Penatration vs fusion in welds

[Glenn]

Someone please explain the difference between penetration and fusion in welding. Many times I hear "good penetration" but not so often the term fusion of the welded metals.

[Rutterbush]

Handbook - Welding Techniques

This says it so much better than I can

Handbook - Weld Defects

[Dodge]

Fusion is what happens when a proper weld is made. The metals fuse together. Penetration is how deep in the weld joint or groove the fusion takes place. If your weld is fused all the way through the parent metals (the two pieces being joined) you have 100% penetration.

[Ten Hammers]

Like Scott sez, fusion welding is the process. Full thickness penetration welds ( 100% welds ) are made by proper prep ( small gap, weld a root, go to other side, back grind into the root or gouge and grind and make a pass). Back grind if necessary( or use a needle scaler) and continue with stringer passes until the cavity is full ( and a smidge over with no undercut or under fill ). Do the same on other side. Full penetration weld that should xray and or bend/hydro test to pass for the man. These welds I speak of are often made with either solid wire or FCAW and perhaps cap gas as well ( outside in the wind ). The welder has enough responsibility to cure any porosity or hole issues immediately but they do not happen often. This is used on carbon or stainless 10 guage to 1" + thick. Tank heads to man ways to legs to inside baffles. Thank you Rutterbush for the links.