What is a fusion welding
To build a frame, it is necessary to join several pieces of different geometries and thicknesses. As these joints have to be safe and light, the most used method is welding (have you ever imagined a bicycle frame where the bars are screwed together?).
Welding is the joining of two metal parts by means of local heating from the point of contact between them to the melting temperature, when the metal liquefies.
Types of welding
Welding techniques vary:
- The purpose of joining the materials and the safety and reliability required in this union.
- Of the types of metals that will be welded: low carbon 1020 and hiten steel, stainless steel, aluminium, titanium or Cromoly alloy, for example).
- The quality and finish that is sought in the union.
- Of costs and productivity.
- The availability of qualified professionals.
Whatever the technique, it is very important to weld according to specified parameters for the objectives of welding. There is a "recipe" for each application, which must be followed by skilled professionals with adequate equipment.
The most common method on bicycle frames is electric arc welding and the most common processes are:
- Coated Electrode, a more usual process where the coated electrode is consumed in the process and the slag from the coating material protects the weld in its solidification. In general, applicable to less noble steels and to thicker parts (heavier frames).
- MIG/MAG (Metal Inert Gas), which makes use of an inert gas (Helium or Argon) to ensure the correct welding atmosphere. Here the electrode is also consumed, depositing material ("dripping metal") in the welded joint. Hiten frames, Cromoly or even Aluminium, can be welded by this method. Advantage: More precise and well finished welding than coated electrode welding.
- TIG (Tungsten Inert Gas), this process also uses an inert gas. In this method the electrode (made out of tungsten) is not consumed. Aluminum and titanium frames are welded by this method. Cromoly can also be welded with TIG, which has the advantage of spending less thermal energy in the welding process (less HAZ, which we will explain below). Although more complex and with lower productivity, TIG welding deliver the best finishing.
Risks of a badly executed weld.
By having an energy input high enough to bring materials to their point of fusion (the metal is "melted"), the weld modifies properties of materials which, when resolidified after they cool down, never return to their original microscopic pattern (their crystalline structure changes). The thermicaly affected area by this heat from the welding process is called the HAZ (Heat Affected Zone). Depending on the technique applied, there may be a weakening of the welded joint in the HAZ. Therefore, many component failures often occur outside the welding region, but still in the region of the heat affected zone. In some cases, heat treatment after welding is necessary to reorganize the crystalline structure.
To avoid these failures, it is important to observe the correct energy to be used in the process. Often a weld with large material addition (coarse weld bead) can be a weak weld. A discrete weld can be stronger when we evaluate the welded component as a whole.
An uneven and rough looking weld can indicate variations in welding parameters and other problems such as the quality of the welding equipment or even inability of the welder. These deformities can generate stress concentrations, higher weld joint weight, micro solidification cracks and cause problems that will materialize in a premature failure of the equipment. In this case, ugly welding can reveal a quality problem and a risk!
Finishing - What is the relationship between a good looking weld and the quality of the weld?
Isto nos leva a outra discussão importante, o acabamento! É possível disfarçar um solda mal feita? Sim. Nem toda solda com boa aparência é uma solda segura. Por vezes soldas com bom aspecto podem esconder uma baixa eficácia da solda.
On the other hand, a good looking weld (a perfect "fish scale") is often a good weld. But be aware, a "buffed" weld can indicate two situations: 1) a well made weld that has been "polished" (with fundamentally aesthetic gains) or 2) a badly made weld that has been "hidden", sometimes even with the use of finishing paste, further increasing the weight. So how to know? It is not possible to identify with the naked eye. It is necessary to use non-destructive testing techniques (NDT) to be able to diagnose whether a "smooth" weld is good or bad (such as magnetic particles, ultrasound, X-ray or penetrating liquid), and it is not possible for the end user to make his own inspection and evaluation. This is why engineers often prefer apparent welds to finished ones.
If the weld is polished and you trust the brand, go for it!, if it is polished and you don't trust the brand, beware! With the bead well visible, observe the appearance, uniform and well finished welds indicate good quality.