How does welding join two pieces of metal?

Welding joins metals by creating a strong bond through heat or pressure, sometimes with a filler material. In fusion welding, heat melts the metals to form a molten pool that cools and solidifies, fusing the parts together. Electric arc or gas flames supply the heat. Solid-phase welding uses pressure, often with heat, to create a bond without melting the base material. The process involves three zones: the weld metal (fusion zone), the heat-affected zone, and the unaffected zone. The weld metal's composition and solidification conditions impact the joint's strength. Various welding methods exist, each with specific uses and techniques. These methods have become essential in modern metal fabrication, offering advantages over older joining methods like riveting.

What happens to the metal when it is heated during welding?

When metal is heated during welding, it undergoes several changes that are critical to the quality of the joint. Most metals react with the atmosphere, leading to rapid oxidation when molten, which can prevent proper bonding and make the joint brittle. To counter this, fluxes and inert atmospheres are used to protect the metal from oxidation. The welding arc creates three distinct zones: the weld metal (fusion zone), the heat-affected zone, and the unaffected zone. The weld metal melts and solidifies, forming a cast structure, while the heat-affected zone experiences microstructural and mechanical property changes due to the welding heat. Welding also induces stresses in the material due to the contraction of the weld metal and the expansion/contraction of the heat-affected zone; these stresses, if not controlled, can cause distortion.

Why do different metals react differently when welded?

Different metals exhibit varying reactions during welding due to their unique physical and chemical properties. These properties affect how metals respond to heat, their affinity for other elements, and their overall weldability . When heated, most metals react with the atmosphere, potentially causing issues like oxidation that weakens the joint. To counter this, welders use fluxes and inert atmospheres to shield the molten metal from the environment. The base metal's structure also plays a crucial role; it can be unaffected, hardened by structural changes, or hardened by precipitation processes during welding. For example, carbon content affects the weldability of carbon steels; aluminum needs fluxes because of its oxide film. The resulting stresses from heating and cooling can cause residual stress, potentially distorting the weldment if not managed through proper techniques and heat treatment.

What challenges can happen in the welded area as it cools?

As the welded area cools, several challenges can arise due to the nature of the welding process. The contraction of the weld metal and the expansion and subsequent contraction of the heat-affected zone induce stresses in the materials. The unheated metal restrains this contraction, leading to a buildup of stress within the joint known as residual stress. If uncontrolled, residual stress can cause bowing or distortion of the weldment. Heat treatment of the entire fabrication is required for some critical applications to remove residual stress. Welding technique, jigs and fixtures, fabrication procedures, and final heat treatment are used to control it. Rapid cooling of the weld metal occurs after the arc passes, resulting in a cast structure with columnar grains extending from the edge of the molten pool to the center of the weld.

How do metallurgists make sure welds are strong and safe?

Metallurgists ensure the strength and safety of welds through careful control and testing. They manage the welding process by selecting appropriate filler materials and using fluxes or inert atmospheres to prevent oxidation. The heat-affected zone is also closely monitored to control structural changes related to temperature exposure and cooling rates. Post-welding, techniques such as heat treatment relieve residual stress, which can cause distortion. Non-destructive testing, including ultrasonic echolocation, is employed to detect flaws, cracks, or corrosion, ensuring structural integrity. These methods help confirm that welds meet required standards.

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Welder in a blue uniform and protective helmet using shielded metal arc welding to join metal pieces, with bright sparks and smoke visible at the welding point.

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welding

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Nov. 4, 2026 •History

Top Questions

What is welding?
Welding is a technique for joining metallic parts by heating them to a suitable temperature, with or without pressure or a filler material. In fusion welding, a heat source, such as electricity or gas, creates a molten pool of metal. Solid-phase processes weld without melting the base material, using pressure and sometimes heat.

The electric arc used in welding is a high-current, low-voltage discharge. Fluxes and inert atmospheres protect the joint from oxidation. The metallurgy of metal joining impacts the joint's functional capabilities and creates three zones: the weld metal, the heat-affected zone, and the unaffected zone. Welding produces stresses in materials that can be controlled through technique, jigs, and heat treatment.
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1. welding: Thermochemical processes
2. metallurgy: Welding
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How does welding join two pieces of metal?
Welding joins metals by creating a strong bond through heat or pressure, sometimes with a filler material. In fusion welding, heat melts the metals to form a molten pool that cools and solidifies, fusing the parts together. Electric arc or gas flames supply the heat.

Solid-phase welding uses pressure, often with heat, to create a bond without melting the base material. The process involves three zones: the weld metal (fusion zone), the heat-affected zone, and the unaffected zone. The weld metal's composition and solidification conditions impact the joint's strength.

Various welding methods exist, each with specific uses and techniques. These methods have become essential in modern metal fabrication, offering advantages over older joining methods like riveting.
2 Britannica Sources
1. welding: Thermochemical processes
2. metallurgy: Welding
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What happens to the metal when it is heated during welding?
When metal is heated during welding, it undergoes several changes that are critical to the quality of the joint. Most metals react with the atmosphere, leading to rapid oxidation when molten, which can prevent proper bonding and make the joint brittle. To counter this, fluxes and inert atmospheres are used to protect the metal from oxidation.

The welding arc creates three distinct zones: the weld metal (fusion zone), the heat-affected zone, and the unaffected zone. The weld metal melts and solidifies, forming a cast structure, while the heat-affected zone experiences microstructural and mechanical property changes due to the welding heat. Welding also induces stresses in the material due to the contraction of the weld metal and the expansion/contraction of the heat-affected zone; these stresses, if not controlled, can cause distortion.
1 Britannica Source
1. welding: Basic principles of welding
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Why do different metals react differently when welded?
Different metals exhibit varying reactions during welding due to their unique physical and chemical properties. These properties affect how metals respond to heat, their affinity for other elements, and their overall weldability.

When heated, most metals react with the atmosphere, potentially causing issues like oxidation that weakens the joint. To counter this, welders use fluxes and inert atmospheres to shield the molten metal from the environment.

The base metal's structure also plays a crucial role; it can be unaffected, hardened by structural changes, or hardened by precipitation processes during welding. For example, carbon content affects the weldability of carbon steels; aluminum needs fluxes because of its oxide film. The resulting stresses from heating and cooling can cause residual stress, potentially distorting the weldment if not managed through proper techniques and heat treatment.
1 Britannica Source
1. welding: Basic principles of welding
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What challenges can happen in the welded area as it cools?
As the welded area cools, several challenges can arise due to the nature of the welding process. The contraction of the weld metal and the expansion and subsequent contraction of the heat-affected zone induce stresses in the materials. The unheated metal restrains this contraction, leading to a buildup of stress within the joint known as residual stress. If uncontrolled, residual stress can cause bowing or distortion of the weldment.

Heat treatment of the entire fabrication is required for some critical applications to remove residual stress. Welding technique, jigs and fixtures, fabrication procedures, and final heat treatment are used to control it. Rapid cooling of the weld metal occurs after the arc passes, resulting in a cast structure with columnar grains extending from the edge of the molten pool to the center of the weld.
1 Britannica Source
1. welding: Basic principles of welding
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do metallurgists make sure welds are strong and safe?
Metallurgists ensure the strength and safety of welds through careful control and testing. They manage the welding process by selecting appropriate filler materials and using fluxes or inert atmospheres to prevent oxidation. The heat-affected zone is also closely monitored to control structural changes related to temperature exposure and cooling rates.

Post-welding, techniques such as heat treatment relieve residual stress, which can cause distortion. Non-destructive testing, including ultrasonic echolocation, is employed to detect flaws, cracks, or corrosion, ensuring structural integrity. These methods help confirm that welds meet required standards.
4 Britannica Sources
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welding, technique used for joining metallic parts usually through the application of heat. This technique was discovered during efforts to manipulate iron into useful shapes. Welded blades were developed in the 1st millennium ce, the most famous being those produced by Arab armourers at Damascus, Syria. The process of carburization of iron to produce hard steel was known at this time, but the resultant steel was very brittle. The welding technique—which involved interlayering relatively soft and tough iron with high-carbon material, followed by hammer forging—produced a strong, tough blade.

In modern times the improvement in iron-making techniques, especially the introduction of cast iron, restricted welding to the blacksmith and the jeweler. Other joining techniques, such as fastening by bolts or rivets, were widely applied to new products, from bridges and railway engines to kitchen utensils.

Modern fusion welding processes are an outgrowth of the need to obtain a continuous joint on large steel plates. Rivetting had been shown to have disadvantages, especially for an enclosed container such as a boiler. Gas welding, arc welding, and resistance welding all appeared at the end of the 19th century. The first real attempt to adopt welding processes on a wide scale was made during World War I. By 1916 the oxyacetylene process was well developed, and the welding techniques employed then are still used. The main improvements since then have been in equipment and safety. Arc welding, using a consumable electrode, was also introduced in this period, but the bare wires initially used produced brittle welds. A solution was found by wrapping the bare wire with asbestos and an entwined aluminum wire. The modern electrode, introduced in 1907, consists of a bare wire with a complex coating of minerals and metals. Arc welding was not universally used until World War II, when the urgent need for rapid means of construction for shipping, power plants, transportation, and structures spurred the necessary development work.

Resistance welding, invented in 1877 by Elihu Thomson, was accepted long before arc welding for spot and seam joining of sheet. Butt welding for chain making and joining bars and rods was developed during the 1920s. In the 1940s the tungsten-inert gas process, using a nonconsumable tungsten electrode to perform fusion welds, was introduced. In 1948 a new gas-shielded process utilized a wire electrode that was consumed in the weld. More recently, electron-beam welding, laser welding, and several solid-phase processes such as diffusion bonding, friction welding, and ultrasonic joining have been developed.

Basic principles of welding

A weld can be defined as a coalescence of metals produced by heating to a suitable temperature with or without the application of pressure, and with or without the use of a filler material.

In fusion welding a heat source generates sufficient heat to create and maintain a molten pool of metal of the required size. The heat may be supplied by electricity or by a gas flame. Electric resistance welding can be considered fusion welding because some molten metal is formed.

Solid-phase processes produce welds without melting the base material and without the addition of a filler metal. Pressure is always employed, and generally some heat is provided. Frictional heat is developed in ultrasonic and friction joining, and furnace heating is usually employed in diffusion bonding.

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The electric arc used in welding is a high-current, low-voltage discharge generally in the range 10–2,000 amperes at 10–50 volts. An arc column is complex but, broadly speaking, consists of a cathode that emits electrons, a gas plasma for current conduction, and an anode region that becomes comparatively hotter than the cathode due to electron bombardment. A direct current (DC) arc is usually used, but alternating current (AC) arcs can be employed.

Total energy input in all welding processes exceeds that which is required to produce a joint, because not all the heat generated can be effectively utilized. Efficiencies vary from 60 to 90 percent, depending on the process; some special processes deviate widely from this figure. Heat is lost by conduction through the base metal and by radiation to the surroundings.

Most metals, when heated, react with the atmosphere or other nearby metals. These reactions can be extremely detrimental to the properties of a welded joint. Most metals, for example, rapidly oxidize when molten. A layer of oxide can prevent proper bonding of the metal. Molten-metal droplets coated with oxide become entrapped in the weld and make the joint brittle. Some valuable materials added for specific properties react so quickly on exposure to the air that the metal deposited does not have the same composition as it had initially. These problems have led to the use of fluxes and inert atmospheres.

In fusion welding the flux has a protective role in facilitating a controlled reaction of the metal and then preventing oxidation by forming a blanket over the molten material. Fluxes can be active and help in the process or inactive and simply protect the surfaces during joining.

Inert atmospheres play a protective role similar to that of fluxes. In gas-shielded metal-arc and gas-shielded tungsten-arc welding an inert gas—usually argon—flows from an annulus surrounding the torch in a continuous stream, displacing the air from around the arc. The gas does not chemically react with the metal but simply protects it from contact with the oxygen in the air.

The metallurgy of metal joining is important to the functional capabilities of the joint. The arc weld illustrates all the basic features of a joint. Three zones result from the passage of a welding arc: (1) the weld metal, or fusion zone, (2) the heat-affected zone, and (3) the unaffected zone. The weld metal is that portion of the joint that has been melted during welding. The heat-affected zone is a region adjacent to the weld metal that has not been welded but has undergone a change in microstructure or mechanical properties due to the heat of welding. The unaffected material is that which was not heated sufficiently to alter its properties.

Weld-metal composition and the conditions under which it freezes (solidifies) significantly affect the ability of the joint to meet service requirements. In arc welding, the weld metal comprises filler material plus the base metal that has melted. After the arc passes, rapid cooling of the weld metal occurs. A one-pass weld has a cast structure with columnar grains extending from the edge of the molten pool to the centre of the weld. In a multipass weld, this cast structure may be modified, depending on the particular metal that is being welded.

The base metal adjacent to the weld, or the heat-affected zone, is subjected to a range of temperature cycles, and its change in structure is directly related to the peak temperature at any given point, the time of exposure, and the cooling rates. The types of base metal are too numerous to discuss here, but they can be grouped in three classes: (1) materials unaffected by welding heat, (2) materials hardened by structural change, (3) materials hardened by precipitation processes.

Welding produces stresses in materials. These forces are induced by contraction of the weld metal and by expansion and then contraction of the heat-affected zone. The unheated metal imposes a restraint on the above, and as contraction predominates, the weld metal cannot contract freely, and a stress is built up in the joint. This is generally known as residual stress, and for some critical applications must be removed by heat treatment of the whole fabrication. Residual stress is unavoidable in all welded structures, and if it is not controlled bowing or distortion of the weldment will take place. Control is exercised by welding technique, jigs and fixtures, fabrication procedures, and final heat treatment.

There are a wide variety of welding processes. Several of the most important are discussed below.

welding

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Basic principles of welding