Mastering the Craft: Resistance Spot Welding Techniques and Setups
Resistance welding Yolo Materials Industry 13th, Sep 2023 15:00pm
Continuing from the previous article, let’s talk about
The Process of Resistance Spot Welding.
Resistance Spot welding is a process for joining metal pieces. It involves pressing the pieces together firmly, then applying an electric current. This current creates heat due to resistance, melting a small section of metal, forming a liquid core.
After turning off the power, pressure is maintained or increased, causing the molten core to solidify and crystallize, creating a strong connection point. Once one spot is welded, the electrode or workpiece moves to the next.
However, during the next weld, some current may flow through the previously welded point, known as the shunt effect. This can weaken the weld quality, so it’s essential to leave some space between weld points.
Weld quality depends on factors like current strength, duration, electrode pressure, and surface cleanliness. Spot welding is ideal for thin metal sheets, whether done one spot at a time or multiple spots simultaneously. It typically involves four stages: pre-press, welding, maintaining pressure, and rest. The key parameters are welding time, current, and electrode voltage. This process is depicted in a timing diagram, illustrating these four stages.
Four Steps of the Resistance Spot Welding
1. Pre-Press Stage
In this phase, we apply force to the electrode. Once the electrode touches the metal piece, we keep a steady pressure to ensure that the electric current flows smoothly when we turn it on. This pre-press stage needs to last for a specific duration.
2. Welding Stage:
This stage is fusion core molding. The effective contact current is required to remain essentially constant or to vary with a small degree of fluctuation. Here, the temperature distribution in the weld zone stabilizes after complex changes. Initially, the heat input between the weldments is much greater than the heat dissipation.
As a result, the temperature in the weld zone climbs rapidly and a high-temperature connection zone is formed at the same time. Since the outside air is blocked from the molten parts in the center of the weld at this time, no oxidation of the weldment material occurs with the air. After a certain period of time, the melting zone area becomes larger, and its plastic ring also follows to become larger, until the heat input and heat dissipation reaches a balanced and stable state.
3. Maintenance Stage:
In this stage the electrodes remain in the same state as in the previous two stages, except that there is no longer a connecting current passing through them. The main purpose of this stage is to accomplish the dissipation of heat in the nucleus in order to cool it down and form a reliable point.
4. Rest Stage：
In this stage the electrodes remain in the same state as in the previous two stages, except that there is no longer a connecting current passing through them. The main purpose of this stage is to accomplish the dissipation of heat in the nucleus in order to cool it down and form a reliable point.
These stages help ensure a successful resistance spot welding process.
Resistance Spot welding electrode common arrangement
These are the common ways to arrange electrodes for spot welding, each with its advantages and considerations.
1. Top and Bottom Touch Method:
This method, depicted in Figure (a), is widely used in general spot welding machines. It involves supplying electricity from both the top and bottom sides of the workpiece. It’s versatile and suitable for welding small and large components.
This approach is particularly effective for welding multiple points around the periphery of both large and small parts.
2. Parallel Method:
As Figure (b), the parallel method feeds electricity from one side while attempting to weld two points simultaneously. This boosts production efficiency. However, there’s a drawback – single-sided feeding can cause a shunt effect, making it impossible to weld when the distance between the welding points is too small. To mitigate this, copper shims can be added under the workpiece to reduce electrical resistance and minimize shunting. Alternatively, a narrow slot can be created between the two welding points on the upper plate of the workpiece to significantly reduce shunt currents, provided the design allows it.
3. Parallel Indirect Method:
Shown in Figure (c), this method involves supplying electricity from one side of the workpiece. It’s used when accessibility is poor on one side of the part or when the workpiece is large with a lengthy secondary circuit. However, this approach is prone to shunting.
To reduce shunting, copper shims can be added on the opposite side and used as single-acting fulcrums.
Resistance Spot welding parameters Influence on welding results
There are several factors that influence the quality of spot welding in power battery assembly. These factors include the resistance between the two welding electrodes, the current distribution during welding, welding current, welding time, electrode pressure, and the choice of welding power source. Among these, welding current, welding time, electrode pressure, and the welding power source have the most significant impact on spot welding quality.
Welding Current:
Welding current affects the heating process in two ways. First, adjusting the current's strength can significantly change the heat generated by the welding equipment. Second, during spot welding, there's a concentration of current at the point where the two pieces are being welded, causing localized heating. This uneven heating can affect the quality of the weld. To control this, you can choose different welding current waveforms, change electrode shapes and sizes, and control current density distribution to achieve the desired weld shape and position.
Electrode Pressure:
Electrode pressure affects the total resistance between the two electrodes and, therefore, the amount of heat generated during spot welding. Additionally, it influences the heat dissipation characteristics of the welded joint. If the electrode pressure is too low, it increases resistance and generates more heat, potentially leading to spattering and poor heat dissipation. On the other hand, excessive electrode pressure can reduce the current density, resulting in insufficient heat and smaller welds. Finding the right electrode pressure is crucial for both energy efficiency and welding quality.
Welding Time:
When welding, some of the current's heat goes into forming the weld (effective heat), while the rest is lost to the surrounding metal and air (loss heat). The effective heat remains constant for a given material and weld volume, regardless of the welding time. However, the loss heat increases as welding time prolongs. It's important to strike a balance to avoid excessive heating or cooling during welding.
Welding Power Source:
The welding equipment consists of three main parts: the main power source, control unit, and mechanical components. The main power source is the most critical component, as it provides the necessary control over the welding process. Choosing a suitable and controllable welding power source is essential for ensuring welding quality.
The next article will talk about how to choose resistance spot welding power supply in detail.
NDB technology
NDB (non-defective bonding) does not require any solder due to. The integration of copper, tungsten, and molybdenum ensures a strong connection with a strength exceeding 130Mpa, without any internal holes.
oxidation resistance
(2) The welding electrode materials is better to have higher oxidation resistance, less tendency to form alloys with weldment materials
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Yolo Materials IndustryContinuing from the previous article, let’s talk about the process of resistance spot welding.
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