Welding Processes Chapter 8

Chapter 8: Joining Processes: Fusion Welding

• Fusion welding is the process of melting and coalescing materials using heat.
• Filler metals are often added to the weld area during the process.
• Autogenous welds are created without filler metals.

Chapter Outline

• Introduction to fusion welding
• Oxyfuel gas welding
• Arc welding processes (nonconsumable electrodes)
• Arc welding processes (consumable electrodes)
• Electrodes for arc welding
• Weld joint, quality, and testing

Introduction

• Fusion welding utilizes heat to join materials.
• Filler metals can be used to augment the joint during welding.
• Welds made without filler materials are termed autogenous welds.

Oxyfuel-gas Welding

• Oxyfuel welding (OFW) is a general term for any welding process using fuel and oxygen to create a flame for heating.
• Oxyacetylene welding (OAW) is a common gas welding process using acetylene and oxygen.
• OAW is typically used in structural metal fabrication and repair work.

Oxyacetylene Welding

• Acetylene and oxygen forms a flame to melt materials at the joint.
• The mixture of gases produces a flame that can be adjusted to be neutral, oxidizing, or reducing.

Acetylene (C₂H₂)

• Acetylene is a commonly used fuel gas in oxyfuel welding due to high temperatures it produces.
• Acetylene reacts with oxygen in a two-stage process producing high temperatures up to 3300°C (6300°F).
• The first stage creates an inner cone, with the second stage creating an outer envelope.

Types of Flames

• Different ratios of acetylene to oxygen produce different flames.
• A 1:1 ratio produces a neutral flame.
• More oxygen produces an oxidizing flame (harmful for some metals).
• Insufficient oxygen results in a reducing/carburizing flame potentially affecting the metal.

Filler Metals

• Filler metals are added to the weld zone during welding in many applications.
• These metals are available in rods or wire form.
• Filler metals can be bare or coated with flux.
• The flux helps retard oxidation of welded parts.
• Slag forms during welding and covers the molten metal, acting as a shield.

Arc Welding Processes

• Welding using electrical energy, to produce an arc between the tip of an electrode and the workpiece causing heat.
• The process often uses alternating current (AC) or direct current (DC).
• The temperature attained is much higher than produced by oxyfuel gas welding.

Arc Welding Processes: Nonconsumable Electrode

• Nonconsumable electrodes, often tungsten (W) electrodes.
• Externally supplied shielding gas is needed to prevent oxidation of the weld zone.

Current and Polarity

• Direct current is often used for welding.
• Straight polarity (DCEN) - workpiece positive, electrode negative.
• Reverse polarity (DCEP) - workpiece negative, electrode positive.
• AC current allows for rapid pulsing of the arc making welding suitable for thick sections needing larger diameter electrodes.

Gas Tungsten Arc Welding (GTAW)

• GTAW, or TIG welding, uses a nonconsumable tungsten electrode.
• Filler metal can be supplied via filler wire.
• A stable arc is maintained by constant current.
• Argon or helium-based shielding gas prevents oxidation.
• Filler metal can be omitted for some applications.

Plasma Arc Welding

• Plasma arcs are concentrated and produce concentrated heat for welding.
• Reaching temperatures up to approximately 33,000°C.
• The plasma is an ionized hot gas.
• Plasma is initiated between a tungsten electrode and orifice by low current pilot arc.
• A filler metal can be used.
• Shielding gas is used (e.g. argon, helium or mixtures).

Two methods of Plasma Arc Welding

• Transferred arc method-workpiece is in the circuit, when the arc moves from the electrode to the workpiece.
• Non-transferred arc method - the arc occurs between the electrode and the nozzle, with the heat transmitted to the workpiece by the plasma gas.

Arc Welding Processes: Consumable Electrode

• Shielded Metal Arc Welding (SMAW)
• Coated electrode supplies the heat for welding and may be in long thin rod form.

Submerged Arc Welding (SAW)

• Weld arc is shielded by granular flux.
• Flux is fed into the weld zone from a hopper.
• Flux prevents spatter and sparks, and suppresses ultraviolet radiation.
• Flux acts as a thermal insulator promoting deep penetration of heat on to the workpiece.

Gas Metal Arc Welding (GMAW)

• GMAW, sometimes referred to as MIG, uses consumable bare wire electrodes.
• The weld area is shielded by inert gases like argon, helium or carbon dioxide.
• Deoxidizers are often present in the electrode metal to prevent oxidation.
• Multiple weld layers can be applied at a single joint.
• Different transfer techniques are possible in GMAW.

Flux-Cored Arc Welding (FCAW)

• FCAW, similar to GMAW except that the electrode is filled with flux.
• Cored electrodes create a more stable arc.
• Produce better weld contour.
• Mechanical properties of the weld are improved.
• Flux is more flexible than SMAW electrodes.

The Weld Joint, Quality, and Testing

• Welds have three distinct zones
• Base metal
• Heat affected zone
• Weld metal (fusion zone).

Solidification of Weld Metal

• Weld metal solidification forms columnar (dendritic) grains in the fusion zone.
• The structure has low strength, toughness and ductility compared to the base structural metal.

Weld Quality – Discontinuities/Imperfections

• Porosity/spheres, caused by gases trapped during welding.
• Slag inclusions, caused by contaminants from the environment or ineffective shielding gas. • Prevented by proper electrode selection, welding techniques, and ensuring ample space for the molten weld puddle, which allows for gas escape.
• Incomplete fusion, caused by inadequate heat input or inappropriate welding techniques.
• Incomplete penetration, caused by inadequate heat input or inappropriate travel speed.
• Prevention by modifying the joint designs, and ensuring that the welding surfaces are clean and fit.
• Weld profile, poor selection of materials or welding practise.
• Prevention by modifying the joint designs and ensuring the surfaces to be welded fit each other properly.

Cracks

• Temperature and composition variations along with thermal stress changes contribute to cracking.
• Prevent by modification of welding operations and procedures, including preheating the welding parts, controlling the cooling speed.

Residual Stress and Distortion

• Localized heating and cooling during welding can cause expansion and contraction, which leads to stresses in the workpiece.
• Potential defects from residual stress include distortion, warping, and buckling of welded parts.

Testing of Welds – Destructive Testing

• Tension , bend, shear, fracture, toughness, creep, and corrosion tests are examples of destructive tests used to assess weld strength and integrity.

Testing of Welds – Nondestructive Testing

• Non-destructive techniques like visual inspection, radiography (X-rays), magnetic particle, liquid penetrant, and ultrasonic testing to evaluate welds for defects without causing damage.