Welding

Joining Processes & Equipment

Chapter 30Fusion Welding Processes

Fusion Welding Processes

Oxyacetylene Flame TypesFigure 30.1 Three basic types of oxyacetylene flames used in oxyfuel-gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing, flame. The gas mixture in (a) is basically equal volumes of oxygen and acetylene. (d) The principle of the oxyfuel-gas welding operation.

Oxyacetylene TorchFigure 30.2 (a) General view of and (b) cross-section of a torch used in oxyacetylene welding. The acetylene valve is opened first; the gas is lit with a spark lighter or a pilot light; then the oxygen valve is opened and the flame adjusted. (c) Basic equipment used in oxyfuel-gas welding. To ensure correct connections, all threads on acetylene fittings are left-handed, whereas those for oxygen are right-handed. Oxygen regulators are usually painted green, and acetylene regulators red.

Pressure-Gas Welding ProcessFigure 30.3 Schematic illustration of the pressure-gas welding process; (a) before, and (b) after. Note the formation of a flash at the joint, which can later be trimmed off.

Arc-welding ProcessesThe process involves:A consumable electrode.A non consumable electrode.An AC or a DC power supply produces an arc between the tip of the electrode and the workpiece to be welded.The arc generates temperatures of about 30,000C,

Non consumable-electrode

Non consumable-electrodethe electrode is typically a tungsten electrode.An externally supplied shielding gas is necessary to prevent oxidation of the weld zone.Typically, direct current is used.Its polarity (the direction of current flow) is important.

polarityStraight polarity:

Also known as direct-current electrode negative (DCEN).The workpiece is positive (anode), and the electrode is negative (cathode).DCEN generally produces welds that are narrow and deep.Reverse polarity:

Also known as direct-current electrode positive (DCEP).The workpiece is negative and the electrode is positive.Weld penetration is less, and the weld zone is shallower and wider.

Shielded-Metal Arc WeldingFigure 30.7 Schematic illustration of the shielded metal-arc welding process. About 50% of all large-scale industrial welding operations use this process.Figure 30.8 A deep weld showing the buildup sequence of eight individual weld beads.

Heat Transfer in Arc WeldingThe heat input in arc Welding is given by the equation.

Where H is the heat input (] or BTU), I is the Weld length,V is the voltage applied, I is the current (amperes),v is the welding speed. The term e is the efficiency of the process and varies from around 75% for shielded metal-arc welding to 90% for gas metal-arc welding and submerged-arc Welding.

The heat input given by Eq. (30.3) melts a certain volume of material, usually the electrode or filler metal, and can also be expressed as

where u is the specific energy required for melting, V," is the volume of material melted, A is the cross section of the Weld

Electrode Designations

Electron Beam WeldingIn electron-beam welding (EBW), developed in the 1960s, heat is generated by high velocity narrow-beam electrons. The kinetic energy of the electrons is converted into heat as they strike the workpiece. The process requires special equipment to focus the beam on the workpiece, typically in a vacuum. The higher the vacuum, the more the beam penetrates, and the greater is the depth-to-width ratio; thus, the methods are called EBWEBW-HV (for high vacuum) and EBW-MV (for medium vacuum); some materials also may also be welded by EBW-NV (for no vacuum).

Welded JointsFigure 30.27 Examples of welded joints and their terminology.

Weld SymbolsFigure 30.28 Standard identification and symbols for welds.

Weld DesignFigure 30.29 Some design guidelines for welds. Source: After J.G. Bralla.

Weld Quality

Porosity(the state of being porous).Slag inclusions.Incomplete Fusion and PenetrationWeld ProfileCracks

Cold WeldingIn cold welding (CW), pressure is applied to the workpieces through dies or rolls.During the joining of two dissimilar metals that are mutually soluble.Brittle intermetallic compounds may form (Section 4.2.2); these will produce a weak and brittle joint.

Friction WeldingFigure 31.3 (a) Sequence of operations in the friction welding process: (1) Left-hand component is rotated at high speed. (2) Right-hand component is brought into contact under an axial force. (3) Axial force is increased; flash begins to form. (4) Left-hand component stops rotating; weld is completed. The flash can subsequently be removed by machining or grinding.Figure 31.4 Shape of the fusion zones in friction welding, as a function of the axial force applied and the rotational speed.

Friction Stir WeldingFigure 31.4 The principle of the friction stir welding process. Alluminum-alloy plates up to 75 mm (3 in.) thick have been welded by this process.

Spot WeldingFigure 31.6 (a) Sequence of events in resistance spot welding. (b) Cross-section of a spot weld, showing the weld nugget and the indentation of the electrode on the sheet surfaces. This is one of the most commonly used processes in sheet-metal fabrication and in automotive-body assembly.