Key Issues: RSW Steel and Aluminium Joints

Key Issues: RSW Steel and Aluminium Joints

Joining Dissimilar Materials, Resistance Welding Steel to Aluminium

Among various dissimilar material combinations, Al-steel is one of the most desirable combination. This section will address the number of fundamental issues faced during al-steel joining. Later section will discuss the current approaches to overcome these issues.

Strong and adherent aluminium surface oxide layer needs to be removed for a sound joint between aluminium and steel. The presence of this oxide layer creates uncertainty in the Resistance Spot Welding (RSW) process and deteriorates the electrodes, hence decreasing electrode life. Moreover, it creates wettability issues between steel and aluminium which is an essential for joint formation. Joint is established once surface oxide layer is broken.

Formation of intermetallic compounds are unavoidable during Al-steel RSW joints. Figure 1 shows the phase diagram between aluminium and iron and from the phase diagram various intermetallics compounds are formed during the RSW process. Formed intermetallics could be iron rich or aluminium rich. Aluminium rich intermetallics are undesirable due to higher hardness compared to iron rich intermetallics. These intermetallics act as stress concentrators, increases the brittleness and making the Al-steel joints more susceptible to cracking.

Al-steel RSW joints. Shows the phase diagram between aluminium & iron and from the phase diagram various intermetallics compounds are formed during the RSW process

Figure 1. Al-Fe phase diagramL-8

 

Another issue is to attain heat balance between aluminium and steel. Compared to steel, aluminium has more electrical & thermal conductivity, and lower bulk resistivity,  usually requiring higher amount of current. A large variation in the melting temperature of the aluminium and steel make the RSW process more complicated. Moreover, the thermal conductivity of aluminium is much higher than steel, and a consequent thermal gradient exists and could result in an isolated nugget formation in steel. These factors make the selection of RSW process parameters more challenging.

Excessive electrode indentation in aluminium is another issue that determines the stress state of the joint during loading. It decreases the resistance to necking and decreases the peak load to failure.

Other defects include formation of voids and porosities due to shrinkage, vaporization, expulsion and lack of wetting. Other section in this menu will discuss the basic to advanced, updated approach utilized to improve the joint formation process between aluminium and steel.

 

Joining Dissimilar Materials

Joining Dissimilar Materials

Joining Dissimilar Materials

Multi-material design approach involves using High-Strength Steels and low-density materials, such as aluminium. The key idea is to utilize the right material for the right application in such a way that it should fulfill the service requirements and achieve mass efficiency at the same time. However, it is challenging to fully utilize multi-material design concepts due to joining issues.

Joining is considered a backbone of any manufactured assembly, since the ultimate reliability and integrity of the manufactured product relies on these joints. Dissimilar materials are difficult to join due to the difference in the physical and thermal properties. For example, one of the most desired combinations is aluminium to steel joints, which is challenging due to the large mismatch in the mechanical properties as shown in Table 1. This Joining section is dedicated specifically to the key joining issues between aluminium and steel.

Table 1:  Aluminium vs. Steel Material Mechanical Properties

Table 1:  Aluminium vs. Steel Material Mechanical Properties

Currently there are three approaches to joining dissimilar materials: solid-state joining, partial solid-state joining and arc joining. During solid state welding, the peak temperature of the process remains below the melting temperatures of the materials to be joined. Examples include diffusion bonding, ultrasonic welding, magnetic pulse welding, friction stir welding and vaporizing foil actuator welding. In partial solid-state welding processes, the peak temperature goes beyond the melting temperature of one of the materials to be joined (e.g. arc brazing & resistance spot welding). The last process (arc welding) has a peak temperature above the melting temperature of both materials to be joined. Practically it is not used anywhere however, it can be utilized in certain applications where the mismatch between the physical properties of the materials to be joined is little.

Among various joining processes, resistance spot welding (RSW) is one of the most widely used joining process in the automotive and aerospace industry due to its ease of automation and high productivity.  Browse the topics below to learn more about joining dissimilar materials.