What is the difference between lack of surface fusion and lack of penetration?

• A. Lack of surface fusion is a cosmetic issue; lack of penetration is structural only
• B. Lack of surface fusion occurs only at the root; lack of penetration is only at the toe
• C. Lack of surface fusion is incomplete bonding at the surface; lack of penetration is insufficient weld through the joint thickness
• D. Lack of surface fusion means excess weld metal at the surface

Answer: (C)

The key idea is how a weld bonds to the base metal versus how deeply it fuses into the joint. Lack of surface fusion means the weld metal did not fuse to the base metal at the surface of the joint—the interface at the surface isn’t bonded, so there’s an incomplete bond right where the metal meets the base. Lack of penetration, on the other hand, means the weld metal did not extend through the full thickness of the joint; the interior of the joint remains unwelded and the weld does not carry load through the full thickness. So, surface fusion issues are about bonding at the surface, while penetration issues are about depth of fusion through the thickness. The best description captures that distinction: incomplete bonding at the surface versus insufficient weld through the joint thickness.