What is the difference between weld metal and filler metal?

What is the difference between weld metal and filler metal? In the first instance, filler metal is supplied to the joint to be welded/joined from an electrode that is consumed in the arc by the welding process, typically Shielded Metal Arc welding (SMAW), Gas Metal Arc welding (GMAW) or Flux Cored/Metal Cored welding (FC/MCAW). It can also be supplied to the joint as a simple filler wire that is not electrically connected as in Gas Tungsten Arc welding (GTAW) and Plasma Welding (PAW).

These electrodes and filler materials are controlled, with their chemical composition, mechanical properties, diameters and hydrogen content, where applicable, being within the bounds of welding standards. In Canada the standard is CSA W 48 and in the United States it is the AWS A 5.0 series.

When this filler material enters the joint space, it does not just fill the area given in the joint preparation, be it a fillet, butt or other type of joint. By definition, in order to assure complete fusion, the heat generated by the electric arc must melt some of the base material which, will then combine with the filler material to form the final joint upon solidification from the melt.

So, the chemical composition of the electrode or filler will be changed in the final joint by the addition of melted metal from the parent material. This change is known as dilution which is defined as the change in composition of the weld metal caused by the mixing of the base metal and/or the previously deposited weld metal. The “dilution” of a welded joint can calculated as follows:

Dilution% = Weight of Parent Metal Melted
                  Weight of Total Melted Metal

The solidified weld metal will display a mixture of chemical and mechanical properties, and its composition will depend on the following:
•    the composition of the electrode
•    the composition of the parent metal
•    chemical reactions between the weld metal, the flux and/or shielding gas

The dilution will vary according to the welding process, the welding procedure and the joint preparation. Some examples are shown in Figure 1 below. This figure shows small dilutions, traced by the red dotted line, for a multipass fillet and fully prepared butt joint with many small passes deposited with low heat input SMAW. The lower part of the figure illustrates larger dilutions for plates partially prepared or square butted and welded with higher heat input processes such as SAW.

Figure 1 Examples of Low Dilution vs High Dilution Situations

On the multipass joint prepared with a V groove, top right of Figure 1, many small, low heat input passes will produce relatively little dilution compared with the square butted plate shown at bottom right. In this case, the amount of parent material melted, shown by the red dotted line, is considerable. In the diagram at bottom right, full penetration joints with unprepared, virtually zero gaps, can be achieved with the high heat input Submerged Arc Welding(SAW) process up to a certain maximum plate thickness. The dilution factor is mostly understood and contained with the welding of similar materials, as the welding consumables are designed to give acceptable joint properties for the majority of dilution situations.

However, it becomes a concern in certain areas, e.g. when welding dissimilar materials or in overlaying stainless steel onto carbon steel. In such cases the final alloy composition of the weld can be compromised and steps taken to address the final alloy content must be taken. This can be by either by judicious selection of filler or the addition of barrier layers when overlaying. These kind of solutions will fall into the realm of welding engineering and welding procedure development.

Mick J Pates, IWE. President, PPC and Associates