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Arc-Welding

Introduction

Arc welding is the fusion of two pieces of metal by an electric arc between the pieces being joined – the work pieces – and an electrode that is guided along the joint between the pieces. The electrode is either a rod that simply carries current between the tip and the work, or a rod or wire that melts and supplies filler metal to the joint.

The basic arc welding circuit is an alternating current (AC) or direct current (DC) power source connected by a “work” cable to the work piece and by a “hot” cable to an electrode. When the electrode is positioned close to the work piece, an arc is created across the gap between the metal and the hot cable electrode. An ionized column of gas develops to complete the circuit.

Basic Welding Circuit

basic welding circuit

The arc produces a temperature of about 3600°C at the tip and melts part of the metal being welded and part of the electrode. This produces a pool of molten metal that cools and solidifies behind the electrode as it is moved along the joint.

There are two types of electrodes. Consumable electrode tips melt, and molten metal droplets detach and mix into the weld pool. Non-consumable electrodes do not melt. Instead, filler metal is melted into the joint from a separate rod or wire.

The strength of the weld is reduced when metals at high temperatures react with oxygen and nitrogen in the air to form oxides and nitrides. Most arc welding processes minimize contact between the molten metal and the air with a shield of gas, vapour or slag. Granular flux, for example, adds deoxidizers that create a shield to protect the molten pool, thus improving the weld.

Advances in Welding Power Source Design and Efficiency

The electricity-consuming device – the key component of the arc welding apparatus – is the power source. Electrical consumption from the approximately 110 000 to 130 000 arc welding machines in use in Canada is estimated at 100 GWh a year.

In the past, power sources used transformer-rectifier equipment with large step-down transformers that made them heavy and prone to overheating. They can be used for only one function, i.e., one type of welding. In the 1990s, advances in power switching semiconductors led to the development of inverter power sources that are multi-functional, lighter, more flexible and that provide a superior arc.

Welding power sources use electricity when welding (arc-on) and when idling. Earlier transformer-rectifier equipment had energy conversion efficiencies that ranged from 40 to 60 percent and required idling power consumption of 2 to 5 kW. Modern inverter power sources have energy conversion efficiencies near 90 percent, with idling power consumption in the order of 0.1 kW.

Modern inverter power sources are gradually replacing transformer-rectifier units. They combine a quick return on investment, and, compared with transformer-rectifier units, are far more portable and easier to operate, are multi-functional rather than mono-functional, create superior arcs and combine higher-quality welds with longer arc-on time.

The Five Most Common Arc Welding Processes

Process Known as Electrodes Shielding Operator skill required Popularity
Shielded metal arc welding SMAW or stick Rigid metal Stick coatings Low Diminishing
Gas metal arc welding GMAW or MIG Solid wire CO2 gas Low Growing
Flux core arc welding FCAW or MIG Hollow wire Core materials Low Growing
Gas tungsten arc welding GTAW or TIG Tungsten Argon gas High Steady
Submerged arc welding SAW Solid wire Argon gas High Steady

GTAW is used chiefly for thin and exotic metals, and SAW is used for thick steel plate.

Power sources produce DC with the electrode either positive or negative, or AC. The choice of current and polarity depends on the process, the type of electrode, the arc atmosphere and the metal being welded.

Energy Efficiency of the Power Source

  • Modern inverter power sources have high energy-conversion efficiencies and can be 50 percent more efficient than transformer-rectifier power sources.
  • Modern inverter power sources for idling power requirements are 1/20th of conventional transformer-rectifier power sources.
  • Modern inverter power sources have power factors that are close to 100 percent; transformer-rectifier power source percentages are much lower, which reduces electricity consumption.
  • Modern inverter power sources are four times lighter and much smaller than transformer-rectifier power sources. They are thus more portable and can be moved by one person instead of four, making it possible to bring the welding equipment to the job, not vice versa.
  • Modern inverter power sources are multi-functional and can be used for GMAW, FCAW, SMAW and GTAW.

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