speeds or high traverse speeds, for example, mean that the weld material is unable to accommodate the extensive deformation during welding. This may result in long, tunnel-like defects running along the weld, which may occur on the surface or subsurface. Low temperatures may also limit the forging action of the tool and so reduce the continuity of the bond between the material from each side of the weld. The light contact between the material has given rise to the name "kissing bond". This defect is particularly worrying, since it is very difficult to detect using nondestructive methods such as X-ray or ultrasonic testing. If the pin is not long enough or the tool rises out of the plate, then the interface at the bottom of the weld may not be disrupted and forged by the tool, resulting in a lack-of-penetration defect. This is essentially a notch in the material, which can be a potential source of fatigue cracks. A number of potential advantages of FSW over conventional fusion-welding processes have been identified: Good mechanical properties in the as-welded condition. Improved safety due to the absence of toxic fumes or the spatter of molten material. No consumables — A threaded pin made of conventional tool steel, e.g., hardened H13, can weld over of aluminium, and no filler or gas shield is required for aluminium. Easily automated on simple milling machines — lower setup costs and less training. Can operate in all positions (horizontal, vertical, etc.), as there is no weld pool. Generally good weld appearance and minimal thickness under/over-matching, thus reducing the need for expensive machining after welding. Can use thinner materials with same joint strength. Low environmental impact. General performance and cost benefits from switching from fusion to friction. However, some disadvantages of the process have been identified: Exit hole left when tool is withdrawn. Large down forces required with heavy-duty clamping necessary to hold the plates together. Less flexible than manual and arc processes (difficulties with thickness variations and non-linear welds). Often slower traverse rate than some fusion welding techniques, although this may be offset if fewer welding passes are required. Important welding parameters Tool design The design of the tool is a critical factor, as a good tool can improve both the quality of the weld and the maximal possible welding speed. It is desirable that the tool material be sufficiently strong, tough, and hard wearing at the