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Welding
This process is one of the most complex interactions of the materials, the joint form, process parameters and finally the functional requirements of the design.
There are three main methods used in welding:
- Conduction limited
- Continuous molten pool
- Key-hole welding
For all welding applications a key factor is the tooling to hold the components.. In general the pieces to be welding should be in good contact and good quality tooling makes this consistently possible.
One area of the process tooling specific to welding is the provision of a gas shielding. For spot and simple seam welding the arrangement can be as basic as a pipe feeding an inert gas (such as argon) to the weld point. However more complex design may be necessary for more critical applications.
Conduction limited welding
This is most common form of laser welding where size and shape of the weld is determined by the thermal characteristics of the materials used. This is normally the type of weld produced when a spot weld is formed. The key feature of conduction limited welding is that each spot weld solidifies before the next spot arrives. The reaction of the material with energy is very rapid, usually being fractions of a millisecond with the laser pulse being several milliseconds. This process requires a high power within the laser pulse.
At the beginning of the pulse, energy is partially reflected and partially absorbed. The energy in the rest of the pulse is absorbed and conducted downwards through the material. The higher peak power, generally the deeper the penetration. As peak power continues to increase, vapourisation begins to occur and a vapour cavity is formed with the molten pool further increasing the penetration. Peak power must not vaporise too much of the material.
In thin materials, the weld along a seam consists of a series of overlapping spot welds. Conduction limited welding is effected by the reflectivity of the material so when individual spot welds are overlapped the energy of the laser pulses will be absorbed more effectively. This results in reduced laser power being need, the amount dependant on the percentage overlap. Provided the individual spots overlap at the interface between the tow base pieces, the weld should be hermetic unless there is porosity or cracking.
Continuous molten pool welding
This welding method requires sufficient average power so that the hear dissipation from the weld zone is matched by the hear input. Once the weld has been commenced, the material in the weld area does not sool down. This method of welding may be difficult when using thin materials as the hear below the weld pool cools rapidly. The process makes use of the change of reflectivity of a material when heated. As the materials does onto cool down between each laser pulse when a pulsed laser is used, a dn as the pulses overlap by a high per centage, the absorption is significantly improved.
Welding speed can be higher that the conduction limited method but the type of resultant weld still depends on several factors. The width of the weld is related to the speed of welding. However width is also dependant on the spot of the focussed beam as well as pulse energy, average power, etc.
The shape of the weld is different to the conduction limited weld. It tends to be ‘U’ shaped rather than a ‘V’. There is greater mixing of the materials. When using a pulsed laser, as the weld pool remains molten the actions of the pulses is to agitate the pool. Also the gas generated during the process has a longer time to escpae giving lower porosity.
Key-hole welding
High intensities are required to allow the formation of a narrow hole in the material. Inside the hole a ‘plasma’ us formed, that couples in the laser beam, transferring energy to the surrounding material. Thus there is a zone of molten metal around the plasma filled hole. The weld is formed as the plasma moves on leaving the residue material to solidify.
With this method, penetration is increased relative to the width fo the weld, resulting in a narrow weld with limited heat affected zone (HAZ) around the weld. The process can only be used with relatively thick materials and the fit-up must be better than for other forms of welding. Laser power is generally high in order to maintain the key-hole.
Shielding gas
Often a shield gas is required at the weld area to prevent oxidisation with argon being commonly used. Individual applications vary widely making the choice of the gas and its method of delivery important. Some application use nitrogen, helium or hydrogen.
Optimising the welding parameters
Optimisation for adequate weld properties and consistency of process can be difficult. The optimisation process requires to approached systematically and with discipline to ensure a balance of process quality and process speed whilst minimising the operational costs.
Process problems to be addressed:
- Porosity and voids
- Cracking within the weld
- Cracking in the join centre line
- Cracking in the heat affected zone
- Lack of fusion
- End point craters
- Uneven top bead
- Undercutting
- Slump of the weld surface
- Contamination