improved, and pads may be arranged in any position. The only downside is that it must make two transformers, were placed on both sides of the weldment, this scheme also known as push-spot welding. Basis having two power-transformer polarity.

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5. The multi-spot welding points on the component when more large-scale production, often using multi-point plan to improve productivity. Multi-spot welding equipment are most of unilateral way i feed in a more flexible way, the secondary circuit from the weldment size to contain the situation at the request of a higher, we can use a push-pull type spot welding program . The general use of a set of simultaneous welding transformer two or four o'clock (the latter two groups of secondary circuit). A multi-spot welding by multiple transformer. At the same time the pressure can be energized at the same time, while the pressure group energized and energized group pressure group three options. According productivity, grid capacity to choose the right program.

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Second, spot welding cycle

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Preloading by spot welding process, welding, and the rest of the four basic procedures to maintain the composition of the welding cycle, the additional program can be increased, if necessary, the basic parameters for the current and the variation with time strongly.

 

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1. preloading (F> 0, I = 0) This stage comprises a rise in the electrodes and a constant pressure of two parts. To ensure a constant pressure when conducting electrodes, preloading time must be guaranteed, especially when the need of continuous welding, the welder must take full account of the time required for movement of agency action, can not be infinitely shorter.

 

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Preloading current aim is to establish a stable channel, in order to ensure the welding process of obtaining reproducible current density. Or stiffness of the thick plate stamping parts, when conditions during this period to increase the pre-load, and then reply to the welding power strongly, so that a constant contact resistance but not too small, to improve the thermal efficiency.

 

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性福宝下载 2. Welding (F = Fω, I = Iω) This stage is the weldment heat melted nugget formation stage. Welding current can be basically unchanged (mean value), also for getting up or step increase. After this period the temperature distribution of the weld zone weldment undergo complex changes tend to be stable. Initially heat input is greater than the heat dissipation, the temperature rises, the connecting region is formed of a high temperature plastic state, and that the center from the atmosphere, does not ensure the subsequent oxidation of the molten metal, and then appear in the center of the melting zone first. With the expansion of the heated melting zone, and its periphery plastic shell (in the metallurgical test piece in a ring so called plastic ring) has expanded outward, and finally reaches a steady state when the input heat balance and heat dissipation. When welding parameters appropriate, available size fluctuation is less than 15% of core melt. During this period can produce the following phenomena:

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⑴ liquid metal stirring the liquid metal when energized by the electromagnetic force generated swirling flow, when the globe-shaped nugget regarded as two extremes, and electrical pole, the direction of movement - and then flows around the equator in part by the Globe then flows through the poles to the equator along the outer flow was closed shape. For the same metal spot welding, stirring only the surface of the oxide film weldment can be minced, but dissimilar metal spot welding, the melt must be sufficiently stirred to obtain a homogeneous core. The conduction time is too short, the stirring is insufficient to produce swirling heterogeneous nugget.

 

 

⑵ Splash Splash by generating period can be divided into early and two late; by producing site can be divided into splash (in between the two weldments) and outer spatter (weldments contact with the electrode side) two.

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The reason the early splash generated roughly: weldment surface cleaning the contact surface pressure distribution is poor or severely uneven, resulting in local current density caused by high early melting the plastic ring at this time because there was no splash protection will occur.

 

 

Measures to prevent early splash include: strengthening the weldment cleanup quality, attention to preload the front. When conditions can be gradually increased up current or preheat current to slow down the rate of heating, melting and causing early to avoid splashing.

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性福宝下载 The reason is that the late spatter: melting core grew excessive, electrode force beyond the scope of an effective role in order to break the plastic ring inside the radial cause splashing in the axial direction to break through the outer surface of the plate caused by splashing. This generally produces a large current, power too long occasion. Shorten the energizing time and decreasing the current methods used to prevent.

 

 

性福宝下载 Splash affect the appearance of the outer surface first, followed by the impact scar corrosion and fatigue resistance. Internal splash remnants likely fall at runtime, such as access to the pipeline (eg pipeline) will cause clogging and other serious accidents.

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⑶ beard when heated to a temperature of the semi-molten nugget edge, when certain materials (such as high temperature alloys) are more low melting point inclusions gather at grain boundaries, this part of the impurities is first melted and squeezed under a pressure of the working electrode the gap was. In the subsequent process, the space can sometimes be full of liquid metal filling, but also may not be filled full, on the metallurgical microstructure of this sample is called a beard, full beard like without filling the crack is a dangerous defect .

 

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3. Maintenance (F> 0, I = 0) at this stage no heat input, a nugget rapid cooling, cooling and crystallization. Crystallization process follows Solidification Theory. Since the small nugget, and sandwiched between the water-cooled electrode, the cooling rate is very high, generally within a few weeks the end of solidification. Because the liquid metal in a closed plastic shell, without force, will produce three Vera shrinkage stress upon cooling, easy to produce shrinkage, cracks and other defects, so when cooled electrodes must maintain sufficient pressure to compress the nugget size, compensation contraction. On the plate, aluminum and high-temperature alloys and other parts want to increase the upset force to achieve prevent shrinkage cracks. At this point must be precisely controlled force plus upsetting moment. The liquid metal prematurely due to a sudden increase in pressure due to the plastic ring is broken, splash; too late due to coagulation defects have been formed and valid. Also slow cooling after heating current, lower solidification rate, it can help prevent shrinkage and cracking.

 

 

4. insert pause (F> 0, I = 0) at this stage in the welding hardened steel using only generally inserted in the maintenance time, when the end of the welding current, the nugget is completely solidified and cooled to complete martensite transformation after , whose aim is to improve the microstructure.

 

 

Third, spot welding parameters

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When frequency AC power supply, spot welding parameters are mainly welding current, welding (power) time, electrode force and electrode dimensions.

 

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1. The welding current is proportional to the square Iω precipitation heat and current, so the performance of the welding current on the most sensitive joints. When other parameters constant, when the current value is less than a nugget can not be formed, after exceeding this value, with the current rapid increase in the nugget increased joint strength increased (Figure 3 AB section), then the amount of the increase due to heat The nugget growth slowed, increasing slowly (in BC, 3) joint strength, as it leads to further increase the current splash, joint strength decreased. It is generally recommended to use for the nugget diameter variation insensitive moderate currents (BC segment) to welding.

 

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性福宝下载 In actual production, fluctuations in the welding current is sometimes very large, the reasons are:

 

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① grid voltage fluctuation or welder itself energized simultaneously; ② ferromagnetic weldment welded loop stretching change; ③ point to the last point before the diversion. In addition to selection of the welding current is less sensitive to changes in parameters, the method to solve the above problem is a feedback control. The most commonly used voltage compensation method, constant current method and group control method. Voltage compensation method can be used in all situations, the constant current method is mainly used for the first case ②, ③ can not be used for the first case, the group control method is only used for the first ① case.

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性福宝下载 2. Welding time tω directly affect the length of time the power-heat input size, in the same period of the current widely used to control spot welding, power-on time is a week (week of 20ms) integer multiples. In other parameters fixed, only the energizing time exceeds a certain minimum value when the nugget began to appear, and then increase power over time, before a rapid increase in the nugget, pull shear also improved. When elected with moderate current, power-on time to further increase the nugget growth slows, becoming constant. However, due to the heating time is too long, tissue deterioration, n tension decreased, causes the plasticity index (ductility ratio Fσ / Fτ) decreased (Fig. 4). After splashing generated current is large elected by, the nugget grow to a certain limit.

 

 

3. The size of the electrode force F electrode pressure on the one hand the impact resistance value, thus affecting how many calories analysis, on the other hand the impact of the weldment to the electrode heat dissipation. Too small electrode force will lead to increased resistance, excessive heat and thermal analysis is poor, causing early splash; too much pressure will cause the electrode resistance decreases, less heat analysis, good heat dissipation, nugget size reduction, especially welding penetration rate dropped significantly. Therefore, from the energy point of view, you should choose not to produce minimal spatter electrode force. This value is the current value of the relevant literature can be widely recommended referring the critical splash curve shown in Figure 5. Have suggested the use of the current operating point of the curve near the critical splash splash-free zone.

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性福宝下载 4. Face the size of its working surface electrode size table below. Currently spot welding mainly cone-shaped and spherical two kinds of electrodes. Frustum-shaped or spherical end surface of the diameter d of the end portion of the size of the circular arc radius R, determines the number of electrodes and the weldment contact area, at the same current, which determines the size of the current density and electrode pressure distribution. General expectation should be used to achieve the desired ratio of about 20% larger diameter Face nugget diameter end size. Second, because the electrode is water-cooled, heat loss on the electrode often up to 50% of the total heat input, and therefore fluctuations or water holes end face distance between the ends of the electrode surface changes will seriously affect the average number of heat emission, thereby cause fluctuations nugget size. Thus requiring the frustum-shaped electrode diameter of each working face increased about 15% during the work must be repaired. The water holes in the depletion of end-to-surface distance to that should be replaced with a new electrode remaining 3 ~ 4mm time.

 

 

When spot welding the parameters influence each other, for most occasions can choose a variety of combinations of parameters.

 

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性福宝下载 Spot welding parameters currently used materials can be found in the information in tabular or graphical computing to find, but the former should be adjusted using test welding according to specific criteria.

 

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Because the material surface condition and clean-up of each batch is different, the workshop net voltage fluctuations, equipment status has changed, in order to ensure the quality of welding, to avoid the bulk defects, often want to obtain prior welding parameters allow fluctuation range. So mass production of the occasion, each batch of material, allowing the device to each newly overhauled subject to fluctuations in spot welding parameter range test, the test procedure is as follows:

 

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1) to determine the quality indicators, such as nugget diameter or a single point of pulling the upper and lower shear.

 

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2) other parameters fixed, as a parameter (such as current) and quality indicators curve, then change one of the fixed parameters (such as power-on time), and then make the welding current curve and quality, thus obtaining the relationship between the family of curves.

 

 

性福宝下载 3) then the formation of these two quality indicators parameter (such as current and time) allows leafy curve fluctuation range of qualified partly for CLC.

 

 

The same can be obtained such as welding current and electrode pressure leafy curve. In the production of the control parameters at the operating point within the curve can be lobed.

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