Welding Inspection & Metallurgy API ICP Self Study Notes

10.3.5 Density
The density of a material is defined as its mass per unit volume. Castings,
and therefore welds, are usually less dense than the wrought material of
similar composition. Castings and welds contain porosity and inclusions that
produce a metal of lower density. This is an important factor employed during
RT of welded joints. The density of a metal is often important to a designer,
but more important to the welder is the density of shielding gases. A gas with
a higher density is more efficient as a shielding gas than one of a lower
density as it protects the weld environment longer before dispersion.