Aluminium
Aluminum is very easy to weld, contrary to popular opinion. Aluminum melts at a fairly low temperature, 1218°F. Like copper, aluminum transmits heat very readily. This makes it difficult to get a small, specific part of the piece hot enough to weld. Welders experience problems in heavy sections because they don’t put enough preheat into the part. (Remember, the heat is continually being dissipated.) Thin or small pieces of aluminum will heat up very easily and, in fact, can very quickly reach the melting point and end up as molten metal on the floor.
The proper way to preheat aluminum is to use a heat indicator that will show you when to start welding. Two simple devices are temperature indicating crayons or soot from the oxy-acetylene flame. When soot is on the welding surface and is heated, it will disappear when the base metal reaches 550°F. For Torch and TIG Welding, preheat to about 550°F before welding. For Arc Welding, preheat to about 350°F. Remember, thinner sections need less preheat and heavier sections need higher preheat To better understand the makeup of aluminum alloys, the following is a list of the various grades of aluminum. The Aluminum Association has set a designation system as follows:
1XXX Series: These are aluminums of 99{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26} or higher purity. They are used primarily in the electrical and chemical industries. These grades are easily welded.
2XXX Series: Copper is the principal alloy in this group. This group provides extremely high strength when properly heat treated. These alloys do not produce as good corrosion resistance and are often clad with pure aluminum or special-alloy aluminum. These alloys are used in the aircraft industry. They are usually
difficult to weld.
3XXX Series: Manganese is the major alloying element in this group. These alloys are non-heat-treatable. Manganese is limited to about 1.5{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26}. These alloys have moderate strength and are easily worked and welded.
4XXX Series: Silicon is the major alloying element in this group. It can be added in sufficient quantities to substantially reduce
melting point and is used for brazing alloys and welding electrodes. Most of the alloys in this group are non-heat-treatable. These grades are easily welded.
5XXX Series: Magnesium is the major alloying element of this group. These alloys are of medium strength. They possess good welding characteristics, good resistance to corrosion, but the amount of cold work should be limited. These grades are easily welded.
6XXX Series: Alloys in this group contain silicon and magnesium which make them heat treatable. These alloys possess medium strength and good corrosion resistance and have good welding properties.
7XXX Series: Zinc is the major alloying element in this group. Magnesium is also included in most of these alloys. Together they result in a heat-treatable alloy of very high strength. This series is used for aircraft frames. This grade is often heat treated to get
higher tensile strengths. They are usually unweldable.
The casting grades of aluminum have a two or three digit identification number, the more common grades being 122, 355 and 356. Most casting grades of aluminum are high in silicon. Silicon makes aluminum thin flow when it is molten so that it will get into all parts of the mold during the casting operation. Silicon enhances weldability, hence aluminum castings are usually easily welded.
As with any casting, care must be taken when large heavy parts are preheated to higher temperatures. Because metal usually gets weaker as it gets hotter, heavy casting can collapse. Very often, steel braces or supports must be placed in the casting to keep it from distorting or collapsing while welding. Occasionally, parts that appear to be aluminum are actually magnesium. Magnesium has the same color and is light in weight like aluminum. To identify the base metal, scrape an area about the size of a penny with a knife blade. Put a drop of soldering or battery acid on the cleaned spot. If the drop starts to bubble and turn black, the part is magnesium. If not, it is aluminum.
Most aluminums, whether they are cast, sheet, or wrought, are easily welded using MG 400. This versatile alloy is used as an arc rod (DC Reverse Polarity) in heavier sections, and as a brazing alloy on thin sections. It is a thin flowing alloy for good fit-up and a bead forming alloy for poor fit-up. If the flux is removed and the core wire cleaned, it can be used as a 4043 type TIG wire.
For aluminum parts that must be brazed with high strength at the lowest possible bonding temperature, use MG 410. This alloy should be used only where good fit-up is the rule. Extremely dirty material is easily brazed using MG 420. This product is a flux cored brazing alloy with an aggressive flux to make easy joints on the dirtiest aluminum. Soldering is easily accomplished using the MG 460 kit. The joint area should be mechanically cleaned by scraping or sanding. Squeeze enough flux onto the surface to cover the entire joint area. For best results, the parts should be preheated with a torch, then a soldering iron should be used to apply the MG 460 alloy to the joint. If a torch is used, it is best to pre-place a piece of MG 460 alloy into the fluxed area and heat the area uniformly until the flux starts to bubble; remove the torch and wait until the flame goes out. Let it cool slowly. Rinse in water to remove the flux residue.
