The internal defect of titanium alloy tube weld is the lack of fusion between the workpiece and the weld metal or between the weld layers. Incomplete penetration weakens the welding section, forms severe stress concentration and greatly reduces the joint strength, which often becomes the source of weld cracking. There is non-metallic slag in the slag weld, which is called slag inclusion. Slag inclusion reduces the working section of weld, forms stress concentration, and reduces the strength and impact toughness of weld.

According to the different mechanism of crack, it can be divided into hot crack and cold crack. Hot cracks occur during the crystallization process from liquid to solid in weld metal, mostly in weld metal. The main reason is that there is a low melting point material (such as FES, melting point 1193 ℃) in the weld, which weakens the connection between grains. When the welding stress is greater, it will simply cause cracks between grains. When there are many impurities such as s and Cu in weldment and welding rod, the hot crack will occur easily.

Hot cracks are distributed along grain boundaries. When the crack passes through the surface and communicates with the outside, it has a significant tendency of hydrogenation. Cold cracks occur in the cooling process after welding, mostly in the base metal or the fusion line between the base metal and the weld. The primary reason is that the quenching arrangement is formed in the heat affected zone (HAZ) or weld. Under the effect of high stress, the crack in the grain is caused. When welding the easily quenched titanium alloy with high carbon content or more alloy elements, the cold crack is most likely to occur. Too much hydrogen in the weld will also cause cold cracks.

Crack is one of the most dangerous defects. In addition to reducing the bearing section, it will also cause severe stress convergence. In the application, the crack will gradually expand and eventually cause damage to the component. Therefore, this kind of defect is usually not allowed in the welding layout. Once it is found, it must be removed for re welding.

When the weld metal with porosity absorbs too much gas (such as H2) or gas (such as CO) due to metallurgical reaction in the bath at high temperature, it is too late to discharge when the bath is cooled and condensed, and it forms cavities in the interior or exterior of the weld metal, which is called porosity. The existence of porosity reduces the useful working section of the weld and reduces the mechanical strength of the joint. If there are penetrating or continuous pores, it will seriously affect the tightness of the weldment. During or after welding, the metal partial cracking in the welding joint area is called crack. Cracks can occur on the weld and also in the heat affected zone on both sides of the weld. Sometimes it happens on the outside of the metal, sometimes it happens inside the metal.