Challenges in Dissimilar Welding & joining

Types of Dissimilar joining

Dissimilar welding and joining is defined when the two materials to be joined are of different nature, their chemical and or physical properties are not the same. These joints can be divided into three different categories that describe the different degrees of complexity present in the joints:

1. Metallurgically compatible dissimilar metal joining – Type I
2. Metallurgically incompatible dissimilar metal joining – Type II
3. Dissimilar material joining – Type III.

These different types of dissimilar joining are described below;

Type 1 – Metallurgically compatible dissimilar metal joining

Type I joining is characterized either by joining metals that are the same, but the crystallographic structure is different or in which the main metal constituent is common but the alloying elements are different (e.g. joining of different Al alloys). Alternatively Type I can also be the joining of two different metals which are metallurgically compatible, do not make any IMC phases in equilibrium conditions. These metallurgical compatible metals can be divided into two further categories, soluble and insoluble systems. Silver and gold are an example of a soluble type 1 dissimilar metallic combination (Figure below).

File:Ag-Au-phase-diagram-greek.svg - Wikimedia Commons
Ag (silver) – Au (gold) phase diagram

The above Ag-Au phase diagram shows which phases are present in equilibrium conditions for any concentration of the two metals, the Ag-Au system does not form any intermediate brittle phases in equilibrium conditions for any weight percentage and it also forms solid solutions in all ranges of self dilutions. This happens when two metals are very close in the periodic table and their crystal structure is similar. Both Ag and Au have a face-centered cubic crystal structure and both elements are placed in the eleventh column of the Periodic table.
Iron and copper is an example of a system that is type I, but is insoluble, these metals do not form any intermediate phases, but they do not present a solid solution overall ranges of self-dilutions. They have two small solid solution regions and show a large region where the two phases co-exist with similar properties, as can be observed in the phase diagram in Figure below.

Solidification Microstructure Evolution of Undercooled Cu-15 wt.% Fe Alloy  Melt

The previously mentioned dissimilar metallic combinations do not show the presence of any brittle intermediate phases. Nevertheless as the thermal and physical properties of the participating metals are different, cold cracking,
galvanic corrosion, and other incompatibilities can be originated in a joint created by such dissimilar systems.

Type 2 – Metallurgically incompatible dissimilar metal joining

Dissimilar metal joining of Type II corresponds to joining of two metals which are metallurgically incompatible and therefore form several intermediate phases over a broad range of composition ratio. Examples of joining of Type II are Fe to Al, Fe to Ti, Al to Ti, etc. The metallurgical incompatibility between these metals can be verified by the intermediate phases present on their corresponding phase diagrams (Figure below).

The Al-Ti equilibrium phase diagram. | Download Scientific Diagram

From above Figure it is possible to identify four intermediate phases at room temperature (Ti3Al, TiAl, TiAl2 and TiAl3) that are formed at equilibrium conditions between Ti and Al. The main issue with the intermediate phases formed is the brittle nature of these compounds. This brittle nature causes a reduction of the mechanical properties and enhances the crack formation within these joints. Phase diagrams are determined in equilibrium conditions, however welding processes are transient. This enables the formation of IM phases different from the phases shown in the phase diagram. Besides the intermediate or intermetallic phase formation the same dissimilar physical and thermal properties identified for type 1 dissimilar joining are also applied to this type of dissimilar joining.

Type 3 – Dissimilar material joining

This type of dissimilar welding does not restrict itself to metallic systems, and so welding and brazing techniques are less applicable. This type of joining is necessary when trying to join dissimilar materials, e.g. composite material to a metal (carbon fiber reinforced polymer to Ti), glass to metal, plastics to metal, etc.

To avoid the extreme dissimilarities presented in this type of joining, fusion welding processes are usually avoided and mechanical joining with rivets and bolts has been used, but the added weight can be a disadvantage. Glues were also used, but there are also limitations in their applicability for higher working temperatures.

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