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When considering the developments in aluminum welding equipment, the friction stir welding of aluminum has probably been the most publicized in recent welding journals and has attracted much attention. The friction stir welding process has many unique and exciting characteristics when used to make certain joints in aluminum alloys. However, in my opinion, friction stir welding is somewhat limited in its application and, while excellent for some components, does not have the versatility of some of the other welding processes. My personal interests are directed toward the development and use of one of the more traditional methods of welding aluminum, the Gas Metal Arc Welding (GMAW) or alternatively named Metal Inert Gas (MIG) welding process. This process has been used for the welding of aluminum for many years. Fairly recently there have been developments with both power supplies and feeding systems used for this welding process. Some of the inherent problems associated with the MIG welding of aluminum, when compared with the welding of steel, are: feedability, incomplete fusion at the start of a weld, and crater or termination cracking at the ends of the weld.
Feedability: This is the ability to consistently feed the spooled welding wire when MIG welding, without interruption, during the welding process. Feedability is probably the most common problem experienced when moving from MIG welding of steel to MIG welding of aluminum. Feedability is a far more significant issue with aluminum than steel. This is primarily due to the difference between the material’s mechanical properties. Steel welding wire is rigged, can be fed more easily over a further distance and can withstand far more mechanical abuse when compared to aluminum. Aluminum is softer, more susceptible to being deformed or shaved during the feeding operation, and, consequently, requires far more attention when selecting and setting up a feeding system for MIG welding.
Feedability problems often express themselves in the forms of irregular wire feed or as burn-backs (the fusion of the welding wire to the inside of the contact tip). In order to prevent excessive problems with feedability of this nature, it is important to understand the entire feeding system and its effect on aluminum welding wire. If we start with the spool end of the feeding system, we must first consider the brake settings. Brake setting tension is required to be backed off to a minimum. Only sufficient brake pressure, to prevent the spool from free-wheeling when stopping welding, is required. Electronic braking systems and electronic and mechanical combinations have been developed to provide more sensitivity within the braking system. Inlet and outlet guides, as well as liners, which are typically made from metallic material for steel welding, must be made from a non-metallic material such as nylon to prevent abrasion and shaving of the aluminum wire. Drive rolls have been developed, often with U-type contours with edges that are chamfered and not sharp, that are smooth, aligned, and provide correct drive roll pressure. Excessive drive roll pressure can deform the aluminum wire and increase friction drag through the liner and contact tip. Contact tip I.D. and quality are of great importance. We are seeing the availability of contact tips made specifically for aluminum welding, with smooth internal bores and the absence of sharp burrs on the inlet and outlet ends of the tips which can easily shave the softer aluminum alloys. Aluminum welding wire is used in both push and pull feeder systems; however, limitations are recognized dependent on application and feeding distance. Push-pull feeder systems for aluminum have been developed and improved to help overcome feeding problems and may be used on more critical/specialized operations such as robotic and automated applications. More recently, the planetary drive push-pull system (ESAB Mongoose System) has become popular for aluminum welding providing an extremely positive feeding system capable of delivering aluminum wire over greater distances with minimum burn back problems.
The Hot Start Feature: Aluminum has a thermal conductivity about 6 times that of steel, and because of this ability to rapidly conduct heat away from the weld area, there has always been an inherent problem, particularly when starting a weld on this material. It is not uncommon to experience incomplete fusion at the start of an aluminum weld because of the material’s high thermal conductivity. One method which can now be used to help overcome this problem, particularly on thicker sections of aluminum used in structural applications, is the use of equipment that has a hot start feature. This feature may allow the user to program the weld starting current characteristics independently from that of the general welding current parameters, thus providing the user with the ability to start the weld with a higher current density for a predetermined period before moving to the general welding conditions for the remainder of the weld. This allows the use of a higher heat input at the beginning of the weld that can help to overcome the dramatic heat sink associated with this material prior to the weld area becoming heated by the welding operation. The result of this technique is to eliminate, or significantly reduce, the probability of incomplete fusion at the start of the weld and thereby improve the life expectancy of welded components subjected to high stress or fatigue loading.
Crater Fill Feature: Other characteristics of aluminum which can provide welding problems are associated with its thermal expansion which is about twice that of steel, and its shrinkage on solidification which is 6% by volume. This can increase both distortion and weld crater size. One common concern when welding aluminum is crater cracking or what is sometimes called termination cracking. When MIG welding with conventional equipment, once the trigger of the welding gun has been released, the arc is extinguished, and no additional filler metal is added to the weld pool to fill the crater. Consequently, if no further precautions are taken, a large crater will be left which will have a higher probability of cracking. Craters can be serious defects and most welding standards require them to be filled and free from cracks. Run-off tabs or other methods of locating weld craters on scrap material away from the weld are not usually practical. However, if the weld pool size can be reduced before the arc is fully extinguished, the resulting crater may be very small or almost eliminated and, consequently, the weld may be free from cracks. In the past a number of welding techniques have been used in an attempt to reduce this termination problem. Reversing the direction of travel at the end of a weld, increasing travel speed to reduce crater size, and providing suitable build-up and remolding the crater area flush with the weld surface by mechanical means are some of the methods which have been used. These methods are often difficult to control, require specialized training, and are not always successful in their objective. More recently, welding equipment has been developed for aluminum welding which has a built-in crater fill feature. This feature is designed to terminate the weld in a gradual manner by decreasing the welding current over a predetermined period as the weld is completed. This feature may be adjustable to enable the user to select the most favorable termination conditions and thereby prevent a crater from forming at the weld termination. Tests have shown this crater fill feature to be extremely user friendly and very effective in eliminating the crater cracking problem.
The Need For Technical Training In Aluminum Welding Technology
The advancement of aluminum in the automotive industry, along with its increased use within the welding fabrication industry in general, has certainly promoted the development of specialized welding equipment design. Correspondingly, the increased use of aluminum welding has promoted the demand within industry for technically competent aluminum welding personnel. The need for welding engineers, technicians, inspectors, supervisors and welders who have experience and technical training in aluminum welding technology has increased. Unfortunately, because aluminum welding has traditionally made up such a small part of the overall welding industry, personnel with such qualifications have been difficult to find. Many of the university’s and technical institutions, which have been involved in welding education, have neglected detailed instruction in aluminum welding technology. Consequently, it is not uncommon to find formally trained welding engineers with very little, if any, experience or in-depth training within this field.
In order to help remedy this problem, and in recognition of the need for technical training and support for those manufacturers who have moved into the aluminum welding industry, AlcoTec Wire Corporation provides specialized training in aluminum welding technology. AlcoTec is located in Traverse City, Michigan, U.S.A., and is recognized as both a world leader in the manufacturing of aluminum welding wire, and the ESAB Aluminum Welding Center of Excellence. AlcoTec’s staff of metallurgical, welding and quality engineers present numerous training courses that combine their many years of aluminum manufacturing experience with a knowledge of the industry equipment, specifications and quality requirements. The courses, which have been developed over many years, are designed to incorporate both the theory and practical hands-on approach to the welding of aluminum alloys. The classroom instructions include an understanding of the various aluminum alloys and their tempers, metallurgical characteristics, chemical compositions, weldability and crack sensitivity. Additional topics covered are filler alloy selection, metal preparation, welding procedures, workmanship, weld discontinuities, welding inspection, quality control, welding processes and equipment, and designing for aluminum welding. The laboratory portions of the programs may provide an opportunity to weld aluminum with both GTAW (TIG) and GMAW (MIG), also to perform equipment evaluation and a variety of inspection and testing functions, bend testing, macro etching, tensile testing, fillet weld fracture tests, dye penetrant testing and radiographic evaluation of welded samples. These training programs have proven to be extremely successful in providing instruction in this expanding and specialized field of aluminum welding technology to both AlcoTec’s customers and ESAB’s sales and technical personnel world-wide.