What is Cast Aluminum
Cast Aluminum is used for making engine blocks, motors casing and many other parts in automobile due to their light weight compare to steel. Aluminum alloys are divided into two major categories: casting alloys and wrought alloys. Aluminum castings are mostly produced by pressure-die, permanent-mold, green and dry-sand, investment, and plaster mold casting methods. Casting alloys also have a system of four-digit designations as given in the below Table:
Aluminum casting alloys have the following characteristics:
1. Good fluidity, which helps in filling thin sections.
2. Low melting point.
3. Rapid heat transfer from the molten aluminum to the mold.
4. Hydrogen is the only gas with appreciable solubility in aluminum and its alloys that can be controlled by processing methods.
5. Many aluminum alloys are relatively free from hot-short cracking and tearing tendencies.
6. Chemical stability.
7. Good “as-cast” surface finish.
8. Many casting alloys have good weldability.
TIG or MIG welding of Aluminum casting
Welding of aluminum casting can be carried out using TIG welding or GMAW (MIG Welding). The selection of the filler wire is based on the casting type or its series i.e. 2000 series of 3000 and so on.
Generally, ER4043 is good to weld many types of aluminum casting used in engine blocks using TIG or MIG welding. ER5356 is another option. TIG or MIG with AC polarity shall be used for any type of aluminum welding either new welds or repair welds.
Stick welding of Cast Aluminum
The cleanliness and minimal surface oxidation of the filler metal are important with all welding processes. Oil, or other organic materials, as well as a heavy oxide film on the rod, will interfere with coalescence of the weld and also are sources of porosity. Because of this, it is necessary to clean the welding rod and electrode before packaging.
Stick welding of cast aluminum is not much popular & stick welding electrodes are available in limited types in the market.
Preheating for aluminum casting welding
Preheating is generally recommended when welding thick cast aluminum. As aluminum has high heat conductivity, heat dissipate very fast so preheating around 200-250 Deg F is good otherwise you can have lack of fusion issues. Control of interpass temperature is not an issue with cast aluminum materials.
Preheat is recommended for thicker sections or whenever sufficient penetration can not be achieved due to high thermal conductivity of the aluminum.
So, if any of the above issues being faced, a preheat temperature of 150 to 200°C can be applied. Also, when repairing 5XXX (Al-Mg) grades, it can crack in the weld or HAZ area. Following a preheat in these conditions is very beneficial to mitigate the cracking inter-alia to using the right filler wire.
TIG- MIG welding parameters for welding
TIG welding parameters for cast aluminum welding are given in the below table.
MIG welding parameters for cast aluminum repair are given in the below table.
Gas tungsten arc welds are most commonly made with alternating current power and pure argon (AWS A5.32 (ISO 14175) – I1) gas shielding. Helium [AWS A5.32 (ISO 14175) – I2] additions to the extent of 25% of the mixture with
argon are used to increase the rate of initial melting and the amount of melting in thick base metal. Helium additions of up to 75% are very effective with square wave AC power supplies and especially with AC frequencies up to 400 Hz. Pure tungsten (AWS A5.12M/A5.12 Class EWP) or zirconiated tungsten (AWS A5.12M/A5.12 Class EWZr-1) electrodes are preferred for AC-GTAW. Lanthanated and ceriated tungsten can also be used as a substitute for pure and zirconiated tungsten. The positive electrode polarity of the AC power provides an arc cleaning action to remove the surface oxide; however, thick aluminum oxides caused by weathering, thermal treatments, or anodic treatments need to be reduced by chemical or mechanical means prior to welding to obtain uniform results and proper fusion.
Cast Aluminum welding with DCEP polarity
Direct current power can also be used to gas tungsten arc weld aluminum. DCEP power can be used to weld sheet gauges; however, a 1/4 in [6.4 mm] diameter tungsten electrode is required to carry the 125 amperes needed to weld 1/8 in [3 mm] thickness, so this polarity is seldom used. DCEN power is used with helium (AWS A5.32 (ISO 14175) – I2)
gas shielding and a thoriated tungsten electrode for welding aluminum-base alloys. This negative electrode polarity provides a deep, narrow melting pattern, which is advantageous for repair of thick weldments or castings and for increased welding speeds in all thicknesses.