What is the MIG/MAG Process?

The MIG/MAG Process was developed to meet the production demands of the war and post war time economies. This process utilises an electric arc power source, continuously fed consumable wire electrode shielded by gas.

A number of terms are used for the process such as:

MIG – Metal Inert Gas Welding

MAG – Metal Active Gas Welding

GMAW – Gas Metal Arc Welding

 

Process Description

The MIG process was first patented for the welding of aluminium in 1949 in the USA. The process uses the heat that is generated by an electric arc formed between a bare consumable wire electrode and the work piece. This arc is shielded by a gas to prevent oxidation of the weld. In the MIG process an inert shielding gas is used to protect the electrode and weld pool from contamination and enhance the arc. The 1950’s saw the UK  popularise this process for aluminium using argon as the shielding gas.

However when welding mild steel, stainless and alloyed steels, argon alone cannot be used as the arc would be unstable. Subsequent development of gas mixtures which include other gases such as CO2, oxygen and helium make it possible to weld these materials. This use of mixture gases or gases other than argon led to the process being referred to as the MAG process. As an example CO2 could be used as a lower cost option compared to argon mixed gases. It was widely used in some industry segments as it offers some positive beneficial weld properties. But it also had some negative aspects such as more weld spatter which could require post weld treatment. Many welders who used CO2 often refer to the process as CO2 welding.

 

The Power Source

The power source converts the mains supply in general single phase (230v) or 3 phase (400v) to a usable welding supply and its output is DC. In a standard MIG power source the relationship between voltage and welding current is termed to be a flat or constant voltage characteristic. In the MIG/MAG process, welding current is determined by wire feed speed, and arc length is determined by power source voltage level (open circuit voltage). Wire burn-off rate is automatically adjusted for any slight variation in the gun to work piece distance, wire feed speed, or current pick-up in the contact tip. For example, if the arc momentarily shortens, arc voltage will decrease and welding current will be momentarily increased to burn back the wire and maintain pre-set arc length. The reverse will occur to counteract a momentary lengthening of the arc.

The electrode (wire) is normally +VE and the work return is normally -VE. However certain consumable wires sometimes require what is called reverse polarity i.e. Electrode -VE work +VE. Typical of these types of wire are cored wires used in hard facing or high deposition and gas less applications.

There are a wide range of power sources available and the mode of metal transfer can be:

  • Dip
  • Globular/Spray
  • Pulsed

 

Typical Welding Range

Wire Diameter         Current (A)    Voltage (V)

0.6mm Wire                30 – 80            15 -18

0.8mm Wire                45 – 180          16 – 21

1.0mm Wire                70 – 180          17 – 22

1.2mm Wire                100 – 200       17 – 22

1.6mm Wire                120 – 200       18 – 22

 

Wire Feed Mechanism

The mechanism has a drive motor and feed roll system to feed the wire. This feed system may be built into the power source (Compact) or a separate unit (SWF). Feed units are either two or four roll drive. The performance of the wire feed system can be crucial to the stability and reproducibility of MIG welding. Roll pressure must not be too high otherwise the wire will deform and cause poor current pick up in the contact tip.

 

Types of Feed Rolls

V Groove Rollers: Are used for hard wires such as steel, stainless steel where the wire shape is not deformed due to tensioner pressure.

U Groove Rollers: Are used for soft wires such as aluminium. This type of wire can easily deform its shape making poor current pick up at the contact tip.

Knurled Rollers: Are used on tubular cored wires which are easily deformed.

 

Gases

The shielding gas used is primarily to shield the weld zone from contamination by air. It also affects the heat of the arc, the stability of the arc and mode of transfer. The gas mixtures vary depending on the gas supplier but generally the following guide lines can be used. Gas flow rates should be approximately 10-30 Ltr/Min.

Steel – Carbon dioxide (CO2) can be used but generally Argon/CO2 mixtures are used as they give improved transfer modes and cleaner finish to the weld

Stainless – Argon/Oxygen mixtures provide an economical solution but many bespoke gases containing helium and CO2 can provide better transfer and weld finish. Non Ferrous materials  such as aluminium, copper nickel and alloys – argon or helium

 

Wire Feed Speed

The wire speed is directly related to the current. The higher the wire speed the more the wire is deposited and hence more current is required to burn off the consumable wire. Wire speed is measured in m/min (metres per min) or sometimes in IPM (inches per minute). The diameter of the wire also forms part of the current demand e.g. a 1.0mm wire feeding at 3 m/min will require less current than a 1.2mm wire feeding at the same rate.

The wire feed is set according to the material to be welded. If the wire feed rate is too high in comparison to the voltage then a “stubbing” effect happens where unmelted consumable contacts the work piece creating large amounts of weld spatter. Too little wire feed in comparison to the voltage will result in a long arc being created with poor transfer and eventual burning back of the wire onto the contact tip.

 

Voltage

Voltage is required to burn off the wire and is often referred to as the “heat” control. The amount of voltage required is related to the wire speed (welding current), the wire diameter, the transfer mode, and the gas type. As with the wire speed defects the same occurs with the incorrect setting of the voltage i.e., too much voltage results in the long arc scenario and too little voltage will result in stubbing.

 

Burn Back

In the event the welder stops welding and all functions of the machines stopped simultaneously then the consumable filler wire would in all likelihood freeze in the weld pool. In order to avoid this happening the burn back feature is present on most machines. This will allow the power to be maintained on the consumable filler wire when it has stopped feeding thereby burning clear of the weld. In some equipment the burn back is preset within the control circuits, others offer an external variable control feature to adjust the time of delay.

 

Other Controls

Other common control features are latching or 2T/4T where the welding can either in 2T mode press the torch trigger to weld and release to stop or in 4T press and release the torch trigger to start, weld without holding the trigger on and stop by pressing and releasing the trigger again. This is particularly useful when welding long weld runs.

A spot welding timer will allow the time of the weld to be set and after the time has expired the operator will have to release the torch switch to re start the weld.

In many of the new technology machines there are numerous other control features made possible by the use of electronics and micro computers such as synergic control, pulse and double pulse etc. These can greatly improve both machine and welder performance and the operator should always read the full instruction manual to gain full knowledge of all the additional features.

 

MIG/MAG Welding Tips & Guide

Some quick reference tips below:

  1. When welding try to use an electrode stick out (the distance between the weld and the contact tip) of around 6-8mm
  2. When welding thin materials try and use smaller wire diameters and thicker materials use thicker wires
  3. Make sure you select the correct wire type for the material to be welded
  4. Always ensure you have the correct size drive rolls and torch liner for the wire size selected
  5. Select the correct gas to achieve the correct weld characteristics and finish
  6. for optimum control of the weld the wire at the leading edge of the weld pool
  7. Before commencing welding ensure a comfortable and stable position
  8. When welding try to keep the welding torch as straight as possible to ensure the best feed
  9. Carry out daily housekeeping on the condition of the welding torch and drive rolls
  10. Keep any consumables clean and dry to avoid contamination such as oxidation and damp

 

Welding Carbon Steels

In order to weld these materials you must:

  • Use the correct welding gas, usually ARGON + CO2 with percentages of argon ranging from 75% upward
  • Using pure CO2 as a protection gas will produce narrow beads, with greater penetration but a considerable increase in spatter
  • Use a welding wire of similar quality as the steel to be welded. it is best to always use good quality wires and avoid welding with rusted wires that could cause welding defects
  • Avoid welding rusted material or that with oil or grease contamination

Welding Stainless

  • Ensure the correct gas mix for stainless steels is used. Usually a protection gas with a high Argon content, containing a small percentage of O2 or carbon dioxide (CO2 – approximately 2%) to stabilize the arc
  • It is important to keep the welding area clean at all times, to avoid contaminating the joint to be welded

Welding Aluminium

In order to weld aluminium you must:

  • Use Pure argon or argon helium mix as the protection gas
  • A welding wire with a composition suitable for the base material to be welded
  • NOTE: If only a torch that has been used for steel is available then you MUST alter the torch as follows
  • Make sure the welding lead is as short as possible – ideally no longer than 3 Mtr
  • Remove ALL consumables – All torch ends and the liner
  • Insert a teflon or carbon fibre liner attach new torch consumables
  • Use wire feed rollers suitable for aluminium wire
  • Adjust the pressure applied on the rollers to as low as possible

 

Welding Problems

Welding Problems Table

 

 

 

 

 

 

 

 

 

 

 

 

Duty Cycle

The European norm (EN 60974-1) states that the time period for one complete cycle is 10 minutes. So in the case of the JASIC Arc 140 Inverter MMA machine which has a duty cycle of 140A at 35% this would mean the output of 180 amps could be maintained for 3 minutes 30 seconds after which a rest period of 6 minutes 30 seconds should be observed, this would represent a complete cycle.

A 100% rating is the continuous rating for the machine, as the duty cycle increases the current decreases.

Important duty cycle things to consider when selecting your machine

  1. The European norm EN 60974-1 states that welding power sources should be capable of delivering their rated output when the ambient air temperature is within the range of -100°C to +400°C. Consider that the normal temperatures in the UK fall into this band with good margins.
  2. The thermal trip will not operate in accordance with duty cycle ratings
  3. When a machine is put into use it will be at ambient air temperature and may take several hours to bring it to “working temperature” before encountering thermal trip problems. It is during this period that the duty cycle is often ignored and heat builds up within the machine
  4. The duty cycles achieved by most manual welding processes are often quite low, for example MMA 10-20% MIG 30-40% TIG 20-25%. These figures should be considered when selecting equipment.
  5. Often a machine is selected which requires operating parameters that are within the range of the machine. But care should be taken to also ensure the machine construction is of suitable design i.e. mechanical components, switching requirements etc.

 

If you have any questions about MIG welding, then please get in touch.

 

The Welders Discount Warehouse Team