Fusion Welding of Aluminum and Its Alloys: Part One

概要:

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.

Characteristics Influencing Joining

There are several characteristic factors, which influence the joining of aluminum by fusion welding, and these are summarized as follows:
  • Pure aluminum has a melting point of 660°C whereas the fusion range of the aluminum alloys is between 520-660°C and because there is no visible color change it becomes difficult to judge when the metal approaches its melting point.
  • The high thermal conductivity of pure aluminum is about five times that of steel, while the aluminum alloys range from three to five times that of steel, and in both cases necessitates high heat inputs to achieve fusion. Because of this thermal conductivity, preheating is sometimes necessary, particularly where thick sections are encountered.
  • When exposed to air, aluminum rapidly develops a tenacious, refractory surface oxide film, which above all other factors related to aluminum welding causes the most trouble. Unless this oxide film is removed before and during welding it will interfere with the coalescence of the parent plate and filler material.

Filler Material Specifications

A major contributory factor towards producing good quality aluminum welds is the use of high quality filler rods and wires of the correct diameter and alloy specification. Moreover, the surfaces must be free from moisture, lubricants, and other contaminants.

Handling, Storage and Usage

The rods and wires must be kept scrupulously clean when handled, especially in the case of TIG welding where the rod is passed through the fingers, the operator’s hands also must obviously be kept clean. Alternatively, white silk gloves can be worn. This precaution also applies when reels of MIG wires are being handled and mounted on to the wire drive units.

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.

Edge Preparations

It is not possible to state categorically which edge preparation should be used for a specific material thickness and welding process. Before deciding on an edge preparation for aluminum welding, there are several influential factors, which have to be considered such as welding position, use or non-use of backing aids, heat sink characteristics, and whether or not back chipping will be required.

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.

Joint Fit-Up

Attention to good fit-up is important to ensure that the production of high quality welds can be achieved. Joint fit-up and edge preparation are closely related and cannot be entirely divorced. Inferior joint fit-up will produce poor welds, no matter how good the edge preparation may be. Excessive root gaps especially on joints which are unsupported can be troublesome because arc burn-through and/or irregular penetration and oxide inclusions can occur.

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.

Joint Accessibility

In addition to the preparation and fit-up, joint accessibility becomes just as important a consideration, to achieve sound welds with the minimum of effort.

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.

Preheating

The need to preheat aluminum prior to joining depends on the welding process, type of alloy, its thickness and ambient temperature. In general, it is accepted that preheating is not usually required with MIG welding until the material thickness exceeds 25 mm. However, some preheat, i.e., 150-200°C will be found beneficial from the point of fact of improving weld quality (with respect to porosity) on material thicknesses in excess of 9 mm.

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.

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