Aluminium welding
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Aluminium welding

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Aluminium welding Aluminium welding Document Transcript

  • ALUMINIUM WELDING: GUIDE TO WELDERS TO AVOID PITFALLS BEYOND DIRECT CONTROL.Before welding work is started it is advisable to check the following so as to ensure an adequatechance of success: Fit upAluminium is very intolerant of poor fit up. Joint gaps that could be quite easily filled in steel aremuch more difficult to fill in aluminium and will seriously compromise the life of the fabrication. Ingeneral a gap of 1 - 1,5 mm should be considered a maximum for quality welding. AlignmentWelds are designed to carry a specific load and any additional load will shorten the life of theweld. Such additional loads are not necessarily a result of over stressing of the finishedweldment but can be the result of unpredicted loads such as bending because of poor jointalignment. Any linear misalignment should be corrected before attempting to weld. HumidityMolten aluminium has a high solubility for hydrogen but on solidification the gas is rejected asbubbles which remain trapped in the weld as spherical porosity. Whilst this does not seriouslyaffect the strength of the weld, it considerably reduces the ductility. Joints should be fullyprotected from moisture, but where humidity cannot be avoided such as in coastal conditionsparticularly during early morning then local preheat of, typically, 100 ˚C can be applied. The useof temperature indicating crayons is helpful to ensure that the correct temperature is attained.This preheat should be followed by normal cleaning procedures. If a preheat is used then itshould be applied with caution since some alloys can suffer a serious loss of properties if too higha temperature is maintained for too long. WindThe gas shield used in the MIG or TIG welding process is quite fragile and easily disturbed bywind. Whilst this is easily taken care of in the workshop by the normal welding screens whichshould be erected to prevent passers by being exposed to the arc, it can become a seriousproblem in site work. Under windy conditions windbreaks must be erected around the weldingarea. Failure of the gas shield allows rapid oxidation of the weld pool, causing oxide inclusionsand porosity.The inert gas is there to provide a controlled ionised path, to protect against Oxygen andHydrogen in the atmosphere which will react with the aluminium pool and, in the case of Argon toassist in maintaining an oxide free pool. Argon is the standard inert gas but above 6 mm sheetthickness Helium is increasingly used in combination with Argon to get a high arc voltage to assistpenetration. Alloy weldability
  • Though it is not the job of the welder to specify the material to be used on a specific job he shouldcheck before starting to weld that he has been supplied with the correct material. This applies tothe filler wire or rods as well as to the material to be welded. Equipment conditionBefore welding it is always good practice to check the equipment condition. This is not only forquality reasons but also to ensure safety. Check the overall condition of the welding machine, thetorch, nozzle and cable for any damage. The contact tip in a MIG torch wears surprisingly quicklyand any play between the wire and the contact tip must be rectified by replacing the tip otherwisethe ability of the welding current to pass into the wire is seriously reduced. This leads to erraticfeeding and reduced weld quality.The gas hose must be securely attached to the regulator and the welding machine or wire feederand must be free from obvious damage such as cracks or splits. The work return cable must befree from damage and securely connected to the machine and the work clamp; it is impossible toproduce quality welds without sound electrical connections all the way round the welding circuit. Weld preparationCorrect weld preparation is vital if quality welds are to be achieved. In most workshops thewelder does not prepare his own joints so he must always check that the weld preparationmatches the drawing on the weld procedure. Attempting to weld without the correct weldpreparation seriously compromises the chances of obtaining a good quality weld and suchdefective preparation must be rectified. Part of weld preparation is cleaning to remove anycontamination from the area to be welded. This is covered in more detail below. Weld significance (Criticality of specific welds)The welder should be familiar with the design of the fabrication in order to ascertain which, if anyof the welds are especially critical so that they can have the utmost care and attention devoted tothem whilst others may tolerate a slightly less stringent approach to the work. Welding plan and sequencingAttempting to weld without the correct welding procedure amounts to guessing how thefabrication is to be welded and seriously reduces the possibility of obtaining a good quality job. Itis essential that the welder adhere strictly to the welding procedure, particularly with regard to thewelding parameters of voltage, current, travel speed and welding sequence since these have agreat influence on the mechanical properties of the weld as well as on distortion. Welding processIt is vital that the welding process specified in the welding procedure is used to carry out the weldsince some joint types and weld preparations are more suited to one particular welding processand it is very difficult to obtain good quality welds by the wrong process. If a welding machinebreaks down then it must be repaired or replaced before the job can proceed; it is not acceptableto substitute for example a TIG machine with a MIG machine. Similarly, if the welding procedurecalls for the weld to be carried out, for example in the down-hand position and a crane is notavailable to turn the work piece then welding must be discontinued until the job can be turned. Ifpreheat has been applied then this must be maintained during the delay but welding must not becarried out in a position other than that called for by the welding procedure. Check procedure is relevantThe welding procedure provided to the welder must be the correct one for the job to be doneotherwise he is guessing how to carry out the weld. Drawing numbers or job numbers as 2
  • appropriate in the particular organisation must be correlated to ensure that the weld procedure isappropriate to the particular job. SettingsThe welding machine must be capable of being set to the welding parameters needed for the job.This means that volt meters and current meters must be in good working order and calibrated toensure accuracy and conformance with the welding procedure. In addition an independentmeasuring device such as a clamp type multi meter with ranges adequate for the welding processshould be available for periodic checks of the welding machine meters; especially when thewelding is carried out on a different machine to that on which the welding procedure wasdeveloped. Normally a welding procedure specifies a small range of parameters and it isessential that the welder adhere to these. CleaningIn any form of aluminium welding cleanliness is vital. Handling clean plate or filler rods withexcessively dirty or greasy gloves for example negates any cleaning and increases the possibilityof defective welds. The use of contaminated cleaning equipment such as greasy wire brusheshas the same effect. Filler wire type, cleanlinessThe wire or filler rods must be the correct type for the job and be clean and free of contamination.Whilst MIG wire is generally supplied clean the spool should preferably be removed from the wirefeeder at the end of the shift and placed in a clean plastic bag overnight. At the very least thewire should be covered overnight to keep it clean. MIG wire is best used within as short a periodas possible after being opened as it, like the base material forms an oxide layer and can thereforecontribute to oxide inclusions in the weld. TIG filler rods should be kept in their original packing,often a plastic tube, until needed. If contamination is suspected then the rods must be wiped witha clean cloth soaked in a solvent such as acetone then pulled through a stainless steel wool pad. Gas type, flow, suitability, purity, contaminationThe type of gas to be used is called out in the welding procedure and the welder must ensure thathe is using the correct gas. Argon or argon helium mixtures are usually used for weldingaluminium and the heat of the arc and therefore the penetration of the weld depend on using thecorrect gas. A helium addition to the gas increases the arc energy so substituting pure argon forthis mixture would reduce the weld penetration, particularly on thicker material and lead todefective welds. The required gas flow must be adhered to since too little gas results in a loss ofshielding causing porosity. Too high a gas flow causes turbulence in the gas shield whichentrains air and also causes porosity. A gas purity of 99.995% is generally needed and thewelder must check that the label on the cylinder indicates suitable gas purity when he is issuedwith a new cylinder. A moisture content of >20 ppm is unsuitable while the preference is farlower, around 2 ppm. Where gas impurity is suspected it is possible that the welding equipmentis actually contaminating the gas. When water-cooled torches are being used water leaks canoccur, allowing moisture to contaminate the gas and cause hydrogen porosity. Leaks in gashoses or connections may also introduce air into the shielding gas. To minimise the effects ofhydrogen in high purity argon gas, a dew point below minus 60oC is often specified to restricthydrogen to below 30 ppm. (An incorrect welding torch angle will also result in an increase inporosity). Oil or grease is never needed on gas fittings and if such contamination occurs then itmust be thoroughly cleaned.Complied by Aluminium Federation of Southern Africa – © AFSA 2006 3