Welding and joining processes

Welding & Joining processes

Process terminology

The European standard, EN 24063:1992 Welding, brazing, soldering and braze welding of metals (Nomenclature of processes and reference numbers for symbolic representative on drawings), assigns a unique number to the main welding processes. These are grouped as follows:

Arc welding

Resistance welding

Gas welding

Forge welding

Other welding processes

Brazing, soldering and braze welding

Each process is identified within the group by a numerical index or reference number. For example, the MIG welding process has a reference number of 131 which is derived as follows:

1 - Arc welding

3 - Gas-shielded metal arc welding

1 - Metal arc inert gas welding

The main arc welding process reference numbers are:

111 MMA with covered electrodes

114 Flux cored wire (self-shielded)

112 Submerged arc

131 MIG (inert gas)

135 MAG (CO2, active gas)

141 TIG

15 Plasma welding

The reference numbers are used as a convenient way of identifying the welding process in documentation such as welding procedure (EN 288) and welder qualffication (EN 287) records.

Process options

Factors which must be taken into account when choosing a suitable welding or joining process are:

Material type

Plate or tubular

Quality and strength requirements

Degree of mechanisation

Capital cost

Although consideration of these factors will identify the most suitable welding process, the choice within a company may be restricted by the cost of implementing a new process, availability of plant or current workforce skill. Welding and joining processes available to the welding engineer can be separated into the following generic types:

Fusion

arc

gas

power beam

resistance

Thermomechanical

friction

flash

explosive

Mechanical

fasteners

Solid state

adhesive

soldering

brazing

The suitability of the processes for welding and joining materials, joint types and components are shown in Table 1.

ProcessIndex no.SteelSSAlButt jointLap jointPlateTubePortabilityManualMechanisedAutomatedSite

Arc1YesYesYesYesYesYesYesYesYesYesYes

Gas3YesPossiblePossibleYesYesYesYesYesYesNoYes

Laser751YesYesPossibleYesYesYesYesNoNoYesNo

Resistance2YesYesYesPossibleYesYesPossiblePossibleYesYesNo

Friction43YesYesYesYesNoYesNoNoNoYesNo

Brazing9NoYesYesNoYesYesPossibleYesYesPossibleYes

FastenersnoneYesYesYesNoYesYesNoPossibleYesYesYes

AdhesivesnoneYesYesYesNoYesYesYesYesYesPossibleYes

In selecting a suitable process, consideration must also be given to the type of application, for example, the portability of equipment, whether it can be used on site, whether it is manual or mechanised, and the overall cost of the welding plant.

Fusion welding processes

When welding using a fusion process, the edges of a component are melted together to form weld metal.

ProcessHeat sourceShieldParentmetalthickness mmDepositionrate Kg/hr

Arc

MMAArcGas/flux1-1001-2

MIGArcGas0.5-1001-8

TIGArcGas0.1-1001-4

SAWArcFlux5-1005-20

ES/EGResistance/arcGas/flux5-100-

StudArc-4-20-

Gas

OxyfuelFlameGas0.6-101-2

Power beam

LaserRadiationGas0.2-100-

EBElectrogasVacuum0.2-100-

Resistance

Spot/SeamArc-0.2-10-

Thermit

ThermitChemicalGas10-100-

Table 2 shows heat source, mode of shielding, thickness range and metal deposition rates for a range of fusion processes. Although fusion welding is one of the simplest joining techniques, problems likely to occur include porosity in the weld metal, and cracking in either the weld or heat affected zone (HAZ). Porosity is avoided by ensuring adequate shielding of the weld pool and, for materials such as aluminium, the addition of filler wire.

Consideration of the joint design and the chemistry of the weld metal will prevent weld metal cracking. HAZ cracking which might be caused by hydrogen, is avoided by using low hydrogen consumables (MMA) and controlling the heat input and the rate of cooling of the parent metal.

The Manual Metal Arc process

Manual metal arc welding was first invented in Russia in 1888. It involved a bare metal rod with no flux coating to give a protective gas shield. The development of coated electrodes did not occur until the early 1900s when the Kjellberg process was invented in Sweden and the Quasi-arc method was introduced in the UK. It is worth noting that coated electrodes were slow to be adopted because of their high cost. However, it was inevitable that as the demand for sound welds grew, manual metal arc became synonymous with coated electrodes. When an arc is struck between the metal rod (electrode) & the work piece, both the rod & work piece surface melt to form a weld pool. Simultaneous melting of the flux coating on the rod will form gas and slag, which protects the weld pool from the surrounding atmosphere. The slag will solidify and cool and must be chipped off the weld bead once the weld run is complete (or before the next weld pass is deposited).

The process allows only short lengths of weld to be produced before a new electrode needs to be inserted in the holder. Weld penetration is low and the quality of the weld deposit is highly dependent on the skill of the welder.

Types of flux/electrodes

Arc stability, depth of penetration, metal deposition rate and positional capability are greatly influenced by the chemical composition of the flux coating on the electrode. Electrodes can be divided into three main groups:

Cellulosic

Rutile

Basic

Cellulosic electrodes contain a high proportion of cellulose in the coating and are characterised by a deeply penetrating arc and a rapid burn-off rate giving high welding speeds. Weld deposit can be coarse and with fluid slag, deslagging can be difficult. These electrodes are easy to use in any position and are noted for their use in the 'stovepipe' welding technique.

Features:

deep penetration in all positions

suitability for vertical down welding

reasonably good mechanical properties

high level of hydrogen generated - risk of cracking in the heat affected zone (HAZ)

Rutile electrodes contain a high proportion of titanium oxide (rutile) in the coating. Titanium oxide promotes easy arc ignition, smooth arc operation and low spatter. These electrodes are general purpose electrodes with good welding properties. They can be used with AC and DC power sources and in all positions. The electrodes are especially suitable for welding fillet joints in the horizontal/vertical (H/V) position.

Features:

moderate weld metal mechanical properties

good bead profile produced through the viscous slag

positional welding possible with a fluid slag (containing fluoride)

easily removable slag

Basic electrodes contain a high proportion of calcium carbonate (limestone) and calcium fluoride (fluorspar) in the coating. This makes their slag coating more fluid than rutile coatings - this is also fast-freezing which assists welding in the vertical and overhead position. These electrodes are used for welding medium and heavy section fabrications where higher weld quality, good mechanical properties and resistance to cracking (due to high restraint) are required.

Features:

low weld metal produces hydrogen

requires high welding currents/speeds

poor bead profile (convex and coarse surface profile)

slag removal difficult

Metal powder electrodes contain an addition of metal powder to the flux coating to increase the maximum permissible welding current level. Thus, for a given electrode size, the metal deposition rate and efficiency (percentage of the metal deposited) are increased compared with an electrode containing no iron powder in the coating. The slag is normally easily removed. Iron powder electrodes are mainly used in the flat and H/V positions to take advantage of the higher deposition rates. Efficiencies as high as 130 to 140% can be achieved for rutile and basic electrodes without marked deterioration of the arcing characteristics but the arc tends to be less forceful which reduces bead penetration.

Power source

Electrodes can be operated with AC and DC power supplies. Not all DC electrodes can be operated on AC power sources, however AC electrodes are normally used on DC.

Welding current

Welding current level is determined by the size of electrode - the normal operating range and current are recommended by manufacturers. Typical operating ranges for a selection of electrode sizes are illustrated in the table. As a rule of thumb when selecting a suitable current level, an electrode will require about 40A per millimeter (diameter). Therefore, the preferred current level for a 4mm diameter electrode would be 160A, but the acceptable operating range is 140 to 180A.

What's new

Transistor (inverter) technology is now enabling very small and comparatively low weight power sources to be produced. These power sources are finding increasing use for site welding where they can be readily transported from job to job. As they are electronically controlled, add-on units are available for TIG and MIG welding which increase the flexibility. Electrodes are now available in hermetically sealed containers. These vacuum packs obviate the need for baking the electrodes immediately prior to use. However, if a container has been opened or damaged, it is essential that the electrodes are redried according to the manufacturer's instructions.

TIG Welding

Tungsten inert gas (TIG) welding became an overnight success in the 1940s for joining magnesium and aluminium. Using an inert gas shield instead of a slag to protect the weldpool, the process was a highly attractive replacement for gas and manual metal arc welding. TIG has played a major role in the acceptance of aluminium for high quality welding and structural applications.

Process characteristics

In the TIG process the arc is formed between a pointed tungsten electrode and the workpiece in an inert atmosphere of argon or helium. The small intense arc provided by the pointed electrode is ideal for high quality and precision welding. Because the electrode is not consumed during welding, the welder does not have to balance the heat input from the arc as the metal is deposited from the melting electrode. When filler metal is required, it must be added separately to the weldpool.

Power source

TIG must be operated with a drooping, constant current power source - either DC or AC. A constant current power source is essential to avoid excessively high currents being drawn when the electrode is short-circuited on to the workpiece surface. This could happen either deliberately during arc starting or inadvertently during welding. If, as in MIG welding, a flat characteristic power source is used, any contact with the workpiece surface would damage the electrode tip or fuse the electrode to the workpiece surface. In DC, because arc heat is distributed approximately one-third at the cathode (negative) and two-thirds at the anode (positive), the electrode is always negative polarity to prevent overheating and melting. However, the alternative power source connection of DC electrode positive polarity has the advantage in that when the cathode is on the workpiece, the surface is cleaned of oxide contamination. For this reason, AC is used when welding materials with a tenacious surface oxide film, such as Aluminium.

Arc starting

The welding arc can be started by scratching the surface, forming a short-circuit. It is only when the short-circuit is broken that the main welding current will flow. However, there is a risk that the electrode may stick to the surface and cause a tungsten inclusion in the weld. This risk can be minimised using the 'lift arc' technique where the short-circuit is formed at a very low current level. The most common way of starting the TIG arc is to use HF (High Frequency). HF consists of high voltage sparks of several thousand volts, which last for a few microseconds. The HF sparks will cause the electrode - workpiece gap to break down or ionise. Once an electron/ion cloud is formed, current can flow from the power source.

Note: As HF generates abnormally high electromagnetic emission (EM), welders should be aware that its use can cause interference especially in electronic equipment. As EM emission can be airborne, like radio waves, or transmitted along power cables, care must be taken to avoid interference with control systems and instruments in the vicinity of welding.

HF is also important in stabilising the AC arc; in AC, electrode polarity is reversed at a frequency of about 50 times per second, causing the arc to be extinguished at each polarity change. To ensure that the arc is reignited at each reversal of polarity, HF sparks are generated across the electrode/workpiece gap to coincide with the beginning of each half-cycle.

Electrodes

Electrodes for DC welding are normally pure tungsten with 1 to 4% thoria to improve arc ignition. Alternative additives are lanthanum oxide and cerium oxide, which are claimed to give superior performance (arc starting and lower electrode consumption). It is important to select the correct electrode diameter and tip angle for the level of welding current. As a rule, the lower the current the smaller the electrode diameter and tip angle. In AC welding, as the electrode will be operating at a much higher temperature, tungsten with a zirconia addition is used to reduce electrode erosion. It should be noted that because of the large amount of heat generated at the electrode, it is difficult to maintain a pointed tip and the end of the electrode assumes a spherical or 'ball' profile.

Shielding gas

Shielding gas is selected according to the material being welded. The following guidelines may help:

Argon - the most commonly-used shielding gas which can be used for welding a wide range of materials including steels, stainless steel, aluminium and titanium.

Argon + 2 to 5% H2 - the addition of hydrogen to argon will make the gas slightly reducing, assisting the production of cleaner-looking welds without surface oxidation. As the arc is hotter and more constricted, it permits higher welding speeds. Disadvantages include risk of hydrogen cracking in carbon steels and weld metal porosity in aluminium alloys.

Helium & Helium/Argon mixtures - adding helium to argon will raise the temperature of the arc. This promotes higher welding speeds and deeper weld penetration. A disadvantage of using helium or a helium/argon mixture is the high cost of gas and difficulty in starting the arc.

Applications

TIG is applied in all industrial sectors but is especially suitable for high quality welding. In manual welding, the relatively small arc is ideal for thin sheet material or controlled penetration (in the root run of pipe welds). Because deposition rate can be quite low (using a separate filler rod) MMA or MIG may be preferable for thicker material and for fill passes in thick-wall pipe welds.

TIG is also widely applied in mechanised systems either autogenously or with filler wire. However, several 'off the shelf' systems are available for orbital welding of pipes, used in the manufacture of chemical plant or boilers. The systems require no manipulative skill, but the operator must be well trained. Because the welder has less control over arc and weldpool behaviour, careful attention must be paid to edge preparation (machined rather than hand-prepared), joint fit-up and control of welding parameters.

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 aluminium, using argon as the shielding gas, and for carbon steels using CO2. CO2 and argon-CO2 mixtures are known as metal active gas (MAG) processes. 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 work piece; 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

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 1.2mm 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.

Submerged-arc Welding

The first patent on the submerged-arc welding (SAW) process was taken out in 1935 and covered an electric arc beneath a bed of granulated flux. Developed by the E O Paton Electric Welding Institute, Russia, during the Second World War, SAW's most famous application was on the T34 tank.

Process features

Similar to MIG welding, SAW involves formation of an arc between a continuously-fed bare wire electrode and the workpiece. The process uses a flux to generate protective gases and slag, and to add alloying elements to the weld pool. A shielding gas is not required. Prior to welding, a thin layer of flux powder is placed on the workpiece surface. The arc moves along the joint line and as it does so, excess flux is recycled via a hopper. Remaining fused slag layers can be easily removed after welding. As the arc is completely covered by the flux layer, heat loss is extremely low. This produces a thermal efficiency as high as 60% (compared with 25% for manual metal arc). There is no visible arc light, welding is spatter-free and there is no need for fume extraction.

Operating characteristics

SAW is usually operated as a fully-mechanised or automatic process, but it can be semi-automatic. Welding parameters: current, arc voltage and travel speed all affect bead shape, depth of penetration and chemical composition of the deposited weld metal. Because the operator cannot see the weld pool, greater reliance must be placed on parameter settings.

Process variants

According to material thickness, joint type and size of component, varying the following can increase deposition rate and improve bead shape.

Wire

SAW is normally operated with a single wire on either AC or DC current. Common variants are:

Twin wire

Triple wire

Single wire with hot wire addition

Metal powdered flux addition

All contribute to improved productivity through a marked increase in weld metal deposition rates and/or travel speeds.

Flux

Fluxes used in SAW are granular fusible minerals containing oxides of manganese, silicon, titanium, aluminium, calcium, zirconium, magnesium and other compounds such as calcium fluoride. The flux is specially formulated to be compatible with a given electrode wire type so that the combination of flux and wire yields desired mechanical properties. All fluxes react with the weld pool to produce the weld metal chemical composition and mechanical properties. It is common practice to refer to fluxes as 'active' if they add manganese and silicon to the weld, the amount of manganese and silicon added is influenced by the arc voltage and the welding current level. The the main types of flux for SAW are:

Bonded fluxes - produced by drying the ingredients, then bonding them with a low melting point compound such as a sodium silicate. Most bonded fluxes contain metallic deoxidisers which help to prevent weld porosity. These fluxes are effective over rust and mill scale.

Fused fluxes - produced by mixing the ingredients, then melting them in an electric furnace to form a chemical homogeneous product, cooled and ground to the required particle size. Smooth stable arcs, with welding currents up to 2000A and consistent weld metal properties, are the main attraction of these fluxes.

Applications

SAW is ideally suited for longitudinal and circumferential butt and fillet welds. However, because of high fluidity of the weld pool, molten slag and loose flux layer, welding is generally carried out on butt joints in the flat position and fillet joints in both the flat and horizontal-vertical positions. For circumferential joints, the workpiece is rotated under a fixed welding head with welding taking place in the flat position. Depending on material thickness, either single-pass, two-pass or multipass weld procedures can be carried out. There is virtually no restriction on the material thickness, provided a suitable joint preparation is adopted. Most commonly welded materials are carbon-manganese steels, low alloy steels and stainless steels, although the process is capable of welding some non-ferrous materials with judicious choice of electrode filler wire and flux combinations.

Electro slag welding

Description

Electro slag welding is a very efficient, single pass process carried out in the vertical or near vertical position and used for joining steel plates/sections in thicknesses of 25mm and above. It was developed by the Paton Institute in the Ukraine in the early 1950s and superseded the very high current submerged arc process for making longitudinal welds in thick-walled pressure vessels.

Unlike other high current fusion processes, electro slag welding is not an arc process. Heat required for melting both the welding wire and the plate edges is generated through a molten slag's resistance to the passage of an electric current.

In its original form, plates are held vertically approximately 30mm apart with the edges of the plate cut normal to the surface. A bridging run-on piece of the same thickness is attached to the bottom of the plates. Water-cooled copper shoes are then placed each side of the joint, forming a rectangular cavity open at the top. Filler wire, which is also the current carrier, is then fed into this cavity, initially striking an arc through a small amount of flux.

Additional flux is added which melts forming a flux bath, which rises and extinguishes the arc. The added wire then melts into this bath sinking to the bottom before solidifying to form the weld. For thick sections, additional wires may be added and an even distribution of weld metal is achieved by oscillating the wires across the joint. As welding progresses, both the wire feed mechanism and the copper shoes are moved progressively upwards until the top of the weld is reached.

Fig.1. Electro slag welding

The consumable guide variant of the process uses a much simpler set-up and equipment arrangement which does not require the wire feed mechanism to climb. In this case, the wire is delivered to the weld pool down a consumable, thick-walled tube which extends from the top of the joint to the weld pool. Support for the molten bath is provided by two pairs of copper shoes, which are moved upwards, leapfrogging each other as welding progresses. The tubular guides can be further supplemented by additional consumable plates attached to the tube. Generally, as the thickness of plate increases, the number of wires/guides increases, approximately in the ratio of one wire per 50mm of thickness,

Current status

In the fabrication industry, the process continues to be used for thick walled pressure vessels, which are post-weld normalised, and for structures such as blast furnace shells and steel ladles which are used at above ambient temperatures. The process is also extensively used for the welding of railway points.

Important current issues

Considerable interest was shown in electro slag welding during the 1970s when ideas for increasing welding speed were investigated. This was seen as an important parameter for increasing productivity and as a way of reducing heat input to improve HAZ and weld metal impact properties.

However, since that time little has been done by way of development. Those developments that have taken place have been limited to the tuning of parameters and tailoring techniques for specific applications.

Benefits

The principal benefits of the process are:

Speed of joint completion; typically 1 hour per metre of seam, irrespective of thickness

Lack of angular distortion

Lateral angular distortion limited to 3mm per meter of weld

High quality welds produced

Simple joint preparation, i.e. flame-cut square edge

Major repairs can be made simply by cutting out total weld and re-welding

Risks

Electro slag welding is not one of the major welding processes because the high heat input generates large, coarse grained weld metal and HAZs which lead to poor fracture toughness properties in these areas. Toughness improvements can only be achieved by post-weld normalising treatment. Additionally, the near parallel-sided geometry of the weld, combined with the coarse grains, can make it difficult to identify defects at the fusion boundary by standard ultrasonic NDT techniques.

The process has considerable potential for increasing productivity. However, its use has been limited because of relatively poor understanding of the process and, for specific applications, the significance of the fracture toughness values. As a result, use of the process has been restricted to a few niche applications.