Norbert Knopp / Heinz Lorenz, Mündersbach and Robert Killing, Solingen, Germany
Introduction
“Higher, further, faster” is today more than just a maxim in top-class sport; there are always new challenges inmodern technology which follow this principle. Whether it’s the height of skyscrapers, the span of bridges or thespeed of trains, materials and welded joints are required to meet increasingly high demands and weldingtechnology is expected to function without any errors.
For example, to realise the technology required for higher speeds and/or lower power consumption, lightweightconstruction techniques have established themselves in vehicle construction (Figure 1). As well as modifieddesigns, this has required in particular the use of aluminium and aluminium alloys of high-strength steels.
Due to its physical properties, aluminium as a material does many different things during welding and evenexperienced steel welders have initial difficulties when required to switch to aluminium. However, thanks tocontinued advancements in power sources, welders have a functional tool for welding aluminium materials. Thisarticle covers this topic.1
Why is welding aluminium different fromwelding steel
Table 1 compares the physical properties of iron andaluminium. It becomes clear that there are significantdifferences between the two metals in someimportant properties for welding [1].
Specifically, the physical properties of aluminium canhave the following negative effects:
to a spray arc even at relatively low currentintensities. Without MIG pulse welding, out-of-position welding on thicker aluminium structureswould be therefore not possible at all.
Due to the low solubility of aluminium in hydrogen inthe solid state, complicated cleaning work of thematerial surface and some pre-heating is required inorder to keep pore formation within reasonable limits.This is because aluminium oxide is hygroscopic.
The points mentioned above indicate that for error-free welds on aluminium, the welder needs to be wellversed in the properties of aluminium, and in terms ofthe technology, only the best is good enough foraluminium welding.
2 The shielding gas is also a critical factorPreviously it was mainly argon that was used as theshielding gas for MIG welding aluminium. Using thisshielding gas means that a quiet, low-spatter sprayarc can be set and the pulse arc can also be used togood effect using argon. The low heat retention andthe poor thermal conductivity of argon create a wide
Figure 1
MIG aluminium welding in vehicle construction
Physical propertyDensity of the metalDensity of the oxideMelting point of the metalMelting point of the oxideSolubility in hydrogen inneutral state1)Heat-conductanceElectrical conductance
1)
Unit
g/cm3g/cm3
°C°C
cm3/100 gW/(cm x K)S x m/min2
Iron
7.853.715391460...1580
80.5810
Aluminium
2.73.466020500.052.235
The density of aluminium is so low that aluminiumoxide is heavier than the metal itself. On the otherhand, the melting point of the aluminium oxide is veryhigh. Both promote the formation of oxide inclusionsin the weld.
The low melting point of aluminium – it melts evenbefore it starts to glow significantly – makes it difficultto weld through the root. The molten pool dropsthrough more easily.
The high degree of thermal conductivity– more thanthree times as high as iron – requires intensive heatfeeding. The undesirable consequences of excessiveheat dissipation may be bonding errors, insufficientsingle layer welding and pores. Large materialthicknesses therefore need to be pre-heated toensure adequate fusion penetration and sufficientexhalation of the weld.
Due to the high electrical conductance of aluminium,the range of the short arc is very small and it changes
immediately on solidification
Differences in the main physical properties
between iron and aluminium in relation to welding
Table 1
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seam in the upper part, which, however, leaves theseam with a narrow, finger-shaped fusion penetrationshape in the lower part.
Helium as a shielding gas does not have thesedisadvantages, thus producing wider and deeperfusion penetration in comparison to argon. Purehelium is rarely used for MIG welding, but insteadargon/helium mixtures containing between 30 to 70% of helium (the remainder being argon).
When deeper fusion penetration is not required, e.g.when welding thinner materials, the greater degree ofheat retention and the improved thermal conductivityof shielding gases containing helium can beconverted at greater welding speeds. Figure 2clarifies this point in a schematic diagram. Thanks tothe high-energy arc, the pre-heating can also bereduced or may not be necessary at all up to certainthicknesses.
Argon/helium mixtures require different parametersettings than when welding with pure argon. Byadding helium to the argon, the electricalconductance of the arc path is reduced. This meansthat at the same wire feed rate, the arc becomesshorter the greater the amount of helium used, if the
Shielding gasArgonArgon/Helium 50/50Argon/Helium 50/50260 A / 27V260 A / 32V260 A / 32VvS100%vS100%vS140%Figure 2Fusion penetration profile with different shielding
gases (material AlMg3, 1.6mm ∅ wire electrode)
arc voltage is not adjusted. At 50 % He / 50 % Ar, thisequates to a voltage increase of around 4-5 Volts.The low density of helium also requires greatershielding gas flow rates to provide sufficient shieldingagainst the atmosphere.
Shielding gases containing helium are moreexpensive than pure argon. However, this is morethan compensated by the higher welding speedpossible and the fact that pre-heating may no longerbe required.
3 Modern machines make welding easier
As mentioned above, only the best equipment isgood enough when welding aluminium because ofthe difficult conditions produced by the material. Thisapplies both to the characteristics of the powersource and in a very specific way to the wire feedunits. Today, digital inverters (Figure 3) are generallyused as power sources for these welding tasks.
There are special demands on all components of thewire feed unit when welding aluminium because thewire electrodes used are very soft and therefore bendvery easily when being fed through. A few figures forcomparative purposes: A wire electrode made frompure aluminium has a level of strength after drawing
which is normally only a little over 100 N/mm2
.Aluminium alloys can at the very best provide a wire
© 2002 EWM HIGHTEC WELDING GmbHFigure 3Inverter power sources for MIG/MAG welding
strength of 300 N/mm2
. In comparison, Si/Mn-alloyedwire electrodes for welding steel with a diameter of
1.2 mm achieve strengths of over 900 N/mm2
[2]. Thebuckling strength of aluminium wire is alsocorrespondingly lower. For this reason, only shorttube packages can be used and the internal wirefeed tube should be made from plastic due to theimproved gliding properties. In addition, it is essentialthat the feed rollers do not damage the soft surfaceof the wire. Greater pressure points between therollers must therefore be avoided. Multi-roller driveunits have proven to be the most useful here (Figure4). Instead of the steel, trapezoidal groove, the driverollers should ideally have a rounded groove.
With longer feed paths, intermediate feeds arerequired or push/pull torches can be used, where thewire is not just advanced by the machine, but is alsodrawn forwards into the torch (Figure 5). With torchesof this type, even thinner wire electrodes made fromaluminium can also be transported across longerdistances without problems. Small spool torches canalso be used for very thin wire electrodes. An evenwire feed is important because irregularities in the
Figure 4View into a wire feed unit with 4-roller drive
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Figure 5Push/Pull torch
wire feed process are reproduced as fluctuations inthe welding parameters.4
Good welding results need the rightwelding parameters
In MIG/MAG welding, two adjustments are normallyalways required. The wire feed rate and thus thewelding current intensity are set on the wire feed unit,and the arc length and therefore the welding voltageare set by choosing a suitable characteristic on thepower source. This manual setting requiresconsiderable experience from the welder. However,not every company always has sufficiently welltrained welders available. Modern MIG/MAG systemsprovide simplified options for setting the weldingparameters.
5 Synergetic setting of welding parametersIt started as early as the 70s with one-dial operationwhere a single rotary potentiometer was used to setthe output by changing the wire feed and the sameadjustment knob in a specific translation ratio wasconnected to infinite characteristic adjustment formodifying the voltage simultaneously. It was alsopossible to correct the operating point to a certainextent using a second knob.
Today a more far-reaching simplification of the
setting process is part of standard modern MIG/MAGsystems.
The ideal working characteristics for frequently usedwelding tasks are saved on the machine. All theoperator of the system then needs to do, forexample, is use buttons to set the material beingwelded, the required wire diameter and theconnected shielding gas. This calls up the ideal pre-programmed working characteristic. The output canbe infinitely adjusted on a rotary dial and individualrequirements relating to the optimum arc length canalso be set using a correction control. Figure 6 showsthe control panel on a modern welding systemequipped with even more sophisticated settings. Inthe centre part, the welding task can be set using jogbuttons. As well as the material, the wire electrodediameter and the shielding gas, it is still possible tospecify whether solid wire or flux-cored wire is usedfor welding, or whether there are special tasks athand such as MIG soldering or deposit welding.
The different levels of electrical conductance of purealuminium, AlSi alloys and AlMg alloys requiremodified welding voltages. For this reason, as can beseen on the control panel, some characteristics arepre-programmed for these material groups, wherebyan argon/helium mixture can also be selected inaddition to pure argon.
As the system is a multi-process system, the requiredchanges to the characteristics are made in the centrepanel, as well those for other processes (TIG, MMA).In the left-hand part of the display, the output canthen be set on the top rotary dial, the centre rotarydial can be used to correct the arc length and thelower dial changes the arc dynamics electronically.More on this later. The current intensity and voltagerelating to the selected operating point are shown onthe display along with the weldable sheet metal
Figure 6Control panel of the PHOENIX 300 EXPERT PULSE welding machine
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thickness. The welding data used can be saved andretrieved at a later time.
6 It’s much easier with a pulse arc
In MIG pulse arc welding, the current and voltagepulse between a low base level and a short-termhigher pulse-shaped level. With the values for thepulse current height and the duration of the pulse setcorrectly, the drip transition takes place in the samerhythm as the pulse frequency (Figure 7). Thisensures virtually spatter-free welding.
The duration of the pulse arc covers the entire rangefrom the lowest to the highest current intensities. Inthis process, the pulse frequency increases with theoutput from around 15 Hz in the lower range toaround 300 Hz at high current intensities.
For MIG welding aluminium the pulse arc isparticularly important because as mentioned above,the duration of the short arc is very small, meaningthat out-of-position welding on slightly thickermaterials is only possible using a pulse arc. The heatfeeding can also be more easily regulated using apulse arc.
Unlike normal MIG welding, with MIG pulse welding agreater number of parameters needs to be set,namely the level of the base current (voltage), pulsecurrent (voltage), base time and pulse time, inaddition to the wire feed. Modern digital arc modelsuse this to calculate other parameters, such as thepulse frequency. As these values are also specific tothe material, the option of synergetic settings (Figure6) offers an additional advantage here. Thesecharacteristics are also saved in the machine forMIG/MAG pulse arc welding.
7 Special programs are the icing on the cakeIn the software on modern MIG/MAG systems, notonly sets of parameters but also entire weldingprograms can be saved and retrieved later on.
Some of these programs are explained in more detailbelow. They normally operate according to a workingcharacteristic saved on the machine, which isselected on the basis of the material, wire electrodediameter and shielding gas.
8 Safe ignition is important
With MIG/MAG welding it is not usual to retract the
Figure 7Drip detachment in MIG/MAG pulse welding
© 2002 EWM HIGHTEC WELDING GmbHend of the wire after the first contact with the surfaceof the workpiece to give the arc space, with theexception of the first few steps in MIG welding. Alsothis is not normally required because the arc burnsfreely at the ignition point due to the high currentdensity at the ignition point itself. With aluminium it’srather a different story. Due to the good electricalconductance, the transition resistance between theend of the electrode and the surface of the workpieceis relatively low and the heating there iscorrespondingly low. It is therefore advisable to igniteusing a current pulse defined in terms of the heightand the time.
If this occurs at too high a wire feed speed, however,the retracting wire extinguishes the arc, which is stillvery small to begin with. This results in repeatedignition processes in quick succession before the arclength is sufficiently ionised and the arc is burningstably. To avoid this, a lower wire feed speed, knownas wire creep, is used for ignition to achieve a gentlepositioning of the wire onto the workpiece.
In modern digital power sources these ignitionprocesses are already programmed for the individualwire and gas combinations.
Once the first arc is burning, the fusion penetrationcontinues to be very low on the still cold basematerial and this can result in fusion penetrationerrors at this stage. For this reason, increasedwelding energy is used for a short time to start thewelding process. How this type of program works isshown in Figure 8. First of all, the shielding gasbegins to flow. This starts the wire electrode movingat the programmed “creep speed” and ignites the arcwhen it comes into contact with the workpiece usingthe programmed ignition current. For an adjustabletime, the welding process is started at increasedwelding power in order to avoid cool points (Pstart).Only then does the machine switch over to the actualworking program (PA).
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Figure 8Ignition program9 The working program (PA)
The appropriate current intensity and voltage for theactual welding process are programmed in theworking program. If required, these values in theworking program can also be changed, e.g. when thearc approaches corners or edges, the current andvoltage can be reduced to avoid overheating.10
Avoiding welding seam irregularities usingan end program
It is desirable to keep the craters that form at the endof the seam as small as possible in order to avoidend-crater shrinkage cavities and cracks. The manualwelder achieves this in conventional systems bymaking circular movements across the end point ofthe seam with the arc extended, before the arcextinguishes. With modern MIG/MAG systems anend program (PEND) can be entered (Figure 9). Theworking current intensity is first run down for aspecific period (slope-down), held at a lower level fora certain time and then the wire feed is turned off. AstIPENDttFigure 9End program© 2002 EWM HIGHTEC WELDING GmbHthe power source is not switched off, however, thearc continues to burn for a time on the wire (wireburn-back) before the power source is switched off aswell. This prevents the end of the wire dipping intothe end-crater and freezing solid inside it. The freeburning phase must not be too long, however,because otherwise the arc melts onto the contactnozzle.
Another disadvantage of an excessively long free-burn time is that a thick drip forms on the end of thewire. The manual welder cuts this using a side cutterbefore the next ignition process. With mechanicalwelding this is not generally possible, however. Herethe thickened end of the wire can result in ignitionfaults. Modern systems therefore also provide theoption of throwing off the drip formed on the end ofthe wire with a final pulse. The result is a properlyspiked end of the wire that can be easily ignited nexttime. If the arc needs to be fully extinguished, a shortpost-flow phase is used for the shielding gas. Onlythen is the end program complete.
11 Special functions
Modern MIG/MAG systems provide even moreoptions that can be used to control the weldingprocess.
12 Setting the arc pressure
As mentioned above, the operating panel of thewelding system shown in Figure 6 provides the optionof adjusting the dynamics. This means that the arccan be adjusted to be harder or softer. This occursvia an adjustable current dynamic, similar to a chokeeffect. In the short arc range, which is of littlesignificance for welding aluminium, this can be usedto accelerate the drip separation process by reducingthe choke effect. This makes the spatter formationslightly greater, but the arc slightly more directionallystable.
Of more interest is the adjustable dynamic in MIGpulse welding. Here the reduction in the choke effectmeans that the pulse current increases. This alsoincreases the effective current intensity resulting fromthe base current and pulse current. Oneconsequence of this is that the concentric magneticfield surrounding the arc is amplified. This constrictsthe arc and increases the pinch effect. The harderarc becomes less easily diverted by magnetic fieldsand generates narrower, deeper fusion penetration.The dynamics must be set according to the relevantconditions, i.e. when welding thinner metal sheets asofter arc is preferable, and with thicker sheets, aharder arc.
13 Super pulse function
In addition to pulses for targeted drip separation inthe range between around 15 and 300 Hertzdepending on the set current intensity, modernMIG/MAG welding systems also provide the option ofpulsing at just a few Hertz (Figure 10). Here thewelding power with adjustable frequency and pulse-duty factor is switched backwards and forwards
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between a high and a low level. This pulsing at lowfrequency has no effect on the drip separationprocess. However, this does produce reduced heatfeeding. This can be used when welding root passes,when welding in the V-up position and when weldingthin sheet metal. Sometimes this pulsing is also usedwhen welding final passes. This produces rough, butvery even seam ripples, as is familiar from TIG pulsewelding (Figure 11). This type of seam appearance issometimes desirable for visual reasons.
14 Online or offline programming
The individual stages in the programs described canbe switched to non-latched or latched mode inmanual welding using torch switches, or inmechanised welding in a time-controlled way usingthe program saved for the process, for example.
The programs can be programmed on the weldingmachine. As programming time is unproductive time,however – no welding can be carried out duringprogramming – the programming work can also becarried out using special software on a computer orlaptop.
15 Conclusion
Modern, digital, inverter-based MIG/MAG systems(Figure 12) not only provide very good weldingproperties, but also excellent ease-of-use during thewelding process. The user can access the expertiseof the machine manufacturer who will have saved notonly the appropriate characteristics (JOBs) fordifferent welding tasks in the system, but who alsoallows entire welding programs to be stored in thepower sources matched to the welding task. This isFigure 10Current output wave with superpulses
especially important when welding aluminium thatdue to some of its physical properties places higherdemands on the exact settings of the weldingsystems. In addition to the use of the JOBs enteredby the machine manufacturer, the user can alsocreate and archive various program sequences andcall them up again later as required.
© 2002 EWM HIGHTEC WELDING GmbHFigure 11Appearance of a MIG seam welded using a low
pulse frequency
Figure 12MIG welding aluminium using push/pull torches
16 Literature
[1] Killing, R.: Was ist beim Schmelzschweißen vonAluminium anders als beim Schweißen von Stahl(The differences between the fusion welding ofaluminium and welding steel). Der Praktiker,H.11/1995, pages 562-567, DVS-Verlag Düsseldorf[2] Killing, R.: Das Beste ist gerade gut genug dafür(Only the best is good enough), Metallbau, H.2/1996,pages 73-75, Caldewey-Verlag, MünchenFirst publication magazine:„Metallbau 9/2002“
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