Aluminum Welding By Dynamic Fabrication
Dynamic Fabrication is your total source for aluminum welding, fabrication, machining, and assembly. We produce high quality work ranging from precision parts to complete assemblies. DFI is certified to weld all metals. We guarantee superior workmanship and quick turn-around. Founded in 1981, Dynamic Fabrication serves a wide variety of industries.
The unique combination of light weight and relatively high strength makes aluminum the second most popular metal that is welded. Aluminum is not difficult to join but aluminum welding is different from welding steels. Aluminum possesses a number of properties that make aluminum welding different than the welding of steels. These are:
- Aluminum oxide surface coating.
- High thermal conductivity.
- High thermal expansion coefficient.
- Low melting temperature.
- The absence of color change as temperature approaches the melting point.
- The normal metallurgical factors that apply to other metals apply to aluminum as well.
Aluminum is an active metal and it reacts with oxygen in the air to produce a thin hard film of aluminum oxide on the surface. The melting point of aluminum oxide is approximately 1926 degress, which is almost three times the melting point of pure aluminum, 660 degrees. In addition, this aluminum oxide film, particularly as it becomes thicker, will absorb moisture from the air.
Moisture is a source of hydrogen which is the cause of porosity in aluminum welding. Hydrogen may also come from oil, paint, and dirt in the weld area. It also comes from the oxide and foreign materials on the electrode or filler wire, as well as from the base metal. Hydrogen will enter the weld pool and is soluble in molten aluminum. As the aluminum solidifies it will retain much less hydrogen and the hydrogen is rejected during solidification. With a rapid cooling rate free hydrogen is retained within the weld and will cause porosity. Porosity will decrease weld strength and ductility depending on the amount.
The aluminum oxide film must be removed prior to aluminum welding. If it is not all removed small particles of un-melted oxide will be entrapped in the weld pool and will cause a reduction in ductility, lack of fusion, and may cause weld cracking. Even home welding enthusiasts who have experience welding steel may find a switch to aluminum welding challenging. Aluminum welding is difficult because of the softness of aluminum wire. It is more difficult to feed. In addition, wire diameters and machine settings normally used for steel may not be appropriate for aluminum welding.
MIG welding aluminum requires different techniques than MIG welding mild steel.
Before You Start Welding
- Material thickness that can be welded with Mig process on aluminum are 14 Ga. and heavier. (How heavy depends on the output capacity of the welder being used.) To MIG weld aluminum thinner than 14 Ga. (.074″) either specialized pulsed MIG or AC TIG welding equipment may be necessary.
- The removal of lubricants from the aluminum base material may be necessary. This is best done with solvents. Consult with your local Miller Welding Distributor for their recommendation.
- Oxide removal should be done after degreasing. This should be done with a stainless wire brush. This can be done with a hand wire brush or with a cup wire brush. If a power wire brush is used keep the RPM’S and pressures low to reduce smearing the surface of the material, which could entrap oxides and impurities under the surface. Always use a wire brush that is used on aluminum only, to keep from contaminating the base material.
The pure metal has a melting point less than 1200ºF and does not exhibit the color changes before melting so characteristic of most metals. For this reason, aluminum does not tell you when it is hot or ready to melt. The oxide or “skin” that forms so rapidly on its surface has a melting point almost three times as high (3200º+F). To add to this confusion, aluminum even boils at a lower temperature (2880ºF) than this oxide melts. The oxide is also heavier than aluminum and, when melted, tends to sink or be trapped in the molten aluminum. For these reasons, it is easy to see why as much as possible of this oxide “skin” must be removed before welding. Luckily, the reverse polarity half of the AC arc does an outstanding job of cleaning off quantities of this oxide ahead of the weld!
Aluminum is an excellent conductor of heat. It requires large heat inputs when welding is begun, since much heat is lost in heating the surrounding base metal. After welding has progressed a while, much of this heat has moved ahead of the arc and pre-heated the base metal to a temperature requiring less welding current than the original cold plate. If the weld is continued farther on to the end of the two plates where there is nowhere for this pre-heat to go, it can pile up to such a degree as to make welding difficult unless the current is decreased. This explains why a foot or hand Amptrol™ (current control) is recommended with your Precision TIG™ 185 or Precision TIG 275 – it enables you to easily change the current while simultaneously welding.
Some aluminum alloys exhibit “hot short” tendencies and are crack sensitive. This means that at the range of temperatures where the liquid alloy is slushy (part solid and part liquid) or just turned solid, it has not quite enough tensile strength to resist the shrinkage stresses that are occurring from cooling and transformation. The proper choice of filler metal and welding procedures along with smaller beads can help eliminate many problems of this kind. Some experts recommend backstepping the first inch or so of each aluminum weld before finishing in the normal direction.
Anyone who has worked with TIG Welders and automated TIG systems, knows the TIG weld quality is highly dependent on retaining the shape and quality of the tungsten tip. Weld start data is critical with automated TIG welding applications. Tip life is improved with a start ramp up to the low current start point then ramp to the operating current
Ensure Good Electrical Connections
The work clamp should be securely attached to the welding piece in an area free from paint and contaminants. To clean the piece, use a degreasing solvent to remove any oil and grease. Be sure that the surface is dry before you weld. Also, do not weld with flammable material nearby, such as a container of solvent or paint. As a second step, use a clean, stainless steel wire brush to remove all oxides from the surface of the aluminum.