Qu'est-ce que le soudage et quelles techniques sont utilisées ?

This article explains what welding is, how the three most common arc welding techniques differ, the advantages and disadvantages of a welded joint, and how to clean steel before and after welding.

What is welding?

Welding is a joining technique used to make two materials into one. In the most common fusion welding processes, the joint is heated until a small portion of the material melts and fuses, usually with a filler metal added; other processes join materials with pressure, or with a combination of heat and pressure. When compatible materials are welded, continuity is created between the two parts. Welding is mainly used for steel, but it is also applied to stainless steel, aluminum and some plastics. For a fusion weld to succeed, the materials must be fusible and metallurgically compatible.

What are the most common welding techniques?

There are many welding techniques. Which one is best depends on the material, the location and the purpose of the part. To explain the differences, we take a closer look at the three most common arc welding methods.

Stick welding (MMA / SMAW)

Stick welding is also known as manual metal arc (MMA) welding, shielded metal arc welding (SMAW) or covered electrode welding. It is mainly used for carbon steel and stainless steel and is rarely used for aluminum. The welder uses a power source with flux-coated electrodes. The electrode core melts into the joint as filler metal while the flux coating melts to form gas and slag, which protect the weld pool from the surrounding air. The slag is a brittle layer that must be chipped and cleaned off the weld bead once the run is complete. Because the electrode coating provides its own shielding, no separate shielding gas is needed, which makes stick welding well suited to outdoor and site work. It is also relatively inexpensive and portable, which makes it popular for repairs, maintenance and jobs where the appearance of the weld is less important.

TIG welding (GTAW)

Another commonly used method is TIG (Tungsten Inert Gas) welding, also called gas tungsten arc welding. Whereas stick welding is relatively quick and easy to learn, TIG welding is generally regarded as the most demanding technique. The arc is formed by a non-consumable tungsten electrode, and when filler metal is needed it is added by hand: one hand holds the torch, the other feeds the filler rod. This makes TIG welding slower and more skill-intensive. On the other hand, TIG welding performed properly gives a very neat, precise weld. When detailed work has to be done and appearance matters, for example on stainless steel and aluminum, TIG is often the preferred technique. Because no flux is used, TIG welding produces no slag and very little spatter.

MIG/MAG welding (GMAW)

The technique generally considered easiest to learn is MIG/MAG welding. MIG (Metal Inert Gas) and MAG (Metal Active Gas) welding differ mainly in the shielding gas used: an inert gas such as argon, or an active gas or mixture containing carbon dioxide or oxygen. The process uses a constant-voltage power source that maintains an arc between a continuously fed wire and the workpiece. The wire is both the electrode and the filler metal, which makes MIG/MAG welding considerably faster than the other two methods. As a result, it is one of the most widely used techniques in fabrication and production. The gas shield is easily disturbed by wind, so site and outdoor work needs screening; self-shielded flux-cored wires are more wind tolerant. Depending on the transfer mode and gas, MIG/MAG can produce more spatter than TIG welding.

What are the advantages and disadvantages of welding?

Welding is not the only way to join two materials. A bolted connection, for example, is also possible. Nevertheless, a welded joint has several advantages over other ways of joining.

First, a properly made welded joint is very strong. Because there is continuity between both parts, the connection is hardly noticeable and the result behaves as a solid whole. In addition, no holes have to be drilled, as they do for bolts and nuts.

Welding also has disadvantages. A welded joint is permanent and cannot be disassembled. That can be an advantage, but when parts have to be separated later it becomes a drawback. Not every combination of materials can be welded easily: similar metals are the simplest, and while many dissimilar combinations, such as carbon steel to stainless steel, can be welded with the right filler metal and procedure, others are difficult or impractical to fusion weld. The heat also changes the metal next to the weld, the heat-affected zone, and the welding techniques described above leave slag, spatter, oxides and other residue that must be cleaned after welding. MontiPower®’s power tools offer a solution for that cleaning.

How do you clean welds with the Bristle Blaster® and MBX®?

The Bristle Blaster® and MBX® from MontiPower® are hand-held tools for surface preparation and cleaning. They remove rust, coatings and dirt, and are also used for weld cleaning: removing slag, spatter residue and discoloration. The rotating bristles clean thoroughly without grinding away base metal. The two tools have different roles:

• Bristle Blaster® for structural steel, pipelines and plate. It cleans weld seams and the heat-affected zone and creates an anchor profile in the same pass, so the weld area can be coated. In documented tests on standard steel it produces cleanliness comparable to ISO 8501-1 Sa 2½ / SSPC-SP 10 and a 65–85 µm Rz profile; results vary with steel grade, rust grade, belt and technique.
• MBX® for vehicle bodywork and thin sheet, where it removes rust, paint and sealers and dresses weld seams without heat or warping. It is not intended for preparing structural steel to a protective-coating grade. See MBX vs. Bristle Blaster for choosing between them.

Cleaning is not only needed after welding. Before welding begins, mill scale, rust, paint, oil and other contamination should be removed from the joint area, because they can cause porosity and other weld defects. On stainless steel and aluminum, use stainless steel belts so carbon steel particles are not embedded in the surface. Used this way, the power tools provide fast and effective cleaning both before and after welding. For methods and standards in more depth, read weld cleaning and post-weld surface preparation.

Frequently asked questions

What are the three main types of welding?

The three most common arc welding processes are stick welding (MMA/SMAW), which uses flux-coated electrodes; TIG welding (GTAW), which uses a tungsten electrode and inert gas; and MIG/MAG welding (GMAW), which feeds a continuous wire electrode through a gas-shielded torch. Each differs in speed, skill level, materials and the cleaning required afterwards.

Why does stick welding produce slag?

The electrode in stick welding is covered with flux. When it melts, the flux forms gas and slag that shield the molten weld pool from the air. The slag solidifies on top of the weld bead and must be chipped and cleaned off after each run, especially before another pass or a coating is applied.

Can MIG welding be done outdoors?

Yes, but the shielding gas is easily blown away by wind, which causes porosity. On site, the work area needs screening from drafts. Self-shielded flux-cored wires are more tolerant of wind, and stick welding, which needs no separate shielding gas, is often preferred for outdoor work.

Why clean steel before and after welding?

Before welding, mill scale, rust, paint and oil in the joint area can cause porosity and other weld defects. After welding, slag, spatter and oxides must be removed, and the weld and heat-affected zone need cleaning and an anchor profile before coating, because welds are a common starting point for coating failure and corrosion.