TIG-Welding (Tungsten Inert Gas) is an arc welding process with non-consumable electrodes (of tungsten) under protection of inert gas, which can be executed with or without filler metal. TIG-welding is one of the most widespread method. It provides high quality joints but it needs specialized workers. At the beginning this welding technology has been developed for the aeronautic industry.
The procedure is based on a torch where it is inserted a tungsten electrode around which flows a protection gas. The operator moves the torch along the joint in order to move the melting bath and positioning the non-consumable tungsten electrode some millimetres away. It is important to avoid that the electrode get in touch with the piece to be welded otherwise the tungsten stick can attach itself to the joint and the welding is interrupted. The necessary equipment for a TIG-welding is made up of
-Power generator (Welding machine)
-Torch made up of:
- Tungsten electrode
- Gas passage
- Insulating sheath
- Electrical conductor
- Electrode holder
-Inert gas supply
-Possible filler metal
One of the main advantages of this technology is that the supply of the material in the melting bath is independent from the thermic supply in the welding, differently from wire welding or gas metal arc welding. This process can be automated and in this way it can be used for the production of welded tubes starting from the metallic strip and for the welding of tube plates of heat exchangers.
The TIG processing is particularly suitable when there is the need to weld small thicknesses, while the welding of more thicknesses (more than 5-6 mm) is not possible. It is often used for the first welding of a joint while the filling is made through different processes. It is suitable for continuous and intermittent welding. This process is not recommended in open spaces because the wind might dissipate protective gases.
Continuous wire welding MIG/MAG
The MIG (Metal-arc Inert Gas) or MAG (Metal-arc Active Gas) welding has been developed after the Second World War and its importance is increasing. One of the first reasons which have permitted this development was the reduction of the electronic products costs.
MIG/MAG welding is a continuous wire process where the protection is guaranteed from a covering gas. This type of process guarantees a high productivity and the presence of gas permits to work without dross (these features guarantee an cheap process in comparison to the electrode- welding).
A MIG/MAG-welding working station is made up of the following components:
- Torch with double function
- Arc current generator
- Wire progress and control mechanism
- Protection gas tank
The presence of all these components increases the price of a MIG/MAG-welding machine in comparison to an electrode welding machine.
Furthermore with continuous wires there is the possibility to have higher current density and so to obtain bigger penetrations, that means a joint filling with less touches.
The MIG/MAG welding is a process which comes from a submerged arc but the operator can keep the welding process under control as in electrode processes (covered electrode and TIG). Other advantages are the missing of dross and the possibility to weld also in not-flat positions.
In the picture a MIG/MAG welding torch, which is made up from the following parts:
- Insulation (white) and threaded insert for the wire guide (yellow)
- Nozzle for protection gas
- Wire guide
- Fuelling spout of protection gas
In some cases the torch has a refrigeration circuit fed with water.
Protection gas has the function of impeding the contact between the melting bath and the atmosphere.
Brazing is the connection of metallic parts with the help of a contribution metal. The contribution metal goes through the parts to be assembled.
Brazing is applied especially when:
- There is the need of containing the heating
- The joints are made up of materials hard to weld
- The parts are of different nature and their welding is impossible
- The aesthetic aspect of the joint is important
According to the fusion temperature of the contribution metal there is the possibility to use different heating ways. The brazing can be made both with means similar to the ones used for the oxyacetylene welding and with the electric heating in order to obtain higher quality joints.
Fusion temperature of the brazing alloy defines the soft and the strong welding.
The most important application fields are:
- Cycle and motorcycles industry
- Household appliances
- Chemical and hydrothermal systems
Physical phenomena through which the brazing is realized are capillary penetration, wetting and the atomic spread of the brazing alloy. The first one is realized by the creation of spaces among the components to merge; the second one takes advantage of the capability of a liquid metal to lean on a surface with small contact corners. The third one is realized through the atomic exchange between the brazing alloy and the basic metal, the metallurgic union.
It is characterized from the use of contribution metals with fusion temperatures smaller than 450°C. The most known is the tin-brazing, widely used in electronic. For this type of brazing a tin-lead alloy (60/40) was used. After the RoHs regulations, it has been substituted with tin-copper (99.3/0.7) and tin-silver (96/4) alloys.
It is characterized from the employment of contribution materials with fusion temperatures higher than 450°C. According to the required features and to the metal to be welded, different alloys can be used. Copper or zinc alloys with fusion temperature between 600 and 700°C are used for the brazing of carbon steel and stainless steel. For the brazing of stainless steel and high nickel content alloys are employed also nickel alloys.
It is characterized from the use of contribution materials with fusion temperatures higher than strong brazing and lower than the fusion point of the joint material. Joints preparation is often similar to the welding. Alloys which are usually used are melting brasses at high temperatures (800 °C). The process is not based on the capillary attraction but firstly on the atomic spread of the contribution alloy in the intergranular structure of the basic material.
Brazing alloys are normally eutectic mixture of two metals. Soft brazing are made employing brazing materials as tin, lead and (or) antimony alloys. For strong brazing are used silver, gold, palladium, copper and nickel alloys. Often for brazing over 1000°C is used nickel.
The main features of brazing alloy are: A) it is able to dunk metals or basic metals; B) it is able to enter in really close spaces (typical size of a space for brazing is 1/10 mm)
Deoxidizers have the property of promoting the wettability by removing the oxides which may develop with high temperatures.