The post Submerged-Arc Welding appeared first on PT. Bumi Teknik Utama.

Shape and depth of Penetration

The shape and depth of penetration of beads obtained with various shielding gases using DCEP polarity

Some shielding gas selections for GMAW of various metals

The post Shape and depth of Penetration appeared first on PT. Bumi Teknik Utama.

Thermal Gouging

Thermal gouging is an essential part of welding fabrication. Used for rapid removal of unwanted metal, the material is locally heated and molten metal ejected – usually by blowing it away. Normal oxyfuel gas or arc processes can be used to produce rapid melting and metal removal. However, to produce a groove of specific dimensions, particularly regarding depth and width, the welder must exercise careful control of the gouging operation. If this does not happen, an erratic and badly-serrated groove will result.

Thermal processes, operations and metals which may be gouged or otherwise shaped:

Note: All processes are capable of cutting/severing operations. Preheat may or may not be required on some metals prior to gouging

Safety

It should be emphasised that because gouging relies on molten metal being forcibly ejected, often over quite large distances, the welder must take appropriate precau- tions to protect himself, other workers and his equipment. Sensible precautions in- clude protective clothing for the welder, shielding inside a specially-enclosed booth or screens, adequate fume extraction, and removal of all combustible material from the immediate area.

Industrial applications

Thermal gouging was developed primarily for removal of metal from the reverse side of welded joints, removal of tack welds, temporary welds, and weld imperfections. Figure 1 illustrates the value of typical back-gouging applications carried out on arc welded joints., while Fig. 2 shows imperfection removal in preparation for weld repair.

Fig.1 Typical back-gouging applications carried out on arc welded joints

Fig. 2 Imperfection removal in preparation for weld repair

The gouging process has proved to be so successful that it is used for a wide spec- trum of applications in engineering industries :

• repair and maintenance of structures – bridges, earth-moving equipment, mining machinery, railway rolling stock, ships, offshore rigs, piping and storage tanks
• removal of cracks and imperfections – blow holes and sand traps in both ferrous and non-ferrous forgings and castings
• preparation of plate edges for welding
• removal of surplus metal – strongbacks, lifting lugs and riser pads and fins on castings, excess weld bead profiles, temporary backing strips, rivet washing and shaping operations demolition of welded and unwelded structures – site work.

Thermal gouging is also suitable for efficient removal of temporary welded attachments such as brackets, strongbacks, lifting lugs and redundant tack welds, during various stages of fabrication and construction work.

Gouging processes

Gouging operations can be carried out using the following thermal processes:

• oxyfuel gas flame
• manual metal arc
• air carbon arc
• plasma arc

The post Thermal Gouging appeared first on PT. Bumi Teknik Utama.

Weldability of Aluminium

Aluminium alloys

Aluminium and its alloys are used in fabrications because of their low weight, good cor- rosion resistance and weldability. Although normally low strength, some of the more complex alloys can have mechanical properties equivalent to steels. The various types of aluminium alloy are identified and guidance is given on fabricating components with- out impairing corrosion and mechanical properties of the material or introducing imper- fections into the weld.

Material types

As pure aluminium is relatively soft, small amounts of alloying elements are added to produce a range of mechanical properties. The alloys are grouped according to the principal alloying elements, Specific com- mercial alloys have a four-digit designation according to the international specifications for wrought alloys or the ISO alpha – nu- meric system.

The alloys can be further classified according to the means by which the alloying ele- ments develop mechanical properties, non-heat-treatable or heat-treatable alloys.

Non-heat-treatable alloys

Material strength depends on the effect of work hardening and solid solution hardening of alloy elements such as magnesium, and manganese; the alloying elements are mainly found in the 1xxx, 3xxx and 5xxx series of alloys. When welded, these alloys may lose the effects of work hardening which results in softening of the HAZ adjacent to the weld.

Heat-treatable alloys

Material hardness and strength depend on alloy composition and heat treatment (solution heat treatment and quenching followed by either natural or artificial ageing pro- duces a fine dispersion of the alloying constituents). Principal alloying elements are found in the 2xxx, 6xxx, 7xxx and 8xxx series. Fusion welding redistributes the harden- ing constituents in the HAZ which locally reduces material strength.

Most of the wrought grades in the 1xxx, 3xxx, 5xxx, 6xxx and medium strength 7xxx (e.g. 7020) series can be fusion welded using TIG, MIG and oxyfuel processes. The 5xxx series alloys, in particular, have excellent weldability. High strength alloys (e.g. 7010 and 7050) and most of the 2xxx series are not recommended for fusion welding because they are prone to liquation and solidification cracking.

Filler alloys

Filler metal composition is determined by:

• weldability of the parent metal
• minimum mechanical properties of the weld metal
• corrosion resistance
• anodic coating requirements

Nominally matching filler metals are often employed for non-heat-treatable alloys. However, for alloy-lean materials and heat-treatable alloys, non-matching fillers are used to prevent solidification cracking.

The choice of filler metal composition for the various weldable alloys is specified in BS 3019 Pt 1 for TIG and BS 3571 Pt 1 for MIG welding; recommended filler metal compositions for the more commonly used alloys are given in the Table.

Imperfections in welds

Aluminium and its alloys can be readily welded providing appropriate precautions are taken. The most likely imperfections in fusion welds are:

• porosity
• cracking
• poor weld bead profile

Porosity

Porosity is often regarded as an inherent feature of MIG welds; typical appearance of finely distributed porosity in a TIG weld is shown in the photograph. The main cause of porosity is absorption of hydrogen in the weld pool which forms discrete pores in the solidifying weld metal. The most common

sources of hydrogen are hydrocarbons and moisture from contaminants on the parent material and filler wire surfaces, and water vapour from the shielding gas atmosphere. Even trace levels of hydrogen may exceed the threshold concentration required to nucleate bubbles in the weld pool, aluminium being one of the metals most susceptible to porosity.

To minimise the risk, rigorous cleaning of material surface and filler wire should be carried out. Three cleaning techniques are suitable; mechanical cleaning, solvent degreasing and chemical etch cleaning.

Mechanical cleaning

Wire brushing (stainless steel bristles), scraping or filing can be used to remove surface oxide and contaminants. Degreasing should be carried out before mechanical cleaning.

Solvents

Dipping, spraying or wiping with organic solvents can be used to remove grease, oil, dirt and loose particles.

Chemical etching

A solution of 5% sodium hydroxide can be used for batch cleaning but this should be fol- lowed by rinsing in HNO3 and water to remove reaction products on the surface.

In gas shielded welding, air entrainment should be avoided by making sure there is an efficient gas shield and the arc is protected from draughts. Precautions should also be taken to avoid water vapour pickup from gas lines and welding equipment; it is recom- mended that the welding system is purged for about an hour before use.

Solidification cracks

Cracking occurs in aluminium alloys because of high stresses generated across the weld due to the high thermal expansion ( twice that of steel) and the sub- stantial contraction on solidification – typically 5 % more than in equivalent steel welds.

Solidification cracks form in the centre of the weld,,

usually extending along the centreline during solidification. Solidification cracks also oc- cur in the weld crater at the end of the welding operation. The main causes of solidification cracks are as follows:

• incorrect filler wire/parent metal combination
• incorrect weld geometry
• welding under high restraint conditions

The cracking risk can be reduced by using a non-matching, crack-resistant filler (usually from the 4xxx and 5xxx series alloys). The disadvantage is that the resulting weld metal may have a lower strength than the parent metal and not respond to a subsequent heat treatment. The weld bead must be thick enough to withstand contraction stresses. Also, the degree of restraint on the weld can be minimised by using correct edge preparation, accurate joint set up and correct weld sequence.

Liquation cracking

Liquation cracking occurs in the HAZ, when low melting point films are formed at the grain boundaries. These cannot withstand the contraction stresses generated when the weld metal solidifies and cools. Heat treat- able alloys, 6xxx, 7xxx and 8xxx series alloys, are more susceptible to this type of cracking.

The risk can be reduced by using a filler metal with a lower melting tem- perature than the parent metal, for ex-

ample the 6xxx series alloys are welded with a 4xxx filler metal. However, 4xxx filler metal should not be used to weld high magnesium alloys (such as 5083) as excessive magnesium-silicide may form at the fusion boundary decreasing ductility and increasing crack sensitivity.

Poor weld bead profile

Incorrect welding parameter settings or poor welder technique can introduce weld profile imperfections such as lack of fusion, lack of penetration and undercut. The high thermal conductivity of aluminium and the rapidly solidifying weld pool make these alloys particu- larly susceptible to profile imperfections.

The post Weldability of Aluminium appeared first on PT. Bumi Teknik Utama.

Plasma Arc Cutting

Plasma arc cutting has always been seen as an alternative to the oxy-fuel process. However, the important difference between the two processes is that while the oxygen-fuel process oxidises the metal and the heat from the exothermic reaction melts the metal, the plasma process operates by using the heat from the arc to melt the metal. The ability to melt the metal without oxidation is essential when cutting metals, such as stainless steel, which form high temperature oxides. The plasma process was therefore first introduced for cutting stainless steel and aluminium alloys. The first plasma torches gave poor quality cuts and the process itself suffered from excessive noise and fume, especially when cutting thicker material. Over the last thirty years, the plasma cutting process has been highly refined and is now capable of producing high quality cuts, at increased speeds, in a wide range of material thicknesses.

Process variants

The basic plasma torch (Fig 1a) consists of a central tungsten electrode for forming the

arc but, unlike in the conventional TIG welding process, the arc is constricted by a fine

bore copper nozzle. This has the effect of increasing the temperature and velocity of

the plasma emanating from the nozzle. The temperature of the plasma is in excess of

20,000K and its velocity can approach the speed of sound [1]. The process variants

(Figs 1b to 1g) have been principally designed to improve the quality of the cut and the

cutting performance, or to reduce operating costs. The most important process

variants are described below.

Dual gas: The process operates basically in the same manner as the conventional system but a secondary gas shield is introduced around the nozzle (Fig 1b). The cutting gas is normally argon, argon/H2 or nitrogen and the secondary gas is selected according to the metal being cut. For example, when cutting mild steel, air or oxygen can be used to increase the cutting speed.

Water injection: Water can be injected radially into the plasma arc (Fig 1c) to induce a greater degree of constriction. The temperature of the plasma is considerably increased (30,000°C) which facilitates higher cutting speeds and, because of the greater constriction of the arc, much improved cut quality. The presence of an annular film of water around the plasma will protect the nozzle bore, reducing nozzle erosion.

Water shroud: The plasma arc can also be operated either with a water shroud (Fig 1d) or even with the workpiece submerged some 50 to 75 mm below the surface of the water. The water will act as a barrier in reducing fume and noise levels. In a specific example of noise levels at high current levels in excess of 115 dB, this can be reduced to about 96 dB with a water shroud and 52 to 85 dB when cutting underwater [2].

Air plasma: The inert or unreactive plasma forming gas (argon or nitrogen) can be replaced with air but this requires a special electrode of hafnium or zirconium mounted in a copper holder (Fig 1e). The use of compressed air instead of the more expensive cylinder gas, makes this variant of the plasma arc process highly competitive with the oxy-fuel process. A variant of the air plasma process is the monogas torch in which air is used for both the plasma and the cooling gas.

It is generally considered that air plasma is more widely applied in general engineering and light fabrication industries, eg, in cutting sheet steel within the thickness range 1 to 20 mm [3]. The more popular materials are C-Mn and stainless steels but the process has also been used for cutting SG (spheroidal graphite) iron and non-ferrous materials [4]. For thin section material of a few millimetres, the process is much faster than oxy-

fuel, but at thicknesses approaching 30 to 40 mm, air plasma becomes relatively slow [5].

The obvious cost advantages of using air in preference to expensive gases (for the plasma and oxy-fuel processes) must be considered when other operating costs have also been taken into account. For example, the air must be fed at a relatively high pressure (typically 150 1/min at 5 bar) and clean.

This will require an adequate size compressor for a line feed supply with suitable filters for dust particles and oil. Additionally, special purpose electrodes will be required and the operating life of the electrodes and nozzles can be severely shortened if there are frequent stop/starts [6].

Low current air plasma torches, typically less than 40A, are particularly attractive for cutting thin sheet material, in that compressed air is used for both the plasma forming gas and cooling the torch. Moreover, the torch head can be held in contact with the surface of the metal being cut (Fig 1f), without the risk of damaging the nozzle by forming a secondary or series arc between the electrode/nozzle and the workpiece. As nitrogen and oxygen suppress the formation of a series arc, compared to argon, contact cutting can be practised with the air plasma system [6]. The process is becoming more widely used for manual cutting of thin sheet components in both C-Mn and stainless steel, where contact cutting greatly deskills the cutting operation.

High tolerance plasma: In an attempt to improve cut quality and to compete with the superior cut quality of laser systems, a number of plasma systems are available commercially which operate with a highly constricted plasma arc; the systems under the generic name high-tolerance plasma arc cutting (HTPAC) are manufactured by Hypertherm, Koike Aronson, Abicor Binzel and Komatsu-Cybernation [7]. The common features of the torches (Fig 1g) are that the oxygen plasma jet is forced to swirl as it enters the plasma orifice and a secondary flow of gas is injected down stream of the plasma nozzle. The Komatsu-Cybernation torch has a separate magnetic field surrounding the arc which stabilises it by maintaining the rotation induced by the swirling gas

It is claimed that the cut quality lies between a conventional plasma arc cut and laser beam cutting, but the cutting speed is significantly lower than conventional plasma arc cutting and approximately 60 to 80% the speed of laser cutting [2]. Cutting

speeds can be several m/min for materials with thicknesses up to 6 mm.

However, for high quality cuts, ie, minimum kerf width, the optimum speed is more typically 1.5 m/min for 1 mm, reducing to about 500 mm/min for 6 mm (C-Mn) sheet material. An example of a high definition system, which is mounted on torch manipulation equipment with an accuracy of ±0.1 mm is shown in Fig 2.

Applications

The plasma process can be used for cutting a wider range of materials than the oxy-fuel process which is largely restricted to C-Mn steels. It is, therefore, not surprising that the plasma arc process is being used increasingly for cutting materials such as stainless steel (Fig 3), aluminium and coated steels. Even in cutting C-Mn steel, the plasma process

can have advantages over the oxy-fuel process such as a higher cutting speed and a narrow heat affected zone.

For example, there is a substantial speed advantage for cutting thin section materials of less than 30 mm but this disappears very quickly as the plate thickness increases.

The gas in the plasma process has a significant effect on performance and it is generally considered that the best quality is achieved with oxygen for mild steel, and nitrogen or argon/ hydrogen for aluminium and stainless steel [8]. However, from an evaluation of gases for cutting C-Mn steel, aluminium and stainless steel, the maximum cutting speeds were obtained with air and argon/35% H2 compared to nitrogen and argon/15% H2 [3]. In a comparison with nitrogen, air plasma was faster for C-Mn steel and aluminium, but slightly slower for stainless steel.

The overall conclusion was that in terms of cut quality and cutting speeds, air plasma would normally be recommended for cutting C-Mn steels in preference to gas plasma, while gas plasma with argon/15% H2 is preferred for cutting stainless steel and aluminium.

Current industrial practice for mixed gas cutting equipment is to use argon 25/35% H2 for cutting stainless steel and aluminium, which is a compromise between quality and cutting speed.

Heavy duty cutting systems can be used for cutting stainless steel up to 130 mm and aluminium up to 150 mm. Argon/H2 mixtures are recommended, but for materials below 75 mm in thickness, nitrogen, air or argon/H2 can be used [8].

The advantages and disadvantages of plasma cutting are summarised below :

Advantages

Can be used with a wide range of materials, including stainless steel and aluminium

High quality cut edges can be achieved, e.g. the HTPAC process

Narrow HAZ formed

Low gas consumable (air) costs

Ideal for thin sheet material

Low fume (underwater) process

Disadvantages

Limited to 50mm (air plasma) thick plate

High noise especially when cutting thick sections in air

High fume generation when cutting in air

Protection required from the arc glare

High consumable costs (electrodes and nozzles)

The post Plasma Arc Cutting appeared first on PT. Bumi Teknik Utama.

TIG Welding

Tungsten inert gas (TIG) welding became an overnight success in the 1940s for joining mag-nesium and aluminium. Using an inert gas shield instead of a slag to protect the weldpool, the process was a highly attractive replace-ment for gas and manual metal are welding. TIG has played a major role in the acceptance of aluminium for high quality welding and struc-tural 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 work-piece 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 inclu-sion 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 elec-trode – 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 begin-ning 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 weld-ing 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’ pro-file.

Shielding gas

Shielding gas is selected according to the material being welded. The following guide-lines 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 tita-nium.

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 sur-face oxidation. As the arc is hotter and more constricted, it permits higher weld-ing speeds. Disadvantages include risk of hydrogen cracking in carbon steels and weld metal porosity in aluminium alloys.

Helium and helium/argon mixtures – adding helium to argon will raise the temperature of the arc. This promotes higher welding speeds and deeper weld penetration. Disadvantages 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 manipu-lative 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.

Equipment for TIG welding

TIG welding process is using an inert gas shield, instead of a slag to protect the weld-pool. This technology is a highly attractive alternative to gas and manual metal arc welding and has played a major role in the acceptance of high quality welding in critical applications.

Essential equipment

In TIG, the arc is formed between the end of a small diameter tungsten electrode and the workpiece. The main equipment components are:

Power source

Torch

Backing system

Protective equipment

Power source

The power source for TIG welding can be either DC or AC but in both the output is termed a drooping, or constant current, characteristic; the arc voltage / welding current relationship delivers a constant current for a given power source setting. If the arc volt-age is slightly increased or decreased, there will be very little change in welding cur-rent. In manual welding, it can accommodate the welder’s natural variations in arc length and, in the event of the electrode touching the work, an excessively high current will not be drawn which could fuse the electrode to the workpiece.

The arc is usually started by HF (High Frequency) sparks which ionise the gap be-tween the electrode and the workpiece. HF generates airborne and line transmitted interference, so care must be taken to avoid interference with control systems and in-struments near welding equipment. When welding is carried out in sensitive areas, a non-HF technique, touch starting or lift arc, can be used. The electrode can be short circuited to the workpiece, but the current will only flow when the electrode is lifted off the surface. There is, therefore, little risk of the electrode fusing to the workpiece sur-face and forming tungsten inclusions in the weld metal. For high quality applications, using HF is preferred.

DC power source

DC power produces a concentrated arc with most of the heat in the workpiece, so this power source is generally used for welding. However, the arc with its cathode roots on the electrode (DC electrode negative polarity), results in little cleaning of the workpiece surface. Care must be taken to clean the surface prior to welding and to ensure that there is an efficient gas shield.

Transistor and inverter power sources are being used increasingly for TIG welding.

The advantages are :

The smaller size makes them easily transported Arc ignition is easier

Special operating features, e.g. current pulsing, are readily included The output can be pre-programmed for mechanised operations

The greater stability of these power sources allows very low currents to be used par-ticularly for micro-TIG welding and largely replaced the plasma process for micro weld-ing operations.

AC power source

For materials such as aluminium, which has a tenacious oxide film on the surface, AC power must be employed. By switching between positive and negative polarity, the periods of electrode positive will remove the oxide and clean the surface.

Disadvantages of conventional, sine wave AC compared with DC are:

The arc is more diffuse

HF is required to reignite the arc at each current reversal

Excessive heating of the electrode makes it impossible to maintain a tapered point and the end becomes balled

Square wave AC, or switched DC, power sources are particularly attractive for welding aluminium. By switching between polarities, arc reignition is made easier so that the HF can be reduced or eliminated. The ability to imbalance the waveform to vary the proportion of positive to negative polarity is important by determining the relative amount of heat generated in the workpiece and the electrode.

To weld the root run, the power source is operated with the greater amount of positive polarity to put the maximum heat into the workpiece. For filler runs a greater propor-tion of negative polarity should be used to minimise heating of the electrode. By using 90% negative polarity, it is possible to maintain a pointed electrode. A balanced posi-tion (50% electrode positive and negative polarities) is preferable for welding heavily oxidised aluminium.

Torch

There is a wide range of torch designs for welding, according to the application. De-signs which have the on/off switch and current control in the handle are often preferred to foot controls. Specialised torches are available for mechanised applications, e.g. or-bital and bore welding of pipes.

Electrode

For DC current, the electrode is tungsten with between 2 and 5% thoria to aid arc ini-tiation. The electrode tip is ground to an angle of 600 to 900 for manual welding, irre-spective of the electrode diameter. For mechanised applications as the tip angle de-termines the shape of the arc and influences the penetration profile of the weld pool, attention must be paid to consistency in grinding the tip and checking its condition be-tween welds.

For AC current, the electrode is either pure tungsten or tungsten with a small amount (up to 0.5%) of zirconia to aid arc reignition and to reduce electrode erosion. The tip normally assumes a spherical profile due to the heat generated in the electrode during the electrode positive half cycle.

Gas shielding

A gas lens should be fitted within the torch nozzle, to ensure laminar gas flow. This will improve gas protection for sensitive welding operations like welding vertical, corner and edge joints and on curved surfaces.

Backing system

When welding high integrity components, a shielding gas is used to protect the under-side of the weld pool and weld bead from oxidation. To reduce the amount of gas con-sumed, a localised gas shroud for sheet, dams or plugs for tubular components is used. As little as 5% air can result in a poor weld bead profile and may reduce corro-sion resistance in materials like stainless steel. With gas backing systems in pipe welding, pre-weld purge time depends on the diameter and length of the pipe. The flow rate/purge time is set to ensure at least five volume changes before welding.

Stick on tapes and ceramic backing bars are also used to protect and support the weld bead. In manual stainless steel welding, a flux-cored wire instead of a solid wire can be used in the root run. This protects the underbead from oxidation without the need for gas backing.

Inserts

A pre-placed insert can be used to improve the uniformity of the root penetration. Its main use is to prevent suck-back in an autogenous weld, especially in the overhead position. The use of an insert does not make welding any easier and skill is still re-quired to avoid problems of incomplete root fusion and uneven root penetration.

Protective equipment

A slightly darker glass should be used in the head or hand shield than that used for MMA welding. Recommended shade number of filter for TIG welding

Shade number Welding current A

9 less than 20

10 20 to 40

11 40 to 100

12 100 to 175

13 175 to 250

14 250 to 400

TIG  WELDING  PARAMETERS

The post Tig Welding appeared first on PT. Bumi Teknik Utama.

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

The post Solid Wire MIG Welding appeared first on PT. Bumi Teknik Utama.

The post The Manual Metal Arc (MMA) Process appeared first on PT. Bumi Teknik Utama.