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Welding
Material strength is always lower within a welded joint, whether the metal involved is titanium,
steel, or aluminum. The drop in ultimate tensile strength (UTS) for 3-2.5 titanium in the
heat-affected zone (HAZ) is roughly 12-15%. Note that UTS drops 40-50% in a high-quality stee
tube. Aluminum also suffers a significant loss, but in many alloys strength can be recovered by
solution heat-treating and aging.
Titanium weld quality depends on many factors:
- Cleanliness has the single biggest impact on weld quality. The surface metal must free
of grease, chlorides, and all contaminants, and the entire weld area must be free of oxygen,
nitrogen, and hydrogen during the process of welding. Even fingerprint oil can contaminate
the weld area, so scrupulous cleansing standards must be maintained at all times.
- Complete penetration of the filler material is critical. Only a skilled welder using
proper equipment on a well-designed joint can assure that the base metal has been properly
fused with the filler material.
- The type of bead plays an indirect role in penetration, and thus in final welded strength.
A smooth bead disperses heat, and makes full penetration harder to achieve. Puddle welds heat
a smaller area, focusing the bead and improving penetration. An excessively thick or uneven
bead will create a harsh transition in relative stiffness between the bead and tube. Since
the weld bead acts as a stress riser in any case, it is best
to minimize the sharpness of the transition area.
- The rate of post-weld cooling theoretically affects weld quality, but there is no evidence
that cooling rate plays a large role in post-weld fatigue strength.
Welding versus Bonding
The loss of strength due to welding begs the question of substituting bonded lugged joints for
welded beads. The primary drawback to bonded construction is added weight. For example, the
titanium lugs used in the Specialized Epic Ultimate carbon fiber/titanium mountain frame,
designed for minimum weight, weigh 1.5 pounds per set. If the frame were built from welded
Merlin Extralight double-butted tubing, the butted sections would weigh a fraction of the
titanium lug set. This relationship is true of any material, whether metal-matrix composite,
aluminum, steel, or carbon fiber.
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