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Prof. Dr. Viktor Pocajt, CEOKey to Metals AG
In the chemical, cryogenic, aerospace, and nuclear industries, aluminum and its alloys are widely used and frequently need to be joined by the application of welding. To fulfill this demand, the gas-shielded TIG and MIG processes, which are capable of producing high integrity welded joints, are used almost exclusively. However, before acceptable welds in aluminum such as those required within these industries can be produced, certain workshop practices and techniques have to be adhered to.
In the chemical, cryogenic, aerospace, and nuclear industries, aluminum and its alloys are widely used and frequently need to be joined by the application of welding. To fulfill this demand, the gas-shielded TIG and MIG processes, which are capable of producing high integrity welded joints, are used almost exclusively.
However, before acceptable welds in aluminum such as those required within these industries can be produced, certain workshop practices and techniques have to be adhered to. First part describes in detail, workshop practices such as handling and storage of filler materials, preheating, oxide removal, choice of weld backing aids and back chipping techniques. Relevant factors appertaining to the selection of edge preparations and joint fit-up are also highlighted. Recommendations for making tack welds and controlling distortion are included.
Filler rods and wire reels, when not in use, must be kept in closed packets and containers and stored in a dry place at a uniform temperature. During production, MIG welding, where appreciable time delay between the welding operations is likely, i.e., work shifts or temporary stoppages, a clean cloth or plastic covering should protect the reels.
Therefore, from the foregoing, one can only generalize on edge preparations for various material thicknesses to be welded by the processes under discussion. Although these preparations have been found suitable in production, they are, of course, open to modifications and it is therefore intended that they be accepted in the light of established practice at this present time.
The exact degree of fit-up, i.e., joint gap/root face combinations and plate mismatch, has yet to be defined for the processes under discussion. It is therefore impossible to state precise tolerance limits although some work on this subject has been carried out for DC helium TIG welding.
The aspect of accessibility can often be overlooked during the initial design stage and, moreover, some designers lack appreciation of the sizes of the welding TIG torch or MIG gun and the arcing characteristics. Unlike manual metal-arc welding of steel for instance, where the electrode can be bent to allow access to a difficult area, TIG torches and MIG guns prevent this practice; although they are available in a wide variety of sizes and some are designed with swivel necks or long extended flexible necks which can be offset to assist ease of access. Invariably though, these torches and guns only carry a restricted current and hence their usefulness, although valuable, is limited.
Apart from the accessibility factor of the TIG torch and MIG gun, the designer should always be aware that it is important for the joint design to allow an unrestricted view of the arc within the preparation.
Often short arc lengths and nozzle-to-work distances are necessary with the gas-shielded processes, which means that the available nozzle diameter may be too large to allow for this achievement. Consequently poor arcing and gas shielding can occur, which will bring about inferior weld deposits. This can often be the case where gusset plates, flanged pipes to vessel shells, stiffeners and fish plates, etc., are called for, especially where they are close together and continuous welds are required.
For AC TIG, preheating is necessary when sections greater than 3-4 mm are welded, to aid fusion without the need to use excessive welding currents.
Date Published: Aug-2005
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