The variation pattern of resistance in the welding zone during the process of medium frequency spot welding is a fundamental theoretical issue in resistance welding. After years of research, the variation patterns of various constituent resistances in resistance welding in cold and hot states have been determined, along with their relationships with factors such as surface condition, electrode force, welding current, etc.
It has further been confirmed that dynamic resistance, which considers voltage and current factors, has a close relationship with the size of the fusion core. A relatively simple voltage monitoring method can better express the state of fusion core growth. Over the past 20 years, people have utilized this fundamental theory, especially the relationship between dynamic resistance and fusion core generation, to develop welding quality monitoring technology, becoming an effective method for spot welding quality monitoring. Currently, dynamic resistance monitoring systems have been widely used in production both domestically and internationally.
Principle of Dynamic Resistance:
The resistance change curve (i.e., dynamic resistance curve) can be obtained through experiments. When metal materials are spot welded, their dynamic resistance curve characteristics can be observed.
In engineering technology, instantaneous values can be treated as follows: taking the welding current as a unit of half-cycle, the variables within half a cycle are considered constant. Or, certain characteristic values of the variables within half a cycle are extracted. Thus, the peak voltage between the two electrodes is defined as the effective current value within half a cycle, and the resistance reached when the peak is achieved is defined as the resistance within half a cycle. This definition effectively excludes the influence of additional electromagnetic fields by taking the peak voltage and includes the heat factor by taking the effective current value. Hence, there is a close relationship between dynamic resistance and fusion core size.
According to the aforementioned resistance curve, its characteristic is that at the beginning of spot welding, the resistance rapidly decreases as the contact resistance disappears. Subsequently, the resistance remains almost unchanged, forming a horizontal line. This segment of the resistance curve does not change with variations in fusion core size. Therefore, materials with such characteristic curves are not suitable for using electronic monitoring methods to detect the quality of welds. Monitoring spot welding quality based on the characteristics of the first type of dynamic resistance curve mainly takes two forms: tracking resistance curve method and resistance change or resistance change rate method.
The tracking resistance curve method utilizes microprocessors and peripheral circuits to first store the dynamic resistance curves of qualified welds or the resistance function relationship determined through experiments. Then, for each weld and each half cycle subsequently welded, the welding current is calculated and adjusted to force the dynamic resistance of the weld in the process of formation to follow the dynamic resistance curve of the qualified welds or the determined resistance function relationship, thereby ensuring the quality of each weld.
This method requires the reception of welding current and peak voltage between the electrodes for each half cycle, and the resistance value for that half cycle should be calculated. It should also be compared with the stored dynamic curve. When a deviation occurs, the welding current should be adjusted in the subsequent half cycle to ensure that the resistance of the weld consistently tracks the dynamic resistance curve of the qualified welds. This method is technically challenging, but with the rapid and accurate calculation capabilities of computers or microprocessors, automatic control can be achieved.
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Post time: Mar-29-2024