Like most deformable aluminum alloy casting aluminum alloys can be, basically, also connecting via fusion welding.
- Hypoeutectic and eutectic silumins - aluminum-silicon alloys.
- Poorly welded and non-welded are cast parts made of alloys of the Al Cu4Ti type due to the high copper content - thermal cracks are formed during welding.
- When welding cast aluminum-magnesium alloys, hot cracking is fought by selecting the right filler wire.
Welding aluminum casting
Modern casting alloys and casting methods give the designer tremendous freedom when designing castings. Nonetheless, welding is becoming more and more important for joining cast aluminum parts. It is used to connect two or more parts., which are convenient to cast separately, for example from two halves, because it is often very difficult or uneconomical to cast such a part in one piece.
Welding is even more widely used in the repair of aluminum cast parts., including, when eliminating casting defects. Welding is actively used to correct dimensional deviations of a cast part., restoration of a worn-out part by welding metal on it, and, of course, to repair broken parts.
Methods and materials for welding cast aluminum
Most often, for welding cast aluminum parts, arc welding with a metal electrode in an inert gas atmosphere (MIG welding) is used and tungsten arc welding also inert gas (TIG welding). Argon is most often used as an inert gas..
MIG welding of cast aluminum
When welding with the MIG method - the method of arc welding in an inert gas atmosphere - an electric arc is constantly burning between the consumable electrode and the workpiece. The process runs at constant current, and the wire electrode acts as a positive pole. Welding is carried out under the protection of an inert gas, to protect the molten area from the harmful effects of oxygen and moisture, in the air. Argon and helium or their mixture are used as protective gases.. More often, However, use cheaper argon.
The MIG welding method is suitable for manual, semi-automatic and automatic welding. When good heat dissipation from the weld is ensured, then you can get a relatively narrow thermal zone of the seam, as well as satisfactory strength and ductility of the weld.
Pulsed MIG welding enables tough welding tasks, for example, welding of walls about 1 mm.
Currently, it is MIG welding that is most often used for welding aluminum. This is due to the simpler control of the welding process, cheaper equipment and lower operating costs.
Tungsten arc welding (TIG)
In TIG welding - inert gas arc welding - an electric arc burns between a non-consumable tungsten electrode and the workpiece. For welding aluminum, alternating electric current is usually used. Welding wire is inserted separately manually or mechanically. Welding is carried out under the protection of an inert gas, to protect the molten area from the harmful effects of oxygen and moisture, in the air. Argon and helium or their mixture are used as protective gases.. Most often, TIG welding is carried out with alternating current with argon, which is cheaper. MIG welding is carried out mainly by hand, but there, where possible, automatic welding is also used.
One option for TIG welding is welding with a negative electrode polarity., how is it done when welding steel, DC and helium protected. Compared to argon, helium has a higher thermal conductivity and this allows a lower current to be used to destroy the oxide film on aluminum.. Consequently, electrode does not overheat. The TIG welding method also has pulse processing options.
TIG or ME?
In relation to the porosity of the weld, the TIG method gives the cleanest seams. One of the drawbacks of the TIG method is too much energy flow into the welding site. This leads to a significant softening of the weld area., what, however, typical for MIG welding. TIG welding is considered more convenient for repairing minor defects in cast aluminum parts.. Compared to MIG welding, However, TIG welding at a lower speed.
Preparation for welding an aluminum cast part
For, to get a good weld, some "rules" must be followed. Preparation methods for welding depend on the welding methods, thickness of the joined walls, type aluminum alloy. Excessive oxide layer on aluminum is sometimes removed by machining. When cleaning the surface with a grinding tool, do not use synthetic grinding discs in order to avoid porosity..
Another possible way to remove oxides from the surface of a part is to etch it.. Grease and dirt from the welding area must be removed in a suitable manner, taking into account possible pore formation.. Parts with thick walls must be preheated before welding.
ISO welding wire 18273
Welding wire is a standardized product. Requirements for the chemical composition of welding consumables for cast aluminum are determined by the international standard ISO 18273 (aka EN 18273). The choice of filler wire depends on the materials, which will be welded. For the most commonly used aluminum cast alloys, such as hypoeutectic and eutectic silumins, as well as thermally hardened alloys such as Al Si10Mg and Al Si5Mg, welding consumables S-Al Si12 and S-Al S5 are recommended.
A great danger in welding is the tendency of many materials to form cracks during the transition from liquid to solid state. These cracks are caused by tensile shrinkage welding stresses., which arise during cooling of the weld. Often, the number of phases with a low melting point in the welding wire is insufficient to heal emerging cracks. By choosing a softer welding wire with a higher proportion of these phases, this hazard can be reduced.. However, it is necessary to keep in mind the decrease in the strength properties of the weld.
Anodizing of welded aluminum castings
Decorative anodizing of welds made from the above mentioned welding alloys is not possible, this will make the weld look much darker than the base aluminum. Anodizing to improve corrosion resistance or adhesion, of course, always possible.
Castings processes for weld rework
Aluminum castings are created using many different castings processes. Some of the more notable ones include:
- die casting,
- investment casting,
- permanent mold casting,
- green sand casting.
Sand castings, investment castings, and permanent mold casting are all considered completely weldable. In this case, molten aluminum is cast into a mold that possesses somewhat poor thermal conductivity properties. So the castings take more time to solidify and all of the gasses from the molten aluminum have a chance to escape out .
But die casting is the odd one out here. The processes used here involve molten aluminum which is pressure-injected into a water-cooled steel mold. Due to the casting cooling down much quicker, in this case, the created gasses are not able to escape. This results in the creation of gas pockets in the castings themselves. These gas pockets make things very inconvenient for welders as you have to grind them out or repair weld them .
There are a few ways to determine whether you are dealing with die casting. The first and most notable way is looking at the surface. If it is smoother than other castings, then it is definitely a die casting.
Recomendations for welding aluminium casting
Welding is another molten metal process. The correct manufacturing steps can be defined, and the details will depend on the specific defect. Weld rework of aluminum castings incudes key steps to achieve high quality welds.
Those steps can be summarized as follows :
- Prepare the weld area. Remove the defect and remove the oxide layer with a brush or solvent.
- Preheat before welding. Preheat generally improves welding results. The usual range is 100–300 0C). Generally, aluminum alloys containing copper are welded at the higher end of that temperature. Castings will usually be welded in the as-cast condition but sometimes defects will not be uncovered until after heat treatment, so they are welded in the T6 or T7 condition. In 200 series alloys or for castings that require extensive repairs, it is recommended that T6 or T7 tempered castings be annealed before welding to avoid cracking.
- Use weld rod that matches the chemistry of the casting or an approved substitute. Chemistry control in welding is as important as it is in melting for casting. Use of a general-purpose rod across all alloy systems will result in reduction of mechanical properties and color mismatches even though welding may be “easier.”
- After welding, the weld needs to be dressed back to part contour. A common complaint of customers about welded castings is the weld has not been cleaned or there is a color difference. If a weld repair is noticeable with casual viewing, the repair has not been done correctly.
- Heat treat after welding. Even though a good weld will have little porosity and a fine structure, mechanical properties will not be met if the repaired casting is not heat treated to the specification. Heat treatment will also eliminate the residual stress that may have been caused by the welding process.
- Welding and joining aluminium castings //Aluminium Casting Alloys – Aleris materials, 2011