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Robot Welding

The essential guide

MIG welding process
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This page provides an overview of the MIG welding process. MIG welding gives high deposition rates and is the preferred welding method for arc welding robots although good results can also be achieved using plasma, TIG and laser welding in some specialist applications.

Frequently used names for this process are:

MIG (Metal Inert Gas)

MAG (Metal Active Gas)

GMA (Gas Metal Arc)

CO2 welding


The process was developed in the USA the 1940’s and originally known as GMA and MIG for welding aluminium alloys using Helium or Argon as a shielding gas. These gases are inert and no chemical reactions take place between the gas and the weld pool. During the 1960 the process was further developed when low cost shielding gases were introduced using a CO2/Argon mixtures. This gas combination is not inert and reactions take place with the weld pool, hence the term MAG describes the process better. However, in Europe the term MIG is most often used for any of the above.

A small diameter electrode wire is fed continuously into the arc from a coil. A welding power source provides power between the electrode and the work piece. Because the wire has a small diameter the current density is high and as a result the wire is consumed in the arc. This is called the burn off rate and this is generally several meters per minute and upward.


In principle the equipment for robot MIG welding is similar to that of manual MIG welding and comprises the following equipment:

A DC (direct current) power source with the positive normally connected to the torch and the work piece to negative.

The wire feed unit.

The flexible hose bundle with torch.

Control devices to control the process (with robot welding this is done via software in the robot controller or power source).

A gas flow control device.

Transfer type Arc (Volt) Current (Amps) Applications

Dip

13 - 23

40 - 120

Light gauge, all positions

Spray

21 - 40

200 and over

High deposition on heavier plaet, gravity and H V position only

Globular

20 - 26

200 - 280

Higer deposition rates than dip transrer with lower heat input than spray transfer

Pulsed arc

16 - 26

60 - 220 (background 38 - 50)

Low heat input for light guage materials, also for aluminium welding

Dip transfer (or short arc)


In this transfer range low current and voltage settings are used to produce a short circuiting arc. Every time the tip of the electrode dips into the weld pool a short circuit is caused resulting in a rapid rise in temperature in the welding wire. The end of the wire is then melted off and an arc is established between the tip of the electrode and the molten pool. This arc maintains the electric circuit and produces enough heat to keep the molten pool fluid. The wire continuous to feed and the tip once again dips into the pool. This sequence of events is repeated at up to 200 times per second. Dip transfer is suitable for positional welding and heat input to the work piece is kept low, which minimises distortion. It is therefore a suitable process for welding thin materials.

Spray transfer


In spray transfer mode, higher voltages and currents are used than for dip transfer. The arc tends to envelop the end of the wire, which becomes slightly tapered giving rise to a stream of small droplets, about the same size as the wire diameter, which are transferred into the molten pool. The high voltage maintains a longer arc length that prevents short-circuiting to take place and current is flowing continuously. With spray transfer high deposition rates can be achieved, but because the weld pool is fairly fluid due to the high levels of heat input, positional welding is not possible.

Globular transfer


This is an intermediate range between dip and spray transfer. The transfer takes place in the form of irregular shaped globules that tend to fall into the weld pool by gravity. Unlike Argon based shielding gases, CO2 will not produce a true spray transfer and globular transfer is the nearest that can be achieved. This process will produce excessive spatter and a weld that looks rather untidy, but can be used if a lower heat input that spray transfer is required.

Pulsed arc welding


Pulsed arc welding is similar to spray transfer but at a much lower levels of heat input. In this transfer mode small droplets are transferred into the molten pool at a regular frequency using pulses of current in the range of spray transfer mode. A lower background current maintains the arc and keeps the end of the wire molten. The process gives excellent weld quality with virtually spatter free welds, which can be particularly useful when welding high alloy steels or if there is a need to control heat input and at the same time provide deep levels of penetration. This transfer mode requires special controls from the power source to set peak current, background current and pulse frequency. “Synergic controls” allows all welding parameters to be selected automatically for a given wire, gas and parent material combination. In this case only the wire feed speed is selected, whilst all other parameters are automatically generated through a computer-controlled interface.

Adjustment for welding


Once the basic welding parameters are selected a manual welder will often apply his skills achieve the required result. Key to robotic welding is to look at the results, analyse the process and only make modifications to a single parameter at the time. Robot programming and programming the process are to different skills. However, modern power sources contain synergic lines of optimised welding parameters and this facilitates achieving good results faster.

 
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