CQI-15 x Resistance Welding Process Assessment

CQI15

Ensuring compliance in the resistance welding process has always been a constant pursuit, especially in the medical/hospital, aerospace, and automotive segments due to the critical control characteristics, technical, and safety requirements that must be met. In resistance welding, there are three main parameters that are applied in a welding cycle:

I. Welding current: Welding current can basically be controlled as follows:

a. Placement of a sensor for current control in the secondary of bi-phase machines with AC/60Hz technology;

b. Current control directly from the inverter controller, in machines with MFDC 1,000Hz welding technology;

II. Electrode force: The proportional valve or pressure switch are components of the pneumatic circuit designed to control the direction, pressure, and/or flow rate of compressed air. Working integrally with the welding controller, this component will control the pressure, as specified in the welding controller's programming. This feature ensures maximum efficiency in the welding process because the welding controller will maintain constant current and pressure between the electrodes, anticipating quality welds within the established parameters.

III. Welding time: In complex processes, acceleration and deceleration ramps can be used to increase the reliability level of the welding process;

Another important parameter is cooling control, as not only the electrodes are subject to high temperatures, but also the parts are subject to compressive stresses and may degrade in service. For this reason, temperature and flow control of the cooling system is a success factor for weld quality and preservation of the machine and its deteriorable components.

Flow switches are designed to detect and monitor the flow of liquids in tanks or pipes, and can also be used to control liquid levels. Compact, with no moving parts and resistant to vibrations, they can be installed in any position in the piping. This system ensures proper cooling of the transformer, busbars, and electrodes, preventing overheating beyond specifications. Since temperature control is an important ally in the preservation and maintenance of resistance welding equipment, these controllers will ensure maximum efficiency of the welding machine, without risk of burning out the transformer or electronic components.

Electrical resistance is the principle that results in four different types of welding:

1. Spot Welding: used in welding processes of carbon steel sheets, generally with the use of zinc phosphate as a surface treatment to improve corrosion resistance.

Spot welding involves a coordinated application of electrical current and electrode force of appropriate magnitudes and durations. The welding current must pass through the electrodes and sheets, heating them until the region between the electrodes melts, to form the weld spot. The welding current is then turned off and the weld spot solidifies, forming a solid metallic joint between the sheets (AWS, 1997, INTERMACHINERY,

2003).

• Advantages:
  • In this type of welding, the base metal does not suffer large heat-affected areas.
  • This type of welding has an easy process.
  • Spot welding has a high production rate, can be applied to join different metals, and is a low-cost process.

2. Projection Welding: In the projection welding process, the parts have protrusions (projections), are overlapped, and the electrodes have a larger contact area than the electrodes in the spot welding process. This process allows multiple joints to be made in a single operation. Used to join small stamped parts, usually nuts, bolts, and pins. It is a variation of the spot welding process in which projections are raised on one or both parts to be joined, where the welding energy is concentrated.

In addition to the controls already mentioned, to increase the reliability of this process, it is recommended to use displacement and position control of the upper head, to ensure that the projection was adequately welded and that there was no welding of an inverted, mixed, or out-of-spec part.

3. Seam Welding: Seam welding is a variation of the resistance welding process, where the resulting welds are an overlap of weld spots, progressively made along the joint by rotational electrodes, where the electrodes are in the form of rollers (discs). It is a type of electrical resistance welding in which two sheets pass through rollers, similar to the rolling process. However, these rollers are energized conductive electrodes, which upon receiving electric current, cause the sheets to heat up due to the Joule effect. In this process, another control characteristic should be the movement speed of the welding discs, with the possibility of acceleration and deceleration ramps.

4. Other control parameters and additional features that qualify this process:

• • Electrode life control (wear): This control is important for maintaining welding parameters and the visual characteristic of the welding area, considering that with each welding cycle, cumulative wear will occur that can compromise its reliability. Thus, the equipment must have a feature that can ensure this control;

• • AQL (Acceptable Quality Level) control: Maximum defective percentage (or maximum number of defects or non-conformities) which, for sampling acceptance purposes, can be considered satisfactory as an average of a process or continuous series of lots. To ensure conformity, performance tests (pull-out, tensile, dimensional, torque, and visual) are carried out, according to severity classification. When the welding equipment itself is equipped with this feature, it enhances and guarantees greater robustness to the welding process;

• • Digital control plan: During the welding process quality planning, critical characteristics are identified, and the respective Control Plan for the welding process is prepared. This feature incorporated into the welding equipment provides permanent access for operators to view the respective work instructions defined for the specific welding process;
  • Remote access: To provide a quick response to non-conformities, this feature sends an email to those responsible, with screenshots of events that occurred during the welding process;

All SUKHA equipment, from standard machines to automated projects, complies with CQI15.

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References: AIAG, Automotive Industry Action Group. Special Process: Welding System Assessment, CQI-15. Accessed on 07/16/2021