Electric resistance welding
Electric resistance welding (ERW) refers to a group of welding processes such as spot and seam welding that produce coalescence of faying surfaces where heat to form the weld is generated by the electical resistance of material vs the time and the force used to hold the materials together during welding. Some factors influencing heat or welding temperatures are the proportions of the workpieces, the coating or the lack of coating, the electrode materials, electrode geometry, electrode pressing force, weld current and weld time. Small pools of molten metal are formed at the point of most electrical resistance (the connecting surfaces) as a high current (100–100,000 A) is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are limited to relatively thin materials and the equipment cost can be high (although in production situations the cost per weld may be as low as $0.04 USD per weld depending on application and manufacturing rate).
Spot welding is a resistance welding method used to join two to three overlapping metal sheets, studs, projections, electrical wiring hangers, some heat exchanger fins, and some tubing. Usually power sources and welding equipment are sized to the specific thickness and material being welded together. The thickness is limited by the output of the welding power source and thus the equipment range due to the current required for each application. Care is taken to eliminate contaminants between the faying surfaces. Usually, two copper electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. When the current is passed through the electrodes to the sheets, heat is generated due to the higher electrical resistance where the surfaces contact each other. As the electrical resistance of the material causes a heat buildup in the work pieces between the copper electrodes, the rising temperature causes a rising resistance, and results in a molten pool contained most of the time between the electrodes. As the heat dissipates throughout the workpiece in less than a second (resistance welding time is generally programmed as a quantity of AC cycles or milliseconds) the molten or plastic state grows to meet the welding tips. When the current is stopped the copper tips cool the spot weld, causing the metal to solidify under pressure. The water cooled copper electrodes remove the surface heat quickly, accelerating the solidification of the weld, since copper is an excellent conductor. Resistance spot welding typically employs electrical power in the form of direct current, alternating current, medium frequency half-wave direct current, or high-frequency half wave Direct current.
If excessive heat is applied or applied too quickly, or if the force between the base materials is too low, or the coating is too thick or too conductive, then the molten area may extend to the exterior of the work pieces, escaping the containment force of the electrodes (often up to 30,000 psi). This burst of molten metal is called expulsion, and when this occurs the metal will be thinner and have less strength than a weld with no expulsion. The common method of checking a welds quality is a peel test. An alternative test is the restrained tensile test, which is much more difficult to perform, and requires calibrated equipment. Because both tests are destructive in nature (resulting in the loss of salable material), non-destructive methods such as ultrasound evaluation are in various states of early adoption by many OEMs.
The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. When high strength in shear is needed, spot welding is used in preference to more costly mechanical fastening, such as riveting. While the shear strength of each weld is high, the fact that the weld spots do not form a continuous seam means that the overall strength is often significantly lower than with other welding methods, limiting the usefulness of the process. It is used extensively in the automotive industry— cars can have several thousand spot welds. A specialized process, called shot welding, can be used to spot weld stainless steel.
There are three basic types of resistance welding bonds: solid state, fusion, and reflow braze. In a solid state bond, also called a thermo-compression bond, dissimilar materials with dissimilar grain structure, e.g. molybdenum to tungsten, are joined using a very short heating time, high weld energy, and high force. There is little melting and minimum grain growth, but a definite bond and grain interface. Thus the materials actually bond while still in the solid state. The bonded materials typically exhibit excellent shear
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电阻焊接
电阻焊接（ERW）是指一组焊接工艺，例如点焊和缝焊，其产生焊接表面的聚结，其中形成焊缝的热量由材料的电阻抗与时间和用于保持材料的力产生一起焊接。影响热或焊接温度的一些因素是工件，涂层或涂层不足，电极材料，电极几何形状，电极压力，焊接电流和焊接时间的比例。在大电流（100-10000A）通过金属时，在大多数电阻点（连接表面）形成少量熔融金属。一般来说，电阻焊接方法是有效的，污染少，但其应用仅限于相对较薄的材料，设备成本可能很高（尽管在生产情况下，焊接成本可能低于每个焊接0.04美元，取决于应用和制造率）。
点焊是一种电阻焊接方法，用于连接两到三个重叠的金属板，螺柱，突起点，电线吊架，一些热交换器翅片和一些管道。通常电源和焊接设备的大小适合特定的厚度和焊接在一起的材料。厚度受焊接电源的输出限制，因此由于每个应用所需的电流使设备的范围限制。注意消除拼接表面之间的污染物。通常，两个铜电极同时用于将金属片夹在一起并使电流通过片材。当电流通过电极到片材时，由于表面彼此接触的较高的电阻而产生热量。由于材料的电阻使铜电极之间的工件产生热量积聚，所以升高的温度导致电阻上升，导致熔池中大部分时间在电极之间。随着热量在不到一秒的时间内散发到整个工件上（电阻焊接时间通常被编程为交流周期或毫秒数量），熔融或塑性状态增长以满足焊接尖端。当电流停止时，铜尖冷却点焊，使金属在压力下固化。水冷铜电极快速去除表面热，加速焊接的凝固，因为铜是优良的导体。电阻点焊通常采用直流，交流，中频半波直流或高频半波直流形式的电力。
如果施加过多的热量或施加得太快，或者如果基材之间的力太小或涂层太厚或太导电，则熔融区域可能延伸到工件的外部，逃避遏制力的电极（通常高达30,000psi）。这种熔融金属的爆发称为排出，当这种情况发生时，金属将变薄，并且具有比不排出的焊缝更小的强度。检查焊缝质量的常用方法是剥离试验。一个替代测试是限制拉伸试验，这是更难以执行，并需要校准设备。由于这两项测试都是具有破坏性的（导致可销售材料的损失），诸如超声波评估之类的非破坏性方法处于许多原始设备制造商早日采用的各种状态。
该方法的优点包括有效的能量使用，有限的工件变形，高生产率，易于自动化，并且不需要填充材料。当需要高剪切强度时，使用点焊优先于更昂贵的机械紧固，例如铆接。虽然每个焊缝的剪切强度高，但是焊点不形成连续接缝的事实意味着整体强度通常显着低于其他焊接方法，限制了该工艺的有用性。它广泛用于汽车行业 - 汽车可以有几千个点焊。称为射击焊接的专门工艺可用于点焊不锈钢。
电阻焊接有三种基本类型：固态，熔合和回流钎焊。在固态键（也称为热压结合）中，具有不同晶粒结构的不同材料，例如，使用非常短的加热时间，高的焊接能量和高的力来连接钼与钨。几乎没有熔化和最小的晶粒生长，但是具有明确的键和晶粒界面。因此，材料实际上仍然处于固态状态。粘合材料通常表现出优异的剪切和拉伸强度，但剥离强度差。在熔接中，具有相似晶粒结构的相似或不相似的材料被加热到两者的熔点（液态）。随后的材料的冷却和组合形成了具有较大晶粒生长的两种材料的“熔核”合金。通常，根据物理特性，在短或长焊接时间内的高焊接能量用于产生熔接。粘合材料通常表现出优异的拉伸，剥离和剪切强度。在回流焊接中，使用诸如金或焊料的低温钎焊材料的电阻加热来连接不同材料或广泛变化的厚/薄材料组合。钎焊材料必须对每个部件“湿”，并且具有比两个工件更低的熔点。所得的结合具有确定的最小晶粒生长的界面。通常，该过程在低焊接能量下需要更长（2至100ms）的加热时间。所得到的粘合剂显示出优异的拉伸强度，但是剥离和剪切强度差“接缝焊接”在这里重定向。对于几何焊接配置，请参见焊接接头。
电阻缝焊是一种在两种类似金属的表面产生焊缝的工艺。接缝可以是对接或重叠接头，通常是自动化过程。与对接焊接不同的是，对接焊通常一次焊接整个接头，并且从一端开始，缝焊逐渐形成焊缝。像点焊一样，缝焊依赖于通常由铜制成的两个电极来施加压力和电流。电极是圆盘形的，随着材料在它们之间通过而旋转。这允许电极保持与材料的恒定接触以产生长的连续焊接。电极也可以移动或辅助材料的运动。
变压器以低电压，大电流交流电源的形式向焊缝提供能量。工件的接头相对于电路的其余部分具有高的电阻，并被电流加热到其熔点。半熔融表面通过产生熔接的焊接压力被压在一起，导致均匀的焊接结构。大多数缝焊机由于产生的热量而通过电极，变压器和控制器组件使用水冷却。接缝焊接产生非常耐用的焊接，因为接头由于施加的热和压力而被锻造。通过电阻焊接形成的正确焊接接头通常比形成它的材料强。
缝焊的常见用途是制造圆形或矩形钢管。缝焊已被用于制造钢饮料罐，但不再用于现代，现代饮料罐是无缝铝。
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