Solid wire MIG welding

Metal inert gas (MIG) welding was first patented in the USA in 1949 for welding aluminium. The arc and weld pool formed using a bare wire electrode was protected by helium gas, readily available at that time. From about 1952 the process became popular in the UK for welding alu-minium using argon as the shielding gas, and for carbon steels using CO₂. CO₂ and argon-CO₂ mix-tures are known as metal active gas (MAG) proc-esses. MIG is an attractive alternative to MMA, offering high deposition rates and high productivity.

Process characteristics

MIG is similar to MMA in that heat for welding is produced by forming an arc between a metal electrode and the workpiece ; the electrode melts to form the weld bead. The main difference is that the metal electrode is a small diameter wire fed from a spool. As the wire is continuously fed, the process is often referred to as semi-automatic welding.

Metal transfer mode

The manner, or mode, in which the metal transfers from the electrode to the weld pool largely determines the operating features of the process. There are three principal metal transfer modes:

Short circuiting / Dip

Droplet / Spray

Pulsed

Short-circuiting and pulsed metal transfer are used for low current operation while spray metal transfer is only used with high welding currents. In short-circuiting or dip transfer, the molten metal forming on the tip of the wire is transferred by the wire dipping into the weld pool. This is achieved by setting a low voltage ; for a 1.2mm diameter wire, arc voltage varies from about 17V (100A) to 22V (200A). Care in setting the voltage and the inductance in relation to the wire feed speed is essential to minimise spatter. Inductance is used to control the surge in current which occurs when the wire dips into the weld pool.

For droplet or spray transfer, a much higher voltage

is necessary to ensure that the wire does not make

contact i.e.short-circuit, with the weld pool ; for a 1.2

mm diameter wire, the arc voltage varies from approximately 27V (250A) to 35V (400A). The molten metal at the tip of the wire transfers to the weld pool in the form of a spray of small droplets (about the diameter of the wire and smaller).

However, there is a minimum current level, threshold, below which droplets are not forcibly projected across the arc. If an open arc technique is

attempted much below the threshold current level, the low arc forces would be insufficient to prevent large droplets forming at the tip of the wire. These droplets would transfer erratically across the arc under normal gravitational forces. The pulsed mode was developed as a means of stabilising the open arc at low current levels i.e. below the threshold level, to avoid short-circuiting and spatter. Spray type metal transfer is achieved by applying pulses of current, each pulse having sufficient force to detach a droplet.

Synergic pulsed MIG refers to a special type of controller which enables the power source to be tuned (pulse parameters) for the wire composition and diameter, and the pulse frequency to be set according to the wire feed speed.

Shielding gas

In addition to general shielding of the arc and the weld pool, the shielding gas performs a number of important functions:

Forms the arc plasma

Stabilises the arc roots on the material surface

Ensures smooth transfer of molten droplets from the wire to the weld pool

Thus, the shielding gas will have a substantial effect on the stability of the arc and metal transfer and the behaviour of the weld pool, in particular, its penetration. General purpose shielding gases for MIG welding are mixtures of argon, oxygen and C02, and special gas mixtures may contain helium. The gases which are normally used for the various materials are:

steels

CO₂

argon +2 to 5% oxygen

argon +5 to 25% CO₂

non-ferrous

argon

argon / helium

Argon based gases, compared with CO₂, are generally more tolerant to parameter settings and generate lower spatter levels with the dip transfer mode. However, there is a greater risk of lack of fusion defects because these gases are colder. As CO₂ cannot be used in the open arc (pulsed or spray transfer) modes due to high back-plasma forces, argon based gases containing oxygen or CO₂ are normally employed.

Applications

MIG is widely used in most industry sectors and accounts for almost 50% of all weld metal deposited. Compared to MMA, MIG has the advantage in terms of flexibility, deposition rates and suitability for mechanisation. However, it should be noted that while MIG is ideal for ‘squirting’ metal, a high degree of manipulative skill is demanded of the welder.

Equipment for MIG MAG welding

The MIG process is a versatile welding technique which is suitable for both thin sheet and thick section components. It is capable of high productivity but the quality of welds can be called into question. To achieve satisfactory welds, welders must have a good knowledge of equipment requirements and should also recognise fully the importance of setting up and maintaining component parts correctly.

Essential equipment

In MIG the arc is formed between the end of a small diameter wire electrode fed from a spool, and the workpiece. Main equipment components are:

Power source

Wire feed system

Conduit

Gun

The arc and weldpool are protected from the atmosphere by a gas shield. This en-ables bare wire to be used without a flux coating (required by MMA). However, the absence of flux to ‘mop up’ surface oxide places greater demand on the welder to ensure that the joint area is cleaned immediately before welding. This can be done using either a wire brush for relatively clean parts, or a hand grinder to remove rust and scale. The other essential piece of equipment is a wire cutter to trim the end of the electrode wire.

Power source

MIG is operated exclusively with a DC power source. The source is termed a flat, or constant current, characteristic power source, which refers to the voltage / welding current relationship. In MIG, 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 workpiece 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 momen-tarily increased to burn back the wire and maintain pre-set arc length. The reverse will occur to counteract a momentary lengthening of the arc.

There is a wide range of power sources available, mode of metal transfer can be:

Dip

Spray

Pulsed

A low welding current is used for thin-section material, or welding in the vertical position. The molten metal is transferred to the workpiece by the wire dipping into the weldpool. As welding parameters will vary from around 100A \ 17V to 200A \ 22V (for a 1.2mm diameter wire), power sources normally have a current rating of up to 350A. Circuit inductance is used to control the surge in current when the wire dips into the weldpool (this is the main cause of spatter). Modern electronic power sources automatically set the inductance to give a smooth arc and metal transfer.

In spray metal transfer, metal transfers as a spray of fine droplets without the wire touching the weldpool. The welding current level needed to maintain the non short-circuiting arc must be above a minimum threshold level ; the arc voltage is higher to ensure that the wire tip does not touch the weldpool. Typical welding parameters for a 1.2 mm diameter wire are within 250A \ 28V to 400A \ 35V. For high deposition rates the power source must have a much higher current capacity : up to 500A.

The pulsed mode provides a means of achieving a spray type metal transfer at current levels below threshold level. High current pulses between 25 and 100Hz are used to detach droplets as an alternative to dip transfer. As control of the arc and metal transfer requires careful setting of pulse and background parameters, a more sophisticated power source is required. Synergic pulsed MIG power sources, which are advanced transistor-controlled power sources, are preprogrammed so that the correct pulse parameters are delivered automatically as the welder varies wire feed speed.

Welding current and arc voltage ranges for selected wire diameters operating with dip and spray metal transfer

Wire feed system

The performance of the wire feed system can be crucial to the stability and reproducibility of MIG welding. As the system must be capable of feeding the wire smoothly, attention should be paid to the feed rolls and liners. There are three types of feeding systems:

Pinch rolls

Push-pull

Spool on gun

The conventional wire feeding system normally has a set of rolls where one is grooved and the other has a flat surface. Roll pressure must not be too high otherwise the wire will deform and cause poor current pick up in the contact tip. With copper coated wires, too high a roll pressure or use of knurled rolls increases the risk of flaking of the coating (resulting in copper build up in the contact tip). For feeding soft wires such as aluminium dual-drive systems should be used to avoid deforming the soft wire.

Small diameter aluminium wires, 1mm and smaller, are more reliably fed using a push-pull system. Here, a second set of rolls is located in the welding gun – this greatly assists in drawing the wire through the conduit. The disadvantage of this system is increased size of gun. Small wires can also be fed using a small spool mounted directly on the gun. The disadvantages with this are increased size, awkwardness of the gun, and higher wire cost.

Conduit

The conduit can measure up to 5m in length, and to facilitate feeding, should be kept as short and straight as possible. (For longer lengths of conduit, an intermediate push-pull system can be inserted). It has an internal liner made either of spirally – wound steel for hard wires (steel, stainless steel, titanium, nickel) or PTFE for soft wires (aluminium, copper).

Gun

In addition to directing the wire to the joint, the welding gun fulfils two important functions – it transfers the welding current to the wire and provides the gas for shielding the arc and weldpool.

There are two types of welding guns: ‘air’ cooled and water cooled. The ‘air’ cooled guns rely on the shielding gas passing through the body to cool the nozzle and have a limited current-carrying capacity. These are suited to light duty work. Although ‘air’ cooled guns are available with current ratings up to 500A, water cooled guns are preferred for high current levels, especially at high duty cycles.

Welding current is transferred to the wire through the contact tip whose bore is slightly greater than the wire diameter. The contact tip bore diameter for a 1.2mm diameter wire is between 1.4 andt 1.5mm. As too large a bore diameter affects current pick up, tips must be inspected regularly and changed as soon as excessive wear is noted. Copper alloy (chromium and zirconium additions) contact tips, harder than pure copper, have a longer life, especially when using spray and pulsed modes.

Gas flow rate is set according to nozzle diameter and gun to workpiece distance, but is typically between 10 and 30 l/min. The nozzle must be cleaned regularly to prevent excessive spatter build-up which creates porosity. Anti-spatter spray can be particu-larly effective in automatic and robotic welding to limit the amount of spatter adhering to the nozzle.

Protective equipment

A darker glass than that used for MMA welding at the same current level should be used in hand or head shields.

Recommended shade number of filter for MIG/MAG welding