High temperature performance. The working temperature of the filament is much higher than the recrystallisation temperature of the tungsten filament, at this time, the filament under its own weight, the filament section between the two hooks will produce the sagging phenomenon, and in serious cases, the filament can be sagged to the point where it touches the glass shell of the bulb. For the tungsten powder metallurgy process mixed with a small amount of potassium silicon and aluminium oxide doped tungsten filament, although the final product of the filament in the silicon and aluminium content is only a few parts per million, potassium content is only a few dozen parts per million, but with this kind of doped tungsten filament for the filament of the sagging degree can be greatly improved. The reason is due to the doped tungsten filament and undoped tungsten filament recrystallisation of the crystal structure is very different. The recrystallised crystal of undoped tungsten filament is basically an equiaxed crystal, whereas the recrystallised crystal structure of doped tungsten filament is a long strip of coarse grains that overlap each other.
From the viewpoint of high-temperature creep theory of metal materials, this kind of thick and long lap structure of recrystallised crystal structure can greatly improve its high-temperature sag resistance. Trace amounts of potassium remaining in the doped tungsten bars form potassium bubble columns parallel to the filament axis during processing, which impede the lateral growth of grains in the recrystallisation process, thus generating a long and thick lap structure.
Incandescent lamp filament sagging both with the doped tungsten filament in the content of added elements and processing technology, but also with the filament production process related to the treatment process. Tungsten filament drawn into the finished filament retains a large number of internal stresses, in the winding into the filament and in the cross-section of the tungsten filament to produce new uneven deformation of the internal stresses. These internal stresses must be eliminated perfectly before the filament is mounted into the shell, otherwise the bulb will twist, deform and sag the filament at the start of ignition. Sagging of the filament will seriously reduce the luminous efficiency of the bulb.
Room temperature performance. The room temperature performance of tungsten filament is shown in the filament winding performance. Tungsten filament due to its long processing process, if the process management is not good, it is easy to make the tungsten filament produces a lot of small cracks or local brittle, so that the winding is easy to break. Due to the cracks caused by the fracture of the winding wire fracture whisker-like, and due to the wire material becomes brittle caused by the fracture of the crystal surface flash-like.
The consistency of filament diameter. Poor consistency of tungsten filament diameter is to make the incandescent bulb photoelectric parameters of an important reason, some will also affect the service life of the bulb.
