What Is the Weldability of Cast Aluminum?

Cast aluminum has a reputation for being difficult to impossible to weld. There is some truth to this reputation, but it isn’t the whole truth. In large part, this is due to many welders having limited experience trying to repair aluminum castings rather than higher specification cast aluminum plates. Aluminum castings are decorative molded pieces similar to cast iron. The difficulty comes from what was added to the aluminum in order to make it castable, usually a lot of silicone. This non-metallic content in the aluminum disrupts even distribution of the heat and causes cracking and other issues when it is heated during welding.

When it comes to the weldability of cast aluminum plates, these issues can also apply. MIC-6 cast aluminum plates, which has been the top tier in tooling plates for decades, also have a substantial amount of silicon. This has meant that many fabricators and welders who needed to repair a jig or assemble some custom tooling wound up watching a very expensive and hard to replace cast aluminum plate crack. The weldability of cast aluminum has traditionally been poor. However, it is not completely unweldable, and different formulations have been continuing to improve its weldability.

Understanding Cast Aluminum vs. Cast Aluminum Plates

It is important to understand what is meant by cast aluminum. When most people think of cast aluminum, they are thinking of any aluminum melted into a liquid state and then cooled into a shape. This melting process is why additives like silicon are needed. Aluminum is an incredibly reactive metal with a particular affinity for oxygen. This substantially increases when aluminum is heated into a molten state, and additives are needed to bond with the oxygen while keeping the aluminum itself largely metallic. This results in a rough, porous, and non-contiguous surface that is characteristic of aluminum castings.

Aluminum tooling plates like MIC-6 broadly fall into the same category. However, its use along with its appearance is very different. Cast aluminum plates are cast under highly-controlled conditions, stressed-relieved, and then finished with a high degree of surface flatness with deviations that can only be measured in nanometers. The reason for this is to get around the disadvantages of rolled aluminum plates where the rolling process itself induces internal stresses. In the intact plate, these internal stressors are balanced and keep the plate dimensionally stable; when the plate is ground or worked, these forces are no longer in balance, and the plate can become distorted. MIC-6 and other cast aluminum are very different from aluminum castings; they are high-spec and precision-machined metal stock with incredible dimensional consistency under mechanical and temperature stress compared to most aluminums.

This stability has made cast aluminum very desirable for processes beyond the aluminum tooling plate it’s really made for. This had consequences where a great deal of money was spent on MIC-6 cast aluminum, machining it, and finding out that it’s extraordinarily difficult to weld into a final assembly. Even when used in the roles it was meant for, the less than perfect weldability of cast aluminum MIC-6 plates poses a problem. A specialized repair is needed if the tooling plate is scratched or damaged, if it can be repaired at all. However, this doesn’t mean that MIC-6 is entirely unweldable—MIC-6 alternatives can actually be highly weldable.

The Weldability of Cast Aluminum Tooling Plates

MIC-6 components can be welded using gas metal arc welding processes like Gas Metal Arc Welding (GMAW/MIG) and Gas Tungsten Arc Welding (GTAW/TIG). Only pure Argon or more expensive Helium are acceptable gasses for welding MIC-6; carbon dioxide or mixes will not work. Friction Stir Welding (FSW) can also be used, but due to the materials like the high-silicon content, the following special considerations need to be kept in mind:

• Filler Material: High-silicon filler rod or wire will need to be used. 4043, 4145, and 5356 are all acceptable for use. 4145 is especially recommended as it’s high (10%) silicon content offers the best compatibility and weldability.
• Gas Coverage: The volume of gas coverage will need to be kept high at 40 to 50 cubic feet per minute in order to avoid oxidation. This is substantially more than the 20 to 30 cubic feet per hour for welding aluminum with GMAW recommended by Miller Welding.
• Heat Control: Due to it’s high silicon content interpass, temperatures should not exceed 150°F to avoid cracking. This is probably the most difficult part of welding MIC-6 as keeping heat within the accepted parameters will require going very fast using GMAW or exceptionally slowly using GTAW.

Even when these requirements are met, welded MIC-6 is not recommended for high-pressure service or duty service environments exceeding 150°F. A temperature below that which is required to bake a ham. In short MIC-6 is not a material that is recommended for welding in industrial applications. However, other cast aluminums are significantly more weldable.

MIC-6 alternatives, like Alca 5, ACP 5080, ATP-5, and K100-S, are all significantly more weldable, and the final welded product is suitable for a wide range of service duties and temperature duties. The reason is that these are all 5000 series aluminum. This type of aluminum has seen its primary use in shipbuilding for hulls due its corrosion resistance and has a long history of being welded that is very similar to other common aluminum types.

Aluminum manufacturers have developed new formulations within this class of aluminum alloys that are conducive to being cast and are competitive both in terms of its dimensional stability and temperature stability. In terms of weldability these cast aluminums far exceed MIC-6. This allows for the fabrication of more complex tooling like molds by joining different machined pieces by welding and opens up the possibility of very highly-machined and dimensionally stable structural aluminum elements. This new weldability for cast aluminum opens up a range of exciting possibilities. Although manufacturers will need to find a reliable supplier that they can count on.