Welding aluminum alloys is more challenging than welding steel alloys. Since aluminum has a higher thermal conductivity and lower melting point as compared to steel, it can suffer from more burn through.
By reading the properties of aluminum and the best aluminum welding practices, you will understand how to weld aluminum.
Feeder wires made from aluminum are softer as compared to steel feeder wires. Therefore, aluminum feeder wires may get tangled in these feeders. The thinner aluminum sections are, in particular, more susceptible to this risk. As a result, welding aluminum can be a bit of a challenge, even for experienced welders.
When it comes to selecting the right aluminum welding technique, it is based on the application requirements and the proficiency level of the welder. It is necessary to first consider the chemical and physical properties of aluminum to understand its welding issues.
Aluminum Properties
The melting point of aluminum is almost half that of steel. Aluminum melts at just 1221 degrees Fahrenheit, while steel melts at 2,500 degrees Fahrenheit.
The oxide layer of aluminum has a much higher melting point of 3,700 degrees Fahrenheit. The aluminum oxide layer is hard and it offers resistance against corrosion and abrasion. Thermal conductivity and porosity are two major issues with aluminum welding.
Hydrogen has high solubility in liquid aluminum. When the aluminum base metal and filler material melt to a liquid form during the welding process, the resultant liquid mixture can absorb hydrogen (the gas forms a solution). When the molten metal begins to solidify, it becomes incapable of holding hydrogen in the homogenous form. Hydrogen bubbles are then formed in the metal, thereby leading to porosity.
A shielding gas mixture of argon and helium is utilized to combat porosity. However, the voltage must be raised to overcome the greater ionization potential. Due to the higher voltage, there will be greater penetration and heat input. Hence, this mixture must be used for thicker aluminum objects.
The 6061 aluminum base metal has a greater susceptibility to cracking owing to its chemistry. Hence, welding it autogenously is very difficult. Using a filler material with similar chemistry will also create problems. A filler metal containing silicon or magnesium must be used to reduce the crack susceptibility of this material.
Aluminum also offers more thermal conductivity than steel. This means that the cool regions of aluminum can quickly absorb heat from the weld pool, which can lead to low weld penetration. Since aluminum has 5 times greater thermal conductivity than steel, it requires a much larger heat input to avoid low welding penetration.
Filler Metal Selection
While selecting the filler metal for aluminum, it is advisable to refer to a selection chart. Different filler metals are recommended for various aluminum alloys, according to the required weld characteristics.
A selection chart contains 8 characteristics that are critical for welding applications.
- Strength
- Crack sensitivity
- Corrosion-resistance
- Ductility
- Toughness
- Post-weld heat treatment
- Post anodizing color match
- Elevated temperature service
You can determine which properties are the most significant for a particular application by considering the requirements of all components. You may then select the filler metal that best suits the required characteristics.
It should be noted that the elevated service temperature for aluminum lies between 150 and 350 degrees Fahrenheit. This, along with other information about the remaining characteristics, is present within the aluminum selection chart.
It is imperative to select the right filler metal based on the application. Going back to the 6061 aluminum, the best filler metal selections are 5356, 4943 and 4043. The 4943/4043 GMAW wire can help to increase weld pool fluidity, improve weldability and reduce porosity, while the 5356 material can boost its strength and toughness.
In addition to selecting the right filler material, it is important to make use of the best aluminum welding practices.
Best Aluminum Welding Practices
To minimize defects and attain the best results with aluminum welding, you should follow these best practices.
Do Not Weave
The weave technique is commonly employed for welding steel. However, it is unsuitable for welding aluminum alloys. The stringer bead is a better option since it allows better fusion and penetration.
For aluminum MIG welding, you must ensure greater heat input and quicker travel speed. Higher travel speed is required due to the physical properties of aluminum. The welding method for aluminum can be best described as ‘fast and hot’. Higher voltage and amperage is needed for aluminum as compared to steel as aluminum has greater thermal conductivity. The weld travel speed must be fast enough to avoid burnthrough problems, especially on thin aluminum sections.
Clean the Surface
Prior to welding, you must clean the aluminum surface so that it is free from moisture, residue, dirt and oil. This will help minimize the chances of porosity and deliver the best results. Acetone is an effective solution for the removal of hydrocarbons from the aluminum surface. s
Scrape Off the Oxide Layer
After the surface is cleaned, use a clean brush made from stainless steel to get rid of the oxide layer before you begin welding. However, make sure that you use light strokes. Heavy brushing may embed the oxide layer deeper into the surface. The brush should be used for aluminum. Do not use it for carbon or stainless steel. You can also use etching solutions and solvents to remove the oxide layer.
After you have used etching solutions, ensure that they are fully removed prior to the start of aluminum welding. You can use a degreaser to remove hydrocarbons from cutting solvents and oils. The degreaser itself should not have any hydrocarbons.
As stated earlier, the melting point of the oxide layer is much higher than the base aluminum metal. Due to its nature, it will act as an insulator. Significantly higher heat levels are necessary to melt the oxide layer if it is not removed. Creating an arc can also be problematic in the presence of an oxide layer.
Since aluminum oxide melts at a temperature above 3,500 degrees Fahrenheit, it will remain in a solid state if there is not enough heat input. This will then become a barrier, which will inhibit the penetration of the welding filler metal.
Higher heat input may increase the risk of burnthrough, leading to porosity. This is because the oxide layer has a tendency to retain moisture.
Store It Correctly
Storing the filler metal and base aluminum metal correctly may help to reduce porosity.
Whenever possible, you should store aluminum sheets indoors. In case you must store aluminum sheets outside, keep the sheets vertical to that water does not accumulate on the surface to form a bigger oxide layer. You should do the same for filler materials.
If filler materials or aluminum base metal are stored outside or in an air conditioned section of the facility, bring them into a shop and allow their temperature to stabilize before welding. Wipe the surface clean to reduce any condensation formed by the temperature difference.
Inspect Consumables
Certain issues with aluminum MIG welding may transpire due to the condition of consumables.
Use hoses and gas lines that are in good condition to minimize the chances of porosity. Ensure that the hose connections are tight so that there is no leakage of air into the line.
Use the right drive rolls and liners. Plastic liners may be a better option for aluminum welding since brass or metal guides and steel liners can result in the abrasion of the soft aluminum wire as it moves forward. Shavings may then begin to accumulate, which can cause feeding problems.
For aluminum applications, U-groove drive rolls are the preferred choice since other kinds of drive rolls may create distortion in the wire.
Keep Track of the temperatures
Refer to the guide for aluminum filler material to ascertain optimal interpass and preheating temperature values. The adverse thermal effects due to section size can be addressed by preheating thick sections before welding them. For aluminum applications, this preheating should be minimal.
Weld cracking can be avoided by preheating the aluminum part to the right temperature. Use a temperature gauge to ensure that the preheating temperature does not become too high. The preheating temperature should be kept below 230 degrees Fahrenheit. Place tack welds at the extremities of the welding area to aid preheating.
If there is a major difference in the thicknesses of the aluminum sections being welded, then the thick section should be preheated to avoid the possibility of cold lapping. If it does occur, you should try run-off and run-on tabs.
Push Welding Technique
When welding aluminum, the gun should be pushed away from the weld pool instead of being pulled. This welding technique will lead to enhanced shielding gas coverage, lower weld contamination and better cleaning action.
Shielding Gas
Due to its desirable penetration profile and cleaning action, argon is the shielding gas of choice for aluminum welding work. While welding aluminum alloys of the 5xxx series, you should use a combination of helium and argon. The helium content should be no more than 75 percent. The use of this mixture will ensure that magnesium oxide formation is minimized.
Welding Wire
While selecting the welding filler material wire, make sure that its melting temperature is comparable to the workpiece base metal. You will get better welding results with smaller melting temperature difference. Feeding is easier with thicker wires. The wire diameter should be around 1/16 in.
Welding thin gauge aluminum sheet is a bit tough and requires extra precaution. You should employ 0.035 in diameter wire, along with pulsed welding technique. The feed speed of the wire should be in the range of 100 – 300 inches per minute.
Convex – Shaped Welds
Crater cracking is the primary cause of most aluminum welding failures. Cracking is the result of rapid thermal expansion that occurs during welding itself and the contraction that happens during cooling.
Concave craters have the highest probability of cracking since the crater surface can contract and split while cooling down. Welding operators should, therefore, resort to a mound or convex shape to resolve this problem. When the weld is cooling, the convex shape will minimize the resultant contraction forces.
Power Source
For MIG welding aluminum, you should first consider pulse or spray-arc welding. For spray-arc welding, you can use constant voltage machines or constant current sources. In spray-arc welding procedure, a small stream of molten material is sprayed along the arc to the base metal from the wire. Constant current machines will give you the best results for thick aluminum sections, which require welding current above 350 A.
Pulse transfer can be done using an inverter power supply. Pulsing functions are built into newer power supplies. In this procedure (pulsed MIG), one drop of molten filler material passes the workpiece from the electrode during every current pulse. With this procedure, it is possible to minimize splatter and use faster travel speeds compared to spray transfer welding.
With this welding process for aluminum, you can also attain better control over heat input and make out-of-position jobs easier. Operators can also use lower wire feed currents and speeds for welding thing gauge aluminum sheets.
Wire Feeder
When transferring from steel MIG welding to aluminum MIG welding, the difference in feed rate is one of the biggest changes that you will encounter. Due to the mechanical properties of aluminum, there may be certain feeding issues not present in the steel filler wire.
Steel is much stronger than aluminum, which is why it can withstand a much greater amount of abuse. It can also be fed more easily over longer distances. Aluminum, on the other hand, is softer and more susceptible to distortion as a result of feeding. The selection of the feed system for aluminum welding requires far more care than steel welding.
There are two very common problems associated with aluminum wire feeding. One problem is the erratic wire feed. The other problem is fusing of the aluminum welding wire with the contact tip of the welding gun.
For the most suitable feed system that can minimize these issues, the brake settings should first be analyzed. The brake setting tension must be enough to prevent free-wheeling in the spool when welding is stopped. It should not be more than this.
For feeding aluminum wires over longer distances, the push-pull technique is the preferred choice. In this method, wire feed cabinet safeguards the aluminum wire from the surroundings. The wire is pushed and guided through the nozzle at a constant speed and force by means of a variable-speed constant-torque motor. The welding gun has a high torque motor, which pulls the wire at a consistent speed.
Newer welding systems now have electronic brake mechanisms, which can minimize stresses on the aluminum wire while maintaining tension at the threshold value.
The same wire feeder is used in some shops to feed both aluminum and steel wires. Teflon liners should be used in this case to ensure a consistent and smooth feeding of the aluminum wire. To keep the wire from getting tangled, use plastic incoming tubes and chisel type outgoing tubes so that the wire receives support as close as possible to the drive rolls. The cable should be kept straight to keep wire feed resistance to a minimum during welding. Aluminum shaving can be minimized by properly aligning guide tubes and drive rolls.
Set the right level of drive roll tension for consistent wire feeding. Excessive tension will distort the wire shape and lead to rough feeding. You can expect uneven feeding with low tension. Excessive and insufficient tension can cause weld porosity and an uneven arc.
Drive rolls now have U-shaped contours with smooth edges, which are chamfered. This produces minimal stresses on the aluminum wire to prevent it from being deformed. Likewise, contact tips are now being specially designed for aluminum welding applications. The internal bore is kept as smooth as possible and sharp edges and burrs, especially at the tip are eliminated.
Welding Gun
For welding aluminum, you should utilize a separate gun liner. Wire chaffing can be minimized by restraining the two liner ends to finish gaps between the gas diffuser and liner. Frequent change of liners can minimize the chances of wire feeding issues arising from abrasive aluminum oxide layer. The contact tip should be about 0.015 greater than the filler metal diameter. This is necessary because the tip may expand to an oval shape while heating and this can restrict wire feeding. For welding currents greater than 200A, a water-cooled gun should be used to prevent wire feeding problems and heat buildup.
Hot Start Feature
To overcome thermal conductivity problems, welding equipment now have the hot start feature. The starting current is usually higher than the welding current used during the bulk of the welding process. The heavier initial current is maintained for a predetermined period.
The current then falls to the normal value for the remainder of the welding process. The heavy initial current provides the high heat input that is necessary to overcome the exceptional thermal conductivity of aluminum. This reduces the likelihood of incomplete fusion, low penetration and subsequent welding defects associated with aluminum joints.