Filler metal is any material melted into a joint to bond two or more pieces of metal together. It fills the gap between the workpieces, solidifies, and creates a permanent connection. Filler metals come in many forms, from thin wire on a spool to flux-coated rods to powder pastes, and choosing the right one is one of the most important decisions in any welding or brazing project.
How Filler Metal Works
During welding, intense heat melts both the filler metal and the edges of the base metal (the pieces you’re joining). The molten filler blends with the base metal, and as everything cools, it fuses into a single solid joint. In brazing, the process is different: only the filler metal melts, not the base metal. The molten filler flows into the narrow gap between parts through capillary action, the same force that pulls water into a paper towel. Brazing filler metals work at temperatures above 840°F (450°C), while arc welding reaches roughly 10,000°F.
Soldering works on the same capillary principle but at even lower temperatures and produces weaker joints. The key distinction is that welding melts the base metal, brazing melts only the filler at moderate heat, and soldering melts the filler at low heat.
Physical Forms for Different Processes
The welding process you use determines the physical form of filler metal you need. Each form is designed to feed into the joint efficiently for a specific type of equipment.
- Spooled wire is used in MIG welding (also called GMAW). Solid wire comes on 33-pound spools in diameters like .035″ and .045″, and the welding machine feeds it continuously through the gun. This makes MIG welding fast and relatively easy to learn.
- Flux-cored wire looks similar to solid MIG wire but has a hollow core filled with flux, a compound that shields the molten metal from contamination. It also comes on spools or in larger 60-pound coils and 220- to 250-pound drums for high-volume work.
- Coated electrodes (stick rods) are straight rods coated in a layer of flux. They range from 3/32″ to 3/8″ in diameter and 9″ to 24″ in length. The flux coating melts during welding to protect the joint and stabilize the arc. This is the classic “stick welding” setup.
- Bare TIG rods are straight, uncoated rods, typically 36 inches long, that you hand-feed into the weld puddle during TIG welding. A separate shielding gas protects the joint instead of flux.
- Brazing alloys come as thin strips, rings, paste, or powder placed between parts before heating.
Common Filler Metal Alloys
Filler metals are manufactured to match virtually every base metal you might need to join. The most widely used categories cover carbon steel, stainless steel, aluminum, nickel alloys, copper alloys, and even titanium and magnesium alloys.
A few examples you’ll see referenced constantly: ER70S-6 is one of the most popular mild steel MIG wires. The “70” indicates 70,000 psi tensile strength, and the “S-6” tells you it’s a solid wire with a specific chemical composition. E71T-11 is a common flux-cored wire for carbon steel that can run without external shielding gas. For aluminum, 5056 wire is frequently used, and 304 stainless steel wire is a go-to for stainless applications. The 4130 wire handles chromium-molybdenum steel, common in aerospace and motorsport tubing.
Matching Filler Metal to Base Metal
The general rule is that your filler metal’s minimum tensile strength should be equal to or greater than the tensile strength of the base metal. This is called “matching” the filler to the base, though the term is somewhat informal since the American Welding Society (AWS) doesn’t formally define it. In practice, matching tensile strengths often results in slightly mismatched yield strengths because hot-rolled steels and as-deposited welds behave differently under load. For most joint types and loading conditions, this is acceptable.
Strict strength matching is only required in one specific scenario under the AWS D1.1 structural welding code: tension loading on complete joint penetration groove welds. For all other weld types and loading conditions, matching filler metal is permitted but not mandatory, giving fabricators more flexibility.
Beyond strength, several other factors drive filler metal selection. The base metal’s chemical composition matters because the filler needs to be compatible to avoid cracking or brittleness. The service environment plays a role too: a joint exposed to corrosive chemicals, high temperatures, or extreme cold needs a filler engineered for those conditions. If post-weld heat treatment is planned, you may need a higher-alloy filler to maintain the required toughness and hardness after the thermal cycle. Without heat treatment, a lower-alloy option may actually perform better because it avoids excessive hardness.
AWS Classification System
The American Welding Society organizes filler metal standards under its A5 specification series. Each number covers a specific combination of base metal and welding process. Carbon steel alone has nearly a dozen A5 specs (A5.1, A5.18, A5.20, A5.36, and others), while stainless steel is covered by A5.4, A5.9, and A5.22, among others. Aluminum falls under A5.3 and A5.10. Nickel alloys, copper alloys, titanium, cast iron, and even magnesium each have their own dedicated specifications.
These classifications encode a surprising amount of information into short alphanumeric codes. The letter and number combinations tell you the tensile strength, welding position, flux type, shielding gas compatibility, and chemical composition. Learning to read them saves time when comparing products or interpreting a welding procedure specification.
Storage and Moisture Control
Filler metals, especially stick electrodes, are sensitive to moisture. Any electrode exposed to humid air for more than a few hours should be dried before use, because moisture in the flux coating introduces hydrogen into the weld. Hydrogen causes cracking, particularly in high-strength steels.
Low-hydrogen electrodes (designations like E7018, E7016, and E7028) are manufactured with a moisture content of just 0.1 to 0.4 percent and are packaged in hermetically sealed or vacuum-packed cans. Once opened, they should be stored in a holding oven at 225 to 300°F. Cellulosic electrodes like E6010 and E6011 need much lower storage temperatures, around 135 to 140°F. Storing both types in the same oven is a problem: the higher moisture content of cellulosic electrodes will migrate into the low-hydrogen coatings, defeating their purpose.
MIG wire and TIG rods are less moisture-sensitive because they lack flux coatings, but they should still be kept in dry, clean environments to prevent surface oxidation that can cause feeding issues or porosity in the finished weld.
Fume Hazards During Use
When filler metal melts, it produces fumes composed of fine metal particles. Most welding fumes contain some percentage of manganese, along with iron and potentially lead depending on the alloy. NIOSH has identified that welders face increased risk of neurological and neurobehavioral health effects from prolonged exposure to these metals. Carbon monoxide, heat, and physical stress compound the risk. Confined-space welding significantly increases manganese fume exposure because ventilation is limited and concentrations build quickly. Proper ventilation, fume extraction, and respiratory protection are essential whenever filler metals are being melted.