Welding Processes: Oxyacetylene, Resistance, and TIG Techniques

Posted on Jun 18, 2025 in Electromechanical Engineering

Oxyacetylene Welding

Oxyacetylene welding is a process where the parts being joined and the material input (if used) are of the same nature. Welding is performed by melting the edges of the pieces to be joined using a flame, which occurs from the combustion of acetylene fuel gas mixed with oxygen oxidizer.

Equipment for Oxyacetylene Welding

  • Acetylene: The fuel gas. It should not be compressed at a pressure exceeding 1.5 bar. For bottling, it is dissolved in acetone. The bottle is painted red, and its nozzle is brown.
  • Oxygen: The combustion gas (gas containing the oxidizing substance that reacts with acetylene to cause combustion). Its nozzle is white.
  • Pressure Regulators: Devices installed on the bottle valves. Their aim is to supply gas at a constant pressure. They consist of two gauges: one for high pressure (reading the pressure in the bottle) and one for low pressure (reading the working pressure).

Resistance Welding

Resistance welding is a pressure welding method where the temperature of the parts to be joined is increased to just below the metal’s melting point. The temperature is generated by passing current between two electrodes.

Resistance Welding Sequence

In the process of circular resistance welding, current flows between the electrodes following these steps:

  1. Closer Period: The time lag between the electrodes’ approximation and the start of current flow.
  2. Welding Period: The duration of current passage.
  3. Forging Period: The time lag between the current cut-off and the lifting of the electrodes.
  4. Cooling Period: The time after the separation of the electrodes.

Resistance Welding Components

  • Power Source: An AC transformer where the electrodes are connected to the secondary winding.
  • Control Panel: Consisting of selectors that regulate different welding parameters.
  • Electrode Holders: Responsible for holding the electrodes.
  • Electrodes: Good conductors of current and temperature, with high mechanical resistance.

Resistance Welding Parameters

  • Welding Time: The period during which current flows between the electrodes and the parts to be joined.
  • Welding Pressure: Pressure exerted by the electrodes, which should be regulated according to the thickness and nature of the materials being joined.
  • Welding Current: The strength needed depends on the parts being joined.

TIG Welding (GTAW)

TIG (Tungsten Inert Gas) welding, also known as Gas Tungsten Arc Welding (GTAW), uses a non-consumable tungsten electrode. The electric arc is established between the electrode and the workpiece, protected by an inert gas (argon, helium, or a mixture).

TIG Welding Equipment

  • Power Source: Composed of:
    • Constant Current Transformer: Supplies constant current for welding. Power to the electrodes must be DC for alloy steels and AC for aluminum.
    • Rectifier: Supplies the necessary DC current.
    • HF Generator: Facilitates arc initiation.
    • Timer: Allows gas flow for a few seconds before and after cutting the arc.
    • Current Reduction Device: Gradually reduces the current.
  • Torch/Electrode Holder: Its function is to hold the electrode, supply current, and ensure gas outlet.

TIG Welding Polarity

  • Direct Current Electrode Negative (DCEN) / Straight Polarity: The negative pole is connected to the electrode, and the positive pole to the workpiece.
  • Direct Current Electrode Positive (DCEP) / Reverse Polarity: The positive pole is connected to the electrode, and the negative pole to the workpiece.
  • Alternating Current (AC): The machine behaves as a mixture of the two polarities, as each half-cycle behaves as DC of each polarity, taking advantage of the good penetration of direct current and the cleaning effect of reverse current.

Key TIG Welding Parameters

  • Type: DC or AC, depending on the material to be welded.
  • Weld Current: Determined by the thickness of the pieces to be welded.
  • Gas Flow: Regulated according to the parts to be welded.
  • Downslope Time: The time regulated so that when the torch switch is deactivated, the current continues flowing through the electrode but decreases in intensity, preventing crater formation in the weld bead.
  • Post-Flow Time: The time gas continues to flow from the torch after welding to cool the electrode and prevent contamination.