Why You Cannot Arc Weld Steel to Aluminum

The major reason that you cannot arc weld steels to aluminum and its alloys is the significant difference in melting points of these two metals. The melting point of aluminum is 660 Deg C and that of carbon steel is around 1370 deg C, so as you try to melt the steel the aluminum just flows away from the attempted joint. As a result, just as the steel is beginning to get warm the aluminum is already molten and will end up as just a mess on your workshop floor.

In addition to the large difference in melting points, neither of these metals could come together to form a satisfactory weld even if arc welding was possible as they would form brittle metallurgical phases in any joint.

Apart from melting points and metallurgy, other significant reasons for the difficulty in joining these two metals together are the differences in thermal conductivity (ability to conduct heat) and thermal expansion (ability to change shape as the temperature changes). To the nearest round number, the thermal expansion of aluminum is twice that of steel and the thermal conductivity of aluminum five times that of steel.

So, the application of arc welding processes to join steel to aluminum is fraught with the problems as detailed above. The use of lasers, use of bimetallic strips and so-called “solid-state” friction welding are some of the approaches taken to weld aluminum to steel. However, these methods are beyond the scope of this short article.

Arc Welding of Austenitic Stainless Steels to Carbon Steels.

At the outset, it is important to understand that there are several different types of stainless steels and, for this short note, the stainless steels we will comment on are the 300 series, commonly defined as the Austenitic Stainless Steels. These encompass the well-known alloys such as 304L, 309L and 316L types and their higher carbon derivatives, 304, 309 etc.

When welding stainless steel to carbon steel, it is critical to pay attention to the chemistry that will form in the molten pool. The 300 series contain several alloying elements, particularly nickel and chromium, in large amounts. These elements will influence the properties of the final joint. Basic carbon steel does not contain significant amounts, if any, of these elements.

When welding, the final chemistry of the joint is not just a product of the consumable but also a product of the amount of base metal incorporated into the weld by the melting of material from both sides of the joint. In effect, and from Figure 1, the final composition of the weld (W) is a mixture of melted metal A (carbon Steel), melted metal B (stainless steel) and the chosen welding consumable itself (consumable C). The effect of some base metal being melted and incorporated into the final weld is called “dilution” and the degree of this dilution will depend on the amount of penetration/melting that the welding procedure produces.

Schematic of Dilution Effect

Figure 1. Schematic of Dilution Effect

A significant problem with the welding of carbon steel to 300 series stainless is the possible production of a hardened weld zone (W) which may crack. This hardening effect is produced by certain alloy elements in the weld pool and is particularly relevant to the chemistry of a carbon steel consumable (ferrite based) and not the chemistry of a 300 stainless steel conumable (austenite based).

These metallurgical identifiers, ferrite and austenite, are called phases. It is important for the practitioner to understand that metallurgical phase development is something that is happening in the cooling weld pool that they are depositing. This phase development, ongoing at a micro level, cannot be seen with the eye but it’s happening and you must be aware of it.

Generally, in the world described above, if we try joining a carbon steel plate to a 300-type stainless plate using carbon steel electrodes we will end up with a weld deposit that is hard and is very crack sensitive. If we choose to weld the same joint using the correct 300 austenitic stainless series electrode we will end up with a joint which is not hard and not subject to the same cracking mechanisms. This is entirely due to the dilution and its differing effect on the chemistries of the carbon steel consumable vs. the austenitic steel consumable.

The conclusion therefore is:

The welding of carbon steel to a 300 series stainless steel with carbon steel electrodes is incorrect and can cause cracking. This simple mistake has been the cause of many problems in industry.

The welding of carbon steel to a 300 series stainless steel with a selected 300 series stainless steel electrode is the correct choice in making an acceptable joint between these two materials

Welding processes, techniques and consumables that are recommended for welding this combination are available in the literature and from suppliers. For example, to weld 304L austenitic stainless steel to carbon steel, the most commonly recommended filler metal is austenitic stainless steel 309L. Process and electrode selection will be covered in a future article.


Author: Mick Pates, President of PPC and Associates