What Is the Depth of Fusion in Welding?

The welding process can be complex, and many variables are often considered. Producing strong and quality weldments (units of welded pieces) requires a good knowledge of welding terms, concepts, and variables. In line with getting adequate knowledge, what is the depth of fusion in welding?

The depth of fusion in welding is the distance that fusion spreads into the base metal or previous pass from the melted surface during welding. You can see it in the cross-section of a weld, and welding requires an adequate depth. 

This article explores how deep a weld should penetrate and the depth of fusion in welding. I also describe how fusion welding works, the characteristics of a good weld, and welding tests.

How Deep Should a Weld Penetrate?

Generally, the weld penetration is anywhere from 5% to 100%, and the strength requirement determines the depth. The thickness of the material also affects the depth of penetration.

For full strength on butt weld joins, you may have to ensure 100% penetration, but it is not practical for filet welds. Fillet welds usually need a 1:1.4 width/depth ratio: a quarter-inch fillet should have a penetration depth of around 5/16 inches.

Softwares like WeldMate can calculate the depth of penetration when you input base metal thickness, material thickness, penetration level seen under a microscope, and joint metal thickness. The software will return a value and you can multiply it by 100 to give you the percentage of penetration.

Factors Affecting the Depth of Penetration or Fusion

Factors affecting the depth of penetration or fusion 

Many variables affect penetration depth, and good penetration for the type of weld metal is the goal. Here are some variables:

  1. Amperage is the determinant factor for penetration, and a higher amperage causes a deeper penetration and vice versa.
  1. Voltage—has a negligible effect on penetration when set in the correct range, and within this range, a higher voltage results in a lower penetration.
  1. Shielding gas—it shouldn’t be the first thing you adjust to increase or decrease penetration, even though it affects penetration.
  1. Contact Tip to Work Distance (CTTWD) is another essential determinant in penetration, and every welding procedure specification should include it. Penetration decreases as CTTWD increases because as CTTWD increases, there’s more resistance and a drop in amperage.
  1. The technique (Weaves or Stingers)—when you weave a bead, you weld into the puddle as you oscillate, and when you go over the pool, you lose penetration. In contrast, running stringers will produce a deeper and more consistent penetration.
  1. Travel speed—a faster rate will cause a drop in the overall heat input and, thus, decrease penetration. Within an acceptable range, slow speed will increase penetration.

Other factors include the use of preheating, the type of base material, electrode diameter, and surface condition of the base material (rust, mill scale, or oil on it.)

What Is the Depth of Fusion in Welding?

Joining two pieces of metals may involve melting parts of their surfaces and fusing them. The depth of fusion or penetration is how far fusion penetrates the previous bead or base metal from the melted surface during welding.

Although many people believe that a strong weld must have a high depth of fusion, it does not hold. Some joints need light penetration, while others have a large depth of fusion. 

Instead of solely hanging on the depth of fusion, a weld’s strength hinges on achieving complete fusion and other factors, depending on the type of weld. 

In a cross-section of a weld, you will see the depth and width of penetration, as well as other features of the penetration profile. The depth of fusion is measured perpendicular to the bevel face—the fusion faces of the completed weld.

How Does Fusion Welding Work?

Fusion welding is a welding process that applies heat to two or more materials. On heating to a melting point, the materials join with no external pressure (except resistance welding.) 

Filler materials may be necessary to close any gaps in the joint you are welding. Fusion welding is grouped according to the source of heat, and they include:

  • Gas: Oxy-Fuel or Oxyacetylene welding.
  • Resistance: Resistance Spot Welding, Resistance Seam Welding.
  • High energy: Electron Beam Welding, Laser beam Welding, Friction Welding.

Induction welding and solid reactant welding are also types of fusion welding. The application of fusion welding includes manufacturing ships, welded pipes, airplanes, automobiles, bridges, and pressure vessels. You can also use it to join plastics and wood.

Characteristics of a Good Weld?

Many structures in the modern world rely on welding: from ships and automobiles to skyscrapers. As an essential construction process, quality and strength are key parameters that must be maintained.

The variety of welds and welding processes generally eliminates hard and fast rules for distinguishing good and bad welds. Regardless, exploring each type of weld can reveal good and bad characteristics. Here are some visible attributes of good and bad welds:

  1. Tungsten Inert Gas (TIG) weld

Good characteristics—patterned, neatly layered tiny welds, no slag or burnout, and visually appealing.

Bad characteristics—erratic beads, wide with no distinct pattern, signs of burnout

  1. Stick weld

Good characteristics—consistent thickness, straight and uniform, no holes, breaks, or cracks in the bead, and no spatter drops.

Bad characteristics—inconsistent bead width, visible spatter, undercutting, or cracking in the bead.

  1. Metal Inert Gas (MIG) welding

Good characteristics—no slag, no cracking or holes, straight and uniform, smooth with no pattern, no dips or craters in the bead, and no breaks in the weld.

Bad characteristics—cracking along the weld, lack of strength, thin bead or a lack of discoloration of the parent metal, and lack of uniformity and straightness.

Quality testing involves exposing the weld to conditions similar to or more than what the structure will experience in the field. You can do a magnaflux test, an x-ray test, an ultrasonic test, a dye penetrant test, etc.

A weld is not necessarily bad because it does not look good. Proper welding tests are necessary to confirm the strength and quality of a weld. 

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