Aluminium welding: the quest for quality
Tony Anderson, AlcoTec Wire Corporation
The use of aluminium in the automotive industry and the development of aluminium welding technology are both continuing to expand. The continued development of aluminium in the industry can be primarily attributed to the material's many desirable physical characteristics.
Aluminium provides its users with a material that is comparatively lightweight, has high strength, versatility - in both extruding and casting - and excellent corrosion resistant characteristics. When we consider this material's physical properties, in conjunction with continually developing environmental issues, such as the need to improve fuel efficiency and encourage recycling, it becomes perfectly understandable why aluminium is increasingly becoming the popular choice for engineers and designers in a variety of automotive applications.
With the advancement of aluminium in the automotive industry, we have seen both the need for development in the welding equipment used for this somewhat specialised material and also the increased demand for technical training in aluminium welding technology.
Equipment characteristics
When considering developments in aluminium welding techniques, the friction stir welding of aluminium has probably been the most publicised. This process has many unique and exciting characteristics when used to make certain joints in aluminium alloys. However, 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.
This discussion will be directed towards the development and use of one of the more traditional methods for welding aluminium: gas-metal arc welding (GMAW) sometimes called the metal-inert gas (MIG) welding process. This process has been used for the welding of aluminium for many years.
Historically, the first aluminium welding operations were of a non-structural nature, performed on heat exchangers, radiators, cooling systems and their associated components. The goal of this type of welding has been primarily the consistent production of welds with minimum leakage rates. Procedure development has revolved around the use of high-silicon-content filler alloys, with minimum consideration for strength characteristics and much emphasis placed on fluidity and the ability to seal weld joints in thin material. More recently, there have been major moves towards the fabrication of structural components such as engine cradles, front and rear suspension frames, drive shafts and wheels.
These welded components can often be subjected to dynamic loading in service and, consequently, can be susceptible to fatigue. Structural welded joints of this nature require very different considerations when developing welding procedures and maintaining a welding quality that is acceptable for their service conditions.
The requirement for higher integrity in structural welded joints in aluminium, along with the recognition of some unique characteristics of aluminium that can produce certain welding discontinuities, has promoted the search for a better understanding of welding equipment characteristics. Some of the inherent problems associated with MIG welding of aluminium and the production of high-integrity structural welds, 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
Feedability 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 aluminium. Choosing a welding wire of high quality and consistency, such as the ESAB-HQ product, can minimise feedability problems. This wire has exceptional surface smoothness, extremely tight diameter control and excellent lubricity, created through the use of a patented feedability process.
Feedability is far more of an issue with aluminium than steel, due to 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 than aluminium. Aluminium 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. Push-pull feeder systems for aluminium have been developed and improved upon to help overcome feeding problems and may be used on more critical/specialised operations such as robotic and automated applications. More recently, the planetary-drive push-pull system (ESAB Mongoose System) has become popular for aluminium welding, providing an extremely positive feeding system capable of delivering aluminium wire over greater distances with minimum burn-back problems.
Hot-start and crater-fill features
Aluminium has a thermal conductivity of about six 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. Incomplete fusion is not uncommon at the start of an aluminium weld because of the material's high thermal conductivity.
One method which can help overcome this problem, 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 the general welding current parameters. This provides 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.
Other characteristics of aluminium that can create welding problems are associated with its thermal expansion, which is about twice that of steel, and its shrinkage on solidification, which is 6 per cent by volume.
This can increase both distortion and weld crater size. One common concern when welding aluminium is crater 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. 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 negligible, leaving the weld free of cracks. More recently, welding equipment has been developed for aluminium welding with a built-in crater fill feature, 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 favourable termination conditions, thereby preventing 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
The advancement of aluminium in the automotive industry, along with its increased use in the welding fabrication industry in general, has promoted the development of specialised welding equipment design. Correspondingly, the increased use of aluminium welding has promoted demand in industry for technically competent aluminium welding personnel.
Although the need for welding engineers and technicians who have experience and technical training in aluminium welding technology has increased, because aluminium welding has traditionally represented such a small part of the overall welding industry, qualified personnel have been difficult to find. Many universities and technical institutions involved in welding education, have neglected detailed instruction in aluminium welding technology. Consequently, it is common to find formally trained welding engineers with very little, if any, experience or in-depth training in this field.
To help remedy this problem, AlcoTec Wire Corporation provides specialised training in aluminium welding technology. AlcoTec is recognised as a world leader in the manufacturing of aluminium welding wire and at the ESAB Aluminium Welding Center of Excellence. AlcoTec's staff of metallurgical, welding and quality engineers offer training courses that combine their many years of aluminium manufacturing experience with a knowledge of the industry equipment, specifications and quality requirements.
Improving weld quality
To successfully improve both aluminium welding quality and productivity, it is important to understand the many features of today's welding equipment and how they may assist us in achieving our objectives. We must use these features to reduce reworking or defective welding and enhance welding efficiency. It is also important to evaluate our human resources to ensure that we have technically competent welding engineering personnel. We must utilise technical training to develop our technical skills and promote welding quality and improved manufacturing efficiency.
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