In fact, the aluminum welding market is expected to  grow at a rate of 5.5 percent annually based primarily on the assumption that the automotive industry will continue to increase its use of aluminum.

But, those experienced in the welding of steel will find aluminum to be a different breed-the normal welding characteristics of steel don’t always apply to aluminum. For example, aluminum’s high thermal conductivity and low melting point can easily lead to burnthrough and warpage problems if proper procedures are not followed.

To understand aluminum, understanding some basics about aluminum metallurgy is critical. Aluminum can be alloyed with a number of different elements, both primary and secondary, to provide improved strength, corrosion resistance and/or general weldability. The primary elements that alloy with aluminum include copper, silicon, manganese, magnesium and zinc. These alloys fall into two classes: heat-treatable or nonheat-treatable.

Heat-treatable alloys are those that can be heated after welding to regain strength lost during the welding process. With the nonheat-treatable alloys it is possible to increase strength through cold working or strain hardening.

The reason why aluminum is becoming specified for so many jobs is its physical properties. For instance, aluminum is three times lighter than steel and yet offers higher strength when alloyed with the right elements.
It can conduct electricity six times better than steel and nearly 30 times better than stainless steel. This high electrical conductivity makes the effect of electrical stick-out in gas metal arc welding (GMAW)  less significant when compared to steel.

In addition, aluminum provides excellent corrosion resistance, is easy to shape and join, and also is non-toxic for food applications. Since it is non-magnetic, arc blow is not a problem during welding. With a thermal conductivity rate that is five times higher than steel and being less viscous, aluminum can easily be welded out-of-position. Aluminum does have its drawbacks, though, since its high thermal conductivity tends to act as a heat sink making fusion and penetration more difficult.

Since aluminum has a low melting point 1,200° F (half that of steel) for the same wire size, the transition current for aluminum is much lower than it is for steel. Also, for the same welding current, the burn-off rate is about twice that of steel.

In terms of chemical composition, aluminum has a high maximum solubility for hydrogen atoms in the liquid form and a low solubility at the solidification point. This means that even a small amount of hydrogen dissolved in the liquid weld metal will tend to escape as the aluminum solidifies and porosity is likely to occur-a great cause of concern during the welding process.

Also, aluminum combines with oxygen to form an aluminum oxide layer instantaneously as it is machined. This layer is very porous and can easily trap moisture, oil, grease and other materials. The oxide provides excellent corrosion resistance, but must be taken off before welding as it prevents fusion due to its high melting point (3,700° F).

Mechanical properties such as tensile strength, yield and elongation are affected by the choice of aluminum base and filler alloys. For groove welds, the heat affected zone (HAZ) dictates the strength of the joint. In nonheat-treatable aluminum alloys, the HAZ will be completely annealed and the HAZ will be the weakest point. Heat-treatable alloys require much longer periods at annealing temperatures combined with slow cooling to completely anneal them so that weld strength is less affected. Such items as preheating, lack of interpass cooling, and excessive heat input from slow, weaving weld passes all increase peak temperature and time at temperature, which means minimum strength levels might not be met.

Today’s quick response inverters using Lincoln’s patented Waveform Control Technology precisely control welding waveforms for more efficient control of droplet transfer. This reduces the amount of spatter caused by the low density of aluminum while a high-energy pulse peak insures proper penetration.

In addition, since variations in chemistry dramatically change an alloy’s physical properties, these custom waveforms can be designed for specific alloys to best suit the physical properties of what is being welded.
Because aluminum has a high maximum solubility for hydrogen in its liquid state and a low solubility at its solidification point, pulsing output waveforms are further designed to minimize arc length by trimming the output as low as possible and reduce the likelihood of porosity.

Lincoln’s Wave Designer software allows welding engineers and operators to manipulate and modify welding waveforms on their PCs as communicated from welding equipment in real time. This creates high quality, tailored performance, when used in conjunction with inverters.

The use of constant current power sources for the gas metal arc welding of aluminum has a long and very successful history. The use of “drooper” output has assisted in the delivery of a high energy axial spray transfer mode for aluminum that responds evenly and consistently with the proper welding current despite changes in arc length. The result of constant current is consistent penetration throughout the length of a given weld.

The evolution of the control of the arc has resulted in the development of software controlled inverter power sources. The use of software to optimize arc characteristics for aluminum GMAW has been taken to a new level at Lincoln Electric and it is known as Waveform Control Technology. A modified constant current output is employed in a very high speed synergic pulsed output that incorporates many of the benefits of Constant Current GMAW for Aluminum. These benefits include the high energy input that occurs during the pulse peak. The pulse peak helps to provide a consistent penetration profile throughout the length of a given weld and the advantages of pulsing also includes reduced spatter levels, improved puddle fluidity with an increase in effective travel speeds, and reduced heat input and lower distortion levels. Although it can be difficult to weld, aluminum has many attributes that make it the material of choice for a host of applications. But, with a good understanding of metallurgy and the latest tools and technology on the market, aluminum can be dealt with successfully.

This article was supplied by the Lincoln Electric Company of Canad Ltd., Toronto, ON
lincolnelectric.ca