Root Concavity
A macroscopic root suck-back is a depression at the weld root from insufficient fill or too much back pressure. It weakens the effective weld section and acts as a notch.
Physicochemical Mechanism of Action
A macroscopic root sag forms when there is not enough weld volume or the back pressure is too high. That makes the metal deform and suck in at the weld root.
Metallurgical Relevance & Welding Risks
A macroscopic root sag weakens the effective joint cross-section and so hurts the mechanical properties of the weld. It acts like a notch and can lead to cracks or fractures, especially under load. It can also slow down the cooling of the weld and cause an uneven structure.
Workshop Practice & Instructions
To avoid macroscopic root sags, make sure the volume is filled properly and keep the back pressure under control. Use suitable welding parameters like temperature and time. If the root sag is too large, it can help to remove the weld and re-weld it.